Optical: systems and elements – Optical amplifier – Particular active medium
Patent
1991-08-01
1994-03-01
Lobo, Ian J.
Optical: systems and elements
Optical amplifier
Particular active medium
372 45, H01S 319
Patent
active
052913287
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to semiconductor laser amplifiers.
Semiconductor laser optical amplifiers have many possible applications in future optical systems, particularly in optical communications systems and optical data processing. Such amplifiers provide high gain with low power consumption, and their single mode waveguide structure makes them particularly suitable for use with monomode optical fibre.
The radiation used in optical communications is not necessarily in the visible region, and the words "optical" and "light" when used in this specification are not to be interpreted as implying any such limitation. Indeed, if silica optical fibres are used as the transmission medium, infra red radiation is of especial usefulness because loss minima occur in such fibres at 1.3 and 1.55 .mu.m approximately.
In optical transmission systems using single longitudinal mode lasers the effects of fibre dispersion may be small, the main limitation on repeater spacing being the signal attenuation due to fibre loss. Such systems do not require a complete regeneration of the signal at each repeater, amplification of the signal is sufficient. Thus semiconductor laser amplifiers can be used as linear optical repeaters for intensity modulated or coherent systems. Linear amplifier repeaters, and in particular travelling wave repeaters, have the additional advantages of being bit rate independent, bidirectional, and capable of supporting multiplex operation.
A semiconductor laser amplifier can be used in an optical receiver to amplify the optical signal before it reaches the photodetector, thereby increasing detection sensitivity. The improvement can be particularly marked for bit rates in excess of 1 Gbit/s, making such amplifiers potentially useful in the development of sensitive wideband receivers for use in future fibre systems.
The performance of laser amplifiers in systems is dependent upon several factors, including the amplifier gain and saturation characteristic, the amplifier optical bandwidth, the stability of gain with respect to bias current and temperature, the amplifier noise characteristics, and sensitivity to input signal polarisation. Clearly, for any practical application it is desirable for gain to be frequency independent, at least within the frequency range of the optical input. This is of course of particular significance where the laser amplifier is to be used in systems involving wavelength division multiplexing (wdm) or in systems where optical carriers of several different wavelengths are used, particularly where several such carriers are used simultaneously. A second desirable characteristic for a practical system is that the gain of the laser amplifier should be independent of the input signal polarisation. This last characteristic is of particular significance in optical fibre systems, since random polarisation fluctuations are currently unavoidable in practical systems. Much research has been carried out in an effort to find laser structures with high gain over a usefully wide optical bandwidth, with suitably low polarisation sensitivity. Thus although usefully high gains, typically between 24 and 29 dB, have been achieved, polarisation sensitivity remains a problem. In particular, although polarisation sensitivities as low as 2 to 3 dB have been achieved, typical sensitivities are between 3 and 6 dB, and the results have been strongly frequency dependent, polarisation sensitivities of 7 dB or more being typical at non-favoured frequencies, thus laser amplifiers cannot satisfactorily be used in wdm systems.
In recent travelling wave semiconductor laser amplifier (TWSLA) systems demonstrations (see for example: Proc. ECOC, Brighton, 1988, pp 163-166, D. J. Malyon et al, and Electron Lett., Vol. 24, 1988, pp 551-552, G. Grosskopf et al) the polarisation of the input signal had to be carefully controlled due to a polarisation-dependent gain. The problem of polarisation-dependent gain appears to be exacerbated in device designs which give high saturated output power. Indeed the TWSLA with the
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Cole Simon
Cooper David M.
Devlin William J.
Isaac Joseph J.
Lealman Ian F.
British Telecommunications
Lobo Ian J.
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