The Research On Optical90°Phase Shift Hybrids For100Gbit/s Coherent Communication Application

Recently，the100Gbit/s coherent optical fiber communication system based onDP-QPSK modulation format has being paid important attention. The optical90°phaseshift hybrid is a key component of coherent optical fiber communication system fordemodulating QPSK optical signal, and the research on the optical90°phase shift hybridhas great economic value and application prospect. The thesis was supported by WuhanTelecommunication Devices Co, Ltd, which is a company belong to Wuhan ResearchInstitute of Posts and Telecommunications, and its main research object is InP base optical90°phase shift hybrid. And the thesis spreads out discussion around the theory, designoptimization, fabrication and test results of the optical90°phase shift hybrid. The mainwork and innovation points are as follows:(1) A tunable90°optical phase shift hybrid structure is proposed. It is composed of a2×4MMI based on pairs interference, a2×2MMI, and a heater pad. The2×4MMIconnects with the2×2MMI directly. There is a tunable heater pad lies on the connectareas of the two MMI couplers, and its width is a quarter of2×4MMI coupler. The localrefractive index can be modified by tuning the current of the heater pad, and lead tooptimized output. The current of the heater pad can be changed accurately with differentoperation wavelengths. And the craftsmanship of the heater pad is very mature and hightolerance. Because the2×4MMI connects with2×2MMI directly, the loss of the tunableoptical90°phase shift hybrid is decreased, the length of it is shorten at the same time. Thesimulation results show that when light launches to single input, the maximum excess lossis0.45dB and the biggest imbalance is0.04dB in the C-band wavelength. And when lightslaunch to dual inputs, the phase deviations are less3°in the C-band. The tolerance analysisshows that the width error tolerance of the tunable optical90°phase shift hybrid is threetimes than the tranditional optical90°phase shift hybrid.(2) An optical90°phase shift hybrid bsase on4×4MMI is systematically researched,included the design, optimize, tolerance analysis, fabrication and tese. The simulationresults show that when light launches to Input1port, the maximum excess loss is0.66dB and the biggest imbalance is0.35dB in the C-band wavelength. And when lights launch todual inputs, the phase deviations are less2°in the C-band. The results indicate that thedesign after optimizing meet the requirements of the OIF. The results of the toleranceanalysis show that the width error tolerance of the optical90°phase shift hybrid is less than0.1μm. The fabrication error of multimode waveguide width can be accurately controlled toless than0.1μm by adjusting the factors of the lithography and etching. The test resultsshow that the center wavelength of the optical90°phase shift hybrid is1.54μm, which isdeviated5nm from design value. When the operation wavelength is1.54μm, the loss of theoptical90°phase shift hybrid is20.8dB, and it includes0.8dB of excess loss and bend loss,6dB of intrinsic loss, and14dB of other losses. The biggest imbalance is0.5dB in the wholeC-band, and it has budget about0.36dB comparing with the requirement of OIF, and thisindicates that the optical90°phase shift hybrid meet the requirements of the design andhave good application prospect.(3) A10Gbit/s coherent optical fiber communication system based on DP-QPSkmodulation format is built up. In the transmitting terminal, the DP-QPSK modulater iscontrolled accurately by manual adjustment, and get optimized eye pattern. In the receivingterminal, the digital signal is processed by off line, and the initial signals are recovered.