GB2178163A

GB2178163A - Detecting flaws in optical fibres

Info

Publication number: GB2178163A
Application number: GB08519020A
Authority: GB
Inventors: Gerald Boothroyd
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-07-27
Filing date: 1985-07-27
Publication date: 1987-02-04
Also published as: GB8519020D0

Abstract

A method and apparatus for detecting flaws in optical fibres comprises passing the fibre through a fibre guidance assembly (1) while directing a light beam from a light source (5, 6) and detecting the light beam after passage through the optical fibre, by a photoelectric detector (7, 8), and detecting (9, 10, 11) a variation in the detective light beam indicative of a visible flaw in the fibre. <IMAGE>

Description

SPECIFICATION Optical fibre cable This invention relates to optical fibre cables, particularly to a method of testing during the manufacture of such a cable.

According to the present invention there is provided a method of testing an optical fibre comprising feeding the fibre along a predetermined path and directing a light beam transversely through the fibre as it passes, detecting any variation in translucency of the fibre, and determining when that variation exceeds a predetermined threshold level.

In order that the invention can be clearly understood reference will now be made to the accompanying drawing which is a block schematic diagram of an optical fibre flaw detector according to an embodiment of the present invention.

The translucency of optical fibre is primarily determined by the characteristics of the coating of the fibre. Variations in this translucency may be caused by: 1. Changes in the composition of coating material or in process conditions; 2. Contamination or material degradation, e.g. inclusions, voids, etc; 3. Damage; 4. Variations in density of applied colour (if any).

In general the material composition and process conditions remain constant throughout a manufacturing run. Therefore the translucency of the optical fibre will remain constant apart from variations caused by factors 2. 3. and 4.

above.

It is an object of the present invention to test for such variations in translucency and reference will now be made to the accompanying drawing.

In the drawing an optical fibre visible flaw detector comprises a fibre guidance assembly 1 having a through-bore 2 through which the optical fibre 3 under test passes prior to being assembled into a cable or a cable subcomponent such as a cable package.

The guidance assembly 1 has a second through-bore 4 which cuts across the throughbore 2 and accommodates an optical source comprising a laser or LED light supply 5 and an optical fibre "snout" 6. A second optical fibre snout 7 detects the light transmitted by the snout 6 and supplies it to a photo-electric detector 8.

During manufacture of the cable the optical fibre 3 passes through the fibre guidance assembly, and as it passes between the light source and photo-detector, any variations in translucency will be detected by a photo-transistor in the photo-electric detector 8, and a consequent variation in the current generated by the photo-transistor will occur.

This current is amplified in a current amplifier 9 and applied to a limit detection circuit 10 which is adjustable by preset potentiometers 10A and 10B to apply a signal to an alarm circuit 11 to raise an alarm should a visible flaw be detected in the fibre 3.

As shown the light source 5 and detector 8 have optical fibre "snouts" 6 and 7 which enable small targets to be detected. Using this technique, visible flaws of typically 0.1 mum to 0.2mm have been detected in uncoloured secondary coated single mode fibre of 0.85mm nominal diameter.

For reliable detection, contrast between a "flaw" and the background, must be such that it produces a current change ratio of at least 50:1. Typical visible flaws in "naturally" coloured secondary coated single mode fibre will produce current change ratios in excess of 200:1. For monitoring the application of coloured dye to the fibre we have found that by singly checking translucency a satisfactory indication of colour density can be achieved using this equipment. The trigger level for 50:1 can be set to represent a density of colour not to be exceeded, and may be different for different colours.

The system bandwidth is limited by the characteristics of the current amplifier 9 and the limit detector circuit 10, and therefore can be selected as required.

The arrangement described is cheap and simple and a number of these arrangements can be provided to check for flaws in respective fibres being laid up into a cable or into a cable package. Conventional visible flaw detection equipment of which we are aware for use on small targets in continuous processes requires the use of complex video and processing equipment which often uses sophisticated systems of focussing optics. Such equipment is necessarily very expensive and we have found that the arrangement described in satisfactory for detecting flaws in single mode optical fibres.

As shown the alarm circuit 11 has a reset input 12 for resetting when the process is started up again after detection of a flaw. Furthermore the alarm output 13 from the alarm circuit can be used to stop the cabling process when a flaw is detected.

1. A method. of testing an optical fibre comprising feeding the fibre along a predetermined path and directing a light beam transversely through the fibre as it passes, detecting any variation in translucency of the fibre, and determining when that variation exceeds a predetermined threshold level.

2. A method as claimed in claim 1 wherein the light beam is directed into the passing fibre from an optical fibre snout coupled to a source of light.

3. A method as claimed in claim 1 or claim 2, wherein the light beam passing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical fibre cable This invention relates to optical fibre cables, particularly to a method of testing during the manufacture of such a cable. According to the present invention there is provided a method of testing an optical fibre comprising feeding the fibre along a predetermined path and directing a light beam transversely through the fibre as it passes, detecting any variation in translucency of the fibre, and determining when that variation exceeds a predetermined threshold level. In order that the invention can be clearly understood reference will now be made to the accompanying drawing which is a block schematic diagram of an optical fibre flaw detector according to an embodiment of the present invention. The translucency of optical fibre is primarily determined by the characteristics of the coating of the fibre. Variations in this translucency may be caused by: 1. Changes in the composition of coating material or in process conditions; 2. Contamination or material degradation, e.g. inclusions, voids, etc; 3. Damage; 4. Variations in density of applied colour (if any). In general the material composition and process conditions remain constant throughout a manufacturing run. Therefore the translucency of the optical fibre will remain constant apart from variations caused by factors 2. 3. and 4. above. It is an object of the present invention to test for such variations in translucency and reference will now be made to the accompanying drawing. In the drawing an optical fibre visible flaw detector comprises a fibre guidance assembly 1 having a through-bore 2 through which the optical fibre 3 under test passes prior to being assembled into a cable or a cable subcomponent such as a cable package. The guidance assembly 1 has a second through-bore 4 which cuts across the throughbore 2 and accommodates an optical source comprising a laser or LED light supply 5 and an optical fibre "snout" 6. A second optical fibre snout 7 detects the light transmitted by the snout 6 and supplies it to a photo-electric detector 8. During manufacture of the cable the optical fibre 3 passes through the fibre guidance assembly, and as it passes between the light source and photo-detector, any variations in translucency will be detected by a photo-transistor in the photo-electric detector 8, and a consequent variation in the current generated by the photo-transistor will occur. This current is amplified in a current amplifier 9 and applied to a limit detection circuit 10 which is adjustable by preset potentiometers 10A and 10B to apply a signal to an alarm circuit 11 to raise an alarm should a visible flaw be detected in the fibre 3. As shown the light source 5 and detector 8 have optical fibre "snouts" 6 and 7 which enable small targets to be detected. Using this technique, visible flaws of typically 0.1 mum to 0.2mm have been detected in uncoloured secondary coated single mode fibre of 0.85mm nominal diameter. For reliable detection, contrast between a "flaw" and the background, must be such that it produces a current change ratio of at least 50:1. Typical visible flaws in "naturally" coloured secondary coated single mode fibre will produce current change ratios in excess of 200:1. For monitoring the application of coloured dye to the fibre we have found that by singly checking translucency a satisfactory indication of colour density can be achieved using this equipment. The trigger level for 50:1 can be set to represent a density of colour not to be exceeded, and may be different for different colours. The system bandwidth is limited by the characteristics of the current amplifier 9 and the limit detector circuit 10, and therefore can be selected as required. The arrangement described is cheap and simple and a number of these arrangements can be provided to check for flaws in respective fibres being laid up into a cable or into a cable package. Conventional visible flaw detection equipment of which we are aware for use on small targets in continuous processes requires the use of complex video and processing equipment which often uses sophisticated systems of focussing optics. Such equipment is necessarily very expensive and we have found that the arrangement described in satisfactory for detecting flaws in single mode optical fibres. As shown the alarm circuit 11 has a reset input 12 for resetting when the process is started up again after detection of a flaw. Furthermore the alarm output 13 from the alarm circuit can be used to stop the cabling process when a flaw is detected. CLAIMS

3. A method as claimed in claim 1 or claim 2, wherein the light beam passing through the fibre under test is detected by an optical fibre detector snout coupled to a photo-electric detector.

4. A method as claimed in any preceding claim, wherein the detected light beam is amplified and applied to a limit detector which determines the threshold level at which an alarm or control circuit is triggered.

5. A method of testing an optical fibre for flaws, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

6. A method of manufacturing an optical fibre cable, wherein in each optical fibre is laid up in the cable or cable package and is tested by the method according to any preceding claim on line as it is laid up in the cable or cable package.

7. An optical fibre or optical fibre cable tested by the method claim in any preceeding claim.

8. Apparatus for testing an optical fibre, comprising a fibre guide in which the optical fibre under test is constrained to pass in a predetermined path, a light source for directing a light beam transversely through the fibre in the fibre guidance assembly, a light detector for detecting the light passing through the fibre in the fibre guidance assembly and a detector circuit for detecting the variation in the detected light beam indicative of a flaw in the fibre.

9. Apparatus for testing an optical fibre, substantially as hereinbefore described an as illustrated in the accompanying drawing.