GB1465329A - Bandwidth control - Google Patents

Abstract

1465329 Optical communications system POST OFFICE 6 June 1974 25166/74 Heading G1A The bandwidth of an optical communications system is controlled by means of a section of liquid filled dielectric optical waveguide 8, silicon tube filled with carbon tetrachloride, disposed in a temperature maintaining device 9. The temperature of the waveguide determines the number of transmission modes allowed. The temperature is retro-actively controlled with respect to a comparison between the ratio of two pre-selected modes on the received bandwidth and a predetermined reference signal. The optical communications signal uses an optical carrier signal into which an analogue signal having a bandwidth to be controlled, is modulated. In a repeater circuit, Fig. 2, the light signal enters the system via the optical waveguide 7, passes via a section 8 of a liquid cored dielectric optical waveguide and is received by a photodiode detector. The detectors output is amplified and is passed to two tuned circuits 13, 14 which are tuned to a frequency W 2 corresponding to the half power point of the cut-off edge of the required bandwidth and a frequency w 1 corresponding to the flat response region of the desired bandwidth. The levels L 1 , L 2 of the frequencies W 1 , W 2 are measured by level detectors 15 and 16 and the results are passed to a divider 17 which produces a signal representative of L 1 /L 2 that is fed to an analogue comparator 18. There it is compared with a reference signal, corresponding to the desired ratio value. The resulting error signal from the comparator 18 is used to reset the temperature of the oven 9 via a temperature reset unit 20 and a servo temperature controller 21. The repeater circuit also comprises a driver circuit 22, connected as shown, which powers and modulates a laser source 23 which is optically connected to an output dielectric waveguide 26. The number of modes excluded by this arrangement is determined by a second oven 25, operating at a pre-set temperature, enclosing a section of liquid cored dielectric waveguide. An arrangement involving a series of repeater circuits is also disclosed.