GB1267964A - Pulse transmission system - Google Patents

Abstract

1,267,964. Light transmission system. STANDARD TELEPHONES & CABLES Ltd. May 28, 1970 [May 30, 1969], No.27476/69. Heading G1A. Binary information is conveyed in the form of pulses of carrier in which the phase of the carrier within a pulse with reference to a notional reference wave conveys the significance. The carrier wave frequency may be optical or quasi-optical, may be generated by a laser, and may be transmitted as the notional reference wave at regular intervals in the signal pulse train. The phase difference of the binary signals may be # or #/2. Repeaters for the system may include one or more self-quenching super regeneration oscillators including phase reconstituting means whereby at least a proportion of phase jitter in incoming pulses in converted by adding to a signal of known phase into amplitude jitter which may be eliminated subsequently by a high level clipper: This arrangement reconstitutes the pulses with reflection of phase in the case of a #/2 phase difference system, and the pulses are only restored to their original phase relationship after passing through an even number of such repeaters. The high level clippers may also be self-quenching super-regenerative oscillators. The synchronizing pulses may be separated by arranging for a proportion of the light to be incident on a Fabry Perot etalon whose fundamental frequency is equal to the PRF of the synchronizing pulse. SQSR operation of an optical system is achieved by providing a laser with optical feedback with delay in the feedback loop to provide the squegging waveform and to achieve frequencies above 1GHz/2 an optically non-linear attenuator which may be a semi-conductor diode is provided in the feedback path of a laser with a steady drive. An SQSR oscillator may be formed by two optically coupled semi-conductor injection laser diodes both subjected to a steady drive but only one being driven sufficiently for it to lase. The arrangement may comprise a singly laser with the current density over one part of the cavity length less than that over another, which may be achieved by means of cutting through one electrode layer and feeding the two portions through different valued resistors. The initial transmitter may consist of a modelocked laser and the phase may be modulated in an electro-optical crystal. An alternative method of separating the 5 synchronizing pulses is to use part of the incoming signal to trigger (fire in the same phase) weakly (without locking) an SQSR whose free running pulse rate frequency is slightly lower than that of the synchronizing pulses and includes a Fabry Perot etalon tuned to the frequency of light and resonant at the PRF of the synchronizing pulses or a low harmonic thereof to provide an auxiliary trigger to augment the incoming trigger and eventually cause locking. Incoming light at 61 is incident on beam-splitter 62 of a repeater Fig. 6 for a system employing a phase separation #, and is amplified by an SQSR device 63 before being passed to a Fabry Perot etalon 65 resonant at the PRF of the synchronizing pulses and an etalon 66 resonant at the overall system PRF which filter the synchronizing pulses and produce a set of reference pulses in which the phase noise is reduced. Any drop in reference pulse amplitude between synchronizing pulses is eliminated in an SQSR device 68 and the output thereof is fed by beam-splitters to SQSR devices 73 and 76 in the former of which the signal with its phase separation reduced from # to #/2 by the combination of output 63 with "a" has amplitude jitter removed by a clipping action and in the latter phase-jitter is removed after its conversion to amplitude jitter by the combination of the output of the former with signal "b". The phase separation is converted back from #/2 to # by combination of the output from SQSR 76 with signal C. In the final receiver of the transmission system, the signal C has a phase and amplitude such as to eliminate pulses of one type of binary signal but merely shift the phase of the other and the output at 78 is applied to a photo-detector. Spurious feedback can be eliminated by the use of Faraday isolators or other attenuators between the devices or by reducing the reflectivity of the SQSR devices by a form of blooming which provides a low reflectance below laising level and a high reflectance above it. This can be achieved by applying a half wavelength thickness of transparent dielectric to one end of a semi-conductor laser and then depositing on this layer a high reflectivity multi-layer stack. The transparent layer may be aluminium doped gallium arsenide.