GB2234605A

GB2234605A - A holographic element

Info

Publication number: GB2234605A
Application number: GB8917455A
Authority: GB
Inventors: David John Mccartney; Giles Russell Chamberlin
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1989-07-31
Filing date: 1989-07-31
Publication date: 1991-02-06
Also published as: GB8917455D0

Abstract

A transmission holographic element (21) comprises a holographic material (24) such as dichromated gelatin coated onto a colour glass substrate (22). The substrate (22) is transparent to light at the reading wavelength e.g. infra-red radiation of the element (21) and absorbent to light at the recording wavelength e.g. visible light of the element (21). in addition, the substrate (22) has a refractive index which is substantially equal to that of the holographic material (24). <IMAGE>

Description

A HOLOGRAPHIC ELEMENT This invention relates te a transmission holographic element and to a method of manufacturing the said transmission holographic element. Such a holographic element is used at a reading wavelength and comprises a volume hologram formed from a holographic material which is sensitive to light of a recording wavelength used to form the holographic element and coated onto a surface of a substrate.

A known method of forming such a holographic element is described in, for example, "Topics in Applied Physics Vol.20: Holographic Recording Materials" Springer-Verlag Berlin, Editted by H.M.Smith in which a first surface of a substrate which is transparent at the reading wavelength is coated with. a holographic material whicn is sensitive to light at the recording wavelength. In order to form the volume hologram, it is necessary to expose the holographic material to two or more crossed beams of light at the recording wavelength and to minimise unwanted reflections of the beams.To minimize these reflections, an absorbing glass which is absorbing at the recording wavelength is positioned adjacent a second surface of the substrate opposite to tne first surface. Ar. index matching liquid is inserted between the substrate and the absorbing glass to minim sue unwanted reflections at the boundary between the substrate and the absorbing glass. Once the volume hologram has been formed, the absorbing glass and the index matching liquid are removed from the substrate, and the transmission holographic element thus formed may be read by exposing the element to light of the reading wavelength.

A disadvantage of this known way of forming a holographic element is that the use of an index matching liquid can be time consuming and inconvenient and may adversely affect the holographic material if it accidently comes in contact with the holographic material. Accordingly, a first aspect of the invention provides a transmission holographic element comprising a substrate having a first surface, and a holographic material adjacent the first surface, the holographic material being sensitive to light of a recording wavelength used to form a volume hologram in the holographic material, and readable at a reading wavelength, characterised in that the substrate is absorbant to light of the recording wavelength and transmissive to light of the reading wavelength.

Advantageously, the substrate has a refractive index which is substantially equal to that of the holographic material.

This reduces the level of reflection at the interface between the holographic material and the first surface of the substrate.

The holographic material may comprise a silver halide, a photopolymer or photoresist, but preferably it is dichromated gelatin. Dichromated gelatin is sensitive to light in the visible region of the electromagnetic spectrum, specifically light having a wavelength of 514nm. Thus, when using cionromated gelatin, the recording wavelength used to form the holographic element is 514nm.

Conveniently, the reading wavelength is in the infra-red region of the electromagnetic spectrum, for example in the range 1200 - 1500no, although other reading wavelengths may be used. The appropriate reading wavelength will be determined by the use for which the holographic element is intended. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a known holographic recording arrangement; and Figure 2 is a schematic representation of a holographic element according to the present invention.

Referring te figure 1, a known holographic recording assembly 1 is shown. The assembly 1 comprises a first glass substrate 2 which is transparent to light at the wavelength at which the holographic element is to be read that is, the reading wavelength. In this example, the holographic element is intended for use in the infra-red region of the electromagnetic spectrum. The glass substrate has a first surface 3 onto which a holographic material 4 has been coated.

In order to form the holographic element, it is necessary to record a hologram in the holographic material 4 by exposing the holographic material 4 to two crossed beams of light at a recording wavelength. Before thus exposing the holographic material, a second substrate 5 formed from glass which is absorbing at the recording wavelength is positioned adjacent a second surface 6 of the first substrate 2 in order to reduce unwanted reflections of the beams. An index matching liquid 7 is inserted between the second surface 6 of the first substrate 2 and the second substrate 5 to minimise unwanted reflections at the boundary between the twe substrates 2,5.The material 4 may now be exposed to light at the recording wavelength in roe to record a hologram in the material 4. In this example, the recording wavelength is 5;4nor, as the material 4 comprises dichromated gelatin. Once the hologram has been recorded, the Index matching liquid 7 and the second substrate 5 are removed leaving a holographic element formed by the holographic material 4 coated on the substrate 2. The holographic element may now be read by exposing the material 4 to light at the reading wavelength.As the substrate 2 is transparent to light of this wavelength, the light will pass through material 4 and substrate 2 and may then be read by an optical receiver (not shown).

Turning now to figure 2, a holographic element 21 according te the present invention is shown. The element 21 comprises a colour glass substrate 22 onto which a holographic material 24 comprising dichromated gelatin has been coated. The colour glass substrate 22 is chosen such that it is transparent to light of the reading wavelength, and also absorbant to light of the recording wavelength. The holographic element may thus be recorded without it being necessary to add a second substrate and index matching liquid to the element, as was the case with the prior art arrangement described hereinbefore. This results from the fact that the substrate 22 is itself absorbant to light of the recording wavelength. Once the element has been recorded, it may be read by exposing the material 24 to light of the reading wavelength. The glass substrate 22 is also chosen such that its refractive index is substantially equal to that of the holographic material. This minimises unwanted reflections at the interface between the material 24 and the substrate 22.

In this specification the term "optical" is intended to refer te that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the vIsible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

Claims

1. A transmission holographic element comprising a substrate having a first surface and a holographic material adjacent the first surface the holographic material being sensitive to light of a recording wavelength used to form a volume hologram in holographic material and readable at a reading wavelength, characterised in that the substrate is absorbant to light of the recording wavelength and transmissive to light of the reading wavelength.

2. A holographic element as claimed in claim 1 wherein the substrate has a refractive index which is substantially equal to that of the holographic material.

3. A holographic element as claimed in claim 1 or claim 2 wherein the holographic material is dichromated gelatin.

4. A holographic element as ciaimed in any one of Claims 1, 2 and 3 wherein the first wavelength is in the visible region of the electromagnetic spectrum, and the second wavelength is in the infra-red region of the spectrum.

5. A transmission holographic element substantially as herelnbefore described with reference t Figure 2 cf the drawings.