JP2012185872A - Information recording and reproduction device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform multi-value recording of light phase information using a device configuration simpler than a conventional one.SOLUTION: In an information recording and reproduction device, a phase is recorded using a void 19 as a minute reflector. A laser pulse from a laser diode 13 is converted into parallel light by a collimator lens 14, and then focused by a lens 15. A refractive index modulator 16 is disposed in a space between the lens 15 and the focal point thereof. The laser light once focused changes into divergent light beyond the focal point, then is converted into parallel light by a lens 17, and finally is focused in a recording possible area 11 by an objective lens 18 with the result that a void 19 is formed. When the refractive index of the convergent light path is changed with the application of a voltage to the refractive index modulator 16, the position of the focal point of the lens 15 is shifted. In such a case, the light outgoing from the lens 17 ceases to be parallel light, turning into weakly convergent or divergent light. This causes the position of the focal point of the objective lens to shift, so that the record depth of the void 19 can be modulated.

Description

  The present invention relates to a method for recording / reproducing information using the phase of light and an information recording / reproducing apparatus embodying the method.

  Some of the terms in the following description use expressions used in Blu-ray Disc (BD). These may have different names in systems other than BD. However, it can be easily read by those skilled in the art.

  Increasing the recording capacity of an optical disc has been realized by increasing the number of recording layers per disc in addition to shortening the wavelength of the light source and increasing the aperture ratio (NA) of the objective lens. BD uses a blue semiconductor laser and a high NA objective lens with an NA of 0.85 to achieve a recording capacity of 50 GB in two layers. Furthermore, in 2010, BDXL having a recording capacity of 100 GB or more was put to practical use by increasing the number of recording layers to 3 to 4 and simultaneously increasing the surface recording density.

  Shortening the recording wavelength and increasing the NA of the objective lens are close to the limit, and it is not easy to greatly improve the surface recording capacity in the future. Therefore, in order to realize the above-described recording capacity, it is one effective solution to further increase the number of recording layers. However, if an attempt is made to increase the number of recording layers with a configuration similar to that of a conventional multilayer optical disc, there is a high possibility that it is difficult to realize a cost reduction per recording capacity. This is because the manufacturing cost and yield of the current multilayer optical disk are exclusively related to the recording layer formation process. That is, the increase in the number of layers is directly linked to the increase in the number of processes, and the final yield is generally determined by the power of the number of layers in the stamper process yield per layer.

  In view of this, an optical disc that does not have a physically defined recording layer, such as a conventional multilayer disc, and a recording technique thereof have been studied. As an example, in the technique described in Patent Document 1, a micro hologram, that is, a minute interference fringe is recorded in a recording region made of a photorefractive material. Since there is no structure that physically defines the recording position in the recording area, the recording position of each micro-hologram is determined by indirectly controlling the focal position of the light (recording light) used for recording. . As another example, Japanese Patent Application Laid-Open No. H10-228867 discloses that recording is performed by forming a void (gap) in a recording area. Patent Document 3 describes a technique for forming a void by using a short pulse laser and a CW laser together during recording. According to these recording methods, it is possible to increase the number of virtual recording layers relatively freely, and it is easy to increase the recording capacity per disc. In the present specification, as described above, for the sake of convenience, the system having no layer that physically defines the recording position in the recording area is collectively referred to as spatial recording.

  When the number of recording layers is increased including the spatial recording described above, the problem is a decrease in the amount of reflected light from the layer being reproduced. Since the output of the recording light source is finite, in order to perform recording on the innermost layer when viewed from the incident surface of the reproduction light with a disk having a large number of recording layers, the transmittance of each recording layer in the middle Is required to be sufficiently high. Conversely, the light reflectance and absorptance of each layer must be sufficiently small. In addition, since the recording sensitivity of the recording film is set high in order to perform recording on a recording layer having a low absorption rate, there is a limit to increasing the power of the emission light (reproduction light) of the pickup during reproduction. is there. For this reason, generally, the amount of light returned from the recording layer during reproduction decreases as the number of recording layers increases. Therefore, a reduction in the signal-to-noise ratio (SNR) of the reproduction signal becomes a problem.

  As a technique to counter the SNR reduction of the reproduction signal, there is a signal amplitude amplification technique using optical interference as described in Patent Document 4. That is, the reproduction signal is amplified by causing the reference light obtained from the light source common to the reproduction light to interfere with the reflected light from the recording layer on the photodetector. In this specification, the reference light and reproduction light obtained from a light source common to such reproduction light and the reproduction optical system are collectively referred to as homodyne detection and homodyne detection systems, respectively. I will do it.

JP 2008-97723 A JP 2009-238285 A JP 2009-238282 A JP 2009-252337 A US Pat. No. 5,173,909

Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 1062-1067

  One of the main performances of the optical disk drive is a data transfer speed during recording and reproduction (hereinafter simply referred to as transfer speed). This is an important performance item particularly when used in the non-consumer field. The transfer speed is primarily determined by the linear recording density and the linear velocity of the disk. Further, the linear velocity of the disk is limited by the realizable rotation speed of the disk. In the case of a disc made of polycarbonate, which is a material used for almost all optical discs and having a diameter of 12 cm, the limit of the rotational speed is considered to be about 10,000 rpm (rotations per minute) in consideration of vibration and deformation.

  The linear recording density is primarily determined by the optical resolution of the reproducing head, and is further determined in consideration of a practical performance margin (margin) and a performance improvement effect by signal processing. The optical resolution is determined by the wavelength of the light source used by the head and the aperture ratio of the objective lens. In other words, the upper limit of the transfer speed of the optical disc drive is determined solely by the upper limit of the disc rotation speed that can be realized and the linear recording density. Since the above matters are known to those skilled in the art, further details are omitted.

  However, as described above, the optical resolution has already reached the limit. In order to improve the transfer speed in this situation, the conventional multi-value recording method for recording information exceeding 1 bit per channel clock is promising instead of binary recording for recording 1 bit per channel clock. is there. In multi-value recording, the recording capacity per unit length also increases, which naturally leads to an increase in the recording capacity of the disc.

  Regarding the multi-value recording method, there is a technique described in Non-Patent Document 1. This is because the recording waveform is improved with respect to the recording film similar to that used in the conventional recording type optical disc medium, and the reflectance is increased from the conventional binary modulation to the maximum eight-value modulation. Increase. However, since the amplitude of the reproduction signal is the same as that of the conventional optical disk, the SNR between signal levels corresponding to each gradation of the reflectance is lowered, so that it is not necessarily suitable for improving the transfer rate.

  As described above, in order to greatly improve the transfer speed as compared with the current technology under the restrictions on the improvement of the rotational speed and linear recording density of the disk, a binary that records 1 bit per channel clock is used. One solution is a multi-value recording method in which information exceeding 1 bit per channel clock is recorded instead of recording. In the following, an information unit recorded in one channel clock is simply called a symbol. Similarly, channel bits are simply referred to as bits within a range that does not cause confusion. Further, data to be recorded before encoding is referred to as user data, and the minimum unit in the binary representation is referred to as user bits. Thus, for example, in binary recording without encoding, one symbol corresponds to 1 bit, that is, 1 user bit, and in the case of 8-level recording without encoding, 1 symbol corresponds to 3 bits, that is, 3 user bits. . The expression “symbol” is a term widely used in the field, but is used in the above-mentioned meaning within a range that does not cause confusion.

As described above, in realizing a multilevel recording / reproducing system, when a signal obtained when reproducing information expresses information only by amplitude modulation, it is difficult to improve the transfer rate. Considering the case of an optical disk using a reflected light intensity change such as DVD or BD, since the interval of the light intensity for determining the value of each symbol is narrowed, the error rate when determining the level of each symbol is 2. It becomes larger than in the case of value recording / reproduction. This becomes more prominent when a circuit system having a bandwidth necessary for high-speed transfer is used, and as a result, it is difficult to realize high-speed transfer.
The present invention provides an optical recording / reproducing system capable of realizing multi-level recording / reproduction capable of improving the transfer rate.

  The present invention has means for forming a minute reflector in the recording medium. Then, multi-value recording is performed by modulating the recording depth of the reflector.

  As means for modulating the recording depth of the minute reflector, means for modulating the optical distance of the light path used for recording is used. Further, according to another method, there is means for modulating the wavelength of the recording light. According to another method, it has a converging optical element having a significant residual spherical aberration and a light beam selecting element. According to another method, the optical element has a variable focal length.

According to the present invention, a state recognized as light phase information during reproduction can be recorded in a recording medium. As a result, it becomes possible to realize a multi-value recording / reproducing apparatus using optical phase information with a mechanism having the same degree of complexity as that of the current optical disc apparatus.
Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

FIG. 3 is a schematic diagram showing the principle of phase recording according to the present invention. The figure explaining the mode of micro hologram recording. The figure explaining the method of the phase recording which applied the micro hologram. Explanatory drawing of micro hologram phase recording. Explanatory drawing of a homodyne detection technique. The figure which shows the example of a modulation | alteration which applied the expansion | swelling of gas. The figure which shows the example of 1 structure which modulates a recording depth using the focal distance modulation of an optical system. The figure which shows the structural example of a secondary mirror. The figure which shows one structural example which modulates a recording depth using the wavelength modulation of a recording light source. An explanatory view of an example of modulating the recording depth by switching the optical path. The figure which shows the structural example of the recording / reproducing apparatus using this invention.

  As a multilevel recording / reproducing method suitable for high-speed transfer, it is conceivable to use information other than amplitude, that is, a phase. As a method for recording a phase in optical recording, a method using a micro-hologram described in Patent Document 1 can be considered. In the method described in Patent Document 1, the micro-hologram is simply used as a minute reflector. As shown in FIG. 2, the micro hologram is focused at the same location by two objective lenses 108a and 108b in which two laser beams emitted from the same light source are opposed to each other, and interference fringes are recorded in the vicinity of the focus. To form a recording mark. At the time of reproduction, the intensity of the reflected light from the recording mark made of this micro-hologram is detected as in the case of the current optical disc.

  In order to record the phase in the micro-hologram, as shown in FIG. 3, one of the two laser beams used for recording may be modulated in accordance with the recording data. The laser beam emitted from the semiconductor laser 101 is converted into parallel rays by the collimator lens 102, and then divided into two by the non-polarization half beam splitter 118, and one of them is focused on the recording medium 1 using the objective lens 108a through the mirror 116a. Tie. The other laser beam passes through the phase modulator 10 and is similarly guided to the recording medium via the mirrors 116b and 116c and the objective lens 108b, and the two laser beams are focused at the same location. Here, when the phase modulator 10 is driven in accordance with the recording data, the position of the interference fringes forming the micro hologram changes in the optical axis direction. In other words, the interference fringe sequence constituting the micro-hologram recorded by this method changes its position according to the recording data sequence. Considering the case where the reproduction light is irradiated to the micro-hologram, the reproduction light is reflected by each interference fringe constituting the micro-hologram, and the micro-hologram can be regarded as one reflector as a whole. Therefore, when the position of the interference fringes constituting the micro-hologram changes in the optical axis direction, the phase of the reflected reproduction light changes. Therefore, the phase recorded on each micro-hologram can be known by discriminating the phase change of the reproduction reflected light. Further, in phase recording using a micro-hologram, there is no physically defined recording surface or recording layer as in a conventional optical disc. However, for the sake of simplicity, for convenience sake, a micro-hologram recorded in a planar shape is referred to as a recording layer or a recording surface. Similarly, a microhologram recorded in a row is called a track.

  However, the phase recording method using a micro-hologram needs to introduce two opposing laser beams into a recording medium, which makes it difficult to put it into practical use. In order to introduce two opposing laser beams into the recording medium, it is necessary to make the light incident from both sides of the recording medium. That is, it is necessary to place two objective lenses on both sides of the medium and control their positions so as to focus on the same location in the medium. Of course, the mechanical complexity is greatly increased compared with the current optical disc.

  Further, when two opposing laser beams are introduced into the recording medium, the depth at which the micro-hologram is recorded is a problem. That is, it is obvious that it is necessary to use an objective lens having a sufficiently large NA in order to form a minute recording mark. On the other hand, it is obvious that the recording medium needs a certain thickness in order to maintain its own mechanical strength. In other words, a thickness of about 1.2 mm, which is equivalent to the current one, is necessary. Even if only the vicinity of the center in the thickness direction of the recording medium is considered, it is necessary to correct the spherical aberration while passing through a very thick cover layer of about 0.6 mm. This is very difficult for those skilled in the art to easily understand. In addition, it can be easily understood by those skilled in the art that when such a thick cover layer is passed, the influence of the disc tilt is extremely large.

  By the way, in the phase recording using a micro hologram, the micro hologram acts as a minute reflector. The phase information is recorded as position modulation in the depth direction of the interference fringes constituting the front and rear micro-holograms. This will be described with reference to FIG.

  FIG. 4 shows the two-dimensional intensity distribution of the standing wave at the condensing location of the signal light and the reference light. The horizontal direction is the signal light, the optical axis direction of the reference light, and the vertical direction is an (arbitrary) direction perpendicular thereto, and the darker the portion, the higher the intensity. 4A shows the intensity distribution when the phase modulation amount is 0, and FIG. 4B shows the intensity distribution when the phase modulation amount is 2πΔl / λ (λ is the wavelength of the signal light and the reference light), that is, It is an intensity distribution when the optical path length is modulated by Δl by the phase modulator. As shown in FIGS. 4A and 4B, the peak position of the standing wave is shifted by Δl / 2 with respect to the optical path length modulation Δl. When light is irradiated to such an intensity distribution recorded as a change in the refractive index of the medium, the light is reflected by a mirror whose position in the optical axis direction has changed by Δl / 2 as shown in FIG. In exactly the same manner as when the light is reflected, the optical path length of the reflected light changes by Δl as shown in FIG. Therefore, the modulated phase at the time of recording is reproduced as the phase of the reproduction light.

  As described above, the phase information is recorded as a position modulation in the depth direction (optical axis direction) of the effective reflector interference fringes. Therefore, a phase-modulated reproduction signal can be obtained by modulating the recording depth of a reflector other than the micro-hologram. In this case, at the time of recording, the phase information to be recorded is once converted into amplitude information (depth modulation information), so that the phase characteristic of light is not used at the time of recording. However, since the intensity of the recording light is sufficiently strong, there is no problem because it is not necessary to consider the influence of noise. In addition, it is not unthinkable to use a mechanical mechanism such as an actuator for driving the objective lens to modulate the recording depth. However, such a mechanical mechanism is not practical because the operating frequency is extremely low.

  In the present specification, the case where recording is performed so that a reproduction signal that has been phase-modulated in this way is also referred to as phase recording.

FIG. 1 is a schematic diagram showing the principle of multilevel recording according to the present invention. In this example, phase recording is performed using a void described in Patent Document 2 as a minute reflector. In the recording medium, a recordable area 11 made of a nitrocellulose resin is formed on a substrate 12. The phase recording process will be described. The laser diode 13 repeats pulse emission with a drive pulse synchronized with the recording clock. The laser pulse emitted from the laser diode is converted into parallel light by the collimator lens 14 and then focused by the lens 15. At this time, the refractive index modulator 16 is inserted in the space that passes through the convergent light beam from the lens 15 to the focal point. As the refractive index modulator 16, for example, a spatial optical phase modulator can be used. That is, by applying a voltage signal to an electro-optic crystal such as LiNbO ₃ , the refractive index is temporally changed according to the magnitude of the applied voltage. Once focused, the laser light becomes divergent light outside the focus. The divergent light is converted into parallel light by the lens 17 and then focused in the recordable area 11 by the objective lens 18, and a void 19 is formed by the pulse laser. In the state where the applied voltage to the refractive index modulator 16 is 0, the focal point of the lens 15 is assumed to be the focal point of the lens 17.

  By the way, when the refractive index in the convergent optical path is changed by applying a voltage to the refractive index modulator 16, the optical distance between them changes, so that the position of the focal point connected by the lens 15 changes. That is, the focal position of the lens 15 moves to the inside or the outside of the focal position of the lens 17. Then, the light emitted from the lens 17 is not parallel light but weakly convergent or weakly divergent light, and the position of the focal point where the laser light is connected by the objective lens 18 is also moved, so that the recording depth of the void 19 can be modulated. .

である。ここで、ｆ₁，ｆ₂はそれぞれレンズ１５及び１７の焦点距離であり、ｄ＝λ／２である。 The required focal position movement amount in the recordable area 11 is approximately λ / 2 (converted to a value in the atmosphere). The focal movement distance Δf at the lens 15 required for this is

It is. Here, f ₁ and f ₂ are the focal lengths of the lenses 15 and 17, respectively, and d = λ / 2.

  Thus, according to the present invention, it is possible to record the phase information of light without introducing light from both sides of the recording medium like a micro-hologram.

Next, a method for reproducing information recorded in phase by forming a reflector in the recordable area of the recording medium will be described.
As means for detecting the phase change of the light reflected by the reflector, a phase diversity type homodyne detection technique (hereinafter referred to as homodyne detection) can be used. These will be described below. First, homodyne detection will be described. However, since those skilled in the art can easily understand homodyne detection and the structure and operation of an optical disk apparatus using the same by referring to Patent Document 4, only the outline necessary for the description of the present invention will be described below. To do.

  FIG. 5 is a diagram for explaining the operation of the homodyne detection system. Light from the semiconductor laser 101 is converted into parallel light by the collimator lens 102 and transmitted through the λ / 2 plate 103 to enter the polarization beam splitter 104. The polarization beam splitter 104 has a function of transmitting almost 100% of p-polarized light (hereinafter referred to as horizontal polarization) incident on the separation surface and reflecting almost 100% of s-polarized light (hereinafter referred to as vertical polarization). At this time, by adjusting the rotation angle around the optical axis of the λ / 2 plate 103, the intensity ratio between the transmitted light and the reflected light can be adjusted. The light that has passed through the polarization beam splitter 104 passes through the λ / 4 plate 106 a and is converted into circularly polarized light, and is condensed on the recording layer on the optical disc 1 by the objective lens 108 mounted on the two-dimensional actuator 107. The reflected light from the optical disk returns to the same optical path, is converted into parallel light by the objective lens 108, and is converted into linearly polarized light whose rotation direction is rotated by 90 ° from the time when it is first incident by the λ / 4 plate 106a. The light enters the splitter 104. Then, since the polarized light is rotated by 90 degrees, it is reflected and enters the condenser lens 113.

  On the other hand, the light emitted from the semiconductor laser 101 and reflected by the polarizing prism 104 is transmitted through the λ / 4 plate 106 b to be converted into circularly polarized light, and enters the mirror 116 mounted on the two-dimensional actuator 107. The light reflected by the mirror 116 returns on the same optical path, and enters the polarization beam splitter 104 through the λ / 4 plate 106b. Since it is converted into linearly polarized light whose 90 ° polarization direction is rotated when it passes through the λ / 4 plate 106b twice in the forward path and the return path, the reflected light is transmitted through the polarization beam splitter 104, and from the optical disk. The reflected light and the optical axis of the reflected light are coaxially incident on the condenser lens 113 with their polarizations orthogonal to each other. The two lights incident on the condenser lens 113 are reflected and transmitted by the non-polarizing beam splitter 118 at a ratio of 1: 1, respectively. The transmitted light is transmitted through the λ / 2 plate 119 and the polarization is rotated by 45 degrees, and then separated by the polarization beam splitter 120 into a horizontal polarization component and a vertical polarization component, and the separated lights are detected by the detectors 121 and 122. Detected by. The light reflected from the non-polarizing beam splitter 118 passes through the λ / 4 plate 123 and then separated into a horizontal polarization component and a vertical polarization component by the polarization beam splitter 124, and the separated lights are detected by the detectors 125 and 126. Is done.

  A process of obtaining an amplified signal by light interference will be described in detail. The light incident on the condenser lens 113 is obtained by coaxially combining the return light from the corner cube 116 that is horizontally polarized light and the return light from the optical disk 1 that is vertically polarized light. Therefore, when the polarization state of light is represented by a Jones vector, the following equation is obtained.

Here E _s is the electric field of the return light from the optical disk, the E _r is the electric field of the return light from the corner cube prism. The first component of the vector represents horizontal polarization, and the second component represents vertical polarization.

  This light is divided into two by a non-polarizing beam splitter, and the transmitted light passes through a λ / 2 plate having a fast axis in the direction of 22.5 degrees when viewed from the horizontal polarization direction. At this time, the Jones vector is as follows.

  Next, since the horizontal polarization component is transmitted by the polarization beam splitter and the vertical polarization component is reflected, the electric fields of the transmitted light and the reflected light are respectively expressed by the following equations.

  On the other hand, the light reflected by the non-polarizing beam splitter passes through a λ / 4 plate having a fast axis in the direction of 45 degrees as viewed from the horizontal polarization direction. At this time, the Jones vector is expressed by the following equation.

  Next, since the horizontal polarization component is transmitted by the polarization beam splitter and the vertical polarization component is reflected, the electric fields of the transmitted light and the reflected light are respectively expressed by the following equations.

  Accordingly, the detection signals of the four detectors 121, 122, 125, and 126 are respectively expressed by the following equations. η is the photoelectric conversion efficiency of the detector.

  Accordingly, the outputs of the analog subtractors 130 and 131 are as follows:

  Here, φ is the phase difference between the reproduction light and the reference light. In the present specification, hereinafter, the phase difference between the reference light and the reproduction light is simply referred to as a phase (the symbol is φ) unless particularly confusing.

  As understood from the equations (13) and (14), the phase difference between the reference light and the reproduction light can be known from the outputs of the two analog subtractors. Therefore, those skilled in the art can easily understand that the phase modulation amount during recording can be reproduced by observing this phase difference in time series. The phase is recorded as the position of the effective reflecting surface of the reflector, and this is reproduced by reflected light. Therefore, the phase modulation amount at the time of recording is recorded as 1/2 of the phase change amount intended at the time of reproduction. It should be understood by those skilled in the art that this should be done.

  In the example of FIG. 1, a refractive index modulator using an electro-optic crystal is taken as an example, but it is naturally possible to use a refractive index modulator of a principle other than this. FIG. 6 shows an example in which gas expansion is applied as an example. That is, the pulse laser 20 emits light in synchronization with the phase modulation signal, and the gas in the gas cell 21 is heated by the output. The focal position is changed by the expansion of the gas on the convergent optical path of the recording light. The amount of change in the focal position is determined by adjusting the output of the pulse laser. Instead of the gas cell, it is also possible to configure a refractive index modulator by disposing a cell in which a liquid is sealed or a solid medium on the converging optical path of the recording light and heating and expanding it on the same principle.

  As another method for modulating the recording depth, it is conceivable to modulate the focal length of the optical element corresponding to the lens 15 in FIG. An example is shown in FIG. 1 is different from the example of FIG. 1 in that the light emitted from the laser diode 13 is focused on the focal point using a reflection optical system instead of the collimating lens 14 and the lens 15. That is, the laser light emitted from the laser diode 13 is reflected by the primary mirror 26 and travels toward the secondary mirror 25. Then, the light is reflected by the secondary mirror 25, passes through the central hole of the primary mirror 26, is focused, and then converted into parallel light by the lens 17. The sub mirror 25 can change the focal length using the piezoelectric effect.

  FIG. 8 shows an example of the structure of such a secondary mirror. FIG. 8 is a cross-sectional view, and the main body includes a piezoelectric body 31. The arrow in the figure indicates the polarization direction of the piezoelectric body. The surface is almost entirely covered with a metal film 32 serving as a reflective surface and an electrode. On the back surface, the electrode 30 covers only a part near the center. When a voltage is applied between the metal film 32 and the electrode 30, the piezoelectric effect causes only the vicinity of the central portion sandwiched between the electrodes to expand and contract. As a result, the curvature of the entire mirror slightly changes and the focal length changes. . Since the required focal position displacement is in units of micrometers, a sufficient amount can be obtained even in such a system. Also, the diameter and thickness of the secondary mirror can be easily reduced, which is advantageous for high-speed driving.

  As another method for modulating the recording depth, it is conceivable to modulate the wavelength of the recording light. As described in Patent Document 5, it is possible to make the wavelength of a laser diode used for a recording light source variable. As a method for controlling the oscillation wavelength, in addition to a method for controlling the temperature, a method for controlling the refractive index of the waveguide constituting the resonator by current injection is also possible.

  For simplicity, consider a thin spherical lens. The focal length f of such a lens can be expressed by the following equation.

Here, n is the refractive index, and r ₁ and r ₂ are the radii of curvature of the two surfaces. Therefore, the focal length change Δf due to the refractive index change is expressed by the following equation.

Here, Δn is a refractive index change with respect to a wavelength change. If n = 1.5, r ₁ = 3 mm, r ₂ = −3 mm, and a lens using a glass material whose refractive index changes by 1 × e ⁻⁴ per 1 nm, the wavelength modulation range is about 0.33 nm. It becomes.

  In the current optical disk drive, a main optical element such as an objective lens may use a chromatic aberration-corrected one. In an achromatic optical system, the amount of movement of the focal point with respect to a change in wavelength of incident light is not always monotonous. Therefore, when the recording depth is modulated using wavelength modulation, it is necessary to design the entire optical system so that the focal length of the objective lens at least monotonously increases as the wavelength increases.

  An example of a configuration for obtaining sufficient recording depth modulation in a smaller wavelength modulation range is shown in FIG. In this example, in order to obtain sufficient recording depth modulation in a smaller wavelength modulation range, a lens made of high dispersion glass is used as a part of the optical system. The laser light emitted from the wavelength tunable laser diode 36 is converted into parallel light by the collimator lens 14 and then focused by the high dispersion lens 35a. The high dispersion lens 35 a is made of a material having a larger dispersion than the collimator lens 14 and the objective lens 18. The divergent light that has passed the focal point is converted into parallel light by another high dispersion lens 35 b and is incident on the objective lens 18. When the oscillation wavelength of the laser is extended, the focal position of the high-dispersion lens 35a on the condensing side approaches the other high-dispersion lens 35b, so that the incident light on the objective lens 18 tends to diverge accordingly, and in the recording medium. The focus position becomes deeper.

  As another method for modulating the recording depth, it is conceivable to change the optical distance by selecting an optical path. An example of this is shown in FIG. The laser diode 13 repeats pulse emission with a drive pulse synchronized with the recording clock. The laser pulse emitted from the laser diode 13 is converted into parallel light by the collimator lens 14 and then focused by the lens 15. A liquid crystal element 40 and a distance adjusting plate 41 are inserted between the lens 15 and its focal point. Once focused, the laser light becomes divergent light outside the focus. The divergent light is converted into parallel light by the lens 17 and then focused in the recordable area by the objective lens 18, and a void 19 is formed by the pulse laser.

  The liquid crystal element 40 is divided into two equal parts. When no voltage is applied, the light emitted from the laser diode 13 that is linearly polarized light is not transmitted through any region. Further, it is driven so that only the upper or lower half transmits the laser light in accordance with the phase modulation signal. The distance adjusting plate 41 is an optical glass plate that occupies only the lower half of the outgoing light path of the lens 15. Therefore, when the lower half of the optical path is selected by the phase modulation signal, the focal point is closer to the light source side in the distance adjustment plate 41 due to the higher refractive index. As a result, the recording depth is modulated.

  For the sake of simplicity, FIG. 10 has been described using a binary example. However, if the number of divided liquid crystal elements is increased and a distance adjusting plate corresponding to the phase assigned to each area is prepared, more phase values can be handled.

  FIG. 11 shows an example of the configuration of the optical disc apparatus. The optical disk 1 is rotated by a spindle motor 52. The optical head 51 includes a light source used for recording and reproduction, an optical system including an objective lens, and the like. Since the apparatus is based on the present invention, the homodyne detection system is used in the reproducing optical system. The pickup seeks with the slider 53. The seek and the rotation of the spindle motor are performed according to instructions from the main circuit 54. The main circuit includes a dedicated circuit such as a signal processing circuit and a feedback controller, a microprocessor, a memory, and the like. The firmware 55 controls the operation of the entire optical disc apparatus. The firmware is stored in a memory in the main circuit.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. In addition, a part of the configuration of a certain embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of a certain embodiment. Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

  Each of the above-described configurations, functions, processing units, processing means, and the like may be realized by hardware by designing a part or all of them with, for example, an integrated circuit. Each of the above-described configurations, functions, and the like may be realized by software by interpreting and executing a program that realizes each function by the processor. Information such as programs, tables, and files for realizing each function can be stored in a recording device such as a memory, a hard disk, or an SSD (Solid State Drive), or a recording medium such as an IC card, an SD card, or a DVD.

1: recording medium,
11: Recordable area, 12: Substrate, 13: Laser diode, 14: Collimator lens, 15: Lens, 16: Refractive index modulator, 17: Lens, 18: Objective lens, 19: Void, 20: Pulse laser, 21 : Gas cell, 25: Secondary mirror, 26: Primary mirror, 30: Electrode, 31: Piezoelectric body, 32: Metal film, 35a, 35b: High dispersion lens, 36: Wavelength tunable laser diode, 40: Liquid crystal element, 41: Distance Adjustment plate, 52: spindle motor, 53: slider, 54 main circuit, 55: firmware, 101: semiconductor laser, 102: collimating lens, 103: λ / 2 plate, 104: polarization beam splitter, 106a, 106b: λ / 4 Plate 107: two-dimensional actuator 108: objective lens 113: condenser lens 116a to 116c: mirror 118: none Polarization half beam splitter, 119: λ / 2 plate, 120: polarization beam splitter, 121, 122: detector, 123: λ / 4 plate, 124: polarization beam splitter, 125, 126: detector

Claims (7)

In an information recording / reproducing apparatus that reads information as a phase change of reproduction light,
An information recording / reproducing apparatus for recording information by modulating a recording depth of a reflector formed in a recording medium. The information recording / reproducing apparatus according to claim 1,
A first laser light source;
An optical system for irradiating a recording medium with laser light generated from the first laser light source;
Modulation means for modulating the depth of focus formed by the optical system in the recording medium by the laser beam;
An information recording / reproducing apparatus comprising: The information recording / reproducing apparatus according to claim 2,
The information recording / reproducing apparatus characterized in that the modulation means includes a refractive index modulator whose refractive index changes according to an applied voltage. The information recording / reproducing apparatus according to claim 2,
The information recording / reproducing apparatus, wherein the modulation means includes a medium whose refractive index changes by light irradiation and a second laser light source for irradiating the medium with light. The information recording / reproducing apparatus according to claim 2,
The information recording / reproducing apparatus according to claim 1, wherein the modulation means includes an optical element provided in the optical system and capable of modulating a focal length. The information recording / reproducing apparatus according to claim 2,
The information recording / reproducing apparatus according to claim 1, wherein the modulation means includes means for controlling an oscillation wavelength of the first laser light source. The information recording / reproducing apparatus according to claim 2,
The information recording / reproducing apparatus, wherein the modulation means includes a plurality of optical paths having different optical distances set in the optical system, and a means for selecting one of the plurality of optical paths.

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