Sami A. Shakir

Recursive numerical solution for nonlinear wave propagation in fibers and cylindrically symmetric systems

An efficient and compact recursive numerical solution of the wave equation is developed and applied to cylindrically symmetric optical systems. Numerical results are given for wave propagation through an aperture and in linear and nonlinear optical fibers. This code is most useful for multiwave mixing and wave propagation in nonlinear media.

Four-Channel, High Power, Passively Phase Locked Fiber Array

We demonstrate passive phasing in a four channel high power passively phase-locked Yb fiber laser array. We achieved an output power of 710W with high fringe visibility from an array of LMA Yb fiber lasers.

Surface smoothing effects of thin film deposition

Deposition of 0.2–0.5-μm thick Cu thin films onto a polished Cu substrate causes a reduction of scatter due to high spatial frequency microroughness by as much as a factor of 10 and a reduction in total integrated scattering by as much as a factor of 4.

Coupled-mode equations for two weakly guiding single-mode fibers

A recently developed, more accurate coupled-mode formulation for arbitrary parallel waveguides is applied to two single-mode weakly guiding coupled fibers. Propagation constants and coupling coefficients are calculated for both identical and dissimilar fiber pairs.

Approximate theory of Stokes amplification and conversion valid at large gain

We predict Stokes-beam divergence, origin, and gain for large-gain Raman amplifiers as functions of pump-beam divergence and pump power in terms of simple analytical expressions. Stokes divergence is found to peak at a pump power twice above threshold. Results are in excellent agreement with exact computer solutions and explain recent experimental observations.

Propagation of non-resonant temporal Raman solitons in the presence of coherence decay: a perturbation theoretical analysis

Abstract The propagation of a soliton in a Raman medium in the presence of collisional decay of coherence and detuning from Raman line center is studied. The spatial dependence of soliton width, position and amplitude is calculated by treating the decay of coherence as a perturbation. Two approaches are used, one based onthe inverse scattering transform and the other employing constants of motion. Both give identical results which are confirmed through the numerical integration of the equations of Raman scattering. These results confirm the phenomena of soliton narrowing and decay, which have been observed experimentally.

MOPA fiber laser with controlled pulse width and peak power for optimizing micromachining applications

An optimized 1064nm MOPA fiber laser configuration is used for generating pulse widths from 10 ns to 250 ns, with pulse repetition frequencies that range from single-shot to 500 kHz, and peak powers up to 10kW. These parameters are independently controlled and used to investigate the effect of peak power and pulse energy on material ablation. Test results are demonstrated for processed silicon and ceramic materials using pulse energies up to 0.5 mJ and peak powers up to 10kW. We demonstrate that pulses with high peak powers have shallow penetration depths, as compared to longer pulses. These experimental results are well correlated with those from a theoretical thermal model for silicon ablation that is based on silicon temperature rise as the incident pulse energy is absorbed.

Increasing The Efficiency Of Stimulated Scattering Phase-Conjugate Mirrors

Stimulated processes, such as Brillouin and Raman Scattering (SBS and SRS), have been used successfully for high power phase conjugation. To achieve high conversion efficiency, usually the intensity of the laser beam needs to be relatively high. We show that SBS or SRS reflectance can be substantially increased by feeding back a portion of the back scattered Stokes energy. Before the feedback Stokes wave is admitted through the exit window of the Brillouin cell, it is strongly diffused by a diffusing plate. This is necessary since the feedback Stokes wave is the phase conjugate of the laser field at the entrance window but not the exit window. The phase conjugate component in the diffused Stokes wave will increase the efficiency of the process dramatically. We show that if losses are neglected, the energy reflection coefficient, R, can achieve a value R = 1 - β, where β is the feedback energy fraction of the Stokes wave. Since can be small compared to unity, in principle, R can be close to unity.

Bursting for Enhanced Ablation of Materials

A significant enhancement in the rate of material removal is demonstrated using a nanosecond-pulsed UV fiber laser in multi-pulsing burst mode, as compared to the case without bursting. Percussion drilling and scribing of thin-film and bulk material tests show that, in general, laser bursts with increased pulse count and reduced pulse spacing show higher rates of material removal. A considerable improvement in removal rate is demonstrated, when bursting is applied to scribing of mono-crystalline silicon (m-Si) and up to 30% in percussion drilling speed. Likewise, improved material removal is demonstrated for scribing of thin film of indium tin oxide (ITO) on glass or metal film on sapphire. Examples of material processing are given with and without bursting at similar experimental conditions of average power, scan speed, and burst/pulse energies. Experimental results included are for m-Si, ITO thin films on glass, and metal films on sapphire.

Optical Filters Synthesis Using Fourier Series

A simple method of multilayer filter synthesis is developed. The method is based on expanding the reflectance amplitude in a Fourier series and deducing the indices of refraction from the Fourier coefficients. The utility of the method is demonstrated by solving a general representative model for optical filters. The simplicity of the method is due to the fact that we deal directly with the reflectance amplitude. The method is applicable to cases of equal or unequal thickness films. To achieve a practicable design solution, the computed refractive indices are scaled and matched to a given set of available materials using the criterion that the error in the Fresnel amplitudes at each interface is a minimum, illustrative design examples are demonstrated.