In my PhD research, I have developed a method of frequency- and polarization shift-free optical phase conjugation(OPC) using counter-propagating dual pumped FWM in fiber. The two counter-propagating pumps create a Bragg grating inside the fiber, which diffracts the forward propagating signal and generates a backward propagating idler wave whose phase is conjugate of signal phase. By placing the pump frequencies symmetrically about signal frequency, we ensure that the idler has the same frequency as that of the signal. Since the signal and idler waves appear at opposite ends, the idler is easily ﬁltered out from the rest of the spectrum. Ideal phase conjugation is achieved at an optimum length of ﬁber for a given pump power. A detailed study of effect of critical design parameters such as nonlinear length, pump power, and detuning between pump and signal on conjugate efficiency and phase-offset has been carried out. This innovation is currently under review by the patent office in the India.
During my stay in Radio and Optical Communication Lab, Dublin City University, I have experimentally verified this phase-conjugation technique in C-band by counter-propagating dual pumped non-degenerate FWM in semiconductor optical amplifier (SOA) as nonlinear media. In contrast to fiber case, frequency-shift free operation cannot be achieved in SOAs due to end facet reflectivity. FWM conversion efficiency of 1% is demonstrated, which is 5-times higher than previously reported value. For the first time, in my knowledge, I directly demonstrated the spectral inversion by OPC using time-resolved chirp measurement of electric field envelope of signal and conjugate waves.
I have also designed an Optical Buffer using Silicon waveguide with tunable delay. This invention is currently under review by patent office in USA.
My PhD work resulted in filling of 2 patents, publication of 3 research articles in referred journal and 7 conference proceedings, including one invited talk.