Holographic recording medium and holographic recording method using the same

1. A holographic recording method comprising:

applying light for recording or reproduction to a holographic recording medium comprising at least a recording layer with a thickness L which comprises a photorefractive index-changing material as a recording layer material, the recording layer material having an absorption coefficient α, wherein the optical density αL expressed by the product of the absorption coefficient α and the thickness L is in a range approximately from 0.3 to 2.0, wherein

the recording is performed by simultaneously applying signal light at a first incident angle and reference light at a second incident angle different from the first incident angle to the recording layer,

the reference light is applied at two or more different angles to the recording layer for the purpose of angle-multiplexed storage, and

first reference light that enters the recording layer at a first angle forms an angle Δθ with second reference light that enters the recording layer at a second angle different from the first angle, wherein the angle Δθ (angular space) satisfies the formula:

Δθ≧ n ·λ( L ·sinθ)

wherein Δθ represents an angular space (deg), n represents an integer of at least 2, λ represents the wavelength (μm) of the signal and reference lights applied at the time of recording, L represents the thickness (μm) of the recording layer, and θ represents an angle (deg) which the first reference light forms with the signal light applied to the recording layer at the time of information recording.

2. A holographic recording method comprising:

applying light for recording or reproduction to a holographic recording medium comprising at least a recording layer with a thickness L which comprises a photorefractive index-changing material as a recording layer material, the recording layer material having an absorption coefficient α, wherein the optical density αL expressed by the product of the absorption coefficient α and the thickness L is in a range approximately from 0.3 to 2.0, wherein

the recording is multiplexed recording in which signal light at a first incident angle and reference light at a second incident angle different from the first incident angle are applied simultaneously to the recording layer,

the multiplexed recording is performed by: maintaining a constant angle formed between the signal light and the reference light; and relatively shifting at least one of the holographic recording medium and the signal and reference lights,

the optical density αL is in a range approximately from 0.3 to 1.1, and

a relative distance between a first place where a first piece of information is recorded in the recording layer and a second different place where a second piece of information is recorded therein is twice the distance (reference amount of change) between the first place and a place where the intensity of diffracted light from the first piece of information exhibits a First Null.

3. A holographic recording method comprising:

applying light for recording or reproduction to a holographic recording medium comprising at least a recording layer with a thickness L which comprises a photorefractive index-changing material as a recording layer material, the recording layer material having an absorption coefficient α, wherein the optical density αL expressed by the product of the absorption coefficient α and the thickness L is in a range approximately from 3.0 to 2.0, wherein

the recording is multiplexed recording in which signal light at a first incident angle and reference light at a second incident angle different from the first incident angle are applied simultaneously to the recording layer,

the multiplexed recording is performed by: maintaining a constant angle formed between the signal light and the reference light; and relatively shifting at least one of the holographic recording medium and the signal and reference lights,

the optical density αL is in a range approximately from more than 1.1 to at most 2.0, and

a relative distance between a first place where a first piece of information is recorded in the recording layer and a second different place where a second piece of information is recorded therein is three times the distance between the first place and a place where the intensity of diffracted light from the first piece of information exhibits a First Null.

4. A holographic recording method comprising:

applying light for recording or reproduction to a holographic recording medium comprising at least a recording layer with a thickness L which comprises a photorefractive index-changing material as a recording layer material, the recording layer material having an absorption coefficient α, wherein the optical density αL expressed by the product of the absorption coefficient α and the thickness L is in a range approximately from 0.3 to 2.0, wherein

the recording is performed simultaneously by applying signal light and reference light at an angle different from that of the signal light to the recording layer, wherein the reference light to passes through a lens to form a spherical reference wave with respect to the recording layer,

the spherical reference wave is applied every time the holographic recording medium is shifted by a constant distance in its surface direction for the purpose of shift-multiplexed storage, and

the constant distance δ (shift space) by which the holographic recording medium is shifted satisfies the formula:

δ≧ n·λz 0 /( L ·sinθ)

wherein λ represents the shift space (μm), n represents an integer of at least 2, λ represents the wavelength (μm) of the signal light and the spherical reference wave applied at the time of recording, θ represents an angle (deg) which the signal light forms with the center optical axis of the spherical reference wave, and z 0 represents the distance (μm) between the recording layer and the lens for forming the spherical wave.