JP2012103351A - Achromatic lens for laser light imaging - Google Patents

Abstract

An achromatic single lens having a total wavefront aberration of 1/10 wavelength (rms) or less in a wavelength band from a wavelength of 1.0 μm to 1.6 μm.
A two-lens structure in which an A lens having a positive refractive power made of a first material and a B lens having a negative refractive power made of a second material are bonded together, or the B lens on both surfaces of the A lens. This is an achromatic lens for laser beam imaging that has a three-lens configuration with lenses attached, and the difference in Abbe number ν _1.31n between the two lenses exceeds 27 as a condition of the lens material of the A lens and B lens. The absolute value of the difference in dispersion ratio is less than 0.005 and the refractive index n _1.31 (1) of the A lens is in the range of 1.42 to 1.59, or the difference between the Abbe numbers ν _1.31 of both lenses is 27. The Abbe number ν _1.31 ratio exceeds 1.40, and the refractive index n _1.31 (2) of the B lens exceeds 1.69.
[Selection] Figure 11

Description

  The present invention relates to an achromatic lens for laser beam imaging. More specifically, a wide wavelength band (1.0 μm including a wavelength band standardized by optical fiber communication using wavelength multiplexing technology and a 1.06 μm band laser wavelength used in many laser processing machines, etc. To 1.6 μm) for a laser beam imaging lens having diffraction limited imaging performance at the same focal position regardless of wavelength with respect to an incident light beam having a numerical aperture of less than 0.12.

  In optical wireless communications, lasers with near-infrared wavelengths (0.78 μm or 0.85 μm) are used in commercial devices, and optical components (lenses and beam splitters) that match this wavelength band are designed and used. Has been produced. However, in order to meet the demand for high speed and large capacity, in addition to the wavelength band of lasers centered on 1.31 μm or 1.55 μm used in trunk optical fiber communication, optical wireless communication devices include: Correspondence to a wide wavelength band including 1.26 μm to 1.61 μm covering all the wavelength bands standardized by wavelength division multiplexing communication is required. In this case, it is desirable to include a 1.06 μm band laser wavelength for which a high-power laser has been developed for a laser processing machine.

  A collimator lens that converts laser light emitted from an optical fiber into parallel light suitable for spatial transmission over long distances, and spatial light is efficiently introduced into the optical fiber. For this purpose, an imaging lens for a fiber coupler, an imaging lens for a beam expander that reduces or enlarges the beam diameter of parallel light, and the like are required. For these lenses, products having diffraction-limited optical characteristics necessary for image formation of laser light in the above-described wavelength band, for example, wavefront aberrations (hereinafter referred to as λ / 10) of 1/10 or less of the wavelength. There is a need for inexpensive manufacturing.

  At this time, if the wavelength band of the laser to be used is 1 μm or less, a commercially available lens such as an aspherical lens or a laminated double lens (doublet) is used with a non-reflective coating suitable for the wavelength band. can do. However, in order to cover all the wavelength bands standardized by wavelength division multiplexing, a lens that is sufficiently achromatic in the wavelength band of at least 1.27 μm to 1.61 μm is required.

  In the near-infrared wavelength region with a wavelength of about 1 μm from visible light, an invention that realizes wideband achromaticity by a doublet or a three-piece bonded lens (triplet) has been disclosed (Patent Document 1, Patent Document) 2). A broadband achromatic lens can be designed by selecting an optimal variety from among anomalous dispersion glasses suitable for achromatization according to this theory.

  However, in the wavelength band from 1.0 μm to 1.6 μm, product development and design results of doublet lenses and triplet lenses with diffraction-limited imaging accuracy necessary for laser beam imaging have been made. Even when optimal design is performed based on a combination of optical glass (abnormal dispersion glass) that has achromatic characteristics for visible light, good characteristics are not always obtained.

  In general, in a microscope objective lens and a telescope objective lens, ray tracing is performed from an incident surface toward a reference surface of a focus as a non-aberration standard, and an optical path difference (OPD) is obtained for a plurality of incident rays, and the maximum value is obtained. Is used, and the square root (rms value) of the optical path difference is obtained for the entire incident light beam and the square root (rms value) thereof is λ / 13 or less. In the following description, as a diffraction-limited imaging condition for laser light, the root mean square of the optical path difference over the entire incident light beam (this value is referred to as total wavefront aberration) is λ / 10 (rms) or less. The condition of the wavefront aberration is assumed.

  In order to combine laser light propagating in a single mode fiber and spatial light, a light beam with a numerical aperture (NA) of about 0.1 to 0.15 is incident at about 1.0 degree in consideration of assembly errors. There is a need for a diffraction-limited imaging optical system that can tolerate deviations from angular optical axes.

  As an imaging optical system with a large NA having diffraction-limited imaging characteristics, there is an application of an aspheric lens in an LD module. In the LD module, since the position of the optical fiber is fixed with respect to the lens, an aspheric lens that exhibits good imaging performance with respect to an axial ray is preferable. Laser light centered on wavelengths 1.31 μm and 1.55 μm is used for high-speed and large-capacity optical communication using a single-mode optical fiber. In conventional LD modules, these two wavelength bands are individually used. An imaging lens having a corresponding aspherical shape is often used (see Patent Document 3).

  However, the conventional aspheric lens has a problem that the astigmatism is not sufficiently corrected for laser light incident obliquely with respect to the optical axis, and the imaging performance deteriorates. Furthermore, the aspherical lens has not realized a so-called broadband achromatic lens in which the focal point is the same in a wide wavelength band. For this reason, the diffraction-limited imaging optical system that requires broadband achromaticity is often a complex and expensive optical system using a large number of lenses.

  On the other hand, an eyepiece of a telescope (beam expander) used in an optical wireless communication apparatus has a wide incident angle from 0.2 degrees to 4.0 degrees in order to allow variation in the incident angle of laser light. On the other hand, it is necessary to maintain the performance of the diffraction limit. From this condition, it is necessary to correct astigmatism within the range of the incident angle. In order to realize these characteristics with a double-bonded doublet or a triple-bonded triplet, a glass material with a high refractive index that has been developed recently, or a glass that exhibits anomalous dispersion even in the infrared region. It is effective to design an achromatic lens using a material.

JP 2003-139916 A JP 2000-98221 A Japanese Unexamined Patent Publication No. Hei 6-289256

  In a conventional doublet or triplet structure achromatic lens using an anomalous dispersion glass exhibiting good achromatic characteristics in the visible light region, good achromatic characteristics cannot always be obtained at infrared wavelengths exceeding 1 μm. In addition, with a conventional aspherical lens that forms an image of laser light for optical fiber communication, it is impossible to achromatic a wide band, and the imaging performance may drop sharply for laser light with a large incident angle. There were many.

Therefore, in the present invention, the Abbe number ν _1.31 that defines the magnitude of chromatic dispersion by setting the center of the wavelength to 1.31 μm while following the conventional design method of the achromatic lens in the visible light region centered on the d-line. By selecting the optimal glass material using this Abbe number ν _1.31 and the partial dispersion ratio of wavelength 0.98 μm and wavelength 1.55 μm (defined in Equation 3 below) as parameters, the future optical wireless An optical wireless communication apparatus characterized by a direct connection with a single mode optical fiber at the same focal position regardless of the wavelength for a wide wavelength band from 1.0 μm to 1.6 μm required for the communication apparatus It is an object of the present invention to provide an inexpensive optical system having a total wavefront aberration of λ / 10 (rms) or less necessary for an internal optical system.

  In the present invention, in order to realize a broadband achromatic lens having the performance necessary for the above-mentioned laser imaging at low cost, a variety suitable for achromatic at infrared wavelengths is selected from the manufactured optical glass. The conditions under which the solution satisfying the diffraction-limited imaging characteristics can be obtained when the lens shape is optimized by the configuration of the bonded lens were studied. At this time, whether or not the wavefront accuracy required for laser imaging can be obtained for the three wavelengths of short wavelength (0.98 μm or 1.06 μm), medium wavelength (1.31 μm), and long wavelength (1.6 μm). Was used as a criterion for evaluation.

  Furthermore, in order to satisfy the imaging performance with a wavefront error of λ / 10 (rms) or less by allowing an optical axis misalignment and a variation of the incident position of the optical system that cannot be avoided when manufacturing the optical system. The allowable range for the error of the incident height and the incident angle was also evaluated. For this purpose, it is necessary to correct not only chromatic aberration, spherical aberration and coma but also astigmatism among the optical aberrations of the lens. The imaging performance when the entrance pupil is at the front focal position of the lens was also evaluated.

で定義したとき、前記第１の材料のアッベ数ν_1.31（１）と前記第２の材料のアッベ数ν_1.31（２）の差が次式

の範囲である。
（２）波長０．９８μｍから波長１．５５μｍにおける色収差の２次スペクトルの大きさに関連する部分分散比θ_1.31を

で定義したとき、前記第１の材料の部分分散比θ_1.31（１）と前記第２の材料の部分分散比θ_1.31（２）の差の絶対値｜Δθ_1.31｜が次式

の範囲であり、前記第１の材料の波長１．３１μｍでの屈折率ｎ_1.31（１）が次式

の範囲である。条件（１）及び条件（２）を満たすことにより、波長１．０μｍから１．６μｍまでの波長域で良好な色消し特性と回折限界の結像性能を持ったダブレットまたはトリプレットが設計できる。この条件は、従来の可視光・近赤外域の広帯域色消しレンズの設計手法と類似しているが、赤外波長域におけるガラス材料の屈折率分散特性は可視光域とは異なるガラス材料が多いため、上記の条件を満たすことが必要である。上記のアッベ数、部分分散比は従来のものとは異なる波長で定義したものであり、上記の条件を満たすガラス材の組み合わせが可視域でも良好な色消し特性を持つとは限らない。 The achromatic lens for laser beam imaging according to claim 1 is the same for incident light having a numerical aperture of less than 0.12 in the wavelength band from 1.0 μm to 1.6 μm regardless of the wavelength. A lens A having a positive refractive power made of the first material and a B lens having a negative refractive power made of the second material were bonded together for the purpose of obtaining a diffraction-limited imaging characteristic at the focal position. It is an achromatic lens satisfying the following condition (1) and condition (2) or condition (3) with a two-lens configuration or a three-lens configuration in which the B lens is bonded to both surfaces of the A lens. It is a feature.
(1) The refractive index at a wavelength of 1.31 μm is n _1.31 , the refractive index at a wavelength of _1.55 μm is n _1.55 , the refractive index at a wavelength of 0.98 μm is n _0.98 , and the chromatic dispersion is based on the wavelength of 1.31 μm. Abbe number ν _1.31 indicating

The difference between the Abbe number ν _1.31 (1) of the first material and the Abbe number ν _1.31 (2) of the second material is

Range.
(2) The partial dispersion ratio θ _1.31 related to the magnitude of the secondary spectrum of chromatic aberration at wavelengths from 0.98 μm to 1.55 μm

The absolute value | Δθ _1.31 | of the difference between the partial dispersion ratio θ _1.31 (1) of the first material and the partial dispersion ratio θ _1.31 (2) of the second material is

The refractive index n _1.31 (1) at a wavelength of 1.31 μm of the first material is

Range. By satisfying the conditions (1) and (2), a doublet or triplet having good achromatic characteristics and diffraction-limited imaging performance can be designed in the wavelength range from 1.0 μm to 1.6 μm. This condition is similar to the conventional method of designing a broadband achromatic lens in the visible light / near infrared region, but the refractive index dispersion characteristics of the glass material in the infrared wavelength region are often different from those in the visible light region. Therefore, it is necessary to satisfy the above conditions. The above Abbe number and partial dispersion ratio are defined at a wavelength different from the conventional one, and a combination of glass materials that satisfy the above conditions does not always have good achromatic characteristics even in the visible range.

の範囲であり、前記第２の材料の波長１．３１μｍでの屈折率ｎ_1.31（２）が次式

の範囲であれば、部分分散比が合致していなくても、色収差による結像性能の劣化を許容範囲に収めることができる。 Next, as an alternative to condition (2),
(3) The ratio of the Abbe number ν _1.31 (1) of the first material to the Abbe number ν _1.31 (2) of the second material is

The refractive index n _1.31 (2) at a wavelength of 1.31 μm of the second material is

In this range, even if the partial dispersion ratio does not match, the deterioration of the imaging performance due to chromatic aberration can be within an allowable range.

A normal achromatic doublet is made of a positive lens made of a glass material having a small refractive index and a large Abbe number such as crown glass and a negative lens made of a glass material having a large refractive index and a small Abbe number such as flint glass. , The Abbe number ν _1.31 with a wavelength of 1.31 μm as a reference is defined as Equation 1, and the partial dispersion ratio θ _1.31 related to the size of the secondary spectrum at the wavelength of 0.98 μm and the wavelength of 1.55 μm. By defining a combination satisfying the condition of the partial dispersion ratio dispersion ratio θ _1.31 from commercially available optical glasses, a lens satisfying the above imaging performance can be manufactured. If this condition is not satisfied, the curvature of the bonding surface becomes too small to realize a lens having a large numerical aperture, and the lens becomes thick, resulting in high costs and no function as an effective lens.

Generally, by increasing the refractive index n _1.31 , the radius of curvature of each lens surface is increased, and the spherical aberration and coma can be reduced. Furthermore, the larger the difference between the Abbe numbers of the two lenses, the smaller the absolute value of the individual refractive powers of the two lenses having positive and negative refractive powers. Can be balanced small. Therefore, in order to obtain diffraction-limited imaging performance with respect to laser light, a glass combination having a larger Abbe number difference than the combination used in a normal achromatic lens is required. Further, in order to reduce spherical aberration, a configuration in which the refractive index of the material of the B lens having negative refractive power is larger than the refractive index of the material of the A lens having positive refractive power is suitable. Furthermore, in order to have a broadband achromatic characteristic, it is necessary to match the partial dispersion ratio with two types of glass.

  It is difficult to satisfy the conditions for completely correcting these chromatic aberrations and the conditions for correcting spherical aberration, coma, and astigmatism with a lens having a doublet or triplet structure with a low degree of design freedom. However, the radius of curvature and surface spacing of each lens surface are uniquely determined. At present, there is no combination of glass materials with a high refractive index, a large Abbe number difference, and equal partial dispersion ratios. In many cases, it is concluded that there is no solution that achieves the limit of imaging characteristics.

  The invention described in claim 1 has a negative refractive power although the partial dispersion ratio is different from the region where the two glass materials in which the refractive index is 1.42 or more and the partial dispersion ratio substantially match are actually present. By increasing the refractive index of the lens material of the B lens to be 1.69 or more, the difference between the Abbe number and the Abbe number ratio in the two regions of the region where spherical aberration and coma can be reduced can be increased. This shows that a lens for imaging a wide wavelength band laser can be designed.

の範囲である。
（５）前記第２の材料の前記屈折率ｎ_1.31（２）と前記第１の材料の屈折率Δｎ_1.31（１）の差が次式

の範囲である。 The achromatic lens for laser beam imaging according to claim 2 is the achromatic lens for laser beam imaging according to claim 1, wherein the achromatic lens for laser beam imaging is the A lens. It is a two-sheet configuration in which the B lens is bonded, and satisfies the condition (2) and the following conditions (4) and (5).
(4) The difference between the Abbe number ν1.31 (1) of the first material and the Abbe number ν1.31 (2) of the second material is

Range.
(5) The difference between the refractive index n _1.31 (2) of the second material and the refractive index Δn _1.31 (1) of the first material is

Range.

  In the achromatic lens shown in claim 2, by using two types of anomalous dispersion glasses that are available, partial dispersion is achieved while increasing the Abbe number difference defined by Equation 8 for the two lenses. Since the ratio can be strictly matched, achromatic performance in a wide wavelength band can be obtained. In addition, by providing a difference in refractive index between two types of glass materials, spherical aberration and coma can be corrected even on the bonding surface. Therefore, even if the refractive index of the material used is not so large, the wavelength is 1.0 μm to 1.6 μm. Diffraction-limited imaging is possible in the wavelength band up to. The upper limit of the refractive index difference is a condition necessary for suppressing the reflection of the bonded surface to 0.3% or less without applying a special antireflection coating.

  When a doublet lens having a short focal length is designed according to the conditions of claim 2, the radius of curvature of the lens surface is reduced, and accordingly, spherical aberration and coma tend to increase. Furthermore, astigmatism increases sharply as the incident angle of light increases, it is not suitable for eyepieces with large variations in incident angle. For this reason, the lens of claim 2 is preferably used for an imaging lens for fiber coupling with a small incident angle variation and an objective lens of a beam expander.

の範囲である。
（７）前記第２の材料の前記屈折率ｎ_1.31（２）と前記第１の材料の前記屈折率ｎ_1.31（１）の差が次式

の範囲である。 The achromatic lens for laser beam imaging according to claim 3 is the achromatic lens for laser beam imaging according to claim 1, wherein the achromatic lens for laser beam imaging is the A lens. The shape of the lens having the B lens bonded on both sides is a three-sheet configuration symmetrical in the optical axis direction, and satisfies the above condition (3) and the following conditions (6) and (7): To do.
(6) The refractive index n _1.31 (2) of the second material is

Range.
(7) The difference between the refractive index n _1.31 (2) of the second material and the refractive index n _1.31 (1) of the first material is

Range.

  The achromatic lens shown in claim 3 has a completely symmetrical arrangement in which the first lens and the third lens bonded to both surfaces of the A lens serving as the second lens are B lenses having the same shape and the same glass material. By using a triplet structure triplet, it is devised so that a lens can be manufactured at low cost by using two types of curvature radii on the lens processing surface while ensuring a minimum degree of freedom for aberration correction. . As in the condition (3) of claim 1, by using a glass material having a high refractive index, the occurrence of spherical aberration is suppressed, the refractive index difference on the bonded surface is made a certain value or more, and the structure is symmetrical in the optical axis direction. By adopting it, spherical aberration, coma and astigmatism are corrected. Furthermore, in the doublet lens configuration of claims 1 and 2, a large coma is generated unless the entrance pupil of the light beam is placed in front of the lens and the imaging performance deteriorates. However, in this configuration, the entrance pupil is located near the front focal point of the lens. Even if it arrange | positions, a favorable optical characteristic can be acquired. This arrangement is the same as that of a so-called image-side telecentric lens, and is therefore optimal as an eyepiece that requires the exit pupil to be positioned away from the lens surface.

  By using the lens according to the present invention in a fiber coupling optical system, a laser beam transmitted through a space of a plurality of wavelengths (eg, 1.06 μm, 1.31 μm, 1.55 μm) and a fiber propagating in a single mode fiber Light can be coupled without loss, and even in the future, even if ultra-high capacity optical wireless communication exceeding 100 Tbps using all of this wavelength band is required, the optical system can be provided at low cost. The scope of application of the present invention is not limited to optical wireless communication, and a wide range of applications can be achieved by using the lens of the present invention for a precise imaging optical system of multi-wavelength laser light, which has conventionally been impossible with a single lens. Can be used in.

Example 1: Optical path diagram of the shape of a doublet lens using a combination of the anomalous dispersion glass of claim 1 and an incident angle of 1 degree (dotted line). Example 2: A doublet lens shape in which broadband achromaticity is realized using the high refractive index glass of claim 1 and an optical path diagram of an incident angle of 1 degree (dotted line). FIG. 6 is a diagram illustrating a relationship between a wavelength of a doublet lens using the high refractive index glass of Example 2 and a focal position change amount. The wavefront aberration characteristic of the doublet lens using the high refractive index glass of Example 2. (A) Incident angle 0.0 degree (B) Incident angle 1.0 degree Example 3: A doublet lens shape using the high refractive index glass of claim 1 and emphasizing wavefront aberration characteristics in a long wavelength band and an optical path diagram of an incident angle of 1 degree (dotted line). 5 shows wavefront aberration characteristics of a doublet lens using the high refractive index glass of Example 3. FIG. (A) Incident angle 0.0 degree (B) Incident angle 1.0 degree (C) Incident angle 2.0 degree Comparative Example 1: Optical path diagram of lens shape and incident angle of 2 degrees (dotted line) using glass having a relatively small difference in Abbe number. The wave aberration characteristic of the doublet lens of the comparative example 1. (A) Incident angle 0.0 degree (B) Incident angle 1.0 degree (C) Incident angle 2.0 degree Comparative Example 2: Optical path diagram of a lens shape and an incident angle of 2 degrees (dotted line) optimally designed using a combination of glasses used for a commercially available infrared laser imaging lens. The wave aberration characteristic of the doublet lens of the comparative example 2. (A) Incident angle 0.0 degree (B) Incident angle 1.0 degree (C) Incident angle 2.0 degree Example 4: Optical path diagram of a doublet lens shape and an incident angle of 4 degrees (dotted line) using a combination of anomalous dispersion glass materials satisfying the conditions (1) and (2) of claim 1 and having a partial dispersion ratio matching. 10 shows the wavefront aberration characteristics of the lens of Example 4. (A) Incident angle 0.0 degree (B) Incident angle 2.0 degree (C) Incident angle 4.0 degree Example 5: Shape of a symmetrical triplet lens using a high refractive index glass material and an optical path diagram of an incident angle of 4 degrees (dotted line). 10 shows the wavefront aberration characteristics of the lens of Example 5. Example 6: Optical path diagram of the shape of a doublet lens using the combination of anomalous dispersion glass of claim 2 and an incident angle of 1 degree (dotted line). FIG. 10 is a diagram illustrating the relationship between the wavelength of a lens of Example 6 and a focal position shift. 10 shows the wavefront aberration characteristics of the lens of Example 6. (A) Incident angle 0.0 degree (B) Incident angle 1.0 degree Example 8: A shape of a triplet lens using the high refractive index glass of claim 3 and an optical path diagram of an incident angle of 4 degrees (dotted line). FIG. 10 is a diagram showing the relationship between the wavelength of a triplet lens of Example 8 and a focal position shift. 10 shows wavefront aberration characteristics of the triplet lens of Example 8. FIG. (A) Incident angle 0.0 degree (B) Incident angle 4.0 degree Example 9: A shape of a triplet lens using the high refractive index glass of claim 3 and an optical path diagram at an incident angle of 4 degrees (dotted line). Example 10: A shape of a triplet lens using the high refractive index glass of claim 3 and an optical path diagram at an incident angle of 4 degrees (dotted line). Example 11: Shape of triplet lens using high refractive index glass of claim 3 and optical path diagram of incident angle of 4 degrees (dotted line). Example 12: A shape of a triplet lens using the high refractive index glass of claim 3 and an optical path diagram at an incident angle of 4 degrees (dotted line).

The invention described in claim 1 will be described below in detail with reference to tables and figures.
(1) Two-element lens In Embodiment 1, an example in which both the condition (1) and the condition (2) are satisfied is shown.

Example 1 The first lens of Example 1 corresponds to the A lens described in claim 1, and the second lens of Example 1 corresponds to the B lens described in claim 1. In this embodiment, an achromatic lens having a focal length of 9.97 mm is designed by using a combination of anomalous dispersion glass having a large Abbe number difference and a matching partial dispersion ratio, although the refractive index is not large. The lens materials used were S-FPL51Y from OHARA Inc. for the A lens and S-NBM51 for the B lens. This lens surface has a radius of curvature, a lens thickness, a refractive index at wavelengths of 0.98 μm, 1.31 μm, and 1.55 μm, an Abbe number ν _{1.31 at} a wavelength of _1.31 μm, and a wavelength from 0.98 μm to a wavelength of 1.55 μm. Table 1 shows lens data such as the partial dispersion ratio θ _1.31 . In the design example, good imaging performance can be obtained even if the third surface of the lens is flat.

  As can be seen from Table 1, the lens of Example 1 satisfies the conditions (1) and (2) and does not satisfy the condition (4) of claim 2. As a result of evaluating the achromatic characteristic of this lens as a focal shift amount, the variation of the focal position with respect to a wavelength change from 1.0 μm to 1.6 μm is 1 μm or less, and almost complete achromatic is realized. However, at a wavelength of 1.0 μm or less, the achromatic characteristics are rapidly deteriorated.

  FIG. 1 shows an optical path diagram and a lens shape when the incident beam diameter is 2.4 mm and the incident angle is 1.0 degree. In the drawings showing lens shapes and optical paths used in the following description, reference numerals 1 to 3 in the drawings denote first to third lenses. Reference numerals (1) to (4) denote surface numbers 1 to 4. The white portion of the lens shape is the A lens, and the hatched portion is the B lens. Hereinafter, the description of the lens number and the surface number will be used in the drawing of the lens.

  Table 2 shows the total wavefront aberration characteristics for beams up to an incident beam diameter of 2.4 mm and an incident angle of 2.0 degrees. The value of the total wavefront aberration is the root mean square (rms value) of the optical path difference (OPD) for the entire light beam composed of a plurality of light beams. Hereinafter, in all the tables of the total wavefront aberration characteristics, the magnitude of the wavefront aberration is expressed as an rms value in units of wavelength.

  Table 2 shows that the designed achromatic lens exhibits diffraction-limited performance up to an incident angle of 1.0 degree, but the wavefront aberration slightly exceeds λ / 10 (rms) when the incident angle reaches 2.0 degrees. I understand that. When the incident angle is 1.0 degree, the geometric optical spot diameter is 2.2 μm (rms), which is sufficiently smaller than the radius (7 μm) of the air ring, which is a standard for the diffraction limit. From this result, this lens is suitable for applications that require achromatic performance in a wide wavelength range of 1.0 μm to 1.6 μm when the incident angle does not change greatly.

  Example 2 An achromatic doublet lens designed using a combination of high refractive index glass manufactured by OHARA so as to satisfy the conditions (1) and (3) shown in claim 1 will be described. The glass material used was S-LAH58 for the first lens and S-NPH2 for the second lens. The lens data of this lens is as shown in Table 3. In this embodiment, the conditions (2) and (4) of claim 2 are not satisfied.

  As a result of manufacturing a lens having a diameter of 6 mm based on this lens data, the focal length was 11.03 mm. FIG. 2 shows a lens shape and an optical path when the incident beam diameter of the laser beam is 2.4 mm and the incident angle is 1.0 degree. This lens was designed with an emphasis on imaging performance at two wavelengths of 1.55 μm and 1.31 μm, and the bending of the entire lens is increased so that the wavelength band with good achromatic characteristics is between the above two wavelengths. It is adjusted to become. For this reason, the imaging performance in the short wavelength band is deteriorated.

  FIG. 3 shows the change of the focal position when the wavelength is changed from 0.9 μm to 1.9 μm. The focal positions of the wavelength 1.55 μm and the wavelength 1.31 μm are the same, and the focal distance is 20 μm or less between the wavelength 1.0 μm and the wavelength 1.9 μm, which is not perfect, but can be used as a laser imaging optical system Achromatic achromatic performance. When the wavelength is 1.4 μm or more, it has chromatic aberration opposite to that of a normal lens, and can be used to correct chromatic aberration of the entire optical system in combination with a normal lens.

  The wavefront aberration characteristics of the lens of Example 2 are shown in FIG. The solid line indicates a wavelength of 1.064 μm, the dotted line indicates a wavelength of 1.31 μm, and the alternate long and short dash line indicates a wavelength of 1.55 μm. The maximum and minimum values of the vertical scale are ± 0.2 wavelengths, and the vertical axis shows the optical path differences OPD (M) and OPD (S) for the incident light on the meridional surface (meridional) and spherical surface (sagittal). Indicates the height (H) of light incident on the lens. 4A shows the wavefront aberration with respect to the incident light beam on the axis, and FIG. 4B shows the wavefront aberration with respect to the light beam having an incident angle of 1.0 degree. The maximum value of OPD with respect to wavelengths of 1.31 μm and 1.55 μm is λ / 20 or less, and has diffraction limited imaging performance. Even at 1.064 μm, the maximum value of OPD is λ / 5 or less. The subsequent wavefront aberration characteristics are based on the above display method.

  Table 4 shows the result of calculating the total wavefront aberration (rms value) of the lens of Example 2 with respect to the incident angle for beams up to an incident angle of 1.0 degree.

  For a wavelength of 1.064 μm, the target wavefront accuracy of λ / 10 (rms) is exceeded, but for wavelengths of 1.3 μm and 1.55 μm, it has good performance of λ / 20 (rms) or less. Yes. Further, the geometric optical spot diameter is 2.0 μm (rms) or less at all wavelengths and incident angles. In this embodiment, since priority was given to removing chromatic aberrations at wavelengths of 1.31 μm and 1.55 μm, the imaging performance deteriorated at a short wavelength (1.06 μm). This is because, unlike Example 1, the difference in the partial dispersion ratio of the glass material is large. Further, when the incident angle is larger than 1.0 degree, the wavefront aberration characteristic is rapidly deteriorated. This is because the entire lens has a large bending and the difference in refractive index between the two types of glass materials is small, and the bonded surface does not contribute much to aberration correction. For this reason, the lens of the present embodiment is suitable for an application with little incident angle fluctuation such as an optical fiber coupler.

  Using a high refractive glass material from OHARA with a large Abbe number difference Δν, chromatic aberration is achieved under the condition that the spherical aberration, coma, and astigmatism are completely corrected for an incident beam with an incident angle of 2.0 degrees and a diameter of 2.4 mm. The results of a comparative study of changes in characteristics are shown below. All the focal lengths of the lenses are 10 mm.

  Example 3 Designed by selecting S-PHM52 having a large Abbe number and strong anomalous dispersion as the first glass material and S-NPH1 having a small Abbe number and a large refractive index as the second glass material. Table 5 shows the lens data. In this embodiment, the conditions (1) and (3) of claim 1 are satisfied, but the conditions (4) and (5) of claim 2 are not satisfied.

  FIG. 5 shows an optical path diagram of the lens shape, the diameter of 2.4 mm, and the incident angle of 2.0 degrees in Example 3.

The wavefront aberration characteristics of the lens of Example 3 are shown in FIG. Although the difference in partial dispersion ratio is large as Δθ = 0.041, the refractive index difference is large as Δn _1.31 = 0.16 and the Abbe number difference is large as Δν = 37.1. Even when corrected, the wavefront aberration for an incident angle of 1.0 degree is λ / 10 or less for all wavelengths. However, when the incident angle is 2.0 degrees, the characteristics on the short wavelength side deteriorate.

  Table 6 shows the result of calculation of the total wavefront aberration for three wavelengths with respect to the beam of the incident angle up to 2.0 degrees for the lens of Example 3.

  All combinations of incident angles of 0.0 to 2.0 degrees and wavelengths of 0.98 μm to 1.55 μm satisfy the condition of λ / 10 (rms) or less, but the incident angle exceeds 2.0 degrees. The total wavefront aberration exceeds λ / 10 (rms). When the incident angle is 2.0 degrees, the geometrical optical spot diameter is 1.1 μm, and the change in focal position is 33 μm for wavelengths from 1.0 μm to 1.6 μm.

“Comparative example of two-element lens”
Comparative Example 1: For comparison with Example 3, a design example that deviates from the conditions of claim 1 will be described. Table 7 shows the lens data of the design results when the Abbe number difference does not satisfy the condition (1) and the first glass material of Example 3 is changed to S-LAL8 having a small Abbe number.

  FIG. 7 shows an optical path diagram of the lens shape of Comparative Example 1, the diameter of 2.4 mm, and the incident angle of 2.0 degrees.

The wavefront aberration characteristics of the lens of Comparative Example 1 are shown in FIG. In this design example, the difference in the partial dispersion ratio of the lens material is as large as Δθ _1.31 = 0.049, and the difference in refractive index and Abbe number is small with Δn _1.31 = 0.07 and Δν _1.31 = 19.2. When spherical aberration, coma, and astigmatism are corrected at 31 μm, chromatic aberration increases on the short wavelength side and on the long wavelength side, and wavefront aberration increases for light rays having a large incident height.

  Table 8 shows the total wavefront aberration characteristics for the incident beam up to an incident angle of 2.0 of the lens of Comparative Example 1 for three wavelengths.

  The lens of Comparative Example 1 has larger aberrations than the characteristics of Example 3 (Table 6), and satisfies the condition of λ / 10 (rms) or less when the incident angle exceeds 2.0 degrees in the short wavelength region. Absent. The geometrical optical spot diameter at an incident angle of 2.0 degrees is 2.0 μm (rms), the change in focal position is 72 μm for wavelengths from 1.0 μm to 1.6 μm, and a single wavelength (for example, 1.31 μm) In the vicinity) can be used as a diffraction-limited imaging lens.

  Comparative Example 2: A design example using the same glass material (Shot) as that of a lens commercially available for laser imaging in the infrared wavelength band is shown. Table 9 shows the lens data of the design results when correcting the spherical aberration, coma and astigmatism when LANK22 is used as the material of the A lens and SFL6 is used as the material of the B lens.

In the lens of Comparative Example 2, the Abbe number is as large as ν _1.31 (2) = 57.5, and the ratio of the Abbe number ν _1.31 is 1.358, which is smaller than 1.40 in the condition (3). When spherical aberration, coma, and astigmatism are corrected at 31 μm, large chromatic aberration occurs. Conversely, when priority is given to correction of chromatic aberration, coma and astigmatism remain.

  FIG. 9 shows the lens shape of Comparative Example 2 and the optical path diagram with a diameter of 2.4 mm and an incident angle of 2.0 degrees. The wavefront aberration characteristics are shown in FIG.

  Table 10 shows the total wavefront aberration characteristics of the lens of Comparative Example 2 with respect to the incident beam up to an incident angle of 2.0 degrees for three wavelengths.

  The aberration characteristics of the lens of Comparative Example 2 are very similar to those of Comparative Example 1 (Table 8), but the imaging performance in a shorter wavelength region is greatly deteriorated compared to Comparative Example 1. When the incident angle is 2.0 degrees, the geometrical optical spot diameter is 2.5 μm (rms), and the change in the focal position is 74 μm for wavelengths from 1.0 μm to 1.6 μm.

  From the comparison of the two-bonded achromatic lens (contrast between Example 3 and Comparative Examples 1 and 2), the difference between the Abbe numbers of the two glass materials for the doublet lens is at least 27.0 or more. In addition, the Abbe number ratio of 1.40 or more is a condition for reducing the total wavefront aberration to λ / 10 (rms) or less in the target wavelength band from 1.0 μm to 1.6 μm. I understand.

(2) Three-element lens The invention of the achromatic lens having a three-piece structure described in claim 1 will be described in detail with reference to tables and drawings.

  Example 4 A description will be given of a case in which the condition of claim 3 is not satisfied with respect to three asymmetrical lenses having a bonded symmetrical configuration that satisfy the conditions (1) and (2) of claim 1.

Condition (2) means that the partial dispersion ratio is substantially the same, and is a condition necessary for achromaticity in a wide wavelength band. Moreover, the diffraction limit performance can be maintained even for a light beam having a large incident angle by the three-element configuration. Table 11 shows the lens data of the design results when S-FPL51 is used for the A lens located at the center of the three-lens configuration from the lens material of OHARA and S-NBM51 is used for the B lens located on both sides of the A lens. In this case, although the partial dispersion ratios are the same, the refractive index n _1.31 of the B lens material 2 at a wavelength of 1.31 μm is relatively small at 1.5934, which does not satisfy the condition (6) of claim 3. .

  FIG. 11 shows the shape of the lens of Example 4, the optical path diagram when the incident beam diameter is 2.4 mm, the incident angle is 4.0 degrees, and the entrance pupil is set at the front focal position. The lens diameter is 5.0 mm.

  The shift amount of the focal position with respect to the wavelength change of 1.0 μm to 2.2 μm of the lens of Example 4 is 1 μm or less, and almost complete achromaticity can be realized. However, at a wavelength shorter than 1.0 μm, the achromatic characteristics are rapidly deteriorated. FIG. 12 shows the wavefront aberration characteristics of the lens of Example 4 with respect to the change in the incident angle with respect to the beam having a beam diameter of 2.4 mm, and Table 12 shows the total wavefront aberration characteristics.

  Example 5: A glass material having a high refractive index that satisfies the conditions (1) and (3) of claim 1 and does not satisfy the condition of claim 3 (A-lens S-LAM3, B-lens S-LAM3) Table 13 shows the lens data of the design result of the achromatic lens with three bonded symmetrical configurations using NPH1). In this case, since the difference in refractive index between the two glass materials is small, the condition (7) of claim 3 is not satisfied.

  FIG. 13 shows the shape of the lens of Example 5 and the optical path diagram at an incident angle of 4.0 degrees. The lens diameter is 5.0 mm.

  FIG. 14 shows the wavefront aberration characteristics of the lens of Example 5 when a beam having a diameter of 2.4 mm is incident, and Table 14 shows the total wavefront aberration characteristics.

  As shown in Table 14, although the performance in the short wavelength band is slightly lowered, the performance close to the diffraction limit even at an incident angle of 4.0 degrees due to the symmetrical triplet configuration using a combination of high refractive index glasses having a small difference in partial dispersion ratio. A wide-band achromatic lens with However, the achromatic characteristics are better in Example 4. Further, as a property common to Embodiments 4 and 5, the lens thickness is larger than the focal length, which is not suitable for a lens having a large aperture or a long focal length. For this reason, it is suitable for uses such as an eyepiece lens and a collimator lens.

  Next, an embodiment according to the combination of anomalous dispersion glass corresponding to claim 2 will be shown.

  Example 6: Lens data of an achromatic lens designed using Schott's anomalous dispersion glass, N-PK51, KZFSN5 in accordance with the conditions of claim 2 are shown. Table 15 shows lens data such as the curvature radius of the lens surface, the lens thickness, the glass material, the refractive index at a wavelength of 1.31 μm, and the Abbe number.

  As a result of manufacturing a lens having a diameter of 5.0 mm according to the design values in Table 15, the focal length was 9.88 mm. FIG. 15 shows the shape of this lens and an optical path diagram when the incident beam diameter of the laser beam is 2.4 mm and the incident angle is 1.0 degree.

  FIG. 16 shows changes in the focal position when the wavelength is changed from 0.7 μm to 1.7 μm for the lens of Example 6. In the wavelength range of 0.8 μm to 1.6 μm, the amount of change in the focal position is 0.5 μm or less, and almost complete achromatic characteristics can be realized. Compared with the characteristic of FIG. 3 which shows the amount of change in the focal position of the embodiment 2 of the claim 1 which does not satisfy the condition (2), the doublet lens has a higher performance by satisfying the condition (2) of the claim 2. It can be seen that a doublet lens having an achromatic characteristic can be realized.

  The wavefront aberration characteristics of the lens of Example 6 are shown in FIG. Although some frames remain, wavefront aberration characteristics of λ / 10 or less are shown at all wavelengths and incident angles up to 1.0 degree.

  Table 16 shows the results of calculating the total wavefront aberration characteristics of the lens of Example 6 for a beam having a beam diameter of 2.4 mm and an incident angle of 1.0.

  If the incident angle is 1.0 degree or less, the wavefront accuracy is λ / 30 (rms) or less at all wavelengths, and the performance of the diffraction limit is satisfied. However, the aberration of this lens increases abruptly as the incident angle of the light beam increases, and when the incident angle exceeds 2.0 degrees, the diffraction-limited imaging does not occur. The geometric optical spot diameter is 1.2 μm (rms) or less at all wavelengths and an incident angle of 1.0 ° or less.

A lens having a performance equivalent to the above design result can be realized by the following combination of glass materials from Schott.
(1) Combination of PK52 and KZFS4 or KZFS11 (2) Combination of FK51 and KZFS4 or KZFSN4

  Example 7: Table 17 shows lens data when designed using the combination of (1) (PK52 and KZFS4).

  The total wavefront aberration of the lens of Example 7 is λ / 20 (rms) or less for all three wavelengths with respect to a beam having a diameter of 2.4 mm and an incident angle of less than 1.0 degree. Further, the change of the focal position with respect to the wavelength of 1.0 μm to 1.6 μm is 1 μm or less, and the geometric optical spot diameter is 1.6 μm (rms). When the amount of change in the focal position of Examples 6 and 7 is compared with the characteristics of the lens of Example 2 (FIG. 3), it can be seen that the achromatic performance is remarkably improved. However, the incident angle that can maintain the diffraction-limited imaging performance is up to about 1.0 degree.

  Next, an embodiment corresponding to claim 3 will be described.

  Example 8: Example in which an achromatic triplet lens having a completely symmetrical configuration in which the first lens and the third lens have the same shape and the same glass material was designed using OHARA high refractive index glass according to the conditions of claim 3 Indicates. As the glass material, S-NPH2 was used for the first lens as the B lens and S-NPH2 as the third lens, and S-LAM3 was used as the second lens as the A lens. Table 18 shows lens data such as the curvature radius of the lens surface, the lens thickness, the refractive index at a wavelength of 1.31 μm, and the Abbe number, which are the design results.

  FIG. 18 shows the shape of the achromatic triplet lens of Example 8 and the optical path when the incident beam diameter is 2.4 mm and the incident angle is 4.0 degrees. The focal length is 10.08 mm. In this lens data, the position of the entrance pupil having a diameter of 2.4 mm is set not at the front surface of the lens but at the front focal position of the lens.

  FIG. 19 shows changes in the focal position when the wavelength is changed from 0.8 μm to 1.8 μm in the lens of Example 8. The amount of change in the focal position is 20 μm or less at a wavelength of 1.0 μm or more, and good achromatic performance can be realized.

  FIG. 20 shows the wavefront aberration characteristics at wavelengths of 1.064 μm (solid line), 1.31 μm (chain line), and 1.55 μm (one-dot chain line).

  Table 19 shows the total wavefront aberration characteristics of the lens of Example 8 in terms of wavelengths for three wavelengths.

  In the achromatic triplet of Example 8, the total wavefront aberration is λ / 10 (rms) or less at all wavelengths and incident angles, which satisfies the diffraction limit performance. The geometrical optical spot diameter is 1.5 μm (rms) or less at all wavelengths and incident angles.

  A lens having the same performance as in Example 8 in which S-NPH2 is combined with the O lens B lens shown in Table 18 and OHARA S-LAM3 is combined with the A lens can be realized by the following combination.

  Example 9: Table 20 shows the lens data of the design results in the case of combining OHARA S-NPH2 with the B lens and OHARA S-PHM52 with the A lens.

  When a beam having a diameter of 2.4 mm and an incident angle of 4.0 degrees is incident on the lens of Example 9, the total wavefront aberration is 0.098λ (rms) or less for all three wavelengths. Further, the change of the focal position with respect to the wavelength of 1.0 μm to 1.6 μm is 17 μm or less, and the geometric optical spot diameter is 1.9 μm (rms). An optical path diagram of the lens shape and the incident angle of 4 degrees is shown in FIG. The lens diameter is 6.0 mm.

  Example 10: Table 21 shows the lens data of the design results when OHARA S-NPH2 is combined with the B lens and OHARA S-LAL58 is combined with the A lens as the glass material.

  When a beam having a diameter of 2.4 mm and an incident angle of 4.0 degrees is incident on this lens, the total wavefront aberration is 0.09λ (rms) or less for three wavelengths. Further, the change of the focal position with respect to the wavelength of 1.0 μm to 1.6 μm is 17 μm or less, and the geometric optical spot diameter is 1.8 μm (rms) or less. The lens diameter is 6.0 mm. The optical path diagram of the lens shape and the incident angle of 4 degrees is shown in FIG.

  Example 11 Table 22 shows the lens data of the design results when a glass material is a combination of OHARA S-NPH2 for the B lens and OHARA S-LAM2 for the A lens.

  The total wavefront aberration of the lens of Example 11 for the beam with the aperture of 2.4 mm and the incident angle of 4.0 degrees is 0.08λ (rms) or less for the three wavelengths. Further, the change of the focal position with respect to the wavelength of 1.0 μm to 1.6 μm is 17 μm or less, and the geometric optical spot diameter is 1.6 μm (rms). The lens diameter is 6.0 mm. An optical path diagram of the lens shape and the incident angle of 4 degrees is shown in FIG.

  Example 12: Table 23 shows lens data of design results when S-NPH1 from OHARA is combined with the B lens and S-PHM52 from OHARA is combined with the A lens.

  The total wavefront aberration of the lens of Example 12 when the aperture is 2.4 mm and the incident angle is 4.0 degrees is 0.11λ (rms) or less for three wavelengths. Further, the change of the focal position with respect to the wavelength of 1.0 μm to 1.6 μm is 15 μm or less, and the geometric optical spot diameter is 2.3 μm (rms). The lens diameter of the lens of Example 12 is 5.0 mm. The optical path diagram of the lens shape and the incident angle of 4 degrees is shown in FIG.

  From the above examples, by using a combination of glass materials that satisfy the conditions described in claim 3, a simple triple-bonded laminated triplet lens that has a diffraction limit even for a large incident angle in a wide wavelength band. It was found that a lens with imaging performance can be realized with a combination of a plurality of glass materials. With this configuration, it is possible to design a wide wavelength band imaging lens suitable for coupling a single mode fiber having an NA of about 0.12. However, the thickness of the entire lens is about the same as the focal length, which is not suitable for a lens having a long focal length or a large aperture. The lens of claim 2 is suitable for this application.

  From the above embodiments, it has been clarified that a diffraction-limited achromatic imaging optical system that conventionally required several special lens configurations can be realized with a single lens having a laminated configuration. A lens having a wavelength longer than 1 μm is often expensive because its use is limited. However, by adopting the design result shown in this embodiment, an optical for laser imaging using a commercially available lens material is used. The system can be manufactured at low cost. Further, the present invention is not limited to optical wireless communication, but has a wide variety of purposes intended for diffraction-limited imaging in a wavelength band from 1.0 μm to 1.6 μm, or from 0.8 μm to 2.2 μm in some embodiments. It is also useful in laser imaging optical systems and is expected to contribute to industrial development using infrared lasers.

DESCRIPTION OF SYMBOLS 1 1st lens 2 2nd lens 3 3rd lens (1) 1st surface (2) 2nd surface (3) 3rd surface (4) 4th surface A A lens B B lens

Claims (3)

A first objective is to obtain diffraction-limited imaging at the same focal point regardless of the wavelength for incident light having a numerical aperture of less than 0.2 in the wavelength band from 1.0 μm to 1.6 μm. A two-lens structure in which an A lens having a positive refractive power and a B lens having a negative refractive power made of a second material are bonded together, or the B lens is bonded to both surfaces of the A lens. An achromatic lens for laser beam imaging, characterized in that the following condition (1) and condition (2) or condition (3) are satisfied.
(1) The refractive index at a wavelength of 1.31 μm is n _1.31 , the refractive index at a wavelength of _1.55 μm is n _1.55 , the refractive index at a wavelength of 0.98 μm is n _0.98 , and the chromatic dispersion is based on the wavelength of 1.31 μm. Abbe number ν _1.31 indicating

The difference between the Abbe number ν _1.31 (1) of the first material and the Abbe number ν _1.31 (2) of the second material is

Range.
(2) Partial dispersion ratio θ _1.31

The absolute value | Δθ _1.31 | of the difference between the partial dispersion ratio θ _1.31 (1) of the first material and the partial dispersion ratio θ _1.31 (2) of the second material is

The refractive index n _1.31 (1) at a wavelength of 1.31 μm of the first material is

Range.
(3) The ratio of the Abbe number ν _1.31 (1) of the first material to the Abbe number ν _1.31 (2) of the second material is

The refractive index n _1.31 (2) at a wavelength of 1.31 μm of the second material is

Range. 2. The achromatic lens for laser beam imaging according to claim 1, wherein the achromatic lens for laser beam imaging has a two-lens configuration in which the A lens and the B lens are bonded together, and the condition ( 2) and the following conditions (4) and (5) are satisfied.
(4) The difference between the Abbe number ν1.31 (1) of the first material and the Abbe number ν1.31 (2) of the second material is

Range.
(5) The difference between the refractive index n _1.31 (2) of the second material and the refractive index Δn _1.31 (1) of the first material is

Range. The achromatic lens for laser beam imaging according to claim 1, wherein the achromatic lens for laser beam imaging has a lens shape in which the B lens is bonded to both surfaces of the A lens in the optical axis direction. It is a symmetrical three-sheet configuration, and satisfies the above condition (3) and the following conditions (6) and (7).
(6) The refractive index n _1.31 of the second material (2) is

Range.
(7) The difference between the refractive index n _1.31 (2) of the second material and the refractive index n _1.31 (1) of the first material is

Range.

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