JPH0772402A - Optical scanner - Google Patents

Abstract

PURPOSE:To provide an optical scanner capable of performing optical scanning with high accuracy by introducing a laser beam having the deep depth of field into a post-objective scanning optical scanner. CONSTITUTION:A laser beam emitted from a light source means 1 is made to be a Bessel beam having the intensity distribution nearly proportional to the square of a zero order Bessel function of a first kind through a Bessel beam generating means 3, the Bessel beam is converged by means of a converging means 4, made incident on a deflecting means 5, introduced directly to a surface 6 to be scanned after reflecting/deflecting by the deflecting means 5 and optically scans the surface 6 to be scanned.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device, and in particular, a Bessel beam having a minute spot and a deep focal depth is used as a laser beam to optically scan a surface to be scanned to record an image with high accuracy. The present invention relates to a post-objective scanning type optical scanning device suitable for, for example, a laser beam printer (LBP).

[0002]

2. Description of the Related Art Conventionally, a laser beam printer (LB
In the optical scanning device such as P), a so-called pre-objective scanning type in which the beam is deflected on the front side of the condensing means (imaging means), that is, the light source means side, and the rear side of the condensing means, that is, the scanned surface side. There is a so-called post-objective scanning type in which the beam is deflected by.

Among them, the pre-objective scanning type optical scanning device optically modulates a laser beam emitted from a light source means in accordance with an image signal, and the optically modulated laser beam is periodically caused by an optical deflector such as a polygon mirror. The image is recorded by deflecting the light into a spot shape and converging it in a spot shape on the surface of the photosensitive recording medium by an image forming optical system (light collecting means) such as an f-θ lens.

Further, the post-objective type optical scanning device optically modulates the laser beam emitted from the light source means, and the light-modulated laser beam is passed through an imaging optical system (light converging means) to an optical deflector. The laser beam reflected and deflected by the optical deflector is guided directly onto the surface to be scanned,
An image is recorded by optically scanning the surface to be scanned.

[0005]

In the conventional pre-objective scanning type, an fθ lens is usually used as an image forming optical system, and the fθ lens is used to correct its fθ characteristic and field curvature aberration. For example, in the case of a high resolution with a small spot diameter, it is necessary to satisfactorily correct spherical aberration, coma aberration and the like at the same time.

Therefore, since the fθ lens system must be constructed by using a plurality of lenses having a large lens aperture,
There is a problem that it is difficult to reduce the cost because the entire apparatus tends to be large-sized, the optical adjustment between the lenses is difficult, and the number of parts is increased.

On the other hand, the post-objective scanning type image forming optical system is required to satisfactorily correct only the on-axis performance as compared with the above-mentioned pre-objective scanning type image forming optical system. Since a lens having a small aperture may be used, it is advantageous to reduce the size and cost of the entire apparatus.

However, when this post-objective scanning type is used, an arc-shaped field curvature peculiar to the scanning type occurs, so that it is formed on the surface to be scanned when the spot diameter is small and the resolution is high. There is a problem in that a part of the spots (point images) that are formed are out of the depth of focus, and the spots are blurred.

Therefore, in the conventional post-objective scanning type optical scanning device, in order to solve this problem, for example, in Japanese Patent Laid-Open No. 63-239417, an imaging lens (imaging optical system) is synchronized with scanning. The field curvature is corrected by making a simple oscillatory motion by the driving means along the optical axis direction.

[0010] However, since it is necessary to relatively heavy objects (imaging lens) is vibrated at a high speed of several KH _Z stroke of approximately 10μm in the optical scanning apparatus, driving means (for example, an actuator that match the purpose ) Is very difficult to realize, the whole apparatus is complicated, and it is very difficult to obtain a low-cost apparatus.

The present invention provides an optical scanning device capable of improving the image quality of a recorded image with a simple structure by optically scanning the surface to be scanned using the Bessel beam generated by the Bessel beam generating means. To aim.

[0012]

The optical scanning device of the present invention comprises:
The laser beam emitted from the light source means is converted into a Bessel beam having an intensity distribution approximately proportional to the square of the 0th-order Bessel function of the first kind through the Bessel beam generating means, and the Bessel beam is condensed and deflected by the condensing means. It is characterized in that the light is made incident on the means and is reflected and deflected by the deflecting means, and then the light is directly guided onto the surface to be scanned to optically scan the surface to be scanned.

In particular, each element is set so that the center position in the optical axis direction of the Bessel beam generated by the Bessel beam generating means is closer to the light source means than the front focus position of the condensing means. There is.

[0014]

Embodiment 1 FIG. 1 is a schematic view of the essential portions of Embodiment 1 of the present invention.

In FIG. 1, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. Reference numeral 2 is a collimator lens, which collimates a laser beam (light beam) emitted from the light source means 1 on the basis of image information after being modulated.

Reference numeral 3 is an axicon as a Bessel beam generating means, which generates a Bessel beam having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the 1st kind having a beam spot with a deep focal depth.

Reference numeral 4 denotes a condenser lens (condensing means) which is composed of a single lens and condenses the Bessel beam to make it enter the deflecting surface (reflecting surface) of the optical deflector 5 as the deflecting means. ing. The optical deflector 5 is composed of a rotary polygon mirror having four deflecting surfaces, and is rotated at a constant speed in the direction of arrow C about a rotation axis O by a driving means (not shown) such as a motor.
Reference numeral 6 denotes a surface to be scanned, which corresponds to the surface of the photoconductor drum in, for example, a copying machine or LBP.

The optical scanning device of this embodiment is of the so-called post-objective scanning type in which the beam is deflected behind the condenser lens 4, that is, on the surface 6 to be scanned.

Next, the Bessel beam will be described.

The Bessel beam is, for example, J. Durnin, J, Op.
t, Soc, Am. A4 (1987) 651. and Japanese Unexamined Patent Publication No. 4-171415, 2 of 0th order Bessel function of the 1st kind.
It has an intensity distribution that is approximately proportional to the power, is a laser beam with an extremely deep depth of focus, and has the feature that the spot diameter is relatively small.

The intensity distribution I ₀ of this Bessel beam is expressed by I ₀ (r) = J ₀ ² (αr) (1) when the central intensity is normalized to 1. Where J ₀ is the zeroth-order Bessel function of the first kind,
r is the distance from the optical axis in the cross section orthogonal to the optical axis.
The parameter α is a parameter that is determined by the angle formed by the light beam forming the Bessel beam and the optical axis and the wavelength of the laser beam.

FIG. 5 is an explanatory view showing the cross-sectional intensity distribution of the Bessel beam on the surface to be scanned.

In this embodiment, as shown in FIG. 1, the laser beam emitted from the semiconductor laser 1 is converted into a plane wave by the collimator lens 2 and is incident on the axicon 3. The laser beams emitted from the axicon 3 interfere with each other to form a Bessel beam B having an intensity distribution approximately proportional to the square of the 0th-order Bessel function of the first kind in the vicinity of a plane (position) A crossing the optical path. .
The Bessel beam B diverges as it moves away from the plane A, but is converged by the condenser lens 4 and condensed in a substantially ring shape near the deflecting surface (reflection surface) 5b of the rotary polygon mirror 5.

In this embodiment, the central position A of the Bessel beam B in the optical axis direction is set to be closer to the semiconductor laser 1 than the front focus position of the condenser lens 4.

The laser beam reflected and deflected by the deflecting surface 5b of the rotary polygon mirror 5 is directly condensed on the surface 6 to be scanned to form a Bessel beam B '.

In this embodiment, the scanned surface 6 and the plane (position) A are set in an optically substantially conjugate relationship, so that the laser beam in the vicinity of the scanned surface 6 is a Bessel beam B '. It has become.

Then, by rotating the rotary polygon mirror 5 in the direction of arrow C in the figure, the surface to be scanned 6 is optically scanned in the main scanning direction with a minute spot diameter as shown by arrow D in the figure, and the surface to be scanned 6 is scanned. The image is two-dimensionally optically scanned (recorded) by moving or rotating in the sub-scanning direction.

As described above, in this embodiment, the laser beam formed in the vicinity of the surface to be scanned is a Bessel beam, and the Bessel beam has a very deep focal depth as described above, and the shaded area shown in FIG. The inside is characterized in that the diameter of the central spot of the Bessel beam hardly changes.

Therefore, regardless of whether the scanning beam scans the central portion or the peripheral portion of the surface 6 to be scanned, both are limited to the allowable value within the depth of focus of the Bessel beam. The diameter of 1 does not change much, and as a result, it is possible to scan with a minute spot over the entire surface 6 to be scanned.

Next, the paraxial relationship of the condenser lens 4 according to the first embodiment of the present invention will be described with reference to FIG. In the figure, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

In the figure, f is the focal length of the condenser lens 4, F1 is the front focal position of the condenser lens 4, F2 is the rear focal position of the condenser lens 4, and F3 is more than the rear focal position F2. It is a position separated by the focal length f toward the surface 6 to be scanned.
The distance from the center position A of the Bessel beam to the front principal point of the condenser lens 4 is a, and the distance from the rear principal point of the condenser lens 4 to the scanned surface 6 through the rotary polygon mirror 5 is b. .

At this time, for example, when a> 0, b> 0, f> 0, in this embodiment, the respective elements are set so that the relationship of 1 / a + 1 / b = 1 / f is substantially established.

Further, in this embodiment, each element is set so that the surface to be scanned 6 is not too far from the condenser lens 4 so that the relationship of a> f is satisfied.

That is, the center position A of the Bessel beam in the optical axis direction is set so as to be located closer to the light source means 1 side than the front focus position F1 of the condenser lens 4.

Further, in this embodiment, since the Bessel beam B generated near the center position A is magnified and imaged on the surface 6 to be scanned at a predetermined magnification, a <2f (that is, b> 2).
The relationship of f) is also satisfied at the same time. This is the scanned surface 6
Means that the position is farther than the position F3.

Next, the optical operation of the present embodiment will be described with reference to specific numerical examples.

In FIG. 1, for example, an axicon having a refractive index N of 1.511 and a prism apex angle α of 170 ° is used, and a semiconductor laser having a wavelength λ of 780 nm is used, and the beam spot diameter is set to 4 mm. Then, the diameter of the central spot of the Bessel beam B formed near the central position A is 1 / e ^{2 of the} central intensity.
Value is about 10 μm, depth of focus is about 44 mm (± 22 mm)
Becomes

At this time, for example, the imaging magnification is doubled (a =
When the magnified image is formed at 1.5f, b = 3f), the diameter of the central spot of the Bessel beam B ′ formed on the surface 6 to be scanned is about 20 μm, and the depth of focus is about 176 mm (±
88 mm).

Since the Bessel beam thus formed on the surface 6 to be scanned is a laser beam having a very deep depth of focus and a small spot diameter, for example, the rotating polygon mirror 5 is used.
Even if a large arc-shaped field curvature peculiar to post-objective scanning with the beam deflection point (the point where the laser beam enters the optical deflector) as the center of curvature occurs, the intensity distribution of the minute spot diameter changes. It can be kept constant without doing so, and thus a high-resolution optical scanning device can be obtained.

Next, the above optical action will be described with reference to FIGS.
Will be explained.

FIG. 3 is an explanatory view showing the field curvature of the optical system of the post-objective scanning type optical scanning device.

In the figure, 6 is a surface to be scanned, 7 is a curved arcuate image surface (image forming position), O is a beam deflection point (a point where a laser beam enters the optical deflector), and R is a beam deflection point. The distance from O to the point where the scanned surface 6 and the optical axis X intersect, X
Is the optical axis, θ is the scanning angle of the laser beam, and ΔL is the amount of curvature of field. The image plane 7 according to the present embodiment is substantially equal to an arc having a radius R with the beam deflection point O as the center.

Normally, the field curvature is defined as the distance along the optical axis, but here, for convenience, the distance measured along the scanning beam from the scanned surface 6 to the image surface 7 is the field curvature ΔL. It is defined as

The field curvature amount ΔL at this time can be calculated by the following equation (2): | ΔL | = R (1 / cos θ−1) (2)

Here, for example, R = 300 mm, θ = 0 °
FIG. 4 shows the result of calculation of the amount of curvature of field ΔL at 30 °. As shown in the figure, the maximum value of the amount of curvature of field ΔL is ΔL = 46 mm when the scanning angle θ = 30 °.

Since this field curvature amount ΔL is settled within the allowable value within the depth of focus (± 88 mm) described above, practical blurring of the spot does not occur.

Therefore, in this embodiment, a minute spot can be formed over the entire surface to be scanned, which enables highly accurate optical scanning.

In the above embodiment, the scanning angle θ is 0.
The scanning surface 6 and the image surface 7 are set to coincide with each other when the scanning angle θ is 0 °, but it is not always necessary to coincide when the scanning angle θ is 0 °. At this time, the surface 6 to be scanned and the image surface 7 may be made to coincide with each other, whereby a margin can be provided in the allowable range of the depth of focus.

Further, the condensing means in the present embodiment does not necessarily have to be composed of a single lens, and may be, for example, a lens system composed of a plurality of lenses.

Further, as a means for generating the Bessel beam, instead of the axicon, for example, an optical system using a thin ring aperture and a lens, a diffraction grating having an optical performance equivalent to that of the axicon, or the like may be used.

In the present embodiment, the rotary polygon mirror is used as the deflector, but the present invention can be applied to the single-face mirror such as a galvano mirror in the same manner as the above-mentioned embodiments.

[0052]

According to the present invention, as described above, by introducing a Bessel beam as a laser beam into a post-objective scanning type optical scanning device, it is possible to optically scan a surface to be scanned with a minute spot having a deep focal depth. Therefore, it is possible to achieve an optical scanning device capable of improving the performance of the device.

[Brief description of drawings]

FIG. 1 is a schematic view of a main part of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining a paraxial relation of the light collecting means according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram for explaining an amount of curvature of field of Example 1 of the present invention.

FIG. 4 is an explanatory diagram showing field curvature of Example 1 of the present invention.

FIG. 5 is an explanatory diagram showing a cross-sectional intensity distribution of a Bessel beam.

[Explanation of symbols]

 1 Light Source Means 2 Collimator Lens 3 Bessel Beam Generating Means (Axicon) 4 Condensing Means 5 Deflection Means 6 Scanned Surface 7 Image Surface ΔL Image Surface Curvature θ Scanning Angle

Claims (2)

[Claims] 1. A laser beam emitted from a light source means is converted into a Bessel beam having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first kind through the Bessel beam generating means, and the Bessel beam is condensed by the condensing means. An optical scanning device characterized in that the light is converged and made incident on a deflecting means, reflected and deflected by the deflecting means, and then directly guided onto a surface to be scanned to optically scan the surface to be scanned. 2. Each element is set so that the center position in the optical axis direction of the Bessel beam generated by the Bessel beam generating unit is closer to the light source unit than the front focus position of the light condensing unit. The optical scanning device according to claim 1.