Mindaugas Radziunas

Low-frequency fluctuations in a semiconductor laser with optical feedback are analyzed with a traveling wave model and its instantaneous and compound-cavity modes. Compound-cavity mode curves, their height and width in the frequency/carrier-density plane, enable fast estimates of parameter ranges where low-frequency fluctuations occur. These ranges cover a larger part of the parameter space than is commonly realized.

Numerical simulations using a traveling wave model are used to predict the radio frequency (rf) bandwidth and relative total rf power of chaotic output from a semiconductor laser (SL) with a delayed optical feedback system. By tailoring the linewidth enhancement factor (LEF) and nonlinear gain compression of the SL, improvements of a factor of two in the 80%-of-the-rf-power bandwidth, five in the rf bandwidth above 40 dB, and ten in the integrated rf power of the chaos, compared to values for a commercial-like SL, are predicted to be possible. The SL parameter values to be targeted will require device designers and fabricators to explore ways to decrease relaxation oscillation damping while, preferably, still achieving a high relaxation oscillation frequency. Higher values of LEF are a feature of poorer quality SLs of the past and can be expected to be achieved by reversion to past device recipes.

We perform modeling and dynamic simulations of all-semiconductor photonic crystal surface-emitting lasers (PCSELs). A two-dimensional photonic crystal consists of a GaAs layer with InGaP features, repeating periodically in both lateral directions. In our dynamic simulations, we demonstrate that photonic crystals with large isosceles triangular features, having a base angle close to 71.5°, enable suppression of higher-order modes and achieve single-mode lasing in large-area all-semiconductor PCSELs under moderate and high pump levels.

A general theory for the intrinsic (Lorentzian) linewidth of photonic-crystal surface-emitting lasers (PCSELs) is presented. The effect of spontaneous emission is modeled by a classical Langevin force entering the equation for the slowly varying waves. The solution of the coupled-wave equations, describing the propagation of four basic waves within the plane of the photonic crystal, is expanded in terms of the solutions of the associated spectral problem, i.e. the laser modes. Expressions are given for photon number, rate of spontaneous emission into the laser mode, Petermann factor and effective Henry factor entering the general formula for the linewidth. The theoretical framework is applied to the calculation of the linewidth-power product of air-hole and all-semiconductor PCSELs. For output powers in the Watt range, intrinsic linewidths in the kHz range are obtained in agreement with recent experimental results.

We report on the design of a photonic crystal surface emitting laser (PCSEL) with an all-semiconductor (InGaP/GaAs) photonic crystal suitable for very-large-area emission and high-power operation. Using coupled-wave theory for PCSELs we model infinite- and finite-size cavity PCSELs and show that a photonic crystal unit cell with square lattice periodicity and a rotated and stretched triangular feature is suitable for the realization of PCSELs with very large areas (1 mm<L < 3 mm for a square cavity of size L × L) while maintaining high mode discrimination between the fundamental laser mode and higher order cavity modes as well as high external efficiency. This was achieved by exploiting a single-lattice photonic crystal unit cell design that minimizes one-dimensional coupling in the photonic crystal, providing a promising alternative to double-lattice PCSELs.

A highly developed traveling wave model for a semiconductor laser system supports sophisticated mode analysis of the coherence collapse regime in semiconductor lasers with delayed optical feedback. The concept of instantaneous optical modes is used. Time-frequency representations of chaotic trajectories are constructed and interpreted. This involves synthesizing the calculated optical modes with their corresponding steady states, analysis of the mode driving and coupling sources, and field expansion into modal components. The results support detailed physical interpretation of the optical and radiofrequency spectra throughout the coherence collapse regime. This includes simulation results for the transition out of coherence collapse at high optical feedback. Also key frequencies observed in the radiofrequency spectrum of the chaotic output are linked to the modes that mix to generate them.

We consider a brightness- and power-scalable rectified polarization beam combining scheme for high-power, broad-area edge-emitting semiconductor laser diodes. The coupling of $2^{m}$ emitters is achieved through Lyot-filtered optical reinjection from a specially designed multi-stage external cavity, which forces individual diodes to lase on interleaved frequency combs with overlapping envelopes. Simulations of up to sixteen coupled emitters and analysis of the calculated beams suggest that, under ideal conditions, a beam coupling efficiency of approximately 90% can be expected. Reducing optical losses within the external cavity is crucial for improving this efficiency in experimental systems.

We present algorithms for constructing and resolving spectral problems for novel photonic crystal surface-emitting lasers with large emission areas, given by first-order PDEs with two spatial dimensions. These algorithms include methods to overcome computer-arithmetic-related challenges when dealing with huge and small numbers. We show that the finite difference schemes constructed using relatively coarse numerical meshes enable accurate estimation of several major optical modes, which are essential in practical applications.