JP2664369B2 - Focusing device using grating lens optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A first grating lens having a predetermined spatial frequency distribution for causing an incident laser beam to intersect axially symmetrically and a first grating lens having a predetermined spatial frequency distribution for converging the crossed beam on an optical disk By combining two grating lenses with two grating lenses, a good beam spot without aberration and a stable focusing performance without deviation can be realized without being affected by the wavelength fluctuation of the incident laser beam. In the newly developed grating lens system, the two in-line grating lenses are provided with a spatial frequency distribution in which the focal length of the image formed by the second grating lens changes in accordance with the change in the oscillation wavelength of the light source. Changing the wavelength is the second
It is possible to control the movement of the image formation position by the grating lens, that is, the focus position. [Industrial Application Field] The present invention relates to a focusing device using a grating lens optical system having a focusing function by combining a grating lens. Recently, in an optical system that requires a function of converging divergent spherical wave light from a coherent light source to one point, for example, an optical head of an optical disk device, (i) miniaturization of the device;
(Ii) Shorter access time, (iii) lower cost, etc.
Use of a compact and mass-produced grating lens is being studied. [Prior Art] FIG. 11 shows a conventional inline grating lens. As shown in FIG. 1 (a), this lens transmits only a parallel light beam having a specific wavelength λ ₀ at one point (focal length, for example).
f ₀ ). Therefore, the above λ
For light having a wavelength λ ₁ (> λ ₀ ) longer than ₀ , aberration occurs as shown in FIG. 3B, and good focusing performance cannot be obtained, and the focal length also changes to f ₁ . . In addition, for a wavelength λ ₂ (<λ ₀ ) having a wavelength shorter than λ ₀ , as shown in FIG. 3C, an aberration occurs as in (b), and the focal length changes to f ₂ . . Such a phenomenon is common to lenses that bend light by diffraction, and applies to any of a volume hologram lens, a surface relief grating lens, and a blazed grating lens. As described above, the grating lens has a property that, when the wavelength used deviates from a predetermined value (λ ₀ ), aberration occurs, the focusing performance is deteriorated, and the focal position is also deviated depending on the type of lens. . Here, focusing only on the change in the focal length due to the change in the oscillation wavelength, it is understood that there is a possibility that the focal length can be controlled by intentionally changing the wavelength. Therefore, for example, in an optical disk device, it is possible to perform focus servo by controlling the oscillation wavelength of a semiconductor laser (hereinafter, referred to as LD), which is a coherent light source. In fact, a focus servo device that controls the focal length by changing the wavelength has been proposed (for example, Japanese Patent Application Laid-Open No. 60-1985).
66337). In this apparatus, as shown in FIG. 12, in response to the focus error of the focused beam, the oscillation wavelength λ _{0 of the} LD ₁₀ is changed to λ ₁ , λ ₂ by the wavelength control system 12 to change the focal length from Q _0.
Q ₁ and Q ₂ movement control. [Problems to be Solved by the Invention] However, in the above-mentioned grating lens, aberration occurs with a change in wavelength, and good focusing performance cannot be obtained, and it is difficult to read out signals from an optical disk. Therefore, there is a demand for the development of a grating lens in which the focal length changes without aberration with a change in the oscillation wavelength. By the way, the applicant of the present application has previously described in Japanese Patent Application No. 61-220870 that it is possible to always obtain a good beam spot free from aberration without being affected by the wavelength fluctuation of the incident light as described above. We have proposed a grating lens optical system consisting of a pair of grating lenses that has a stable and accurate focusing performance. The present invention uses this grating lens optical system to provide a highly accurate and highly reliable focusing device that satisfies the requirements of light weight, small size, and low cost, and that does not involve the occurrence of aberration due to wavelength fluctuation. The purpose is to do. [Means for Solving the Problems] In order to achieve the above object, according to the present invention, the first divergent spherical wave light emitted from the coherent light source is incident.
In-line type grating lens (11) and the first
A second in-line grating lens that focuses diffracted light transmitted through the in-line grating lens at a predetermined point on the same straight optical axis; Having a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, diffracted light from any two points that are symmetric with respect to the optical axis crosses on the optical axis and is incident on the second in-line grating lens, The second in-line type grating lens has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, focuses the crossed diffracted light at the predetermined point, and forms the first and second in-line type grating lenses. The grating lens has a spatial frequency distribution in which the focal length of the image formed by the second grating lens changes according to the change in the oscillation wavelength of the light source. In addition, there is provided a focusing apparatus characterized in that the focal position is controlled by changing the oscillation wavelength. [Operation] The laser beam (wavelength λ ₀ ) from the laser light source incident on the grating lens optical system is diffracted by the first grating lens in the grating lens system. At that time, the diffracted light crosses the optical axis symmetrically. Next, these diffracted lights are diffracted by the second grating lens,
Focus on one point (focus) Q ₀ . Here, when the oscillation wavelength changes from λ ₀ to λ, the first and second grating lenses have a spatial frequency distribution in which the focal length of image formation by the second grating lens changes according to the change in the oscillation wavelength of the light source. It converges on a point Q different from the case of λ ₀ without aberration. Thus, when the oscillation wavelength is changed, the focal length changes, and focusing is achieved. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, the configuration of a grating lens system that plays an important role in the present invention will be briefly described with reference to FIGS. The detailed structure of the grating lens system is disclosed in Japanese Patent Application No. 61-220870. In FIG. 9, the grating lens systems are the first and second
The in-line type grating lenses 11 and 12 are arranged on the same optical axis ▲ ▼, and the spherical wave diverging from the point P (coherent light source) on the optical axis is shifted to the optical axis side by the first grating lens 11. After being diffracted and once crossing the optical axis, it is focused by the second grating lens 12 to a predetermined point Q on the optical axis. 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 any two points symmetric with respect to the optical axis intersects on the optical axis. . Further, the second grating lens 12 has a predetermined spatial frequency distribution rotationally symmetric with respect to the optical axis, and the crossed diffracted light is reflected at one point Q on the optical axis.
Focusing on In the above configuration, consider the paths of two light beams having different wavelengths λ ₀ and λ ₁ (λ ₀ <λ ₁ ) incident on the arbitrary point of the first in-line grating lens from the same direction. First, the light of wavelength λ ₁ is diffracted by the first in-line type grating lens 11 at a larger angle than the light of wavelength λ ₀ , and after all of these diffracted lights intersect with the optical axis, the second The light reaches the in-line grating lens 12. Goal of these lights, in the optical axis the same radius on around the, yet its distance from the optical axis is farther than the light of the wavelength lambda ₀ is more of a wavelength lambda ₁ of light. Next, these lights are diffracted by the second in-line grating lens 12, at this time,
Since the light having the wavelength λ ₁ is diffracted at a larger angle than the light having the wavelength λ ₀ , the interval between the two lights gradually narrows and finally intersects at one point Q. Therefore, by giving the two in-line grating lenses a predetermined spatial frequency distribution, the point where the two lights intersect can be located at the designated point Q on the optical axis. The above can be said for light incident on any point of the first in-line grating lens 11,
In addition, since the spatial frequency distribution is rotationally symmetric with respect to the optical axis, the incident divergent spherical wavelength is focused on the predetermined point Q on the optical axis even if the wavelength changes, so that aberrations and focal position shifts occur. It will not happen. Next, a specific method for determining the spatial frequency distribution of the grating lenses 11 and 12 will be described below with reference to FIG. Note that the distance between the point P and the first grating lens 11 is 11 and the two grating lenses
The distance between 11 and 12 is d, and the distance between grating lens 12 and point Q is l2. (I) First, consider a light ray having a wavelength λ ₀ that emits a point P and reaches the outermost point R 1 of the grating lens 11. This light beam is diffracted at the point R1 and reaches the center point r1 (= 0) of the grating lens 12, where it is further diffracted and reaches the point Q (solid line a in FIG. 7). Then, assuming the above-described optical path (P → R1 → r1 → Q), the points R1, r
The spatial frequencies F1 and f1 at 1 are determined. (Ii) Next, consider a case where the wavelength has changed from λ ₀ to λ ₁ (> λ ₀ ). Ray of wavelength lambda ₁ advanced to the point R1 from the point P is at point R1, the wavelength is diffracted at an angle greater than when the lambda _0, reaches the point r2 on grating lens 12 (dashed line b). Here, the condition that the wavelength is focused at a point Q even when a lambda _1, the spatial frequency at the point r2
f2 is determined. (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 light beam that reaches the point Q diffracted at the point r2 comes (solid line c). If the point on the grating lens 11 is R2, the spatial frequency F2 at the point R2 is determined from the condition that the diffracted light at the point R2 reaches the point P. (Iv) re-consider the case where the wavelength becomes lambda _1, (ii) above
The point r3 (not shown) on the grating lens 12 and its spatial frequency f3 are obtained in the same manner as described above. Then, the wavelength is returned to λ ₀ , and the grating lens 1 is set in the same manner as in (iii) above.
A point R3 (not shown) on 1 and its spatial frequency F3 are obtained.
By repeating the processes (ii) and (iii) until the point Rn (n = 1, 2, 3,...) Reaches the center of the grating lens 11, the grating lenses 11, 1
The radial spatial frequency distribution at 2 is determined. Note that the diameter of the second grating lens 12 is determined at the position of the 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 ₁ different from the reference wavelength λ ₀ , It can be focused on point Q. A focusing device is realized by using the above-described grating lens optical system of the present invention.
An embodiment of the present invention will be described with reference to the drawings. In FIG. 1, the basic structure of the grating lens is the same as that shown in FIG. 6, and a first grating lens 11 and a second grating lens 12 are arranged at the optical axis position. The divergent spherical wave of wavelength λ ₀ from the coherence light source 10 (point P) is applied to a first grating lens (hereinafter referred to as GL ₁ ) 1
1 and each ray is diffracted toward the optical axis side according to its spatial frequency, intersects with the optical axis, and then enters a _second grating lens (hereinafter referred to as GL ₂ ) 12 where each ray follows the GL ₂ spatial frequency Diffracted and focused at point Q ₀ . GL ₁ 11 if the wavelength is changed to lambda ₁ in the above configuration, GL ₂ 12 of GL ₁ divergent spherical wave having a wavelength lambda ₁ is from the point P by appropriate choice as follows spatial frequency 11, GL _The light is diffracted by ₂ 12 and can be focused to a point Q different from Q ₀ at the wavelength λ ₀ without aberration. As the light source 10, for example, a known wavelength-variable semiconductor laser can be used. Shows the GL ₁ 11, GL ₂ 12 method for determining the spatial frequency of the present invention in Figure 2. Basically, it is completely the same as the case of FIG. 7, and therefore, the description of the same parts will be simplified in order to avoid complexity. In FIG. 2, the distance between the point P and GL ₁ 11 is l ₁ , GL ₁ 11
The distance between the GL ₂ 12 d, the distance GL ₂ 12 and the point Q ₀ and l _2. Consider a light beam a having a wavelength λ ₀ that reaches from the point P to the outermost point R ₁ of GL ₁ . The ray a is reached at the center r ₁ of GL ₂ is diffracted by the R _1, and to further reach the diffracted point Q _0. By assuming the optical path P → R ₁ → r ₁ → Q ₀ of the ray a, the point
The spatial frequencies F ₁ and f _{1 of} R ₁ and r ₁ are determined. Next, when the wavelength is λ ₀
Is changed from λ ₁ to λ ₁ (> λ ₀ ). The light ray that travels from P to R ₁ is diffracted at R _{1 at a} diffraction angle different from the wavelength λ ₀ in a different direction, and reaches a point r ₂ of GL ₂ 12 (broken line b). Here, the point Q ₀ when the wavelength λ ₁ △ l ₂ only differs from Q
The spatial frequency f ₂ in r ₂ is determined from the condition that focused on. Here, returning to the case where the wavelength is λ ₀ again, it is determined at which point on GL _{1 the} light ray diffracted at the point r ₂ and reaching the point Q ₀ comes from (light ray C). When a point on its GL ₁ and R _2, the point from the condition that the light from the diffracted light point P at R ₂ R ₂
Spatial frequency F ₂ is determined in. Considering the case where the wavelength becomes λ ₁ again, the third point on GL ₂ and its spatial frequency r
₃ and f ₃ are obtained, and then the wavelength is returned to λ ₀ to obtain a third point on GL ₁ and its spatial frequencies R ₃ and F ₃ . Repeating the above process, Rn (n = 1,2, ... ) is the spatial frequency distribution of the radial to the GL _1, GL ₂ calculated to reach the center of GL ₁ is determined. The lens radius of GL ₂ is determined by the value of r _n as in the case of FIG. FIG. 2 shows a state in one section including the optical axis, but the lens is symmetric with respect to the optical axis. By determining the spatial frequency distribution of the grating lenses 11 and 12 as described above, the light emitted from the point P can be focused to a point Q different from the point Q ₀ without aberration according to the change in the wavelength λ. I can do it. Note that λ is changed to λ ₁ , λ ₂ , λ ₃ …
By defining Q ₁ , Q ₂ , Q _3, ... In the same manner as described above, the focal length can be moved according to the wavelength change. FIG. 3 shows a specific radial spatial frequency distribution of the grating lenses 11 and 12 determined as described above. This is the numerical aperture (NA) of GL ₁ = 0.3, NA of GL ₂ = 0.5,
l ₁ = 3.1 mm, d = 2.5 mm, l ₂ = 1.7 mm, λ ₀ = 830 nm, λ ₁ = 83
It is a calculation result of each spatial frequency F, f along the radial direction of the grating lenses 11 and 12 when 0.3 nm and △ l ₂ (= Q ₀ −Q) = 0.3 μm. The wavelength difference λ ₁ −λ ₀ = 0.3 nm corresponds to one mode of the semiconductor laser. Next, the characteristics of the grating lenses 11 and 12 having the above-described spatial frequency distribution will be clarified with reference to FIGS. First, as shown in FIG.
The diffraction function of No. 2 is divided into a side between them and a side of a spherical wave. Then, the grating lenses 11 and 12 can be equally divided into two grating lenses 11a and 11b; 12a and 12b, respectively, by a plane wave traveling in the optical axis direction. The same time, F also in the spatial frequency, f, respectively _{F a, F b; f a} , is divided into _{f b, F = F a +} F b, f = f a + f b becomes relationship is established. Here, the grating lens 11a,
Numeral 12a denotes an in-line type grating lens for focusing a plane wave at one point, which is equivalent to the conventional one shown in FIG. The other side of the grating lens 11b contrast, in 12b, the spatial frequency F _b, f _b is, having in the area between the lens center and the lens outer periphery, as shown in FIG. 5 the maximum value (MAX) It has a special distribution. From these facts, the characteristics of the grating lenses 11 and 12 can be explained as shown in FIG. That is, an in-line type grating lens 11a for focusing a plane wave,
The spatial frequency distribution of 12a is combined with the axially symmetric spatial frequency distribution (spatial frequency distribution of the grating lenses 11b and 12b) having the maximum value between the lens center and the lens outer periphery as a compensation element, and the grating lenses respectively 1
It can be said that there are 1,12 spatial frequency distributions. FIG. 7 is a view showing an experimental result on the relationship between the wavelength change amount (量 λ) and the focal point movement amount (△ l ₂ ) in the present invention. The horizontal axis represents the wavelength change △ λ (nm), and the vertical axis represents the focal shift △ l ₂
(Μm). By design, the movement amount of the focal length is 1μm when wavelength is 1nm changes expressed by _{l 20 = 0.3μm, l 20 =} 0.3
The experiment was performed for three times of μm, 0.6 μm, and 0.9 μm. This indicates that at l ₂₀ = 0.3 μm, the focal point surely moves at a ratio of approximately 1: 1 with respect to the wavelength change. Further, l ₂₀ = 0.6μm, was found to focus the wavelength change Similarly, the case of the l ₂₀ = 0.9 .mu.m is reliably moved in a fixed relationship. If the moved focal point has no aberration, the usefulness of the present invention is confirmed. Therefore, in order to confirm the wavelength fluctuation compensation effect of the grating lens optical system of the present embodiment, the values of l ₂₀ = 0.3 μm, 0.6 μm, and 0.9 μm in FIG.
In each case, the aberration that occurs when the wavelength of the semiconductor laser light is shifted from 830 nm was expressed as an RMS value in the wavefront aberration. The result is shown in FIG. The spatial frequency distribution of the grating lenses 11 and 12 was determined so that the grating lenses 11 and 12 had no aberration at the wavelengths of 830 nm and 830.3 nm. In FIG. 8, the horizontal axis represents △ λ (nm), and the vertical axis represents RMS wavefront aberration. In FIG. 8, Marechal's Criterion (RMS wavelength aberration 0.07
Considering λ), an allowable wavelength variation range is obtained. (L ₂₀
= 0.3μm the _{△ λ = ± 18nm, l 20} = 0.6 (μm) in at least _{△ λ = ± 20nm, l 20} = 0.9 (μm) in △ λ = ± 20n
It has been found that up to m is within the rms wavefront aberration criterion which can be regarded as stigmatic. Also it can be seen that the wavelength-varying l ₂₀ becomes larger rms wavefront aberration is reduced even if it occurs. From the above, it was confirmed that the grating lens system of the present invention can move the focal point without aberration. The most reliable method for producing the grating lens according to the present invention is a method using electron beam lithography, and a desired grating lens can be produced by inputting a calculated value of the spatial frequency as data. In this case, by blazing the grating lens, the efficiency of the grating lens can be further increased. Also,
It is also possible to create holographically by creating a desired wavefront with an optical element. In this case, 2
By increasing the spatial frequency band by reducing the distance d between the two lenses, the efficiency of the lenses can be increased. [Effects of the Invention] According to the focusing apparatus of the present invention, while maintaining the advantages of the grating lens optical system, there is no need for a mechanical driving part at all, and simply changing the oscillation wavelength does not cause any aberration without changing the focal length. Control can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a basic configuration of the present invention, FIG. 2 is a diagram showing a method of determining the spatial frequency of the grating lens shown in FIG. 1, and FIG. 3 is a diagram shown in FIG. Showing an example of the spatial frequency distribution in the radial direction of the grating lenses 11 and 12 obtained by the method described above, FIG. 4 is an explanatory diagram of the spatial frequency distribution characteristics in the above embodiment, and FIG. 5 is shown in FIG. FIG. 6 shows an example of the spatial frequency distribution of the grating lens, FIG. 6 shows the characteristics of the spatial frequency distribution of the grating lens shown in FIG. 4, and FIG. 7 shows the relationship between the amount of change in wavelength and the amount of focus movement. FIG. 8, FIG. 8 is a diagram showing the relationship between the RMS wavefront aberration and the semiconductor laser wavelength change amount in the above embodiment, FIG. 9 is a diagram showing the basic principle of the grating lens optical system used in the present invention, and FIG. The gray shown in FIG. FIGS. 11 (a), 11 (b), and (c) are diagrams showing the functions and disadvantages of a conventional in-line grating lens, and FIGS. FIG. 2 is a schematic configuration diagram showing a focus servo system. 11: First in-line grating lens, 12: Second in-line grating lens.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Fumio Yamagishi               Fujitsu, 1015 Ueodanaka, Nakahara-ku, Kawasaki-shi               Inside the corporation (72) Inventor Hiroyuki Ikeda               Fujitsu, 1015 Ueodanaka, Nakahara-ku, Kawasaki-shi               Inside the corporation (72) Inventor Yushi Inagaki               Fujitsu, 1015 Ueodanaka, Nakahara-ku, Kawasaki-shi               Inside the corporation                (56) References JP-A-59-160166 (JP, A)                 JP-A-60-66337 (JP, A)

Claims (1)

(57) [Claims] First in-line grating lens on which divergent spherical wave light emitted from a coherent light source is incident (11)
And a second in-line grating lens (12) for converging the diffracted light transmitted through the first in-line grating lens to a predetermined point is disposed on the same straight optical axis; The in-line type grating lens has a predetermined spatial frequency distribution rotationally symmetric with respect to the optical axis,
Diffracted light from any two points symmetric with respect to the optical axis crosses on the optical axis and is incident on the second in-line grating lens, and the second in-line grating lens rotates with respect to the optical axis. Has a symmetric predetermined spatial frequency distribution,
Focusing the crossed diffracted light to the predetermined point, wherein the first and second in-line grating lenses have a spatial frequency at which the focal length of the image formed by the second grating lens changes according to the change in the oscillation wavelength of the light source. A focusing device that has a distribution and controls the focal position by changing the oscillation wavelength.