Study On Key Technologies For Coherent Optical Phase Modulated OFDM In Fiber System

Coherent optical OFDM (Orthogonal Frequency Division Multiplexing)has been a hot topic in optical fiber transmission. By combing coherentdetection and OFDM technology, it has higher receiving sensitivity andspectral efficiency. In addition, it can achieve complex modulation formats infiber systems, which enhance the spectral efficiency by increasing systembitrates. With the help of sophisticated digital processing algorithm, variouskinds of transmission impairments can be compensated. Experiments havedemonstrated the feasibility of1Tb/s coherent optical OFDM signal’stransmission on7000km Raman fiber. Real-time application of coherentoptical OFDM has been studied, too. In this dissertation, key technology ofcoherent optical phase modulated OFDM in fiber system has been studied,with the following results:An optical phase modulated OFDM is proposed, in order to overcome thedemands of precise bias voltage control of dual MZM and narrow linewidthlasers in coherent optical amplitude modulated OFDM systems. We proposerestoring phase of the optical field to demodulate this type of optical OFDMsignal. By using IF-OFDM (intermediate frequency OFDM) signal, theinfluence of laser phase noise can be filtered off by low-stop filtering. On thisbasis, the transmission system is constructed. Through theoretical analysisand VPI simulation, we verify the feasibility and effectiveness of this method.The results have shown that this type of optical OFDM is affected seriouslyby fiber nonlinearity in long-haul transmission systems. By suppressing powerof the optical signal into fiber link, the deterioration of fiber nonlinearity canbe reduced and better transmission effects achieved.The experimental platform for coherent optical phase modulated OFDMin fiber system has been established and its performance is experimentallystudied. We use single PD (photodiode) and a DFB laser with5MHz linewidth. There is no pilot inserted in OFDM signals and the influence of laserfrequency difference is not considered in digital processing. On back to backconditions, BER can be controlled under10-6when4QAM (4QuadratureAmplitude Modulators) modulation format is used, which corresponds abitrate of10Gb/s. When using16QAM modulation format, BER is about10-5corresponding a bitrate20Gb/s. While the response bandwidth at the receiveris limited to7.5GHz and laser frequency difference increased to200MHz, theBER has no significant deterioration. Through suppressing power of theoptical signal into fiber loop, we reduce the deterioration of fiber nonlinearity.While power of the optical signal into fiber loop at-1dBm, BER can becontrolled under10-3after800km transmission. The experimental resultsshow that the coherent optical phase modulated OFDM is a kind of low-costsolution compared to the traditional optical amplitude modulated one. It hasthe advantage of low requirement for laser linewidth and simple digitalprocessing algorithm. The disadvantage reflects in its relatively lower spectralefficiency of optical spectrum and sensitivity to fiber nonlinearity. Thissystem is appropriate for long and mediate distance transmission applicationsituation, can be used in metro transmission and access systems.