JPH0792528B2 - Grating lens optical system - Google Patents

Description

DETAILED DESCRIPTION [Outline] Two grating lenses, a first grating lens that crosses an incident laser beam axially symmetrically and a second grating lens that focuses the crossed beam on an optical disk, are combined. As a result, in the newly developed grating lens system capable of achieving stable focusing performance without aberrations without being affected by the wavelength fluctuation of the incident laser beam, at least one of the first and second grating lenses By providing a means for partially changing the light use efficiency of the lens, a uniform light intensity distribution is secured and a good beam spot is realized.

[Industrial application field]

The present invention relates to a grating lens optical system having a focusing function by combining grating lenses.

Recently, in an optical system that requires a function of focusing divergent spherical wave light from a coherent light source (semiconductor laser) to one point, for example, an optical head of an optical disk device, (i) downsizing of the device, (ii) access time In order to realize the shortening of (3) and (iii) cost reduction, the use of a grating lens that is thin, weighing, small, and mass-produced in comparison with conventional optical elements is being studied.

[Conventional technology]

A conventional in-line type grating lens is shown in FIG. The lens 20 has a function of converging, for example, only a parallel light flux having a specific wavelength λ ₀ into one point, as shown in FIG. Therefore, for light having a wavelength λ (> λ ₀ ) longer than λ ₀ , aberration is generated as shown in FIG. 6B, the focal length is deviated by Δf, and good focusing performance is obtained. Disappear. Aberrations similarly occur for wavelengths shorter than λ ₀ .

Such a phenomenon is common to lenses that bend light by diffraction, and is applicable to any of volume hologram lenses, surface relief grating lenses, and blazed grating lenses.

As described above, the grating lens has a property that when the used wavelength deviates from a predetermined value (λ ₀ ), an aberration occurs and the focusing performance deteriorates, and the focal position also deviates depending on the type of lens. .

The oscillation wavelength of the currently used semiconductor laser varies depending on individual variations, changes due to temperature, changes due to drive current value, and the like, but even for a laser having a wavelength of 830 nm, the wavelength changes within a range of about ± 12 nm. Therefore, when a semiconductor laser is used as a light source, the conventional grating lens cannot be used in a high precision optical system in which aberration is not allowed.

Therefore, the applicant of the present application has previously found that in Japanese Patent Application No. 61-220870, even if there is a wavelength variation of the incident light as described above, it is not affected by the variation and always obtains a good beam spot without aberration. We have proposed a grating lens optical system that is capable of accurate and stable focusing performance.

This grating lens optical system basically focuses a first grating lens that axially symmetrically intersects a laser beam from a laser light source and a first grating lens that focuses diffracted light that has passed through the first grating lens to a predetermined point (focus point). Two grating lenses are arranged on the optical axis, and by appropriately selecting the spatial frequencies of these two grating lenses, it is possible to absorb wavelength fluctuations and prevent aberrations as described later.

[Problems to be solved by the invention]

However, the following problems have been newly found in such a grating lens optical system. That is, in the above-mentioned grating lens optical system, since the laser beam intersects between the first and second grating lenses, the intensity distribution of the beam emitted from the second grating lens is inevitably in the conventional single grating lens. Different from the distribution. Even if the grating lens system has no aberration, the diffraction-limited beam spot diameter obtained as a result of beam focusing by the second grating lens changes according to this distribution. Therefore, for example, when the above-mentioned grating lens optical system is used for the optical head, the light intensity distribution becomes very important because the spot diameter is desired to be as small as possible.

An object of the present invention is to obtain a good minimum beam diameter by considering the beam intensity distribution in a grating lens optical system, which has not been noticed at all, and making the intensity distribution as uniform as possible.

[Means for solving problems]

In order to achieve the above object, according to the present invention, a first grating lens having a predetermined spatial frequency for axially crossing a laser beam from a laser beam and a diffracted light transmitted through the first grating lens are provided. A second grating lens having a predetermined spatial frequency for focusing on a predetermined point is arranged on the same optical axis, and light rays from any two points symmetrical to the optical axis of the first grating lens are It is incident on the second grating lens while intersecting on the axis, and the light use efficiency of the lens is partially applied to at least one of the first and second grating lenses so that the intensity distribution of the laser light is made uniform. It is characterized in that a means for changing to is provided.

[Work]

The means for partially changing the light use efficiency of the lens relatively reduces the light use efficiency of a portion having a large intensity according to the intensity distribution of the beam. Thereby, the intensity distribution is made uniform, and a small beam spot whose cross section is substantially circular is imaged at a predetermined point.

〔Example〕

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to FIG.

First, the structure of the grating lens system, which is the premise of the present invention, will be briefly described with reference to FIGS.
The detailed structure of this grating lens system is disclosed in Japanese Patent Application No. 61-220870.

In FIG. 6, the grating lens system includes the first and second grating lens systems.
In-line type grating lenses 11 and 12 are arranged on the same optical axis (dashed line), and a point P on the optical axis
The spherical wave diverging from the (coherent light source) is diffracted to the optical axis side by the first grating lens 11 and once intersects with the optical axis, and then is focused at a predetermined point Q on the optical axis by the second grating lens 12. It was made to let.

The first grating lens 11 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and diffracted light from arbitrary two points that are symmetrical with respect to the optical axis intersect on the optical axis. . The second grating lens 12 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and the crossed diffracted light is focused at one point Q on the optical axis.

Consider the paths of two lights having different wavelengths λ ₀ and λ (λ ₀ <λ) that are incident on one arbitrary point of the first in-line type grating lens in the same direction in the above-mentioned configuration. First, the first in-line type grating lens diffracts the light of wavelength λ at a larger angle than the light of wavelength λ ₀ , and after all of these diffracted lights intersect the optical axis, the second in-line type Reach onto the grating lens. As for the distance of the arrival point of these lights from the optical axis, the light of wavelength λ is farther than the light of wavelength λ ₀ . next,
These lights are diffracted by the second in-line type grating lens. At this time, since the light of wavelength λ is diffracted at a larger angle than the light of wavelength λ ₀ , the distance between the two lights gradually narrows. Iki, and finally meet at one point. Therefore, by giving the two in-line type grating lenses a predetermined spatial frequency distribution, the point where the two lights intersect can be placed at a designated point on the optical axis.

The above can be said for light incident on any point of the first in-line type grating lens, and since the spatial frequency distribution is rotationally symmetric with respect to the optical axis, the wavelength of incident divergent spherical wave light changes. Even if it is done, it is focused on the above-mentioned predetermined one point on the optical axis, so that aberrations and focal position shifts do not occur.

Next, a specific method of determining the spatial frequency distribution of the grating lenses 11 and 12 will be described below with reference to FIG. 7 in (i) to (iv). The distance between the point P and the first grating lens 11 is 1, the distance between the two grating lenses 11 and 12 is d, and the grating lens 12 and the point Q are
The distance from and is 2.

(I) First, consider a ray having a wavelength λ ₀ that emits a point P and reaches a point R1 on the outermost periphery of the grating lens 11. This ray is diffracted at the point R1 and reaches the center point r1 (= 0) of the grating lens 12, where it is further diffracted to reach the point Q (solid line a in FIG. 6). Then, by assuming the above-mentioned optical path (P → R1 → r1 → Q), the point R1, r
The spatial frequencies F1, f1 at 1 are determined.

(Ii) Next, consider the case where the wavelength changes from λ ₀ to λ (> λ ₀ ). A ray of wavelength λ that has traveled from point P to point R1 is diffracted at point R1 at a larger angle than when the wavelength is λ ₀ and reaches point r2 on grating lens 12 (broken line b). Here, the spatial frequency f2 at the point r2 is determined under the condition that the light is focused on the point Q even when the wavelength is λ.

(Iii) Returning to the case where the wavelength is λ ₀ , it is possible to reversely determine from which point on the grating lens 11 the ray diffracted at the point r2 and reaching the point Q comes from (solid line c). If the point on the grating lens 11 is R2, the spatial frequency F2 at the point R2 is determined under the condition that the diffracted light at the point R2 reaches the point P.

(Iv) Considering the case where the wavelength becomes λ again, the point r3 (not shown) on the grating lens 12 and its spatial frequency f3 are obtained in the same manner as (ii) above. Then, the wavelength is returned to λ ₀ and the grating lens 1 is used in the same manner as in (iii) above.
Find the point R3 (not shown) on 1 and its spatial frequency F3.
In this way, by repeating the above steps (ii) and (iii) until the point Rn (n = 1,2,3, ...) Reaches the center of the grating lens 11,
The radial spatial frequency distribution at 2 is determined. The diameter of the second grating lens 12 is determined by the position of the point rn.

By determining the spatial frequency distributions of the grating lenses 11 and 12 as described above, the light emitted from the point P is
Even if the wavelength λ is different from the reference wavelength λ ₀ , it can be focused on the point Q without aberration.

The present invention is directed to how to make a beam converged at a point Q into a favorable and stable small-diameter beam in the above-mentioned grating lens optical system.

It is known that the semiconductor laser beam generally has an elliptical shape 50 as shown in FIG.

First, the inventor of the present application, the light intensity distribution and spot (Q) in each of the grating lenses 11 (GL ₁ ) and 12 (GL ₂ ) when focusing the semiconductor laser beam in the grating lens optical system shown in FIG. The light intensity distribution at the point) was calculated and confirmed experimentally (FIG. 3). The numerical aperture (NA) of both GL ₁ and GL ₂ was NA = 0.2.

The divergent wave of the laser beam has a different light intensity distribution depending on the direction. The coefficient coef. Is defined based on the case where the area where the light intensity is 1 / e ² or more of the maximum value is captured by GL ₁ . That is,
Coef. = 1.0 at the standard time, and coef. = 2 when the beam is captured with a width twice the 1 / e ² width. Since the laser beam has different distributions in two directions orthogonal to each other, coef.x and coef.y are considered in consideration of the coefficient in each of the x direction and the y direction in the Cartesian coordinates. Elliptical beam 50 (Fig. 1) with coef.x = 1.0, coef.y
= 2.0, the light intensity distribution in GL ₁ is 4th.
It becomes as shown in FIGS. Since it is an elliptical beam, the y-direction coef.
Twice that in the direction. In GL ₂ after the light rays intersect, they change as shown in (c) and (d), respectively, and the spot diameters become as shown in (e) and (f). That is, 3.6μ in the x direction
It is 4.7 μm in the m and y directions. It should be noted that the conventional Gaussian beam is coef.x = coe with a single grating lens with NA = 0.2.
When focused and focused at fy = 1.0, the spot diameter was about 4.2 μm in both x and y directions. As described above, in the grating lens system targeted by the present invention, the light intensity distribution of the beam is not rotationally symmetric with respect to the optical axis, so the spot diameter becomes larger than the value conventionally obtained in one direction (y direction). The problem was confirmed.

In order to solve the above problems, the basic idea of the present invention is to attenuate the light use efficiency (diffraction efficiency) in the y-axis direction and bring it closer to that in the x-axis direction in order to make the intensity distribution uniform.

FIG. 1 shows an embodiment of the present invention. FIG. 1 (a) is a schematic view of GL ₁ of the grating lens system shown in FIG. 6, and its sectional view taken along the line I--I is shown in FIG. 1 (b). Shown in.

It is known that in a grating lens, the light use efficiency (diffraction efficiency) changes according to the depth of the grating groove, and the light use efficiency decreases as the grating groove becomes shallower. Therefore, the first embodiment of the present invention is realized by making the grating groove depth of GL ₁ non-uniform as a means for partially changing the light use efficiency.

In Figure 1, approximately 4 equal parts by the line S _1, S _2, S ₃ along a circular GL ₁ in the long axis direction of the elliptical beam, S ₁ and the center area A ₁ of the area between the S ₃ The outer side of S _{1 and} the outer side of S ₃ are defined as the outer peripheral area A ₂ . In FIG. 1, S ₂ passing through the center of the lens corresponds to the short axis of the elliptical beam 50. The center area A ₁ including the lens center has a high aspect ratio grating groove 31 in order to obtain high light use efficiency, but the outer peripheral area A ₂ has a shallow groove groove 32 to reduce the light use efficiency. . The optimum groove depth depends on the pitch of the lattice stripes (depending on the NA of the grating lens), the refractive index of the medium (optical tech, etc.), and the cross-sectional shape of the groove (rectangular or sine-shaped), and is optimal according to each condition. The value is determined.

Using GL ₁ having a lattice groove as shown in FIG.
When the grating lens system shown in the figure is constructed, the diffraction-limited spot diameter at the focus is improved as compared with the conventional case (FIG. 4) in which GL ₁ does not have an efficiency distribution.

The improvement effect is shown in FIG. Figure 3 is similar to the case of FIG. 4, GL ₁ in the case where the line I-I direction of the elliptical light intensity distribution of the laser beam first view is incident indicated by an ellipse of major axis, GL ₂ , Shows the result of trial calculation of the light intensity distribution of the beam spot.

In FIGS. 3 and 4, the absolute value of the strength (mountain height) itself is not important, and the relative mountain height is a problem. Therefore, it is meaningless to compare the absolute values of the intensities in FIGS. 3 and 4.

(G) and (h) of Fig. 3 are distributions immediately after diffracted by GL ₁ ,
The case where the grating grooves at both ends of the lens are shallowed so that the light use efficiency is 30% of the central area A ₁ including the lens center is shown. Note that coef.x = 1.0 and coef.y = 2.0.

3 (i) and (j) are distributions in the vicinity of GL _2, and compared with (c) and (d) in FIG. 4, the intensity on the optical axis is suppressed and the intensity on the lens outer peripheral side is relatively increased. You can see that it is being done. Figures 3 (k) and (l) are the distributions in the spot, where the spot diameter is 3.4 μm in the x direction and 4.0 μm in the y direction, which is smaller than the value of 4.2 μm in the case of FIG. 4 and the conventional single grating lens. You can see that

An electron beam drawing method is suitable as a specific method for producing the grating lens. The depth of the groove can be easily adjusted by changing the dose amount as is known. Also, by making a stamper from a thick board, mass production is possible by duplication.

The embodiment shown in FIG. 1 is useful for, but not limited to, surface relief grating lenses.

FIG. 2 shows another embodiment of the present invention. Instead of partially changing the depth of the grating groove as shown in FIG. 1, a means for attenuating the light intensity is added to one side of the grating lens substrate, preferably to the surface opposite to the grating groove, so that the third The effect shown in the figure is obtained. Specifically, for example, the semitransparent mirror 70 may be formed by vapor deposition of Al, or the interference filter film 70 may be formed. Needless to say, these light attenuating film or the semitransparent mirror 70 is applied to the portion corresponding to the outer peripheral area A ₂ shown in FIG. The embodiment shown in FIG. 2 can be applied to either a surface relief type or a volume type grating lens.

The means for partially changing the light use efficiency of the above-mentioned lens is the second grating lens GL ₂ or the first,
The same purpose can be achieved by providing the second grating lenses GL ₁ and GL ₂ together.

〔The invention's effect〕

As described above, according to the present invention, in a grating lens system in which aberration does not occur even if there is a wavelength fluctuation, by providing the grating lens with means for partially changing the light use efficiency, the intensity distribution of light is made uniform. Can be achieved, and good and stable minute spots can be obtained.

[Brief description of drawings]

FIG. 1 shows a grating lens according to an embodiment of the present invention. FIG. 1 (a) is a plan view and FIG. 1 (b) is I in (a).
FIG. 2 is a sectional view taken along the line I, FIG. 2 is a view similar to FIG. 1 (b) showing another embodiment of the present invention, and FIG. 3 shows the light intensity distribution of the grating lens and the beam spot in the present invention. FIG. 4 is a view similar to FIG. 3 in the prior art,
FIG. 5 is a diagram showing a defect of the conventional grating lens,
FIG. 6 is a diagram showing a basic configuration of a grating lens optical system which is a premise of the present invention, and FIG. 7 is a diagram explaining a method of determining a spatial frequency of the grating lens shown in FIG. 11 …… First grating lens, 12 …… Second grating lens, 21 …… Grating lens light.

Front page continuation (72) Inventor Yushi Inagaki 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa, Fujitsu Limited (72) Inventor Satoshi Maeda 1015 Kamedota, Nakahara-ku, Kawasaki, Kanagawa (f), Fujitsu Limited (56) Reference References JP 62-18502 (JP, A) JP 59-160166 (JP, A)

Claims (5)

[Claims] 1. A first grating lens having a predetermined spatial frequency for intersecting a laser beam from a laser light source with respect to an axis and a predetermined grating for focusing diffracted light transmitted through the first grating lens on a predetermined point. A second grating lens having a spatial frequency is arranged on the same optical axis, and light rays from any two points symmetrical to the optical axis of the first grating lens intersect on the optical axis and the second grating lens is arranged. And a means for partially changing the light use efficiency of the first and second grating lenses so as to make the intensity distribution of the laser light uniform. Characteristic grating lens optical system. 2. The grating lens optical system according to claim 1, wherein the grating lens is changed so that the light use efficiency of the central portion is higher than the light use efficiency of the peripheral portion. system. 3. The device according to claim 1, wherein the means for partially changing the light use efficiency of the lens is formed by partially changing the depth of the grating groove of the grating lens. The described grating lens optical system. 4. The grating lens according to claim 1, wherein the means for partially changing the light use efficiency of the lens is formed by a partial light attenuating film provided on the grating lens. Optical system. 5. The grating lens according to claim 1, wherein the means for partially changing the light use efficiency of the lens is formed by a partially semitransparent mirror provided in the grating lens. Optical system.