JP2006154444A - Hologram recording medium, hologram recording device, and hologram recording method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording medium for which holograms to be recorded are appropriately selectable, a hologram recording device, and a hologram recording method. <P>SOLUTION: A birefringent layer which has birefringence and changes the polarization state of at least either of signal light and reference light is arranged between a recording layer and a reflective layer of the hologram recording medium. The birefringent layer changes the polarization state of at least either of the signal light and the reference light, which are different in the incident direction to each other, thereby making it possible to select the hologram to be recorded. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram recording medium, a hologram recording apparatus, and a hologram recording method for recording information using a hologram.

Development of a hologram recording apparatus that records data using a hologram is in progress.
In the hologram recording apparatus, modulated signal light (with data superimposed) and unmodulated reference light are generated from laser light, and these are irradiated to the same location on the hologram recording medium (hologram medium). As a result, the signal light and the reference light interfere on the hologram recording medium, a diffraction grating (hologram) is formed at the irradiation point, and data is recorded on the hologram recording medium.
Hologram recording media include a reflective type and a transmissive type depending on whether or not a reflective layer is added to the recording layer. The reflection-type hologram recording medium has advantages over the transmission-type hologram recording medium, such as easy simplification of the optical system and easy use of the current optical disk servo system due to the similarity of the optical system.

In a reflection type hologram recording medium, a transmission hologram by interference between signal light and reference light, a transmission hologram by interference between reflected signal light and reflected reference light, and a reflection type by interference between reflected signal light and reference light A hologram, a reflection hologram by interference between signal light and reflected reference light, and four types of holograms can be recorded.
These four types of holograms depend on whether the signal light and reference light used for recording / reproduction are transmitted components that remain incident on the hologram recording medium or reflected components reflected by the reflective layer. The selectivity for the shift amount at the time of shift multiplex recording on the medium is different. For example, a hologram recorded by a transmission component of signal light has a defocus amount doubled due to a shift of the hologram recording medium at the time of reproduction, and the S / N ratio is reduced, compared with a reflection component.
Further, the transmission hologram and the reflection hologram are formed vertically and horizontally with respect to the plane direction of the hologram recording medium, respectively. Accordingly, when the hologram recording medium is made of a material that causes a dimensional change such as shrinkage during recording, such as a photopolymer, the method of changing the shape of the hologram differs between the transmission type and the reflection type. For this reason, the transmission light and the reflection hologram have different reproduction light emission angles, which causes noise.
In such a reflection-type hologram recording medium, a technique for preventing interference between transmitted light and reflected light by inserting a quarter-wave plate between the recording layer and the reflective layer is disclosed (for example, Patent Documents). 1).
US Patent Application Publication No. 2003/0039001

However, the above-described technique only prevents interference between reflections of the four holograms, and the use of a quarter wavelength plate further increases the cost of the hologram recording medium.
In view of the circumstances as described above, an object of the present invention is to provide a hologram recording medium, a hologram recording apparatus, and a hologram recording method capable of appropriately selecting a hologram to be recorded in accordance with a recording method or the like.

A. The hologram recording medium according to the present invention is polarized and has a hologram recording layer on which signal light and reference light having different incident directions are incident and records a hologram, and is laminated on the hologram recording layer, and has birefringence. And a birefringent layer that changes a polarization state of at least one of the signal light and the reference light, and a reflective layer that is laminated on the birefringent layer and reflects the signal light and the reference light. It is characterized by doing.
A birefringent layer that has birefringence and changes the polarization state of at least one of signal light and reference light is disposed between the recording layer and the reflective layer of the hologram recording medium. The hologram to be recorded can be selected by changing the polarization state of at least one of the signal light and the reference light having different incident directions from each other in the birefringent layer.

(1) The birefringent layer may be made of a birefringent material having an axial property in a substantially layer direction of the birefringent layer.
By using a birefringent material having an axial property in the substantially layer direction of the birefringent layer, different polarization states can be imparted to signal light and reference light having different incident directions. For example, the polarization state of light incident in the axial direction of the birefringent material does not change, and the polarization state of light incident obliquely in the axial direction changes.
As the birefringent material, a polymer material such as plastic can be used.

(2) Linearly polarized light can be used for signal light and reference light.
The polarization directions of the signal light and the reference light can be approximately parallel or approximately vertical.

1) The polarization direction of the signal light may be substantially the same before and after passing through the birefringent layer, and the polarization direction of the reference light may be different by approximately 90 °.
At this time, if the polarization directions of the signal light and the reference light are substantially parallel, a hologram due to interference between the transmission component of the signal light and the transmission component of the reference light, and a hologram due to interference between the reflection component of the signal light and the transmission component of the reference light To be recorded. If the polarization directions of the signal light and the reference light are substantially perpendicular, a hologram due to interference between the reflection component of the signal light and the reflection component of the reference light, and a hologram due to interference between the transmission component of the signal light and the reflection component of the reference light are generated. To be recorded.

2) Before and after passing through the birefringent layer, the polarization direction of the signal light differs by approximately 90 °, and the polarization direction of the reference light may be substantially the same.
At this time, if the polarization directions of the signal light and the reference light are substantially parallel, a hologram due to interference between the transmission component of the signal light and the transmission component of the reference light, and a hologram due to interference between the transmission component of the signal light and the reflection component of the reference light are obtained. To be recorded. If the polarization directions of the signal light and the reference light are substantially perpendicular, a hologram due to interference between the reflection component of the signal light and the reflection component of the reference light, and a hologram due to interference between the reflection component of the signal light and the transmission component of the reference light are generated. To be recorded.

3) Before and after passing through the birefringent layer, the polarization directions of the signal light and the reference light may be different by about 90 °.
At this time, if the polarization directions of the signal light and the reference light are substantially parallel, a hologram due to interference between the transmission component of the signal light and the transmission component of the reference light, and a hologram due to interference between the reflection component of the signal light and the reflection component of the reference light are obtained. To be recorded. If the polarization directions of the signal light and the reference light are substantially perpendicular, a hologram due to interference between the reflection component of the signal light and the transmission component of the reference light, and a hologram due to interference between the transmission component of the signal light and the reflection component of the reference light are generated. To be recorded.

(3) Circularly polarized light can be used for the signal light and the reference light.
Various hologram recordings due to interference between the transmission component and the reflection component of the signal light and the reference light can be reduced, and the S / N ratio can be improved.

(4) The signal light and the reference light may be incident on the birefringent layer at an incident angle of 20 ° or less and 25 ° or more and 35 ° or less, respectively.

B. A hologram recording apparatus according to the present invention includes a light separation element that separates laser light emitted from a laser light source into signal light and reference light, a modulator that modulates the separated signal light, and a hologram that records a hologram. A recording layer, a birefringence layer that is laminated on the hologram recording layer, has birefringence, and changes the polarization state of at least one of the signal light and the reference light, and is laminated on the birefringence layer. And a condensing system for entering and condensing the modulated signal light and the separated reference light at different angles to a hologram recording medium having a reflection layer that reflects the signal light and the reference light, and It is characterized by comprising.
The hologram to be recorded can be selected by changing the polarization state of at least one of the signal light and the reference light having different incident directions from each other in the birefringent layer.

B. A hologram recording method according to the present invention includes a step of separating laser light emitted from a laser light source into signal light and reference light, a step of modulating the separated signal light, a hologram recording layer, and a birefringent layer And a step of causing the modulated signal light and the separated reference light to enter and collect at different angles on a hologram recording medium having a reflection layer; and The step of changing one of the polarization states through the birefringent layer, the step of reflecting the signal light and the reference light that have passed through the birefringent layer by the reflecting layer, and the reflecting layer are reflected by the reflecting layer. A step in which the signal light and the reference light pass through the birefringent layer again to change the polarization state of one of the signals; and the signal light and the reference light that have passed through the birefringent layer reappear on the hologram recording layer. Characterized by comprising the steps Isa, a.
The hologram to be recorded can be selected by changing the polarization state of at least one of the signal light and the reference light having different incident directions from each other in the birefringent layer.

  As described above, according to the present invention, it is possible to provide a hologram recording medium, a hologram recording apparatus, and a hologram recording method capable of appropriately selecting a hologram to be recorded.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a hologram recording apparatus according to an embodiment of the present invention.
The hologram recording apparatus records and reproduces information on the hologram recording medium 101 and includes an optical unit 100.
The optical unit 100 includes a recording / reproducing light source 111, a collimating lens 112, a polarization beam splitter 113, a mirror 121, a pinhole 122, a spatial light modulator 123, a mirror 124, a dichroic mirror 125, a concave lens 126, an objective lens 127, and a Faraday element 131. 132, polarization beam splitter 133, imaging device 134, half-wave plate 135, 136, mirror 141, shielding plate 142, phase modulation element 143, servo light source 151, collimator lens 152, grating 153, beam splitter 154, It has a light lens 155, a cylindrical lens 156, a light receiving element 157, and a servo drive unit 158.

The hologram recording medium 101 is moved or rotated by a driving unit (not shown), and the image of the spatial light modulator 123 can be recorded as a number of holograms.
A hologram recording medium 101 includes substrates 102 and 103, a recording layer 104, a birefringent layer 105, and a reflective layer 106, and is a recording medium that records interference fringes due to signal light and reference light.
The substrates 102 and 103 are layers for protecting the recording layer 104 and the like from the outside. The substrate 102 functions as a light-transmitting layer that transmits the signal light and the reference light. For example, APO (amorphous polyolefin) or PMMA (polymethyl methacrylate) can be used. These materials are suitable for the substrate 102 because of their low birefringence. The substrate 102 may be made of glass or PC (polycarbonate) whose birefringence is lowered to the limit. Any of APO, PMMA, PC, and glass may be used for the substrate 103. Here, since signal light and reference light are not incident on the substrate 103, a PC having birefringence may be used, and it becomes easy to use an inexpensive PC.

The recording layer 104 records the interference fringes as a change in refractive index (or transmittance), and any material that changes the refractive index (or transmittance) according to the intensity of light can be used. It can be used regardless of whether it is an organic material or an inorganic material.
As the inorganic material, for example, a photorefractive material whose refractive index changes according to the exposure amount by an electro-optic effect such as lithium niobate (LiNbO ₃ ) can be used.
As the organic material, for example, a photopolymerization type photopolymer can be used. In the photopolymerization type photopolymer, in the initial state, monomers are uniformly dispersed in the matrix polymer. When this is irradiated with light, the monomer is polymerized at the exposed portion. As the polymer is formed, the monomer moves from the surroundings, and the concentration of the monomer changes depending on the location.
When the refractive index (or transmittance) of the recording layer 104 changes according to the exposure amount, an interference fringe (hologram) caused by the interference between the reference light and the signal light is recorded as a change in the refractive index (or transmittance). It can be recorded on the medium 101.

The birefringent layer 105 is made of a material having birefringence, and changes the polarization state of at least one of recording light and reference light that passes through the birefringent layer 105. Details of this will be described later.
The reflective layer 106 reflects the recording light and reference light that have passed through the birefringent layer 105. For example, a dielectric multilayer film in which light-transmitting materials having different refractive indexes are alternately stacked can be used for the reflective layer 106.

The recording / reproducing light source 111 is a laser light source, and for example, a laser diode (LD) having a wavelength of 405 [nm] or an Nd-YAG laser having a wavelength of 532 [nm] can be used.
The collimating lens 112 is an optical element that converts the laser light emitted from the recording / reproducing light source 111 into parallel light.
The polarization beam splitter 113 is an optical element that splits the parallel light incident from the collimator lens 112 into signal light and reference light. The polarization beam splitter 113 emits s-wave signal light toward the mirror 121 and p-wave reference light toward the mirror 141.

The mirrors 121, 124, and 141 are optical elements that reflect incident light and change its direction.
The pinhole 122 is an optical element that narrows the beam diameter of the signal light to a desired diameter.
The spatial light modulator 123 is an optical element that superimposes data by spatially (in this case, two-dimensionally) modulating signal light. As the spatial light modulator 123, a transmissive liquid crystal element which is a transmissive element can be used. It is also possible to use a reflection type element such as a DMD (Digital micro mirror), a reflection type liquid crystal, or a GLV (Grating Light Value) element for the spatial light modulator.

The dichroic mirror 125 is an optical element for making light used for recording / reproducing (laser light from the recording / reproducing light source 111) and light used for servo (laser light from the servo light source 151) the same optical path. The dichroic mirror 125 transmits the recording / reproducing light from the recording / reproducing light source 111 in response to the difference in laser light wavelength between the recording / reproducing light source 111 and the servo light source 151, and the servo from the servo light source 151. Reflects light. The surface of the dichroic mirror 125 is subjected to a thin film treatment such that the recording / reproducing light is totally transmitted and the light used for servo is totally reflected.
The concave lens 126 is a lens for making the convergence of the signal light different from the reference light. Since only the signal light passes through the concave lens 126, the condensing depth of the signal light and the reference light on the hologram recording medium 101 is different.
The objective lens 127 is an optical element for condensing both the signal light and the reference light on the hologram recording medium 101.

The Faraday elements 131 and 132 are optical elements for rotating the polarization plane. The polarization plane of the s-polarized light incident on the Faraday element 131 is rotated by 45 °, and is returned to the original s-polarized light by the Faraday element 132.
The polarization beam splitter 133 is an optical element that reflects the return light (reproduction light) that is transmitted from the Faraday element 131 and reflected by the hologram recording medium 101 and returned from the Faraday element 132. This is realized by a combination of the Faraday elements 131 and 132 and the polarization beam splitter 133.
The image sensor 134 is an element for inputting an image of reproduction light. For example, a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) device can be used.

  The half-wave plates 135 and 136 are optical elements for adjusting the polarization directions of the incident signal light and reference light, respectively. By adjusting the polarization directions of the signal light and the reference light with the half-wave plates 135 and 136, the change in the polarization state in the birefringent layer 105 of the hologram recording medium 101 is combined with the change in the polarization state of the hologram recording medium 101. It is possible to select the hologram to be recorded. Details of this will be described later.

  As the signal light and the reference light pass through the half-wave plates 135 and 136, their polarization directions are rotated by 90 °. The presence or absence of rotation of the polarization direction can be adjusted by the angle of the half-wave plates 135 and 136. By appropriately rotating the half-wave plates 135 and 136, the polarization direction is not rotated, or 90 ° It is possible to rotate. In addition, the polarization direction can be switched between the presence and absence of rotation depending on whether or not the half-wave plates 135 and 136 are inserted into the optical path.

The rotation of the polarization direction can be controlled by the rotation of the half-wave plates 135 and 136 or the presence or absence of insertion into the optical path. For example, the wave plates 135 and 136 can be inserted into the optical path to rotate the polarization direction by 90 °, or the wave plates 135 and 136 can be excluded from the optical path so as not to rotate the polarization direction. Alternatively, a rotation mechanism that rotates the wave plates 135 and 136 can be provided to rotate the polarization direction by 90 ° or not.
In this way, by controlling the rotation of the polarization direction by the half-wave plates 135 and 136, the polarization direction of the signal light and the reference light incident on the hologram recording medium 101 can be matched (parallel), or the polarization direction. Can be made different (vertical).

The shielding plate 142 is an optical element for shielding a part of the reference light so as not to overlap the recording light. Here, the central part of the beam is shielded.
The phase modulation element 143 is an optical element for giving the reference light a random phase or a certain phase pattern, and may be called a phase mask. For the phase modulation element 143, a ground glass, a diffuser, or a spatial phase modulator may be used. It is also possible to use a hologram element in which a phase pattern is recorded. Light having a phase pattern is generated by reproduction from the hologram element.

The servo light source 151 is a light source for performing servo control such as tracking servo and focus servo, and emits laser light having a wavelength different from that of the recording / reproducing light source 111. The servo light source 151 is, for example, a laser diode, and has a low sensitivity with respect to the hologram recording medium 101, for example, 650 nm is used as the oscillation wavelength.
The collimating lens 152 is an optical element that converts the laser light emitted from the servo light source 151 into parallel light.
The grating 153 is an optical element for dividing the laser light emitted from the collimator lens 152 into three beams, and is composed of two elements. Laser light is divided for servo control.

The beam splitter 154 is an optical element that transmits the laser light emitted from the grating 153 and reflects the return light that is reflected from the hologram recording medium 101 and returned.
The condensing lens 155 is an optical element for condensing the return light from the beam splitter 154 on the light receiving element 157.
The cylindrical lens 156 is an optical element for converting the beam shape of the laser light emitted from the condensing lens 155 from a circular shape to an elliptic shape.
The light receiving element 157 is an element for receiving return light and outputting a tracking error signal for tracking servo control and a focus error signal for focus servo control.
The servo drive unit 158 is a drive mechanism for driving the objective lens 127 by the tracking error signal and the focus error signal generated from the output of the light receiving element 157 to perform tracking control and focus control, and a driving coil 161. , 162.

(Operation of hologram recording device)
Hereinafter, an outline of the operation of the hologram recording apparatus will be described.
A. Recording Outline of the operation of the hologram recording apparatus during recording will be described.
The laser light emitted from the recording / reproducing light source 111 is converted into parallel light by the collimator lens 112 and split by the polarization beam splitter 113 into s-wave signal light and p-wave reference light.

  The signal light is reflected by the mirror 121, has a desired beam diameter by the pinhole 122, and is spatially intensity-modulated by the spatial light modulator 123. The laser light modulated by the spatial light modulator 123 passes through the Faraday element 131, the polarization beam splitter 133, and the Faraday element 132 (if the half-wave plate 135 is inserted in the optical path, the half-wave plate). 135 also passes through a concave lens 126 that is reflected by the mirror 124 and adjusts the focal point on the hologram recording medium 101.

The reference light transmitted through the polarization beam splitter 113 is reflected by the mirror 141 (if the half-wave plate 136 is inserted in the optical path, then passes through the half-wave plate 136), and the beam is reflected by the shielding plate 142. Only the central part of the beam is cut off to form the desired beam. For this reason, it is not reflected by the mirror 124 and has the same optical path as the signal light.
The objective lens 127 condenses the recording light and the reference light at substantially the same location on the hologram recording medium 101, so that interference fringes are formed on the hologram recording medium 101. As a result, the information spatially modulated by the spatial light modulator 123 is recorded on the hologram recording medium 101 as a hologram. The light incident on the same axis is shifted in focal length in the medium, and information on the spatial light modulator is recorded as a hologram as interference fringes of two lights.

Here, the hologram recording medium 101 or the entire unit 100 is moved (shifted) in a plane, and the signal light and the reference light are incident on substantially the same location, whereby data by spatial modulation is recorded on the hologram recording medium 101 as a hologram. be able to. The amount of movement (shift amount) required at this time depends on the phase pattern pitch of the phase modulation element 143.
Further, the phase modulation element 143 or the entire unit 100 is moved (shifted) in a direction (depth direction) perpendicular to the surface of the hologram recording medium 101, and signal light and reference light are incident thereon, thereby recording data by spatial modulation. be able to.

B. At the time of reproduction An outline of the operation of the hologram recording apparatus at the time of reproduction will be described.
During reproduction, the signal light is blocked and only the reference light is incident on the hologram recording medium 101.
The reference light emitted from the recording / reproducing light source 111 and transmitted through the polarization beam splitter 113 is reflected by the mirror 141 (if the half-wave plate 136 is inserted in the optical path, it passes through the half-wave plate 136). Only the central part of the beam is blocked by the shielding plate 142. Thereafter, the reference light passes through the dichroic mirror 125 and becomes reference light having a phase pattern similar to that at the time of recording by the phase modulation element 143 and enters the hologram recording medium 101.
Here, the state of insertion / exclusion of the half-wave plate 136 into the optical path is not necessarily the same as that during recording. In modes (3), (4), (5), and (6) described later, if the reference light is incident in the same polarization direction as that during recording, the polarization directions of the signal light and the reproduction light (diffracted light) are shifted. For this reason, the state of insertion / exclusion of the half-wave plate 136 in the optical path is made different between reproduction and recording so that the polarization direction of the reference light is rotated by 90 degrees during recording and reproduction.

When reference light having the same phase pattern as that at the time of recording enters the hologram recording medium 101, diffracted light (reproduced light) is generated from the hologram recorded on the hologram recording medium 101.
The generated reproduction light follows an optical path opposite to that of the signal light, passes through the objective lens 127, the concave lens 126, and the dichroic mirror 125, and is reflected by the mirror 124.
The direction of polarization of the reproduction light reflected by the mirror 124 is rotated by the Faraday element 132. As a result, the reproduction light emitted from the Faraday element 132 is reflected by the polarization beam splitter 133 and converted into an electrical signal corresponding to the spatial two-dimensional data in the spatial light modulator 123 by the imaging element 134. The output from the image sensor 134 is binarized by a signal processing unit (not shown) and converted into time-series binarized data.

Here, the hologram recording medium 101 or the entire unit 100 is moved (shifted) in a planar manner, and desired data is reproduced by causing the reference light to enter the place on the hologram recording medium 101 to be reproduced.
Further, the data multiplexed in the depth direction can be taken out by moving (shifting) the phase modulation element 143 or the entire unit 100 in a direction (depth direction) perpendicular to the surface of the hologram recording medium 101.

(Details of signal light and reference light near the hologram recording medium 101)
FIG. 2 is a schematic diagram showing details of signal light and reference light in the vicinity of the hologram recording medium 101. Reference light is incident so as to surround the signal light.
Alternatively, two reference lights may be incident on the signal light from both sides as shown in FIG. Further, the reference light can be incident only from one side, not from both sides. There may be three or four reference beams.
In the hologram recording apparatus shown in FIG. 1, it is assumed that the signal light and the reference light incident on the hologram recording medium 101 are linearly polarized light. However, circularly polarized light can be used instead of linearly polarized light. .

A plurality of holograms a to d as described below may be recorded on the hologram recording medium 101. As will be described later, since the hologram recording medium 101 includes the birefringent layer 105, the holograms to be recorded are limited. As a result, an improvement in the S / N ratio is realized.
Hologram a: Transmission hologram with transmission component of signal light and transmission component of reference light Hologram b: Transmission hologram with reflection component of signal light and reflection component of reference light Hologram c: Reflection component of signal light and transmission component of reference light Reflection hologram by d hologram d: Reflection hologram by transmission component of signal light and reflection component of reference light Here, transmission component of signal light and reference light is transmitted through the substrate 102 out of signal light and reference light, A component that is not reflected by the reflective layer 106. Further, the signal light and reference light reflected components are components reflected by the reflective layer 106 in the signal light and reference light.

When all of the holograms a to d are recorded on the hologram recording medium 101, a signal is reproduced by adding the diffracted lights from the respective holograms a to d (reproducing light).
It is sufficient that the diffracted light from these holograms a to d is accurately aligned, but various such as a shift in the focus direction or track direction, or a tilt angle shifts due to rotation or movement of the hologram recording medium 101. A misalignment occurs. Since the selectivity of each of the holograms a to d is different due to these deviations, the S / N ratio of the reproduction light is deteriorated.
As can be seen, limiting the holograms a to d to be recorded (for example, recording two or only one of the holograms a to d) improves the S / N ratio of the reproduction light (diffracted light). It is expected.

Further, the transmission hologram and the reflection hologram are formed vertically and horizontally with respect to the plane direction of the hologram recording medium, respectively. Accordingly, when the hologram recording medium is made of a material that causes a dimensional change such as shrinkage during recording, such as a photopolymer, the method of changing the shape of the hologram differs between the transmission type and the reflection type. For this reason, the transmission light and the reflection hologram have different reproduction light emission angles, which causes noise.
That is, it can be expected that the S / N ratio of the reproduction light (diffracted light) is improved by selecting only one of the transmission hologram and the reflection hologram.

Here, it is shown that the S / N ratio of the reproduction light (diffracted light) can be changed depending on which of the holograms a to d is selected as a recording hologram.
As in the hologram recording apparatus shown in FIG. 1, when phase correlation multiplexing in which a random phase or amplitude pattern is applied to the reference light is used, the margin in the defocus direction becomes as severe as several tens of μm. At that time, the reflection component of the reference light becomes a double pass (reciprocates) in the hologram recording medium 101, and therefore the margin becomes 1/2.
In holograms b and d, the reference beam becomes a double path during reproduction, and the margin in the defocus direction is severe. In other words, taking the defocus margin into consideration, the holograms a and c are useful because the margins are wider than the holograms b and d.

  Further, in the hologram a, the diffracted light generated from the hologram a by the incidence of the transmission component of the reference light at the time of reproduction is reflected by the reflection layer 106 and becomes reproduction light from the hologram recording medium 101. On the other hand, in the hologram c, the diffracted light generated from the hologram a by the incidence of the reflection component of the reference light during reproduction becomes the reproduction light as it is. Comparing these two holograms a and c, it is generally expected that the efficiency of the hologram c which is output by diffracting the reflection component of the reference light is worse. However, since the quality of the S / N ratio can vary depending on the recording conditions of the hologram, the S / N ratio of the hologram a does not necessarily exceed the hologram c.

Although it has been shown above that it is useful to select the holograms a and c from the holograms a to d, other selections may be useful depending on conditions. For example, the recording efficiency may be better when recording is performed only with the transmission type holograms a and b or only with the reflection type holograms c and d.
As described above, which of the holograms a to d is selected is preferably changed as appropriate according to the recording conditions. As will be described later, two holograms can be appropriately selected and recorded from the holograms a to d by the modes (1) to (6).

(Details of the birefringent layer 105)
As described above, since the hologram recording medium 101 includes the birefringent layer 105, a hologram to be recorded can be selected from the holograms a to d.
The birefringent layer 105 changes the polarization state of at least one of the signal light and the reference light. As an example, when the signal light and the reference light are linearly polarized light, the polarization direction of the reflected light of only the reference light is set to 90 [deg. ] Rotate.

FIG. 4 is a diagram summarizing methods (modes (1) to (6)) in which any two holograms among the holograms a to d can be selected and recorded.
Mode (1): The polarization direction of the light wave (either signal light or reference light, signal light in FIG. 1) incident from the oblique direction on the birefringent layer 105 is rotated by 90 °, and the signal light and reference light at the time of incidence are rotated. The polarization direction of the incident light is matched.
Mode (2): The polarization direction of the light wave (either the signal light or the reference light, or the signal light in FIG. 1) incident from the oblique direction by the birefringent layer 105 is rotated by 90 °, and the incident light of the signal light and the reference light The polarization direction is changed by 90 °.
Mode (3): The polarization direction of the light wave (either signal light or reference light, reference light in FIG. 1) incident perpendicularly to the birefringent layer 105 is rotated by 90 °, and the signal light and reference light at the time of incidence are rotated. The polarization direction of the incident light is matched.
Mode (4): The polarization direction of the light wave (either signal light or reference light, reference light in FIG. 1) incident perpendicularly to the birefringent layer 105 is rotated by 90 °, and the incident light of the signal light and the reference light The polarization direction is changed by 90 °.
Mode (5): The polarization direction of the light wave (both signal light and reference light) incident on the birefringent layer 105 obliquely and vertically is rotated by 90 °, and the polarization direction of the incident signal light and reference light is incident. Match.
Mode (6): The polarization direction of the light wave (both signal light and reference light) incident on the birefringent layer 105 obliquely and vertically is rotated by 90 °, and the polarization direction of incidence of the signal light and the reference light is 90 °. Make it different.

In mode (1), holograms a and c are recorded, but holograms b and d are not recorded. The hologram b is due to interference between the reflection components of the signal light and the reference light, but no interference occurs because the polarization directions thereof are different by 90 degrees. Further, the hologram d is due to interference between the transmission component of the signal light and the reflection component of the reference light, but interference does not occur because their polarization directions are different by 90 degrees.
In mode (2), the hologram a is not recorded by the transmission components of the signal light and the reference light, and the hologram b by the reflection components is recorded. For the reflection hologram, the hologram c is not recorded, but the hologram d is recorded.
Further, in modes (3) to (6), holograms a and d, holograms b and c, holograms a and b, and holograms c and d are recorded, respectively.

As shown below, modes (1) to (6) can be selected according to the characteristics of the hologram recording apparatus.
-To improve the selectivity of diffracted light by shifting the defocus direction of reproduction: Mode (1)
-To improve the margin for image degradation on the CCD due to a shift in the defocus direction of reproduction: Mode (4)
・ When recording with only transmission hologram: Mode (5)
・ When recording only with a reflection hologram: Mode (6)

Each of the transmission hologram and the reflection hologram is formed vertically and horizontally with respect to the plane direction of the hologram recording medium 101. Therefore, the hologram recording medium 101 is made of a material that causes a dimensional change such as shrinkage during recording, such as a photopolymer. When configured, the two holograms have different ways of changing the shape, and the output angles of the reproduction light from the two differ, causing noise. By selecting only one of the transmission hologram and the reflection hologram using either mode (5) or (6), the S / N ratio of the reproduction light is improved.
Here, the transmission hologram has a wider margin with respect to a change in dimension. Reflective holograms are very narrow in wavelength selectivity and are suitable when wavelength multiplexing is desired. Modes (5) and (6) are appropriately selected according to these circumstances.

The following properties 1 and 2 are listed as properties required for the material constituting the birefringent layer 105.
1. 1. The intrinsic birefringence value is high to some extent and can be controlled to some extent. High birefringence As the birefringent layer 105, a polymer material (polymer) such as polycarbonate (PC), polystyrene, polyphenylene ether, polyvinyl chloride, polymethyl methacrylate, polyethylene terephthalate, or polyethylene can be used. These materials also have the advantage of being relatively inexpensive and easily available.

The properties required for the birefringent layer 105 differ depending on the above modes (1) to (6).
In order to realize the modes (1) and (2), the polarization state of the reference light and the signal light needs to be different depending on the incident angle due to the birefringence of the birefringent layer 105. Specifically, it is necessary to satisfy the following formula (1).
| (Ns · L / cos θs) − (nr · L / cos θs) | = N · (λ / 2)
n = ns (1)

Here, θs: incident angle of signal light in the birefringent layer 105
θr: incident angle of reference light in the birefringent layer 105
L: thickness of the birefringent layer 105
n: Average refractive index of the birefringent layer 105
ns: Refractive index at incident angle θs
nr: Refractive index at incident angle θr
N: An arbitrary integer.

If the condition deviates from the above equation (1), it means that the polarization direction is not deviated by 90 degrees, and therefore an unnecessary hologram (interference) is generated by the deviation.
When the incident light is not parallel light but convergent light, the incident angle θ is not fixed to one value, but at this time, the incident angle is defined with reference to the optical axis. By doing so, interference can be prevented to the maximum.

In modes (3) and (4), the following expression (2) must be satisfied.
| (Ns · L / cos θs) − (nr · L / cos θs) | = N · (λ / 2)
n = nr (2)

In the modes (5) and (6), the following expression (3) needs to be satisfied.
ns · L / cos θs = N · (λ / 2)
nr · L / cos θs = N · (λ / 2) (3)

A. Experimental example 1 (when both signal light and reference light are linearly polarized)
The case where both the signal light and the reference light are linearly polarized will be described in detail.
5A and 5B are diagrams illustrating an example of a birefringence characteristic with respect to a change in the incident state and a schematic state when linearly polarized light is incident on the birefringent layer 105. FIG. A case where a PC substrate is used as the birefringent layer 105 is assumed.
Here, it is assumed that the birefringent layer 105 can be represented by a uniaxial refractive index ellipsoid (nx, ny, nz) parallel to the layer direction of the birefringent layer 105. That is, the main refractive index nz is parallel to the layer direction of the birefringent layer 105, and the main refractive indexes nx and ny in the X and Y directions are equal to each other (nx = ny).
When the incident light is incident vertically (incident angle 0 °), only the main refractive indexes nx and ny in the X and Y directions (the direction parallel to the birefringent layer 105) are affected, and the signal light and the reference light The polarization direction is the same as before incidence. However, as the incident angle increases, the influence of the main refractive index nz appears, and the polarization direction changes. Specifically, the incident angle ± 30 [deg. ], The polarization direction changes by 90 °.

Therefore, when a PC substrate is used as the birefringent layer 105, the incident angle of the signal light is set to −20 [deg. ] To +20 [deg. ], And the incident angle of the reference light is −35 [deg. ] To -25 [deg. ] (Or +25 [deg.] To +35 [deg.]), Only the polarization direction of the reference light can be rotated by approximately 90 °.
In this case, the reflection component of the signal light (the signal light reflected by the reflection layer 106) has a deflection direction that is substantially the same as the transmission component of the signal light (the signal light before being reflected by the reflection layer 106). The reflection component of the reference light has a deflection direction of 90 [deg. ] Deviation. Therefore, the hologram b is not recorded because the signal light reflection component and the reference light reflection component do not interfere with each other. Further, the reflection hologram d due to interference between the transmission component of the signal light and the reflection component of the reference light is not recorded. Thus, by selecting and recording the hologram, it is possible to avoid the recording of an extra hologram that causes noise and to improve the S / N ratio.

B. Experimental example 2 (when both signal light and reference light are circularly polarized)
FIG. 6 is a diagram illustrating an example of birefringence characteristics with respect to a change in incident angle when circularly polarized light is incident on the birefringent layer 105. A case where a PC substrate is used as the birefringent layer 105 is assumed.
From FIG. 6, when a PC substrate is used as the birefringent layer 105, the incident angle of the signal light is −18 [deg. ] To +18 [deg. ], And the incident angle of the reference light is −35 [deg. ] To -25 [deg. ] (Or +25 [deg.] To +35 [deg.]), It is possible to change only the polarization state of the reference light.

  In this case, the reflection component of the signal light (the signal light reflected by the reflection layer 106) is in a deflected state (circularly polarized light) that is substantially the same as the transmission component of the signal light (the signal light before being reflected by the reflection layer 106). However, the reflection component of the reference light becomes linearly polarized light due to the influence of the birefringence of the birefringent layer 105, and the transmission component of the reference light is different from circularly polarized light. Therefore, the interference between the reflected component of the signal light and the reflected component of the reference light is reduced by several tens of percent compared to the case where the reflected light is reflected by the circularly polarized light as it is. Although it is difficult to prevent hologram recording by completely rotating the polarization direction by 90 degrees as in the case of linearly polarized light indicated by A, it is beneficial in terms of reducing the number of holograms that cause noise.

It is a schematic diagram showing the hologram recording device which concerns on one Embodiment of this invention. It is a schematic diagram showing the details of an example of signal light and reference light in the vicinity of the hologram recording medium. It is a schematic diagram showing the detail of the other example of the signal light in the hologram recording medium vicinity, and reference light. It is the figure which put together the system (mode) which can select and record any two holograms among several holograms. It is a figure showing an example of the birefringence characteristic with respect to the typical state when a linearly polarized light enters the birefringent layer, and the change of an incident angle. It is a figure showing an example of the birefringence characteristic with respect to the change of an incident angle when circularly polarized light injects into a birefringent layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical unit 101 Hologram recording medium 102,103 Substrate 104 Recording layer 105 Birefringence layer 106 Reflection layer 111 Recording / reproducing light source 112 Collimating lens 113 Polarizing beam splitter 121 Mirror 122 Pinhole 123 Spatial light modulator 124 Mirror 125 Dichroic mirror 126 Concave lens 127 Objective lens 131 Faraday element 132 Faraday element 133 Polarizing beam splitter 134 Imaging element 135, 136 1/2 wavelength plate 141 Mirror 142 Shielding plate 143 Phase modulation element 151 Light source for servo 152 Collimating lens 153 Grating 154 Beam splitter 155 Condensing lens 156 Cylindrical lens 157 Light receiving element 158 Servo drive unit 161, 162 Coil

Claims (13)

A hologram recording layer that is polarized and receives signal light and reference light having different incident directions and records a hologram; and
A birefringent layer that is laminated on the hologram recording layer, has birefringence, and changes the polarization state of at least one of the signal light and the reference light;
A reflective layer laminated on the birefringent layer and reflecting the signal light and reference light;
A hologram recording medium comprising: The hologram recording medium according to claim 1, wherein the birefringent layer is made of a birefringent material having an axial property in a substantially layer direction of the birefringent layer. The hologram recording medium according to claim 2, wherein the birefringent material is a polymer material. The hologram recording medium according to claim 1, wherein the signal light and the reference light are linearly polarized light. The hologram recording medium according to claim 4, wherein the polarization directions of the signal light and the reference light are substantially parallel. The hologram recording medium according to claim 4, wherein the polarization directions of the signal light and the reference light are substantially perpendicular. The hologram recording medium according to claim 4, wherein the polarization direction of the signal light is substantially the same before and after passing through the birefringent layer, and the polarization direction of the reference light is different by approximately 90 °. The hologram recording medium according to claim 4, wherein the polarization direction of the signal light differs by approximately 90 ° before and after passing through the birefringent layer, and the polarization direction of the reference light is substantially the same. The hologram recording medium according to claim 4, wherein polarization directions of the signal light and the reference light differ by approximately 90 ° before and after passing through the birefringent layer. The hologram recording medium according to claim 1, wherein the signal light and the reference light are circularly polarized light. The hologram recording medium according to claim 1, wherein the signal light and the reference light are incident on the light-transmitting layer at an incident angle of 20 ° or less and 25 ° or more and 35 ° or less, respectively. A light separating element that separates laser light emitted from the laser light source into signal light and reference light;
A modulator for modulating the separated signal light;
A hologram recording layer for recording a hologram, a birefringence layer that is laminated on the hologram recording layer, has birefringence, and changes a polarization state of at least one of the signal light and the reference light, and the birefringence layer. The modulated signal light and the separated reference light are incident and condensed at different angles on a hologram recording medium that is laminated on a refractive layer and has a reflective layer that reflects the signal light and the reference light. A condensing system;
A holographic recording apparatus comprising: Separating the laser light emitted from the laser light source into signal light and reference light;
Modulating the separated signal light;
Incident and condensing the modulated signal light and the separated reference light at different angles on a hologram recording medium having a hologram recording layer, a birefringent layer, and a reflective layer;
The focused signal light and reference light pass through the birefringent layer and the polarization state of either one changes;
The signal light and the reference light that have passed through the birefringent layer are reflected by the reflective layer;
The signal light and the reference light reflected by the reflective layer pass through the birefringent layer again to change the polarization state of any one of the steps;
The signal light and the reference light that have passed through the birefringent layer are reincident on the hologram recording layer;
A hologram recording method comprising:

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