P.J.R. Laybourn

Operating regimes of GaAs-AlGaAs semiconductor ring lasers: experiment and model

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.

Unidirectional bistability in semiconductor waveguide ring lasers

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.

Alternate oscillations in semiconductor ring lasers

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.

An integrated fluorescence array as a platform for lab-on-a-chip technology using multimode interference splitters

We present the design and fabrication of 1-to-N multimode interference (MMI) splitters, suitable for use in integrated optical fluorescence array sensing, with particular applications in lab-on-a-chip (micro-TAS) technologies. Electron beam irradiation of germanium-doped flame hydrolysis deposited silica was used to define the MMI waveguide regions. The splitters were integrated with microfluidic channels to form direct-excitation fluorescence sensor chips for use at visible wavelengths. Characterization of the waveguides shows that predictable splitting ratios can be achieved. Two devices are presented: a 1/spl times/2 splitter integrated with one analytical chamber and a 1/spl times/4 array device for multipoint excitation. A photomultiplier tube was used to assess the analytical performance of the chip, in response to standard aliquots of fluorophore (31 nM to 1.25 /spl mu/M).

Integrating optics and microfluidics for time-correlated single-photon counting in lab-on-a-chip devices

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.

Fiber-to-waveguide coupling using ion-milled grooves in lithium niobate at 1.3-μm wavelength

A new method for fiber-Ti:LiNbO(3) waveguide coupling is described. Ion-milled grooves in LiNbO(3) permit repeatable, rigid location of fibers that have been chemically etched and then polished. The constituent loss mechanisms are analyzed and waveguide parameters optimized to minimize insertion loss resulting from modal mismatch by using experimental data on the optical fields. A total insertion loss of -3.1 dB has been measured for the coupler.

Control of unidirectional operation in semiconductor ring lasers

We have measured unidirectional laser operation in a semiconductor ring laser, controlled through the biasing of a coupled waveguide cavity. The devices fabricated consist of a circular ridge waveguide 2 /spl mu/m wide formed on InGaAs/InGaAsP and GaAs/AlGaAs material systems. The direction of the dominant mode switches as the laser current is increased, and can also be switched by altering the bias of the coupled guide. The subordinate mode is completely suppressed, spectral measurements indicating that any output is purely spontaneous emission.

Towards optical fluorescence sensing using multimode interference splitters

One to N MMI splitters suitable for use in optical sensors have been fabricated using electron beam irradiation of germanium doped flame hydrolysis deposited (FHD) silica (a technique that we have previously used for producing waveguides) (Garcia-Blanco et al., 2001). The splitters have been integrated with microfluidic chambers to form a direct-excitation fluorescence sensor chip for use at visible wavelengths.

Coupled-mode equations for monolithic semiconductor ring lasers: numerical simulations and experimental results

This work performs numerical and experimental studies of coupled mode dynamics for monolithic semiconductor ring lasers. This study presents numerical analysis and experimental results of the different operating regimes, namely: bidirectional CW, bidirectional with harmonic oscillations of the mode intensities, and unidirectional.

Operating regimes and optical bistability in InGaAs/InGaAsP and GaAs/AlGaAs ring lasers

It has recently been found that the coexistence of the two counterpropagating modes in the laser cavity may affect the optical stability and lead to a large variety of operating regimes. In this work we report on the ring cavity mode behaviour in ridge-waveguide semiconductor lasers fabricated on GaAs/AlGaAs and InGaAs/InGaAsP material systems. Investigation on the unidirectional operating regime has shown a drastic improvement in the device performance and frequency stability with respect to bidirectional operation. In addition, we found that a semiconductor ring laser (SRL) operating in the unidirectional regime behaves as an optical bistable device between the two counterpropagating modes.