P. Shum

Theoretical analysis of modulation response and second-order harmonic distortion in vertical-cavity surface-emitting lasers

A rate-equation model is developed, with the consideration of size effects, to analyze the steady state and dynamic behavior of index-guided vertical-cavity surface-emitting lasers. The size dependence of spatial hole burning, cavity loss, as well as thermal resistance of device cavity are taken into account. Using this model, the influence of size effects on the amplitude modulation response and second-order harmonic distortion are studied. It is found that a laser with a small core radius exhibits better modulation response and less harmonic distortion than that of a large waveguide device, however, there is a tradeoff between the output power and modulation efficiency of the lasers.

Switching dynamics of short optical pulses in a nonlinear directional coupler

It has been shown recently that the propagation of short optical pulses in a nonlinear directional coupler consisting of two parallel waveguides or fibers should be described more generally by a pair of coupled nonlinear equations that take into account the dispersion properties of the coupling coefficient. In this paper, we use the Fourier series analysis method to solve these equations, and study the effects of a dispersive coupling coefficient on the switching dynamics of short optical pulses in a nonlinear directional coupler. We demonstrate, with numerical examples, that a dispersive coupling coefficient could cause significant pulse distortion and affect the threshold switching intensity. The effects of introducing frequency chirps in the input pulses are also discussed.

Numerical analysis of nonlinear soliton propagation phenomena using the fuzzy mesh analysis technique

A novel numerical technique, the fuzzy mesh analysis technique, is developed to study the nonlinear propagation phenomena of solitons in an optical fiber. The main advantage of this technique is the variation of mesh size with the shape of soliton pulses along the propagation distance such that: 1) the calculation efficiency can be enhanced and 2) the number of sampling points can be greatly reduced. It is shown that the fuzzy mesh analysis technique is capable of analyzing the propagation phenomena of high-power solitons, pulse compression, and soliton interaction in an efficient manner.

Analysis of a DPSK soliton transmission system

Abstract We present a closed form expression to analyse the error performance of a differential phase-shift keying (DPSK) digital modulation format in an optical soliton communication system. In the analysis we include the effect of Gordon-Haus jitter, which is mainly due to the amplified spontaneous emission noise of the amplifiers. Gordon-Haus jitter is found to be less significant when the bit period is 15 times greater than the jitter variance and becomes more significant when the signal-to-noise ratio is greater than 20 dB. Due to a good bit error rate (BER) performance (BER ≈ 10−20at SNR= 21 dB), the DPSK modulation format has a potential application for a high-bit-rate and low-noise soliton communication system.

Improvement of Fourier series analysis technique by time-domain window function

We demonstrate that by adding a time-domain window function to the recently developed Fourier series analysis technique can reduce the propagation error in solving the nonlinear soliton propagation equation. With suitable modification of window function parameters, the number of sampling points as well as computational time required for the calculation can be minimized even with higher order dispersion terms taken into consideration.

Dynamic analysis of erbium-doped optically pumped waveguide lasers using a time-domain travelling wave model

A simple and straightforward dynamic model of Er3+-doped optically pumped waveguide lasers using a time-domain algorithm is developed. The theoretical model is based on (a) time-dependent rate equations of a quasi-two-level system for the population densities and (b) time-dependent travelling wave equations for the pump and signal power which are solved simultaneously in the time domain. In addition, the longitudinal distribution of spontaneous emission noise is also taken into consideration. Using the fast Fourier transform, the spectrum profile of the time-domain signal power is calculated. In this paper, the transient responses of population densities, pump and signal power are investigated for Er3+-doped Ti:LiNbO3 waveguide lasers. To demonstrate the sophistication of this time-domain model, the influence of the cross-coupled effect of the travelling waves due to the optical feedback of an etched grating is also studied. Dynamic response of the waveguide laser with and without a grating are compared.

Self-sustained pulsation in vertical-cavity surface-emitting lasers under external optical feedback

The generation mechanism of self-sustained pulsation in vertical cavity surface emitting lasers is analyzed theoretically. The influence of self-focusing, diffraction loss as well as optical feedback is taken into consideration. It is shown that the condition of self-sustained pulsation can be modified significantly by optical feedback. In addition, it is possible to obtain self-sustained pulsation in vertical cavity surface emitting lasers by using external mirror.

Nonlinear soliton propagation by use of the split-step reconstruction technique

A split-step reconstruction technique is proposed to solve a nonlinear wave equation. A nonlinear wave equation can be segmented into a set of linear equations that can be solved in the time domain by use of the split-step reconstruction technique. With this technique, one can avoid the propagation errors that occur as a result of nonlinear wave equation splitting. We propose an adaptive mesh control to increase the speed with which nonlinear wave equations can be calculated.

Efficient analysis of high power nonlinear soliton propagation

Due to the periodic variations of high power solitons, ultra-sharp pulses are created, and the analysis of the propagation behavior is computer intensive because of the requirement of large sampling density, in order to make the analysis more effective, we have implemented a time domain analysis technique to analyze the propagation behavior of high power solitons in an optical fiber. For a relatively small number of sampling points, the distribution of sampling points at different point of the fiber has been controlled effectively according to the shape of soliton pulse. We found that by a real time effective control of sampling distributions, the propagation behavior of high power solitons can be studied more effectively and the required number of sampling points is drastically reduced.

Numerical Simulation Models of Soliton Systems

This paper describes the development of numerical simulation models of soliton systems using a newly developed Fourier Series Analysis Technique to analyze the nonlinear Schrodinger equation of soliton propagation. Generation of sub-picosecond solitons in an active mode-locked fiber ring laser with amplitude and phase modulators and soliton pulse compression mechanisms using dispersion decreasing fiber are investigated.