JPH0564772B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] The present invention is a grating lens optical system that focuses diverging spherical wave light to one point using two in-line grating lenses, and which By giving a predetermined frequency distribution to two rays that are symmetrical with respect to the optical axis in the space between both lenses, and converging them to one point, even against wavelength fluctuations of the incident light, This makes it possible to obtain a good beam spot free of aberrations and a stable focal position free of deviation, thus making it possible to put the grating lens into a wide range of practical applications.

[Industrial application field]

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

Nowadays, optical systems that require the function of converging diverging spherical wave light from a coherent light source to a single point, such as the optical head of an optical disk device, are becoming more and more compact. , (iii) 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.

[Prior art]

A conventional in-line grating lens is shown in FIG. As shown in FIG. 2A, this lens has a function of converging only a parallel beam of light having a specific wavelength λ ₀ onto one point, for example. Therefore, for light having a wavelength λ (>λ ₀ ) longer than the above-mentioned λ ₀ , aberrations occur as shown in FIG. Furthermore, aberrations similarly occur for wavelengths shorter than λ ₀ .

This phenomenon is common in lenses that bend light through diffraction, such as the volume hologram lens shown in Figure 9 (a), the surface relief grating lens (b), and the blazed grating lens (Figure 9). This also applies to c) in the same figure. In addition, in the case of an off-axis grating lens as shown in Fig. 10, even if there is no aberration when the wavelength is λ ₀ as shown in Fig. 10a,
When the wavelength changes, not only aberrations occur, but also the focal point shifts from the optical axis, as shown in Figure b.

In this way, grating lenses have the property that if the used wavelength deviates from a predetermined value (λ ₀ ), aberrations will occur and the focusing performance will deteriorate, and depending on the type of lens, the focal position will also shift. I have it.

[Problem that the invention seeks to solve]

In the optical head of an optical disk device, a semiconductor laser is used as a coherent light source. Semiconductor lasers generally include single mode lasers and multimode lasers, and their oscillation wavelengths are shown in FIGS. 11a and 11b, respectively. Since a conventional general optical head uses a normal optical lens, changes in the wavelength of the laser beam do not cause changes in the beam spot that would affect reading, writing, etc. on the optical disk. Therefore, both the above-mentioned single mode laser and multimode laser can be used. On the other hand, when attempting to use a conventional grating lens for the optical head, only a single mode semiconductor laser can be used because the influence of wavelength changes is large as described above.

However, even single-mode semiconductor lasers have the characteristic that their oscillation wavelength changes depending on temperature. FIG. 12 shows the relationship between the oscillation wavelength and case temperature in a single mode semiconductor laser (provided that the laser output is constant). As is clear from the figure, (a) the wavelength changes gradually and continuously as the temperature changes, (b) the wavelength changes discontinuously at a certain temperature _t (hereinafter referred to as a mode hop), (c )There is a phenomenon in which two or more wavelengths exist at a certain temperature _t2 .

Therefore, in an optical head using a grating lens, even if a single mode semiconductor laser is used as the light source, when the temperature changes, the wavelength will also change due to the above phenomenon, so the gray In some cases, the quality of the focusing spot produced by the focusing lens deteriorates, and furthermore, the focal position changes. These things lead to an enlargement of the beam diameter of the spot on the optical disk medium, occurrence of track deviation, occurrence of focus deviation, etc. In particular, when the wavelength changes due to the mode hop shown in FIG. 12, the change is discontinuous, so there is a problem that the current servo mechanism cannot follow it at all.

For this reason, some attempts have been made to maintain the temperature of the semiconductor laser at an appropriate value by providing an external temperature control function, but since the drive current of the semiconductor laser differs when writing to and reading from an optical disk, the temperature at the junction part has changed rapidly,
Therefore, accurate control cannot be achieved using an external temperature adjustment mechanism.

Therefore, as a countermeasure to the wavelength change of a semiconductor laser, a grating lens optical system was proposed in which the spot on the optical disk does not change even if the wavelength changes (Japanese Patent Application Laid-Open No. 160166/1983). Its configuration is shown in FIG. This optical system has a configuration in which two off-axis grating lenses 1 and 2 are arranged with their optical axes shifted from each other, and the grating lenses 1 and 2 diffract light in a zigzag pattern, thereby changing the wavelength of the light. This is an attempt to cancel each diffraction angle variation due to the change. However, even in this configuration, the effects of aberrations and focal position deviations (see Figure 10) specific to grating lenses appear, and the amount of compensation required for the optical head (track deviation ±0.02 μm, focus deviation ±0.2 μm) cannot be realized. In addition, 2
By setting the distance D between the two grating lenses 1 and 2 to almost zero, it becomes possible to compensate for the track deviation, but the focus deviation can be
It is not possible to reduce the working distance to 0.2μm (for example, the working distance
In the case of 1.8 mm, a 1 nm change in wavelength will result in a focus shift of 6 μm). For these reasons, it is impossible to put the grating lens optical system into practical use as an optical system for an optical head.

In view of the above-mentioned problems, the present invention makes it possible to obtain a good beam spot without aberrations and a stable focal position that does not deviate from a predetermined point even when the wavelength of the incident light varies. An object of the present invention is to provide a grating lens optical system that can be applied to various fields including optical heads of optical disk devices.

[Means for solving problems]

The grating lens optical system of the present invention includes a first in-line grating lens into which diverging spherical wave light emitted from a coherent light source enters, and a second in-line grating lens which focuses diffracted light transmitted through the grating lens on a predetermined point. The first in-line grating lens and the second in-line grating lens each have a predetermined spatial frequency distribution that is rotationally symmetrical with respect to the optical axis. The in-line grating lens causes the diffracted lights from two arbitrary points symmetrical about the optical axis to intersect on the optical axis, and the second in-line grating lens focuses the intersected diffracted lights on the one predetermined point mentioned above. It was designed to let you do so.
The predetermined spatial frequency distribution is the sum of the spatial frequency distribution of the in-line grating lens that converges plane waves, and an axially symmetrical spatial frequency distribution that has a maximum value between the center of the lens and the outer periphery of the lens.

[Effect]

In the above configuration, consider the paths of two lights having mutually different wavelengths λ ₀ and λ (λ ₀ <λ) that are incident on any one point of the first in-line grating lens from the same direction. First, by the first in-line grating lens, light with a wavelength λ is diffracted at a larger angle than light with a wavelength λ ₀ , and after both of these diffracted lights intersect with the optical axis, they are diffracted into a second in-line grating lens. reach above the in-line type grating lens. The arrival points of these lights are on the same radius centered on the intersecting axis, and the distance from the optical axis for light with wavelength λ is farther than light with wavelength λ ₀ . 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 λ ₀ , so the two lights are The distance between them gradually narrows, and eventually they intersect at one point. By the way, 2
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 predetermined 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 one point on the optical axis, and therefore no aberrations or focal position shifts will occur.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of the present invention. This embodiment has a configuration in which first and second in-line type grating lenses 11 and 12 are arranged on the same optical axis (dotted chain line), and a point P on the optical axis
The first spherical wave diverges from a (coherent light source)
The beam is diffracted toward the optical axis by the second grating lens 11, once intersects with the optical axis, and then focused at a predetermined point Q on the optical axis by the second grating lens 12.

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. be. Also,
The second grating lens 12 has a predetermined spatial frequency distribution that is rotationally symmetrical with respect to the optical axis, and the crossed diffracted light is focused on one point (point Q) on the optical axis. .

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. 2. Note that the distance between the point P and the grating lens 11 is l ₁ , the distance between the two grating lenses 11 and 12 is d,
Let the distance between the grating lens 12 and point Q be _l2 .

(i) First, point P is emitted and grating lens 1
Consider a ray of wavelength λ ₀ that reaches point P ₁ on the outermost circumference of 1. This ray is diffracted at point P ₁ and reaches point r ₁ (=0) at the center of grating lens 12, where it is further diffracted and reaches point Q (solid line a in Figure 2). . Then, by assuming the above-mentioned optical path (P→R ₁ →r ₁ →Q), the spatial frequencies F ₁ and f ₁ at the points R ₁ and r 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 R ₁ is diffracted at point R ₁ at a larger angle than when the wavelength is λ ₀ and reaches point r ₂ on grating lens 12 (dashed line b). . Here, the spatial frequency f ₂ at point r ₂ is determined from the condition that the light is focused on point Q even when the wavelength is λ.

(iii) Returning to the case where the wavelength is λ ₀ , it is possible to conversely find from which point on the grating lens 11 the light ray that is diffracted at point r ₂ and reaches point Q comes from (solid line c). The grating lens 1
If the point on point 1 is R ₂ , the diffracted light at point R ₂ is the point P.
From the condition that , the spatial frequency F ₂ at point R ₂ is determined.

(iv) Considering the case where the wavelength becomes λ again, point r ₃ on the grating lens 12 is determined in the same way as in (ii) above.
(not shown) and its spatial frequency _f3 . Then, the wavelength is returned to λ ₀ and the point R ₃ (not shown) on the grating lens 11 and its spatial frequency F ₃ are determined in the same manner as in (iii) above. In this way the point Rn
(n = 1, 2, 3, ...) by repeating the steps (ii) and (iii) above until reaching the center of the grating lens 11, the spatial frequency distribution in the radial direction in the grating lenses 11, 12 is is determined. Note that the diameter of the second grating lens 12 is determined at the position of point r _o .

As described above, the grating lens 11,
By determining the spatial frequency distribution of 12, even if the light emitted from point P has a wavelength λ different from the reference wavelength λ ₀ , it can be focused on point Q without aberration.

FIG. 3 shows a specific spatial frequency distribution of the grating lenses 11 and 12 determined as described above. This means that l ₁ = 8 mm, d = 10 mm, l ₂ =
3.4 mm, λ ₀ =830 nm, and λ = 830.3 nm. These are the calculation results of the respective spatial frequencies F and f along the radial direction of the grating lenses 11 and 12. Note that the wavelength difference λ−λ ₀ =0.3 nm corresponds to one mode of the semiconductor laser. Here, λ ₀ =
When the spatial frequency distribution was calculated by fixing 830 nm and changing λ to various values, it was found that even when λ=λ ₀ ±5 nm, the results almost completely matched those in FIG. 3. From this, λ ₀ =830nm and λ
If the spatial frequency distribution is determined for λ = 830.3 nm, there will be no aberration even for light with λ = 830 ± 5 nm, and there will be no focal position shift. Further, if the value of d is shortened, the same effect can be obtained for light of a larger wavelength. For example, if d=5 mm, there will be no aberration for light of λ=830±7 nm.

Next, the characteristics of the grating lenses 11 and 12 having the above-described spatial frequency distribution will be made clearer using FIGS. 4 to 6.

First, as shown in FIG. 4, the diffraction function of the grating lenses 11 and 12 is divided into the side between them and the spherical wave side. Then, the grating lenses 11 and 12 can be equivalently divided into two grading lenses 11a and 11b; 12a and 12b, respectively, by the plane wave traveling in the optical axis direction. At the same time, F and f are divided into F _a , F _b , fa and f _b respectively in terms of spatial frequency, and the relationships F=F _{a +} F _b and f=f _a +f _b are established. Here, grating lenses 11a, 12
Reference character a is an in-line type grating lens that focuses plane waves to one point, and is equivalent to the conventional grating lens shown in FIG. On the other hand, in the grading lenses 11b and 12b on the other side,
As shown in FIG. 5, the spatial frequencies F _b and f _b have a special distribution with a maximum value (MAX) in the region between the lens center and the lens outer periphery.

From this, grading lens 11,
The 12 features can be explained as shown in FIG. That is, in the spatial frequency distribution of the in-line grating lenses 11a and 12a that focus plane waves, there is an axisymmetric spatial frequency distribution (spatial frequency of the grating lenses 11b and 12b) that has a maximum value between the lens center and the lens outer periphery. It can be said that the spatial frequency distributions of the grating lenses 11 and 12 are obtained by adding the distributions (distribution) as compensation elements.

Therefore _, in order to confirm the wavelength fluctuation compensation effect _of the grading lens optical system of this example, in FIG.
The aberrations that occur when the wavelength of semiconductor laser light is shifted from 830 nm are expressed as RMS values of wavefront aberrations. The results are shown in FIG. In addition,
The grating lenses 11 and 12 have a wavelength of 830n.
The spatial frequency distribution was determined so that there would be no aberration for wavelengths of m and 830.3 nm.

In FIG. 7, if Marechal's Criterion (RMS wavelength aberration 0.07λ) is considered as a standard that can be considered practically aberration-free, an allowable wavelength variation range can be obtained. If d=5mm, at least 830
±7nm, 830±6nm at d=10mm, 830±6nm at d=20mm
This shows that beam spot quality can be maintained against wavelength fluctuations of 830±4 nm. Note that there is no focal position shift.

From these results, according to this example, it is possible to obtain a good beam spot and a stable focal position even for wavelength changes caused by temperature changes in semiconductor lasers, mode hops, and even for multimode lasers. As a result, the practical use of grating lenses in, for example, optical heads can be realized. In addition, when the output light intensity of a semiconductor laser has a Gaussian distribution, the light focused by a conventional optical lens system also has a Gaussian distribution, but the light focused by this example has very good uniformity. Since it has an intensity distribution, it can be said that it is advantageous in obtaining minute spots.

The most reliable method for producing the grating lens according to the present invention is by electron beam lithography, and by inputting the calculated value of the spatial frequency as data, a desired grating lens can be produced. In this case, by blazing the grating, the efficiency of the grating lens can be further improved. It is also possible to create a desired wavefront holographically by creating a desired wavefront with an optical element. In this case, the efficiency of the lenses can be increased by reducing the distance d between the two lenses and increasing the spatial frequency band.

Note that the grading lens optical system of the present invention is not only applicable to the optical head of an optical disk device, but also to various optical systems that require the function of converging diverging spherical wave light from a coherent light source to one point. applicable.

〔Effect of the invention〕

According to the grating optical system of the present invention, it is possible to obtain a good beam spot without causing aberrations and a stable focal position that does not deviate from a predetermined point. It has become possible to put it into practical use in various fields.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure shows a grating lens 11 according to the same embodiment,
3 is a diagram showing an example of the spatial frequency distribution in the radial direction of the grating lenses 11 and 12 obtained by the method shown in FIG. 2, and FIG. An explanatory diagram of the spatial frequency distribution characteristics in the example, FIG. 5 is a diagram showing an example of the spatial frequency distribution of the grating lenses 11b and 12b shown in FIG. 4, and FIG. 6 is a diagram showing an example of the spatial frequency distribution of the grating lenses 11b and 12b shown in FIG. FIG. 7 is a diagram showing the characteristics of the spatial frequency distribution of Nos. 11 and 12.
A diagram showing the relationship between RMS wavefront aberration and semiconductor laser wavelength, FIGS. 8a and 8b are diagrams showing the functions of a conventional in-line grating lens and its drawbacks,
Figures 9a to 9c are schematic configuration diagrams showing various conventional grating lenses, Figures 10a and b are diagrams showing the functions and drawbacks of conventional off-axis grating lenses, and Figures 11a and b are diagrams showing the oscillation wavelength of a single mode semiconductor laser and a multimode semiconductor laser, respectively. Figure 12 is a diagram showing the temperature dependence of the oscillation wavelength in a single mode semiconductor laser. Figure 13 is a diagram of a conventional grating lens. FIG. 2 is a configuration diagram showing an optical system. 11...First in-line type grating lens, 12... Second in-line type grating lens.

Claims (1)

[Claims] 1. A first in-line grating lens 11 into which diverging spherical wave light emitted from a coherent light source enters, and diffracted light transmitted through the first in-line grating lens to a predetermined point. A grating lens optical system comprising a second in-line grating lens 12 for converging plane waves on the same optical axis, wherein the first in-line grating lens is an in-line grating lens for converging plane waves. It has a rotationally symmetrical spatial frequency distribution with respect to the optical axis, which is formed by adding an axially symmetrical spatial frequency distribution having a maximum value between the lens center and the lens outer periphery to the spatial frequency distribution, and is symmetrical with respect to the optical axis. It is a lens that causes diffracted light from two arbitrary points to intersect on the optical axis and enters the second inline type grating lens, and the second inline type grating lens is an inline type grating lens that converges plane waves. It has a rotationally symmetrical spatial frequency distribution with respect to the optical axis, which is obtained by adding an axially symmetrical spatial frequency distribution having a maximum value between the lens center and the lens outer periphery to the spatial frequency distribution of the grating lens, and the intersection A grating lens optical system, characterized in that the grating lens is a lens that converges the diffracted light to the predetermined one point.