Feasibility of Building an Optical Phase-Lock Loop Using Semiconductor Lasers

This thesis reports on the work carried out in designing and attempting to build an optical heterodyne phase- lock loop using semiconductor lasers as the local oscillator and as the transmitter. It is shown that homodyne detection of PSK is the most sensitive modulation/demodulation format a communication system can use. Receiver sensitivities for various schemes are evaluated. Performance improvements of more than 10dB are realisible in a coherent scheme. In a coherent system it is also possible to send thousands of signals simultaneously down one piece of fibre providing they are separated in frequency by a set amount, say 5GHz. This use of the bandwidth is called frequency division multiplexing and in monomode fibre could result in increasing channel capacity by more than 10,000 fold. Design guidelines are determined for building optical phase lock loops. Laser phase noise, shot noise and loop propagation delay are determined to be the main criteria which must be taken into account when designing a coherent reciever. Specifically if a receiver is to operate with a 10-9 bit error rate and suffer no more than 1 dB power penalty as a result of phase noise degradation then the bit rate to laser linewidth ratio must be in excess of 2000. In addition to this the absolute stability requirement for a phase lock loop is ontaud ≤ 0.736 where on is the bandwith of the loop and taud is the loop propagation delay. In reality if ontaud is greater than 0.2 or 0.3 then severe penalties are incurred. The Hitachi HLP1400 laser diode which was used in this project is fully characterised. It is shown that it is not suitable as the optical source for a phase- lock loop as it has too large a linewidth, greater than 5MHz, and it does not have a broad continuously tunable tuning range. Several techniques are presented which can be used to enhance the operation of laser diodes. In particular compound cavity and external cavity laser modules were designed and built up around HLP1400 diodes. These modules had some vastly improved characteristics with the external cavity having a linewidth of less than 100kHz and a tuning range in excess of 10nm. The compound cavity although it had a reduced linewidth was extremely sensitive to any alteration in current, temperature or optics. Results are quoted for the performance of other semiconductor laser devices incorporating different structures such as Distributed Feedback and Distributed Bragg Reflector diodes. The results of the experimental programme including the design and build of the laser modules and the attempts at phase locking two devices are presented. Phase lock was not observed during the experimental programme. The main reason for this was that small amplitude, 1 - 10nm, low frequency vibrations of the external cavity of the laser module resulted in low frequency fluctuations of magnitude 10 - 100MHz. The loop could not cope with such frequency variations. This was partly due to the loop bandwidth being restricted to less than 50MHz because of the size of propagation delay and partly due to the frequency tuning control of the external cavity diode. The experimental programme terminated when it was found that the spectral properties of the lasers being used deteriorated to the point where they were useless. Upon close inspection it was found that the lasers spectral profile had dramatically altered. Under free running conditions it displayed a spectrum similar to that which might be expected from a compound cavity structure. Subsequent to the termination of the experimental programme other groups worldwide have designed and built phase-lock loops using semiconductor lasers. These loops operate in the manner predicted by the design criteria developed during this project