JPS62212940A - Optical pickup - Google Patents

Abstract

PURPOSE:To contrive miniaturization, light weight and simplicity of the structure in an optical pickup by making a wavelength plate to change a polarizing condition to separate a reflecting signal light from a surface to be detected into a surface relief type hologram. CONSTITUTION:A 1/4 wavelength plate 4h composed of a surface relief type hologram is provided at the rear surface of a glass plate 13. When an X axis is in parallel to a P polariza tion and a Y axis is in parallel to an S polarization, the polarizing surface of a P polarization 15p after the laser light is collimated by a hologram 10 for collimating is orthogonal in the direction of a lattice groove 16 of a hologram 11 for polarization separation, and therefore, the surface transmits the hologram 11 for polarization separation. Since the direction of a lattice groove 14 of a hologram wavelength plate 4h is inclined to 45 deg. to the polarizing surface of the P polarization 15p, the P polarization 15p transmits the hologram wavelength plate 4h, and then, it is rotated to 45 deg. and converted to a circle polarization. In the condition of the circle polarization, it is made incident on an optical disk 6, and the circle polarization after reflection is made incident in the direction reverse to the hologram wavelength plate 4h. At such a time, it is further rotated to 45 deg. and converted to the S polarization. Since the S polarization is in parallel to the direction (Y direction) of the lattice groove 16 of the hologram 11 for polarization separation, it is diffracted and guided to a detecting device 8.

Description

[Detailed Description of the Invention] [Summary] The present invention aims to reduce the size and weight of an optical pickup, simplify the structure, and reduce costs by replacing the conventional 174-wave plate made of mica or the like. In this method, a surface relief grating is used.

[Industrial Application Field] When information is optically read from a disk medium in a laser disk device, the reading is performed by moving an optical system for an optical pick amplifier at high speed.

Therefore, efforts are being made to make optical pickups smaller and lighter in order to shorten access time, and to simplify structures in order to reduce costs. In order to meet such demands, research is being carried out on using holograms in place of conventional glass lenses and the like. The present invention
The present invention relates to an optical pick amplifier using such a hologram.

[Conventional technology]

FIG. 7 is a side view of an optical pickup using a conventional optical lens. The light emitted from the semiconductor laser LD is converted into parallel light by a collimating lens 1, passes through an optical path of a circular correction prism 2 - a polarizing beam splitter 3 - a quarter wavelength plate 4, and is focused by an objective lens 5. The optical disc 6 is irradiated with light. The reflected light that has read the information is sent to the objective lens 5.
Pass through the optical path of -1/4 wavelength plate 4 -> polarizing beam block 3 - condensing lens 7 - photodetector 8, and by the photodetector 8.
converted into an electrical signal. Note that the focus position adjustment of the lens 5 is performed by a drive coil 9.

As described above, conventional optical pickup amplifiers must use a large number of group lenses, and the optical pickup is large and heavy, which poses a problem in high-speed driving. Furthermore, assembly is difficult because a large number of optical lenses must be arranged while spatially adjusting their positions.

Therefore, in Japanese Patent Application No. 60-260317 filed on November 20, 1985, the applicant of the present invention proposes to use a hologram instead of a group lens as shown in FIGS. 8 and 9. proposed. In FIG. 8, 10 is a phase type collimating hologram, 11 is a surface relief type hologram, and 12 is an objective hologram. The phase type hologram 10 functions as the collimating lens 1 and the circular correction prism 2 shown in FIG. 7, and the surface relief type hologram 11 functions as the polarizing beam splitter 3 shown in FIG.

The objective hologram 12 replaces the group lens 5 in FIG.

As shown by the solid line, the diverging light emitted from the semiconductor laser LD is collimated by a phase-type hologram 10 for collimation, diffracted toward the optical disk 6 side, transmitted through a surface relief-type hologram 11, and then passed through a 174-wave plate 4. It is converted into circularly polarized light. The light is then focused onto the optical disk 6 by the objective hologram 12.

The signal light reflected from the optical disk 6 is collimated by the objective hologram 12 and then collimated by the objective hologram 17, as shown by the broken line.
The light is returned to linearly polarized light by the four-wavelength plate 4, polarized by the surface relief hologram 11, and converged onto the detector 8.

In FIG. 9, a surface relief type hologram lla controls the focusing effect toward the optical disk 6 side and a polarization separation effect, and a phase type hologram 10a controls the focusing effect toward the detector 8 side.

In this way, the functions of a heavy and large group lens and polarizing beam splitter 3 can be realized with a small hologram.

[Problem that the invention seeks to solve]

However, the 174 wavelength plate 4 uses a wavelength plate made of an anisotropic crystal material such as mica, as in the past. Therefore, AR cocoing is necessary to suppress reflection loss. Furthermore, it requires high-precision machining of the material, such as cutting angles, making it unsuitable for mass production. Therefore, it was difficult to reduce costs. The technical problem of the present invention is to solve these problems in conventional optical pickups,
The 174-wavelength plate can also be replaced with a hologram that is easy to create.

[Means for solving problems]

FIG. 1 is a side view illustrating the basic principle of an optical pickup according to the present invention. 4h is a 174-wavelength plate made of a surface relief type hologram, and is provided on the back surface of the glass plate 13. The surface relief grating is produced holographically using photoresist as a recording medium, or transferred and produced by beam etching using the same surface relief type resist hologram as a mask, or using the same surface relief type resist hologram as a master.
A copy made by injection molding or photopolymer method is used. On the front side of the glass plate 13, a hologram 11 for polarization separation, which is a surface relief type hologram similar to the conventional one, is provided. If the light incident from the semi-2IE body 1z-fLD is P-polarized, the direction of the grating groove of the polarization separation hologram 11 is perpendicular to the polarization plane of the P-polarized light, and the grating groove of the hologram wave plate 4h is the polarization separation hologram. It is inclined at 45 degrees with respect to the lattice groove of No. 11.

Both holograms 4h and 11 may be provided on a common radix 13, or may be provided on separate substrates and stacked back to back. 10 is a collimating hologram, and 12 is an objective hologram.

[Effect]

FIG. 2 (al~(C1 is a diagram explaining the operation of the optical pick amplifier according to the present invention, (al is a plan view of a 174-wave plate 4h made of a surface relief type hologram, (bl is a fa
bb sectional view in Figure 1, (C1 is a perspective view. (
C) As shown in the figure, the Y axis is the collimating hologram 1
1, and the X axis is perpendicular. On the other hand, the y-axis is
It is parallel to the groove 14 of the hologram wave plate 4h, and the X axis is perpendicular. The opening angle between the Y-axis and the y-axis (X-axis and X-axis) is 45 degrees. Further, ny is an equivalent refractive index for light whose incident polarization plane is parallel to the y-axis, and nx is an equivalent refractive index for light whose incident polarization plane is parallel to the x-axis. Such optical anisotropy found in surface relief gratings is called structural birefringence.

Assuming that the X-axis is parallel to the P-polarized light and the Y-axis is parallel to the S-polarized light, the laser beam collimates into the collimating hologram 1.
The polarization plane of the P-polarized light 15p after being collimated at zero is perpendicular to the direction of the grating grooves 16 of the polarization separation hologram 11, so that it is transmitted through the polarization separation hologram 11. Also P
Since the direction of the grating grooves 14 of the hologram wave plate 4h is inclined by 45 degrees with respect to the polarization plane of the polarized light 15p, the P polarized light 15
When the light p passes through the hologram wave plate 4h, it is rotated by 45 degrees and converted into circularly polarized light. The circularly polarized light enters the optical disk 6, and the reflected circularly polarized light enters the hologram wave plate 4h from the opposite direction. At this time, it is further rotated by 45 degrees and converted into S-polarized light. Since the S-polarized light is parallel to the direction of the grating grooves 16 (Y direction) of the polarization separation hologram 11, it is diffracted and guided to the detector 8.

〔Example〕

Next, examples will be used to explain how the optical pickup according to the present invention is actually implemented. The quarter-wave plate 4h made of a hologram uses a surface relief hologram created by exposing and developing a photoresist by causing two lights to interfere with each other.

Alternatively, a surface relief hologram can be reproduced as a mask or master.

In order to configure the wavelength plate 4h with a hologram, it is necessary to make the pitch of the surface relief finer than that of the collimating hologram 11, and to select the pitch and the depth of the grooves of the surface relief to predetermined values.

Next, these selection methods will be explained.

FIG. 3 shows the relationship between birefringence Δn and duty ratio in a rectangular relief, where the horizontal axis is the ratio of relief width W to pitch d, and the vertical axis is birefringence Δn. Birefringence Δn is large in a region where the w/d ratio is about 0.2 to 0.6, and since the refractive index differs depending on the material of the surface relief hologram, the birefringence Δn also differs depending on the material. For example, in the case of a photopolymer with a refractive index n of 1°5, the birefringence Δ
n is also small, forming a gentle curve. A photoresist suitable for a surface relief hologram has a refractive index of about 1.66.

In this way, the surface relief hologram material and w/
Once d is determined, the grating depth h and phase difference are determined from the characteristic diagram of FIG. FIG. 4 shows the relationship between the phase difference Δφ obtained by birefringence and the grating depth h, where the horizontal axis is the grating depth h and the vertical axis is the phase difference. Both characteristic lines show that as the grating depth h increases, the phase shift increases.

The characteristic line on the right side of the figure is for a photoresist with a refractive index of n=1.66, and the characteristic line on the left side is for a photoresist with a refractive index of n-2, OO
It is the material of Wavelength λ = 0.78 μm, grating pitch d =
At 0.25 μm, in order for the relief grating to act as a 174-wave plate, h = 1°22. for n = 1.66.
17 m, n=2. h=o for oo, 61 II
A grid depth of m is required. In this way, 〇 to birefringence is 0
．． When Δn is as large as 32, a 174-wave plate can be constructed with the grating depth h of the surface relief hologram shallow < 0.61 μm, but when Δn is as small as 0.16, the grating depth h is reduced to 1 The depth must be .22 μm.

FIG. 5 shows the relationship between the reflectance and the grating depth in the case of a rectangular relief hologram, where the horizontal axis is the ratio of the grating depth h to the grating pitch d, and the vertical axis is the reflectance (χ). ta) is for P-polarized light, and (b) is for S-polarized light. In either case, if the grating depth h is extremely small and the grating pitch d is extremely large and flat, that is, if h/d is O, then
The reflectance also reaches its maximum, but after that, the reflectance repeatedly increases and decreases periodically as h/d increases. Therefore, in order to reduce the reflectance for both P-polarized light and S-polarized light, a value of h/d with a small reflectance is selected in both (a) and (b).

The grating depth h = 1.22 p m (h
/d=4.9), the reflectance is 3.0χ at the arrow a2 position for S-polarized light, and 0.60χ at the arrow a position for P-polarized light. This reflectance can be further reduced by considering the relief shape.

Of course, even if the surface relief shape is rectangular,
It may have a sinusoidal shape or other cross-sectional shape.

FIG. 6 shows an example of a method for creating a surface relief type hologram. First, a photoresist 18 is laminated on the hologram material 17 (as shown in 81), and a diffraction grating 19 is created by exposing and developing the photoresist 17 using interference fringes. When the mask is subjected to beam etching as in tel, the lower side of the groove 20 of the mask 19 is deeply etched as shown by 21. The hologram 22 thus duplicated can also be used instead.

Alternatively, the mask hologram 19 or the duplicated hologram 22 can be used as a mold, and the molding can be performed by transfer using an injection molding method or a photopolymer method.

〔Effect of the invention〕

As mentioned above, by constructing the 174-wave plate with a surface relief hologram, it is extremely easy and inexpensive to manufacture compared to the conventional 174-wave plate that is processed with high precision from anisotropic crystal material. can.

In addition, by combining resist mask and beam etching,
Furthermore, effects such as being able to be transferred and produced on materials with high durability can be obtained. Moreover, it is also possible to reduce the reflectance to 1% or less, in which case an AR coat is unnecessary.

[Brief explanation of drawings]

FIG. 1 is a side view explaining the basic principle of the optical pickup according to the present invention, FIG. 2 is a diagram explaining the operation of the optical pickup according to the present invention, and FIG. 3 is a diagram showing birefringence and duty ratio in a rectangular relief. FIG. 4 is a diagram showing the relationship between phase difference and grating depth, FIG. 5 is a diagram showing the relationship between reflectance and grating depth, and FIG. 6 is a diagram showing an example of the method for creating a surface relief type hologram. Figure 7 is a side view of a conventional optical pickup amplifier, and Figures 8 and 9 are conventional holographic optical pickup amplifiers.
FIG. In the figure, LD is a semiconductor laser, 6 is an optical disk, and 8 is a semiconductor laser.
10 is a photodetector, 10 is a collimating hologram, 11 is a polarization separation hologram, 4h is a hologram wave plate, 14.
16 indicates a grating groove, respectively. Patent Applicant: Fujitsu Limited Agent, Patent Attorney Minoru Aoyagi ＼Cha≦I of January (Ichi Jsu Suppression Figure 1): Jibi Yui and [F] Sarashi (C) Jin l] To1 Imihiri1 no Iγ publication] Sakubd (χ) Koritsu! (%) *Stand y To create a grass relief hoggewam Figure 6 Figure 7

Claims (4)

[Claims] (1) An optical pickup that collimates semiconductor laser light, makes it incident on a detection surface via a wavelength plate, and guides the reflected light to the detector side using a polarization separation means, in which the reflected signal light from the detection surface is An optical pickup characterized in that a wave plate that changes the polarization state to separate the light is realized by a surface relief hologram (4H). (2) The optical pickup according to claim (1), wherein the surface relief hologram (4h) is produced by holographic exposure using photoresist as a recording medium. (3) Using the above wavelength plate as a mask using a holographically produced surface relief type resist hologram,
Claim No. 3 is characterized in that it is composed of a surface relief type grating transferred and produced by beam etching.
The optical pickup described in section 1). (4) A patent claim characterized in that the wavelength plate is constituted by a surface relief grating that is reproduced by an injection molding method, a 2P method, etc. using a holographically created surface relief resist hologram as a master. Range number (1)
Optical pickup described in section.