Electrical and Computer Engineering ETDs

Publication Date

2-2-2016

Abstract

The nonlinear interaction (NLI) between modes in a semiconductor laser or amplifier is one of the techniques to obtain slow light (ng >> 1), fast light (-1 < ng < 1), phase conjugated reflection (also known as backward light (ng < 0)) and supercontinuum emission (also known as critically anomalous dispersion (ng = 0)), where ng is the group index. In the nonlinear interaction (NLI) mechanism, the interaction between a driving wave (mode) which is of sufficiently high intensity, and a probe wave (mode) results in modification of complex permittivity of the probe wave, if the frequency detuning between the two waves falls into the spectral range of parametric interaction. Consequently, in the vicinity of a strong driving wave (mode), the optical parameters such as the phase index, group index, linewidth, and the gain of a neighboring probe (weak) wave are strongly perturbed. The unusual perturbation of group index leads to slow light, fast light and backward propagating light depending on the frequency detuning between the two waves. Fast light can have important applications in interferometry, to make very sensitive interferometric sensors for detection of nano displacement, ultra-low vibration, rotation rate and even very weak gravitational waves. The slope sensitivity of an active integrated ring laser gyro (RLG) is directly proportional to the size of the ring and inversely proportional to the group index of the medium filling the cavity. By reducing ng in the nonlinear regime of laser operation, slope sensitivity can be improved by manifolds without having to increase the size of the gyro, which will lead to monolithic integration. In this dissertation work, the numerical modeling and simulation of NLI in semiconductor ridge waveguide (RWG) lasers is performed and perturbations of phase index, group index, gain, linewidth are calculated. The modeling is based on theory of nonlinear interaction of waves' developed for plane waves and is applied to RWG lasers. The RWG laser structures are based on GaN/InGaN, GaAs/AlGaAs, InGaAs/GaAs/AlGaAs, InGaAsP/InP material systems, to cover the most important spectral range of semiconductor lasers starting from λ = 0.4 μm to λ = 1.55 μm, respectively. The photonic design software RSoft's FemSIM that uses finite element method is used for design and modeling of NLI in RWG and calculation of effective phase index. The induced dispersion and effective group index ng are then calculated by differentiation of the effective phase index of guided probe wave over the optical frequency. The calculations indicate that the magnitude of NLI depends on the driving wave intensity, frequency detuning between driving and probe waves, lifetime of carriers in the laser system, optical confinement factor, driving wave wavelength and linewidth broadening factor of the laser structure. An important result of this work is observation of frequency points where ng passes zero line, known as the points of critically anomalous dispersion (CAD). In the spectral vicinity of CAD points, superluminal group velocity is observed, which can be used for improving the sensitivity of ring laser gyros and other interferometric sensors. In this dissertation, we established possibility of superluminal behavior (-1 < ng < 1) through NLI in all of the above mentioned RWG laser structures, and obtained the conditions necessary to observe this behavior. Also, the CAD frequencies are plotted by varying the driving wave intensities, where the data form a closed loop. The closing of the loop is due to suppression of NLI with an increase in driving wave intensity; at high intensities, the stimulated recombination rate increases, leading to an increase in overall relaxation rate, which in turn decreases the NLI. The CAD loops are calculated for all the RWG lasers mentioned above. The CAD loops provide information on lower and upper threshold intensity of the driving wave, and on frequency detuning between the driving and probe waves for observation of superluminal group velocity. Another important result is that the perturbed linewidth increases infinitely at the CAD point, leading to supercontinuum emission or white light, which can be exploited for very broadband laser emission applications. We also fitted our numerically calculated perturbed delay and advance due to NLI with the experimental data of perturbed delay and advance of an optical pulse in a semiconductor optical amplifier based on InGaAsP/InP, and identified the CAD point for the very first time.'

Keywords

Semiconductor lasers--Mathematical models., Optical amplifiers--Mathematical models., Nonlinear optics.

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Eliseev, Peter

Second Committee Member

Sheik-Bahae, Mansoor

Third Committee Member

Balakrishnan, Ganesh

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