Research On DQPSK Demodulator Based On Planar Lightwave Circuit

In Wavelength division multiplexing (WDM) systems, rapid increase of bit rateper-channel has brought great challenges to the whole systems, such as decreasingtolerance to Chromatic dispersion (CD) and Polarization mode dispersion (PMD) as wellas nonlinear impairments, higher requirement on Optical signal-to-noise ratio (OSNR), etc.As a result, some corresponding technologies have been put forward and studied toovercome such limitations, among which advanced optical modulation formats is one ofthe most effective methods.Recently, phase-modulated formats such as Differential quadrature phase-shift keying(DQPSK) are being extensively studied. DQPSK techniques have shown great robustnessto CD, PMD and nonlinear impairments in WDM systems, so the transmission distancecan be extended. In DQPSK receivers, an optical demodulator is necessary to convertphase modulation to intensity modulation, so the information coded in carrier phase can beextracted, and optical demodulators are the most important devices in DQPSK systems.This thesis focuses on the research and design of optical DQPSK demodulators, the mainwork is given as follows.(1) A novel kind of DQPSK demodulators which requires only a Mach-ZenderInterferometer (MZI) and a2×490°hybrid are designed to Demodulate the signal.Thuswe need only one frequency stabilization control loop,and the demodulator can be morecompact and less expensive which is compared with the traditional two MZI-basedDQPSK demodulator.Then, the theoretical analysis and the simulation by using FD-BPMalgorithm of the proposed demodulator and the sub-structures of the demodulator such asthe3dB direction coupler, FSR,2×490°hybrid and the metal thin film heater isillustrated.(2) We fabricate the demodulator by using this single MZI-base waveguide structure,and the definition of key parameters and the measuring methods of DQPSK demodulator isdescribed in detail. The analysis of the principle of reducing the polarization dependent frequency shift(PDFS) by using half-wave plate is Emphasis on. On this basis, weproposed the optimized design of PDFS, and verified by experiments.Meanwhile, we proposes two optimization designs to replace the expensive polymertype half-wave plate by the quartz half-wave plate. One is that adding taper-shapedwaveguide structure in the position where is the half-wave plate inserted, the other is thatadding Period segment waveguide(PSW) structure in the position where is the half-waveplate inserted. Both methods can reduce the excess insertion loss which is caused by theincreasing of the thickness of the quartz half-wave plate. We analyse these optimizeddesigns theoretically and simulate the proposed designs by using FD-BPM algorithm,fabricate the chip by using these waveguide structure designs,and verify the performancewith the experiment. Finally, we introduce the experimental test of tuning time constant,experimental results show that tuning time of PLC-based DQPSK demodulator is only500swhich is farer more better than fiber-based DQPSK demodulator and crystal-baseddemodulator, and is more suitable for commercial use.