JP2005235342A - Optical information recording / reproducing device - Google Patents

Abstract

The present invention relates to an optical information recording / reproducing apparatus capable of recording and reproducing three-dimensionally, and optical information capable of adjusting the amount of interlayer crosstalk so that the generation of interlayer crosstalk can be made substantially the same in the upper and lower layers in the optical axis direction. A recording / reproducing apparatus is provided.
A light source for emitting recording light or reproducing light, an objective lens for condensing the light emitted from the light source on an information recording medium capable of three-dimensional recording and reproduction, and the information recording medium A light detector 19 for detecting light from the light source 21, a pinhole 14 for reducing the crosstalk between the information recording medium 21 and the information recording medium 21 disposed in the optical path to the light detector 19, and the light source 20 includes a balance correction means 10 for generating spherical aberration, arranged in any one of the optical paths from 20 to the pinhole 14, and adjusts the balance of the interlayer crosstalk amount of the information recording medium 21 with respect to the optical axis direction. To do.
[Selection] Figure 1

Description

  The present invention relates to an optical information recording / reproducing apparatus capable of recording / reproducing three-dimensionally, and more particularly to an optical information recording / reproducing apparatus capable of adjusting an interlayer crosstalk amount of an information recording medium in an optical axis direction.

  An optical information recording / reproducing apparatus is an apparatus for recording / reproducing information on / from an optical information recording medium such as an optical disk such as a compact disk (CD) or DVD or an optical card memory.

  A conventional optical information recording / reproducing apparatus corresponding to a multi-layer optical information recording medium having a plurality of three-dimensional layers as shown in FIG. 1.

  Light 122a having a high peak power emitted from the recording light source 120a becomes parallel light 108 by the collimator lens 116, and a desired recording layer 103b of the multilayer optical disk 121 that can be recorded and reproduced three-dimensionally by the objective lens 106. (Converged light 107) and recorded as a recording bit using a nonlinear phenomenon such as a two-photon absorption process.

Light 122b having a low peak power emitted from the reproduction light source 120b is condensed on the recording bits of the desired recording layer 103b (converging light 107) by the objective lens 106, and the reflected light is collected by the detection lens 111. The signal can be reproduced by detecting the light (detection convergent light 117) and detecting it with the photodetector 119. The pinhole 114 is arranged at the condensing point of the detection lens 111, and the light passing through the pinhole 114 is detected by the photodetector 119, so that it is unnecessary from another recording bit from the upper and lower layers 103a and 103c of the recording layer 103b. Crosstalk (interlayer crosstalk) that is reflected light can be reduced, and the quality of the reproduction signal can be improved. Such an optical system can improve the resolution in the optical axis direction as described above, and is known as a confocal optical system.
Yoshimasa Kawada: “Three-dimensional optical memory using femtosecond laser”, Optronics pp. 138-142 (2001)

  However, in the configuration of the conventional optical information recording / reproducing apparatus, the resolution in the optical axis direction can be improved if the diameter of the pinhole 114 is reduced. However, according to the study by the inventors, the shallower optical axis ( It was found that the interlayer crosstalk from the layer 103c of the layer close to the lens 106 is generally larger than the interlayer crosstalk from the layer 103a having the deeper optical axis.

  The present invention has been made to solve the above-described problems in the prior art, and relates to an optical information recording / reproducing apparatus capable of recording / reproducing three-dimensionally. It is an object of the present invention to provide an optical information recording / reproducing apparatus capable of adjusting the amount of inter-layer crosstalk that can be balanced and substantially the same.

  In order to solve the above-described conventional problems, the present invention provides a light source that emits recording light or reproducing light, and an objective lens that condenses the light emitted from the light source onto an information recording medium that can be recorded and reproduced three-dimensionally. A light detector for detecting light from the information recording medium, a pinhole for reducing interlayer crosstalk between the information recording medium and the information recording medium disposed in an optical path to the light detector, and the light source An optical information recording / reproducing apparatus comprising a balance correction unit arranged in any one of the optical paths from to the pinhole.

  Thereby, an optical information recording / reproducing apparatus capable of adjusting the interlayer crosstalk amount of the information recording medium in the optical axis direction can be realized with respect to the optical information recording / reproducing apparatus capable of recording / reproducing three-dimensionally.

  As described above, according to the present invention, an optical information recording / reproducing apparatus capable of three-dimensional recording / reproducing is provided with an interlayer that can cause the generation of interlayer crosstalk to be substantially the same in the upper and lower layers in the optical axis direction. An optical information recording / reproducing apparatus capable of adjusting the amount of crosstalk can be realized.

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

(Embodiment 1)
First, the optical information recording / reproducing apparatus according to the first embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 1 is a side view showing the basic configuration of an optical information recording / reproducing apparatus according to Embodiment 1 of the present invention and the state of light propagation, and FIG. 2 uses a pinhole in the optical information recording / reproducing apparatus of the same embodiment. FIG. 3A is a configuration diagram for explaining the reduction characteristic of interlayer crosstalk of an information recording medium (only main components are displayed), and FIG. 3A is a sectional view of a balance correction means of the optical information recording / reproducing apparatus in the embodiment, FIG. 3B is a cross-sectional view of the balance correction means of the optical information recording / reproducing apparatus according to another embodiment of the present embodiment, and FIG. 3C is the balance correction means of the optical information recording / reproducing apparatus according to still another embodiment of the same embodiment. 4 is a sectional view, FIG. 4 is a light intensity distribution of a focused spot on the pinhole surface in the optical information recording / reproducing apparatus of the embodiment, and FIG. 5 is a balance correction method in the optical information recording / reproducing apparatus of the embodiment. It detected intensity for explaining the effect as a relationship diagram of the recording layer interval [Delta] d.

  The optical information recording / reproducing apparatus of the present embodiment includes two types of light sources 20a and 20b for recording and reproduction, and a beam splitter 18a and a collimator lens in the optical path from the light sources 20a and 20b to the information recording medium 21. 16, a beam splitter 18b, a rising mirror 12, a spherical aberration correction element 13, and an objective lens 6 are disposed. The information recording medium 21 is a recording unit 3 in which recording layers 1 and intermediate layers 2 are alternately deposited on a substrate 9 (in FIG. 2, the recording unit 3 is formed with six recording layers 1a to 1f and intermediate layers 2a to 2e). And has a multilayer structure in which a protective layer 4 is provided on the surface, but there is a structure capable of recording and reproducing three-dimensionally without a multilayer structure, for example, one thick recording layer. A bulk structure may be used.

  In the optical path from the beam splitter 18 b to the photodetector 19, which is the return path, a focus / track error signal detection element 15, a balance correction means 10, a detection lens 11, and a pinhole 14 that reduces crosstalk of the information recording medium 21 are arranged. Has been. The balance correction means 10 can be arranged in any one of the optical paths from the light source 20 to the pinhole 14. However, the balance correcting means 10 is provided in the optical path to the photodetector 19 by providing a beam splitter 18 b as a beam branching element. However, this is desirable because it does not affect the outward optical system.

  The light source 20a is a semiconductor pulse laser light source for recording having a pulse width of, for example, 100 femtoseconds to 10 nanoseconds and a wavelength of 0.65 μm. The light source 20b is a semiconductor laser light source for reproduction having a wavelength of 0.405 μm, for example. By making the wavelength of the reproduction light source 20b shorter than the wavelength of the recording light source 20a, there is an effect that the density can be increased in the two-photon absorption recording described below. However, operation is possible even if the light source for reproduction and recording has a wavelength of, for example, 0.405 μm. In this case, the laser is oscillated with a pulse during recording to emit a laser beam having a large peak power, and is continuously oscillated during reproduction. Thus, a laser beam having a small peak power is oscillated. In this case, the configuration is simple because only one light source is required.

  As shown in FIG. 1, the optical information recording / reproducing apparatus according to the present embodiment emits light, which is pulse laser light having a relatively large peak power, emitted from the recording light source 20a in the y-axis direction during recording. 22 a passes through the beam splitter 18 a, becomes substantially parallel light by the collimator lens 16, passes through the beam splitter 18 b as a beam branching element, and is bent in the −z-axis direction by the rising mirror 12. The laser beam (parallel light) 8 bent in the −z-axis direction passes through the spherical aberration correction element 13 and is recorded by the objective lens 6 having a numerical aperture NA = 0.85 and a focal length of 2 mm, for example. The desired recording layer 1 of the recording unit 3 through the protective layer 4 of the medium 21 (in FIG. 1, three recording layers are shown for simplification, and six recording layers 1a to 1f are shown in FIG. 2) The recording bit 5 as shown in FIG. 2 is recorded on the recording layer 1 using a nonlinear phenomenon such as a two-photon or multi-photon absorption process.

  At this time, since the thickness of the recording unit 3 through which the convergent light 7 passes varies depending on the recording depth, information recorded in the recording unit 3 by the spherical aberration correction element 13 provided in the optical path from the light source 20 to the objective lens 6. If the spherical aberration correction element 13 performs recording while controlling the amount of spherical aberration according to the recording depth of the bit 5, a good information bit 5 can be formed. The spherical aberration correction element 13 can be configured by a liquid crystal element having a variable refractive index distribution, a beam expander in which a distance between the two lenses in the optical axis direction is variable by an actuator by combining a concave lens and a convex lens.

  If a recording material that absorbs at a half wavelength of the recording wavelength (in the case of two-photon absorption) is used as the recording layer 1 by using recording utilizing a two-photon or multi-photon absorption process, for example, several hundred mW to several When recording light having a relatively high peak power of W or more, for example, 100 femtoseconds to 10 nanoseconds and a small pulse width, only a portion (condensing point) where the power density of the light condensed by the objective lens 6 is high. As a result, the wavelength is halved, absorption occurs in the recording material, and the recording bit 5 is recorded. Since absorption occurs only at the condensing point in this way, light is not attenuated so much even in a deep recording layer, which is suitable for a three-dimensional optical memory such as a multilayer memory.

  The recording bit 5 is recorded three-dimensionally using the change in the optical constant of the recording layer 1. In this embodiment, the recording bit 5 is recorded mainly using the change in the refractive index of the photosensitive material of the recording layer 1. ing. By utilizing the change in refractive index, light absorption loss can be reduced. In addition, since recording is performed using light absorption in the phase change material, it is not suitable for recording with a large number of layers, but it can be used as a recording layer of a multilayer optical disc having about 2 to 6 layers.

  At the time of reproduction, the emitted light 22b, which is a laser beam emitted from the reproduction light source 20b, is bent in the y-axis direction by the beam splitter 18a, and becomes substantially parallel light by the collimator lens 16 and is transmitted through the beam splitter 18b. Then, the optical path is bent in the −z-axis direction by the rising mirror 12. Then, the laser beam 8 bent in the −z-axis direction passes through the spherical aberration correction element 13 and is focused (laser) on the recording bit 5 of the recording layer 1 of the recording unit 3 of the information recording medium 21 by the objective lens 6. 7) Light.

  The laser beam 7 reflected by the recording bit 5 is folded in the opposite direction, passes through the objective lens 6, the spherical aberration correction element 13, and the rising mirror 12 in order, and the optical axis is bent in the z-axis direction by the beam splitter 18b. The light is split into a plurality of lights by the diffractive focus / track error signal detection element 15, passes through the balance correction means 10 that generates spherical aberration, and becomes convergent lights 17 and 17 ′ by the collimator lens 16. The rim intensity on the detection lens 11 is 0.8, for example. The convergent light 17 as the reproduction light passes through the pinhole 14 and a signal is detected by the photodetector 19a. The convergent light 17 ′ that is a branched focus / track error signal is detected by another photodetector 19 b without passing through the pinhole. The convergent light 17 ′, which becomes a focus / track error signal, can detect the focus and track error signals by a conventional method such as an astigmatism method and a three-beam tracking method, respectively, by a configuration that does not pass through a pinhole.

  The focal length of the detection lens 11 is, for example, 33 mm, and the Airy disk diameter on the photodetector 19 side is, for example, 9.6 μm. The pinhole 14 is installed at a substantially focal position of the detection convergent light 17. However, by providing the pinhole 14, as shown in FIG. 2, the upper and lower layers 1a to 1d in the optical axis direction of the desired recording layer 1e. Crosstalk (interlayer crosstalk) light, which is unnecessary reflected light from the other recording bits 5a to 5d, 5f, etc. irradiated by the laser light 7a converged by the objective lens 6 from 1f, also enters the pinhole 14 outside. Since the light is distributed (see the light beam indicated by the broken line and the alternate long and short dash line) and the light does not enter the pinhole 14, there is an effect of reducing the interlayer crosstalk. Further, the same effect can be obtained by detecting the detection convergent light 17a with the minute light detector 19a in which the light receiving portion of the light detector has a pinhole diameter instead of the pinhole 14.

  In the present embodiment, the size of the pinhole 14 is set to be not more than 5 times the Airy disk diameter of the respective detection converged light 17, for example, the problem described below with a layer interval Δd of the recording layer 1 of 5 to 8 μm. It was possible to improve the quality of the reproduced signal up to a level (interlayer crosstalk amount ≦ 30 dB).

  If the size of the pinhole 14 is reduced, the interval Δd between the recording layers 1 can be further reduced. However, if the pinhole 14 is too small, the amount of light entering the pinhole 14 is reduced or the optical system is affected by the environmental temperature. Since the converging light 17 may be distorted and deviate from the center of the pinhole 14, it is necessary to consider them.

  More specifically, in the present embodiment, it has been found that there is no practical problem if the interlayer crosstalk amount from the upper and lower layers is reduced to 0.032 times, which is 30 dB. When the balance correction means 10 is not provided, the distance Δd between the recording layers 1 is obtained with reference to 30 dB. When the diameter of the pinhole 14 is equal to the Airy disk diameter on the photodetector 19 side, for example, 9.6 μm, the light amount is Δd decreased by 30 dB was −1.5 μm and 1.8 μm when the refractive index of the intermediate layer 2 was 1.5. Therefore, the interval between the recording layers 1 needs to be 1.8 μm at the minimum. Similarly, when the diameter of the pinhole 14 is twice the diameter of the Airy disk on the photodetector 19 side, for example, 19.2 μm, the Δd at which the light amount decreases by 30 dB was −3.0 μm and 3.3 μm. . Accordingly, the minimum distance between the recording layers 1 is 3.3 μm. The pinhole 14 is three times the Airy disk diameter on the photodetector side. For example, in the case of 28.8 μm, Δd at which the light amount decreases by 30 dB was −4.4 μm and 4.8 μm (FIG. 5). See the dashed line). Therefore, the interval between the recording layers 1 needs to be at least 4.8 μm. That is, regardless of the size of the pinhole 14 diameter, about 10 to 20% is larger than the crosstalk from the distant layer 1d because the amount of interlayer crosstalk from the recording layer 1f close to the objective lens 6 becomes larger. There was a balance.

  As shown in FIG. 3A, from the central part to the outer peripheral part, the surface shape is concave (region A), convex (region B), and substantially rotationally symmetric, and positive third-order spherical aberration (3 When the balance correction means 10 is used, which is a phase element that causes the following spherical aberration (SA3) to be 0.02λ (for example, the depth of the central depression is 0.054 μm), the pinhole 14 is In the case of 3 times the airy disk diameter (28.8 μm) on the photodetector side, as shown by the solid line in FIG. 5, the detection intensity is reduced when Δd ≧ 1.3 μm, and Δd ≦ −1. As a result, the inventors have found that the interlayer crosstalk characteristics are balanced, and as a result, Δd at which the amount of light decreases by 30 dB is the same value as −4.7 μm and 4.7 μm. became. Therefore, the interval Δd between the adjacent recording layers in the recording layers 1a to 1f needs to be 4.7 μm at the minimum, and the interval between the recording layers can be made smaller than the configuration of the conventional example. Spherical aberration generated by a phase element having a concave area A in the central portion has a function of a concave lens in the vicinity of the optical axis, and thus has a sign in the direction in which the focal length of the light beam near the optical axis becomes longer It is to generate.

  The light intensity distributed on the surface of the pinhole 14 when the interval Δd of the recording layer 1 is ± 4.8 μm is shown. In FIG. 4, r indicates the radial distance of the pinhole 14 from the origin when the center of the pinhole 14 is the origin. From FIG. 4, when the third-order spherical aberration changes from 0 (represented by the alternate long and short dashed lines) to 0.02λ (solid line and broken line), Δd decreases the light intensity in the positive direction and reduces the light intensity in the negative direction. It was found that the strength can be improved, and the effect increases as the aberration increases. Conversely, in the case of negative third-order (−third-order) spherical aberration (the balance correction means 10 has a convex shape (region A) and a concave shape (region B) as it goes from the central portion to the outer peripheral portion), It was found that when Δd is positive, the light intensity is improved, and when Δd is negative, the light intensity is decreased. Therefore, if the sign and magnitude of the third-order spherical aberration are changed in the direction in which the detection intensity is desired to be balanced (generally, the direction in which Δd is negative and the light intensity is improved), the correlation intensity of the correlation crosstalk, which is the detection intensity, is changed. It was found that the value can be adjusted in the direction of the optical axis.

  Actually, the balance correction means 10 as shown in FIG. 3 (a) can be produced by machining a mold and injection-molding from the mold. In another embodiment of the present invention, a shape as shown in FIG. 3B in which the surface of such a phase element is approximated by a step-shaped element may be used. In that case, a well-known binary optical element manufacturing technique that repeats photolithography and etching processes that are mass-productive can be used.

  In yet another embodiment of the present embodiment, as shown in FIG. 3 (c), the balance correction means 10 has a concave (region A) convex (region B) surface shape as it moves from the center to the outer periphery. When the fifth-order spherical aberration (fifth-order spherical aberration: SA5) is generated in the concave (region c) shape, the correlation crosstalk value can be adjusted as in the case of the positive third-order spherical aberration. Met. For example, since SA5 = −0.01λ and almost SA3 = 0.02λ, the balance correction means 10 has a higher balance correction effect as higher order fifth-order aberrations are generated.

  Similarly, when the surface shape is convex and concave and a + 5th order spherical aberration is generated, it functions in the direction of improving the light intensity when Δd is positive, as in the case of −SA3.

  Furthermore, the surface shape is uneven, the + 7th order spherical aberration, and the surface shape is uneven, the higher-order aberrations such as the −9th order spherical aberration are also in the negative Δd direction. It works in the direction of improving the light intensity. Similarly, Δd is positive for higher-order aberrations such as a −7th order spherical aberration and a −9th order spherical aberration and a + 9th order spherical aberration. It works in the direction of improving the light intensity in the direction. However, the higher the aberration, the greater the effect of balance correction, but the more complicated the surface shape and the more accurate the alignment, so the balance correction means 10 is about ± 3rd to ± 7th order. It is desirable to generate up to spherical aberration.

  The inventors have also found that the amount of aberration generated by the balance correction means 10 depends on the diameter of the pinhole 14, and it is desirable to increase the amount of aberration as the pinhole diameter increases.

  The direction of the balance correction unit 10 may be either, and the positions of the balance correction unit 10 and the focus / track error signal detection element 15 may be reversed.

  Furthermore, if a spherical aberration is generated in the entire optical system including the objective lens 6 so as to also serve as the balance correction means 10, the configuration can be simplified. The optical system including the objective lens 6 and the entire information recording medium 21 may be configured to generate spherical aberration and also serve as the balance correction means 10. The spherical aberration correction element 13 can also be configured to leave a certain amount of spherical aberration and also serve as the balance correction means 10.

(Embodiment 2)
Next, the optical information recording / reproducing apparatus according to the first embodiment of the present invention will be described with reference to FIG. 6, focusing on differences from the first embodiment.

  FIG. 6 is a side view showing the basic configuration of the optical information recording / reproducing apparatus in Embodiment 2 of the present invention and the state of light propagation.

  In the optical information recording / reproducing apparatus of the present embodiment, the detection lens 11 ′ also serves as a balance correction unit. That is, since the balance correction means is not required, there is an effect that the configuration of the optical system is simple and alignment is not required.

  As a specific surface shape of the detection lens 11 ′, a shape in which a concavo-convex shape as shown in FIG. 3 is superimposed on a normal convex lens shape is difficult. Once the mold is made, it can be mass-produced by injection molding, so the price can be reduced.

(Embodiment 3)
Next, the optical information recording / reproducing apparatus according to the third embodiment of the present invention will be described with reference to FIG. 7, focusing on differences from the first embodiment.

  FIG. 7 is a side view showing the basic configuration of the optical information recording / reproducing apparatus according to Embodiment 3 of the present invention and the state of light propagation.

  In the present embodiment, as shown in FIG. 7, the collimator lens 16 is disposed after the beam splitter 18b, and the detection lens is eliminated. Further, the balance correction means 10 ″ and the focus / track error signal detecting element 15 are integrated and integrated. By doing so, the configuration is simple and the positioning becomes easy.

  The objective lens, the collimator lens, and the detection lens used in the above embodiment are named for convenience and are the same as commonly used lenses.

  In the above embodiment, an optical disk is described as an example of the information recording medium. However, the information recording / reproducing apparatus is designed to reproduce a plurality of media having different specifications such as thickness and recording density. Application to a card-like, drum-like, or tape-like product is also included in the scope of the present invention. The optical information recording / reproducing apparatus of the present invention also includes a confocal microscope using a confocal optical system that cannot record information but can reproduce it.

  The optical information recording / reproducing apparatus according to the present invention relates to an optical information recording / reproducing apparatus capable of three-dimensional recording / reproducing, and relates to an interlayer that can make the generation of interlayer crosstalk substantially the same in the upper and lower layers in the optical axis direction. The present invention can be used for an optical information recording / reproducing apparatus capable of adjusting the amount of crosstalk.

1 is a side view showing the basic configuration of an optical information recording / reproducing apparatus according to Embodiment 1 of the present invention and the state of light propagation; FIG. 3 is a configuration diagram for explaining a reduction characteristic of interlayer crosstalk of an information recording medium using pinholes in the optical information recording / reproducing apparatus according to Embodiment 1 of the present invention. (A) is a cross-sectional view of the balance correction means of the optical information recording / reproducing apparatus in Embodiment 1 of the present invention. (B) is a cross-sectional view of the balance correction means of the optical information recording / reproducing apparatus in another embodiment of the present invention ( c) is a cross-sectional view of the balance correction means of the optical information recording / reproducing apparatus in still another embodiment of the present invention. The figure which shows the light intensity distribution of the condensing spot on a pinhole surface in the optical information recording / reproducing apparatus of Embodiment 1 of this invention. In the optical information recording / reproducing apparatus of Embodiment 1 of the present invention, the relationship between the detection intensity and the recording layer interval Δd for explaining the effect of the balance correction means The side view which shows the basic composition of the optical information recording / reproducing apparatus in Embodiment 2 of this invention, and the mode of light propagation Side view showing the basic configuration of the optical information recording / reproducing apparatus in Embodiment 3 of the present invention and the state of light propagation Side view showing the basic structure of a conventional optical information recording / reproducing apparatus and the state of light propagation

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Recording layer 2 Intermediate | middle layer 3 Recording part 4 Protective layer 5 Recording bit 6 Objective lens 7 Laser beam 8 Parallel light 9 Substrate 10 Balance correction means 11 Detection lens 12 Raising mirror 13 Spherical aberration correction element 14 Pinhole 15 Focus / track error Signal detection element 16 Collimator lens 17 Detection convergent light (reproduction signal)
17 'detection convergent light (focus / track error signal)
18a Beam splitter 18b Beam splitter (beam splitter)
19a Photodetector (for reproduction signal)
19b Photodetector (for focus / track error signal)
20a Light source (for recording)
20b Light source (for playback)
21 Information recording medium 22a Emission light 22b Emission light

Claims (24)

A light source that emits recording light or reproduction light, an objective lens that condenses the light emitted from the light source on an information recording medium that can be recorded and reproduced three-dimensionally, and light detection that detects light from the information recording medium And a pinhole for reducing the crosstalk between the information recording medium and the information recording medium in the optical path from the light source to the pinhole. An optical information recording / reproducing apparatus comprising a balance correction unit. A light source for emitting recording light or reproducing light, an objective lens for condensing the light emitted from the light source on an information recording medium capable of three-dimensional recording and reproduction, and detecting light from the information recording medium. A light detector for reducing interlayer crosstalk of the information recording medium by setting it to a size not more than five times the airy disk diameter of the light to be detected, and any one of the optical paths from the light source to the detector. An optical information recording / reproducing apparatus comprising: a balance correction unit arranged. In the optical path from the light source to the objective lens, a beam branching element for branching the light from the information recording medium and guiding it to the photodetector is provided, and the balance correction means and the pinhole are respectively connected from the beam branching element to the photodetector. The optical information recording / reproducing apparatus according to claim 1, wherein the optical information recording / reproducing apparatus is provided in the optical path up to. A beam branch element is provided in the optical path from the light source to the objective lens to branch the light from the information recording medium and guide it to the photodetector, and the balance correction means is provided in the optical path from the beam branch element to the photodetector. The optical information recording / reproducing apparatus according to claim 2 provided. A collimator lens is provided in the optical path from the light source to the objective lens to make the light from the light source substantially parallel, and the light from the beam branch element is condensed in the optical path from the beam branch element to the photodetector. 4. The optical information recording / reproducing apparatus according to claim 3, further comprising a detection lens, wherein the pinhole is provided in an optical path from the detection lens to the light detector. A collimator lens is provided in the optical path from the light source to the objective lens to make the light from the light source substantially parallel, and the light from the beam branch element is condensed in the optical path from the beam branch element to the photodetector. The optical information recording / reproducing apparatus according to claim 4, further comprising a detection lens. 7. The optical information recording / reproducing apparatus according to claim 5, wherein the detection lens also serves as a balance correction unit. 3. The optical information recording / reproducing apparatus according to claim 1, wherein the entire optical system including the objective lens also serves as balance correction means. The optical information recording / reproducing apparatus according to claim 1, wherein the optical system including the objective lens and the entire information recording medium also serve as balance correction means. 2. The optical information recording / reproducing apparatus according to claim 1, wherein the diameter of the pinhole is not more than five times the airy disk diameter on the photodetector side. The optical information recording / reproducing apparatus according to claim 1, wherein the balance correction unit generates spherical aberration. The optical information recording / reproducing apparatus according to claim 11, wherein the balance correction unit generates a spherical aberration having a sign in a direction in which a focal length of the light beam near the optical axis becomes longer. The optical information recording / reproducing apparatus according to claim 11, wherein the amount of aberration generated by the balance correction unit depends on the pinhole diameter or the size of the photodetector. The optical information recording / reproducing apparatus according to claim 13, wherein the amount of aberration generated by the balance correction unit increases as the pinhole diameter or the size of the photodetector increases. 12. The optical information recording / reproducing apparatus according to claim 11, wherein the balance correction means generates substantially only third-order spherical aberration. The optical information recording / reproducing apparatus according to claim 11, wherein the balance correction unit generates substantially only fifth-order spherical aberration. The balance correction means is a substantially rotationally symmetric phase element in which the shape of the surface becomes an uneven shape or an uneven shape such as an uneven shape or an uneven shape such as an uneven shape as it goes from the center to the outer periphery. Optical information recording / reproducing apparatus. The optical information recording / reproducing apparatus according to claim 17, wherein the surface of the phase element is approximated by a step-shaped element. 3. The optical information recording / reproducing apparatus according to claim 1, further comprising a spherical aberration correction element in an optical path from the light source to the information recording medium, and correcting spherical aberration mainly occurring in the recording medium. The optical information recording / reproducing apparatus according to claim 19, wherein the spherical aberration correction element causes a certain amount of spherical aberration to remain and also serves as balance correction means. 21. The optical information recording / reproducing apparatus according to claim 20, wherein the spherical aberration correction element also has a spherical aberration having a sign in a direction in which the focal length of the light beam near the optical axis becomes shorter, and also serves as a balance correction unit. A focus / track error signal detection element is provided in an optical path from the information recording medium to the photodetector, and the light from the information recording medium is branched by the element and guided to the photodetector without passing through the pinhole. 2. The optical information recording / reproducing apparatus according to 1. The light source comprises: a reproduction light source that emits reproduction light; and a recording pulse laser light source that emits recording light, wherein the wavelength of the reproduction light source is shorter than the wavelength of the recording light source. 2. The optical information recording / reproducing apparatus according to 2. 3. The optical information recording / reproducing apparatus according to claim 1, wherein recording is performed using a two-photon or multi-photon absorption process.

Images