Noise and Saturation Properties of Fiber Optical

Doktorsavhandling, 2006

Fiber Optical Parametric Amplifiers (FOPAs) are based on a highly efficient Four-wave Mixing process in highly nonlinear fibers and have gained a lot of interest since it was shown that a high gain can be obtained. They are multi-functional devices that are not only limited to amplifying signals, but their wide range of applicability also include wavelength conversion, pulse-generation, optical sampling and also regeneration at very high bit-rates. The foremost property that makes this amplifier unique is its very fast response time, which make the signal gain respond to any change in pump power almost instantaneously. The FOPA was long thought of as an ideal amplifier in terms of noise performance. We show, however, that the noise around the pump itself is transferred to the signal. The fast response time makes this source of noise particularly large compared with other amplifiers exhibiting similar properties. This leads to stringent requirements on the laser. In this thesis we have developed a theory for the noise figure of FOPAs, both in single-pumped and dual-pumped implementations and show very good agreement with experimental measurements. Furthermore, in order to launch the pump powers necessary for high gain, the pump sources need to be phase-modulated in order to reduce the effects of stimulated Brillouin scattering. This may introduce intensity fluctuations on to the signal and may therefore affect the amplifiers performance. We quantify this effects and also derive theory of how it affects the bit-error rate (BER) in digital data. It is shown that its effect on the BER is much lower then previously expected. Also, optical signal regeneration is based on a nonlinear relationship between the input and output optical power. FOPAs exhibit such a relationship when operated in the saturated gain regime. We simplify previous gain saturation theories and present experimental measurements proving our theory. A simplified expression is useful in the development of regenerators and in the design of multi-wavelength amplification, where cross gain saturation may affect the amplifier performance.