Theoretical and experimental investigations of the nonlinear refractive index in an Erbium-doped fibre is presented. The transient response of this fibre is also examined. The switching speed can be improved by using a signal power at a wavelength close to the resonant wavelength between the excited state and the ground state.
Erbium-doped optical fibre (EDOF) is an important platform to study the intensity-dependent nonlinear propagation and nonlinear polarisation rotation, optical switching and four-wave mixing. The source thesis contains both theoretical and computational work. In this paper the work is presented in the format of a research paper that is published by the University Grants Commission. The nonlinear coefficient is expressed in terms of the nonlinear refractive-index coefficient, the operating wavelength and the effective mode area, and the resulting nonlinear phase response is investigated. The consideration of the passive attenuation and the active gain in the theoretical model, and the computational analysis carried out on the basis of MATLAB are discussed. The nonlinear coefficient varies from 1.758 to 1.471 W−1 km−1 over the wavelength range 1480-1630 nm, while at 1550 nm it varies from 2.189 to 1.368 W−1 km−1 for effective mode area increasing from 50 to 80 µm2. The nonlinear phase shift goes up to 0.160 rad at 100 mW, compared to 0.016 rad at 10 mW. For the same input-power range, the nonlinear polarisation rotation increases from 2.4° to 24.0°. The optical-switching analysis shows a transmission of 90% and an extinction ratio of 9.54 dB at 100 mW. Four-wave-mixing analysis shows that the relative FWM response increases from 0.10 to 1.00 when the input power is increased from 10 to 100 mW and decreases from 0.90 to 0.15 when the assumed chromatic-dispersion value is changed from -20 to +15 ps/nm/km. The results show the inter-related roles of nonlinear coefficient, optical power, wavelength, polarisation evolution and chromatic dispersion in erbium-doped fibre systems. Erbium-doped fibre, nonlinear coefficient, Kerr nonlinearity, nonlinear polarisation rotation, optical switching, four-wave mixing, chromatic dispersion.
We present a theory that enables us to account for the pump-induced change of refractive index in a rare-earth-doped fiber. Experimental results carried out on an erbium-doped fiber with cw pump and cw signal operation confirm the accuracy of this theory at the steady state. Both theoretical and experimental results show that significant index changes can be obtained for the order of 1–2 mW of cw optical pump or signal power. In addition we studied the transient dynamics of the resonance-enhanced refractive index in a wide range of pulse widths from millisecond to tens of femtoseconds. The result shows that a fast response of refractive-index changes is achievable since the effects of spontaneous emission and nonradiative decay are negligible in the short-pulse case.
We perform a study of extremely slow pulse propagation speed in densely-doped erbium fibers by using a detuned signal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneous broadened absorption profile of the erbium-doped fiber. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. The interparticle interaction between the nearest neighboring active particles due to the high density of erbium ions is considered. The result shows that the nonlinear refractive index can be greatly increased which leads to the smaller group velocity and the larger time delay.