JPH0427901A - Birefringence diffraction grating type polarizer - Google Patents

Abstract

PURPOSE:To obtain a birefringence diffraction grating type polarizer which has a deep grating and has a narrow grating pitch in the wavelength variance allowable range and has a high diffraction efficiency and is stable against the wave-length variance by setting respective elements of a birefringence diffraction grating type structure so that they satisfy specific formulas. CONSTITUTION:An incidence-side diffraction grating 5 has a depth T1, and an exit-side diffraction grating 3 has a depth T2. The refractive index in the grating direction of a high-refractive index part, that in the direction orthogonal to the grating direction of this part, that in the grating direction of a low- refractive index part, that in the direction orthogonal to the grating direction of this part, that of the center in the grating direction, and that of the center in the direction orthogonal to the grating direction of the incidence-side diffraction grating are denoted as n11, n12, n11', n12', N11, and N12 respectively. Those of the exit-side diffraction grating 3 are denoted as n21, n22, n21', n22', N22', and N22 respectively. Diffraction gratings are so constituted that respective elements satisfy formulas (A) to (D), and thereby, the grating depth is deep and the grating pitch is narrowed in the wavelength variance allowable range.

Description

[Detailed description of the invention] Industrial applications The present invention Used in optical pickup, etc. The present invention relates to a birefringent grating type polarizer.

BACKGROUND OF THE INVENTION Conventionally, there is a birefringent grating type polarizer of this type as shown in FIG. This was announced as ``29a-ZH-10 birefringence grating type polarizer'' in the Spring 1988 Proceedings of the Japan Society of Applied Physics, p. 848, and is a crystal with optical anisotropy, here LiNb0. The output side diffraction grating 3 is formed by performing periodic ion exchange of, for example, 1007111 pitches on the surface including the optical axis of the substrate l to form a proton exchange region 2, and a dielectric film 4 is formed on the proton exchange region 2. It has a structure in which an incident side diffraction grating 5 is formed by loading. In proton exchange region 2, the refractive index is 0.09 pm for extraordinary rays with a wavelength of 113 μm.
increases, and decreases by about 0.04 for ordinary rays. Therefore, the phase difference of the ordinary ray in the proton exchange region 2 can be expressed as
By canceling each other out, the polarizer becomes a polarizer that allows the ordinary rays to travel straight and only the extraordinary rays to diffract. That is, such a birefringent diffraction grating type polarizer 6 separates two orthogonal linearly polarized lights into diffracted light and transmitted light. Here, the incident light including the ordinary ray component and the extraordinary ray component is divided into O1±1 double diffracted light.

Therefore, such a birefringent diffraction grating type polarizer 6 can be used for reading magneto-optical signals in a magneto-optical disk device. Figure 4 shows an application to optical pickup for magneto-optical disks, which was announced as ``Hologram optical element with analyzer function for r30a-PB-3 magneto-optical disk heads'' in the Proceedings of Japan Society of Applied Physics, Fall 1989, p. 972. This is an example. In this optical pickup, a laser beam emitted from a semiconductor laser 7 passes through a lens 8, a polarizing beam splitter 9, and an objective lens 10, and is focused onto a magneto-optical disk 11, and the light reflected from the magneto-optical disk 11 is returned to the objective lens. 0, is reflected by a polarizing beam splitter 9 and separated from the incident light, passes through a lens 12, and is separated and imaged on a light receiving element 14 by a hologram optical element 12. This hologram optical element 12 utilizes the birefringent diffraction grating type polarizer function shown in FIG. 3, and as shown in FIG.
2 and is used to detect focus errors, tracking errors, and magneto-optical signals. Here, the magneto-optical signal is detected based on the difference between the total light amount of the +1st order light and the 11th order light and the light amount of the 0th order light.

On the other hand, holograms for reading magneto-optical signals can also be realized by making the pitch finer. In other words, by appropriately controlling the wavelength, pitch, and depth of the surface relief grating, the diffraction efficiency of TE and TM polarized light differs ('87 Autumn Proceedings of Japan Society of Applied Physics, p. 70).
(See 3). No. 68! I shows an exaggerated cross-sectional shape of such a surface relief type grating 14, in which pitch A=0.32 pm, depth d=o, 254a,
When the wavelength λ of the incident light is 0.83 μm, the characteristics of the diffraction efficiency and grating pitch dependence are as shown in FIG. In the figure, the solid line shows the O-order TM, the -dot chain line shows the 0-order TE, the two-dot chain line shows the first-order TE, and the broken line shows the first-order TM. It is also known that in surface relief gratings, even higher efficiency can be obtained if the grating depth is made into grooves that are approximately twice as deep as the pitch.

Problem to be Solved by the Invention However, in a magneto-optical pickup using a birefringent diffraction grating 6 as shown in FIG.
Since it is as large as 0p, it is due to the Raman-Nath diffraction phenomenon. Therefore, the diffraction efficiency is as low as about 34% at most. Furthermore, even if the ratio is 68% by detecting both the +1st-order light and the 1st-order light, since the light-receiving elements for the +1st-order light and the -1st-order light are separate, the noise of each light-receiving element The C/N is also generated independently, and the C/N is also equivalent to that at a diffraction efficiency of 34%. Therefore, a high C/N cannot be expected.

In addition, in a magneto-optical pickup using a surface relief type grating 14, which is a fine pitch diffraction grating, as shown in FIG. The angle of failure varies greatly, which poses a practical problem. This point will be explained using the following equation expressing the diffraction angle. That is, when the grating pitch is Λ, the wavelength is λ, the incident angle is θi, and the diffraction angle is 00, the following formula holds true. That is, as the pitch A becomes minute, the coefficient 1/A of the wavelength λ becomes larger. Therefore, it can be seen that the diffraction angle θ0 changes greatly when the incident wavelength ^ changes.

For example, when the incident angle θ1=30° and λ=A,
When the wavelength λ changes by 1% (a change value that can occur in a practical semiconductor laser), the diffraction angle θ0 becomes 30.664° from the above equation. As a result, the optical axis movement on the photodetector is 340 μm.
reach even. For example, considering that the allowable range of optical axis movement for focus error detection using the astigmatism method is about 20 μm, it can be seen how large the amount of forward axis movement is.

If the above points are collectively illustrated, the result will be as shown in Fig. 8.

That is, in the case of the birefringent diffraction grating type polarizer 6, although the influence of wavelength fluctuation is within the permissible range, the diffraction efficiency is low;
In the case of the fine pitch diffraction grating 14, although the diffraction efficiency is high, the influence on wavelength fluctuation exceeds the permissible value (however, the grating depth is set to the depth at which the maximum diffraction efficiency is obtained at each pitch).

Means for Solving the Problem Consists of a birefringent diffraction grating consisting of an input side diffraction grating and an output side diffraction grating, where the grating pitch is Λ, the wavelength of the incident light is λ, and the amount of change in diffraction angle allowed per unit amount of wavelength change. is X, the depth of the input side diffraction grating is T1, the depth of the output side diffraction grating is T,
Regarding the incident side diffraction grating, the refractive index in the grating direction of the high refractive index part is n11, the refractive index in the grating direction is 1, the refractive index in the grating direction of the low refractive index part is n11, and the refractive index in the grating perpendicular direction is n12. ', the central refractive index in the grating direction is N11, the central refractive index in the orthogonal direction to the grating is N l l, and the refractive index in the grating direction of the high refractive index portion of the output side diffraction grating is 0.
21. The refractive index in the direction perpendicular to the lattice is n83, the refractive index in the lattice direction of the low refractive index part is n33, and the refractive index in the direction perpendicular to the lattice is n.
12', the center refractive index in the lattice direction is N t + , and the center refractive index in the direction perpendicular to the lattice is N. When closed, each specification is A pressure]7ゝ ゛°゛゛°°°°°°°°°°゛°
°゛°′°pieces n12' T, ten n12' Ts #
It was constructed so as to satisfy the following equations: n(+L + n12T, - Kawa (c).

In the active birefringence grating structure, each of the specifications is as described in (A) above.
By satisfying the formulas (D) to (D), the grating pitch is fine within a range where the grating depth is deep and wavelength fluctuations can be tolerated, resulting in high diffraction efficiency and stability against wavelength fluctuations.

Embodiment An embodiment of the present invention will be explained based on FIGS. 1 and 2. Components that are the same as those shown in FIGS. 3 to 8 are indicated using the same reference numerals. In order to eliminate the drawbacks of the two conventional methods mentioned above and take advantage of the advantages of both, this embodiment is based on a birefringent grating type polarizer 6 structure, but the grating depth is increased and the grating pitch is increased. The wavelength fluctuation is made finer within an allowable range, and in the characteristics shown in Figure 8, the intermediate region C between the two is obtained.
It was designed to realize the following.

First, conditions for the grating pitch A that can tolerate wavelength fluctuations are derived as follows. First, the incident angle is θi, and the diffraction angle is θO.
5.If the wavelength of the incident light is λ and the grating pitch is 8, the diffraction conditional expression is sinθi + sinθ0=λ/A...
・・・・・・・・・・・・It is expressed as (1). now,
If the change in the diffraction angle when the wavelength fluctuation Δλ occurs is Δθ0, then equation (1) becomes sinθi + 5in(θ0+Δθ
0)=(λ+Δλ)/A (2). Here, when formula (2) is expanded by setting Δθ0 x θ0, it becomes sinΔθ0#Δθ0%cO8Δθ0#l. Substituting equation (1) into equation (3), Δθo cos
θ0=Δλ/A ・・・・・・・・・・・・
......(4). That is, Δθ01 Δ^ Acosθ0 (5) holds true. Here, if the diffraction is Bragg diffraction, θ1
=00, so from equation (1), λ sinθo = sinθi=-A holds true. From equation (6), ・・・・・・・・・・・・・・・・・・・・・・・・(
6) cosθo=J1-sin"Oo=J1-(λ/2A)'
7). Substituting equation (7) into equation (5) yields. From this equation (8), it is possible to calculate a grating pitch of eight that can tolerate wavelength fluctuations.

For example, if the distance from the diffraction grating is 30 mm, the allowable optical axis movement length on the photodetector is 20 μm (general tolerance for focus error detection), and the wavelength is 0.83 μm, then Δθ0/Δλ
'=6.7 x 10-' [radl, and A that satisfies this is A#15 μm. After all, if the allowable diffraction angle change per unit wavelength change is X, then Rma ■πW<
It is sufficient to satisfy the following condition (A).

Next, conditions for obtaining high diffraction efficiency will be considered.

First, the Q value of the diffraction grating is Q=2πλT/noA'', where T is the grating depth and no is the central refractive index.
It is represented by (9).

The highest diffraction efficiency obtained when Q<1 is at most 33.9
% (see "Light Wave Electron Optics" published by Corona Publishing Co., Ltd., written by Hiroshi Nishihara, p. 118). In the case of a birefringent diffraction grating, the center refractive index of the resist (=proton exchange region 2) is n8, the depth is TR, and LfNbO
.

Letting the center refractive index of (=substrate 1) be nLs and the depth be TL, then the following equation is obtained. Here, nR==1166, T,=0.4pm
, nL=2. 2, T, = 5.3pm, Δ=100
p+++, λ=113pm, Q=2.2X
10-'', which is the region of Q<1.

Therefore, in this example, in order to obtain high diffraction efficiency, Q>
Make it 1. In the case of Q>1, considering the polarization in the direction orthogonal to the diffraction grating in the birefringence grating type polarizer 6 configuration of FIG. 1 corresponding to FIG. 3, the following equation is obtained. However, the materials used are the above-mentioned resist and LiNb.
0. Therefore, in general terms, as shown in FIG.
The depth of the diffraction grating 5 on the incident side is T, the refractive index of the high refractive index portion in the grating direction is n,1, the refractive index in the direction perpendicular to the grating is nl12, and the refractive index of the low refractive index portion in the grating direction is n11, the refractive index in the direction orthogonal to the lattice is n12', the center refractive index in the lattice direction is N11, the center refractive index in the direction orthogonal to the lattice is N
l m, the refractive index in the grating direction of the high refractive index part of the output side diffraction grating 3 is 021, the refractive index in the grating direction perpendicular to the grating is nyo3, and the refractive index in the grating direction of the low refractive index part is n3 in the grating perpendicular direction. The refractive index is 03□, and the central refractive index in the lattice direction is N3. , the central refractive index in the direction orthogonal to the grating was set at N3°.

In this way, if the diffraction gratings 3 and 5 are configured to satisfy equations (A) and (B), they will be resistant to wavelength fluctuations and have high diffraction efficiency. However, in the case of the birefringent diffraction grating type that is the subject of this example, the diffraction efficiency is high for light polarized in the direction perpendicular to the grating, and low diffraction efficiency is used for light polarized in the grating direction, and the magneto-optical signal is read. It has characteristics. That is, the first
In the figure, it is necessary for the polarized light in the grating direction to behave as if there were no diffraction grating, so the optical lengths of the diffraction grating perceived by rays Q and Q, which have a distance of 1/2 of the grating pitch, must be the same. Must be. Therefore, for polarized light in the lattice direction, n++' T+ 10 nm% T, #n12T, +
n12T, ・Refractive index n12 that satisfies -(c)
, n12, n12.

It is necessary to select a material that has nyo1 (n12'#l if the incident side is a surface relief type and is in contact with the outside air), and realize T12 T by manufacturing according to Figure 3. (This is a feature of the birefringent grating type polarizer 6 as shown in FIG. 3).

For example, n12=n12=1,66,n12'
=n'=1, n12=2,246, n12'=2
．． 286, n12=2.29, and n12'=2.20. Then, N11=N
12= (1,66+1)/2=1133, N11=(
n*+ + nm+') / 2" 2. 266,
N11=(n12+n12')/2=2.245, and by substituting these values into formula (C), T, = 16.5T, ...
・・・・・・・・・・・・(11) Furthermore, if the wavelength λ used is λ=0.83 μm and the grating pitch A is 15 μm obtained from equation (A), then T, ) −1,688T + 93.86 ・・
・・・・・・・・・・・・・・・(12)

When these formulas (II) and (12) are expressed in a graph, the result is as shown in FIG. The intersection P in the figure is (T12 T,)=
(5, 33, 879). In the structure shown in Figure 3, this value is λ = 113 μm, proton exchange depth = 5.3 μm.
m, resist film thickness = 400 nm, grating pitch = 100 μ
m, diffraction angle = 0.74°, extraordinary ray loss = 29.1 dB
, ordinary ray loss = 114 dB (these losses include Fresnel loss of 113 dB), polarization separation degree = 27.7 dB, this is the minimum T l l T yo, and P in Figure 2
It may be the part of the solid line above and to the right of the point. Such a design can be made arbitrarily depending on the material.

Note that the polarized light to be diffracted may be in the grating direction. This can also be changed depending on the material used. Further, the refractive index of the refractive index diffraction grating may change continuously. In this case, the light ray Q12Q has a maximum or minimum refractive index (either the light ray Q12Q may be used). Further, the directions of incidence and emission are relative, and it goes without saying that the same effect can be obtained even if the direction of incidence is reversed.

Effects of the Invention The present invention has a birefringent diffraction grating type structure as described above, which is configured so that each of its specifications satisfies the formulas (A) to (D). The grating pitch is fine within a range that allows wavelength fluctuations, and the diffraction efficiency is high, making it possible to provide a polarizer that is stable against wavelength fluctuations caused by external conditions and the like.

[Brief explanation of drawings]

Fig. 1 is a principle structural diagram showing an embodiment of the present invention, Fig. 2 is a characteristic diagram, Fig. 3 is a perspective view showing a conventional birefringence grating type polarizer structure, and Fig. 4 is its magneto-optical structure. An optical system configuration diagram showing an example of application to a pickup, FIG. 5 is a perspective view of the hologram optical element and light receiving element, FIG. 6 is a structural diagram showing a conventional example of a fine pitch diffraction grating, and FIG. 7 is a λ/ d-diffraction efficiency characteristic diagram, and FIG. 8 is an A/λ-diffraction efficiency and diffraction angle change/wavelength change characteristic diagram. 3... Output side diffraction grating, 5... Input side diffraction grating.

Claims (1)

[Claims] Consisting of a birefringent diffraction grating consisting of an incident side diffraction grating and an output side diffraction grating, the grating pitch is Λ, the wavelength of the incident light is λ, and the allowable amount of change in diffraction angle per unit amount of change in wavelength is X, the depth of the input side diffraction grating is T_1, the depth of the output side diffraction grating is T_2, the refractive index of the high refractive index portion of the input side diffraction grating in the grating direction is n_1_1, and the refractive index in the direction orthogonal to the grating is n_
1_2, the refractive index in the lattice direction of the low refractive index part is n_1_1
', the refractive index in the direction orthogonal to the grating is n_1_2', the center refractive index in the grating direction is N_1_1, the center refractive index in the direction orthogonal to the grating is N_1_2, and the refractive index in the grating direction of the high refractive index part of the output side diffraction grating is n_2_1. , the refractive index in the direction perpendicular to the lattice is n_2_2, the refractive index in the lattice direction of the low refractive index portion is n_2_1', and the refractive index in the direction perpendicular to the lattice is n_2_2'.
, when the central refractive index in the lattice direction is N_2_1 and the central refractive index in the orthogonal direction to the lattice is N_2_2, each dimension is 1/Λ√1-(λ/2Λ)^2<X... (A )2
πλ/Λ^2(T_1/N_1_2+T_2/N_2_
2)>1... (B) n_1_1'T_1+n_2
_1'T_2≒n_1_1T_1+n_2_1T_2・
...(C)N_1_2=n_1_2+n_1_2'
/2, N_2_2=n_2_2+n_2_2'/2...
A birefringent diffraction grating type polarizer, characterized in that it satisfies the following formulas (D).