JP4120686B2 - Optical head device - Google Patents

Description

  The present invention relates to an optical head device, and more particularly to an optical head device used for recording and reproducing information on an optical recording medium.

  In an optical head device that records and reproduces information on an optical recording medium such as a CD-R or CD-RW, the amount of light collected on the optical recording medium by an objective lens or the like is large when information is recorded. It is necessary to make it smaller during playback. Conventionally, in order to satisfy this need, the amount of light emitted from the semiconductor laser has been changed by changing the injection current to the semiconductor laser that emits light.

However, depending on the semiconductor laser used, when the injection current is reduced to reduce the amount of emitted light, there is a problem that noise increases or the amount of emitted light becomes unstable.
An object of the present invention is to provide an optical head device having excellent information recording and reproducing characteristics by changing the amount of light collected on an optical recording medium without changing the amount of light emitted from a semiconductor laser to be used. .

The present invention includes a light source, condensing means for condensing light emitted from the light source on the optical recording medium, and a photodetector for detecting reflected light from the optical recording medium of the condensed emitted light. In the optical head device, a liquid crystal element is disposed in the optical path between the light source and the light condensing means, and the liquid crystal element has a transparent electrode formed on each opposing surface of the two transparent substrates, and further between the transparent electrodes. The liquid crystal layer is sandwiched, the direction of the liquid crystal molecules is spirally twisted around the axis of the thickness direction of the liquid crystal layer, and a voltage control device is provided so that a voltage can be applied to the liquid crystal layer on the transparent electrode of the liquid crystal element. A polarizer is disposed in the optical path between the liquid crystal element and the optical recording medium, and the polarizer is configured to reduce the amount of light emitted toward the optical recording medium when light incident on the polarizer is emitted. It is changed according to the polarization direction of incident light, and it is a semiconductor laser as a light source. Without changing the output of The emitted light, when the light emission amount of the light source is constant, information of the light amount P ₁ of the optical recording medium to be condensed on the optical recording medium for reproducing information of an optical recording medium an optical head, characterized that you adjust the level of the applied voltage to the liquid crystal layer by the voltage controller as the ratio amount P ₂ to be focused is 0.2 to 0.8 for recording Providing equipment .

  In addition, one of the alignment directions of the liquid crystal molecules at the interface between the liquid crystal layer and the two transparent substrates in contact with the liquid crystal layer is parallel to or orthogonal to the polarization direction of the light incident on the liquid crystal element. The above-described optical head device is provided.

  Furthermore, the direction in which the angle formed by each of the two alignment directions of the liquid crystal molecules at the boundary surface between the liquid crystal layer and the two transparent substrates in contact with the liquid crystal layer is bisected is the polarization direction of light incident on the liquid crystal element. The above-described optical head device is provided that is parallel or orthogonal to the above.

  In the present invention, a liquid crystal element in which twisted nematic liquid crystal in which liquid crystal molecules are twisted and aligned between two transparent substrates constituting the liquid crystal element and a polarizer are combined with an optical head device. Arrange. At the time of this arrangement, the liquid crystal element is arranged on the light source side with respect to the polarizer. With this configuration, the amount of light on the optical recording medium can be changed without changing the output of the light emitted from the semiconductor laser, which is a light source, by simply applying a voltage to the electrodes formed on the liquid crystal element. Thus, an optical head device having excellent information recording and reproducing characteristics of the optical recording medium can be obtained. In particular, the reproduction characteristics are excellent and reproduction with low noise is possible.

FIG. 1 shows an example of an optical head device of the present invention. The light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2 and passes through the liquid crystal element 3. A voltage can be applied to the liquid crystal element 3 from the outside using the voltage control device 11. The light that has passed through the liquid crystal element 3 passes through the polarizer 4. The polarizer 4 changes the amount of light emitted toward the optical recording medium according to the polarization direction of the incident light when the light incident on the polarizer is emitted, and includes a polarization beam splitter, a prism, or a wire grating. A lattice or the like can be used. In the example of FIG. 1, a polarization beam splitter is used as the polarizer. Further, the polarizer 4 can be attached to and integrated with the transparent substrate on the condenser lens 5 side of the liquid crystal element 3.
The light transmitted through the polarizer 4 is transmitted through the λ / 4 plate 8 and then condensed on the optical recording medium 7 by the condenser lens 5 mounted on the actuator 6.

  The light reflected by the optical recording medium 7 travels in the reverse direction of the optical path. In the example of FIG. 1, since a polarizing beam splitter is used as the polarizer 4, the light traveling backward in the optical path is reflected by the polarizer 4 and then collected by the condenser lens 9 and reaches the photodetector 10. To do. At this time, by applying a plurality of different voltages to the liquid crystal element 3 (actually, to the transparent electrode formed on the surface of the transparent substrate constituting the liquid crystal element), the polarization direction of the light transmitted through the liquid crystal element 3 is changed. The amount of light that passes through the polarizer 4 and is condensed on the optical recording medium can be changed.

Ratio amount P ₂ which is focused for recording the information of the light amount P ₁ of the optical recording medium to be condensed on the optical recording medium for reproducing information of the optical recording medium is from 0.2 to 0. A value in the range of 8 is preferable, but a value in the range of 0.5 to 0.8 is more preferable. With this light quantity ratio, information can be recorded sufficiently by setting the light quantity to 100% when information is recorded on the optical recording medium, while the light quantity is in the range of 20 to 80% with respect to the light quantity during recording. Further, a value in the range of 50% to 80% is preferable because information can be read while taking a large S / N without writing information on the optical recording medium.

  In the optical head device of the present invention, the amount of light emitted from the semiconductor laser 1 is changed by passing through the polarizer 4. Therefore, in the forward path, the light passes through the liquid crystal element 3 before passing through the polarizer 4. Need to pass. That is, the liquid crystal element 3 needs to be disposed on the optical path semiconductor laser 1 side (light source side) with respect to the polarizer 4.

  The liquid crystal element according to the present invention shown in FIG. 2 includes, for example, transparent electrodes 103 and 104 such as ITO for applying voltage to the liquid crystal layer 108 on the transparent substrates 101 and 102 such as plastic and glass, and an alignment film such as polyimide. 105 and 106 are applied. After sealing the outer periphery of the liquid crystal element with a sealing material 107 such as acrylic resin or epoxy to form a cell, the liquid crystal is sealed to form a liquid crystal element. The liquid crystal used is, for example, a twisted nematic liquid crystal. The pretilt angle of the liquid crystal molecules is not particularly limited, but is preferably about 0 to 15 degrees, and particularly preferably 2 to 5 degrees from the viewpoint of the response speed and temperature characteristics of the liquid crystal.

  This liquid crystal element will be described with reference to FIG. 3 which is a plan view of FIG. 2 (viewed from the transparent substrate 101 to the transparent substrate 102). The liquid crystal element is formed by sealing the periphery of the transparent substrate with a sealant 205 (107), injecting liquid crystal to form a liquid crystal layer 204 (108), and then sealing the injection port with UV adhesive 203. The In addition, this liquid crystal element can apply a voltage to the liquid crystal layer from the outside via the electrode terminal extraction portion 206.

  The alignment directions 201 and 202 of the liquid crystal molecules on the boundary surface between the two transparent substrates 101 and 102 and the liquid crystal layer 204 are inclined by the pretilt angle but are substantially parallel to the transparent substrate surface and are shown in FIG. Are at an angle to each other. For this reason, the major axis direction of the liquid crystal molecules 109 sealed in the liquid crystal element has a structure that is continuously twisted around the axis of the liquid crystal layer 204 in the thickness direction between the transparent substrates 101 and 102. There is no particular limitation on the twisting angle, but a value in the range of 20 to 45 degrees is preferable so that the amount of light set by the voltage does not have a large temperature dependence and a fast response speed characteristic can be obtained.

  Particularly limited to the relationship between the polarization direction of incident light to the liquid crystal element 3, the alignment direction of liquid crystal molecules on each transparent substrate (interface between the transparent substrate and the liquid crystal layer) constituting the liquid crystal element, and the transmission axis direction of the polarizer However, when the alignment direction of the liquid crystal molecules on one transparent substrate is parallel or orthogonal to the polarization direction of the incident light to the liquid crystal element 3, the amount of light set by the voltage does not show a large temperature dependence. Highly preferred.

  For example, the polarization direction of the incident light to the liquid crystal element 3 and the liquid crystal molecule alignment direction on the transparent substrate on the semiconductor laser side of the liquid crystal element are aligned and arranged in parallel. The alignment direction of liquid crystal molecules is twisted, for example, 45 degrees parallel to the substrate surface between the two transparent substrates of the liquid crystal element. When the retardation value of the liquid crystal is adjusted and no voltage is applied to the liquid crystal layer, the polarization direction of the light transmitted through the liquid crystal element is rotated by 45 degrees with respect to the incident polarization direction.

  The direction of the transmission axis of the polarizer (the axis that gives the maximum transmittance when the polarization direction of the incident light coincides) is aligned with the polarization direction of the semiconductor laser. When a voltage is applied to the liquid crystal element, the alignment direction of the liquid crystal molecules is aligned with the electric field direction, so that the light from the semiconductor laser that has passed through the liquid crystal element is transmitted without changing the polarization direction. Therefore, the light can pass through the polarizer as it is and 100% of the light can be transmitted.

  When no voltage is applied, the light from the semiconductor laser that passes through the liquid crystal element changes its polarization direction due to the twisted liquid crystal, so that it deviates from the transmission axis direction of the polarizer and the amount of light that passes through the polarizer decreases. Therefore, by adjusting the magnitude of the voltage applied to the liquid crystal element, the amount of light transmitted through the polarizer can be adjusted without changing the output of the semiconductor laser.

  Further, when the direction that bisects the angle between the two alignment directions of the liquid crystal molecules on each transparent substrate is parallel or orthogonal to the polarization direction of the incident light to the liquid crystal element 3, The response of the modulated light quantity with respect to the applied voltage is improved, and further low voltage driving can be achieved.

The relationship between the voltage applied to the liquid crystal element and the amount of light collected on the optical recording medium will be described in detail. FIG. 9 is a graph showing the light intensity on the optical recording medium with respect to the voltage applied to the liquid crystal element in the present invention, using temperature as a parameter. The light intensity shows a minimum value at a minimum value voltage V _{L, and} at a voltage higher than V _L The light intensity increases monotonously.

  First, in order to increase the light intensity on the optical recording medium as much as possible, a voltage as high as possible can be applied to the liquid crystal element so that the polarization direction of the light passing through the liquid crystal element matches the transmission axis direction of the polarizer. preferable. However, when it is difficult to apply a high voltage due to the limitation of the voltage control device, it is preferable to apply from 4 Vrms to 6 Vrms, for example, because a light intensity of 90% or more is obtained.

Next, in order to weaken the light intensity on the optical recording medium, a voltage in the vicinity of _VL may be applied. However, _VL varies with the temperature of the liquid crystal element due to the temperature characteristics of the liquid crystal. FIG. 9 shows voltage characteristics at temperatures of 0, 25, 40, and 70 degrees, and V _L shifts to a lower voltage side as the temperature increases. When a liquid crystal element is used in an optical head device, it is not preferable that the light intensity varies with temperature.

  As a method of making the temperature dependence of the light intensity constant, there is a method of adjusting an applied voltage based on temperature information obtained by a temperature detection element such as a thermistor. Although this method can make the temperature dependence almost constant, it is necessary to add a temperature detecting means and a means for controlling the voltage according to the temperature, and the cost increases.

The second method is a method in which a voltage substantially equal to the minimum value voltage V _L at an approximately intermediate temperature in the operating temperature range of the optical head device is applied constant regardless of the temperature. The optimum value of the constant voltage varies depending on the element conditions such as the temperature characteristics of the liquid crystal material used and the twist angle of the liquid crystal, but ± 0 of the minimum voltage V _L at any temperature from 30 degrees to 45 degrees. What is necessary is just to determine in the range of .3Vrms. As shown in FIG. 9, it is difficult to completely suppress the temperature dependence of the light intensity by this method, but it is extremely effective as a method of keeping the temperature dependence low without using the temperature adjusting means.

  In the above description, 100% of light is transmitted when a voltage is applied. However, depending on the direction of the transmission axis of the polarizer, 100% of light can be transmitted when no voltage is applied.

"Example 1"
First, regarding the liquid crystal element, in FIGS. 4 and 5 (the liquid crystal element in FIG. 4 is viewed from the transparent substrate 301 to the transparent substrate 302), the glass transparent substrates 301 and 302 having a thickness of 0.53 mm are sputtered. A transparent conductive film made of ITO was formed to a thickness of 30 nm by the method and patterned by photolithography and wet etching to form transparent electrodes 303 and 304.

  On the transparent electrodes 303 and 304 of the transparent substrates 301 and 302, polyimide films 305 and 306 having a thickness of about 50 nm were applied by a flexographic printing method and then baked. The polyimide films 305 and 306 were rubbed with a cloth. At this time, the liquid crystal molecules 309 are twisted around the axis in the thickness direction of the liquid crystal layer 308 so that the alignment directions 401 and 402 of the liquid crystal molecules on each transparent substrate surface form an angle of 25 degrees. .

  An epoxy sealant 307 (405) was printed on the transparent substrate 301 by a screen printing method. The epoxy sealant 307 (405) has a fiber spacer with a diameter of 3 μm for maintaining a predetermined cell gap (3% in terms of mass ratio, the same applies hereinafter), and conductivity between the transparent substrates 301 and 302. In order to obtain the properties, 2% of acrylic spheres having a diameter of 3.5 μm having a conductive coating on the surface were mixed.

After the transparent substrates 301 and 302 were overlapped and aligned, a cell was formed by pressure bonding at a temperature of 170 ° C. and a pressure of 6 × 10 ⁴ N / m ² . A liquid crystal having a difference Δn = 0.1 between ordinary light refractive index and extraordinary light refractive index is injected into the prepared cell by a vacuum injection method to form a liquid crystal layer 308 (404), and the injection port is sealed with a UV adhesive 403. A liquid crystal element having an outer shape of 8 mm × 10 mm square was prepared. This liquid crystal element can apply a voltage to the liquid crystal layer from the outside via the electrode terminal extraction portion 406.

  The produced liquid crystal element was incorporated into an optical head device as shown in FIG. The liquid crystal element 3 was installed between a polarization beam splitter, which is a polarizer 4 disposed in the optical head device, and the collimating lens 2, and this liquid crystal element was controlled by an output voltage from the voltage control device 11. When no voltage is applied to the liquid crystal element, the alignment direction of the liquid crystal molecules on the semiconductor laser side transparent substrate of the liquid crystal element and the transmission axis direction of the polarization beam splitter are both coincident with the polarization direction of the incident light to the liquid crystal element. Arranged.

  Light emitted from the semiconductor laser 1 is transmitted in the order of the collimator lens 2, the liquid crystal element 3, the polarizing beam splitter as the polarizer 4, and the λ / 4 plate 8, and then transmitted through the condenser lens 5 held by the actuator 6. It is condensed on the optical recording medium 7. The condensed light is reflected by the optical recording medium 7, passes through the condenser lens 5 and the λ / 4 plate 8 in this order, and after the polarization direction is changed by 90 degrees, is reflected by the polarization beam splitter and is reflected by the condenser lens 9. It was led to the detector 10.

  At this time, when a voltage was applied to the liquid crystal element 3 and a photodetector was placed at the position of the optical recording medium 7 and the amount of the collected light was measured, the voltage applied to the liquid crystal element was 0 V as shown in FIG. 79% when the voltage is 5V, and 99% when the voltage is 5V (the amount of light emitted from the semiconductor laser is 100%), and the amount of light collected on the optical recording medium 7 is changed by the voltage applied to the liquid crystal. I was able to.

  When information is recorded on the optical recording medium using this optical head device, a voltage of 5 V is applied to the liquid crystal element 3 so that the amount of light collected on the optical recording medium 7 is 99%. When reproducing information, the output of the semiconductor laser 1 is not changed, and the amount of light collected on the optical recording medium 7 without applying a voltage to the liquid crystal element 3 is 79%. Thus, information was reproduced with low noise.

"Example 2"
A liquid crystal element having an outer dimension of 8 mm × 10 mm was prepared by the same procedure as described in Example 1 (FIG. 8). However, the orientation directions 801 and 802 of the liquid crystal molecules on the upper and lower transparent substrate surfaces are at an angle of 43 degrees. The produced liquid crystal element was incorporated in the optical head device as shown in FIG. At this time, the polarization direction 803 of the incident light to the liquid crystal element and the transmission axis direction of the polarization beam splitter are both arranged to coincide with the direction that bisects the angle formed by each alignment direction of the liquid crystal molecules. 8 that are the same as those in FIG. 5 indicate the same elements as those in FIG.

  Light emitted from the semiconductor laser 1 is transmitted in the order of the collimator lens 2, the liquid crystal element 3, the polarizing beam splitter as the polarizer 4, and the λ / 4 plate 8, and then transmitted through the condenser lens 5 held by the actuator 6. It is condensed on the optical recording medium 7. The condensed light is reflected by the optical recording medium 7, passes through the condenser lens 5 and the λ / 4 plate 8 in this order, and after the polarization direction is changed by 90 degrees, is reflected by the polarization beam splitter and is reflected by the condenser lens 9. It was led to the detector 10.

  When a voltage was applied to the liquid crystal element 3 and a light detector was placed at the position of the optical recording medium 7 to measure the amount of the collected light, a change in the amount of light with respect to the voltage applied to the liquid crystal element was observed as shown in FIG. It was done. Since the operating temperature range of the optical head device fabricated this time was from 0 to 70 degrees Celsius, the voltage that gives the minimum value of the light amount at the position of the optical recording medium 7 at 40 degrees Celsius, which is almost in the middle of these temperatures. Two voltages, 2 V and 4 V that is saturated and maximum, were used as drive voltages. For this reason, as described in the second method, the temperature fluctuation of the liquid crystal element at the time of low transmitted light amount when driven at 1.2 V can be suppressed. The result of suppressing the temperature fluctuation is shown in FIG. In addition, since the polarization direction of the incident light to the liquid crystal element and the transmission axis direction of the polarization beam splitter are arranged so as to coincide with the direction that bisects the angle formed by the alignment directions of the liquid crystal molecules, the voltage is as low as 4V. It was possible to obtain a light amount of almost 100%.

  When information is recorded on the optical recording medium using this optical head device, a voltage of 4 V is applied to the liquid crystal element 3 to increase the amount of light collected on the optical recording medium 7, and information is also transmitted from the optical recording medium 7. Is reproduced, the output of the semiconductor laser 1 is not changed, and a voltage of 1.2 V is applied to the liquid crystal element 3 to reduce the amount of light collected on the optical recording medium 7 to approximately 40%, thereby reducing the noise. The information was played back.

  In the optical head device of the present invention, the amount of light on the optical recording medium is changed without changing the output of the emitted light of the semiconductor laser, which is the light source, by simply applying a voltage to the electrode formed on the liquid crystal element. Thus, an optical head device having excellent information recording and reproducing characteristics of the optical recording medium can be obtained.

1 is a conceptual diagram illustrating an example of an optical head device of the present invention. The side view which shows an example of the liquid crystal element in this invention. The top view of the liquid crystal element of FIG. 2 is a side view showing a liquid crystal element in Example 1. FIG. The top view of the liquid crystal element of FIG. 6 is a graph showing the relationship between the voltage applied to the liquid crystal element and the light intensity on the optical recording medium in the optical head device of Example 1. 4 is a graph showing an example of the relationship between the voltage applied to the liquid crystal element and the light intensity on the optical recording medium in the optical head device of the present invention. FIG. 6 is a plan view showing a liquid crystal element in Example 2. 6 is a graph showing the relationship between the voltage applied to the liquid crystal element and the light intensity on the optical recording medium in Example 2 using temperature as a parameter. 6 is a graph showing the relationship between the temperature and the light intensity on the optical recording medium in Example 2.

Explanation of symbols

1: Semiconductor laser 2: Collimating lens 3: Liquid crystal element 4: Polarizer 5: Condensing lens 6: Actuator 7: Optical recording medium 8: λ / 4 plate 9: Condensing lens 10: Photo detector 11: Voltage control device 101, 102, 301, 302: Transparent substrates 103, 104, 303, 304: Transparent electrodes 105, 106: Alignment films 107, 205, 307, 405: Sealing materials 108, 204, 308, 404: Liquid crystal layers 109, 309: Liquid crystal molecules 201, 202, 401, 402, 801, 802: liquid crystal molecule orientation directions 305, 306: polyimide film 203, 403: UV adhesive 206, 406: electrode terminal extraction portion 803: polarization direction of incident light

Claims (3)

In an optical head device comprising a light source, condensing means for condensing light emitted from the light source on an optical recording medium, and a photodetector for detecting reflected light from the optical recording medium of the condensed emitted light ,
A liquid crystal element is disposed in the optical path between the light source and the light condensing means. The liquid crystal element has a transparent electrode formed on each opposing surface of the two transparent substrates, and a liquid crystal layer is sandwiched between the transparent electrodes. , The direction of the liquid crystal molecules are spirally twisted around the axis of the thickness direction of the liquid crystal layer,
A voltage control device is connected to the transparent electrode of the liquid crystal element so that a voltage can be applied to the liquid crystal layer, and a polarizer is disposed in the optical path between the liquid crystal element and the optical recording medium,
This polarizer changes the amount of light emitted toward the optical recording medium when the light incident on the polarizer is emitted, depending on the polarization direction of the incident light, and changes the output of the emitted light from the semiconductor laser as the light source. without let, the ratio amount P ₂ which is focused for recording the information of the light amount P ₁ of the optical recording medium to be condensed on the optical recording medium for reproducing information of the optical recording medium is 0.2 an optical head apparatus characterized that you adjust the level of the applied voltage to the liquid crystal layer by the voltage control unit so as to 0.8. One of the alignment directions of the liquid crystal molecules at the interface between the liquid crystal layer and the two transparent substrates in contact with the liquid crystal layer is parallel to or orthogonal to the polarization direction of the light incident on the liquid crystal element. the optical head apparatus according to claim 1 Symbol placement are. The direction that bisects the angle formed by the two alignment directions of the liquid crystal molecules at the interface between the liquid crystal layer and the two transparent substrates in contact with the liquid crystal layer is parallel to the polarization direction of the light incident on the liquid crystal element. the optical head apparatus according to claim 1 Symbol placement has or is orthogonal with.

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