GB2171538A - Making reflection holograms - Google Patents

Making reflection holograms Download PDF

Info

Publication number
GB2171538A
GB2171538A GB08604077A GB8604077A GB2171538A GB 2171538 A GB2171538 A GB 2171538A GB 08604077 A GB08604077 A GB 08604077A GB 8604077 A GB8604077 A GB 8604077A GB 2171538 A GB2171538 A GB 2171538A
Authority
GB
United Kingdom
Prior art keywords
coating
coherent light
reflection
reflection hologram
hologram
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB08604077A
Other versions
GB2171538B (en
GB8604077D0 (en
Inventor
Richard Keith Howard
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Allard Way Holdings Ltd
Original Assignee
GEC Avionics Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by GEC Avionics Ltd filed Critical GEC Avionics Ltd
Publication of GB8604077D0 publication Critical patent/GB8604077D0/en
Publication of GB2171538A publication Critical patent/GB2171538A/en
Application granted granted Critical
Publication of GB2171538B publication Critical patent/GB2171538B/en
Expired legal-status Critical Current

Links

Classifications

    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03HHOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04Processes or apparatus for producing holograms
    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03HHOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04Processes or apparatus for producing holograms
    • G03H1/0402Recording geometries or arrangements
    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03HHOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04Processes or apparatus for producing holograms
    • G03H1/0402Recording geometries or arrangements
    • G03H2001/0415Recording geometries or arrangements for recording reflection holograms
    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03HHOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04Processes or apparatus for producing holograms
    • G03H1/0402Recording geometries or arrangements
    • G03H2001/0415Recording geometries or arrangements for recording reflection holograms
    • G03H2001/0417Recording geometries or arrangements for recording reflection holograms for recording single beam Lippmann hologram wherein the object is illuminated by reference beam passing through the recording material

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Holo Graphy (AREA)

Abstract

A reflection hologram is made by exposing an optically transparent light sensitive coating (13) carried on one surface of an optically transparent substrate (11) to coherent light transmitted through the substrate and the coating from a coherent light source and relying on coherent light back reflected within the coating from an interface (21) between the coating and an external medium (29) interfering with coherent light directly transmitted through the coating to create a latent image for a reflection hologram, and processing the sensitive coating to convert the latent image to a reflection hologram. <IMAGE>

Description

SPECIFICATION Holograms This invention relates to reflection holograms and to a method of making them.
Over the last few years many optical systems have made use of holographic optical elements to save space, weight or accomplish optical effects impossible by conventional methods. One particular group of holograms used as wavelength selective mirrors are known as reflection holograms. These holograms consist of a recording of a set of interference planes within a suitable recording medium creating modulations in refractive index or absorption. These modulations act to reflect only light for which the optical path difference between the planes is a multiple of the incident light's wavelength. This means that since the path difference is angularly dependent, that the wavelength of the reflected light is also dependent on angle and obeys the Bragg condition.
A = constant cos 6 Where A=Wavelength of light reflected 6= Internal angle of incidence of reflected light to hologram planes.
It is known to produce reflection holograms employing what is referred to as back reflection. Where it is desired to have the interference planes of the reflection hologram substantially parallel to the surface of a light sensitive coating in which the hologram is formed, the use of back reflection techniques during the exposure stage offers significant advantages in that stability requirements for the hologram are more easily met and the complexity of the optical system employed in the hologram production is significantly reduced.
In known forms of the back reflection technique, a single beam is used to expose the sensitive coating and an interfering beam, i.e.
the object beam, is derived from the single beam by reflection at a reflective mirror surface placed close to the sensitive coating. A virtual image of the hologram construction point is formed behind the reflective surface, and interference planes are formed in the sensitive coating by constructive and destructive interference between the incident single beam and the object beam, such planes lying substantially parallel to the coating surface. Commonly, the reflective mirror surface is 'index matched' with the sensitive coating using an index matching fluid between the mirror surface and the surface of the coating.
There are certain drawbacks in this known back reflection technique. Firstly, the hologram quality is dependent on mirror surface quality.
Secondly, the sensitive coating is, in particular, liable to damage when it is brought into place with respect to the mirror. Thirdly, the index matching fluid must be selected not only to match the index of the sensitive coating, but also to be non-reactive in relation to that coating, and a discrepancy in the refractive index of the fluid may give rise to interference fringes. Lastly, there must be a stable geometrical relationship between the mirror surface and the sensitive coating during the exposure period. This last is a problem where an expanded beam technique is employed.
Scanning methods as described in UK Patnet Specification No. 2071866 and US Patent Specification No. 4395088 serve to mitigate this drawback but the problem remains nevertheless and the requirement to use the scanning method in order to relax the severity of this requirement is a penalty to be paid.
It is an object of the present invention to provide a method of making a reflection hologram which overcomes the above-mentioned problems.
According to the present invention there is provided a method of making a reflection hologram comprising: exposing an optically transparent light sensitive coating carried on an optically transparent substrate to coherent light transmitted through the substrate into the coating from a coherent light source; causing said coherent light within the coating to interfere with coherent light from said source back reflected into said coating to create a latent image for a reflection hologram within the coating; and processing the coating to convert the latent image to a reflection hologram; wherein said back reflected light is obtained substantially entirely by reflection within said coating at an interface between said coating and an external medium.
It will be appreciated that in a method according to the invention the light sensitive coating preferably has a high surface quality, that is to say, a surface substantially free from local irregularities.
In one particular embodiment of the invention the light sensitive coating comprises a dichromated gelatin coating, the processing of the coating then involving the removal of unreacted dichromate from the coating followed by dehydration of the resultant gelatin coating.
Preferably the light sensitive coating, whether dichromated gelatin or any other nature, is formed on the substrate using a casting technique.
The invention also extends to a reflection hologram made using the aforedescribed method.
It will be understood that reflection holograms made by a method according to the invention have all the virtues of holograms produced by the prior art back reflection technique in which a mirror is employed in producing the object beam, but avoid the pitfalls, previously mentioned, encountered with the said prior art method.
The invention will now be further explained, and one method in accordance with the invention described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 illustrate diagrammatically, greatly enlarged and not to scale, a known method of making a reflection hologram using a reflective mirror surface to produce the back reflected interference or object beam; and Figure 3 illustrates the method in accordance with the invention.
Referring to Figs. 1 and 2, in the known method a glass substrate 11 carries an optically transparent dichromated thin uniform gelatin coating 13, between 10 and 40 microns in thickness, on one 15 of its major surfaces. A full reflective surface 17 of a mirror 19 is juxtaposed to the coating 13. Space between the surface 17 and the adjacent surface 21 of the coating 13 is occupied by a refractive index matching fluid 23, commonly, xylene.
In creating the reflection hologram incident light from a coherent light source, e.g. a laser 25, after passing through a construction point 27 (Fig. 2) is, after tranmission through the glass substrate 11 and the coating 13, reflected at the mirror surface 17. The reflected light, the object beam, appears, as a result of the refractive properties of the elements, to emanate from a virtual construction point 27z, behind the mirror 19. The whole arrangement is designed and configured to produce, within the gelatin coating 13 an interference pattern between the incident and object beams thereby to create a latent image for a reflection hologram consisting of generally coplanar, strictly speaking parabolic, regions of constructive interference separated in the direction of thickness of the coating by regions of destructive interference.Whereas, over a small region of the gelatin coating 13 the planes may to a first order be regarded as being substantially parallel to the mirror surface 17, the spacing between the planes at any position is dependent on the angle of incidence of the coherent light used to create the hologram at that position. So, where the coherent light is a parallel beam i.e. the construction point 27 is relatively distant from the coating 13, the planes are parallel at all positions within the coating 13, where the construction point 27 is close to the coating 13 the planes are parallel locally but the parallelism varies with angle subtended by the relevant point of the coating 13, to the construction point 27.
It will be appreciated that the area of the hologram may be controlled by control of the area of the beam incident on the substrate 11 and coating 13, and/or scanning the beam.
The drawbacks of the known back reflective method described above with reference to Figs. 1 and 2 have been mentioned previously.
Referring now to Fig. 3 the method according to the present invention relies not on the presence of a mirror to effect formation of an object beam by back reflection but instead relies on the back reflection within the coating 13 of a small proportion of the coherent light incident at the interface between the surface 21 of the gelatin coating 13 and the external medium 29, normally air, after transmission first through the glass substrate 11 followed by transmission through the coating 13. The remainder (typically about 96%) of the incident light is transmitted from the coating 13 to the external medium 29.
Following exposure and consequent formation of the latent image, the dichromated gelatin coating is processed by being subjected to a development and fixing step analogous to the development and fixing of an image in a photographic emulsion layer.
Since the intensity of the object beam is relatively low, the latent image of the interference pattern from which the hologram is to be formed has a lower amplitude than a corresponding pattern produced using a full reflective mirror surface as previously described.
However, after processing of the latent image in a similar manner to that conventionally used in the back reflective method of forming reflection holograms, a reflection hologram of equivalent quality and performance to those produced using the mirror back reflection is obtained.
In a typical method according to the invention the gelatine coating is sensitised by immersion in a solution of ammonium dichromate, typically 5% by weight concentration.
Excess solution is allowed to drain from the gelatin which is then dried in air at about 40% relative humidity for up to two hours. The gelatin is then exposed to the laser beam, which is typically obtained from an argon laser having a wavelength of 488 nanometres or 514 nanometres. After exposure the coating is processed by washing it in running water at about 20"C for ten minutes after which it is immersed in a 50% mixture of isopropyl alcohol (IPA) and water for three minutes, and then subsequently in pure IPA for a further three minutes, after which excess IPA is removed using an absorbent lens tissue. The gelatin is subsequently dried and baked in a warm oven at about 70"C or more about ten to fifteen minutes or more.
The advantages of a method according to the invention are as follows: Firstly, since the refractive surface is physically a feature of the coating itself, mechanical instabilities during exposure are substantially eliminated, and the need for vibration isolation arrangements is also obviated.
Secondly, the elimination of the use of a mirror reduces the risk of touching the sensi tive coating and consequent damage to the coating.
Thirdly, the elimination of a mirror also removes the dependency of the quality of recording on the mirror quality.
Fourthly, the avoidance of the need to use an index matching fluid avoids any problems regarding both its selection and safety.
Fifthly, the method greatly reduces the number of spurious fringes which can occur and reduces the contrast of any remaining fringes, such fringes often creating serious problems when a mirror reflective surface with indexing fluid is used.
Sixthly, the present method is totally compatible with all forms of scanning.
There are two further matters that need to be addressed when using a method according to the invention.
Since the surface of the gelatin coating is used as a mirror, the quality of the final hologram is dependent on the optical quality of the coating, and the coating therefore needs to be of high optical quality. There are various techniques which allow a high quality surface to be incorporated in the sensitive coating at the time of coating and substrate, but the best of these, and the one used to most effect, is that of casting. This was developed by B. M. Crawford. (See Notes on Applied Science No. 20-published by National Physical Laboratory 1060, Small Scale preparation of fine grain (colloidal) Photographic Emulsions).
In the method described in the above referenced Notes an emulsion is cast between a surface treated with a water repellent chemical and a substrate. A spacer is used to determine the final thickness of the layer and the surface quality is determined by a treated casting plate.
Since, also, the contrast of the interference planes is low a large amount of the exposure energy has no contribution towards the hologram. With some forms of sensitive coating however, such as dichromated gelatin, this energy may be usefully used in creating some optical hardening, resulting in substantial physical i.e. geometric stability, within the layer.
The use of ultra violet radiation heretofore employed for this purpose may be dispensed with in a method according to the invention, or the quantity of such ultra violet radiation may be substantially reduced. The subsequent chemical processing may then be adapted to compensate accordingly to produce the required hologram parameters.

Claims (7)

1. A method of making a reflection hologram comprising: exposing an optically transparent light sensitive coating carried on an optically transparent substrate to coherent light transmitted through the substrate into the coating from a coherent light source; causing said coherent light within the coating to interfere with coherent light from said source back reflected into said coating to create a latent image for a reflection hologram within the coating; and processing the coating to convert the latent image to form a reflection hologram; wherein said back reflected light is obtained substantially entirely by reflection within said coating at an interface between said coating and an external medium.
2. A method according to Claim 1 wherein said external medium is air.
3. A method according to Claim 1 or Claim 2 wherein the sensitive coating is a coating of dichromated gelatin.
4. A method according to Claim 3 wherein said processing comprises removal of unreacted dichromate from the coating followed by dehydration of the resultant gelatin coating.
5. A method according to any one of the proceding claims wherein the sensitive coating is formed on the substrate using a casting technique.
6. A method of making a reflection hologram substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.
7. A reflection hologram made by a method according to any one of the preceding claims.
GB08604077A 1985-02-27 1986-02-19 Holograms Expired GB2171538B (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB8505096 1985-02-27

Publications (3)

Publication Number Publication Date
GB8604077D0 GB8604077D0 (en) 1986-03-26
GB2171538A true GB2171538A (en) 1986-08-28
GB2171538B GB2171538B (en) 1988-06-22

Family

ID=10575189

Family Applications (1)

Application Number Title Priority Date Filing Date
GB08604077A Expired GB2171538B (en) 1985-02-27 1986-02-19 Holograms

Country Status (1)

Country Link
GB (1) GB2171538B (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0585941A2 (en) * 1992-09-03 1994-03-09 Nippondenso Co., Ltd. Process for making holograms and holography device

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0585941A2 (en) * 1992-09-03 1994-03-09 Nippondenso Co., Ltd. Process for making holograms and holography device
EP0585941A3 (en) * 1992-09-03 1994-09-21 Nippon Denso Co Process for making holograms and holography device
US5898511A (en) * 1992-09-03 1999-04-27 Nippondenso Co., Ltd. Process for making holograms and holography device

Also Published As

Publication number Publication date
GB2171538B (en) 1988-06-22
GB8604077D0 (en) 1986-03-26

Similar Documents

Publication Publication Date Title
US4815800A (en) Flare reduction in holograms
US6236476B1 (en) Apparatus for making a high quality reflection type holographic optical element
US4013465A (en) Reducing the reflectance of surfaces to radiation
US4960311A (en) Holographic exposure system for computer generated holograms
US3580657A (en) Blazed surface hologram
EP0046218B1 (en) Method and apparatus for production of holographic optical elements
US4469407A (en) Laser apodizing filter
EP0346869B1 (en) High efficiency holograms by multiple-layer holography
EP0215839B1 (en) Efficient holograms and method for making same
US5330264A (en) Method and apparatus for copying and manufacturing hologram plate
EP0043993B1 (en) Method and assembly for holographic exposure
US4687720A (en) Side lobe suppression in holograms using pre-exposure
JPH0424712B2 (en)
EP0415245B1 (en) Holographic exposure system to reduce spurious hologram noise
US5315417A (en) Low noise transmission holographic optical element
EP0214226B1 (en) Process for improving holographic efficiency
US3601017A (en) Method of suppressing interference fringes in photosensitive material
US4343874A (en) Method of constructing a hologram on a photoresist detector
GB2171538A (en) Making reflection holograms
US5052765A (en) Scanning fiber optic holographic exposure and feedback system
Sjölinder Bandwidth in dichromated gelatin holographic filters
US5052764A (en) System and method for forming a holographic exposure with a simulated source
SU1163342A1 (en) Transformer of scale of optic image with respect to single coordinate
RU1791791C (en) Method of taping translations of holographic mirrors
Rallison SBIR Phase 2, Final Report on Contract DAAB07-87-C-F022 (RALCON Corp.)

Legal Events

Date Code Title Description
732E Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977)
732E Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977)
PE20 Patent expired after termination of 20 years

Effective date: 20060218