JP5171219B2 - Optical pickup device and optical reproduction device - Google Patents

Description

  The present invention relates to an optical pickup device and an optical reproducing device, and is particularly suitable for use in reproducing a recording medium having a plurality of recording layers in the stacking direction.

  In recent years, with the increase in capacity of recording media, optical recording media having a plurality of recording layers in the stacking direction have been developed. For example, Patent Document 1 below discloses an optical recording medium in which a plurality of recording layers and one servo layer are stacked. In this recording medium, a flat recording layer without guide tracks is formed, and a servo layer having guide tracks is formed thereon.

At the time of reproduction, a focus servo signal and a tracking servo signal are generated based on a guide track formed on the servo layer. By controlling the position of the beam spot on the recording layer based on these servo signals, the beam spot is positioned at a desired position on the scanning locus.
JP 2004-335060 A

  In this type of recording medium, the capacity of the recording medium can be increased by reducing the interval between the recording layers. On the other hand, however, if the interval between the recording layers is reduced, unnecessary light (stray light) reflected by the recording layer other than the reproduction target enters the photodetector and adversely affects the reproduction signal.

  The present invention has been made to solve such a problem. Even when a large number of recording layers are provided in the stacking direction, the present invention effectively suppresses the adverse effects of stray light and can reproduce the recording medium smoothly and satisfactorily. It is an object of the present invention to provide a pickup device and an optical reproducing device.

  In view of the above problems, the present invention has the following features.

An optical pickup device according to a first aspect of the present invention includes a light source that emits reproduction light, a first optical system that converges the reproduction light on a recording layer in a recording medium, and the reproduction light reflected by the recording layer. , A second optical system that superimposes reference light on the reproduction light reflected by the recording layer, and an optical path length of the second optical system that is disposed in the second optical system. An optical path adjustment unit that changes according to a signal, and the optical path adjustment unit includes an optical element that receives the reference light and changes a refractive index with respect to the reference light according to the control signal. To do.

  According to the present invention, when the control signal is applied to the optical path adjusting unit in a state where the reproduction light is converged on the reproduction target recording layer, the first optical system and the second optical system are The relationship between the optical path lengths changes. In this process, when the optical path length of the second optical system is matched with the optical path length where the reproduction light (signal light) from the reproduction target recording layer and the reference light interfere with each other, the signal light and the reference light are mutually connected. Interfering with each other, high intensity light is guided to the photodetector. Thereby, a high level signal is output from the photodetector. Therefore, by controlling the optical path adjustment unit so that this signal is output from the photodetector, it is possible to reproduce the recording layer as the reproduction target.

  If the distance between the recording layers exceeds the distance range where the reproduction light and the reference light interfere with each other (coherence distance: details will be described in the embodiment), the reproduction target recording layer is reproduced. At this time, unnecessary reproduction light (stray light) reflected by the upper and lower recording layers sandwiching the recording layer does not interfere with each other. Therefore, even if this stray light is incident on the photodetector, its intensity is several steps smaller than the intensity of the signal light amplified by interference with the reference light, and therefore the influence of stray light on the reproduction signal is significant. It will be small.

  As described above, according to the present invention, even when a large number of recording layers are arranged in the stacking direction, adverse effects due to stray light can be effectively suppressed, and the recording medium can be reproduced smoothly and satisfactorily. The present invention exhibits a desired effect when reproducing a recording medium in which the distance between the recording layers exceeds the coherence distance as described above.

  According to a second aspect of the present invention, in the optical pickup device according to the first aspect, the second optical system separates a part of the reproduction light emitted from the light source from an optical path of the first optical system. A beam splitter that generates the reference light and a mirror unit that reflects the separated reference light and returns it to the beam splitter, and the optical path adjustment unit returns to the beam splitter after being separated by the beam splitter The optical path length of the reference light up to is changed.

According to a third aspect of the present invention, in the optical pickup device according to the first or second aspect, the optical path adjusting unit includes an optical fiber on which the reference light is incident and a piezoelectric that changes a total length of the optical fiber in accordance with the control signal. An element is provided. Here, for example, the optical path adjustment unit can be configured to wind an optical fiber around a piezoelectric element (piezo element) whose diameter changes in accordance with an applied signal.
According to a fourth aspect of the present invention, in the optical pickup device according to the first or second aspect, the optical path adjustment unit includes an optical fiber on which the reference light is incident and a piezoelectric that changes a total length of the optical fiber in accordance with the control signal. An element is provided. Here, for example, the optical path adjustment unit can be configured to wind an optical fiber around a piezoelectric element (piezo element) whose diameter changes in accordance with an applied signal.

An optical regenerator according to a fourth aspect of the invention is output from the optical pickup device according to any one of the first to third aspects, an optical path control circuit that controls the optical path adjustment unit, and the photodetector. And a reproducing circuit for reproducing a signal. According to the present invention, the same effect as in the first aspect can be obtained.

  According to a fifth aspect of the present invention, in the optical reproduction apparatus according to the fourth aspect, the optical path control circuit is configured to cause the reproduction light from a plurality of recording layers within the focal depth of the reproduction light to interfere with the reference light. The optical path length of the second optical system is changed, and the reproducing circuit relies on signals output from the photodetector when the reproducing light from the plurality of recording layers interferes with the reference light. The recording data recorded on the recording layer is reproduced.

  According to the present invention, recorded data can be reproduced simultaneously from a plurality of recording layers within the focal depth. Therefore, it is possible to increase the efficiency or speed of the reproduction operation.

  As described above, according to the present invention, even when a large number of recording layers are provided in the stacking direction, the recording medium can be smoothly and satisfactorily reproduced by effectively suppressing the adverse effects of stray light.

The effects and significance of the present invention will become more apparent from the following description of embodiments. However, the following embodiment is merely an exemplary form for implementing the present invention, and the present invention is not limited to the following embodiment.

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

  FIG. 1 shows a configuration of a recording medium 10 according to the embodiment. FIG. 2 shows a cross-sectional structure of the recording medium 10.

  As shown in the figure, the recording medium 10 has a plurality of stages (four stages in FIG. 1) each including a servo layer 13, six space layers 14, and five recording layers 15 between a substrate 11 and a surface layer 12. It is comprised by.

  The substrate 11 and the surface layer 12 are made of a translucent material such as polycarbonate, polyolefin, or acrylic. The servo layer 13 is formed of a material having a high reflectance with respect to servo laser light (hereinafter referred to as “servo light”) and a low reflectance with respect to reproduction laser light (hereinafter referred to as “reproduction light”). The space layer 14 is made of an ultraviolet curable resin or a transparent film material in which an adhesive is applied on both sides. The recording layer 15 is made of a uniform and flat semi-transmissive film having a predetermined thickness.

  One servo layer 13 is for positioning reproduction light on five recording layers 15 existing between the servo layer 13 and the next servo layer 13 adjacent in the surface layer 12 direction.

  The servo layer 13 has a large number of linear and parallel guide tracks. In the recording layer 15, recording marks are arranged in a straight line along the guide track at a position directly below the guide track. The recording mark formation region has a higher reflectance than the recording mark non-formation region. For example, the refractive index with respect to the reproduction light is different between the recording mark formation region and the non-formation region.

  Note that the pitch of the recording layers 15 in the stacking direction is set larger than the coherence distance described later.

  FIG. 2A is a plan view of the recording medium 10. As illustrated, the recording medium 10 is divided into a plurality of recording areas 10a. FIG. 2B is a partially enlarged view of FIG. In the servo layer 13 of each recording area 10a, a predetermined number of linear guide tracks parallel to the X direction in FIG. 5B are arranged at a constant pitch in the Y axis direction. In the recording layer 15 of each recording area 10a, as described above, recording marks are arranged at positions immediately below the respective guide tracks of the servo layer 13 along the corresponding guide tracks.

  FIG. 3 is a diagram illustrating a basic configuration of an optical system that reads a recording signal from the recording medium 10.

  Reproduction light emitted from a low-coherence light source (SLD: Super-Luminescent Diode, hereinafter referred to as “SLD”) 21 is separated by a non-polarizing beam splitter (hereinafter referred to as “beam splitter”) 22 such as a half mirror, and the recording medium 10 and the mirror 23 are irradiated. The reproduction light (signal light) reflected by the reproduction target recording layer 15 passes through the beam splitter 22 and is irradiated to the photodetector 24. Further, the reproduction light (reference light) reflected by the mirror 23 is reflected by the beam splitter 22 and applied to the photodetector 24. Therefore, the photodetector 24 is irradiated with the signal light in a state where the reference light is superimposed.

Here, the optical path length from the reproduction target recording layer 15 to the light receiving surface of the photodetector 24 (the optical path length of the signal light), and the optical path length from the mirror 23 to the light receiving surface of the photodetector 24 (the optical path length of the reference light). ) Is within the range of the coherent distance Δlc, the signal light and the reference light interfere with each other, and the intensity of the light irradiated on the photodetector 24 increases. The emission spectrum of the SLD 21 has a Gaussian distribution. When the full width at half maximum is Δλ and the center wavelength is λc, the coherence distance Δlc is
Δlc = {(2ln2) / 2π} × (λc ² / Δλ) (1)
Given in.

  Therefore, if the difference between the optical path length L1 and the optical path length L2 in FIG. 3 is within the coherent distance Δlc obtained by Expression (1), the intensity of the signal light reflected by the recording layer 15 as the reproduction target increases. It will be.

  If the pitch of the recording layers 15 in the stacking direction is larger than the coherence distance Δlc, the reflected light (stray light) from the recording layer 15 adjacent to the recording layer 15 to be reproduced does not interfere with the reference light. . Therefore, if the pitch of the recording layer 15 is set in this way, even if stray light is incident on the photodetector 24, the intensity thereof is several times that of the signal light amplified by interference with the reference light. Step down. Therefore, the influence of stray light on the output signal of the photodetector 24 is remarkably small.

  When reading a signal from the target recording layer 15, the optical path length L1 may be adjusted so that the difference from the optical path length L2 with respect to the target recording layer 15 is within the coherent distance Δlc. This adjustment can be performed, for example, by changing the distance between the mirror 23 and the beam splitter 22 by displacing the mirror 23. Thus, by adjusting the optical path length L1, the signal light from each recording layer 15 can be made to interfere with the reference light, and the recording signal on each recording layer 15 can be read.

  FIG. 4 is a diagram schematically showing a recording signal reading method according to the present embodiment.

  As shown in FIG. 5A, in the present embodiment, the objective lens is configured so as to cover all the recording layers 15 between the servo layers 13 where the focal depth of the reproduction light is adjacent. In the following description, the five recording layers 15 between the servo layers 13 are referred to as layers L1, L2, L3, L4, and L5 in order from the incident side of the reproduction light. In addition, a light beam in the range of the focal depth is particularly referred to as a focal light beam.

  When the optical path length L1 in FIG. 3 is changed by displacing the mirror 23 with the drive signal shown in FIG. 3C, for example, in the state of FIG. As shown in FIG. 4B, at the timing when the difference between the optical path length L2 and the optical path length L1 for each layer is within the coherence distance Δlc (hereinafter referred to as “coherence timing”), light detection is performed. The output from the vessel 24 increases in a pulse shape.

  Therefore, as shown in FIG. 4B, the time window ΔT is set so as to include the coherent timing, and each layer is detected by detecting whether a pulse is generated in the output from the photodetector 24 within the time window ΔT. It is possible to read the signal recorded in In FIG. 5B, at the position of the focal light beam, there are recording marks in the layers L1, L3, and L4, and no recording marks are present in the layers L2 and L5. Therefore, the signal “1” is read from the layers L1, L3, and L4, and the signal “0” is read from the layers L2 and L5.

  The time window ΔT can be set as follows, for example. That is, a reference position where the recording marks on the layers L1 to L5 are arranged in a line in the stacking direction is provided, and the focal beam of the reproduction light is positioned at this reference position. In this state, the mirror 23 is displaced by the scan signal shown in FIG. 4C, and the pulse generation timing (coherence timing) during the recording layer scan period is detected. Then, during the recording layer scanning period, the time window ΔT is set in correspondence with this timing.

  At the time of reproduction, the position of the focused light beam of the reproduction light is intermittently shifted bit by bit along the guide track of the servo layer 13. Then, the mirror 23 is scanned as described above between the shift of the focal light flux and the next shift, and the recording signal at that position is read from each layer and stored in the memory. In this way, the recording data of each layer is sequentially mapped on the memory. In parallel with this, the recording data is sequentially read from the memory so as to follow the data sequence and demodulated to generate and output a reproduction data string.

  FIG. 5 is a diagram illustrating a specific configuration example of the optical pickup device. FIG. 4A is a plan view of an optical system excluding the quarter wavelength plate 106, the servo objective lens 107, and the reproduction objective lens 116, and FIG. 5B shows the rising mirror 105 and the quarter wavelength plate. 106 is a side view of the optical system in the portions of the servo objective lens 107 and the recording / reproducing objective lens 116. FIG.

  In the figure, reference numerals 101 to 109 denote servo optical systems, and reference numerals 111 to 118 denote signal reading optical systems.

  The semiconductor laser 101 emits servo light having a predetermined wavelength. The diffraction grating 102 divides the servo light into three beams. The polarization beam splitter 103 substantially totally reflects the servo light incident from the diffraction grating 102 side, and substantially totally transmits the servo light incident from the collimator lens 104 side. The collimating lens 104 converts servo light into parallel light.

  The raising mirror 105 reflects the servo light incident from the collimating lens 104 side toward the servo objective lens 107. The raising mirror 105 reflects the reproduction light incident from the expander 114 side toward the reproduction objective lens 116.

  The quarter-wave plate 106 converts the servo light incident from the rising mirror 105 side into circularly polarized light, and also converts the servo light (reflected light from the recording medium 10) incident from the servo objective lens 107 side, The light is converted into linearly polarized light orthogonal to the polarization direction when traveling toward the servo objective lens 107. The servo objective lens 107 converges the servo light on the servo layer 13.

  The anamorphic lens 108 introduces astigmatism into the servo light reflected by the polarization beam splitter 103 (reflected light from the recording medium 10). The photodetector 109 receives the servo light converged by the anamorphic lens 108 and outputs a detection signal. The photodetector 109 is provided with a four-divided sensor that receives servo light. The photodetector 109 is arranged so that the optical axis of the servo light passes through the intersection of the sensor dividing lines of the four-divided sensor.

  The SLD 111 emits reproduction light with low coherence. As described above, in the SLD 111, the emission spectrum has a Gaussian distribution, and the full width at half maximum Δλ is such that the coherence distance Δlc obtained by the equation (1) is smaller than the pitch distance between the recording layers 15 of the recording medium 10. And the center light wavelength λc are set.

  The collimator lens 112 converts the reproduction light emitted from the SLD 111 into parallel light. The half mirror 113 transmits half of the reproduction light converted into parallel light by the collimator lens 112 and reflects the other half.

  The expander 114 is a combination of a concave lens and a convex lens, and one of the lenses is driven in the optical axis direction by the actuator 115. Here, the actuator 115 includes a motor, a worm gear, and the like, and is driven according to a servo signal for positioning the focus light beam of the reproduction light on the layers L1 to L5 as described above. The reproduction light that has passed through the expander 114 is reflected by the rising mirror 105 toward the reproduction objective lens 116.

  The reproduction objective lens 116 converges reproduction light on the layers L1 to L5. That is, the reproduction objective lens 116 is configured such that the focal depth of reproduction light covers the layers L1 to L5.

  The delay element 117 changes the optical path length between the half mirror 113 and the mirror 118 according to the control signal. The delay element 117 is constituted by, for example, a liquid crystal element whose refractive index with respect to reproduction light changes according to an applied voltage (control signal). The mirror 118 reflects the reproduction light transmitted through the delay element 117 in a direction opposite to the incident direction. As described above, this reflected light is used as reference light.

  In the configuration of FIG. 3, the optical path length of the reference light is changed by displacing the mirror 23, but in the configuration of FIG. 5, the reference light is driven by driving the delay element 117 without displacing the mirror 118. The optical path length changes. For example, when the delay element 117 is a liquid crystal element, the refractive index of the liquid crystal element with respect to the reference light can be changed according to the applied voltage (control signal), thereby changing the optical path length of the reference light. Can do.

  The condenser lens 119 converges the reflected light from the recording medium 10 that passes through the half mirror 113 and the reflected light (reference light) from the mirror 118 reflected by the half mirror 113. A photodetector (for example, APD: Avalanche Photo Diode) 120 receives the light converged by the condenser lens 119 and outputs a signal corresponding to the light intensity.

  The quarter-wave plate 106, the servo objective lens 107, and the reproduction objective lens 116 are mounted on a common holder 131. Here, the holder 131 is driven in the focus direction and the tracking direction by the objective lens actuator 132. The objective lens actuator 132 includes a conventionally known coil and a magnetic circuit, and the coil is mounted on the holder 131.

  When the servo signal is supplied to the objective lens actuator 132, the quarter-wave plate 106, the servo objective lens 107, and the reproduction objective lens 116 are displaced integrally with the holder 131 in the focus direction and the tracking direction. .

  FIG. 6 shows a configuration of an optical reproducing apparatus that reproduces the recording medium 10.

  As shown in the figure, the optical reproducing apparatus includes a laser driving circuit 201, an optical pickup device 202, a signal calculation circuit 203, a demodulation circuit 204, a memory control circuit 205, a memory 206, a decoder 207, and a servo circuit 208. , An XY stage 209 and a controller 210.

  The laser drive circuit 201 supplies drive signals to the semiconductor laser 101 and the SLD 111 in the optical pickup device 202 in order to output servo light and reproduction light at a constant power during reproduction.

  The optical pickup device 202 includes the optical system shown in FIG. The signal arithmetic circuit 203 generates a focus error signal and a tracking error signal based on the output from the photodetector 109 (for servo) shown in FIG. 5 and outputs them to the servo circuit 208. The signal arithmetic circuit 203 amplifies and removes noise from the output from the photodetector 120 (for reproduction) shown in FIG. 5 to generate a reproduction signal, and outputs this to the demodulation circuit 204.

  The demodulator circuit 204 generates binary data “1” and “0” from the reproduction signal input from the signal arithmetic circuit 203, and outputs this to the memory control circuit 205. The memory control circuit 205 writes data to and reads data from the memory 206 in accordance with instructions from the controller 210. The memory 206 is a writable and readable memory such as a RAM (Random Access Memory).

  The memory control circuit 205 writes the binarized data input from the demodulation circuit 204 in a predetermined area in the memory 206. In addition, the memory control circuit 205 sequentially reads the binarized data written in the memory 206 so as to follow the reproduction sequence, and outputs it to the decoder 207. The decoder 207 performs decoding processing such as error correction on the binarized data input from the memory control circuit 205 to generate reproduction data, and sequentially outputs it to the subsequent circuit.

  The servo circuit 208 generates a focus servo signal and a tracking servo signal from the focus error signal and tracking error signal input from the signal arithmetic circuit 203 and outputs them to the objective lens actuator 132 in the optical pickup device 202. Further, the servo circuit 208 outputs a drive signal for displacing the focal light beam of the reproduction light in units of bits along the guide track as described above to the XY stage 209. In addition, the servo circuit 208 drives the actuator 115 of the optical pickup device 202 as will be described later during the reproducing operation.

  The XY stage 209 drives the recording medium 10 in a direction parallel to the recording layer 15 (XY plane direction in the figure). The XY stage 209 is provided with a holding unit that holds the recording medium 10 without deviation, and the recording medium 10 is attached to the holding unit. The XY stage 209 drives the mounted recording medium 10 in the XY plane direction in units of bits according to a drive signal from the servo circuit 208.

  By driving the XY stage 209 in the X direction, the reproduction light is displaced along the guide track, and by driving the XY stage 209 in the Y direction, the reproduction light is transferred from one guide track to another. Displacement to the guide track. The XY stage 209 can be finely driven to such an extent that the recording medium 10 can be displaced in bit units of a recording signal. For example, a piezo element is used as a driving source.

  The controller 210 includes a CPU (Central Processing Unit) and a built-in memory, stores various data in the built-in memory, and controls each unit according to a preset program.

  During reproduction of the recording medium 10, servo light is drawn into the servo layer 13 corresponding to the reproduction target layer group with the expander 114 in the optical pickup device 202 set to the initial position. At this time, the XY stage 209 is driven, and the servo light is positioned in a desired recording area 10 a on the recording medium 10. Thus, the servo light is drawn into the desired servo layer 13 in the desired recording area 10a.

  In the state in which this pull-in is performed, the focal beam of the reproduction light is in a state of being deviated by a certain distance in the optical axis direction from the position covering all the reproduction target layer groups (layers L1 to L5). The servo circuit 208 outputs a signal for shifting the focal light beam in the optical axis direction by this deviation amount to the actuator 115 in the optical pickup device 202. As a result, the expander 114 is driven from the initial position, and the focal luminous flux of the reproduction light is drawn to a position that covers all the reproduction target layer groups (layers L1 to L5). Thus, when the pull-in process for the focal light flux of the reproduction light is completed, the XY stage 209 is driven, and the focal light flux of the reproduction light is positioned at the reproduction start position in the recording area 10a.

  Thereafter, the focal beam is sequentially shifted bit by bit along the guide track. Then, the delay element 117 is driven between the shift of the focal beam and the next shift, and the optical path length of the reference light is changed. Here, the change in the optical path length of the reference light is between the signal light from the layers 1 to 5 and the reference light as in the case of the change in the optical path length by the mirror 23 described with reference to FIG. In the range where interference occurs sequentially. By this scanning operation, the recording signal recorded in each layer at that position is read and output from the signal arithmetic circuit 203 to the demodulation circuit 204.

  The demodulation circuit 204 generates binarized data from the signals based on each layer and outputs them to the memory control circuit 205. The memory control circuit 205 sequentially stores the input binarized data in the memory 206. In this way, the binarized data of each layer is mapped in the memory 206.

  In parallel with this, the memory control circuit 205 sequentially reads the binarized data from the memory 206 and outputs it to the decoder 207 in accordance with the data sequence. The decoder 207 decodes the input binarized data to generate reproduction data, and sequentially outputs it to the subsequent circuit.

  FIG. 7 is a diagram showing a processing flow during the reproduction operation.

  When the reproduction operation is started, servo light and reproduction light are emitted (S101). Next, the focus position of the servo light is drawn to a position corresponding to the reproduction start position of the target track (S102). Then, the expander 114 is driven, and the focal beam of the reproduction light is drawn to a position that covers all the layer groups (layers L1 to L5) on the target track (S103).

  Thereafter, the above-described scanning operation by the delay element 117 is performed (S104), and the binarized data based on the recording signal on each layer is mapped to the memory 206 (S105). If the reproduction operation is not completed (S107: NO), it is determined whether the reading position is at the end position of the guide track (S108). Here, if the reading position is not at the end position of the guide track (S108: NO), the XY stage 209 is driven one step in the X direction, and the focal beam of the reproduction light is the guide track by one bit of the recording signal. (S110). Thereafter, the process returns to S104, and signal reading of the layer group (layers L1 to L5) at the next step position is performed.

  When this operation is repeated and signal reading is performed up to the end position of the guide track (S108: YES), the XY stage 209 is driven and the focal beam of the reproduction light is moved to the start position of the next guide track. (S109). Then, returning to S104, signal reading of the layer group (layers L1 to L5) in the next guide track is performed.

  In parallel with the signal reading, the binary data is sequentially read from the memory 206 and decoded so as to follow the data sequence (S106). The reproduction data generated by the decoding process is sequentially output to the subsequent circuit.

  As described above, according to the present embodiment, the delay element 117 is driven in a state in which the focus light flux of the reproduction light is positioned in the reproduction target layer group (layers L1 to L5), so that the layer group (layers L1 to L1) is driven. The recorded data can be reproduced simultaneously from L5). Therefore, it is possible to increase the efficiency or speed of the reproduction operation.

  Further, since the layers are formed such that the distance between the layers exceeds the coherence distance Δlc, when reproducing the reproduction target layer, unnecessary reproduction light reflected by the upper and lower layers sandwiching the layer ( (Stray light) and reference light do not interfere with each other. Therefore, even if this stray light is incident on the photodetector, its intensity is several steps smaller than the intensity of the signal light amplified by interference with the reference light, and therefore the influence of stray light on the reproduction signal is significant. It will be small.

  As described above, according to the present embodiment, even when a large number of layers are arranged in the stacking direction, adverse effects due to stray light can be effectively suppressed, and the recording medium can be reproduced smoothly and satisfactorily.

  If a recording signal is recorded so that each layer corresponding to the same guide track is connected from the end of one layer to the beginning of the next layer, at least one guide track is stored in the memory 206. The binarized data can be continuously supplied to the decoder 207 by providing a capacity sufficient to buffer the binarized data based on the layer group (layers L1 to L5) corresponding to. Therefore, if the recording signal is recorded on the recording medium 10 in this way, the capacity of the memory 206 for storing the binarized data can be reduced, and the cost can be reduced.

  By the way, as can be seen from the above equation (1), the coherence distance Δlc decreases as the full width at half maximum Δλ of the reproduction light increases. Accordingly, the distance between adjacent layers can be reduced as the full width at half maximum Δλ of the reproduction light is increased. Therefore, in order to increase the capacity of the recording medium, it is desirable that the full width at half maximum Δλ of the reproduction light is large. From this point of view, in the present embodiment, the SLD 111 is used as a light source of reproduction light.

  As another method of increasing the full width at half maximum Δλ of the reproduction light, a method of causing the laser light to emit light and shortening the pulse width can also be used. When this method is used, a femtosecond laser such as a fiber laser capable of emitting an ultrashort pulse laser beam can be used as a light source for reproducing light.

  FIG. 8 is a diagram showing an optical system of an optical pickup device when a fiber laser is used. In the figure, reference numeral 141 denotes a fiber laser. In this optical system, an optical path adjustment unit 143 including an optical fiber 143a and a piezo element 143b is arranged instead of the delay element 117 in FIG. Here, the optical path adjustment unit 143 has a configuration in which an optical fiber 143a is wound around a piezo element 143b whose diameter changes in accordance with an applied signal.

  After the reproduction light emitted from the fiber laser 141 is converted into parallel light by the collimator lens 112, half of the light is reflected by the half mirror 113 and the other half is transmitted through the half mirror. The optical path of the reproduction light after being reflected by the half mirror 113 is the same as in the case of FIG.

  The reproduction light transmitted through the half mirror 113 is collected by the condenser lens 142 and enters the optical fiber 143a. The reproduction light emitted from the optical fiber 143 a is converted into parallel light by the condenser lens 144 and reflected by the mirror 118. The reproduction light (reference light) reflected by the mirror 118 reverses the optical path when traveling from the half mirror 113 to the mirror 118, and half of the light is reflected by the half mirror 113. Thereafter, the reference light is collected on the photodetector 120 by the condenser lens 119.

  In this optical system, by changing the signal applied to the piezo element 143b, the overall length of the optical fiber 143a changes, and the optical path of the reference light changes. Therefore, by adjusting the signal applied to the piezo element 143b by the servo circuit 208 shown in FIG. 6, the optical path length difference between the signal light from each layer in the recording medium 109 and the reference light can be changed. Similar to the above embodiment, signals on each layer can be read by interference between both signals.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications other than those described above can be made to the embodiments of the present invention.

  For example, in the above embodiment, the recording medium 10 is a card-like medium, but it can also be a disk-like medium having a guide track spirally. In the above embodiment, the depth of focus covers all the layers (layers L1 to L5) between the servo layers 13, but the depth of focus is limited to only a predetermined number of layers among the layers between the servo layers 13. You may make it cover. Furthermore, in the above embodiment, the number of the recording layers 15 existing between the servo layers 13 is five, but other number of recording layers 15 may be arranged between the servo layers. In addition, the present invention can also be applied to an optical pickup device and an optical reproduction device that reproduce information from a recording medium without a servo layer.

  The embodiments of the present invention can be appropriately modified in various ways within the scope of the technical idea shown in the claims.

The figure which shows typically the cross-section of the recording medium which concerns on embodiment The figure which shows typically the planar structure of the recording medium which concerns on embodiment The figure which shows the figure which shows the basic composition of the optical pick-up apparatus which concerns on embodiment The figure which shows typically the reading method of the recording signal which concerns on embodiment The figure which shows the specific structural example of the optical pick-up apparatus which concerns on embodiment The figure which shows the structure of the optical reproduction apparatus which concerns on embodiment The figure which shows the process flowchart at the time of reproduction | regeneration operation | movement which concerns on embodiment The figure which shows the other specific structural example of the optical pick-up apparatus which concerns on embodiment

Explanation of symbols

10 Recording media 21 SLD
22 Beam splitter 23 Mirror 24 Photo detector 111 SLD
113 half mirror 114 expander 116 objective lens for reproduction 117 delay element 118 mirror 120 photodetector 114 fiber laser 143 optical path adjustment unit 143a optical fiber 143b piezo element 202 optical pickup device 203 signal arithmetic circuit 204 signal processing circuit 205 memory control circuit 206 memory 207 Decoder 208 Servo circuit

Claims (5)

A light source that emits reproduction light;
A first optical system for focusing the reproduction light on a recording layer in a recording medium;
A photodetector for receiving the reproduction light reflected by the recording layer;
A second optical system that superimposes reference light on reproduction light reflected by the recording layer;
An optical path adjustment unit that is arranged in the second optical system and changes an optical path length of the second optical system according to a control signal ;
The optical path adjustment unit includes an optical element that receives the reference light and changes a refractive index with respect to the reference light according to the control signal.
An optical pickup device characterized by that. In claim 1,
The second optical system includes:
A beam splitter for separating the part of the reproduction light emitted from the light source from the optical path of the first optical system and generating the reference light;
A mirror unit that reflects the separated reference light and returns it to the beam splitter;
The optical path adjustment unit is
Changing the optical path length of the reference light from being separated by the beam splitter to returning to the beam splitter;
An optical pickup device characterized by that. In claim 1 or 2,
The optical path adjustment unit includes an optical fiber on which the reference light is incident, and a piezoelectric element that changes a total length of the optical fiber according to the control signal.
An optical pickup device characterized by that. An optical pickup device according to any one of claims 1 to 3 ,
An optical path control circuit for controlling the optical path adjustment unit;
A reproducing circuit for reproducing a signal output from the photodetector;
Having
An optical regenerator. In claim 4 ,
The optical path control circuit changes the optical path length of the second optical system so that the reproduction light from the plurality of recording layers within the focal depth of the reproduction light and the reference light interfere with each other.
The reproduction circuit reproduces the recording data recorded in each recording layer based on a signal output from the photodetector when the reproduction light from the plurality of recording layers interferes with the reference light.
An optical regenerator.

Images