GB2035594A - Light guide-light modifying device connector - Google Patents

Abstract

A connector for interconnecting light guiding devices comprises a substrate (1) in the form of a flexible strip of a light-transmitting material having a refractive index of less than 2.00 and mounted thereon at least one flexible strip (2) of a light-transmitting material having a refractive index of greater than 2.00. The connector is for providing a readily made low loss connection between a light guide (3) of low refractive index and a light-modifying device (5) of high refractive index comprising a light-modifying element (6). The substrate (1) may comprise a strip of polyethersulphone and the strip (2) may be narrower and comprise a strip of a chalcogenide glass e.g. As2 S3, Ge or Si. <IMAGE>

Description

SPECIFICATION Improvements in light guiding devices This invention relates to light guiding devices.

In the following description the word "light" is used to include electromagnetic radiation of frequencies lower than those of visible light extending into the infrared.

In the technique of sending signals by means of electromagnetic waves of frequencies in the visible light and infrared ranges incorporating fibres transparent to such waves it is necessary to intercalate in the light circuit various devices such as light switches, amplifiers, modulators and other devices for altering the characteristics of the waves transmitted. The operative parts of such devices are constructed of materials which are transparent to the frequencies of the waves.While the light guides for conveying the waves from point to point are almost universally formed of glasses of various kinds having refractive indices in the region of 1.6 it is not practicable to form the light-modifying devices of such glasses because such devices operate as a result of changes in the refractive index of the lightcarrying materials produced by impressed electric fields and in the glasses most suitable for the conveyance of light waves the electric fields required to produce useful changes in refractive index are so high that extremely high voltages are required to create them. Thus it is necessary to use in such devices materials which are amendable to changes in their refractive index by the use of much more moderate voltages.Materials currently used in light-modifying devices and which make possible operation at moderate voltages are doped GaAs, CdS and Ti-diffused LiNbO3. These materials have however high refractive indices somewhere in the region of 2.2 to 2.4.

It is possible to transmit light from a light guide having a refractive index of about 1.6 to the input of light-modifying devices of high refractive index by abutting the end of the light guide against the lightreceiving input of the device but this gives rise to considerable technical problems in that the abutting ends require to be accurately cut and polished if a reasonable efficiency of transfer is to be achieved and this method is simply not usable if a light guide is to be tapped at an intermediate point for a reason described later. It is also a disadvantage of this method of connection that a light guide which may be of a length running into kilometres requires to be directly connected into the light-modifying device.

It is possible to transfer light from one light guide to another or from a light guide to the light input channel of a light-modifying device which is itself a special form of light guide by laying one light guide alongside the other in as close contact as possible but there is a high power loss between the two in the transfer if there is a wide difference in the respective refractive indices of the two light guides such as the difference between 1.6 of normal optical glass and 2.2 to 2.4 of the light-conducting material of which light-modifying devices are usually made, because the phase velocities inherent in materials of widely different refractive index differ too much from one another to provide any coincidence at any value for the modes of propagation of the light.

Communicating by light waves would be much easier if there were available some form of connector for connecting a light guide of low refractive index to a light-modifying device of high refractive index which would permit the transfer of light from one of the other at high efficiency. It is an object of the present invention to provide a device which can be used to provide such a connection and which can also function as a light guide of any desired length which has the capability of easy connection to other light guides and connection of branch light guides.

A connector according to the invention comprises a substrate in the form of a flexible strip of lighttransmitting material having a refractive index of less than 2.00 and mounted thereon a flexible strip of light-transmitting material the refractive index of which is greater than 2.00.

The light-transmitting material having a refractive index greater than 2.00 may be a chalcogenide glass.

Chalcogenite glasses based on the heavier chalcogen elements sulphur, selenium and tellurium have high refractive indices. An account of such glasses is given in Inorganic Glass-Forming Systems by H. Rawson published by Academic Press Inc.

(London) Ltd.

It has been found that the most convenient wave lengths of light for communication purposes lie in the infrared range and particularly in the range about 0.633 to 1.15 m. For use within this range the light transmitting strip may be formed of the chalcogenide glass arsenictrisulphide (As2S3) which has a refractive index of 2.435 atX = 1.15,am and the flexible substrate may be a strip of polyethersulphone the refractive index of which is 1.65 at A = 0.633,am.

Where the infrared range is to be used the lighttransmitting filament may be of a material other than glass, for example for use at low temperatures it may be of germanium or silicon both of which are transparent to infrared light of certain wavelengths and have indices of refraction well above 2.00 at these wavelengths. Ge and Si are unsuitable at high temperatures because of the thermal generation in them of free carriers.

An example of manufacture of a connector accord ing to the invention is given below.

EXAMPLE A block of As2S3 was cut and polished to dimen sions 5 mm x 22 mmx 75 mm and drawn in a tubular electrical furnace at 245 C to form a strip 225 clam thick 1,000,um wide. Asheetofpolyethersulphone was cut to dimensions 0.9 mmx 10 mm x75 mm and two smaller lengths of the strip of As2S3 were placed on opposite sides of the strip of polyethersulphone.

The substrate with the As2S3 strips on each side was placed between polished carbon blocks and was subjected to a compressive load of 2509 while being maintained at a temperature of 245"C for 60 minutes.

The As2S and the substrate now fused to one another were then drawn together at a temperature of 245 C and art a pulling rate of 200 mm/minute. It was found that the fused assembly drew well to a high ratio of draw down to dimensions of the As2S3 of 3.5,am wide. The reason for using two strips of As2S3 one on each side of the substrate was to minimize distortion of the substrate during fusion and drawing.

A connector of the required length was cut from the drawn assembly and was attached to a lightmodifying device by laying a length of the lighttransmitting strip at one end of the strip parallel to the light input channel of the light-modifying device and preferably as close to the channel as possible.

The overlap necessary for efficient light transfer is in proportion to the gap between the connector and the light input channel. The smaller the gap the smaller the overlap necessary. Preferably the connector and the light-modifying channel are placed into contact with one another and fastened to one another by adhesive of refractive index close to that of the subs trade to minimize dispersion. By this means it is possibleto achieve gaps as small as 0.7 clam. The overlap necessary at such small gaps is itself small. As the propagating modes of As2S3 and any of the materials already mentioned for the light-modifying devices overlap in phase velocity there ia high efficiency of light transfer, as much as 96%, between the connector and the light-modifying device.At the thickness of the light-transmitting strip mentioned the strip operates on several modes and the ideal is to use a strip of single mode operation. Such a strip can be obtained by drawing to a sufficient extent thus reducing the thickness of the strip. The connector may be connected to the normal light guide by abutting the free end face of the light transmitting strip against the input end face of the light guide. This is comparatively easy to do where the light guide is being connected to a flexible connector but is not easy to do where the light guide is to be con nected to a rigid input channel of a light-modifying device.

Although the light-transmitting strip is mentioned mainly for use as a connector it may be used in its own right as an efficient lighttransmitter and it has the advantage that it is possible to take easilytap- pings from it at any intermediate point with high efficiency of light transfer using a length of the same material with short portions of the main light guide and the branch light guide laid in contact with another.

The invention also contemplates the provisions of light-modifying devices having the light input and output channels connected as described to connectors as described, the connectors being useful for easy connection to light-transmitting and receiving devices. The material of the connector according to the invention is particularly compatible with the light-emitting portions of semiconductor laser devices and a light guide according to the invention can be connected directly to such laser devices by laying a portion of the light guide alongside and in contact with the light emitting portion of the laser It is thus easy to form a combination of a laser and a laser light-modifying device using as a connector a light guide according to the invention.

As a connector will normally be only a few centimetres long the light-transmitting strip will normally be ieft bare but ifthe device of the invention is to be used as a light guide of a length measured in metres or, although used as a connector, requires for some reason to have a length measured in metres the light-transmitting strip may be encased in a sheath of material of low refractive index. Inthis case at the place where the device of the invention is to be connected to a light-modifying device the sheath will be removed leaving the light-transmitting strip bare.

A practical embodiment of the invention is illus- trated diagrammatically in the accompanying drawings in which Fig. 1 is a side view of a connector, Fig.

2 is a cross section of the connector and Fig. 3 shows how the connector is used to cormect a light guide to a light-modifying device.

In the drawings 1 denotes a substrate of a material of refractive index which is less than 2.00 and 2 denotes light-transmitting strips formed of material of refractive index greater than 2.00 fused to the substrate 1.3 denotes a light guide one end of which is stripped of the sheath normally provided on light guides to expose the light-carrying core 4.5 denotes a light-modifying device the light-modifying element of which is indicated by 6. A connector according to the invention is shown connecting the light guide 3 and the light-modifying device 5. One of the lighttransmitting strips 2 of the connector is abutted at one end to the core 4 of the light guide 3 and at the other end to the light-carrying element 6 of lightmodifying device 5 respectively, the strip 2 being fastened to the core 4 and to the light-transmitting element 6 by adhesive of refractive index close to that of the substrate 1. The other light-transmitting strip 2 is unused. If desired said unused lighttransmitting strip may be removed from the substrate but this is normally unnecessary. As mentioned earlier in the specification two strips were applied to the substrate one on each surface merely to minimize distortion of the substrate during manufacture of the connector.

Claims (9)

1. A connector for connecting a light guide of low refractive index to light-modifying device of high refractive index comprises a substrate in the form of a flexible strip of light-transmitting material having a refractive index of less than 2.00 and a flexible strip of light-transmitting material the refractive index of which isgreaterthan 2.00 mounted on said substrate.

2. A connector as claimed in claim 1 in which the tight-transmitting material having a refractive index greater than 2.00 is a chalcogenide glass, i.e. a glass based on an element selected from one of the heavier chalcogen elements sulphur, selenium and tellurium.

3. A connector as claimed in claim 1 in which the flexible light transmitting strip is formed of the chalcogenide glass arsenictrisulphide (As2S3) and the flexible substrate is a strip of polyethersulphone.

4. A connector as claimed in claim 1 for use in the infrared range of light in which the flexible strip is formed of germanium.

5. A connector as claimed in claim 1 for use in the infrared range of light in which the flexible strip is formed of silicon.

6. A method of making a connector for connecting a light guide of low refractive index to a lightmodifying device of high refractive index comprising forming a block of arsenictrisulphide (As2S3), polishing the block and subsequently hot drawing it to form an elongated strip, cutting a sheet of polyethersulphone to provide a strip wider than the strip of As2S3, placing two lengths of As2S3 strip one on each side of the strip of polyethersulphone with all the strips orientated in the same direction, placing the assembly of strips between polished carbon blocks and subjecting the assembly to a compressive load applied normal to the planes of the strips while maintaining the assembly at a temperature and for a length of time sufficient to permit the three strips to bond to one another, then hot drawing the assembly to produce a composite strip of reduced thickness and cutting a connector of desired length from the composite strip.

7. A method of making a connector for connecting a light guide of low refractive index to a lightmodifying device of high refractive index comprising cutting a block of arsenic trisulphide (As2S3) to dimensions 5 mm x 22 mm x 75 mm, polishing the block and subsequently hot drawing it at 245"C to form a strip 225 elm thick x 1,000,am wide, cutting a sheet of polyethersulphone to dimensions 0.9 mm x 10 mm x75 mm and placing two lengths of the strip of As2S3 on opposite sides of the strip of polyethersulphone, then placing the strip of polyethersulphone with the As2S3 strips on each side between polished carbon blocks and subjecting the assembly to a compressive load of 2509 while maintaining it at a temperature of 245"C for 60 minutes, then drawing the assembly the components of which are now fused together at a temperature of 245"C and at a pulling rate of 200 mm/minute down to dimensions of the AS2S3 of 3.5,am thick by 14 clam wide and cutting a connector of the desired length from the drawn assembly.

8. A connector for connecting a light guide of low refractive index to a light-modifying device of high refractive index as claimed in claim 1 and substantially as described.

9. A method of making a connector for connecting a light guide of low refractive index to a lightmodifying device of high refractive index substantially as described with reference to the accompanying drawings.