|M.Sc Student||Gershikov Alexander|
|Subject||Tunable Sources Based on Narrow Band Fiber Parametric|
|Department||Department of Electrical and Computer Engineering||Supervisor||PROFESSOR EMERITUS Gad Eisenstein|
The subject of this work is the creation of an efficient tunable source in the 2 ?m wavelength region.
Two major non linear effects are relevant to our work, Four Wave Mixing, and Raman scattering. Four wave mixing takes place in two main modes of operation, those depend on the dispersion of the pump. A pump propagating in the anomalous dispersion regime, causes broad band parametric gain, while a pump propagating in the normal dispersion regime, enables narrow band amplification provided that the next even dispersion term is negative. Recently, it was understood that the narrow band parametric gain properties, mainly the high tunability of the gain in wavelength regions far from the pump can serve well for creating coherent light sources.
Several major obstacles such as parasitic nonlinear effects, and enormous fiber losses due to absorption near 2 ?m were overcome.
We started from building an oscillator using the common dispersion shifted fiber as the nonlinear media. During the experiment we obtained clear tunable parametric oscillations, which were accompanied by Raman Scattering, which had to be removed, because it hampered the parametric processes and produced noisy background.
Two approaches for removing Raman were suggested, for short nonlinear fibers with length that comparable to fiber beat length, matching the polarization of the feedback wave to the pump wave enhanced parametric the effect, and so eliminates the Raman signal. For long nonlinear fibers, matching the pump pulses repetition rate to allows resonance of only of Raman, or parametric oscillations; this was proven successful. However the conversion efficiency of these oscillators was relatively low, about several percents.
The main obstacle for tunable oscillator at 2?m was the inability to pump in with high enough power at a far enough wavelength from fiber zero dispersion wavelength. By using a fiber with zero dispersion wavelength of 1.59 micron, we succeed to obtain oscillations in the 1.97-2.14 ?m range. Previous methods for eliminating Raman were not successful for this combination. A third approach - inserting into the loop an active filter based on Thulium doped fiber was implemented. The filter eliminated the Raman signal and the short wavelength oscillations, and enhanced the long wavelength oscillations, obtaining output powers of up to 20 Watt, at wavelengths between 1.97 and 2.14 ?m. The high output power and oscillations in the presence of cavity losses of up to 63db testify for excellent parametric process efficiency.