JPH0777923A - Hologram fixing method - Google Patents

Abstract

PURPOSE:To obviate the generation of an adverse influence on formation of a hologram reconstructed image at the time of fixing a hologram to a hologram recording medium by irradiating a hologram recording material with fixing light in such a manner that the integrated exposure quantity in each part within the material is made uniform. CONSTITUTION:An angle modulating means for continuously changing the irradiation angle to the hologram with lapse of time in case of use of the same illuminating optical system as reference light B1 is provided. The exposure to the hologram is executed while the irradiation angle of the fixing light C1 changes if this means is operated. While interference fringes are primarily generated within the hologram recording medium 1, the forming position thereof eventually changes with lapse of time if the wavelength and the irradiation angle are within the diffraction characteristic range of the light in such a case. The final integrated exposure within the hologram recording medium 1 is, therefore, eventually averaged. The hologram recording medium 1 after fixing is thus exposed uniformly in the respective parts therein.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for fixing a hologram to an optically reactive hologram recording material such as a photopolymer.

[0002]

2. Description of the Related Art In recent holographic techniques, particularly holographic lithography techniques, etc., hologram forming materials are used to perform a process such as forming a hologram reproduced image while maintaining the same positional relationship after forming and recording a hologram. It is preferable not to move (the recording medium containing) during these steps.

Therefore, as a material for recording a hologram, it is common to use a material which does not require a developing process, such as a step of removing a hologram recording medium. A hologram recording material capable of fixing a hologram or the like, that is, a hologram recording material exhibiting so-called optical reactivity is mainly used.

As a hologram recording material exhibiting such properties, a photopolymer or the like is generally known, and when this is used as a hologram recording material, after forming and recording interference fringes forming a hologram. In order to fix holograms (interference fringes), a fixing step of uniformly exposing the entire surface of the hologram recording material has been performed.

In the conventional fixing process, it is considered that fixing light having a sufficient intensity for denaturing the recording material may be uniformly applied to the entire surface of the hologram recording medium for fixing exposure, and special attention should be paid. I wasn't told. Therefore, as a simple fixing method, it is often the case that only the reference light is used as the fixing light after the hologram recording by directly using the irradiation means of the reference light or the like used at the time of recording the interference fringes forming the hologram.

[0006]

However, it has been found that the use of the same light flux as the reference light as the fixing light as described above causes the following problems. First, due to the characteristics of the holography technique, when a hologram that has been formed once is irradiated with the same reference light, it is diffracted by this hologram.
Here, since a light source with good coherence (generally a laser beam) is used as the light source of the reference light, the diffracted light by the reflected light in the hologram recording medium and the fixing light (the same as the reference light) itself have a finger in the pie.

Due to the internal interference of the fixing light itself, an intensity distribution of the fixing light is generated in the hologram recording material, and therefore, an intensity distribution is also generated in the fixing exposure amount in each portion in the hologram recording material, so that the entire exposure is uniform. There arises a problem that (fixing) cannot be realized. As a result, adverse effects (such as non-uniform change in refractive index) associated with the intensity distribution of fixing exposure occur in the hologram recording material after fixing, so that the contrast of the hologram itself decreases, and when a reproduced image is formed from the hologram,
It was found that there is a problem that the diffraction efficiency of the reproduction light in the hologram deteriorates.

Further, in some cases, interference fringes due to interference between diffracted light and the fixed light in the hologram at the time of fixing occur, but when this is formed in the hologram recording material, the interference fringes forming the original hologram are formed. Means that another interference fringe will exist. Therefore, the diffraction efficiency of the diffracted light (reproduced image forming light) in the hologram of the reproduced light is deteriorated, and the diffracted light due to the internal interference fringes is generated as noise light, so that the image forming characteristic of the hologram reproduced image is also deteriorated. It turned out to be a thing.

Further, when a hologram is recorded by a so-called total reflection holography technique which forms a hologram by utilizing the total internal reflection light in the hologram recording medium, the recorded hologram is irradiated with the same light flux as the reference light. Then, a standing wave is generated, so that an illuminating means of object light may be used as the fixing light. In this case, it was found that a new hologram was generated by the same fixing light as the object light, and it was found that a new noise was generated from this.

These phenomena will be described in more detail by taking a case where a photopolymer is used as a recording material as an example. Among the recording materials exhibiting optical reactivity, a photopolymer records interference fringes (holograms) formed in the hologram recording material by utilizing the fact that a monomer is polymerized by irradiation of light to be polymerized.

Since the polymerization of the monomer proceeds according to the intensity (exposure amount) of the irradiated light, the intensity of the interference fringes generated by the interference between the object light and the reference light during the exposure during hologram formation. Polymerization proceeds according to the distribution. At the position where the interference fringes are formed, the refractive index changes due to the monomer being polymerized, so that the intensity distribution of the interference fringes is recorded (hologram recording) as a refractive index distribution in real time.

However, since the unpolymerized monomer remains as it is, if the reproducing light is irradiated as it is, the remaining monomer reacts and the hologram just recorded on the photopolymer is unstable. is there. For this reason, it is necessary to perform a fixing step of stabilizing the hologram by uniformly exposing the entire surface of the photopolymer of the hologram recording medium to polymerize the unpolymerized residual monomer. After this fixing process, the hologram is in a stable state and does not change even when irradiated with reproducing light.

The light used for the fixing exposure may be any light exhibiting reactivity with the photopolymer, that is, light having a wavelength having a sensitivity for causing polymerization of the monomer. Therefore,
It is conceivable to prepare a separate light source for fixing exposure in addition to the recording light source for interference fringes during hologram formation, but it is the simplest and cheapest to use the light source and optical system used for recording interference fringes as it is for fixing exposure. That's the method.

Here, since the object light generally does not have a uniform intensity in the optical axis cross section, the reference light is suitable as a light flux for uniformly exposing the entire surface. When the hologram that has already been formed is irradiated with the same light flux as this reference light,
Diffracted by the already formed refractive index distribution (hologram), the object light (a light flux traveling on the same path as the object light) is reproduced.

The reproduced object light interferes with the reference light itself which is the fixing light (similar to when the hologram is formed), and causes new interference fringes in the hologram recording medium. Therefore,
The hologram recording medium is irradiated with different intensities for each place due to the influence of internal interference, and thus it is not possible to perform exposure with uniform intensity.

Here, new interference fringes generated at this time are:
Ideally, it should be exactly the same as the original interference fringe (hologram) originally recorded. Even in this case, the fixing exposure is performed again with a light beam having the same intensity distribution as the interference fringe intensity distribution at the time of hologram formation recording.
As a result, the remaining portion of the monomer is irradiated with lower illuminance, and the original purpose of fixing, which is to polymerize the remaining monomer, is not fulfilled.

Furthermore, in reality, since the photopolymer itself undergoes changes such as changes in the average refractive index and shrinkage due to the effects of exposure during hologram formation and uniform exposure during fixing, the reference light is irradiated as fixing light. New interference fringes generated at this time are formed at positions slightly deviated from the original interference fringes (hologram).

The new interference fringes serve to reduce the contrast of the original interference fringes and deteriorate the diffraction efficiency of the hologram. Further, in some cases, unnecessary diffracted light may be generated during reproduction, and noise light may be generated in the reproduced image forming light from the hologram.

For the above reasons, uniform exposure with the same light flux as the reference light is not preferable, and similarly uniform exposure with the same light flux as the object light or light conjugate with the reference light or the object light (reproduction light) is performed. Similarly, it is not so preferable as fixing light.

The present invention has been made in view of the above problems, and when fixing a hologram by irradiating the hologram recording medium with fixing light, the fixing is performed so that the hologram reproduced image is not adversely affected. It is intended to provide a method.

[0021]

In order to achieve the above object, the invention described in claim 1 of the present invention is such that an interference fringe forming a hologram is recorded on an optically reactive hologram recording material, and then a fixing light is applied. A method of fixing a hologram by irradiating the hologram recording material is characterized in that the fixing light is irradiated so that the integrated exposure amount of each part in the hologram recording material becomes uniform.

According to a second aspect of the present invention, there is provided a method of fixing a hologram by irradiating fixing light after recording interference fringes constituting the hologram on an optically reactive hologram recording material, Provided is a hologram fixing method characterized in that at least one condition of an incident angle to the hologram, a wavelength, a wavefront shape, and coherence satisfies a condition that diffracted light at the hologram does not occur. .

According to the third aspect of the invention, there is provided a method of fixing the hologram by irradiating a fixing light after recording the interference fringes constituting the hologram on the optically reactive hologram recording material, Provided is a hologram fixing method characterized in that fixing light is irradiated while changing at least one condition of an incident angle to the hologram, a wavelength, a wavefront shape, and coherence during a fixing exposure operation on the hologram. .

[0024]

Since the present invention is constructed as described above, it has the following effects. The invention described in claim 1 of the present application is a method of fixing a hologram by irradiating fixing light after recording interference fringes constituting the hologram on an optically reactive hologram recording material, It is characterized in that the fixing light is irradiated so that the integrated exposure amount in each portion of the recording material becomes uniform.

By irradiating the fixing light so that the integrated exposure amount becomes uniform in each part of the hologram recording material, the optical reaction of the hologram recording material also occurs uniformly, and the modified state after fixing is uniform in each part. It will be like. As a result, the hologram composed of the recorded interference fringes is stably present in the hologram recording medium, and each portion is modified by a uniform optical reaction, so that the hologram has a high contrast.

Therefore, when the hologram fixed by the fixing method according to the present invention is irradiated with the reproducing light, the reproduced image forming light diffracted here does not impair the diffraction characteristics and further ghost light or the like is not generated. It is possible to reproduce a clear reproduced image with high contrast.

Next, in the invention described in claim 2, at the time of irradiating the optically reactive hologram recording material with the fixing light, at least one of an incident angle to the hologram, a wavelength, a wavefront shape, and coherence is selected. It is characterized in that the fixing light is irradiated so as to satisfy the condition that the diffracted light at the hologram does not occur. That is, in the present invention, uniform fixing exposure is realized in each part of the hologram recording medium by irradiating light that is not diffracted by the hologram as fixing light.

The conditions under which the diffracted light at the hologram does not occur are:
For example, when a light beam having the same wavelength as that of the reference light is used, the fixing light may be emitted at an angle at which diffracted light does not occur in the hologram, although it depends on the recording conditions of the hologram. further,
It is also conceivable to select a light flux having a wavelength at which diffracted light does not occur in consideration of the distance between interference fringes forming a hologram (for example, different oscillation wavelength of laser light) and irradiate.

On the other hand, even if a diverging wave or a converging wave (a spherical wave or the like) is applied as the fixing light instead of a parallel light (a plane wave or the like) like the reference light, diffracted light is less likely to be generated, and an interference fringe based on this is generated. Does not happen. In addition, if an incoherent light beam is applied instead of a light beam such as a reference light having good coherence, the interference does not occur even if diffracted light occurs, so that no intensity distribution occurs in the hologram recording medium. .

Next, in the invention described in claim 3, at the time of irradiating the optically reactive hologram recording material with the fixing light, at least one of the incident angle to the hologram, the wavelength, the wavefront shape, and the coherence is selected. It is characterized in that the fixing light is irradiated while changing the conditions during the fixing exposure operation for the hologram.

In the method according to the present invention, by changing the fixing light during exposure, even if interference fringes or the like due to the diffracted light by the fixing light and the hologram are generated, by changing the position thereof, the hologram is consequently obtained. By uniformly averaging the exposure amount in the recording medium in each part, uniform fixing exposure is realized.

According to this method, even if the hologram does not diffract, the influence of the internally reflected light, scattered light, etc. can be homogenized, and even if interference with the light beam diffracted by the hologram occurs. By changing the incident angle or wavelength of the fixing light, the affected part due to diffraction or interference can be changed,
It is assumed that adverse effects will not occur only in specific areas.

[0033]

The present invention will be described in more detail with reference to the following examples. In this example, a method of fixing a hologram recording medium using a photopolymer as a hologram recording material as a material exhibiting optical reactivity will be described. Although the present invention is not limited to this photopolymer, a photopolymer which is generally widely used as a so-called development-free hologram recording material will be described as an example.

As described above, when the same light flux as the reference light is used as the fixing light, there is a problem that an intensity distribution or the like occurs inside the recording material due to the influence of diffraction etc. in the hologram.
Such an adverse effect can be solved by using a light flux that does not inherently generate diffracted light or interference as the fixing light, thereby eliminating the intensity distribution due to the fixing light. Further, even if the diffracted light or interference occurs primarily, the intensity distribution can be averaged by changing the position, so that the adverse effect on the reproduction can be eliminated. Specific examples of these will be described below.

First, when the hologram formed and recorded on the hologram recording material is a volume hologram,
By utilizing the diffraction characteristics of the volume hologram, it is possible to select fixing light that does not generate diffracted light. As shown in FIGS. 3 and 4, it is known that the diffraction efficiency in the volume hologram has an angle selectivity and a wavelength selectivity, and by utilizing this diffraction efficiency in reverse, an irradiation without diffracted light is generated. A light beam having an angle or a wavelength may be irradiated as the fixing light.

As shown in FIG. 1, the light flux (object light A ₁ or reference light B ₁₎ used for forming and recording a hologram is used as the fixing light C _1.
When a light beam having the same wavelength as ₁ ) is used, there is an advantage that a light source and the like can be shared, but the incident angle to the hologram (recording medium 1) is different from that at the time of hologram formation so that diffracted light at the time of fixing does not occur. Are preferably different.

In practice, after the hologram is formed and recorded, the individual diffraction efficiency characteristics (see FIG. 3) of the hologram at the time of fixing are judged in consideration of the hologram deformation and the like, and whether the diffracted light is generated in the hologram or not It is preferable to perform the fixing exposure by selecting the irradiation angle at which the intensity becomes weak.

When using the irradiation optical system of the reference light,
By providing an angle adjusting means capable of adjusting at least the final incident angle to the hologram, the irradiation angle is changed (FIG. 3b) or the hologram (recording medium) itself is tilted to change the irradiation angle (FIG. 3c). Etc. are possible. Other types of fixing light may be used as long as the fixing light can emit a light beam having the same wavelength (wavelength, etc.) as that of the reference light at an angle different from that at the time of hologram formation. (See Figure 1)

Next, by utilizing the wavelength selectivity among the diffraction characteristics of the hologram, it is possible to prevent the generation of diffracted light even if the fixing light is irradiated from the same angle as the reference light. The wavelength selectivity of the diffracted light in the hologram indicates that the diffracted light is generated only in a certain wavelength range centered on the wavelength used for hologram recording and recording. Then, no diffracted light is generated.

Therefore, as shown in FIG. 4, in consideration of the wavelength selectivity of the individual hologram obtained from the wavelength of the reference light at the time of forming the hologram, the fixing light is separated from the wavelength range separated from the wavelength of the reproduction light by a certain amount or more. By selecting and irradiating, diffracted light will not be generated at the same irradiation angle as the reference light or at a different irradiation angle.

In this method, a light source of another wavelength for fixing light is provided in addition to the interference fringe recording light source (used for reference light or object light), or the laser light source used as the interference fringe recording light source is different. This can be easily realized by selecting and using a light beam having an oscillation wavelength.

Next, since the incident angle to the hologram can be partially changed by changing the wavefront of the fixing light, this can be used to apply the angle selectivity of the diffraction efficiency described above. It is possible. That is, even if the same irradiation optical system as the reference light is used, it is conceivable that a diverging or converging optical system or the like is inserted in the optical path to irradiate the fixing light as a diverging wave or a converging wave.

When the wavefront of the fixing light is changed in this way, when the fixing light is irradiated on the hologram, the incident angle varies depending on the irradiation position, so that the diffracted light from the hologram hardly occurs. Even if the diffracted light is generated, the interference between the diffracted light and the fixing light is unlikely to occur. However, since the irradiation angle changes continuously depending on the irradiation position,
In some cases, diffracted light may be generated, but in such a case, it is advisable to combine it with a means for changing with time described below.

In order to prevent the intensity distribution due to internal interference from occurring, it is conceivable to use a light beam having low coherence as the fixing light. If the light flux has a low coherence, even if diffracted light from the hologram occurs, it does not interfere with other fixing light, so that internal interference does not occur. Therefore, even if the wavelength and the irradiation angle are within the range of the diffraction characteristics (for example, the reference light is used), the fixing exposure with uniform intensity can be performed on each part in the hologram recording medium.

Here, since the reference light and the object light are originally supposed to form interference fringes, laser light or the like having good coherence is used. Therefore, it is not preferable to apply these light fluxes to the fixing light as they are, but if a diffusion plate or the like is inserted in these optical paths, the light flux transmitted therethrough becomes a light flux with low spatial coherence.

For example, as shown in FIG. 2, by irradiating the same light flux C ₂ as the reference light as the fixing light C ₂₂ through the diffusion plate 9, the fixing exposure is performed without causing the interference inside the hologram recording medium. You will be able to. When the object for hologram formation is a two-dimensional mask, the mask and the diffusing plate are exchanged after forming and recording the hologram, and the same light flux as the object light (mask irradiation light) is used as the fixing light C.
Irradiate as ₂ . In consideration of the uniformity of the entire irradiation area, it is preferable to use the rotary diffusion plate.

Next, an embodiment will be described in which the fixing light is irradiated while changing the irradiation condition during the fixing exposure operation.
First, a method of continuously changing by changing the irradiation angle of the fixing light will be described. If the same illumination optical system as that of the reference light is used, an angle modulation means for continuously changing the irradiation angle to the hologram with time is provided, and if this is operated during the fixing process, the irradiation angle of the fixing light is changed. The hologram is exposed while changing.

In this case, if the wavelength and the irradiation angle are within the above-mentioned diffraction characteristic range, although the interference fringes are primarily generated in the hologram recording medium, the formation position thereof changes with time. Becomes For this reason, the final integrated exposure amount in the hologram recording medium is averaged, and uniform exposure is performed in each part in the hologram recording medium after fixing.

When the wavelength and irradiation angle are selected so that diffracted light does not occur, naturally, the intensity distribution due to internal interference does not occur, but the influence of reflected light does not occur at all. It cannot be said that there is an advantage that fixing exposure can be performed by averaging these adverse effects.

This method is particularly effective when fixing a hologram formed by total reflection holography. In the total reflection holography, when the reference light is used as the fixing light, the fixing light passes through the hologram, then is totally reflected on the surface of the hologram recording medium, and again passes through the hologram. Therefore, a standing wave due to these two lights may be generated in the hologram recording medium and uniform exposure may not be realized. However, by changing the irradiation angle of the fixing light as described above, the standing wave is not generated. Also moves, and as a result, the integrated exposure amount is averaged in each part.

Further, in the case of total reflection holography, noise generated when the object light is used as the fixing light can be avoided by this method. About the principle of this noise generation,
This will be described with reference to FIG. First, consider the case where a hologram is formed and recorded by using a two-dimensional mask 57 having a predetermined opening as an object.

When the mask 57 is illuminated with the object light A ₃ , the reference light B is applied only to the portion where the mask-transmitted diffracted light A ₃₁ reaches.
_The reflection type hologram 53 is recorded by interfering with ₃ . Then, after removing the mask 57, using the object light (mask illumination light) as the fixing light C _3, a portion fixing light C ₃ has been diffracted by the hologram 53, corresponding to the reference light (reference light and the same route The light B ₃₁ which is a light flux of) is reproduced. This diffracted light B
_{After 31} is totally reflected on the surface of the hologram recording medium 51,
A new reflection type hologram 55 is formed and recorded by interfering with the fixing light C ₃ .

In the state where this new hologram (reflection hologram) 55 is recorded, the original hologram (reflection hologram) 53 is generated by the light (reproduction light) D ₃ which is conjugate with the reference light.
Consider the case of playing.

[0054] When irradiated with reproducing light D _3, reproduced light D ₃ that is totally reflected by the hologram recording medium 51 surface reproduces the image 60 of the mask aperture is diffracted by the hologram 53. At this time, at the same time, the reproduction light D ₃ is also diffracted by the reflection hologram 55 recorded at the time of fixing, and an image 62 having another different aperture is reproduced. The reproduced image 62 formed by the reflection hologram 55 is not an image to be reproduced originally, and therefore appears as noise when the hologram is reproduced.

As described above, in the total reflection holography, when the object light is used as the fixing light, it is an unavoidable problem that an image reproduced laterally is generated as noise in addition to the original reproduced image.

Since this lateral shift amount is about the thickness of the hologram recording medium, it cannot be ignored if the object is a fine pattern. Further, if there is a layer such as a cover glass on the surface of the hologram recording medium, the lateral shift amount becomes larger, and the noise of the reproduced image becomes a problem even if it is not a fine pattern. Furthermore, it should be noted that this noise image may be generated by a light beam traveling in the same direction as the object light, even if it is not exactly the same as the object light.

Such a noise image is produced by forming and recording a reflection type hologram which causes it. However, by changing the irradiation angle and the like as described above, the position change of the formed hologram is caused. Since the intensity distribution is generated and the intensity distribution is homogenized, the hologram having the intensity distribution of the interference fringes is not recorded as a result, and the generation of noise light can be prevented. It is needless to say that the same effect can be obtained by using the light flux that originally does not generate the diffracted light as the fixing light as in the above-described embodiment.

Next, the same effect as the angle change can be obtained by the means for irradiating the fixing light having the angular width by using the incoherent light source having the size. This uses, for example, as a secondary light source, a condensing area in which a light flux from a laser light source is condensed on a rotating diffusion plate or the like with a predetermined size instead of a pin spot, and the light flux from this secondary light source is used as a Koehler illumination system. By using it as fixing light, fixing light having an irradiation angle width can be emitted.

Similarly, when a light source having a wavelength width is used, the interference fringes due to the light of each wavelength are averaged, so that the exposure intensity distribution does not occur in the hologram recording medium, which adversely affects hologram reproduction. Will not occur.

The fixing method according to each embodiment described above is
Each of them has its own effect, but a combination of two or more can further enhance the effect. Further, the hologram recording material is not limited to the photopolymer,
A recording material having a property of being modified according to the amount of exposure, that is, a recording material exhibiting so-called optical reactivity can be applied to other materials.

In each of these fixing methods, a mechanism for a fixing exposure optical system for uniform fixing exposure and a control device are incorporated into the hologram recording device, so that the uniform exposure process is automatically performed after the hologram exposure. You can enter. In this case, since it can be shared with other lighting means and the like, the manufacturing cost can be suppressed.

[0062]

As described above, according to the present invention, since the hologram recording material is uniformly fixed and exposed at each portion,
While the hologram contrast is maintained properly,
It is possible to obtain a stable hologram that does not change due to reproduction light.

Further, since no new interference fringes are recorded by the fixing light, it is possible to prevent the diffraction efficiency of the hologram from being lowered by the irradiation of the reproducing light and the generation of noise such as a ghost image.

Further, since the light source and the optical system of the other certifying means can be shared as the fixing light illuminating means, the manufacturing cost of the apparatus can be suppressed and the fixing means can be incorporated into the conventional hologram forming apparatus by a simple improvement. There is also an advantage.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a state in which an angle is changed in a fixing light irradiation state in a fixing method according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state in which diffused light transmitted through a diffusion plate is irradiated as fixing light in a fixing method according to an embodiment of the present invention.

FIG. 3 is a diagram showing an angle selection characteristic of diffracted light in a hologram according to an embodiment of the present invention.

FIG. 4 is a diagram showing wavelength selection characteristics of diffracted light in a hologram according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram illustrating the principle of noise generation in total reflection holography.

[Explanation of symbols]

 1, 21, 51 ... Hologram recording medium, 57 ... Mask, 9 ... Diffusion plate, 53 ... (Reflective type) hologram (original hologram), 55 ... Reflective hologram (noise factor)

Claims (3)

[Claims] 1. A method for fixing a hologram by irradiating a fixing light after recording interference fringes constituting the hologram on an optically reactive hologram recording material, wherein each part in the hologram recording material is A fixing method for holography, which comprises irradiating a fixing light so that the integrated exposure amount in step 1 becomes uniform. 2. A method of fixing a hologram by irradiating a fixing light after recording interference fringes constituting the hologram on an optically reactive hologram recording material, wherein the incident angle to the hologram is: A method for fixing a hologram, characterized in that the fixing light is irradiated so that at least one of the conditions of wavelength, wavefront shape and coherence satisfies the condition that diffracted light at the hologram does not occur. 3. A method for fixing a hologram by irradiating a fixing light after recording interference fringes constituting the hologram on an optically reactive hologram recording material, wherein the incident angle to the hologram is: A method of fixing a hologram, wherein irradiation of fixing light is performed while changing at least one condition of wavelength, wavefront shape, and coherence during a fixing exposure operation of the hologram.