The effect of laser noise on an optical ofdm system

In recent years, researchers and network equipment manufacturers have focused on the development of a 100 Gb/s optical transmission systems to cater the ever increasing demands for bandwidth. To cater the demands, optical orthogonal frequency division multiplexing (O-OFDM) has been proposed. O-OFDM has been shown as a promising technique to increase spectral efficiency with its ability to compensate for chromatic and polarisation mode dispersion. There are two forms of receiver used for long haul O-OFDM links: coherent-OOFDM (CO-OFDM) and direct detection O-OFDM (DDO-OFDM). The CO-OFDM system is known to be limited by the laser’s phase noise, where the system requires a narrow linewidth laser at both transmitter and receiver, where these lasers phase must track one-another. In the DDO-OFDM system, an optical carrier is transmitted along with the subcarriers so both originate from the same laser, and both experience the same degradations along the fibre. Thus, the linewidth, or equivalently phase-noise, requirements are far less stringent than for CO-OFDM systems. However, given that laser phase noise will be converted to intensity noise (PM-to-IM conversion noise) along a dispersive fibre, there will be some effect on performance in a DDO-OFDM system. The question is whether this is significant, given that the fibre dispersion is not optically compensated as in conventional lightwave systems. In this thesis, laser phase noise effect upon detection in a DDO-OFDM system is shown to impose broad noise pedestal around a subcarrier for higher frequency subcarriers when there is a high accumulated dispersion. The noise pedestal can introduce inter-subcarrier-interference between the adjacent subcarriers. This is different in a CO-OFDM system, where the phase noise is independent of the subcarriers’ frequency and common to all subcarriers. This thesis focuses on the DDO-OFDM system using external modulation at the transmitter. A transmission system was developed to study the effects of the laser relative intensity noise (RIN) and phase noise over a long-haul transmission, and to show the importance of the laser’s characteristics. Experimental measurements on a commercial fixed-wavelength distributed feedback (DFB) laser investigated its RIN, linewidth and light vs. current (L-I) characteristics. The effect of external modulation of this laser was investigated. With the externally-modulated transmitter, the laser chirp is ignored. The linewidth measurement was done using self-homodyne technique and validated using a high-resolution spectrophotometer. The measurements showed inter-dependency between the linewidth and RIN, which agreed with theory. A laser linewidth emulator was developed and demonstrated experimentally. This was done by phase modulating a semiconductor’s laser output to broaden its linewidth. With this laser, the linewidth can be made independent from the other laser’s noise characteristics. Simulations and experiments were performed to study the interaction of linewidth and fibre dispersion. The interaction produces noise conversion from phase modulation to intensity modulation, where the noise PSD was obtained. An analytical equation of the PM-to-IM’s upper-bound was derived and plotted on the noise PSD’s amplitude. When the laser was modulated with RF tones, a noise pedestal around each of the tones was obtained. The peak power of the noise is shown to increase with the increased of the tone’s frequency. This is due to the phase walk-off of the high frequency subcarrier from the carrier is larger than the low frequency subcarrier. The peak also increases when a wider linewidth was used. This noise was shown to cause phase rotations of the received symbols and reduces the Q performance of the DDO-OFDM system. To reduce the effect of the linewidth in the DDO-OFDM system, an experiment using a simple delay line to match the delay experienced by the subcarrier was performed. The result shows improvement of the subcarriers’ Q performance. Finally, this thesis also presented the performance of the DDO-OFDM system in transmitting high-speed data of 120 Gbit/s. This experiment involved several important device improvements in order to achieve a good quality and wide RF-signal bandwidth. In this transmission, a variant of the DDO-OFDM system employing self-coherent polarization-diversity receiver design was developed.