JPS63223703A - Grating lens optical system - Google Patents

Abstract

PURPOSE:To permit realization of a good beam spot by partially changing the light utilizing efficiency of at least one of 1st and 2nd grating lenses. CONSTITUTION:A means for partially changing the light utilizing efficiency of at least one of the 1st grating lens having a prescribes space frequency to intersect the laser light from a laser light source axisymmetrically and the 2nd grating lens 12 having the prescribed space frequency to focus the diffracted light transmitted through the 1st grating lens to a prescribed one spot are provided to a grating lens optical system disposed with the above-mentioned 1st grating lens and 2nd grating lens on the optical axis. The means for partially changing the light utilizing efficiency is realized by, for example, forming the depth of the grating grooves of grooves GL1 to nonuniform depths. More specifically, the means for partially changing the light utilizing efficiency of the lens is realized by relatively and partially decreases the light utilizing efficiency of the parts where the intensity is larger according to the intensity distribution of the beam. The intensity distribution is thereby uniformized and the good and extremely small beam diameter is obtd.

Description

[Detailed Description of the Invention] [Summary] Two grating lenses: a first grating lens that crosses an incident laser beam axially symmetrically, a second grating lens that converges this crossed beam onto an optical disk, and In the newly developed grating lens system, which is able to achieve stable focusing performance without aberrations without being affected by wavelength fluctuations of the incident laser beam by combining the first and second grating lenses, By providing means for partially changing the preemption efficiency of the lens on either side, a uniform light intensity distribution is ensured and a good beam spot is achieved.

[Industrial application field]

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

Recently, in optical systems that require the function of converging diverging spherical wave light from a coherent light source (semiconductor laser) to a single point, such as the optical head of an optical disk device, improvements have been made to (i) miniaturization of the device, and (ii) access. time reduction, (ii
i) In order to reduce costs, the use of grating lenses, which are thinner, lighter, smaller, and more easily mass-produced than conventional optical elements, is being considered.

[Conventional technology]

A conventional in-line grating lens is shown in FIG. 5. This lens 20, as shown in FIG. . Therefore, for light with a wavelength λ (>λ0) longer than the above-mentioned λO, aberrations occur as shown in FIG. 3(b), the focal length shifts by Δf, and good focusing performance cannot be obtained. Furthermore, aberrations similarly occur for wavelengths shorter than λ0.

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

As described above, grating lenses have the property that when the wavelength used deviates from a predetermined value (λ0), aberrations occur and the focusing performance deteriorates, and depending on the type of lens, the focal position also shifts.

The oscillation wavelength of currently used semiconductor lasers varies within a range of about ±12 rv due to individual variations, changes due to temperature, changes due to drive current value, etc., even if the laser has a wavelength of 830n-. Therefore, when a semiconductor laser is used as a light source, conventional grating lenses cannot be used in high-precision optical systems where aberrations cannot be tolerated.

Therefore, the applicant of this application first applied for patent application No. 61-220870.
In the specification, there is provided a grating lens optical system that can always obtain a good beam spot without aberrations even if there is a wavelength fluctuation of the incident light as described above, and has accurate and stable focusing performance. proposed.

This grating lens optical system basically includes a first grating lens that intersects laser light from a laser light source axially symmetrically, and a second grating lens that focuses the diffracted light transmitted through the first grating lens on a predetermined point (focal point). By appropriately selecting the spatial frequencies of both grating lenses, it is possible to absorb wavelength fluctuations and prevent the occurrence of aberrations, as will be described later.

[Problem that the invention seeks to solve]

However, the following new problem has been found in such a grating lens optical system.

That is, in the grating lens optical system described above,
Since the laser beam is crossed between the first and second grating lenses, the intensity distribution of the beam emitted from the second grating lens is necessarily different from the distribution in a conventional single grating lens. Even if the grating lens system has no aberration, the bit diameter of the second grating lens changes according to this distribution. Therefore,
For example, when using the above-mentioned grating lens optical system in an optical head, it is desired to make the spot diameter as small as possible, so the light intensity distribution becomes very important.

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

[Means for Solving the Problems] In order to achieve the above object, the present invention includes a first grating lens having a predetermined spatial frequency that allows laser light from a laser light source to intersect axially symmetrically; In a grating lens optical system, a second grating lens having a predetermined spatial frequency that focuses the diffracted light transmitted through the first grating lens on a predetermined point is arranged on the optical axis. The present invention is characterized in that at least one of the lenses is provided with means for partially changing the light usage efficiency of the lens.

[For production]

The means for partially changing the light use efficiency of the lens relatively partially reduces the light use efficiency of a portion where the intensity is high according to the intensity distribution of the beam. As a result, the intensity distribution is made uniform, and a small beam spot with a nearly circular cross section is imaged at a predetermined point.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIG. 1 and subsequent figures.

First, the configuration of the grating lens system, which is the premise of the present invention, will be briefly explained with reference to FIG. 6.7. The detailed structure of this grating lens system is disclosed in the above-mentioned Japanese Patent Application No. 61-220870.

In Fig. 6, the grating lens system has the first and second
This is a configuration in which in-line grating lenses 11 and 12 are arranged on the same optical axis (dotted chain line), and a spherical wave diverging from a point P (coherent light source) on the optical axis is
The second grating lens 11 diffracts the light toward the optical axis, and once it crosses the optical axis, the second grating lens 1
2 to focus the light on a predetermined point Q on the optical axis.

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

In the above configuration, consider the paths of two lights having mutually different wavelengths λO1λ (λ0<λ) that are incident on any one point of the first in-line grating lens from the same direction. First, light with a wavelength λ is diffracted by a first in-line grating lens at a larger angle than light with a wavelength λ0, and after both of these diffracted lights intersect with the optical axis, they are passed through a second in-line grating lens. reach above the lens. The distance from the optical axis of the arrival point of these lights is longer for light with wavelength λ than for light with wavelength λ0. Next, these lights are diffracted by the second in-line grating lens, but at this time, the light with wavelength λ is diffracted at a larger angle than the light with wavelength λO, so the distance between the two lights gradually becomes smaller. They become narrower and eventually intersect at one point. Therefore, by providing the two in-line grating lenses with 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 about the light incident on any point of the first in-line grating lens, and since the above spatial frequency distribution is rotationally symmetric with respect to the optical axis, the wavelength of the incident diverging spherical wave light is Even if the light changes, it will be focused on the predetermined point on the optical axis, and therefore no aberrations or focal position shifts will occur.

Next, a specific method for determining the spatial frequency distribution of the grating lenses 11 and 12 will be described below in (i) to (iv) using FIG. Note that the distance between point P and the first grating lens 11 is 11. The distance between the two grating lenses 11 and 12 is d.

The distance between the grating lens 12 and the point Q is assumed to be 12.

(i) First, consider a ray of wavelength λ0 that is emitted from point P and reaches point R1 at the outermost periphery of grating lens 11. This ray is diffracted at point R1 and grating lens 1
2, and is further diffracted to reach point Q (solid line a in Fig. 6), then the above-mentioned optical path (P-R1-rl→Q) By assuming that, the spatial frequencies Fl and fl at points R1 and rl are determined.

(ii) Next, consider the case where the wavelength changes from λO to λ (>λ0).The wavelength λ that progresses from 0 point P to point R1
At point R1, the light ray is diffracted at a larger angle than when the wavelength is λO, and reaches point r2 on grating lens 12 (dashed line b).

Here, the spatial frequency f2 at the point r2 is determined from 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 λO, it is possible to conversely find from which point on the grating lens 11 the ray that is diffracted at point r2 and reaches point Q comes from (solid line C), and the grating lens 11 If the upper point is R2, the spatial frequency F2 at point R2 is determined from the condition that the diffracted light at point R2 reaches point P.

(iv) Consider the case where the wavelength becomes λ again, and consider (ii)
), the point r3 on the grating lens 12 is
(not shown) and its spatial frequency f3 is determined. Then, the wavelength is returned to λ0, and the point R3 (not shown) on the grating lens 11 and its spatial frequency F3 are determined in the same manner as in (iii) above. In this way, point Rn (n=
1 , 2 , 3', ...) above (ii) and (iii) until they reach the center of the grating lens 11.
) By repeating the process, the radial spatial frequency distribution in the grating lenses 11 and 12 is determined. Note that the diameter of the second grating lens 12 is determined at the position of point rn.

By determining the spatial frequency distribution of the grating lenses 11 and 12 as described above, even if the light emitted from the point P has a wavelength λ different from the reference wavelength λ0, it can be transmitted to the point Q without aberration. can be focused on. - The present invention is directed to how to make a beam focused on point Q into a good, stable, small-diameter beam in the above-mentioned grating lens optical system.

It is known that the cross-sectional shape of a semiconductor laser beam is generally not circular but elliptical 50 as shown in FIG.

First of all, the inventor of the present application will explain how each grating lens 11 (GL, ) and 12 (
GLg) and the light intensity distribution and spora l-(Q point)
The light intensity distribution was calculated and confirmed experimentally (Figure 3). Note that the numerical aperture (NA) of both GL+ and GLt was set to NA -0.2.

The light intensity distribution of the diverging wave of a laser beam differs depending on the direction. The part where the light intensity is 1/e8 or more of the maximum value is GL,
The coefficient coef is defined based on the case where That is, the reference time is coef, wl, 0, and 1/
e” If you want to take in the beam with a width twice the width, use coe.
Let f,=2. Since the distribution of the laser beam is different in two orthogonal directions, coefficients are considered for each of the X direction and the y direction in the orthogonal coordinates and are set as coef, x, coef, y. The elliptical beam 50 (Fig. 1) is coef, x-1,
When taken at 0, coef, and y-2, 0, the light intensity distribution at GL is shown in FIG. 4(a).

Since the beam is elliptical as shown in (b), the coef in the y direction corresponding to the major axis direction is twice that in the X direction corresponding to the minor axis direction. After the rays intersect, the OL changes as shown in (C) and (d), respectively, and the spot diameter becomes (e
), Cf), that is, 3.6 μmS in the X direction.
It is 4.7 μm in the y direction. In addition, the conventional NA -0,
A single grating lens of 2 co-generates a Gaussian beam.
When taken in and focused at ef, x=coef, y-1,0, the spot diameter was about 4.2 μm in both the x and y directions. As mentioned above, in the grating lens system targeted by the present invention, the light intensity distribution of the beam is not rotationally symmetrical with respect to the optical axis, so the spot diameter becomes larger in one direction (X direction) than the value conventionally obtained. The problem was confirmed.

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

FIG. 1 shows an embodiment of the present invention, and FIG. 1(a) is a schematic diagram of GL of the grating lens system shown in FIG. 6, and FIG. Shown below.

It is known that in a grating lens, the efficiency for first use (diffraction efficiency) changes depending on the depth of the grating grooves, and that the shallower the grating grooves, the lower the efficiency for first use. Therefore, in the first embodiment of the present invention, the depth of the lattice grooves of the GL is made non-uniform as a means for partially changing the efficiency of first use.

In FIG. 1, the circular GL is divided into approximately four equal parts by lines Sl, St, and Ss along the long axis direction of the elliptical beam,
The area between S and S is defined as a central area At, and the area outside Sl and S is defined as an outer peripheral area At. In FIG. 1, S2 passing through the center of the lens corresponds to the short axis of the elliptical beam 50. The central region A+ including the center of the lens has grating grooves 3 with a high aspect ratio as in the past in order to obtain high efficiency for first use.
1 is in the outer area A! In this case, the lattice grooves 32 are made shallow to reduce the efficiency of first use. The depth of the groove is determined by the pitch of the plaid (
depending on the NA of the grating lens), the refractive index of the medium (optical dichroic, etc.), the cross-sectional shape of the groove (rectangular or sinusoidal)
The optimum value differs depending on the conditions, and the optimum value is determined according to each condition.

, using a GL having grating grooves as shown in FIG.
When the grating lens system shown in FIG. 6 is constructed, the diffraction-limited spot diameter at the focal point is improved compared to the conventional case (FIG. 4) in which the GL does not have an efficiency distribution.

The improvement effect is shown in FIG. As in the case of FIG. 4, FIG. 3 shows the elliptical light intensity distribution of the laser beam as an ellipse whose major axis is along the I-I line in FIG. 1. GL, GL, , which shows the trial calculation results of the light intensity distribution of the beam spot.

In Fig. 3.4, the absolute value of the intensity (height of the peak) itself is not important, but the relative height of the peak is what matters. Therefore, it is meaningless to compare the absolute values of the intensities in FIGS. 3 and 4.

(g) and (h) in Figure 3 are the distributions immediately after diffraction by GL, and the light usage efficiency is 30% in the central area A1 including the lens center.
%, the grating grooves at both ends of the lens are made shallower. Note that coef, x=1, 0, coef
, y=2.0.

Figure 3 (i) (j) shows the distribution in the vicinity of the GL, and when compared with Figure 4 (c) and (d), the intensity on the optical axis is suppressed and the intensity on the outer peripheral side of the lens is relatively high. I can see that it's being done. Figure 3 (k) (1) shows the distribution in the spot, and the spot diameter is 3.4 μm in the X direction and 4.0 μm in the X direction.
It can be seen that this is smaller than the value of 4.2 μm in the case of FIG. 4 and the conventional single grating lens.

As a specific method for producing the grating lens, electron beam lithography is suitable. The depth of the groove can be easily adjusted by changing the dose scale as is known. Also, by creating a stand bar from the master disc, it is possible to mass produce copies.

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

FIG. 2 shows another embodiment of the invention. Rather than partially changing the depth of the grating grooves as shown in Figure 1, by adding a means (=j) for attenuating the light intensity to one side of the grating lens substrate, preferably to the surface opposite to the grating grooves. , the effect shown in Fig. 3 was obtained.Specifically, for example, A
The semitransparent mirror 70 may be formed by vapor deposition of I, or a film 70 like an interference filter may be formed.These light attenuating films or semitransparent mirrors 70 may be formed in the outer peripheral area A! shown in FIG. Of course, it is applied to the parts corresponding to the
The embodiment of FIG. 2 can be applied to either a surface relief type or a volume grating lens.

Incidentally, the means for partially changing the preemption efficiency of the above-mentioned lens is the second grating lens CI.

or the first and second grating lenses GL.

The same purpose can be achieved even if it is provided on both sides of G[5□.

〔Effect of the invention〕

As described above, according to the present invention, in a grating lens system in which no aberration occurs even when there is a wavelength variation, by providing a means for partially changing the light usage efficiency in the grating lens, the light intensity distribution is uniform. It is possible to obtain a good and stable micro spot.

[Brief explanation of drawings]

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

Claims (1)

[Claims] 1. A first grating lens (11) having a predetermined spatial frequency that allows laser light from a laser light source (23) to intersect axially symmetrically, and diffracted light transmitted through the first grating lens. In a grating lens optical system in which a second grating lens (12) having a predetermined spatial frequency that focuses on a predetermined point is arranged on the optical axis, at least one of the first and second grating lenses has a second grating lens (12) that has a predetermined spatial frequency focused on a predetermined point. Means for partially changing the light usage efficiency of (3)
1, 32, 70). 2. The grating according to claim 1, wherein the means for partially changing the light usage efficiency of the lens is formed by partially changing the depth of the grating grooves of the grating lens. lens optics. 3. The grating lens optical system according to claim 1, wherein the means for partially changing the light usage efficiency of the lens is formed by a partial light attenuation coating provided on the grating lens. 4. Claim 1, characterized in that the means for partially changing the light usage efficiency of the lens is formed by a partially translucent mirror provided on the grating lens.
The grating lens optical system described in section.