JPH041940A - Optical head device - Google Patents

Abstract

PURPOSE:To obtain an optical head device which has a position error of an optical spot smaller than the allowable value on an optical recording medium by using two specific prisms. CONSTITUTION:A semiconductor laser 1 is provided together with a collimator lens 2, the 1st and 2nd prisms 4 and 5, on objective lens 10, and a photodetector 22. The incident and refracted angles are referred to as PHI1 and theta1 on a 1st incident surface of a light beam with the incident and refracted angles referred to as PHI2 and theta2 on a 2nd incident surface respectively. Then the increments of the refraction rates of both prisms 4 and 5 are referred to as DELTAn1 and DELTAn2 respectively when the wavelength of the light beam increases by 1mm at a point near the center wavelength. Under such conditions, the conditions of an equation I are satisfied. As a result, the angle error value due to the change of the wavelength of the light beam is reduced in a prism radiation state. Then the position error of an optical spot can be reduced less than the allowable value on an optical recording medium 12.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical head device for recording and reproducing information on an optical recording medium.

(Prior Art) In an optical head device for recording and reproducing information on an optical recording medium, a light beam emitted as diverging light from a semiconductor laser as a light source usually has a radiation angle in a direction parallel to the p-n junction surface. is narrower than the radiation angle in the direction perpendicular to the pn junction surface. In order to form a minute spot with a diameter of about 1 pm on an optical recording medium, it is necessary to convert the cross-sectional shape of this light beam into a substantially circular shape and make it incident on the objective lens. In read-only optical head devices, a collimator lens with a small numerical aperture is used to convert the light beam emitted from a semiconductor laser into parallel light. A commonly used method is to obtain a light beam.

However, this method suffers from a large loss in the amount of light in the collimator lens, and only a low optical output can be obtained on the optical recording medium. is not applicable. In these optical head devices, a collimator lens with a large numerical aperture is used in order to reduce the loss of light amount in the collimator lens. In this case, the cross-sectional shape of the light beam transmitted through the collimator lens becomes an ellipse whose short axis is parallel to the pn junction surface of the semiconductor laser and whose long axis is perpendicular to the pn junction surface. Therefore, a means is required for enlarging the diameter of this ellipse in the minor axis direction to obtain a light beam having a substantially circular cross-sectional shape. As such means, for example,
A prism described in Japanese Patent No. 53775 is known.

FIG. 5 shows an example of the configuration of a conventional magneto-optical optical head device using this prism. A light beam emitted as diverging light from a semiconductor laser 1 as a light source is converted into parallel light by a collimator lens 2, and then enters a prism 3. The entrance surface of this prism 3 is perpendicular to the p-n junction surface of the semiconductor laser 1, and is provided so that the incident light beam is refracted by a predetermined angle, and the exit surface is such that the light beam refracted by the entrance surface is approximately It is installed so that it emits vertically. After passing through the prism 3, the light beam is enlarged only in its diameter in the direction parallel to the pn junction surface of the semiconductor laser 1 (in the direction parallel to the plane of the paper in FIG. 5), and its cross-sectional shape becomes approximately circular. At this time, if the refractive index of the prism 3 with respect to the wavelength of the incident optical beam is n, and the magnification rate of the diameter of the optical beam in the prism 3 in the direction parallel to the p-n junction surface is k, then the incident angle φ on the incident surface and The refraction angle θ (θ is equal to the apex angle of the prism 3) may be determined to satisfy the following. For example, if the wavelength of the light beam is 780 nm and 2.0, and if BK7 is used as the material for the prism 3, then n=1.510722, so φ=66.6° and θ=37.4°. Become. The light beam whose cross-sectional shape has been converted into a substantially circular shape by the prism 3 passes through the beam splitters 6 and 7, and then
It is converted into convergent light by the objective lens 10, and the magneto-optical recording medium 1
1. The reflected light from the magneto-optical recording medium 11 passes through the objective lens 10 in the opposite direction and is converted into parallel light again. A part of the light is reflected by the beam splitter 7 and the rest passes through the beam splitter 7. The reflected light from the beam splitter passes through the 1/2 wavelength plate 18 and the plane of polarization becomes 45°.
After rotation, the polarizing beam splitter 19 separates the light into transmitted light and reflected light.

The transmitted light and reflected light of the polarizing beam splitter 19 are converted into convergent light by converging lenses 20 and 21, respectively, and then received by photodetectors 22 and 23. Information recorded on the magneto-optical recording medium 11 is reproduced by taking the difference between the outputs from the photodetectors 22 and 23. on the other hand,
A portion of the light transmitted through the beam splitter 7 is reflected by the beam splitter 6. The reflected light from the beam splitter 6 is converted into convergent light by a converging lens 13, and then converted to a convergent light by a half mirror 14.
The light is separated into transmitted light and reflected light. The reflected light from the half mirror 14 is received by a two-split photodetector 16 via a knife edge 15.

By taking the difference between the outputs from the two light-receiving surfaces of the two-split photodetector 16, a focus error signal is detected using the principle of the known Knifezi method. The transmitted light of the half mirror 14 is directly received by the two-split photodetector 17. By taking the difference between the outputs from the two light-receiving surfaces of the two-split photodetector 17, a track error signal is detected using the principle of the known push-pull method.

FIG. 6 shows an example of the configuration of a conventional magneto-optical optical head device using two prisms similar to those shown in FIG. Here the fifth
The prism 3 in the figure has been replaced by a first prism 4 and a second prism 5 of the same material and shape. The entrance surfaces of the first prism 4 and the second prism 5 are both perpendicular to the pn junction surface of the semiconductor laser 1, and are provided so that the incident light beams are refracted by a predetermined angle in opposite directions, The exit surfaces are both provided so that the light beam refracted by the entrance surface exits substantially perpendicularly. The refractive indices of the first prism 4 and the second prism 5 with respect to the wavelength of the incident light beam are nl and n2, respectively, and the p-n junction surface of the diameter of the light beam in the first prism 4 and the second prism 5. The magnification factor in the parallel direction is 1 for each
, k2, the incident angle φ□ and the refraction angle θ0 at the entrance surface of the first prism 4, the incidence angle Φ2 and the refraction angle θ2 at the entrance surface of the second prism 5 (θ1 and θ2 are respectively the first prism 4 and the apex angle of the second prism 5) may be determined to satisfy the following. For example, if the wavelength of the light beam is 780 nm, 1 = 2.0, k1 = 2, and if BK7 is used as the material for both the first prism 4 and the second prism 5, then n1 = n2 = 1
．． 510722, so φ1=φ2=53.2° and θ1=θ2: 32.0°. In this case, the light beam that enters the first prism 4 and the light beam that exits from the second prism 5 are approximately parallel, making the design of the optical head device easier than in FIG. 5. The configuration of other parts is the same as that in FIG. 5, so the explanation will be omitted.

(Problems to be Solved by the Invention) In the conventional optical head device described above, when the wavelength of the light beam incident on the prism changes due to ambient temperature or the like, the refractive index of the prism also changes accordingly. Therefore, an angular deviation occurs in the light beam emitted from the prism,
Accordingly, there is a problem in that a positional shift occurs in the light spot formed on the optical recording medium. p- of semiconductor laser
When the direction parallel to the n-junction plane is perpendicular to the track of the optical recording medium, the positional shift of the optical spot is observed as a track offset. In this case, the allowable amount of positional deviation is relatively large, about ±0.1 pm. On the other hand, when the direction parallel to the pn junction plane of the semiconductor laser is parallel to the track of the optical recording medium, the positional shift of the optical spot is observed as jitter of the reproduced signal. The tolerance for positional deviation in this case is quite small. For example, when recording is performed on a 5.25-inch optical recording medium with 2-7 modulation, the shortest pit length at the innermost circumference is 0.7611 m. If the tolerance for the jitter of the reproduced signal is ±2%, the tolerance for the positional deviation of the optical spot is ±0.015 μm.

FIG. 3 shows how the prism 3 of the optical head device shown in FIG. 5 changes in angle when it is emitted from the prism due to a change in the wavelength of the light beam. When the wavelength of the light beam is 1n near the center wavelength
The amount of increase in the refractive index of the prism 3 when m increases is Δn,
If the amount of angular deviation at the time of prism emission at that time is α, then a=lΔntanθ1 holds true.

For example, if the center wavelength of the light beam is 780 nm, the prism 3
The material is BK7, the angle of incidence and angle of refraction are Φ = 66.6°
, when θ=37.4°, Δn=−2,09X10=, so α=1.60X10−5. Here, assuming that the amount of change in the wavelength of the light beam is 5 nm and the focal length of the objective lens 10 is 3.0 mm, the amount of positional deviation of the light spot on the optical recording medium is 0.2411 m, which greatly exceeds the allowable amount.

FIG. 4 shows the angular deviation in the first prism 4 and second prism 5 of the optical head device shown in FIG. 6 when the prisms are emitted due to a change in the wavelength of the light beam. The amount of increase in the refractive index of the first prism 4 and the second prism 5 when the wavelength of the light beam increases by 1 nm near the center wavelength is Δn0, Δ-1, respectively, and the amount of angular shift when the prism exits at that time is If α, then holds true.

For example, the center wavelength of the light beam is 780 nm, the materials of the first prism 4 and the second prism 5 are both BK7, the incident angle and the refraction angle are φ□=φ2=53.2°, θ1=θ2:32.0 In the case of .DELTA.n1=.DELTA.n2=-2.09 0-5, α=3.83X10-6. Here, similarly, the amount of change in the wavelength of the light beam is set to 5n
m, and the focal length of the objective lens 10 is 3.0 mm,
The amount of positional deviation of the light spot on the optical recording medium is 0.057
It becomes pm. In this way, by using two prisms, the amount of positional deviation can be made considerably smaller, but it still exceeds the allowable value.

The purpose of the present invention is to solve such conventional problems, and to reduce the amount of angular deviation when the light beam is emitted from the prism due to wavelength changes, and as a result, the amount of positional deviation of the light spot on the optical recording medium is tolerable. The object of the present invention is to provide an optical head device that is small in size or less.

(Means for Solving the Problems) An optical head device of the present invention includes a semiconductor laser, a collimator lens that converts a light beam emitted from the semiconductor laser as a diverging light into parallel light, and a collimator lens that converts the light beam into parallel light. a first prism having a first entrance surface into which the light beam is incident and a first exit surface from which the light beam exits, the first entrance surface being the p-
The first exit surface is perpendicular to the n-junction surface and is provided so that the incident light beam is refracted by a predetermined angle, and the first exit surface allows the light beam refracted by the first entrance surface to exit approximately perpendicularly. a second prism having a first prism provided as shown in FIG. , the second incident surface is perpendicular to the p-n junction surface of the semiconductor laser and is provided so that the incident light beam is refracted by a predetermined angle in the opposite direction to the first incident surface, and The second exit surface includes a second prism provided so that the light beam refracted by the second entrance surface exits substantially perpendicularly, and a second prism that converts the light beam emitted from the second prism into convergent light. An optical head device comprising: an objective lens that converts and irradiates onto an optical recording medium; and a photodetector that receives a light beam reflected from the optical recording medium, the optical head device comprising: The first prism when the angle and refraction angle are Φ1 and θ1, respectively, the incidence angle and refraction angle at the second incident surface are Φ2 and θ2, respectively, and the wavelength of the light beam increases by 1 nm in the vicinity of the center wavelength. When the amount of increase in the refractive index of the second prism is Δn1 and Δn2, respectively, the following is satisfied.

(Function) By satisfying the following conditions, the optical head device of the present invention can change the position of the light spot on the optical recording medium, assuming that the amount of change in the wavelength of the light beam is 5 nm and the focal length of the objective lens is 3.0 mm. The amount of deviation is 0.0
15 pm, which can be kept below the allowable value. Furthermore, by satisfying the condition φ1-θ1=Φ2-θ2, the light beam entering the first prism and the light beam exiting from the second prism become approximately parallel, so as in the case of Fig. 6, Designing an optical head device is easy.

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

FIG. 1 shows the structure of an embodiment of a magneto-optical optical head device of the present invention. The configuration of the parts other than the first prism 4 and the second prism 5 is the same as that shown in FIG. 6, so a description thereof will be omitted. In Figure 1, for example, the wavelength of the light beam is 780n.
In the case where m is 1 and 2 = 2.0, if BaSF4 is used as the material for the first prism 4 and BK7 is used as the material for the second prism 5, n1 =
1.640350, n = 1.510722, so Φ = 49.9°, Φ2 = 54.9°, θ1 = 27.8
°, θ2=32.8°. At this time Δn1=-3.74 mowing O1 Δn2==-2,09 mowing 0-
5, the amount of angular deviation at the time of prism output is α=1.
25 cut 0-8 and total 0-8 Rui, if F8 is used as the material of the first prism 4 and BK7 is used as the material of the second prism 5, nl =
1.585694, n2=1.510722, so Φ1=51°1°, Φ2=54.3°, θ1=29.4
°, θ2=32.5'. At this time Δn1=-3, 38×10, Δn2:== 2-09
Since it is X10-5, the amount of angular deviation at the time of prism output is α=1.26X10-7.

BaSF4 has the problem of being a little expensive because it is not commonly used, but F8 does not have that problem.

Also, for example, when the wavelength of the light beam is 780 nm, 1 = 2 =
2.5, if 8F9 is used as the material of the first prism 4 and BK7 is used as the material of the second prism 5, n1=1
．． 642089, n2:=: 1-510722, so φ1=55.2°, φ2=60.3°, θ□=30.0
°, θ2=35.1°. At this time, M=-4,19X10, Shima=-2,09X10-5
Therefore, the amount of angular deviation at the time of prism exit is α = 4.1
It becomes 2X10-8.

Also, for example, when the wavelength of the light beam is 830 nm, 1 = 2 =
2.0, if F3 is used as the material for the first prism 4 and BK7 is used as the material for the second prism 5, n□=1.
600638, -=1.509744, so φ, = 50.7°, φ2 = 54.4°, θ = 28.9°
, θ2=32.6°. At this time, M=-3,10X10, Shima=-1,84X10
-5, the amount of angular deviation at the time of prism exit is α = 4
．． It becomes 88xlO-8.

Also, for example, when the wavelength of the light beam is 830 nm, 1 = 2 =
2.5, if SF5 is used as the material of the first prism 4 and BK7 is used as the material of the second prism 5, n1=1
．． 657424, -= 1.509744, so φ1=55.0°, φ2=60.5°, θ1=29.6
°, θ2=35.2°. At this time,: -3,76X10, Shima = -1,84X10
-5, the amount of angular deviation at the time of prism exit is α = 1
．． 04xlO-7.

FIG. 2 shows the structure of an embodiment of the write once type or phase change type optical head device of the present invention. The first prism 4 and the second prism 5 are the same as those shown in FIG. 1. The light beam whose cross-sectional shape has been converted into a substantially circular shape by the first prism 4 and the second prism 5 passes through the polarizing beam splitter 8 and the quarter-wave plate 9, and then is converted into convergent light by the objective lens 10. The light is irradiated onto a write-once type or phase change type optical recording medium 12.

The reflected light from the optical recording medium 12 passes through the objective lens 10 in the opposite direction, is converted into parallel light again, passes through the quarter-wave plate 9, and is then completely reflected by the polarizing beam splitter 8.

The reflected light from the polarizing beam splitter 8 is converted into convergent light by a converging lens 13, and then separated into transmitted light and reflected light by a half mirror 14. The reflected light from the half mirror 14 is received by a two-split photodetector 16 via a knife 15.

By taking the difference between the outputs from the two light-receiving surfaces of the two-split photodetector 16, a focus error signal is detected using the principle of the known Knifezi method. The transmitted light of the half mirror 14 is directly received by the two-split photodetector 17. By taking the difference between the outputs from the two light-receiving surfaces of the two-split photodetector 17, a track error signal is detected using the principle of the known push-pull method. Furthermore, by summing the outputs from the two light-receiving surfaces of the two-split photodetector 17, information recorded on the optical recording medium 12 is reproduced.

In the embodiments described above, BK7 is used as the material of the second prism 5, so if the material of the beam splitter 6 in FIG. 1 and the polarizing beam splitter 8 in FIG. It is also possible to integrate these with the second prism 5. Of course, materials other than BK7 may be used as the material for the second prism 5.

(Effects of the Invention) As described above, according to the present invention, the amount of angular deviation when the light beam is emitted from the prism due to the wavelength change is small, and as a result, the amount of positional deviation of the light spot on the optical recording medium is small. can be suppressed to below a permissible value, and since the light beam incident on the first prism and the light beam emitted from the second prism are substantially parallel, it is possible to realize an optical head device that is easy to design.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of a magneto-optical optical head device of the present invention, FIG. 2 is a diagram showing the configuration of an embodiment of a write-once type or phase change type optical head device of the present invention, and FIG. 5 is a diagram showing the angular deviation when the prism is emitted due to a change in the wavelength of the light beam in the prism of the optical head device shown in FIG. 5, and FIG. Fig. 5 is a diagram showing an example of the configuration of a conventional magneto-optical optical head device using a prism;
FIG. 6 is a diagram showing an example of the configuration of a conventional magneto-optical optical head device using two prisms. In the figure, 11.・Semiconductor laser, 2...Collimator lens, 3
... Prism, 4... First prism, 5... Second prism, 6, 7... Beam splitter, 8...
・Polarizing beam splitter, 9...174 wavelength plate, 10
...Objective lens, 11...Magneto-optical recording medium, 12.
... Optical recording medium, 13... Converging lens, 14... Half mirror 115... Naifetsuji, 16, 17...
・2-split photodetector, 18...172 wavelength plate, 19...
- Polarizing beam splitter, 20, 21... Converging lens, 22.23... Photodetector. FIG. 1 1. Semiconductor laser 2: collimator lens 4, first prism (BaSF4) 10. objective lens 11: magneto-optical recording medium

Claims (1)

[Claims] A semiconductor laser, a collimator lens that converts a light beam emitted from the semiconductor laser as diverging light into parallel light,
A first prism having a first entrance surface into which the light beam converted into parallel light by the collimator lens enters, and a first exit surface through which the light beam exits, the first entrance surface being the The first exit surface is perpendicular to the p-n junction surface of the semiconductor laser and is provided so that the incident light beam is refracted by a predetermined angle, and the first exit surface is provided so that the light beam refracted by the first entrance surface is a second prism having a first prism provided to emit light approximately perpendicularly; a second incident surface into which the light beam emitted from the first prism enters; and a second exit surface from which the light beam exits. The prism is such that the second incident surface is perpendicular to the p-n junction surface of the semiconductor laser, and the incident light beam is refracted by a predetermined angle in the opposite direction to the first incident surface. a second prism provided at the second prism, and the second exit surface is provided so that the light beam refracted by the second entrance surface exits substantially perpendicularly; and the light emitted from the second prism. In an optical head device including an objective lens that converts a beam into a convergent light and irradiates it onto an optical recording medium, and a photodetector that receives a light beam reflected from the optical recording medium, the first incident of the light beam The incident angle and refraction angle on the surface are Φ_1 and Θ_1, respectively, and the incidence angle and refraction angle on the second incidence surface are Φ_2, respectively.
, Θ_2, and when the wavelength of the light beam increases by 1 nm near the center wavelength, the amount of increase in the refractive index of the first prism and the second prism is Δn_1 and Δn_1, respectively.
n_2, Φ_1-Θ_1=Φ_2-Θ_2 and |Δn_2tanΘ_2-Δn_1tanΘ_1(co
An optical head device characterized by satisfying sΦ_2/cosΘ_2)|<10^-^6.