JP2008262660A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head device in which the quantity of light to be condensed to a disk can be stabilized and eventually the stable reading or writing of information can be performed. <P>SOLUTION: The optical head device includes a semiconductor laser 11 which emits linearly polarized light, a PBS 12 which separates the exit light thereof to two optical paths, a photodetector 13 which monitors the light quantity of the separated first light (monitor light), and a condenser lens 15 which condenses the separated second light (recording/reproducing light) to a disk D. The PBS 12 varies in the light quantity separation ratio of separating the optical paths by a polarization direction of the incident light. Further, the optical head device includes a first quarter-wave plate 17, a circularly polarized light selective reflection element 18 which approximately reflects the first circularly polarized light rotating the first light in a first rotation direction and approximately transmits the light of the second circularly polarized light rotating in an opposite direction, and an optical photodetector 16 which detects the reflected light from the disk D. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical head device for recording or reproducing information, or recording and reproducing information on an optical recording medium such as an optical disk.

  Information is written (hereinafter referred to as “record”) on an information recording surface of an optical recording medium (hereinafter referred to as “disk”) such as an optical disk such as a CD or DVD and a magneto-optical disk (hereinafter referred to as “record”), or information on the information recording surface is read. An example of a conventional optical head device (hereinafter referred to as “reproduction”) will be described with reference to FIG. 8 (see, for example, Patent Document 1).

  The optical head device 100 includes a semiconductor laser 101 as a light source, an analyzer 102, a beam splitter 103, a light receiving element 104 for monitoring, a quarter wavelength plate 105, a condenser lens 106, a recording lens 106, And a light receiving element 107 for reading.

  In this optical head device 100, the light emitted from the semiconductor laser 101 is condensed on the disk D through the condenser lens 106, and the return light reflected by the disk D is detected by the light receiving element 107. Information can be recorded and reproduced.

  By the way, in order for the optical head device 100 to perform a stable recording operation, it is necessary that the amount of laser light focused on the disk D is stable, that is, the semiconductor laser light must have a stable amount of light. Become. For this reason, conventionally, a part of the semiconductor laser light is separated by the beam splitter 103 in the middle of the path toward the disk D, and the separated light is detected by the light receiving element 104 for monitoring. The amount of light of the semiconductor laser 101 is controlled according to the above.

  Next, stabilization of the amount of laser light focused on the disk D will be described in detail.

First, light of the semiconductor laser 101 that is linearly polarized light passes through the polarization filter 102 and then enters the beam splitter 103 in a predetermined polarization direction. Here, the beam splitter 103 is a so-called polarization beam splitter (PBS), and has a characteristic in which reflectance and transmittance are different depending on the polarization direction of incident light. Therefore, the semiconductor laser light incident on the beam splitter 103 is focused on the disk D according to the polarization direction and the reflectivity and transmittance with respect to the polarization direction of the beam splitter 103, and on the light receiving element 104 for monitoring. Separated into two with the guided light. Therefore, by arranging the analyzer 102 between the beam splitter 103 and the light receiving element 104 to absorb unnecessary components in the monitored light, the light quantity of the light receiving element 104 to be monitored can be controlled to a constant value. . In this way, the amount of light collected on the disk D is stabilized by controlling the amount of light of the semiconductor laser 101.
JP 2003-67961 A

  By the way, in the above-described optical head device, it is desirable that the separation ratio between the light guided to the monitoring light receiving element 104 and the light focused on the disk D is constant and does not vary. However, during the operation of the semiconductor laser 101, the polarization direction of the emitted light may fluctuate and the separation ratio may vary. In such a case, the amount of light collected on the disk D becomes unstable, which causes a problem in reading or writing information in a stable state.

  Further, since the analyzer 102 selectively transmits and absorbs linearly polarized light, the durability of the analyzer 102 itself is insufficient, is easily deteriorated, and the light quantity cannot be stably supplied over a long period of time. There was a problem with sex. Furthermore, since it is difficult for the absorption analyzer to have wavelength selectivity that changes the transmittance according to the wavelength of incident light, there is a problem that it cannot be optimized with the wavelength of incident light.

  The present invention has been made in view of the above circumstances, and provides an optical head device that can stabilize the amount of light collected on the disk D, and thus can read or write information in a stable state. With the goal.

  The present invention has been made to solve the above problems, and is an optical head device that reads information on an optical recording medium or records information on the optical recording medium, and includes a light source and light from the light source. A beam splitter that separates at least the first optical path and a second optical path that is different from the first optical path, a first photodetector that monitors the amount of light in the first optical path, An objective lens for condensing the light of the second optical path onto the optical recording medium; and a second photodetector for detecting reflected light from the optical recording medium, wherein the beam splitter includes the first The light quantity separation ratio, which is the ratio of the light quantity of the second optical path to the light quantity of the optical path, differs depending on the polarization state of the light incident on the beam splitter, and further, the optical head device is provided between the light source and the beam splitter. In the light path of The first circularly polarized light is substantially reflected on one of the optical paths between the beam splitter and the first photodetector, and rotated in the opposite direction to the first circularly polarized light. An optical head device comprising: a circularly polarized light selective reflection element including a circularly polarized light selective reflection layer that substantially transmits second circularly polarized light.

  According to the above configuration, the light emitted from the semiconductor laser as the light source is converted into circularly polarized light, and unnecessary circularly polarized light components are reflected and removed by the circularly polarized light selective reflection element. On the other hand, the ambient light of the optical head device and the emission power of the laser were changed by receiving the transmitted light of the necessary circularly polarized component with a first photodetector (hereinafter referred to as “front monitor PD”). Since the change in the amount of light to the front monitor PD due to the change in the polarization direction of the emitted light from the laser can be suppressed, the amount of light written to the optical recording medium (hereinafter referred to as “optical disk”) can be accurately controlled. Further, since the circularly polarized light selective reflection layer included in the circularly polarized light selective reflection element reflects unnecessary circularly polarized light instead of absorbing it, the element is not damaged by the light absorbing action.

  Also provided is an optical head device in which a modulation element for modulating light from the light source into circularly polarized light is disposed in an optical path between the light source and the circularly polarized light selective reflection layer of the circularly polarized light selective reflection element. .

  According to the above configuration, the circularly polarized light can be incident on the circularly polarized light selecting element by the modulation element that modulates the circularly polarized light in accordance with the polarization state of the light source. For example, an effect can be obtained by arranging an appropriate modulation element that converts circularly polarized light into an elliptically polarized light source.

  The light source is linearly polarized light, and an optical head device in which a first quarter-wave plate is disposed in an optical path between the light source and the circularly polarized light selective reflection layer of the circularly polarized light selective reflection element. I will provide a.

  According to the above configuration, linearly polarized light can be easily converted to circularly polarized light by using a known quarter wave plate. Further, it is preferable to integrate the first quarter-wave plate with the circularly polarized light selective reflection element because space can be saved and reliability is improved.

  The circularly polarized light selective reflection layer of the circularly polarized light selective reflection element provides an optical head device formed of cholesteric phase liquid crystal or cholesteric phase polymer liquid crystal.

  The above configuration is preferable because it can easily realize selective reflection of circularly polarized light in a specific rotational direction. Further, a cholesteric phase polymer liquid crystal is preferable because the liquid crystal can be solidified and temperature characteristics and reliability are easily secured.

The light source emits at least a light having a first wavelength and a light having a second wavelength different from the first wavelength, and the circularly polarized light selective reflection element has the first wavelength having the first wavelength. Of the first circularly polarized light, and the second circularly polarized light of the first wavelength, the first circularly polarized light of the second wavelength, and the second circularly polarized light. Provided is an optical head device that transmits both of them.

  According to the above configuration, for example, an optical disc such as a BD or HD-DVD that performs recording / reproducing with light having a wavelength near 400 nm, a DVD that performs recording / reproducing with light having a wavelength near 660 nm, or recording / reproducing with light having a wavelength near 780 nm. It can be applied to an optical head device having a light source that emits light of two or more different wavelengths corresponding to a plurality of standard optical discs such as a CD. Even when the outgoing polarization characteristics of the laser differ when the ambient temperature or outgoing power changes, it is possible to obtain a circle-changing reflective element that is optimal for each wavelength. As a result, a change in the amount of light to the front monitor PD can be suppressed, which is preferable because the amount of optical disk writing light can be accurately controlled.

  In addition, the beam splitter is a light whose change in the light amount separation ratio due to the polarization state of the light incident at the first wavelength is larger than the change in the light amount separation ratio due to the polarization state of the light incident at the second wavelength. A head device is provided.

  According to the above configuration, when the change of the light amount separation ratio according to the polarization direction of the incident light is set to “polarization dependency”, the light beam is emitted from the light source by reducing the polarization dependency of the beam splitter with respect to the second wavelength. Even when the initial solid state variation in the polarization direction of the second wavelength is large, a stable light amount to the front monitor PD can be obtained, and the optical disk writing light amount can be controlled with high accuracy. In addition, even when there is a disc having a large birefringence of the optical disc to be recorded / reproduced at the second wavelength, the light quantity to the second photodetector for receiving the reflected light having the information on the optical disc is stabilized, and good reading / writing is possible. Since it is realizable, it is preferable.

  The circularly polarized light selective reflection element is disposed in an optical path between the light source and the beam splitter, and a second quarter-wave plate is disposed in the optical path between the circularly polarized light selective reflection element and the beam splitter. An optical head device is provided.

  According to the above configuration, unnecessary linearly polarized light components are eliminated, and only the required linearly polarized light components are incident on the beam splitter. Since there is no dependency and the change in the light quantity to the front monitor PD can be suppressed, the optical disk writing light quantity can be controlled with high accuracy, which is preferable.

  Further, the present invention provides an optical head device in which the circularly polarized light selective reflection element is disposed in an optical path between the light source and the beam splitter, and the circular polarized light selective reflection element and the three-beam diffraction grating are integrated.

  According to the above configuration, only necessary linearly polarized light can be efficiently diffracted into light for three beams. Further, not only the three-beam diffraction grating is integrated with the circularly polarized light selective reflection element, but also the first quarter-wave plate and the second 1/2 according to the polarization state of the light source and the optical characteristics of the beam splitter. The integration with the four-wave plate is preferable because the number of parts can be reduced. The three-beam diffractive element generally performs rotation adjustment around the optical axis during assembly adjustment of the optical head device. A conventional element that integrates an analyzer that selectively transmits and absorbs linearly polarized light into a three-beam diffractive element, detects the direction of linearly polarized light from the light source when rotating the three-beam diffractive element. While the relative angle with the polarization direction that transmits the photon changes and the amount of light that passes through the analyzer changes, the circularly polarized light selective reflection element of the present invention and the three-beam diffraction element can be integrated. This is preferable because the transmittance for circularly polarized light does not change.

  According to the present invention, the amount of light to the front monitor PD hardly changes with respect to the change in the polarization state of the light source, and the light intensity at the time of recording information on or reproducing information from the optical disk can be stabilized. An optical head device having an effect can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(First embodiment)
FIG. 1 shows an optical head device 1 according to a first embodiment of the present invention. The optical head device 1 includes a semiconductor laser 11 as a light source, a beam splitter 12, a light receiving element 13 for monitoring, A third quarter-wave plate 14, a condenser lens 15 as an objective lens, a light receiving element 16 for reading recorded information, a first quarter-wave plate 17, and a circularly polarized light selective reflection element. 18. In the figure, a symbol D indicates a disk which is an optical recording medium.

  The semiconductor laser 11 is used as a light source that generates light for reading or writing information on the disk D, and emits coherent light having a predetermined natural wavelength (λ). The light source 11 is not limited to linearly polarized light but may be elliptically polarized light or circularly polarized light. However, in order to efficiently use the second light directed to the disk D, linearly polarized light is preferable.

  The beam splitter 12 is composed of a polarized beam splitter (PBS; hereinafter referred to as “PBS”), and the light emitted from the semiconductor laser 11 is monitored by using the polarization direction. To the first light (hereinafter referred to as “monitor light”) and the second light (hereinafter referred to as “recording / reproducing light”) toward the disk D. Further, the PBS used in the first embodiment has a different light quantity separation ratio that separates into two optical paths by changing the direction of incident linearly polarized light. The present invention is not limited to such a configuration using PBS, but is separated into two optical paths by changing the ratio of the components of the incident light in the clockwise circularly polarized light and the counterclockwise circularly polarized light. For example, a cholesteric mirror having a characteristic of varying the light amount separation ratio may be disposed. In this case, an optical head device that can be controlled by monitor light can be realized by arranging a circularly polarized light selective reflection element between the cholesteric mirror and the front monitor PD to reflect light in any rotation direction.

  The light-receiving element 13 for monitoring constitutes a first photodetector that monitors the amount of the monitor light described above, and the first quarter-wave plate 17 is placed on the first light path on the first light path. It is installed at a position where light of

  The third quarter-wave plate 14 is configured such that the return light passes through the beam splitter 12 and is projected onto the light receiving element 16 by combination with the PBS 12. In other words, the return light is reflected by the PBS 12 and travels backward to avoid returning to the semiconductor laser 11. The third quarter-wave plate 14 generates circularly polarized light by transmitting the linearly polarized light transmitted through the PBS 12, and transmits linearly polarized light reflected by the disk D to generate linearly polarized light. is doing.

  The condensing lens 15 condenses the light transmitted through the third quarter-wave plate 14 toward the information recording surface of the disk D.

  The light receiving element 16 constitutes a second photodetector that detects reflected light from the disk D. The light receiving element 16 of this embodiment reads the information recorded on the disk D by making the second light transmitted through the third quarter-wave plate 14 and the PBS 12 incident after being reflected by the disk D. For example, a pin photodiode (PD) is used.

  The first quarter-wave plate 17 constitutes an optical filter that (selectively) blocks transmission of first circularly polarized light, which will be described later, in combination with the circularly polarized light selective reflection element 18.

  The circularly polarized light selective reflection element 18 substantially reflects the first circularly polarized light that rotates in the first rotational direction with respect to the recording / reproducing light, and rotates in the direction opposite to the first circularly polarized light. The second circularly polarized light is generally transmitted, and is disposed in the optical path of the monitor light between the first quarter wavelength plate 17 and the light receiving element 13.

  Next, the circularly polarized light selective reflection element 18 provided in the optical head device 1 according to the present embodiment will be described in detail with reference to FIG. 2 which is a sectional view showing the configuration of the circular polarization selective reflection element 18. In this configuration, the traveling direction of light from the light source incident on the beam splitter is changed by 90 ° and guided to the optical disk. However, the optical disk may be arranged in the straight traveling direction of light from the light source. In this case, the monitoring photodetector is disposed on the optical path where the traveling direction of the light from the light source is changed by 90 °.

  The circularly polarized light selective reflection element 18 of this embodiment includes two light transmissive substrates 18A and 18B, and a circularly polarized light selective reflection layer 18C sandwiched therebetween.

  The circularly polarized light selective reflection layer 18C formed between the translucent substrates 18A and 18B transmits the first circularly polarized light (α1) rotating in the first direction (for example, counterclockwise), and the second The second circularly polarized light (α2) rotating in the direction (for example, clockwise) is reflected. Here, the translucent substrates 18A and 18B are preferably made of glass or resin, and glass is particularly preferred as the substrate because of its low birefringence. In the present embodiment, the circularly polarized light selective reflection layer 18C has a structure sandwiched between the translucent substrates 18A and 18B. However, the circularly polarized light selective reflection layer 18C may be formed on one surface of a single translucent substrate, or a substrate is used. It does not have to be. It is preferable to use a substrate because the rigidity and wavefront aberration show good values.

  As the circularly polarized light selective reflection layer 18C, cholesteric liquid crystal or polymer liquid crystal is preferably used. A cholesteric phase liquid crystal or a nematic phase liquid crystal to which a chiral material is added can be used. In addition, solidifying the liquid crystal not only reduces the temperature dependence, but also eliminates the need for a seal structure to prevent liquid crystal leakage. Therefore, a cholesteric phase polymer liquid crystal obtained by polymerizing and solidifying a polymerizable liquid crystal composition can be used. It is preferred to use.

  A cholesteric phase polymer liquid crystal is obtained by polymerizing and solidifying a polymerizable liquid crystal composition containing a nematic phase liquid crystal having at least one of a polymerizable property and a chiral agent by ultraviolet irradiation, heating, or the like.

  The cholesteric phase liquid crystal has a helical twist, and when the helical pitch P is substantially equal to the wavelength λ of the incident light, the second circularly polarized light that enters from the helical axis direction and rotates in the same direction as the helical twist direction ( The first circularly polarized light (α1) that reflects α2) and rotates in the opposite direction is transmitted. The central wavelength λc and bandwidth Δλ of the second circularly polarized light (α2) to be reflected are given by the following [Equation 1].

  That is, the cholesteric phase liquid crystal has the following properties with respect to circularly polarized light having a wavelength λ that rotates in the same direction as the helical twist direction of the liquid crystal.

(I) In the case of λc−Δλ / 2 <λ <λc + Δλ / 2;
Circularly polarized light within the reflection wavelength band of this range is reflected.
(II) When λ ≦ λc−Δλ / 2 or λc + Δλ / 2 ≦ λ;
Transmits circularly polarized light outside the reflection wavelength band in this range.

  The cholesteric phase liquid crystal functions as an isotropic medium having transparency with respect to circularly polarized light rotating in the direction opposite to the spiral twist direction of the liquid crystal regardless of the wavelength of incident light.

Next, a method for manufacturing the circularly polarized light selective reflection element 18 will be described.
1. First, each transparent substrate used as the translucent board | substrates 18A and 18B is prepared, and after apply | coating a polyimide film to those one surfaces, this polyimide film is baked. Thereafter, alignment processing such as rubbing is performed to form alignment films on the transparent substrate.
2. Next, a polymerizable liquid crystal composition is injected between the two light-transmitting substrates 18A and 18B facing each other with the alignment film facing inside at an interval of 6.2 μm.
3. Thereafter, the polymerizable liquid crystal composition is irradiated with ultraviolet rays to be polymerized and solidified to form a circularly polarized light selective reflection layer 18C made of a cholesteric phase polymer liquid crystal.

  For example, a clockwise polymerization chiral agent is added to a polymerizable nematic phase liquid crystal having an ordinary light refractive index no = 1.56 and an extraordinary light refractive index ne = 1.68 so that a helical pitch P becomes 0.405 μm. By solidification, a polymer cholesteric liquid crystal layer that reflects clockwise circularly polarized light (α2) having a wavelength in the range of a center frequency λc = 663 nm and a bandwidth Δλ = 49 nm can be obtained.

  As shown in FIG. 2, the circularly polarized light selective reflection element 18 according to the first embodiment manufactured by the above-described process includes, among lights having a wavelength λ1 = 660 nm applied to recording / reproduction of a disk D such as a DVD, as shown in FIG. Only clockwise circularly polarized light (α2) is reflected, and counterclockwise circularly polarized light (α1) and linearly polarized light are transmitted.

  The thickness of the cholesteric phase polymer liquid crystal layer exhibits wavelength selectivity if it is about 5 times the helical pitch, but in practice it is preferably 10 times or more of the helical pitch, and more reflective. In order to increase the value, it is desirable to set it to 20 times or more of the helical pitch.

  However, if the thickness of the cholesteric phase polymer liquid crystal layer exceeds 50 μm, domains may be formed in the liquid crystal layer and the alignment may be disturbed. Therefore, it is necessary to make the thickness 50 μm or less and 30 μm or less, which is easy to manufacture. It is desirable to do.

  As described above, according to the circularly polarized light selective reflection element 18 of the first embodiment, the ordinary light refractive index, the extraordinary light refractive index of the cholesteric phase liquid crystal (that is, the circularly polarized light selective reflection layer 18C) forming the light shielding portion, Of circularly polarized light having a wavelength determined based on the helical pitch, circularly polarized light (α2) rotating in the same direction as the helical pitch of the cholesteric phase liquid crystal is reflected, and circularly polarized light (α1) rotating in the opposite direction is reflected. ) Can be transmitted.

  Next, the operation of the optical head device 1 will be specifically described with reference to FIG. 1 showing the configuration of the optical head device 1 according to the present embodiment and FIG. 2 which is a conceptual cross-sectional view thereof.

  In the optical head device 1 according to the present embodiment, the light emitted from the semiconductor laser 11 as the light source passes through the third quarter-wave plate 14 and the condenser lens 15 as the objective lens, and the disk D as the optical recording medium. However, a part of the light is separated by the PBS 12 installed in the optical path, and the partially separated light is a light receiving element 13 for monitoring (hereinafter referred to as “front monitor PD13”) as a first photodetector. It is possible to monitor the emission state of the laser light from the semiconductor laser 11 by projecting light onto the semiconductor laser 11. The light condensed on the disk D by the condensing lens 15 is reflected by the information recording surface of the disk D and guided to the second photodetector 16 so that the information on the optical disk D can be read.

  The PBS 12 is a light in which the ratio of the light amount of recording / reproducing light toward the disk D to the light amount of the monitor light toward the front monitor PD 13 (hereinafter referred to as “light amount separation ratio”) of the light emitted from the semiconductor laser 11 is incident on the PBS 12. It has different characteristics depending on the polarization direction. In the optical head device 1 according to the present embodiment, the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 are installed between the PBS 12 and the front monitor PD 13 for the reason described later. Yes.

  In this optical head device 1, the intensity of light collected on the optical disk D is controlled to a predetermined value by monitoring the amount of light to the front monitor PD 13. It is important that the light quantity separation ratio is always constant. However, the polarization state of the semiconductor laser 11 as the light source often changes with time depending on the environmental temperature and the emission power.

  As described above, the PBS 12 has different transmission and reflection separation ratios depending on the polarization direction of incident light, that is, S-polarized light (perpendicular to the paper surface) and P-polarized light (parallel to the paper surface). For this reason, since the light quantity separation ratio changes when the state of polarized light emitted from the light source changes, even if the operation of the semiconductor laser 11 is controlled so that the light quantity of the front monitor PD is always a constant value, it reaches the optical disk D. The amount of light to be generated cannot be always constant. Therefore, in the present embodiment, this problem can be solved by arranging the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 between the PBS 12 and the front monitor PD13.

This will be specifically described with an example of the characteristics of the PBS 12.
For example, when S-polarized light (perpendicular (Z) polarized light in FIG. 1) is incident, the reflectance is 90% (the reflected light is to the optical disc D) and the transmittance is 10% (the transmitted light is to the front monitor PD13). Thus, a description will be given of a PBS having a reflectance of 10% and a transmittance of 90% when P-polarized light (polarized light parallel to (Y) in FIG. 1) is incident.

Here, when laser light having an S-polarized component of substantially 100% is emitted from the semiconductor laser 11, the light quantity separation ratio 9 (10% is directed to the front monitor PD and 90% is directed to the optical disk by the PBS 12. = 90% / 10%). Here, if the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 are not installed, the polarization state of the laser light emitted from the semiconductor laser 11 changes due to a temperature change or the like, for example, S-polarized light. When the component changes to 95% and the P-polarized component changes to 5%, the ratio of the light separated into the front monitor PD by the PBS 12 is
(I) 9.5% (0.95 × 0.1), which is 10% of the S-polarized light component which is 95% of the entire emitted light,
(Ii) Out of the P-polarized light component that is 5% of the whole emitted light, 90% of the component is 4.5% (= 0.05 × 0.9).
And 14%.

  Accordingly, the light traveling toward the front monitor PD is increased from the original 10% to 14%, that is, 1.4 times, and the light amount separation ratio is 6.1 (= 86% / 14%), which changes greatly. Further, the remaining light that is separated into the optical disk D out of the entire laser light is 86% (= 0.95 × 0.9 + 0.05 × 0.1). Thus, the amount of light received by the front monitor PD greatly fluctuates depending on the emission state of the semiconductor laser 11.

  On the other hand, in the present embodiment, the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 are installed on the optical path of the monitor light immediately after the PBS 12. Therefore, the light traveling toward the front monitor PD 13 after passing through the PBS 12 is first converted into circularly polarized light by the first quarter-wave plate 17. At this time, the S-polarized light component and the P-polarized light component transmitted through the PBS 12 become circularly polarized light whose rotation directions are opposite to each other (for convenience, they will be referred to as “S circularly polarized light” and “P circularly polarized light”, respectively). . Here, the circularly polarized light selective reflection element 18 of the present embodiment is configured to selectively transmit S circularly polarized light and reflect P circularly polarized light (that is, in FIG. 2, the incident light (α1) is reflected. S circularly polarized light and incident light (α2) is P circularly polarized light). Thereby, the monitor light transmitted through the circularly polarized light selecting element 18 becomes only the S circularly polarized light component, and 9.5% of the entire emitted light can reach the front monitor PD13.

  Thereby, in this embodiment, the light quantity separation ratio is 9.05 (= 86% / 9.5%), which is almost the same as the light quantity separation ratio when the polarization of the light source does not rotate. Therefore, the amount of light collected on the optical disc D can be sufficiently controlled by controlling the amount of light of the front monitor PD13.

  Thus, in the present embodiment, by using the circularly polarized light selective reflection element 18 and the first quarter-wave plate element 17, the front monitor PD light quantity can be stabilized. In the present invention, a characteristic is used in which the transmittance at which light travels straight through the PBS and the reflectance at which the traveling direction changes are different depending on the polarization state of the light incident on the PBS. For example, as a characteristic of PBS that separates light into the front monitor PD side and the optical disc side, the ratio of transmittance and reflectance between S-polarized light and P-polarized light is 1.02 or more or 0.98 or less Then, control is facilitated by the front monitor PD, preferably 1.05 or more and 0.95 or less, and more preferably 1.1 or more and 0.9 or less, thereby enabling more accurate control. Similarly, when a cholesteric mirror is used, it can be controlled by using a transmittance ratio and a reflectance ratio of clockwise circularly polarized light and counterclockwise circularly polarized light in the above numerical range.

  Here, as a matter of course, the polarization characteristic of the PBS 12 is not limited to the above example, but the same effect can be obtained in any case. There is no problem even if the light from the light source that has passed through the PBS 12 is arranged to guide the optical disc D and the reflected light to the front monitor PD. Further, if the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 are integrated with the optical element 19 as shown in FIG. 3, for example, the number of parts can be reduced and the cost can be reduced. Therefore, it is more preferable. In FIG. 3, the incident lights α1 and α2 are shown as linearly polarized light before entering the first quarter-wave plate element 17.

  Further, in the present invention, a configuration in which a polarizer that absorbs specific linearly polarized light is used instead of the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 of the present embodiment is also conceivable. However, in the absorption type polarizer, in order to absorb strong laser light, the temperature of the polarizer rises, and the polarizer has deteriorated, particularly for a short wavelength light source such as a blue laser. There is a problem that the polarizer itself undergoes a chemical change due to light energy, resulting in a decrease in transmittance and a decrease in the polarization dependency of the transmittance. On the other hand, since the circularly polarized light reflecting element of the present invention performs polarization selection by reflecting light instead of absorbing it, there is no problem like an absorptive polarizer.

  In the present invention, a configuration using a polarization diffraction element that transmits and diffracts a specific linearly polarized light is also conceivable. In this case, there is no absorption and there is no problem like an absorption polarizer, but this is excellent. However, in order to selectively diffract unnecessary polarization components, the diffracted light does not enter the front monitor PD. It is necessary to increase the diffraction angle. In particular, when the front monitor PD is a large size or when the distance between the polarization diffraction element and the front monitor PD is short, it is necessary to make the diffraction angle very large, and there are restrictions on the arrangement. On the other hand, when the circularly polarized selective reflection element of the present invention is used, unnecessary light can be reflected, and light that passes straight through can be guided to the front monitor PD. Is preferable.

  Further, in the optical head device that requires a circularly polarized light selecting element having wavelength selectivity as in the second embodiment to be described later, the above-described absorption polarizer and polarized light selective diffractive element cannot sufficiently cope. There was also a problem.

  In addition, there is a polarization beam splitter (PBS) using an optical multilayer film as another polarization selection element. In this case, incident light is incident obliquely (for example, 60 degrees) with respect to the polarization beam splitting surface, or has three angles. It is necessary to make a polarizing beam splitter prism like a prism attached. In that case, the thickness of the element (in the optical axis direction) is large, and the element size cannot be reduced, which is a major obstacle to miniaturization of the optical head device, and the prism has problems such as high cost. . On the other hand, the circularly polarized light selective reflection element of the present invention can function at normal incidence in order to reflect circularly polarized light, and is therefore suitable for downsizing of the optical head device.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. FIG. 4 shows an optical head device 2 according to the second embodiment. The optical head device 2 can reproduce and record two types of optical disks D (for example, DVD and CD). A semiconductor laser 21 as a light source, a beam splitter 23, a monitoring light receiving element 13, a third quarter-wave plate 14, a condensing lens 15, a light receiving element 16, and a first light receiving element 16. 1 quarter wave plate 17 and circularly polarized light selective reflection element 22.

  The semiconductor laser 21 is composed of a light source that emits light of two types of wavelengths, a first wavelength and a second wavelength that is different from the first wavelength. Λ1) of red light and CD 780 nm (second wavelength λ2) near-infrared light, and two-wavelength lasers capable of emitting light of two wavelengths.

  The circularly polarized light selective reflection element 22 substantially reflects the circularly polarized light of the first wavelength (λ1) in the first rotational direction, and the circularly polarized light of the first wavelength (λ1) in the second rotational direction and The circularly polarized light having the second wavelength (λ2) in the first and second rotational directions is substantially transmitted. As shown in FIG. 5, the light transmitting substrate 22A and the light transmitting substrate 22B are transmitted. And a circularly polarized light selective reflection layer 22C sandwiched between them.

  Next, the circularly polarized light selective reflection element 22 having a function of making the circularly polarized light selection characteristics different for light of two different wavelengths will be described with reference to FIG.

  For light of the first wavelength (λ1), circularly polarized light (β1) in the first rotational direction is reflected, and circularly polarized light (β2) in the second rotational direction is transmitted. Further, the light having the second wavelength (λ2) is transmitted through both the circularly polarized light (β3) in the first rotational direction and the circularly polarized light (β4) in the second rotational direction. However, as described above, the first wavelength (λ1) is the 660 nm band for DVD and the second wavelength (λ2) is the 780 nm band for CD.

  Next, the method for producing the circularly polarized light selective reflection element 22 is substantially the same as that of the first embodiment. For example, a polymerizable nematic having an ordinary light refractive index no = 1.56 and an extraordinary light refractive index ne = 1.68. By adding a clockwise polymerization chiral agent to the phase liquid crystal in an amount such that the helical pitch P is 0.405 μm, and polymerizing and solidifying, a clock having a wavelength in the range of the center frequency λc = 663 nm and the bandwidth Δλ = 49 nm is obtained. A polymer cholesteric liquid crystal layer that reflects surrounding circularly polarized light, that is, a circularly polarized light selective reflection layer 22C can be obtained.

  As shown in FIG. 5, the circularly polarized light selective reflection element 22 of the first embodiment manufactured by the above process includes, among the lights of the first wavelength (λ1 = 660 nm) applied to DVD recording / reproduction, as shown in FIG. Only clockwise circularly polarized light (β1) is reflected, and counterclockwise circularly polarized light (β2) and linearly polarized light are transmitted. Further, light other than the wavelength band of the bandwidth Δλ centered on the wavelength λ1 can be transmitted almost regardless of the polarization state. For example, light (β3, β4) in the band of the second wavelength (λ2 = 780 nm) for CD can be transmitted regardless of the polarization state.

  Next, the optical head device 2 provided with the circularly polarized light selective reflection element 22 of the second embodiment will be described with reference to FIG.

  The optical head device 2 has substantially the same configuration as that of the optical head device 1 schematically shown in FIG. 1, but, as described above, the light source 21 is light having wavelengths of 660 nm for DVD and 780 nm for CD (hereinafter, respectively). , “DVD wavelength” and “CD wavelength”). Here, a two-wavelength laser that can emit light of two wavelengths from one laser chip will be described. However, a hybrid laser in which individual laser chips that emit light of two different wavelengths are close to each other, or two laser chips It is also possible to combine the light from the light with a multiplexing element.

  Since the cover of a CD disk is thick, a disk cover having a large refractive index anisotropy or a large in-plane distribution is often observed. When such a disk D is recorded / reproduced, the polarization state of the return light from the disk D changes with the individual difference of the disk D or the rotation of the disk D due to the refractive index anisotropy of the disk cover. Therefore, if a beam splitter 23 having a large polarization dependency is used, the amount of light to the light receiving element 16 is greatly changed by the refractive index anisotropy of the disk D, which is not preferable. For this reason, it is preferable that the beam splitter 23 has a large polarization dependence at the DVD wavelength and no polarization dependence at the CD wavelength, that is, a light quantity separation ratio of the S polarization component and the P polarization component is equal.

  In order to suppress the semiconductor laser 21 at a low cost, a semiconductor laser having a large individual difference in the output polarization direction of light having a CD wavelength may be used as the light source. In the case of an optical head device using such a light source, CD light having a CD wavelength of a circularly polarized light selective reflection element is configured to be different in transmission and reflection with respect to the rotational direction of circularly polarized light. On the contrary, the amount of light of the front monitor PD changes due to the change in the polarization state of the wavelength light, which is not preferable.

  Therefore, an example of the characteristics of the beam splitter 23 (hereinafter referred to as “PBS23”) will be shown and described in more detail.

  For DVD wavelength (λ1) light, for example, when S-polarized light (polarized light perpendicular to the paper surface of FIG. 4) is incident, the reflectance is 90% (the reflected light is directed to the disk D) and the transmittance is 10%. On the other hand, the PBS 23 having the characteristics of 10% reflectance and 90% transmittance when P-polarized light (polarized light parallel to the paper surface of FIG. 4) is incident will be described. .

  Accordingly, when 100% of the light whose S polarization component of the DVD wavelength is substantially 100% is emitted from the semiconductor laser 21, 10% is polarized to the front monitor PD and 90% is polarized to the disk D by the PBS 23. Separation is performed at a separation ratio of 9 (= 90% / 10%). Note that the PBS 23 used here has a separation characteristic of 90% reflectance (reflected light to the optical disc D) and 10% transmittance (transmitted light to the front monitor PD), regardless of the polarization state, with respect to light having a CD wavelength. It shall have.

  Regarding the light of DVD wavelength, since the same effect as the first embodiment is obtained, the description thereof is omitted. On the other hand, regarding the light of CD wavelength (λ2), when the polarization state of the semiconductor laser 23 changes due to a temperature change or the like, for example, the S polarization component changes to 70% and the P polarization component changes to 30%, the PBS 23 separates the light to the front monitor PD. Since the emitted light is 10% of the total emitted light, the sum of the S-polarized component is 7% (= 0.70 × 0.1) and the P-polarized component is 3% (= 0.30 × 0.1). Of 10%.

  It is assumed that a circularly polarized light selective reflection element having a characteristic in which light separated in the direction of the front monitor PD13 by the PBS 23 is selected for transmission and reflection according to the rotation direction of circularly polarized light is also used for light of CD wavelength. . In this case, considering the monitor efficiency, it is preferable to have a characteristic that transmits S circularly polarized light and reflects P circularly polarized light. Therefore, if a circularly polarized light selective reflection element having such characteristics is used, it is incident on the front monitor PD. This is 7% of the entire emitted light. On the other hand, the light separated into the disk D is 90% regardless of the polarization state due to the above-described characteristics. Therefore, the separation ratio of the light toward the front monitor PD greatly changes to 12.9 (= 90% / 7%) when the polarization state of the semiconductor laser 21 changes as described above. The same problem arises even in a configuration in which light having a necessary polarization state is incident on the front monitor PD using a conventional absorption analyzer.

  On the other hand, in the present embodiment, the circularly polarized light selective reflection element 22 is configured to transmit 100% of CD wavelength light regardless of the polarization state, as shown in FIG. As a result, even if the polarization state of the semiconductor laser 21 changes from (S-polarized light 100%, P-polarized light 0%) to (S-polarized light 70%, P-polarized light 30%) as described above, the front monitor PD13 is operated by the PBS 23. The light that is separated into 10% of the light that is the sum of the S-polarized component 7% (0.70 × 0.1) and the P-polarized component 3% (= 0.30 × 0.1) is the front monitor PD. Can lead to. For this reason, the light quantity separation ratio does not change to 9 (= 90% / 10%) even if the polarization state of the semiconductor laser 21 changes as described above.

(Third embodiment)
Next, a third embodiment of the present invention will be described. FIG. 6 shows an optical head device 3 according to the third embodiment. In this optical head device 3, the first quarter-wave plate 17 and the circularly polarized light selective reflection element 18 in the first embodiment are shown. Instead, a circularly polarized light selective reflection element 31 is provided.

  In this circularly polarized light selective reflection element 31, the circularly polarized light selective reflection element and the three-beam diffraction element are integrated, and are arranged in the optical path between the semiconductor laser 11 as the light source and the beam splitter 33. The light quantity separation ratio of the beam splitter 33 that separates the optical path is such that the light with the first wavelength (λ1) that is the DVD wavelength is light with respect to the light with the second wavelength that is the CD wavelength (λ2). Therefore, the polarization dependency is large.

  FIG. 7 shows a configuration example of the circularly polarized light selective reflection element 31 when linearly polarized light is incident from the light source 11 and integrated with the three-beam diffraction grating as the third embodiment. Here, the first and second quarter-wave plates 31C and 31D have a configuration in which a circularly polarized light selective reflection layer 31E is sandwiched. That is, in the case of this embodiment, the circularly polarized light selective reflection element 31 includes the translucent substrate 31A and the translucent substrate 31B, and the first 1/4 sandwiched between the translucent substrates 31A and 31B. A wave plate 31C, a second quarter wave plate 31D, and a circularly polarized light selective reflection layer 31E are provided. The first quarter-wave plate 31C and the second quarter-wave plate 31D do not have to be integrated with the circularly polarized light selective reflection element, but are integrated in order to save space and improve reliability. And preferred. The light from the light source 11 is not limited to linearly polarized light but may be in a polarization state such as elliptically polarized light. At this time, if the first quarter-wave plate 31C is a modulation element having a function of modulating to circularly polarized light, 1 / You may use not only a 4 wavelength plate.

  As the circularly polarized light selective reflection layer 31E, the same one as used in the first embodiment or the second embodiment can be used.

  Further, in the present embodiment, a diffraction grating (grating) 32 for generating three beams (that is, 0th-order and ± 1st-order diffracted light) is formed on the surface of the transparent substrate 31B.

  Therefore, according to the present embodiment, with respect to the first linearly polarized light (γ1) that is S-polarized light (having polarization (Z) perpendicular to the paper surface in FIG. 7) emitted from the semiconductor laser 11 that is a light source, The light is converted into circularly polarized light by the first quarter-wave plate 31C, transmitted through the circularly-polarized selective reflection layer 31E, and converted again into linearly-polarized light (γ1) by the second quarter-wave plate 31D. The Thereafter, the light is diffracted by the diffraction grating 32 to become three beams (0th-order light and ± 1st-order light) and separated by the beam splitter 33 into monitor light directed to the front monitor PD13 and recording / reproduction light directed to the disk D.

  Here, the circularly polarized light selective reflection element 31 functions as an optical element capable of selectively transmitting only linearly polarized light (γ1) of linearly polarized light. Therefore, the polarization state of the semiconductor laser 11 exhibits a P-polarized light component orthogonal to the S-polarized light due to a temperature change or the like (having P-polarized light parallel to the paper surface), that is, the P-polarized light shown in FIG. In the case of the second linearly polarized light (γ2), the circularly polarized light is reflected by the circularly polarized light selective reflection layer 31E and cannot be transmitted even if it is circularly polarized by the first quarter-wave plate 31C.

  Next, the case where the beam splitter 33 is the same as that of the first embodiment, that is, the same characteristic as the PBS 12 will be described.

  That is, when S-polarized light (polarized light perpendicular to (Z) in FIG. 6) is incident on the beam splitter 33, the reflectance is 90% (the reflected light is directed to the disk D) and the transmittance is 10% (the transmitted light is front). To monitor PD13). Further, when P-polarized light (polarized light parallel to (Y) in FIG. 6) is incident, the reflectance is 10% and the transmittance is 90%.

  Here, when all S-polarized light is emitted from the semiconductor laser 11, the beam splitter 33 causes 10% to be directed to the front monitor PD and 90% to be applied to the optical disc D, so that the light amount separation ratio is 9 (= 90% / 10%). Separated by

  Next, the light beam is split into three beams by a three-beam diffraction grating. Here, a case where the circularly polarized light selective reflection element 31 of the present invention is not provided will be described.

  When the polarization state of the semiconductor laser 11 changes due to a temperature change or the like, the S-polarized component changes to 95% and the P-polarized component changes to 5%. The monitor light transmitted through the beam splitter 33 and separated into the front monitor PD13 This is 14%, which is the sum of S polarization component 9.5% (= 0.95 × 0.1) and P polarization component 4.5% (= 0.05 × 0.9). Therefore, the recording / reproducing light reflected by the beam splitter 33 and separated toward the disk D becomes the remaining 86% (= 0.95 × 0.9 + 0.05 × 0.1), and goes to the front monitor PD13. The light increases 1.4 times from 10% to 14%, and the light quantity separation ratio is 6.1 (= 86% / 14%), which is greatly changed.

  On the other hand, in the case of the optical head device 3 including the circularly polarized light selective reflection element 31 according to the present embodiment, the following operation occurs.

  When the polarization state of the semiconductor laser 11 changes, the light of an unnecessary polarization component generated by this change has a rotation direction opposite to that of the desired polarization by the first quarter-wave plate 31C shown in FIG. It is converted into circularly polarized light and reflected by the circularly polarized light selective reflection layer 31E. Therefore, the light transmitted through the circularly polarized light selective reflection element 31 is substantially only in a desired polarization direction, and the polarization state incident on the beam splitter 33 does not change even if the polarization direction of the semiconductor laser 11 changes. For this reason, the amount of light collected on the disk D can be sufficiently controlled by controlling the amount of light of the front monitor PD13.

  Further, when the circularly polarized light selective reflection element 31 and the diffraction grating 32 are integrated as in the present embodiment, the optical head device 3 is assembled in order to align the groove direction of the diffraction grating 32 with the radial direction of the disk D. Adjust the rotation. In the element that selectively absorbs or diffracts the linearly polarized light (γ1) described above, when this element is rotated, the polarization state of the transmitted light is naturally different. On the other hand, since the circularly polarized light selective reflection element of the present invention is integrated, the polarization characteristics do not change even when rotated with respect to the optical axis, and the transmitted polarization state does not change, which is preferable.

  The present invention is not limited to the embodiment described above, and can be implemented in various forms without departing from the gist of the present invention.

  The optical head device of the present invention can stabilize the amount of light collected on the disk D, and thus has the effect of being able to read or write the disk in a stable state, such as a CD or DVD, Is useful for an optical head device capable of recording / reproducing a Blu-ray disc or HD-DVD.

1 is a configuration diagram showing an optical head device according to a first embodiment of the present invention. Sectional drawing which shows the circularly polarized light selective reflection element with which this optical head apparatus was equipped Sectional drawing which shows schematic structure of the modification of the circularly polarized light selective reflection element The block diagram which shows the optical head apparatus based on the 2nd Embodiment of this invention. Sectional drawing which shows the circularly polarized light selective reflection element with which this optical head apparatus was equipped The block diagram which shows the optical head apparatus based on the 3rd Embodiment of this invention. Sectional drawing which shows the circularly polarized light selective reflection element with which this optical head apparatus was equipped Configuration diagram showing a conventional optical head device

Explanation of symbols

1, 2, 3 Optical head device 11, 21 Semiconductor laser (light source)
12, 23, 33 Beam splitter (PBS)
13 Light-receiving element for monitoring (first photodetector)
14 Third quarter wave plate 15 Condensing lens (objective lens)
16 light receiving element (second photodetector)
17 1st 1/4 wavelength plate 18 Circularly polarized light selective reflection element 18A, 18B Translucent substrate 18C Circularly polarized light selective reflection layer 22 Circularly polarized light selective reflection element 22A, 22B Translucent substrate 22C Circularly polarized light selective reflection layer 31 Circularly polarized light Selective reflection element 31C First quarter wave plate 31D Second quarter wave plate 31E Circularly polarized light selective reflection layer D Disc (optical recording medium)
α1 First circularly polarized light α2 Second circularly polarized light β1 Circularly polarized light in the first rotational direction for light of the first wavelength β2 Circularly polarized light in the second rotational direction for light of the first wavelength β3 Circularly polarized light in the first rotational direction for light β4 Circularly polarized light in the second rotational direction for light of the second wavelength γ1 S-polarized light (first linearly polarized light)
γ2 P-polarized light (second linearly polarized light)

Claims (8)

An optical head device for reading information on an optical recording medium or recording information on an optical recording medium,
A light source;
A beam splitter that receives light from the light source and separates the light into at least a first optical path and a second optical path different from the first optical path;
A first photodetector for monitoring the amount of light in the first optical path;
An objective lens for condensing the light of the second optical path onto the optical recording medium;
A second photodetector for detecting reflected light from the optical recording medium,
In the beam splitter, a light amount separation ratio that is a ratio of a light amount of the second optical path to a light amount of the first optical path varies depending on a polarization state of light incident on the beam splitter,
Furthermore, the optical head device comprises:
The first circularly polarized light is substantially reflected on either the optical path between the light source and the beam splitter or the optical path between the beam splitter and the first photodetector, An optical head device comprising: a circularly polarized light selective reflection element including a circularly polarized light selective reflection layer that substantially transmits the second circularly polarized light rotating in the opposite direction to the first circularly polarized light. The optical head device according to claim 1,
An optical head device in which a modulation element that modulates light from the light source into circularly polarized light is disposed in an optical path between the light source and the circularly polarized light selective reflection layer of the circularly polarized light selective reflection element. The optical head device according to claim 1, wherein:
The optical head device, wherein the light source is linearly polarized light, and a first quarter-wave plate is disposed in an optical path between the light source and the circularly polarized light selective reflection layer of the circularly polarized light selective reflection element. The optical head device according to any one of claims 1 to 3,
The circularly polarized light selective reflection layer of the circularly polarized light selective reflection element is an optical head device formed of cholesteric phase liquid crystal or cholesteric phase polymer liquid crystal. The optical head device according to any one of claims 1 to 4,
The light source emits at least light having a first wavelength and light having a second wavelength different from the first wavelength;
The circularly polarized light selective reflection element substantially reflects the first circularly polarized light having the first wavelength, the second circularly polarized light having the first wavelength, and the second wavelength having the second wavelength. An optical head device that substantially transmits both first circularly polarized light and second circularly polarized light. The optical head device according to claim 5,
The beam splitter is an optical head device in which a change in the light amount separation ratio due to a polarization state of light incident at the first wavelength is larger than a change in the light amount separation ratio due to a polarization state of light incident at the second wavelength. . The optical head device according to any one of claims 1 to 6,
The circularly polarized light selective reflection element is disposed in an optical path between the light source and the beam splitter;
An optical head device in which a second quarter-wave plate is disposed in an optical path between the circularly polarized light selective reflection element and the beam splitter. The optical head device according to claim 7,
The circularly polarized light selective reflection element is disposed in an optical path between the light source and the beam splitter;
An optical head device in which the circularly polarized light selective reflection element and a three-beam diffraction grating are integrated.

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