Acousto-optic Nonreciprocal Phase Shifter Based On Two-Mode Fiber

Nonreciprocal phase shifter is widely used not only in basic optic devices, suchas optic isolator, or circulator, but also in optic channel power equalization, opticwavelength conversion and optic sensor system. The traditional nonreciprocal phaseshifter is implemented based on the magneto-optic affection or birefringence affection.The dissertation is targeted to offer nonreciprocal phase shift for NALM or NOLMstructure, choosing two-mode fiber acoustic-optic device scheme. And in order toimplement the two-mode fiber acoustic-optic nonreciprocal phase shifter and increaseits performance, the investigation is made through basic theory simulation, devicedesign and experiment.The two-mode fiber acoustic-optic nonreciprocal phase shifter consists of twoacoustic-optic frequency shifters same in characteristic and mode strippers, etc. Socharacteristic of the acoustic-optic frequency shifter and the identification of twoacoustic-optic frequency shifters' characteristics are very important. Based on theacousto-optic mode coupling theory, the principle of the two-mode fiberacousto-optic nonreciprocal phase shifter is demonstrated. Two acoustic-opticfrequency shifters with same acoustic driver frequency and high transformefficiencies are implemented, through solving a serial of difficulties, such astwo-mode fiber design, elimination of acoustic reflection, the tune between the opticcenter wavelength and the matched acoustic-wave frequency, the process of etchingthe fiber outer diameter, and the wavelength tunable characteristic using fiberlongitudinal strain. Then the resulted nonreciprocal phase shifter with 3dB dynamicloss and 8nm (1550nm-1558nm) 1dB bandwidth is implemented.It's pointed out in theory that when both acoustic-optic frequency shifters'transform efficiencies equal 50%, the forward and backward transmissions havemaximum amplitude and a shift in wavelength representing the nonreciprocal phaseshift amount. The Fourier analyzing method calculating the nonreciprocal phase shiftamount based on the forward and backward transmissions is also proposed, and isused in experiment. It's also pointed out in theory and experiment that the ripples ofthe nonreciprocal phase shifter's transmission originated from the fact that the actualacoustic-optic frequency shifter's transform efficiency is less than 100% and theacoustic-optic couple process is polarization-dependent.The etched three-segment cascaded tunable comb based on single-mode fiber isput forward and implemented by incorporating design idea of the two-mode fiberacoustic-optic nonreciprocal phase shifter and the advantage of single-mode fiberacoustic-optic device. And the relations between the comb filter characteristic and thecascaded structure parameters are also investigated.