Optical fibre-loop buffers

© 1996 Dr. Adam Matthew DicksonThis thesis contains a detailed investigation of fibre-loop buffers. Fibre-loop buffers will be required in all-optical packet-switching networks which may be the basis of future telecommunications networks. The term "all-optical" or "Photonic" refers to the fact that this buffer stores this data in an optical form. At no stage is optical data converted from an optical into an electrical form or vice-versa. The buffer investigated in this thesis achieves all-optical data storage using an optical fibre delay-line (hence “fibre-loop”). Data entering the buffer passes into the input of this delay-line. Some time later (typically less than a microsecond) the data appears at the output of the delay-line. The data can be passed back into the input of this delay-line and the process repeated, thereby extending the storage time. Optical power loss is inevitable as stored data make repeated round-trips through the fibre delay-line. A semiconductor optical amplifier (SOA) is inserted in series with this delay-line to provide all-optical amplification and compensate for this power loss. Unfortunately, a SOA also generates amplified spontaneous emission noise (ASE) which successively adds to and degrades stored data as it makes multiple passes through the SOA. The presence of ASE therefore limits the maximum number of roundtrips (recirculations) that data can be stored in the memory loop. This thesis contains new experimental data showing the accumulation of ASE. A characteristic of the fibre-loop buffer is that small changes in the gain of the SOA can have a cumulative effect on the power level of stored data after a few recirculations. Such large changes of power level must be avoided if the fibre-loop buffer is to be reliable storage system with predictable characteristics. One cause of such gain changes is gain compression which causes the gain of a SOA to decrease at high input power levels. Previous researchers have utilised the negative-feedback effect caused by gain compression to stabilise the power level of stored data in a fibre-loop buffer, in what is a partial answer to the above requirement. Near-travelling-wave SOAs also possess Fabry-Pérot ripple in their gain spectra which is caused by residual end-facet reflections. This ripple is shown in this thesis to also affect the power levels of stored data as well as making the performance (i.e. the maximum storage time) of a fibre-loop buffer dependent on its wavelength. It is shown in this thesis that the thermal characteristics of the SOA active region also influence the power levels of stored data. Both the gain and Fabry-Pérot ripple characteristics depend on the temperature of the active region. This latter quantity is in turn dynamically coupled to the bias current level (with microsecond to millisecond time constants) of the SOA. It is shown in this thesis that since the bias current level is likely to vary in a complicated manner with time in a practical application, the cumulative effect of the SOA thermal characteristics on the power level of stored data can be large and unpredictable unless corrective measures are taken. The phenomena described above are complicated by the carrier recombination dynamics in the SOA, which affects the degree of gain compression and also the Fabry-Pérot ripple characteristics on sub-nanosecond time scales. It is shown in this thesis that the dynamic behaviour of gain compression significantly distorts high bit-rate data as well as affecting the bit-error-rate (BER) at subsequent detection. It is shown by experiment in this thesis that all of the phenomena described above affect fibre-loop buffer performance to a significant degree. These experiments have been performed using a prototype fibre-loop buffer constructed by the author. Optical component characteristics (the SOA, as well as other components) which significantly affect the operation of the prototype fibre-loop buffer are discussed in detail. This thesis also contains a time-domain model of the prototype fibre-loop buffer which incorporates all of the above phenomena. This model successfully (and quantitatively) accounts for all of the observed behaviour of the prototype buffer. The time-domain model, having been proven, is then used to predict the performance of the prototype fibre-loop buffer under realistic operating conditions at data-rates up to 40 Gbit/s. This model is also used to predict the performance of an improved fibre-loop buffer design using a strained-quantum-well SOA. It is also shown using the time-domain model that the use of gain compression to stabilise power levels requires a fibre-loop buffer to operate in such a way that it provides sub-optimal storage times. An active feedback mechanism is shown be a better way of guaranteeing repeatable operation of a fibre-loop buffer. This feedback mechanism monitors the power level of stored data and adjusts the gain of the memory loop by changing the bias current level of the SOA