Semiconductor optical amplifiers to extend the reach of passive optical networks.
PhD thesis, University of Glasgow.
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This thesis reports on Semiconductor Optical Amplifiers (SOAs) and their use in optical communication systems; in particular improving the reach of Passive Optical Networks (PON).
Following a comprehensive overview of the components of optical communication systems a PON is introduced and the standard of Gigabit-PON (GPON) explained. The concept of extending the reach of GPON through the introduction of amplification is presented and the business drivers of the telecommunication operators detailed.
The physics of SOAs are described followed by the parameters used to characterise them. Carrier dynamics of SOAs are explained and the methods of measurement of the carrier dynamics are detailed including the spectrogram technique. This method simultaneously measures the gain and phase recovery which is desirable for applications in long range telecommunications which require unchirped signals with a fast response for both gain and phase.
Parameters of commercially available SOAs are compared with the requirements to extend the reach of PONs. Following this the fabrication tolerances for SOAs insensitive to polarisation dependent gain (PDG) are modelled. Results from SOA modelling showed that the greatest contributing factor to PDG variation was the active region thickness error. In the context of bulk production this requires a realistic tolerance of ~10nm to maintain PDG of ~1dB. A polarisation insensitive high gain SOA is designed and experimentally measured. This SOA is measured in the context of GPON and shown to extend the reach of the current standard by a record margin of 28dB. The limitation of the improvement is attributed to gain modulation sourced intersymbol interference (the patterning effect).
The patterning effect has been reported in literature to be reduced through the introduction of SOAs with an active region made from quantum dot (Qdot) material. A comparative study of the gain and phase recovery time and alpha factor of various dimensional SOAs is presented. Using the spectrogram method it is shown that reducing the power and increasing the bias of the SOA can reduce the carrier recovery time. A Qdot active region SOAs is shown to considerably reduce the gain recovery time compared to a bulk SOA of similar length. The active region of the Qdot SOA alludes to a faster carrier recovery time which could be beneficial to extend the reach of PONs without patterning. However as these are more difficult to fabricate in mass production it is unknown if they are a viable solution on a commercial scale.
In the context of GPON a low alpha factor is desired for minimizing chirp and phase nonlinearities during amplification of short pulses. An alpha factor study is presented and the Qdot SOA was measured to have the lowest alpha factor which could be beneficial for reducing chirp in 10G-PON.
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