JPH0583909B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for producing a hologram, and particularly to a method for producing a hologram in which an object beam and a reference beam are obtained by hologram reproduction.

[Prior art]

A hologram lens can be obtained by creating a hologram of a point light source using holography technology. The hologram lens has the advantage of being a flat plate-shaped thin film lens with a thickness of approximately several microns, and that a large number of lenses can be mass-produced on the same flat plate using the step-and-repeat method.
For this reason, hologram lenses are used as optical elements in optical systems that utilize laser light, such as condensing lenses in optical heads of optical disk devices and collimation lenses for converting divergent light beams from semiconductor lasers into parallel light beams. It is proposed to do so.

The optical system of the optical head section of an optical disk device has a condensing hologram on the front side of the disk substrate so that signals recorded on the back side of the disk substrate, which is usually about 1.1 mm thick, can be read through the disk substrate. The lens is placed. The hologram lens is placed with an air gap of about 1 mm from the disk substrate to prevent collisions due to vibration of the disk, and a cover glass or protective layer of an appropriate thickness is interposed to prevent dust from adhering.

FIG. 1 shows an optical system for producing a hologram lens used in such an optical system. In the figure, a part of monochromatic light 2 emitted from a laser light source 1 passes through a semi-transparent mirror 3, is reflected by a reflecting mirror 4, is focused on a pinhole 16 by a microscope objective lens 15, and is transmitted through the pinhole 16. The light passes through the collimation lens 17 and becomes a parallel beam 18,
The light passes through the parallel plate 9 and enters the hologram sensitive material 11 coated on the hologram substrate 10 . This is the reference light. On the other hand, the light beam reflected by the semi-transparent mirror 3 is reflected by the reflecting mirror 5 and is reflected by the microscope objective lens 7.
The light is focused on the pinhole 8 by
The light transmitted through becomes a diverging light beam 12, and the parallel plate 9
The light passes through and enters the hologram sensitive material 11. This is object light. The object beam 12 becomes a divergent beam with spherical aberration by the parallel plate 9, and this and the reference beam form interference fringes at the position of the hologram sensitive material 11.
recorded in A hologram lens is obtained by developing this.

When using the hologram lens thus produced, a laser beam having the same wavelength as that used during production is made to enter the hologram 11 as a parallel light beam at the same angle as the parallel light beam 18 but in the opposite direction.
The light diffracted by the hologram 11 becomes a convergent light beam with the spherical aberration given to the object light at the time of manufacture, and after passing through the cover glass and disk substrate, a light spot is formed at a position corresponding to the pinhole 8 at the time of manufacture of the hologram. occurs.

In this way, by using light of the same wavelength during fabrication and use, it is possible to perform complete wavefront reconstruction with substantially no aberration using the hologram lens.

In particular, when a volume phase hologram is fabricated using dichromate gelatin or the like as the hologram sensitive material 11, the diffraction efficiency of the hologram can be improved to almost 100%, and the light utilization efficiency is sufficiently high. .

By the way, as a light source in an optical system using a hologram, it is preferable to use a semiconductor laser, which is small, lightweight, and does not require a special modulator. The oscillation wavelength range of such a semiconductor laser is usually from the near-infrared region to the infrared region (0.78 μm or more). Therefore, when producing a hologram lens as described above using this semiconductor laser and performing image reproduction using the same, it is necessary to use a hologram sensitive material having an effective sensitivity at 0.78 μm or more. A hologram sensitive material sensitive to this wavelength range includes a silver salt sensitive material sensitized to infrared light. However, since the hologram produced using this sensitive material is an absorption type hologram, it has a drawback of having a low diffraction efficiency of about several percent. Furthermore, although the diffraction efficiency can be improved to some extent by methods such as bleaching, there is a limit to this method as well.

Therefore, in order to improve the diffraction efficiency, it is necessary to employ a volume type phase hologram. Dichromate gelatin is a typical sensitive material used for producing such holograms. However, the effective sensitivity range of this sensitive material is limited to a maximum of 0.55 μm green light as it is, and even with special dye sensitization,
It can only be sensitive to red light of 0.6 μm. Furthermore, there is no known sensitive material for volume holograms that has effective sensitivity in the near-infrared and infrared regions.

For this reason, a semiconductor laser cannot be used when producing a volume phase hologram, and a laser with a shorter wavelength is used. When a hologram manufactured in this way is used in an optical system using a semiconductor laser, the wavelength of the light differs between the time of manufacture and use, so the image cannot be formed without aberration, so in some cases it may be necessary to correct the aberration. Is required. When creating a hologram by adding aberrations to the light beam produced in advance in consideration of aberrations caused by differences in the wavelength of light during use, the reference wave and object wave are created using independent optical systems, and It is necessary to place it in the space of Therefore, the setting accuracy of the hologram production optical system is extremely strict.

Although the above description has been made regarding a hologram lens that condenses a parallel light beam to a single point, other difficulties arise with a hologram lens that converts a diverging light beam into a convergent light beam. FIG. 2 is a diagram showing an optical system for producing a hologram lens that condenses a luminous flux diverging from one point onto another point. In the figure, the reference light beam 18 needs to converge on a point 20 in the air without aberration after exiting the hologram substrate 10. However, since this light beam 18 passes through the parallel plate 9 and the hologram substrate 10, aberrations occur due to these optical elements.
In order to cancel this aberration, for example, a single lens 17 and a cylindrical lens 1 are installed in front of the parallel plate 9.
It is necessary to provide means such as installing each of the optical lenses 7' obliquely to the optical axis. However, in reality, it is quite difficult to design such an optical system, manufacture optical elements, and arrange them with high precision.

Furthermore, in the hologram production optical system as shown in FIGS. 1 and 2, since the parallel plate 9 is placed immediately in front of the hologram sensitive material 11, a harmful ghost image is generated due to this. That is, the object wave beam 12
A part of the light beam 13 is reflected by the second surface and then the first surface of the parallel plate 9, and a part of the object wave light beam 12 is reflected by the surface of the hologram sensitive material 11 and then reflected by the second surface of the parallel plate 9. The resulting light beam 13' enters the hologram sensitive material 11, and a harmful ghost image is recorded.
These ghost images are reproduced when the hologram lens is used, resulting in the generation of unnecessary ghost light and a decrease in diffraction efficiency.

[Purpose of the invention]

SUMMARY OF THE INVENTION In view of the above-mentioned prior art, it is an object of the present invention to provide a method for manufacturing a hologram that can be implemented with a simple configuration and does not require particularly strict setting accuracy.

Another object of the present invention is to fabricate a hologram that can be reproduced with virtually no aberration and with high diffraction efficiency when the wavelength of light when the hologram is fabricated is different from the wavelength of the light when it is used. It is.

[Summary of the Invention] According to the present invention, the above object is to perform predetermined diffraction of a hologram used with light of a wavelength different from the wavelength of light at the time of fabrication by an object beam hologram and a reference beam hologram, respectively. In a method for manufacturing a hologram using light, the object beam hologram and the reference beam hologram are provided at different positions on the same plane of the same substrate, and the illumination light flux manufactured by the same optical system is used to This is achieved by a hologram manufacturing method characterized by illuminating an object beam hologram and the reference beam hologram.

[Embodiments of the invention]

Prior to explaining the contents of the present invention, the principle of Bragg diffraction will be explained with reference to FIGS. 3a and 3b. It will be explained that it is possible to create a hologram that can satisfy the bragg condition.

FIG. 3a shows Bragg diffraction at wavelength λ ₁ when the hologram 11 is used. In the figure, when the ray 21 that is incident on the interference fringe 20 at pitch d ₁ at an angle φ ₁ satisfies the Bragg condition, it becomes a ray 21' that emerges from the interference fringe at an angle φ ₁ as a result of diffraction. The relationship in this case is expressed as 2d ₁ sinφ ₁ =λ ₁ /N ₁ (1), where N ₁ is the refractive index of the hologram sensitive material at wavelength λ ₁ .

To create interference fringes that satisfy equation (1) using light with a wavelength λ ₂ that is different from the wavelength λ ₁ used, the wavelength λ ₂
Letting the refractive index of the hologram sensitive material be N ₂ , light should be incident at an angle φ ₂ that satisfies 2d ₂ sinφ ₂ =λ ₂ /N ₂ (2). The situation is shown in FIG. 3b. The pitch of the interference fringes at the time of manufacture was set to _d2 , which is _different from d1 at the time of use, in order to take into account changes in the size of the sensitive material during hologram development. After development, the hologram has interference fringes with a pitch of d ₁ , as shown in FIG. 3a. Light rays 2 from two directions at an angle φ ₂ to form interference fringes 20 as shown in FIG. 3b.
2 and 22', a hologram having interference fringes that satisfies the bragg condition during use can be obtained.
There is a specific relationship between d ₁ and d 2 in equations (1) and ( ₂ ), but if we set d ₁ = d ₂ for simplicity, then equations (1) and (2) From the formula, φ ₂ is sinφ ₂ = {(λ ₂ /N ₂ )/(λ ₁ /N ₁ )}sin φ ₁ (3
) is determined to satisfy the following. Also, since |sinφ ₂ |1, it is necessary to satisfy |μ·sinφ ₁ |1 as μ≡(λ ₂ /N ₂ )/(λ ₁ /N ₁ ).

As explained above, even if the wavelength of the light when using the hologram is different from the wavelength of the light when producing the hologram, two beams of light are incident on each point of the hologram sensitive material from appropriate directions when producing the hologram. By doing so, it is possible to produce a hologram that satisfies the Bragg condition at each point during use and can generate diffracted light in a predetermined direction.

Hereinafter, specific examples of the present invention will be described.

FIG. 4 is a diagram showing an optical system when a hologram lens is used in the optical head section of an optical disk device. In the figure, a parallel light beam 26 with a wavelength λ ₁ is incident on a hologram substrate 28 and a hologram 29 is abbreviated as
It is 100% diffracted, passes through the cover glass or protective film 30, is emitted into the air, becomes a convergent light beam 27, and is focused on the back surface 31' of the disk 31.

FIG. 5 is a diagram showing an optical system for manufacturing a hologram lens used in the optical system of FIG. 4. In the figure, a reference wave beam 33 and an object wave beam 34 having a wavelength λ ₂ are made incident on a hologram sensitive material 32 . Here, the reference wave beam 33 and the object wave beam 34
is determined in accordance with the above-mentioned principle so as to obtain a hologram that can be focused with substantially no aberration by diffraction as shown in FIG. 4 when the hologram is used. As a method for producing such a light beam, it is preferable to use a computer generated hologram (CGH) since using a lens system would result in a complicated system.

The method using CGH will be explained below.

Conventionally, in order to produce a CGH, an enlarged hologram was produced using an X-Y plotter, and the hologram was optically reduced and exposed. However, with this method, errors such as errors in the optical system and its arrangement were introduced into the reduction process, making it impossible to obtain a CGH with sufficiently satisfactory performance, especially for high NA holograms. However, in recent years, with improvements in semiconductor integrated circuit manufacturing technology, it has become possible to perform microfabrication using electron beam exposure (EB exposure). If this EB exposure method is used, fine patterns can be formed on the hologram sensitive material, and since reduction using an optical system is not necessary, a high-performance CGH can be obtained. EB exposure CGH
Sensitive materials for use include poly-methyl methacrylate (p-MMA), poly-t-butyl methacrylate (p-BMA), and poly-methacrylic anhydride (p-MMA).
MA, AN), etc., and can produce gratings with submicron line widths.

By using such a CGH, a desired luminous flux can be obtained. An example of a hologram production optical system using such a CGH is shown in FIG. In the figure, numerals 38 and 40 are parts of the light beams from the laser light source that are made into parallel light beams using a beam expander or the like, and are incident on holograms 42 and 43, respectively, produced on the substrate 37. These two parallel light beams 38 and 40 are shown by extracting a part of one parallel light beam, and the parallel light beam 38 represents the light beam incident on the reference wave hologram 42, and the parallel light beam 40 represents the light beam incident on the object wave hologram. represents the luminous flux incident on 43. A part of the light beam incident on the reference wave hologram 42 becomes a transmitted light beam 39, and a part undergoes first-order diffraction and becomes a reference wave light beam 33. Similarly, a part of the light flux incident on the object wave hologram 43 becomes a transmitted light flux 41, and a part undergoes first-order diffraction and becomes an object wave light flux 34. Interference fringes formed by the reference wave light beam 33 and the object wave light beam 34 within the hologram sensitive material 32 on the hologram substrate 28 are recorded. Here, 35 is a mask for allowing the light flux to enter only a predetermined portion of the hologram 36 or the hologram sensitive material 32. Further, 35' is a mask provided on the substrate 37 to block unnecessary transmitted light beams 39 and 41 and higher-order diffracted light beams 50 and 51 generated from the reference wave hologram 42 and object wave hologram 43, Therefore, unnecessary light does not reach the hologram sensitive material 32.

The required hologram grid pitch and its direction can be determined by the grid equation. As the coordinate axes, the x axis is perpendicular to the hologram surface, and the y axis and z axis are in the in-plane direction, and the direction cosines of the incident and exit light beams are (l ₁ , m ₁ , n ₁ ) and (l ₂ , m ₂ , n ₂ ), and the pitches of the lattice in the y and z directions are P _y and P _z , respectively, then the following relationship holds true.

P _y = λ _c / (m ₂ - m ₁ ) (4) P _z = λ _c / (n ₂ - n ₁ ) (5) where λ _c is the wavelength of the laser beam. (4) and
The lattice pitch at each point on the hologram is determined by equation (5). This lattice pitch can be calculated for any point on the hologram, and if we consider a group of curves that smoothly connect the hologram as a whole, this becomes the necessary hologram shape.

FIG. 7 shows how the hologram 36 used in the optical system of FIG. 6 is observed from the x-axis direction. 44 is a coordinate origin, and 42 and 43 are a reference wave hologram and an object wave hologram, respectively. This hologram is created using an optical system as shown in Fig. 4, in which an incident parallel light beam of 0.78 μm wavelength is
In order to fabricate a hologram 29 that is used by making the hologram 29 incident at an incident angle of 75 degrees, in the production optical system shown in FIG. It is used. In this design example, the object wave hologram 43 has a grating pitch of 0.6 to 130 μm, the number of gratings is 5000, and the size is 4 × 4 mm, and the reference wave hologram 42 has a grating pitch of 1.7 to 6 μm.
The number of pieces is 5,600 pieces, the size is 12 x 1.4 mm, and each
This is a value that can be achieved with CGH using EB exposure.

Next, the arrangement accuracy of the hologram production optical system according to the embodiment of the present invention as described above will be explained. Calculating the placement accuracy of the hologram production optical system such that the wavefront aberration is λ/4 (λ is the wavelength of the light during use) or less within the NA used in the hologram usage state (Figure 4), The CGH
The relative positioning accuracy between the holographic material and the hologram sensitive material may be ±100 μm. Furthermore, in this example, the object wave CGH
Since the CGH and reference wave CGH are fabricated on the same substrate, in principle no placement error occurs between them. On the other hand, when using an object wave optical system and a reference wave optical system that are independent from each other, the relative positioning accuracy between these optical systems needs to be ±30 μm, and the relative surface tilt needs to be ±3′. Further, the relative positioning accuracy between these optical systems and the hologram sensitive material must be ±30 μm.

In the above embodiment, two
Although we have explained the case where CGH is placed on the same board by repeatedly applying the method explained above,
A large number of sets of two CGHs can be arranged and produced, and by using this, a large number of holograms can be produced by one holographic exposure.

[Effects of the present invention]

According to the hologram manufacturing method of the present invention as described above, the object wave hologram and the reference wave hologram are provided on the same substrate, and the substrate and the hologram sensitive material need only be arranged in parallel, which simplifies the arrangement method. .

Further, according to the method of the present invention, even if the object wave and the reference wave have aberrations, strict placement accuracy of the fabrication optical system is not required.

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams showing a conventional hologram production optical system. FIG. 3a is a diagram showing Bragg diffraction of light by a hologram, and FIG. 3b is a diagram showing hologram production by interference of light. Figure 4 is a diagram showing the optical system when using the hologram lens, Figures 5 and 6 are diagrams showing the optical system when it is manufactured, and Figure 7 is a diagram showing the optical system used in the optical system in Figure 6. FIG. 2 is a plan view of a computer generated hologram. 1... Laser light source, 7, 15... Microscope objective lens, 8, 16... Pinhole, 9... Parallel plate, 10... Hologram substrate, 11... Hologram or hologram sensitive material, 17... Collimation Lens, 17'... Cylindrical lens, 28
... Hologram substrate, 29 ... Hologram, 32
...Hologram sensitive material, 35,35'...Mask,
36...Hologram, 37...Substrate, 42...Reference wave hologram, 43...Object wave hologram.

Claims (1)

[Scope of Claims] 1. A method for producing a hologram, in which a hologram used with light of a wavelength different from the wavelength of the light at the time of production is produced using predetermined diffracted lights from an object beam hologram and a reference beam hologram, respectively. The object beam hologram and the reference beam hologram are provided at different positions on the same plane of the same substrate, and the object beam hologram and the reference beam hologram are provided with an illumination light beam produced by the same optical system. A method for producing a hologram characterized by illuminating both sides of the hologram. 2. The hologram manufacturing method according to claim 1, wherein a plurality of sets of the object beam hologram and the reference beam hologram are arranged on the same substrate.