Theory and experiments of single-mode ridge waveguide GaAs-AlGaAs semiconductor ring lasers are presented. The lasers are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Bidirectional operation reveals that just above threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave. As the injected current is increased, a new regime appears where the intensities of the two counterpropagating modes undergo alternate sinusoidal oscillations with frequency in the tens of megahertz range. The regime with alternate oscillations was previously observed in ring lasers of the gas and dye type, and it is here reported and investigated in semiconductor ring lasers. A theoretical model based on a mean field approach for the two counterpropagating modes is proposed to study the semiconductor ring laser dynamics. Numerical results are in agreement with the regime sequence experimentally observed when the injected current is increased (i.e., bidirectional continuous-wave, bidirectional with alternate oscillations, unidirectional). The boundaries of the different regimes are studied as a function of the relevant parameters, which turn out to be the pump current and the conservative and dissipative scattering coefficients, responsible for an explicit linear coupling between the two counterpropagating field modes. By a fitting procedure, we obtain good numerical agreement between experiment and theory, and also an estimation for the otherwise unknown scattering parameters.

We report on fabrication and characterization of single-longitudinal-and transverse-mode semiconductor ring lasers. A bifurcation from bidirectional stable operation to a regime with alternate oscillations of the counterpropagating modes was observed experimentally and is theoretically explained through a two-mode model. Analytical expressions for the onset and the frequency of the oscillations are derived, and L I curves numerically evaluated. Good quantitative agreement between theory and measurements made over a large number of tested devices is obtained.

Large-diameter ridge-guided semiconductor lasers weakly coupled to a straight output waveguide show unidirectional operation and directional bistability at currents up to about twice the threshold. The direction of lasing in the ring may be controlled by biasing contacts at either end of the coupled guide.

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The authors describe the integration of low-loss optical waveguides with lab-on-a-chip structures to produce an integrated optical-microfluidic platform for time-correlated single-photon counting of fluorescent molecules. Waveguides were fabricated using electron beam densification of planar silica on silicon, eliminating any requirement for depositing upper cladding silica layers. Microfluidic channels were dry etched directly through the waveguides and the device was sealed using a poly(dimethylsiloxane) gasket. Time-resolved fluorescence lifetime measurements of the fluorophore nile blue were used as a model system to demonstrate the operation of the microfluidic device, with dye concentrations as low as 1.5nM (equivalent to <6000molecules) being measured.