Theoretical And Experimental Study On Fiber Optical Parametric Amplifiers

Fiber optical parametric amplifiers(FOPAs) will become essential devices in future dense wavelength-division multiplexed (DWDM) all-optical networks because of their several advantages: high gain, high saturated output power, low noise, flexible use of signal wavelengths, a broad gain bandwidth,ultrafast intrinsic gain response . In this dissertation, we focus on the saturated gain, gain flat, gain bandwidth, optimum fiber length, pulse signal propagation and application of FOPA. Several theoretical and experimental studies have been carried out as followings:(1) Principal theory for FOPAs is investigated. The coupled nonlinear Schr?dinger (NLS) equations that govern wave propagation in a FOPA are derived from basic propagation equation. The methods to improve the gain and gain bandwith of FOPAs are presented based on analyzing the variation of the parametric gain with the fiber parameters, fiber length, FOPA configuration and pump power. Gain spectrums are studied experimentally. Experiment results coincide well with theoritical results.(2) Saturated gain property of FOPAs is studied. Two pithy and powerful analytical expressions for saturated signal gain and signal output power of FOPAs are educed with numerical analysis.It comes to a conclusion that saturated signal gain of fiber optical parametric amplifiers is related to pump and signal power while saturated signal output power only rely on pump power, but both have nothing to do with nonlinear coefficient and fiber length, which is quite different from small-signal gain but accords well with law of conservation of energy.(3) For the first time, the concept of optimum fiber length is introduced, namely the minimum fiber length needed for the maximum signal gain or signal output power with other parameters fixed. By solving the coupled NLS equations that govern the fiber parameter process with numerical integration as well as adopting control variable method, the optimum fiber length as a function of nonlinear coefficient, input pump power and input signal power is profoundly studied. Finally, coefficients are figured out with least square method, and a pithy and useful analytical expression for the optimum fiber length is presented as well. Compared with existing experimental result, this analytical expression can be soundly used to optimize fiber length for FOPAs. (4) A novel superstructure fiber Bragg grating (SFBG) -based comb gain equalizer for FOPA is proposed. Based on layer-peeling inverse scattering technique, adopting independent channel-by-channel apodizing-windowing method rather than integral apodizing-windowing procedure, the desired SFBG is figured out successfully. Reflection spectrum, group time delay and group time delay ripple are analyzed by the method of transformation matrix. The results show that this SFBG satisfied all requirements of designing target. Using the designed SFBG, the gain variation of the FOPA is flattened to be within±0.4dB for 16 ITU-T channels and the channel isolation is as high as 35dB.(5) On amplifying of picosecond pulses in FOPAs, group-velocity dispersion (GVD) and third-order dispersion (TOD) effects are numerically investigated. It comes to a conclusion that any dispersion-induced modifying of the signal (idler) pulse shape will be coupled to the idler (signal) and result in corresponding modifying of the idler (signal) pulse shape. It is extremely helpful for further experimental study.(6) Taken into account pump depletion, walk-off effect, dispersion and nonlinear effect, amplification of strong pulse signal in FOPAs are numerically studied by solving the coupled NLS equations with split-step Fourier method. Calculation results show that a very interesting phenomenon occurred, that is one signal pulse splitting into two sub-pulses. At the same time, the unique strong pulse signal amplification characteristics are finely explained by saturated gain properties of FOPA, and the potential applications of pulse splitting phenomenon in pulse compression, signal sampling and time-division multiplex system are pointed out as well.(7) A simple technique for widely tunable wavelength conversion between picosecond pulses based on Four-wave mixing in highly nonlinear fiber is demonstrated experimentally. When the signal wavelength is fixed near zero-dispersion wavelength of the highly nonlinear fiber to act as the pulsed pump, the newly generated idler wavelength can be tuned continuously towards the short-wavelength side while the wavelength of a continuous tunable laser is tuned towards the long-wavelength side.