JPH08160330A - Optical scanning device - Google Patents

Abstract

PURPOSE: To compensate optical performance so as to maintain the performance in both main scanning direction and sub-scanning direction in an optical system including a resin lens complying with the increase of resolving power of a laser beam printer, etc. CONSTITUTION: In an optical system composed of a light source 1, an incident optical system 2, 4, a deflector 5, a scanning optical system 7, 8 and a medium to be scanned 10, the scanning optical system 7, 8 is an anamorphic scanning optical system including at least one optical element made of resin having a positive power, the incident optical system is an anamorphic optical system composed of a first optical system 2 connected to the light source 1 each other by a fixing member 3 and a second optical system 4 making a divergent light beam from the light source nearly a parallel beam in the main scanning direction and form the image in the vicinity of the deflector in the sub-scanning direction and an optical element made of resin having a negative power in the sub-scanning direction is included in either the first optical system or the second optical system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-resolution optical scanning device, and more particularly to an optical scanning device in which back focus change due to temperature change is corrected.

[0002]

2. Description of the Related Art In recent years, with the spread of laser printers and the like, scanning optical systems are increasingly required to be small in size, inexpensive, and high in performance. Therefore, an aspherical lens is often used to reduce the number of lenses and to make it compact, but in order to obtain this aspherical lens at low cost, the use of a plastic lens is indispensable.

However, when the lens of the scanning optical system is made of plastic, the influence of the change in the refractive index and the linear expansion due to the temperature change is large. Therefore, when a certain degree of resolution is required, the plastic lens is used to improve the environment of use. Performance may not be maintained due to fluctuations, especially temperature changes. In addition, due to the influence of the wavelength change of the light source, when the temperature changes with respect to the reference design temperature, the focus position moves independently in the main scanning direction and the sub scanning direction on the image plane of the scanning optical system.

At this time, the change of the field curvature around the medium to be scanned due to the temperature change tends to shift in the defocus direction while substantially maintaining the shape of the field curvature of the scanning optical system at the reference design temperature. . Further, the movement of the focus position, that is, the change in back focus, monotonically increases or monotonically decreases with respect to the temperature change. If the amount of focus movement exceeds the allowable depth of beam diameter in consideration of the amount of curvature of field of the scanning optical system in the image height direction, the performance cannot be maintained.

The beam spot diameter, which affects the performance due to temperature change, is more susceptible to the smaller depth, that is, the smaller the beam waist diameter. If the beam waist diameter is made smaller, the depth becomes narrower even with the same focus movement amount, and the depth becomes smaller due to the focus movement amount, and accuracy cannot be maintained.

In order to solve this problem, the change of the image plane of the scanning optical system with respect to the temperature change tends to shift with the curvature of the image plane in the image height direction substantially maintaining its shape. It can be corrected by the incident optical system on the light source side.

As an example based on such a design concept, in Japanese Patent Laid-Open No. 1-92714, resin lenses are provided in the incident optical system and the scanning optical system, respectively, and one of them has a positive power and the other has a negative power. As a result, the back focus change with respect to the temperature change of the scanning optical system is offset in the sub-scanning direction.

In Japanese Patent Laid-Open No. 5-341215, an anamorphic lens in which an incident optical system makes parallel light in the main scanning direction and convergent light in the sub scanning direction, and a sub scanning direction having almost no power in the main scanning direction. It consists of a single cylindrical lens with negative power.

JP-A-5-341216 and JP-A-5-3
In No. 41217, the incident optical system is fixed by a negative rotationally symmetric plastic lens included in the collimator lens and an anamorphic plastic lens having a positive power only in the main scanning direction, or by a light source and a plastic member. By using a combination of a collimator lens and an anamorphic plastic lens that has a positive power only in the main scanning direction, focus fluctuation due to temperature change is suppressed only in the incident optical system in the main scanning direction, and the temperature of the entire optical system is decreased in the sub scanning direction. Focus fluctuations due to changes are suppressed.

Further, in Japanese Patent Application Laid-Open No. 2-161410, the expansion and contraction of a member connecting a collimator of an incident optical system and a light source, the refractive index change and expansion and contraction of an optical material, and the back focus change with respect to the temperature change of the light source are integrated. , The back focus change with respect to the temperature change of the scanning optical system in both the main scanning direction and the sub scanning direction is balanced and corrected so that the depth can maintain the performance in each of the main scanning direction and the sub scanning direction. .

However, recently, there has been a demand for further downsizing and cost reduction of a laser beam printer or the like corresponding to a higher resolution, and it is also necessary to use a resin for a scanning optical system corresponding to a model corresponding to a higher resolution. ing. Therefore, the beam spot diameters in the main scanning direction and the sub scanning direction must be made smaller, and the depth must be made smaller in both directions, which cannot be dealt with by the above conventional technique.

Specifically, in the case of Japanese Patent Laid-Open No. 1-92714, in the sub-scanning direction, the back focus change of the scanning optical system with respect to the temperature change is made by a resin-made cylindrical lens having a negative power in the incident optical system. Although it can be corrected, in the main scanning direction, this incident optical system has almost no effect on the temperature change, and the back focus change of the scanning optical system due to the temperature change cannot be corrected. For this reason, it is insufficient as a measure for designing a high-resolution scanning optical system.

JP-A-5-341215 and JP-A-5-3
No. 41216 and JP-A-5-341217, as an effect, correction of temperature change of the scanning optical system can be made only in the sub-scanning direction as in JP-A-1-92714, and temperature change of the scanning optical system in the main scanning direction. The effect of remains.

Japanese Unexamined Patent Publication No. 2-16140 is intended to correct changes in both main scanning and sub-scanning due to back focus change due to temperature change of a collimator fixed to each other by a light source and a fixing member. Generally, the scanning optical system is affected by the difference in the back focus change amount in the main scanning direction and the sub scanning direction due to the temperature change of the scanning optical system, and the difference in the vertical magnification in each direction in the entire system including the incident optical system and the scanning optical system. Since the influence of the back focus change amount due to the temperature change of the light source and the collimator on the back focus change amount on the image plane of the system is different in the main scanning direction and the sub-scanning direction, the target of the back focus change due to the temperature change of the collimator Most of the values differ between the main scanning direction and the sub scanning direction. On the other hand, since the collimator is optically rotationally symmetrical about the optical axis, the back focus change due to the temperature change of the collimator is the same value in both main scanning and sub scanning, and cannot correspond to two different target values.

[0015]

However, since the beam waist diameter has been large in the related art, the permissible depth for maintaining the performance has a slight width. As shown in FIG. 2A, the back focus fluctuation range of the collimator due to temperature change has a target value with a wide range in each direction (in the figure, 2 · Δm in the main scanning direction and 2 · Δs in the sub scanning direction). ) Had. Therefore, even if the target values in the main scanning direction and the sub-scanning direction are different, the overlapping portion in the width in the main scanning direction and the width in the sub-scanning direction (for example, in the range from b-Δs to a + Δm in the figure). ) Occurs, and it should have been set as the target range of the back focus change of the collimator due to the temperature change.

On the other hand, in order to cope with higher resolution, the beam diameter of each of the main scanning and the sub scanning must be set small, and as a result, the allowable depth for maintaining the performance becomes narrow. As shown in FIG. 2 (b), the allowable width in each direction of the back focus variation range of the collimator due to temperature change becomes small. As a result, the width in the main scanning direction and the width in the sub scanning direction do not overlap as shown in FIG. 2B, and there is no usable range of the back focus change amount of the collimator that can maintain the performance with respect to the temperature change in both directions. Will end up.
For this reason, in this state, it was impossible for the conventional technique to perform correction so as to maintain the performance in both the main scanning direction and the sub scanning direction with respect to the temperature change.

With the increase in resolution of laser beam printers and the like, the allowable depth of the beam waist is narrowing in both the main scanning direction and the sub-scanning direction, and in addition, the scanning optical system includes a resin lens. The image plane position moves along the optical axis due to the temperature change. At this time, if the back focus position of the scanning optical system moves due to temperature change in a narrow depth, the conventional technique as described above cannot compensate the beam diameter on the fixed scanning medium,
It has not been able to sufficiently meet the recent demand for higher resolution. The present invention intends to obtain an optical scanning device which can meet the demand for higher resolution in an optical system including a resin lens.

[0018]

The optical scanning device of the present invention comprises:
Includes a light source, an incident optical system, a deflector, a scanning optical system, and a medium to be scanned, and the light emitted from the light source is deflected by the deflector via the incident optical system to form an image on the medium to be scanned via the scanning optical system. In the optical scanning device that scans on the medium to be scanned, the scanning optical system is an anamorphic optical system including at least one resin optical element having a positive power, and the incident optical system is a light source and a fixing member. Is an anamorphic optical system fixed to each other by a first optical system that makes the divergent light emitted from the light source in the main scanning direction substantially parallel light, and the light from the light source through the first optical system. , A second optical system which forms an image near the deflector in the sub-scanning direction, and a resin optical element having negative power in the sub-scanning direction in either the first optical system or the second optical system. It is characterized by including.

ΩoM is the beam waist diameter on the optical axis in the main scanning direction on the image plane of the scanning optical system, ωoS is the beam waist diameter on the optical axis in the sub scanning direction on the image plane of the scanning optical system, and Mm is The absolute value of the lateral magnification of the entire optical system in the scanning direction (main scanning), Ms is the direction perpendicular to the scanning direction (sub-scanning direction)
Absolute value of lateral magnification of all optical systems, ΔfB ₂ M (ΔT), ΔfB
₂ S (ΔT) is the back focus change in each of the main scanning direction and the sub scanning direction by the scanning optical system only when the temperature change is ΔT, and λ is (0.18 / λ). ^{[(ωoM 2 / Mm 2) +} (ωoS 2 / Ms 2) ] _{≦ | △ fB 2 M (△} Tmax) / Mm 2 - △ fB 2 S (Tmax) / Ms 2 | or (0.18 / lambda) - ^{[(ωoM 2 / Mm 2) +} (ωoS 2 / Ms 2) ] _{≦ | △ fB 2 M (△} Tmin) / Mm 2 - △ fB 2 S (Tmin) / Ms 2 | is preferably satisfied. However, the definition of the beam diameter is the diameter of the intensity of 1 / e ² of the beam profile, and ΔTm
ax (≧ 0) and ΔTmin (≦ 0) represent the upper and lower limits T ₀ + ΔTmin ≦ T ₀ ≦ T ₀ + ΔT max of the temperature change amount with respect to the temperature range of the operation specification with respect to the reference design temperature T ₀ .

The above-mentioned incident optical system, specifically, the first optical system is a collimator optical system whose optical surface is rotationally symmetric with respect to the optical axis, and a second optical system located behind the collimator as seen from the light source. Is an anamorphic optical system that has a resin optical element that has negative power in the sub-scanning direction, has almost no power in the main scanning direction, and has positive power in the sub-scanning direction. The light emitted from the light source is substantially parallel light in both main scanning and sub-scanning directions.

Alternatively, the first optical system has a function of making the light from the divergent light source substantially parallel light in the main scanning direction and substantially functions as a collimator, and has a negative power in the sub scanning direction. Is an anamorphic optical system having an anamorphic optical element made by, the second optical system is an anamorphic optical system having positive power in the sub scanning direction as a whole, and the light emitted from the first optical system is the main scanning The light is almost parallel in the direction and divergent in the sub-scanning direction. At this time, the anamorphic optical element made of resin having a negative power in the sub-scanning direction may be a cylindrical lens.

[0022]

When the scanning optical system includes a resin optical element having power, the influence of the temperature change of the scanning optical system on the back focus change is composed of all glass optical elements, not including the resin optical element. It is considerably larger than the optical system. In the case of a resin, generally, when the temperature rises above the reference design temperature, the refractive index of the resin becomes lower than the reference design and the resin itself tends to expand. In addition, a light source such as a semiconductor laser has a longer oscillation wavelength as the temperature rises. As a result, as can be said for a glass lens, in a resin optical element having a positive power, the back focus tends to be extended due to the temperature rise. At this time, the curvature of field around the scanned medium of the scanning optical system with respect to the image height at the reference design temperature is
There is a tendency for defocus shift by the amount of change in back focus while the curved shape of the field surface is substantially maintained.

Therefore, when the scanning optical system includes a resin optical element and the total power of the resin optical elements in each of the main scanning direction and the sub-scanning direction is positive, it is assumed that the light traveling direction is positive. ΔfB ₂ M (ΔT) / ΔT ≧ 0 (1) ΔfB ₂ S (ΔT) / ΔT ≧ 0 (2) Here, ΔfB ₂ M (ΔT) and ΔfB ₂ S (ΔT) represent the back focus change amount due to only the scanning optical system when the temperature changes by ΔT in the main scanning direction and the sub scanning direction, respectively.

As described above, the curvature of field of the scanning optical system only shifts in the defocus direction while maintaining its shape with respect to temperature changes. Focus changes can be corrected. At this time, the back focus change amount due to the temperature change of the scanning optical system in the incident optical system is corrected in each of the main scanning direction and the sub-scanning direction, and each of the temperature changes due to the temperature change when the incident optical system and the scanning optical system are combined. The back focus change amount in the direction must be suppressed within a range in which the performance can be maintained. That is, back focus change amounts in the main scanning direction and the sub scanning direction on the medium to be scanned of the entire system including the incident optical system and the scanning optical system with respect to the temperature change ΔT, ΔfBm (ΔT) and ΔfBs ( ΔT) is a range δ (main scanning direction δm, sub scanning direction δs) in which the performance described below can be maintained.
Must satisfy the following relationship. | ΔfBm (ΔT) | ≦ δm (3) | ΔfBs (ΔT) | ≦ δs (4)

The ranges in which the performance can be maintained, δm and δs, are the one-side depths Xom and Xos in each direction where the performance can be maintained, from the processing errors of the scanning optical system, the main scanning direction due to the placement error, and the one-side margins Pm and Ps in the sub-scanning direction. And the main scanning direction that occurs in the design,
It can be defined as a value obtained by subtracting the one-side designed field curvature margins Dm and Ds with respect to the image height direction in the sub-scanning direction. δm = Xom-Pm-Dm (5) δs = Xos-Ps-Ds (6) Here, the one-side depths Xom and Xos capable of maintaining the performance are the beam waist diameter on the optical axis of the scanning optical system, and its allowable value. Determined by beam variation and beam allowable depth. The reason that one side of the depth is defined here is that many optical devices have a specification that the operating temperature changes in both the positive direction and the negative direction with respect to the reference design temperature.

The allowable beam depth on the optical axis can be obtained as follows. Assuming that the beam is a Gaussian beam, the beam spot diameter ω around the position where the beam is focused is

[Equation 1] Is represented by Here, λ is the wavelength of the beam, ω ₀ is the beam waist as shown in FIG. 3, and x is the distance from the beam waist position to the position where the predetermined beam spot diameter (ω) is obtained. Further, the definition of the beam diameter represents the diameter of the intensity of 1 / e ² of the beam profile. here,
The allowable beam diameter for the beam waist ω ₀ is ωs
Then, one side X ₀ of the depth that allows the beam diameter for the depth 2X _{0 at} that time is ω = ω in the equation (7).
s, x = X _0, and the one-sided allowable depth X ₀ is

[Equation 2] Here, ε = ωs / ω ₀ : Beam tolerance coefficient.

In the formula (8), the requirement for the beam diameter variation becomes strict, especially for high resolution, and the beam tolerance coefficient must be set to ε = 1.1. ε = 1.
4, the relationship between the beam waist ω ₀ and the allowable depth on one side X ₀ when the wavelength λ used is 780 nm is shown in FIG. As shown in FIG. 4, as an example of an optical system using a conventional plastic lens, the beam waist diameter on the optical axis is 70 μm in the main scanning direction and 100 μm in the sub scanning direction.
If m is ε = 1.1, the permissible depth on one side of the main scanning was about 2.3 mm, and the depth for sub-scanning was about 4.5 mm. When the scanning is 50 μm and the sub-scanning is 60 μm, the main scanning one side depth is about 1.1 mm, and the sub-scanning one side depth is about 1.7 m.
It can be seen that it is considerably small as m. Summarizing the above in the main scanning direction and the sub scanning direction,

(Equation 3) Becomes Where ω ₀ M is the beam waist diameter on the optical axis in the main scanning direction around the scanned medium of the scanning optical system, and ω ₀ s is on the optical axis in the sub-scanning direction around the scanned medium of the scanning optical system. Beam waist diameter.

The one-side designed field curvature margins Dm and Ds in the main scanning direction and the sub-scanning direction are based on the design values of the scanning optical system. Usually, in the scanning optical system, the curvature of field is generated around the medium to be scanned in the image height direction under the influence of the entrance and exit of the surface of the reflecting surface of the rotary polygon mirror that is the deflector. Recently, a proposal for a scanning optical system that corrects the influence of the entrance and exit of the surface of this rotating polygon mirror has been announced for high resolution, but this may also cause some field curvature to remain in the image height direction. is there. When the image height becomes large, aberration occurs in the beam and the beam diameter becomes large, and the depth at that image height tends to be smaller than that on the axis. This one-sided field curvature margin is the entire image height corresponding to the print width of the scanning optical system on the medium to be scanned, and the one-sided amount of curvature of field variation and the decrease in depth due to aberration in the beam having the image height. It represents.

The processing errors in the main scanning direction and the sub-scanning direction, and the placement error margins Pm and Ps are image heights due to actual working errors of optical elements of the optical scanning device or placement errors of optical elements of the optical scanning device. Represents the amount of field curvature with respect to. Especially, when it becomes compatible with a high-resolution optical scanning device having a small beam diameter, this influence becomes large.

The margin of the one-sided design field curvature margin in the main scanning direction and the sub-scanning direction, the processing error, and the placement error margin is small, especially in a high-resolution optical scanning device. Therefore, at least the scanning optical system is used. It is necessary to allow 1/2 of the on-axis depth of. Dm + Pm≡X ₀ m / 2 (11) Ds + Ps≡X ₀ s / 2 (12) The ranges δm and δs in which the performance can be maintained are expressed by equation (5), (6).
From the formula, the formula (9), the formula (10), the formula (11), and the formula (12)

[Equation 4] Can be represented by From this equation, when the scanning optical system has a higher resolution, the axial beam waist diameter becomes smaller, and the ranges δm and δs where the performance can be maintained become smaller in proportion to the square of the beam waist diameter, and the allowable width becomes smaller. I understand.

As means for correcting by the incident optical system,
The incident optical system is fixed to each other by a light source and a fixing member, and comprises a first optical system for making divergent light emitted from the light source into substantially parallel light in the main scanning direction, and the first optical system from the light source. A second optical system, which is a cylindrical optical system having a power in the sub-scanning direction, which forms an image of light in the vicinity of the deflector in the sub-scanning direction. In addition to this, the first optical system or the second optical system. In any of the above, if an optical system of a type including a resin optical element having a negative power in the sub-scanning direction is used, the back focus change amount due to the temperature change of the entire optical system in each main scanning and sub-scanning direction There is a possibility that each direction can be zero. The following three factors are considered to be the major factors that the incident optical system influences the back focus change around the medium to be scanned in the entire optical system due to the temperature change. Influence of back focus change around the medium to be scanned due to change in the distance between the light source and a part of the incident optical system due to expansion and contraction of the fixing member due to temperature change ΔT Main scanning direction ΔfBML (ΔT) Sub Scanning direction ΔfBSL (ΔT) Influence of back focus change around scanned medium due to wavelength change due to temperature change of light source Main scanning direction ΔfBMλ (ΔT) Sub scanning direction ΔfBSλ (ΔT) Incident due to temperature change Expansion and contraction of optical elements in the optical system,
Effect of back focus change around scanned medium due to refractive index change Main scanning direction ΔfBMN (ΔT) Sub scanning direction ΔfBSN (ΔT)

A back focus change amount ΔfBm (Δ in the main scanning direction and sub scanning direction due to a temperature change ΔT around the medium to be scanned in the entire optical system including the incident optical system and the scanning optical system.
T) and ΔfBs (ΔT) are the back focus change amount ΔfB ₂ M in the main scanning direction and the sub scanning direction due to the temperature change ΔT of the scanning optical system due to the influence of the three temperature changes of the incident optical system.
Since (ΔT) and ΔfB ₂ S (ΔT) are added for each direction, ΔfBm (ΔT) = ΔfBML (ΔT) + ΔfBMλ (ΔT) + ΔfBMN (Δ T) + ΔfB ₂ M (ΔT) (15) ΔfBs (ΔT) = ΔfBSL (ΔT) + ΔfBSλ (ΔT) + ΔfBSN (ΔT) + ΔfB ₂ S ( ΔT) can be represented by (16). By utilizing such a relationship, the influence of the temperature change of the scanning optical system is corrected by the incident optical system,
The back focus change amount due to the temperature change in the entire optical system must be suppressed within a range in which a predetermined performance can be maintained.

In the optical scanning device of the present invention, the incident optical system is fixed by the light source and the fixing member to each other at least up to the optical system portion having a function of making the light emitted from the light source in the main scanning direction substantially parallel light. There is. One of the light source and the first optical system of the incident optical system, which are fixed to each other by the fixing member.
An example is shown in FIG. In FIG. 5, reference numeral 1 is a semiconductor laser as a light source, 2 is a first optical system of an incident optical system for converting divergent light into substantially parallel light in the main scanning direction, and 2'is a lens frame of the optical system. The lens frame of this optical system and the semiconductor laser are fixed to each other by three fixing members. Here, the fixing member 3 is made of a material having a coefficient of linear expansion larger than that of the lens frame 2'and the spacer due to temperature change.

Since the linear expansion coefficient of the member with respect to the temperature change is usually positive, the fixing member expands when the temperature rises, and the distance between the lens frame 2'of the optical system and the light source 1 slightly increases. As a result, the light source moves in the negative direction with respect to the traveling direction of the light when viewed from the incident optical system. The distance between the mounting portion of the light source to the fixing member and the mounting portion of the lens frame to the fixing member is lx (the spacing lx is the distance between the mounting portion of the light source to the fixing member and the mounting portion of the incident optical system). Actually, it depends on the mounting structure.I experimented several times to calculate the value corresponding to this interval,
As a result, it is known that it is almost equal to the focal length of the incident optical system in the main scanning direction. ), Where L is the linear expansion coefficient of the fixing member, and ΔS (ΔT) is the amount of movement of the light source when viewed from the incident optical system when the temperature changes by ΔT, ΔS (ΔT) =- L·lx · ΔT (17)

When the absolute value of the lateral magnification in the main scanning direction in the entire system is Mm and the absolute value of the lateral magnification in the sub-scanning direction is Ms, the light source enters the light source due to the expansion and contraction of the fixing member due to the temperature change ΔT. Back focus change in the main scanning direction around the medium to be scanned due to the change in the distance from the optical system ΔfBML
(ΔT) and change in the sub-scanning direction ΔfBSL (ΔT) are ΔfBML (ΔT) = Mm ² · ΔS = -Mm ² · L·lx · ΔT (18) ΔfBSL (ΔT) = Ms ² · ΔS = −Ms ² · L·lx · ΔT (19) Since the linear expansion coefficient with respect to temperature change is L ≧ O and the distance lx ≧ 0 between the light source and the optical system, ΔfBML (ΔT) / ΔT ≦ O in the main scanning direction (20) Sub-scanning direction ΔfBSL (ΔT ) / ΔT ≦ O (21). From (1) and (2), the change in back focus due to the temperature change of the scanning optical system having a positive power becomes positive,
From the expressions (15), (16), (20), and (21), the influence of the fixing member that attaches the light source and the optical system to the temperature change is a negative change in the back focus around the medium to be scanned of the scanning optical system. , Tend to cancel each other out.

When the light source is a semiconductor laser, the oscillation wavelength slightly changes due to the temperature change. As a result, the refractive index of the optical material in the incident optical system changes, and the back focus position around the medium to be scanned in the scanning optical system also changes due to the change. When viewed in the main scanning direction, the incident optical system has a function of making the light beam emitted from the light source substantially parallel light, so that the back focus position thereof is close to infinity. Therefore, it is difficult to discuss the back focus change position due to the temperature change as it is with only the incident optical system. To simplify this discussion, in the main scanning direction, when parallel light whose wavelength has changed due to temperature change is incident on the incident optical system from the side opposite to the light source, The back focus change amount due to the wavelength change may be obtained, and the back focus change on the medium to be scanned due to the above influence may be calculated from the relationship of the vertical magnification of the entire system including the incident optical system and the scanning optical system.

With respect to the influence of the wavelength change of the light source of the incident optical system in the main scanning direction, the back focus change amount in the periphery of the light source of the incident optical system when parallel light of which the wavelength is changed is incident from the opposite side of the light source, If ΔfB ₁ Mλ (ΔT), the back focus change ΔfBMλ due to the influence of the wavelength change of the incident optical system around the medium to be scanned in the main scanning direction
(ΔT) has a relationship of ΔfBMλ (ΔT) = ΔfB ₁ Mλ (ΔT) · Mm ² (22).

When viewed in the sub-scanning direction, the light emitted from the light source is once condensed near the deflector via the incident optical system. Therefore, the back focus change near the deflector of the incident optical system due to the wavelength change of the light source caused by the temperature change is Δf.
As B ₁ Sλ (ΔT), the absolute value of the lateral magnification of the incident optical system in the sub-scanning direction is M ₁ s, and the absolute value of the lateral magnification of the scanning optical system is M ₂ s.
Then, the back focus change ΔfBSλ (Δ due to the wavelength change of the incident optical system around the medium to be scanned in the sub-scanning direction
T) can be represented by ΔfBSλ (ΔT) = ΔfB ₁ Sλ (ΔT) · M ₂ s ² (23). The magnification relationship is Ms = M ₁ s · M ₂ s (24). The above is the effect of the back focus change due to the wavelength change caused by the temperature change of the incident optical system on the periphery of the medium to be scanned.

The back focus change due to the wavelength change of the light source caused by the temperature change of the incident optical system can be further decomposed into the following forms. fB ₁ M, fB ₁ when S is referred to as back focus value at the incident optical _{system, △ fB 1 Mλ (△ T} ) / △ T ≒ ∂fB 1 M / ∂λ · ∂λ / ∂T = Σ∂fB 1 M / ∂ni ・ ∂ni / ∂λ ・ ∂λ / ∂T (25) △ fB ₁ Sλ (△ T) / △ T = Σ∂fB ₁ s / ∂ni ・ ∂ni / ∂λ ・ ∂λ / ∂T (26) Here, ni represents the refractive index of the i-th optical element of the incident optical system, and λ represents the wavelength of the light emitted from the light source.

(∂λ / ∂T) is positive when a semiconductor laser is used as a light source because the wavelength slightly rises when the temperature rises. (∂λ / ∂T ≧ 0) Further, (∂ni / ∂λ) is negative for ordinary optical materials. (∂ni / ∂λ≤0) Looking at the main scanning direction, Σ (∂fB ₁ M / ∂ni ・ ∂
ni / ∂λ) can be changed to positive or negative depending on the power arrangement of the optical element. Therefore, the selection of the optical material, the power allocation, and the like may be determined in combination with the influence of the temperature change due to other factors. However, if the power allocation is also controlled in order to control the influence of this wavelength change, the number of optical elements will increase unnecessarily and the cost will increase. This incident optical system has positive power as a whole in both the main scanning direction and the sub scanning direction. Therefore, if the influence of the wavelength change is not controlled by the power allocation, the influence of the wavelength change of the optical element having the positive power in the incident optical system tends to be larger, and the influence of the optical element having the positive power of the incident optical system tends to be larger. Since ∂fB ₁ M / ∂ni is negative,
Expression (25) is likely to be positive. The same applies to the sub-scanning direction of the equation (26). In this way, it was necessary to control the back focus change in the medium to be scanned due to the wavelength change due to the temperature change while taking into consideration the influence due to the temperature change due to other factors not the influence due to the wavelength change. The optical material of the optical element of the incident optical system should be selected.

The optical material expands and contracts due to temperature changes, and its refractive index changes. Influence of main focus direction ΔfBMN (ΔT) and sub-scan direction influence of back focus change on the medium to be scanned due to this change in the incident optical system.
fBSN (ΔT) is ΔfBMN (ΔT) = ΔfB ₁ MN (ΔT) · Mm ² (27) ΔfBSN (ΔT) = ΔfB ₁ SN (ΔT) · M ₂ s ² (28) Becomes Here, in the same manner as described in the explanation of the influence of the wavelength change of the light source caused by the temperature change, ΔfB ₁ MN (Δ
T) represents the back focus change of the incident optical system around the light source when parallel light is incident on the incident optical system in the main scanning direction from the direction opposite to the light source, and ΔfB ₁ SN (Δ
T) represents the back focus change amount around the deflector of the incident optical system in the sub-scanning direction.

At this time, the back focus change ΔfB ₁ MN (ΔT) due to the temperature change in the main scanning direction of the incident optical system is
Change in the refractive index of the optical material due to temperature changes, and further decomposed because due expansion and contraction of the optical _{material, △ fB 1 MN (△ T} ) / △ T ≒ Σ∂fB 1 M / ∂ni · ∂ni / ∂T + Σ∂fB ₁ M / ∂Di ・ ∂Di / ∂T + Σ∂fB ₁ M / ∂Smj ・ ∂Smj / ∂T (29) Here, ni represents the refractive index of the material of the i-th optical element, Di represents the axial thickness of the i-th optical element, and Smj represents the j-th main scanning side optical element surface. ∂Di / ∂T and ∂Sj ∂ / ∂T are the effects of the linear expansion coefficient Lni of the i-th optical material having the j-th surface ∂Di / ∂T = Lni · Di (30) and ∂Smj / ∂T is ∂Smj / ∂T = ∂Rmj / ∂T = Lni · Rmj (31), assuming that the jth optical surface of the ith optical element is a spherical surface having a radius of curvature Rmj in the main scanning direction. In this case, it is a spherical surface, but in the case of an aspherical surface, it may be calculated by proportional expansion / contraction of the linear expansion coefficient.

Here, the back focus change ∂fB ₁ M / ∂Di due to the axial thickness change of the optical element is smaller than the other factors, and therefore ∂fB ₁ M / ∂Di / ∂Di / ∂T ~ 0. (32) It can be said. As a result, the formula (29) is expressed as ΔfB ₁ MN (ΔT) / ΔT = Σ∂fB ₁ M / ∂ni ・ ∂ni / ∂T + Σ∂fB ₁ M / ∂Smj ・ ∂Smj / ∂T (33) Can be set. The same applies to the sub-scanning direction. If the state of the j-th surface in the sub-scanning direction is Ssj, ΔfB ₁ SN (ΔT) / ΔT = Σ∂fB ₁ S / ∂ni ・ ∂ni / ∂T + Σ∂ fB ₁ S / ∂Ssj / ∂Ssj / ∂T (34)

When the optical element is made of resin, the linear expansion coefficient Lni due to temperature change is much larger than when the optical material is made of glass, and ∂Smj / ∂T and ∂Ssj / ∂T are large. When the optical material is made of resin, the change in refractive index due to temperature change ∂ni / ∂T is also larger than that of glass. The optical element having the j-th negative power and the j + 1-th surface not having the power is made of resin,
When the temperature rises, the back focus tends to be shortened due to the influence of the expansion of the j-th surface, and in addition to this, the refractive index decreases when the temperature rises, and since it has negative power, the back focus further decreases. This is the direction in which the focus becomes shorter. From the above, different back focus changes are made in the main scanning direction and the sub-scanning direction of the incident optical system due to the temperature change, and the temperature change of the scanning optical system is corrected as a whole. In particular, by using at least one optical element made of resin and having different powers for the main scanning and the sub scanning, it is possible to independently correct each of the main scanning direction and the sub scanning direction.

From the above, the equations (15) and (16) of the temperature change of the entire system considering the three factors as the influence of the temperature change of the incident optical system on the medium to be scanned are given by the equations (18) and ( 19), the formula (22), the formula (23), the formula (27), and the formula (28), ΔfBm (ΔT) = Mm ² · (−L·lx · ΔT + fB ₁ Mλ (ΔT) + Δ fB ₁ MN (△ T)) + △ fB ₂ M (△ T) (35) △ fBs (△ T) = -Ms ²・ L ・ lx ・ △ T + M ₂ s ²・ (△ fB ₁ Sλ (△ T ) + ΔfB ₁ SN (ΔT)) + ΔfB ₂ S (ΔT) (36) In this way, in order to correct the back focus change due to the temperature change of the scanning optical system in the main scanning direction and the sub scanning direction of the scanning optical system by the incident optical system independently in each direction, first, the above three influences in the main scanning direction are applied. In consideration of the above three influences in the sub-scanning direction, the back focus change of the scanning optical system due to the temperature change in the main scanning direction is almost canceled by selecting the material of the fixing member and the optical material. In addition to the selection in the main scanning direction, the power of the resin optical element having a negative power in the sub scanning direction of the incident optical system is selected to reduce the back focus change of the scanning optical system due to the temperature change in the sub scanning direction. The incident optical system may be designed in a procedure that almost cancels it.

From the relationship between the depth of the scanning optical system in each of the main scanning direction and the sub-scanning direction, the magnification of the entire system, and the back focus change amount due to the temperature change of the scanning optical system, a conventional technique, for example, Japanese Unexamined Patent Publication No. Whether the known means disclosed in 161410 is applicable is determined. When the present invention is used, the cost is often higher than that in JP-A-2-161410. Therefore, if the known means can be used, it is preferable to adopt this. On the contrary, the present invention is useful when known means cannot be applied. Therefore, it is necessary to set conditions under which the present invention is useful, that is, conditions to which known means cannot be applied.

The known means uses an optical system in which an incident optical system is composed of a collimator fixed to each other by a light source and a fixing member, and a cylindrical lens made of glass having a positive power in the sub-scanning direction. Effects of changes in the light source and the fixing member that mounts the collimator due to changes in the temperature of the incident optical system Effects of changes in the wavelength of the light source caused by changes in temperature Effects of expansion, contraction, and changes in the refractive index of optical elements in the incident optical system due to changes in temperature By controlling the three temperature change factors, the back focus change due to the temperature change ΔT of the scanning optical system, ΔfB ₂ M (ΔT) in the main scanning direction, and ΔfB ₂ S (ΔT in the sub scanning direction.
T) is corrected to change the back focus of the entire optical system including the incident optical system and the scanning optical system, and ΔfBm in the main scanning direction.
(ΔT) and ΔfBs (ΔT) in the sub-scanning direction are kept within the ranges δm and δs where the performance in each direction can be maintained. That is, | ΔfBm (ΔT) | ≦ δm (37) | ΔfBs (ΔT) | ≦ δs (38) must be satisfied.

The incident optical system is constructed as follows. That is, the light emitted from the light source becomes substantially parallel light in the main scanning direction and the sub scanning direction via the collimator. An optical system that has almost no power in the main scanning direction and, through a glass cylindrical lens having a positive power in the sub scanning direction, collimates the main scanning direction as it is in the vicinity of the deflector and condenses it in the sub scanning direction. It is a system. Therefore, in order to correct the back focus change due to the temperature change of the scanning optical system in the incident optical system, the change due to the temperature change of the collimator attached by the light source and the fixing member is controlled by using the above three factors. The back focus change amount that combines the above three factors due to the temperature change ΔT of the collimator is ΔfBCOL (ΔT). This value is because the collimator is optically rotationally symmetrical about the optical axis, so
And the values in the sub-scanning direction are the same.

The cylindrical lens having a positive power in the sub-scanning direction has no power in the main scanning direction, and the light passing through the cylindrical lens in the main scanning direction is almost parallel light. Therefore, in the main scanning direction. Is not affected by temperature changes due to the cylindrical lens. The change in back focus due to the temperature change ΔT of the cylindrical lens in the sub-scanning direction is ΔfBCY (ΔT)
And the absolute value of the main scanning lateral magnification of the entire system is Mm, the absolute value of the sub-scanning lateral magnification is Ms, and the absolute value of the sub-scanning lateral magnification of the scanning optical system is M ₂ s, the temperature change in the entire optical system Back focus change amount due to ΔT [ΔfBm (Δ in the main scanning direction
T) and ΔfBs (ΔT)] in the sub-scanning direction are as follows. Main scanning direction ΔfBm (ΔT) = ΔfBCOL (ΔT) · Mm ² + ΔfB ₂ M (ΔT) (39) Sub-scanning direction ΔfBs (ΔT) = ΔfBCOL (ΔT) · Ms ² + △ fBCY (△ T) ・ M ₂ s ² + △ fB ₂ S (△ T) (40)

Here, in the known example, the cylindrical lens is made of glass, the back focus change amount of the cylindrical lens is small, and the sub-scanning vertical magnification of the scanning optical system is smaller than that of all other systems. Δf in the equation 40)
BCY (△ T) ・ M ₂ s ² term is △ fBCOL (△ T) ・ Ms ² + △ fB ₂ S (△ T) ≫ △ fBCY (△ T) ・ M ₂ s ² (41), which can be set as ΔfBCY (ΔT) · M ₂ s ² ˜0. As a result, the sub-scanning direction becomes from the formula (40) to the sub-scanning direction ΔfBs (ΔT) = ΔfBCOL (ΔT) · Ms ² + ΔfB ₂ S (ΔT) (42). From equations (39) and (42), the temperature compensation method of the scanning optical system by the incident optical system using the known means is the back focus change amount Δ due to the temperature change ΔT of the collimator in both the main scanning direction and the sub scanning direction. It can be seen that it depends on one value of fBCOL (ΔT).

The condition to which the known means can be applied is that the back focus change amount ΔfBCOL (ΔT) of the collimator when the temperature change ΔT simultaneously satisfies the expressions (39) and (42).
The target value of is always present in the guaranteed temperature change range ΔTmin to ΔTmax corresponding to the temperature guaranteed range.
This has been described with reference to FIG. 2, but when it is shown again in FIG.
As in (a), the main scanning ΔfBCOL (Δ
This is a case where the range of T) and the range of ΔfBCOL (ΔT) satisfying the expression (42) overlap. 6 (b-1) and FIG.
The case (b-2) represents the case where there is no ΔfBCOL (ΔT) that simultaneously satisfies the expression (39) in the main scanning direction and the expression (42) in the sub-scanning direction, within the range where the present invention is required. is there.

In order to derive the conditions for applying the present invention, the range to which the known means can be applied is first calculated again. This is because the back focus change due to the temperature ΔT of the entire optical system in each direction of equations (39) and (42) is (3
Range δ in which the performance shown in the equations (7) and (38) can be maintained
Since it must be suppressed to m and δs, (37), (3
Rewriting equation 8), main scanning direction −δm ≦ ΔfBCOL (ΔT) · Mm ² + ΔfB ₂ M (ΔT) ≦ δm (43) Sub scanning direction −δs ≦ ΔfBCOL (ΔT) · Ms ² + ΔfB ₂ S (ΔT) ≦ δs (44) When further deformed, the main scanning direction − (δm + ΔfB ₂ M (ΔT)) / Mm ² ≦ ΔfBCOL (ΔT) ≦ (δm−ΔfB ₂ M (ΔT)) / Mm ² (45) Sub scanning direction − (Δs + ΔfB ₂ S (ΔT)) / Ms ² ≦ ΔfBCOL (ΔT) ≦ (δs−ΔfB ₂ S (ΔT)) / Ms ² (46)

The present invention should be applied to the equation (45) in both the main scanning direction and the sub-scanning direction as shown in FIGS. 6 (b-1) and 6 (b-2). The condition is that there is no target value of ΔfBCOL (ΔT) that simultaneously satisfies the expression (46). That is, equation (45), (46)
From the formula, in the case of FIG. 6 (b-1) (δm−ΔfB ₂ M (ΔT)) / Mm ² ≦ − (δs + ΔfB ₂ S (ΔT)) / Ms ² (47) FIG. 6 (b− In the case of 2), (δs-ΔfB ₂ S (ΔT)) / Ms ² ≦-(δm + ΔfB ₂ M (ΔT)) / Mm ² (48), which is the formula (47) or (48). If any of the ranges is satisfied, known means cannot be applied,
This is within the scope of the present invention.

By further modifying the equations (47) and (48), δm / Mm ² + δs / Ms ² ≤ | ΔfB ₂ M (ΔT) / Mm ² −ΔfB ₂ S (ΔT) / Ms ² | (49) The performance maintaining range of δm and δs in the above formula (49) is the main scanning,
The form described by the equations (13) and (14) is obtained within a range in which the substantial performance can be maintained by subtracting the field curvature margin by design, the processing error, and the placement error margin from the allowable beam depth of the sub-scan. Further, for higher resolution, the beam acceptance coefficient must be at least ε = 1.1. Substituting such an item into the equation (49), (0.18 / λ) · [(ω ₀ M ² / Mm ² ) + (ω ₀ S ² / Ms ² )] ≦ | ΔfB ₂ M (ΔT ) / Mm ² −ΔfB ₂ S (ΔT) / Ms ² | (50). Where ω ₀ M is the beam waist diameter around the scanned medium on the optical axis of the scanning optical system in the main scanning direction, and ω ₀ S is around the scanned medium on the optical axis of the scanning optical system in the sub-scanning direction. The beam waist diameter of λ, λ represents the wavelength of the light emitted from the light source.

If ΔTmax (≧ 0) and ΔTmin (≦ 0) represent the upper and lower limits of the amount of temperature change with respect to the reference design temperature T _{0 with} respect to the temperature range of the operating specification, then T ₀ + ΔT min ≦ T ₀ ≦ T ₀ + ΔTmax (51) Temperature change amount ΔT satisfying the equation (50) within this temperature range
If there is even a little, it becomes impossible to use the known means, which is a condition that requires the means of the present invention. The back focus change of each optical system with respect to the normal temperature change is
It increases or decreases monotonically with temperature changes. Therefore, the lower limit temperature T ₀ + ΔTmin (that is, ΔT = ΔTmin) and the upper limit temperature T of the operating temperature guarantee range of this optical scanning device are set.
_{If any of 0} + ΔTmax (that is, ΔT = ΔTmax) satisfies the expression (50), the condition for the present invention is required. This can be expressed by an equation: (0.18 / λ) · [(ω ₀ M ² / Mm ² ) + (ω ₀ S ² / Ms ² )] ≦ | ΔfB ₂ M (ΔTmax) / Mm ² −Δ fB ₂ S (ΔTmax) / Ms ² | (52) or (0.18 / λ) · [(ω ₀ M ² / Mm ² ) + (ω ₀ S ² / Ms ² )] ≦ | ΔfB ₂ M (ΔTmin) / Mm ² −ΔfB ₂ S (ΔTmin) / Ms ² | (53).

In the present invention, the first optical system, which is a collimator whose optical surface is rotationally symmetrical with respect to the optical axis, is attached to the fixed member together with the light source, and the sub-scanning direction behind the collimator when viewed from the light source. Consisting of a second optical system including a resin cylindrical lens with negative power
An incident optical system, which is an anamorphic optical system having a positive power in the sub-scanning direction as a whole, is used. The back focus change in the main scanning direction due to the temperature change of the scanning optical system is corrected by the influence of the above-mentioned three factors due to the temperature change of the collimator optical system. At this time, in the sub-scanning direction, the correction is insufficient only by the correction by the collimator, and the shortage is corrected by the power of the cylindrical lens having the negative power of the cylindrical optical system. In the case of this optical device, since the light emitted from the collimator is parallel light in both main scanning and sub-scanning, the collimator optical system and the cylindrical optical system are infinite optical systems, respectively, and the two optical systems are designed separately. It is an optical device that allows evaluation of actual equipment and makes it easy to adjust the layout.

As another configuration, when viewed from the light source,
An anamorphic first optical system, which is fixed by a fixing member together with the light source, makes the light from the divergent light source almost parallel in the main scanning direction and has a resin anamorphic optical element with negative power in the sub scanning direction. , And the second optical system composed of a cylindrical optical system having a positive power in the sub-scanning direction as a whole. This device measures the back focus change of the scanning optical system due to the temperature change in the main scanning direction, the influence of the temperature change of the fixing member mounted together with the light source including the resin anamorphic optical element, and the wavelength of the light source caused by the temperature change. Back focus change of the scanning optical system due to temperature change in the sub-scanning direction is corrected by the effect of change, the refractive index change in the optical system including the anamorphic optical element made of resin due to temperature change, and the effect due to expansion and contraction of the optical element. Is a device for performing correction by the influence of the negative power of the anamorphic lens in addition to these three influences. The cylindrical optical system may use a cylindrical lens made of glass having a positive power, and does not have a great influence on the temperature change.

If this anamorphic optical element has a positive power in the main scanning direction, it can be applied only to the scanning optical system in which the back focus change due to the temperature change in the main scanning direction is small, but that of the optical element of the first optical system. May reduce the number. If this anamorphic optical element has negative power in the main scanning direction, the back focus change amount in the main scanning direction due to temperature change is large, and the expansion due to temperature change of the fixing member exerts a correction effect on this change,
This is useful as an additional correction amount for the scanning optical system that cannot be corrected only by contraction. If the anamorphic optical element has almost no power in the main scanning direction, that is, in the case of a cylindrical lens having negative power in the sub-scanning direction,
Easier to process than anamorphic lenses.

[0059]

EXAMPLES Examples of the incident optical system and the scanning optical system of the present invention will be described below. FIG. 1 is a diagram combining an example of an incident optical system and an example of a scanning optical system described later. This optical system includes a laser light source 1, an incident optical system including a laser light source, a first optical system 2 attached to a fixing member 2 ', and a cylindrical lens system 4 which is a second optical system, and a rotating device which is a deflecting device. The scanning optical system includes a polygon mirror 5 and resin lenses 7 and 8.

Light from the laser light source 1 passes through the first optical system 2 of the incident optical system, the cylindrical lens system of the incident optical system or the second optical system 4 and enters the rotary polygon mirror 5.
At this time, in the vicinity of the rotary polygon mirror, the light beam is almost parallel in the main scanning direction and once in the sub scanning direction. The rotating polygonal mirror 5 is arranged such that the center of rotation 6 is displaced from the optical axis 9 of the scanning optical system. The light reflected by the rotating polygon mirror 5 is
An image is formed on the surface to be scanned 10 by the imaging lenses 7 and 8 of the scanning optical system, and in the sub-scanning surface, the reflecting surface of the rotary polygon mirror and the surface to be scanned are in a substantially geometrically conjugate relationship. Table 1 shows an example of the scanning optical system (6 or later in FIG. 1) at this time. The reference design wavelength at this time is 780 nm. Note that the virtual pupil in the polygon mirror is plane number 1.

As shown in FIG. 7, the scanning optical system shown in Table 1 has 2
The scanning first lens 7 has an S2 surface which is a spherical surface, the scanning first lens 7 has an S3 surface, and the scanning second lens 8 has an S5 surface.

(Equation 5) φ ² = y ² + z ² C * = C + A ₂ C *: Axisymmetric aspherical surface represented by paraxial curvature. As shown in FIG. 8, the S4 surface is an asymmetric anamorphic for correcting the field curvature in the sub-scanning direction caused by the position of the virtual pupil at each deflection angle being different due to the entrance and exit of the reflecting surface of the rotary polygon mirror. In the surface, the change in the radius of curvature in the cross section in the sub-scanning direction includes a surface asymmetric with respect to the distance h from the optical axis, and the light from the laser light source incident on the rotating polygon mirror and the optical axis of the scanning optical system are The angle is α, the radius of the inscribed circle of the rotary polygon mirror is Rp, the number of surfaces is n, the radius of curvature in the main scanning direction cross section including the optical axis of the optical axis asymmetric surface is R, and the sub scanning direction cross section including the optical axis. When the radius of curvature at is ₀ , the main scanning side including the rotation center of the rotary polygonal mirror about the optical axis of the scanning optical system is the − side, and the opposite side is the + side, as shown in FIG. And − are asymmetrical, and are separated from the optical axis by + h in the main scanning direction. Radius of curvature r in the sub-scan section parallel to the optical axis at the position
h (+) and rh (−) are rh (±) = r ₀ + {1 + k (±) · Rp · α / n} · R · [1-cos {sin ⁻¹ (h / R)}] (55 ) Is represented. The print width of this scanning optical system is approximately A4.
The height is 220 mm corresponding to the size, and the image height that guarantees the performance is from -110 mm to +110 mm.

By using the asymmetric surface as described above, this scanning optical system corrects the field curvature due to the influence of the entrance and exit of the surface of the rotary polygon mirror. The image planes in the main scanning direction and the sub-scanning direction at this time are shown in FIG. 9A, and the beam diameter on the reference image plane in each direction is shown in FIG. 9B. At this time, the beam waist diameter on the optical axis of the scanning optical system in the vicinity of the surface to be scanned of the scanning optical system is set to 48 μm in the main scanning direction and 56 μm in the sub-scanning. Is also available. Here, the definition of the beam diameter represents the diameter of the intensity of 1 / e ² of the beam profile. The allowable beam depths in the main scanning direction and the sub-scanning direction on the optical axis of this scanning optical system are X ₀ m and X ₀ s, the beam waist diameter ω ₀ M on the optical axis in the main scanning direction is 0.048 mm, Since the beam waist diameter on the optical axis in the scanning direction is ω ₀ S = 0.056 mm and the wavelength is λ = 780 nm, ε = 1.1 is set and X ₀ m = 1.0 mm X from Equations (9) and (10) _{Since 0} s = 1.4 mm, the axial depth of the scanning optical system is smaller than that of the conventional one.
Further, looking at FIG. 9A showing the state of the field curvature with respect to the image height of this scanning optical system, in the range of the image height −110 mm,
In the main scanning direction, the range is 0.4 mm with respect to the image height, and in the sub scanning direction, the range is 1.2 mm with respect to the image height. Therefore, the field curvature margin of the design value is at least 0.2 mm in the main scanning direction and 0.6 mm in the sub scanning direction. Including this, considering the depth reduction due to the aberration in the beam that occurs by design when the image height is deviated from the optical axis, and the arrangement and the processing margin, one-sided range in which the performance can be maintained as described above, Main scanning direction δm, Sub scanning direction δ
Since s requires at least 1/2 of the allowable beam depth on the optical axis, δm = 0.5 mm and δs = 0.7 mm. That is, in the whole system including this scanning optical system,
The margin of temperature change must be kept below this performance maintenance margin.

The standard design temperature of this scanning optical system is 25 degrees, and the operating temperature guarantee range is usually at least 10 degrees to 50 degrees.
A range of degrees is needed. Here, the influence of the temperature change of the scanning optical system is the lower limit of 10 degrees (temperature change amount ΔT =
Compared to −15 degrees, the upper limit is 50 degrees (temperature change ΔT = +
Since the influence of back focus and the like due to the temperature change of the optical system is monotonically increasing or decreasing, the upper limit of 50 degrees, ie, ΔTmax = + 25 degrees, which is greatly influenced by the temperature change, will be discussed. If the temperature rises by 25 ° C. with respect to the reference design temperature, the image plane as shown in FIG. 10A is obtained due to the change in the refractive index of the resin and the linear expansion coefficient. As compared with FIG. 9A, the image planes of main scanning and sub-scanning have image plane position shifts as described below while maintaining substantially the same curved image plane shape. Main scan ΔfB ₂ M (ΔT = + 25 degrees) = + 1.725 mm Sub-scan ΔfB ₂ S (ΔT = + 25 degrees) = + 1.499 mm Where ΔfB ₂ M (ΔT = + 25), ΔfB ₂ S (Δ
T = + 25) represents the influence of the scanning optical system alone on the back focus when ΔT = + 25 ° C. is raised from the reference design temperature. At this time, the refractive index change Δn with respect to the temperature change ΔT is Δn / ΔT = −12 × 10 ⁻⁵ / ° C. Further, the linear expansion coefficient L ₂ of the lens constituent material is L ₂ = 7 × 10 ⁻⁵ / ° C. Temperature The wavelength change Δλ of the light source with respect to the change ΔT was calculated as Δλ / ΔT = 0.2 nm / ° C. Therefore, a light source having a wavelength of 780 nm at the reference design temperature emits light having a wavelength of 785 nm when the temperature rises by 25 degrees from the reference design temperature. As described above, ε = 1.1 (allowable beam diameter main scanning 52.8 μm,
When sub-scanning is 61.6 μm), main scanning direction is about 1 m
There is only one side depth of m, about 1.4 mm in the sub-scanning direction, and the shift amount of the field curvature becomes larger than the allowable beam diameter in both the main scanning direction and the sub-scanning direction. Actually, FIG. 10B shows the beam diameter on the reference image plane when the temperature rises by 25 ° C. from the reference design temperature. Looking at this figure, the beam diameter has changed considerably, and it cannot be used as it is. As described above, in the high resolution scanning optical system, the incident optical system shown in the following embodiments must be used for correction.

Here, regarding the main scanning direction and the sub-scanning direction,
Table 2 shows the specifications of the incident optical system and the scanning optical system so that the beam waist diameters in each direction are 48 μm and 56 μm. The incident optical system represents the range from the light source 1 to the vicinity of the rotating polygonal mirror 5 in FIG. 1, and the light emitted from the light source 1 becomes almost parallel light in the optical systems 2 to 4 in the main scanning direction and the optical system in the sub scanning direction. From 2 to 4, an image is formed near the rotary polygon mirror. From the specifications of the scanning optical system, the beam waist diameter in the main scanning direction on the optical axis of the scanning optical system is ω ₀ M = 48 μm, and the beam waist diameter in the sub-scanning direction is ω ₀ S = 56 μm. When the difference ΔTmax = + 25 degrees from the upper limit of the guaranteed operating temperature range of ΔTmax = + 25 degrees is substituted into the expression (52), (0.18 / λ) · [(ω ₀ M ² / Mm ² ) + (ω ₀ S ² / Ms
² )] = 7.48 × 10 ⁻³ (mm) ≦ | ΔfB ₂ M (ΔT) / Mm ²
−ΔfB ₂ S (ΔT) / Ms ² | = 1.02 × 10 ⁻² (mm), which satisfies the expression (52). As a result, it has been found that this scanning optical system cannot be temperature-corrected by known means, and therefore the incident optical system according to the present invention must be used.

The following is an example of the present invention of an incident optical system adapted to the scanning optical system of Table 1 with the specifications described in Table 2 above. Example 1 In Examples shown in Tables 3 (a) and 3 (b), a collimator in which an optical surface of which an incident optical system is fixed to each other by a light source and a fixing member is rotationally symmetric with respect to an optical axis, and sub-scanning is performed. The anamorphic optical system has a resin optical element that has negative power in the main scanning direction and positive power in the sub scanning direction as a whole. The back focus change of the scanning optical system with respect to temperature changes in the main scanning direction and the sub scanning direction. This is an example of forming a means for correcting. Symbols in the table are: Ri: i-th vertex radius of curvature from the light source side di: i-th lens surface distance from the light source side ni: wavelength of the i-th lens material from the light source side 780 nm
Refractive index ni ': wavelength of the i-th lens material from the light source side is 785 nm
And the refractive index at

Table 3 (a) shows data in the main scanning direction,
Table 3 (b) shows data in the sub-scanning direction. A cross-sectional view of this optical system is shown in FIG. 11A for the main scanning direction and FIG. 11B for the sub-scanning direction. The collimator is shown in Table 3 (a),
In (b), it comprises an optical system from the first surface to the sixth surface, and this collimator lens frame is fixed to the light source by a fixing member. Aluminum is used as the material of the fixing member. At this time, the collimator lens frame is made of a material having a smaller linear expansion coefficient with respect to temperature change than the material of the fixing member. The anamorphic optical system includes seventh to tenth surfaces, of which the ninth and tenth surfaces are resin cylindrical lenses. The focal length of the collimator is fCOL = 8.707 mm, and the focal length of the anamorphic optical system in the sub-scanning direction is fCY = 93.129.
mm, and accordingly, the magnification in the sub-scanning direction in the first optical system becomes M ₁ s = 10.70 times.

By combining this incident optical system and the above scanning optical system, the back focus change amount of the entire optical system when the temperature ΔT changes by +25 degrees is calculated. In the main scanning direction of the incident optical system, due to the refractive index change and expansion / contraction of the optical material when the temperature rises by 25 degrees, the light near the light source when parallel light is incident on the incident optical system from the direction opposite to the light source Back focus change amount ΔfB ₁ MN (ΔT = + 25
Degree) is ΔfB ₁ MN (ΔT = + 25 degrees) = 1.56 × 10 ⁻⁴ mm In the main scanning direction of the incident optical system, the incident optical system is affected by the wavelength change of the light source when the temperature rises by 25 degrees. On the other hand, the amount of change in the back focus near the light source when parallel light is incident from the direction opposite to the light source ΔfB ₁ Mλ (ΔT = + 25
Degree) is ΔfB ₁ Mλ (ΔT = + 25 degrees) = 1.863 × 10 ⁻³ mm

In the sub-scanning direction of the incident optical system, when the temperature rises by 25 degrees, the refractive index in the optical material and the change amount of the back focus due to expansion and contraction ΔfB ₁ SN (ΔT = + 25
Degree) is ΔfB ₁ SN (ΔT = + 25 degrees) = − 0.929 mm In the sub-scanning direction of the incident optical system, the back focus change amount ΔfB ₁ Sλ (due to the wavelength change of the light source when the temperature rises 25 degrees. △ T = + 25 °) is _{△ fB 1 Sλ (△ T =} + 25 °) = + a 0.233Mm.

In order to obtain the influence of the fixing member that fixes the light source and a part of the incident optical system to each other, the temperature change ΔT = +
When the equation (17) is used to obtain the amount of movement ΔS of the light source as seen from the incident optical system when changing by 25 °, ΔS = −L·lx · ΔT. Here, the linear expansion coefficient of the fixing member is L = 23 × 10 ⁻⁶ / ° C. from the linear expansion coefficient of aluminum. As described above, lx is set to the focal length f ₁ M of the incident optical system in the main scanning direction = 8.707 mm. As a result, the above formula △
S is ΔS = −5.01 × 10 ⁻³ (mm).

For the refractive index change amount Δn / ΔT and the linear expansion coefficient Ln depending on the temperature of each lens in the incident optical system, the back focus change amount is calculated using the following numerical values. Lens No. Surface No. Δn / ΔT (/ ° C) Ln (/ ° C) 1 1, 2 2.8 × 10 ^-6 74 × 10 ^-7 2 3, 4 6.6 x 10 ^-6 59 x 10 ^-7 3 5, 6 3.0 x 10 ^-6 62 x 10 ^-7 4 7, 8 2.8 x 10 ^-6 74 x 10 ^-7 5 9, 10 −12 × 10 ⁻⁵ 6 × 10 ⁻⁵ However, at this time, the lens surface distance has a small influence on the back focus as described in the equation (32),
Calculation is performed without applying linear expansion. The amount of wavelength change Δλ / ΔT of the light source with respect to temperature is Δλ / ΔT = 0.2 nm / ° C. Therefore, Δλ (ΔT = 25 ° C) = 5 nm
And λ (ΔT = 25 ° C.) = 785 nm.

Absolute main scanning lateral magnification of the entire optical system Mm = f ₂ M / f ₁ M = 22.970 times (Mm ² = 527.6 times) Sub scanning lateral magnification absolute value of the scanning optical system M ₂ s = 0.988 times, the absolute value of the sub-scanning lateral magnification of the entire optical system is Ms = M ₁ s · M ₂ s = 10.568 times (Ms ² = 111.7 times), and the temperature changes by ΔT = + 25 degrees. At this time, the back focus change amount of the scanning optical system is ΔfB ₂ M (ΔT = 25 degrees) = 1.725 mm in the main scanning direction and ΔfB ₂ S (ΔT = 25 degrees) = 1.499 mm in the sub scanning direction. By substituting these into the equations (35) and (36), the value of the back focus change amount when the temperature rises by 25 degrees and the main scanning direction fBm in the entire optical system including the scanning optical system and the incident optical system. (ΔT = + 25 degrees) and the sub-scanning direction fBs (ΔT = + 25 degrees) are as follows. Main scanning direction ΔfBm (ΔT) = Mm ² · {-L·lx · ΔT + ΔfB ₁ M λ (ΔT) + ΔfB ₁ MN (ΔT)} + ΔfB ₂ M (ΔT) = 527.6・ (-5.01 × 10 ^-3 + 1.863 × 10 ^-3 + 1.56 × 10 ^-4 ) +1.725 mm = 0.147 mm Sub-scanning direction △ fBs (△ T) = -Ms ²・ L ・ lx ・ △ T ＋ M ₂ s ²・ {△ fB ₁ S λ (△ T) ＋ △ fB ₁ SN (△ T)} ＋ △ fB ₂ S (△ T) ＝ -111.7 ・ 5.01 × 10 ^-3 ＋ (0.988) ²・ (0.233 -0.929) + 1.499 = 0.260mm

The change in the back focus of the entire optical system must be within the range where the performance can be maintained.
Therefore, the expressions (3) and (4) must be satisfied. The range in which the performance of this scanning optical system can be maintained is δm = 0.5 mm in the main scanning direction and δs = 0.7 mm in the sub scanning direction. | ΔfBm (ΔT = + 25 degrees) = 0.147 mm | ≦ 0.5 mm | ΔfBs (ΔT = + 25 degrees) = 0.260 mm | ≦ 0.7 mm, which satisfies the equations (3) and (4). This allows
In this embodiment, it is understood that the back focus change of the scanning optical system due to the temperature change can be sufficiently corrected by the incident optical system in each of the main scanning direction and the sub scanning direction.

(Example 2) In the examples shown in Tables 4 (a) and 4 (b), the incident optical system is fixed to each other by the light source and the fixing member, and the light from the divergent light source is made into substantially parallel light in the main scanning direction. And an anamorphic first optical system having an anamorphic lens having negative power in the sub-scanning direction, and a second optical system consisting of an anamorphic optical system having positive power in the sub-scanning direction as a whole. In this embodiment, the means for correcting the back focus change due to the temperature change of the scanning optical system with respect to the temperature change in the main scanning direction and the sub scanning direction is constituted. Here, as the anamorphic lens, a cylindrical lens that is easy to process is adopted. Table 4 (a) shows data in the main scanning direction, and Table 4 (b) shows data in the sub-scanning direction. A cross-sectional view of the optical system of this embodiment is shown in FIG. 13A for the main scanning direction and FIG. 13B for the sub-scanning direction. In the main scanning direction, the optical system from the first surface to the eighth surface in Table 1 has a function of making the light emitted from the light source substantially parallel light, and in the sub-scanning direction, the entire incident optical system includes the light source. It has the function of focusing the light emitted from the periphery of the rotating polygon mirror. Of these, the third surface to the fourth surface are resin cylindrical lenses having a power of almost 0 in the main scanning direction and a negative power in the sub scanning direction,
From the 9th surface to the 10th surface, the power in the main scanning direction is almost 0,
It is a cylindrical lens having a positive power in the sub-scanning direction.

The lens frame of all the optical elements of this incident optical system is fixed to the light source by a fixing member.
Aluminum is used as the material of the fixing member. At this time, the coefficient of linear expansion of the material of the lens frame due to temperature change is smaller than that of the material of the fixing member. Also, the focal length in the main scanning direction fCOL = 8.707m
m, the magnification in the sub-scanning direction in the incident optical system is M ₁ s =
It becomes 10.70 times.

By combining this incident optical system and the above scanning optical system, the back focus change of the entire optical system when the temperature rises by +25 degrees is calculated. Using the calculation method of Example 1, main scanning direction ΔfB ₁ MN (ΔT = + 25 degrees) = − 7.75 × 10 ⁻⁴ mm ΔfB ₁ Mλ (ΔT = + 25 degrees) = 2.149 × 10 ⁻³ mm Sub-scanning direction ΔfB ₁ SN (ΔT = + 25 degrees) = − 1.042 mm ΔfB ₁ Sλ (ΔT = + 25 degrees) = 0.303 mm. Also, the coefficient of linear expansion due to the temperature change of the fixing member is
L = 23 × 10 ⁻⁶ / ° C., the distance lx from the mounting portion of the light source to the fixing member to the mounting portion of the incident optical system to the fixing member is the same as in Example 1, the focal point in the main scanning direction of the incident optical system. In terms of distance, lx = 8.707 mm. As a result, the amount of movement ΔS of the light source is ΔS = −5.01 × 10 ⁻³ (mm) when the temperature change ΔT viewed from the incident optical system is +25 degrees.

The back focus change amount is calculated using the following numerical values for the refractive index change amount Δn / ΔT and the linear expansion coefficient Ln depending on the temperature of each lens in the incident optical system. Lens No. Surface No. Δn / ΔT (/ ° C) Ln (/ ° C) 1 1, 2 2.8 × 10 ^-6 74 × 10 ^-7 2 3, 4 -12 x 10 ^-5 6 x 10 ^-5 3 5, 6 5.3 x 10 ^-6 86 x 10 ^-7 4 7, 8 3.0 x 10 ^-6 62 x 10 ^-7 5 9, 10 2.8 × 10 ⁻⁶ 74 × 10 ⁻⁷ However, at this time, the lens surface distance is calculated without linear expansion because the influence on the back focus is small. Also, the amount of wavelength change of the light source with respect to temperature Δλ
/ ΔT is Δλ / ΔT = 0.2 nm / ° C. Therefore, at ΔT = 25 ° C, Δλ = 5 nm
Becomes

Absolute main scanning lateral magnification of the entire optical system Mm = f ₂ M / f ₁ M = 22.970 times (Mm ² = 527.6 times) Sub scanning lateral magnification absolute value of the scanning optical system M ₂ s = 0.988 times, the absolute value of the sub-scanning lateral magnification of the entire optical system is Ms = M ₁ s · M ₂ s = 10.568 times (Ms ² = 111.7 times), and the temperature changes by ΔT = + 25 degrees. At this time, the back focus change amount of the scanning optical system is ΔfB ₂ M (ΔT = 25 degrees) = 1.725 mm in the main scanning direction and ΔfB ₂ S (ΔT = 25 degrees) = 1.499 mm in the sub scanning direction. By substituting these into the equations (35) and (36), the value of the back focus change amount when the temperature rises by 25 degrees and the main scanning direction fBm in the entire optical system including the scanning optical system and the incident optical system. (ΔT = + 25 degrees) and the sub-scanning direction fBs (ΔT = + 25 degrees) are as follows. Main scanning direction ΔfBm (ΔT) = Mm ² · {-L·lx · ΔT + ΔfB ₁ M λ (ΔT) + ΔfB ₁ MN (ΔT)} + ΔfB ₂ M (ΔT) = 527.6・ (-5.01 × 10 ^-3 + 2.149 × 10 ^-3 -7.75 × 10 ^-4 ) + 1.725mm = -0.193mm Sub-scanning direction △ fBs (△ T) = -Ms ²・ L ・ lx ・ △ T + M ₂ s ² · {△ fB ₁ S λ (△ T) + △ fB ₁ SN (△ T)} + △ fB ₂ S (△ T) = -111.7 ・ 5.01 × 10 ^-3 + (0.988) ²・ (0.303 -1.042) + 1.499 = 0.218 mm

The back focus change in the entire optical system must be within the range where the performance can be maintained. The range in which the performance of this scanning optical system can be maintained is δm in the main scanning direction.
= 0.5 mm and δs = 0.7 mm in the sub-scanning direction ‖ΔfBm (ΔT = + 25 degrees) = − 0.193 mm‖ ≦ 0.5 mm ‖ΔfBs (ΔT = + 25 degrees) = 0.218 mm ‖ ≦ 0.7 mm, and it can be seen that in this embodiment, the back focus change of the scanning optical system due to the temperature change can be sufficiently corrected by the incident optical system in each of the main scanning direction and the sub scanning direction.

[0079]

[Table 1] Example of scanning optical system Surface number R (mm) r ₀ (mm) d (mm) n (780nm) n '(785nm) 1 ∞ 29.0 2 -67.865 6.5 1.51922 1.51913 3 -40.774 92.0 4 -290.000 32.380 3.0 1.51922 1.51913 5 -606.715 103.5 aspherical coefficients third surface _{k = -0.36231 A 4 = -0.41334 ×} 10 -7 P1 = 4 A 6 = -0.30407 × 10 -9 P2 = 6 A 8 = −0.87238 × 10 ⁻¹⁴ P3 = 8 A ₁₀ = + 0.49223 × 10 ⁻¹⁹ P4 = 10 Fifth surface k = −0.13137 A ₄ = −0.21906 × 10 ⁻⁶ P1 = 4 A ₆ = + 0.15502 × 10 ⁻¹⁰ P2 = 6 A ₈ = −0.14610 × 10 ⁻¹⁴ P3 = 8 A ₁₀ = + 0.55053 × 10 ⁻¹⁹ P4 = 10 Incident ray angle α = 1.5708 (rad) Rotation polyhedral Radius of inscribed circle Rp = 17.3205 (mm) Number of rotating polygon mirror surfaces n = 6 Scanning optical system main scanning method Focal length f ₂ M = 200 mm subscanning direction lateral magnification M ₂ s = -0.988 asymmetric surface shape k (-) = - 1.125 k (+) = - 1.020 using the wavelength lambda = 780 nm

[0080]

[Table 2] Specifications of incident optical system and scanning optical system (scanning optical system) Focal length in main scanning direction f ₂ M = 200 mm Absolute value of lateral magnification in sub scanning direction M ₂ s = 0.988 times Print width Image height -110 mm Image height +110 mm Change in back focus of scanning optical system when temperature ΔT = + 25 ° changes Main scanning direction ΔfB ₂ M (ΔT = 25 °) = 1.725 mm Sub scanning direction ΔfB ₂ S (ΔT = 25 degrees) = 1.499 mm Focal length in the main scanning direction of the incident optical system f ₁ M = 8.707 mm (NA 0.30) Absolute value of lateral magnification in the sub scanning direction M ₁ s = 10.70 times Overall main scanning lateral of the optical system Absolute magnification Mm = f ₂ M / f ₁ M = 22.970 times (Mm ² = 527.6 times) Sub-scanning lateral magnification absolute value Ms = M ₁ s · M ₂ s = 10.568 times (Ms ² = (111.7 times)

[0081]

Table 3 Example 1 (a) Main scanning direction lens surface No. Ri di ni ni '1 ∞ 6.00 1.51072 1.51062 2 ∞ 1.00 3 -17.349 1.50 1.79323 1 .79301 4 -6.801 0.20 5 ∞ 1.50 1.76203 1.76185 6 -18.509 3.00 7 ∞ 1.50 1.51072 1.51062 8 ∞ 1.00 9 ∞ 1.50 1.48595 1.48585 10 ∞ (b) Sub-scanning direction Lens surface No. Ri di ni ni '1 ∞ 6.00 1.51072 1.51062 2 ∞ 1.00 3 -17.349 1.50 1.79323 1.79301 4 -6.801 0.20 5 ∞ 1.50 1.76203 1.76185 6 -18.509 3.00 7 19.320 1.50 1.51072 1.5 062 8 ∞ 1.00 9 ∞ 1.50 1.48595 1.48585 10 28.501

[0082]

Table 4 Example 2 (a) Main scanning direction Lens surface No. Ri di ni ni '1 ∞ 4.00 1.51072 1.51062 2 ∞ 1.00 3 ∞ 1.00 1.48595 1.48585 4 ∞ 1.00 5 -12.007 1.50 1.67496 1.67473 6 -5.954 0.25 1.50 1.76203 1.76185 8 -14.424 4.09 ∞ 1.50 1.51072 1.51062 10 ∞ (b) Sub-scanning direction Lens surface No. Ri di ni ni '1 ∞ 4.00 1.51072 1.51062 2 ∞ 1.00 3 -13.000 1.00 1.48595 1.48585 4 ∞ 1.00 5 -12.007 1.50 1.67496 1.67473 6 -5.954 0.20 7 ∞ 1.50 1.76203 1.76185 8 -14.424 4.00 9 23.554 1.50 1.51072 1.51062 10 ∞

[0083]

As is apparent from each embodiment, the optical scanning device of the present invention fixes the incident optical system and the light source, and the thermal expansion of the fixing member and the anamorphic optics in the incident optical system and the scanning optical system. By comprehensively considering the optical system, it is possible to keep the focal point movement in the vicinity of the surface to be scanned within a predetermined range with a simple configuration even with temperature changes at the specification limit. The system made it possible to realize a scanning optical system capable of maintaining high resolution unprecedented regardless of changes in environmental temperature.

[Brief description of drawings]

FIG. 1 is an optical layout diagram showing an overall configuration of an optical scanning device of the present invention.

FIG. 2 is an explanatory diagram of a permissible range of a change in back focus of a collimator due to a change in temperature.

FIG. 3 is an explanatory diagram of a shape near a beam waist of a Gaussian beam.

FIG. 4 is a graph showing a relationship between a beam waist and an allowable depth on one side. Is.

FIG. 5 is a cross-sectional view showing an example of a structure in which a light source and an incident optical system are mutually fixed by a fixing member.

FIG. 6 is an explanatory diagram of a method for setting an allowable range of back focus change of a collimator due to temperature change.

FIG. 7 is a sectional view showing an example of a scanning optical system in the scanning device of the present invention.

8 is an explanatory diagram of a shape of an anamorphic surface used in the scanning optical system shown in FIG.

9 (a) of the scanning optical system shown in FIG. 7 is an image plane,
(B) is a diagram showing a beam diameter.

FIG. 10 (a) of the scanning optical system when a temperature change occurs
Is an image plane, and (b) is a diagram showing a beam diameter.

FIG. 11A is a cross-sectional view of Example 1 of the incident optical system of the present invention in the main scanning direction, and FIG. 11B is a cross-sectional view in the sub-scanning direction.

FIG. 12A is a sectional view of a second embodiment of the incident optical system of the present invention in the main scanning direction, and FIG. 12B is a sectional view in the sub scanning direction.

[Explanation of symbols]

1 Light source 2 First optical system in incident optical system
2'Mirror frame 3 Fixing member 4 Second optical system
5 Rotating polygon mirror 6 Rotation center 7,8 Imaging lens
9 Optical axis 10 Scanned surface

Claims (5)

[Claims] 1. A light source, an incident optical system, a deflector, a scanning optical system, and a medium to be scanned, the light emitted from the light source is deflected by the deflector via the incident optical system, and scanned via the scanning optical system. In an optical scanning device that scans a scanned medium by forming an image on the medium, the scanning optical system is an anamorphic optical system including at least one resin optical element having positive power, and the incident optical system is A first optical system that is an anamorphic optical system that is fixed to each other by a light source and a fixing member, and that makes divergent light emitted from the light source in the main scanning direction to be substantially parallel light,
A second optical system which forms an image of light from the light source through the first optical system in the vicinity of the deflector in the sub-scanning direction, and is provided in either the first optical system or the second optical system. An optical scanning device comprising an optical element made of resin having a negative power in the sub-scanning direction. 2. The method of claim 1] (0.18 / λ) · ^{[(ωoM 2 / Mm 2) +} (ωoS 2 / Ms 2) ] _{≦ | △ fB 2 M (△} Tmax) / Mm 2 - △ fB 2 S (Tmax ) / Ms ² | or (0.18 / λ) · ^{[(ωoM 2 / Mm 2) +} (ωoS 2 / Ms 2) ] _{≦ | △ fB 2 M (△} Tmin) / Mm 2 - △ fB 2 S ( The optical scanning device according to claim 1, wherein Tmin) / Ms ² | is satisfied. Here, ωoM is the beam waist diameter on the optical axis in the main scanning direction on the imaging plane of the scanning optical system, ωoS is the beam waist diameter on the optical axis in the sub-scanning direction on the imaging surface of the scanning optical system, and Mm is the main Absolute value of lateral magnification of all optical systems in the scanning direction, Ms is absolute value of lateral magnification of all optical systems in the sub-scanning direction, ΔfB ₂ M (ΔT), ΔfB ₂ S (ΔT)
Is the back focus change in the main scanning direction and the sub scanning direction by only the scanning optical system when the temperature change is ΔT,
λ represents the wavelength used. However, the definition of the beam diameter is the diameter of the intensity of 1 / e ² of the beam profile, and ΔTmax
(≧ 0) and ΔTmin (≦ 0) represent the upper and lower limits T ₀ + ΔTmin ≦ T ₀ ≦ T ₀ + ΔTmax of the temperature change amount with respect to the temperature range of the operation specification with respect to the reference design temperature T ₀ . 3. The first optical system, which is a collimator in which an incident optical system is fixed to each other by a light source and a fixing member, and has a rotationally symmetric optical surface with respect to the optical axis, and the collimator of the collimator when viewed from the light source. It has a resin optical element that has negative power in the sub-scanning direction behind it, and consists of an anamorphic second optical system that has almost no power in the main-scanning direction and positive power in the sub-scanning direction as a whole. 2. The optical device according to claim 1, wherein the light emitted from the collimator becomes substantially parallel light in both main scanning and sub scanning directions. 4. The incident optical system is fixed to each other by a light source and a fixing member, and in the main scanning direction, the light from the divergent light source is made into substantially parallel light to substantially act as a collimator, and in the sub scanning direction. An anamorphic first optical system having a resin-made anamorphic optical element having a negative power, and a second optical system which is an anamorphic optical system having a positive power in the sub-scanning direction as a whole, the first optical system. The optical scanning device according to claim 1, wherein the light emitted from the light source is substantially parallel light in the main scanning direction and divergent light in the sub scanning direction. 5. The optical device according to claim 4, wherein the anamorphic optical element made of resin is a cylindrical lens having a negative power in the sub-scanning direction.