JPH0580736B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical head device that optically reads information from a recording surface of an information carrier, and particularly to an optical head device that is small and lightweight and suitable for mass production.

Conventionally, information carriers have been used to record information using changes in the refractive index or reflectance of the recording surface,
Various types of media, such as so-called magneto-optical recording media in which information is recorded magnetically, are known. When reading information from such information carriers, light is incident on these recording surfaces, and the reflected light from the recording surface is separated from the incident light by a light splitter installed in the optical path of the incident light. This is done by guiding it to a detector.
Generally, the light splitter is made up of two right angle prisms whose diagonal surfaces are joined. However, such a light splitter requires complicated processing and alignment adjustment during production, and has the disadvantage that it is difficult to reduce the cost.
Moreover, since the shape is almost cubic, it has been a factor that prevents the optical head device from being made thinner.

On the other hand, examples in which a volume type diffraction grating is used in place of the prism type light splitter are described in Japanese Patent Application Laid-Open No. 155508/1982 and US Pat. No. 3,622,220. The structure of an optical head device using such a volume type diffraction grating is shown in FIG. In Figure 1,
A parallel light beam 2 emitted from a light source section 1 including a laser light source and a collimator lens is
The light is incident on the volume type diffraction grating 3 placed at an angle of 45°. Here, the light beam 2 is S-polarized light having a vibration plane in a direction perpendicular to the plane of the drawing. Moreover, the volume type diffraction grating 3 is
If the wavelength of the incident light is λ, the pitch is approximately λ/
1.414 and deflects the beam 2 with a diffraction angle of 90°. At this time, the diffraction effect for S-polarized light is approximately
At 100%, the diffraction efficiency for P-polarized light is almost 0%. Therefore, most of the light beam 2 is diffracted and λ/
The light passes through the four-plate 4 and becomes circularly polarized light, which forms a spot on the information recording surface 7 of the optical disk 6 by the objective lens 5.

Information is recorded on the information recording surface 7 by changes in reflectance, etc., and the reflected light is modulated in light amount according to the information. This reflected light is transmitted through lens 5, λ/
The light passes through the four-plate 4 and becomes P-polarized light, which is not diffracted by the diffraction grating 3 and enters the photodetector 8, where the information is read.

However, in the configuration shown in Figure 1, the diffraction angle of the volume type diffraction grating is set to approximately 90°, so
It cannot be used to read information from magneto-optical recording media. The reason for this will be explained below. 1st
In the figure, when information is magnetically recorded on the information recording surface 7, the λ/4 plate 4 is removed from the optical path. The S-polarized light flux incident from the diffraction grating is reflected as a light flux whose plane of polarization is rotated (force-rotated) by the same amount in the opposite direction depending on whether the magnetization direction of the information recording surface 7 is upward or downward. Since this force-rotation angle is a minute angle of about 1 degree, the reflected light is mostly an S component and contains a small P component of different directions and equal magnitudes. Since the diffraction grating 3 has a diffraction angle of 90 degrees, most of the S component of the reflected light is diffracted in the direction of the light source section 1. On the other hand, only the P component generated by the force-rotation is transmitted as it is and enters the photodetector 8. As mentioned above, the P component depending on the magnetization direction has completely opposite directions and the same magnitude, so no matter what azimuth angle the analyzer is installed in front of the photodetector 8, the amount of light transmitted through it will be in the magnetization direction. It is the same regardless of the location, and no information can be read from it. As described above, a light splitter having appropriate polarization characteristics is essential for reading information from a magneto-optical recording medium, but such a light splitter cannot be realized with conventional configurations.

Furthermore, in conventional optical head devices, the incident light and the diffracted light form an angle of 90° with each other inside the volume diffraction grating. Therefore, in the normal form, the incident light is totally reflected on the surface of the diffraction grating, and although it is not shown in Fig. 1, in reality, as shown in the above-mentioned US patent specification, a volume type structure is formed between the right angle prisms. This required a configuration in which the diffraction grating was placed, and it was not possible to take full advantage of the small and lightweight characteristics of the diffraction grating.

An object of the present invention is to provide an optical head device that can efficiently read magnetically recorded information using a volume type diffraction grating.

The above objects of the present invention are achieved by using a volume diffraction grating with a diffraction angle in the range of 22° to 80° or 100° to 158° in the light splitter of the optical head device.

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

FIG. 2 shows the first part of the optical head device according to the present invention.
FIG. 2 is a schematic configuration diagram showing an example. The light emitted from the semiconductor laser 11 is turned into a parallel beam by the collimator lens 12 and enters the light splitter 13 . This light splitter 13 consists of two parallel flat plates 14 and 16.
and a volume type diffraction grating 15 formed therebetween. Further, the light from the semiconductor laser 11 is set to become P-polarized light with respect to the light splitter 13. The linearly polarized light (P-polarized light) transmitted through the light splitter 13 passes through the objective lens 18 and becomes a focused beam, forming a spot of about 1 μm on the recording surface 20 on which information is magnetically recorded via the substrate 19. Here, the reflected light reflected by the recording surface 20 is
The light is modulated into light whose polarization plane is rotated in the opposite direction depending on the recorded information (ie, due to a change in the magnetization direction). This reflected light is returned to the objective lens 18
The light beam enters the light splitter 13 through the beam, and is diffracted by the volume diffraction grating 15 at a predetermined angle θ in the range of 22° to 80° or 100° to 158°.

In other words, the volume diffraction grating 15 is set so that the incident light 26 satisfies the Bragg condition of the following equation.

λ=2ndcosθ ₀ λ: Wavelength of incident light n: Average refractive index of volume type diffraction grating d: Grating pitch θ ₀ : Bragg angle (θ ₀ = θ/2 θ: diffraction angle) Volume type diffraction grating 15 will be explained later. Power as stated -
Since the rotational component (S-polarized light) is set to have a predetermined diffraction efficiency higher than that of the incident light component (P-polarized light), the apparent force-rotation angle of the diffracted light increases. This diffracted light is transmitted to the analyzer 2
1, the rotation of the polarization plane is converted into a change in light intensity,
It is detected by a photodetector 23 via a sensor lens 22.

Note that the optical head device requires tracking control so that the spot always scans the track on the recording surface correctly, and focusing control that aligns the focal position of the objective lens with the recording surface. Such control can be performed by combining with conventionally known control methods. For example, if the sensor lens 22 is an anamorphic optical system and the photodetector 23 is a 4-split photodetector, the light intensity distribution of the light incident on the photodetector changes depending on the focusing state of the spot on the recording surface 20. However, by detecting this change on the divided light-receiving surfaces, a focus error signal can be obtained. This method is generally well known as the astigmatism method.

FIG. 3 is a partial cross-sectional view of the light splitter 13 used in the embodiment shown in the second figure. The volume diffraction grating 15 is sandwiched between parallel plates 14 and 16, and is composed of a layer 24 with a high refractive index and a layer 25 with a low refractive index. Incident light 26 from the recording surface is diffracted by the diffraction grating 15 and becomes diffracted light 27. If the diffraction grating 15 substantially satisfies the Bragg condition for the incident light 26, most of the energy of the diffracted light 27 will be concentrated in a predetermined direction as indicated by the diffraction angle θ. Furthermore, the diffraction efficiency of P-polarized light and S-polarized light changes depending on the setting of this diffraction angle θ. The present invention attempts to read magnetically recorded information with a high S/N ratio by utilizing the polarization characteristics of this volume type diffraction grating.

Now, assume that P-polarized light that is incident on the recording surface of a magneto-optical recording medium, undergoes Kerr rotation, and is reflected, is diffracted by a volume type diffraction grating and detected. At this time,
Since the force-rotation component generated by the magnetic force-effect of the recording surface is the S component, the higher the diffraction efficiency of the volume type diffraction grating for S-polarized light, the higher the detected signal S/N.
The ratio will improve. Further, the apparent force-rotation angle changes depending on the ratio of the P-polarized light and the S-polarized light that are diffracted.
Therefore, volume type diffraction grating Diffraction grating for S-polarized light |
r _s | Diffraction efficiency for P polarized light with ² as 100% |
By varying r _p | ² , the C/N ratio of the detected magneto-optical signal (S/N ratio at the center frequency of the signal
N ratio) was calculated. The results are shown in FIG. Here, the axis shows the diffraction efficiency for P-polarized light, and the vertical axis shows the C/N ratio of the detection signal in dB relative to the peak.
(decibel) displayed. In order to obtain a detection signal suitable for reading information, the decrease in the C/N ratio must be suppressed to within -6 dB from the maximum, which is acceptable.
The range of r _p | ² is from 0.03 to 0.86.

The polarization dependence of volume holograms is described in detail in the paper below.

H.Kogelnik“Coupled Wave Theory for
Thick Hologram Grating”: Bell Syst.Tech.
J.48 (1969) 2909−2947 A volume phase hologram without absorption, and
In the case of noslant (the grating is perpendicular to the substrate surface) and a Bragg angle of incidence of θ ₀ , the diffraction efficiency for S-polarized light is η _s = Sin ² (πn ₁ d/λ cos θ ₀ ) = Sin ² (κd /cosθ ₀ )...(1) Here, κ=(πn ₁ )/λ.

Next, the diffraction efficiency for P-polarized light is expressed as κ in equation (1) above.
By replacing κ 1 with κ ₁ , it is given by η _p =Sin ² (κ ₁ d/cosθ ₀ ).

Further, in the case of noslant (φ=π/2), κ ₁ =−κcos2(θ ₀ −φ) becomes κ ₁ =κcos2θ ₀ , and the diffraction efficiency for P-polarized light is expressed by the following equation.

η _p = Sin ² (κdcos2θ ₀ /cosθ ₀ ) …(2) Therefore, taking the ratio (2)/(1) of each diffraction efficiency, η _p /η _s = Sin ² (Acos2θ ₀ )/Sin ² (A) Here, A=κd/cosθ ₀ .

Add this formula to the sine series (SinX=X−X ³ /3!+
X ^5/5 ! −…) using the first-order approximation, η _p /η _s = (Acos2θ ₀ /A) ² = cos ² 2θ ₀ …(3), and the ratio of diffraction efficiency is the cos of twice the Bragg incident angle θ ₀ . It is expressed as a square.

By the way, in this specification, as the diffraction angle θ,
Since the incident light and the diffracted light form an angle as shown in FIG. 3, θ ₀ =θ/2.

Substituting this into the above equation (3) yields η _p /η _s = cos ² θ.

As described above, the ratio of the diffraction efficiencies of P-polarized light and S-polarized light in the volume diffraction grating is approximately expressed as cos ² θ using the diffraction angle θ. If θ=80° or θ=100°, cos ² θ=0.03, and θ=22° or θ=
If it is 158°, cos ² θ=0.86. Therefore, from the results shown in FIG. 4, a volume diffraction grating used as a light splitter for reading magnetic information must have a diffraction angle in the range of 22° to 80° or 100° to 158°.

In addition, in the above, the diffraction angle was determined from the permissible range of magneto-optical reading, but in order to detect the information signal well, the decrease in the C/N ratio is -3 dB as shown in Figure 4.
It is desirable that it be within the range. The P-polarized light diffraction efficiency at this time is 0.30 to 0.70, and as above, cos ² θ=
From 0.30, θ=72°, 108°, cos ² θ=0.7, θ=33°,
147° is required. Therefore, for reading magnetic information, it is more desirable to use a volume diffraction grating with a diffraction angle of 33° to 72° or 108° to 147°.

FIG. 5 is a diagram showing an example of a method for manufacturing a volume type diffraction grating used in the present invention. The light beam emitted from the laser light source 28 is reflected by a mirror 29 and then split by a beam splitter 30. divided 2
The light beams pass through mirrors 31 and 41 to microscope objective lenses 32 and 42 and collimator lenses 33 and 43, respectively.
The beam diameter is expanded by a beam expander system composed of the following, and becomes parallel light beams 34 and 44. These parallel light beams 34 and 44 are connected to the substrate 36
The light enters the volume hologram sensitive material 35 coated on top from different angles and interferes to form three-dimensional interference fringes. The interference fringes thus exposed to the volume hologram sensitive material 35 are recorded in the form of a change in refractive index through development processing, thereby forming a volume diffraction grating. Any hologram sensitive material can be used here, such as dichromate gelatin.

FIG. 6 is a schematic diagram showing a second embodiment of the optical head device of the present invention. The P-polarized light emitted from the semiconductor laser 51 is turned into a parallel light beam by the collimator lens 52, and enters the light splitter 53 composed of parallel flat plates 54 and 56 and a volume type diffraction grating 55. The linearly polarized light (P
The polarized light is focused by an objective lens 58 onto a spot having a diameter of about 1 μm on a recording surface 60 on a substrate 59. On the recording surface 60 on which information is magnetically recorded, the reflected light 57 whose polarization plane has been rotated (force-rotated) according to the information enters the light splitter 53 again, and is reflected by the volume type diffraction grating 55. It is diffracted at a predetermined diffraction angle in the range of 22° to 80° or 100° to 158°. This diffracted light 22 has an increased apparent force-rotation angle as in the first embodiment, and is guided while repeating total reflection on the surface of the parallel plate 54 or 56.
The light enters a four-split photodetector 63 provided on the end face of the light splitter 53. An analyzer 61 is provided immediately in front of the photodetector 63, and converts the magneto-optical signal into a change in the amount of light.

FIG. 7 shows a view of the light splitter 53 shown in FIG. 6 from the semiconductor laser side. The volume type diffraction grating 55 of this embodiment has a conical grating, and has a lens function to condense the diffracted light 62. Further, the four-split photodetector 63 has four light-receiving surfaces arranged in series in the direction of the paper. The light intensity distribution on the photodetector 63 changes depending on the focusing state of the spot on the recording surface. For example, the objective lens 58
When the focal position of the beam coincides with the recording surface 60, the reflected light 57 becomes parallel light, and the diffracted light 62 becomes a solid line in FIG.
It is incident in the shape shown in b. In addition, the objective lens 58
When the light comes too close to or too far away from the recording surface, the reflected light 57 becomes a diverging light or a convergent light, and the diffracted light 62 becomes a dotted line or a broken line, respectively, in FIG. 63, the shape is shown as 62c or 62a, respectively. The principle of detecting a focus error signal using such a change in the shape of the light beam will be explained in detail below.

Figures 8a, b, and c are views of the four-split photodetector 63 from the light incident side, with b showing the in-focus state and a and c showing the out-of-focus state. Here, 63a, 63b, 63
c and 63d indicate the divided light-receiving surfaces, and the shape of the incident light beam is as described above, 62a, 62b, 6
It changes to 2c. Light receiving surfaces 63a, 63b, 63
Letting the outputs from the differential amplifiers 63d and 63d be Ia, Ib, Ic, and Id, respectively, the differential amplifier 64 can be calculated by performing the calculation (Ib + Ic) - (Ia + Id) in the electrical system shown in Figure 9a. At the output terminal 65, a focus error signal as shown in FIG. 9b is obtained. In FIG. 9b, the horizontal axis shows the distance between the objective lens and the recording surface (focus error) when the in-focus position is zero, and the vertical axis shows the signal output. Autofocus is made possible by moving the objective lens 58 or the entire optical head relative to the disk along the optical axis of the incident light via an actuator (not shown) in accordance with the obtained focus error signal.

Next, the principle of auto-tracking in the embodiment shown in FIG. 6 will be explained. Assuming that a groove 59a is formed in the substrate 59 of the information carrier as shown in FIGS. 10a, b, and c, the incident light beam is focused near the groove 59a by the objective lens 58. Here b
In this case, a spot appears on the target groove,
Symbols a and c indicate that the spot occurs to the right or left of the groove, respectively. The light beam reflected by the recording surface 60 on the substrate 59 contains tracking information due to diffraction or scattering at the groove 59a. When the reflected light is received in the four-split photodetector 63 shown in FIG. It changes as shown in Figure 11 a, b, and c.
Therefore, by performing the calculation (Ia + Ib) - (Ic + Id) in the electrical system as shown in Figure 12a, the output terminal 67 of the differential amplifier 66 will have a signal as shown in Figure 12b. A tracking error signal is obtained. In FIG. 12b, the horizontal axis shows the tracking error and the vertical axis shows the signal output. Auto-tracking becomes possible by driving a tracking actuator (not shown) in accordance with the obtained tracking error signal and moving the objective lens perpendicular to the optical axis. Here, a case has been described in which a groove as a guide track is formed in advance on the substrate 59, but even if there is no such groove, a portion (recording track) on which information is recorded on the recording surface 60, The amount of light passing through the detector 61 differs from other parts due to the above-mentioned magneto-optic effect, and an imbalance occurs in the light amount distribution on the photodetector 63 depending on the positional relationship between the recording track and the spot. Therefore, even in such a case, as shown in FIG.
A tracking error signal can be similarly obtained by calculating the output of each light receiving surface.

Further, in the embodiment shown in FIG. 7, the diffracted light from the diffraction grating 55 is diffracted as a focused light, but a diffraction grating that produces a lens effect in this way may be used in the configuration shown in FIG. 3, for example. This can be achieved by making the grid into a conical shape. When using optical means to produce the diffraction grating 55, the first
An optical system as shown in Figure 3 can provide a focusing effect. In FIG. 13, parallel light beams 71 and 72 emitted from the same laser light source and split by an optical system (not shown) are incident parallel to the rotation axis 75 into conical mirrors 73 and 74, respectively, which share the rotation axis 75. do. 2 reflected by each conical mirror
The two light beams form a conical wavefront having a focal line on the rotation axis 75 and are incident on the hologram sensitive material 77 on the substrate 76 . At this time, the interference fringes generated in the region 78 on the surface of the photosensitive material have a three-dimensional conical shape with the rotation axis 75 as the center of rotation. Therefore, by developing the interference fringes thus exposed, a diffraction grating having a focusing effect as shown in FIG. 7 is formed.

As explained above, although the present invention is used for reading magnetically recorded information, the optical head device of the present invention can also be used as is for writing information to a magneto-optical recording medium. As is well known, a magneto-optical recording medium has a recording layer made of a magnetic film that is magnetized in one direction, and the direction of magnetization of the part heated to the Curie point by light irradiation is reversed. Information is recorded by. Therefore, in the configuration of the present invention described above, by making it possible to modulate the light beam from the light source in accordance with recorded information, it is possible to realize an optical head device that can be used for both writing and reading. Further, the present invention is not limited to the above-described embodiments, but can be applied to any optical head device using an optical splitter.

As is clear from the above description, the present invention provides an optical head device for reading magnetic information, by using a volume type diffraction grating having a predetermined diffraction angle in the light splitter, (1) S of the information signal to be read. /N ratio is improved.

(2) The optical splitter is made into a flat plate type, making it possible to make the optical head device smaller and lighter.

It has the following effects.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the configuration of a conventional optical head device, and FIG. 2 is a first diagram of the optical head device according to the present invention.
A schematic configuration diagram showing an embodiment, FIG. 3 is a partial cross-sectional view of a light splitter used in the present invention, and FIG. 4 shows the P-polarized light diffraction efficiency of the diffraction grating used in the present invention and the C/N of the detection signal.
FIG. 5 is a diagram illustrating an example of the method for manufacturing the volume type diffraction grating used in the first embodiment,
FIG. 6 is a schematic diagram showing the configuration of the second embodiment of the present invention, FIG. 7 is a diagram of the light splitter shown in FIG. 6 viewed from the semiconductor laser side, and FIG. 8 a, b, and c show focus errors. Figures 9a and 9b are diagrams showing the electrical system for focus error detection and the focus error signal, respectively. Figures 10a, b, and c are diagrams showing the light spot on the recording surface, respectively. Figures 11a, b, and c are diagrams showing changes in the amount of light on the photodetector, and Figures 12a and b are diagrams showing the electrical system for tracking error detection and the tracking error signal, respectively. , FIG. 13 is a diagram showing an example of a method for manufacturing a volume type diffraction grating used in the second example. 11... Semiconductor laser, 12... Collimator lens, 13... Light splitter, 14, 16... Parallel plate, 15... Volume type diffraction grating, 18... Objective lens, 19... Substrate, 20... Recording surface, 21...analyzer, 22...sensor lens, 23...photodetector.

Claims (1)

[Claims] 1. Light is incident on a recording surface on which information is magnetically recorded, and a light splitter placed in the optical path of the incident light guides the reflected light from the recording surface to a photodetector, producing a magneto-optical effect. In the optical head device that reads the information using An optical head device characterized by certain things.