Mirvais Yousefi

Dynamical behavior of a semiconductor laser with filtered external optical feedback

We report on a theoretical analysis of the dynamical performance of a semiconductor laser under the influence of delayed weak filtered external optical feedback. The filter widths considered range from 1 to 100 GHz. The analysis concentrates on the well known low-frequency fluctuations (LFFs) regime, in which LFFs occur in the absence of filtering. As expected, filtering the feedback light stabilizes the system in general. LFF can already be suppressed for moderately broad filters (25-50 GHz). In that case, the system was found to operate on the maximum gain mode with a small amplitude limit cycle. We show how the filtering can, in principle, be used for targeting the laser on the maximum gain mode.

Experimental and theoretical study of filtered optical feedback in a semiconductor laser

We report on the systematical investigation of the steady-state regime and the dynamical behavior of a semiconductor laser subject to delayed filtered optical feedback. We study a Fabry-Perot (FP) interferometer type of filter placed in the external feedback loop of a diode laser. The effects of the filter on the locking of the diode laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled laser and FP cavity system. We also present an experimental stability diagram that summarizes the dynamical behavior of the system.

Experimental and theoretical study of semiconductor laser dynamics due to filtered optical feedback

We report experimental results on the nonlinear dynamical response of a semiconductor laser subjected to time-delayed (>5 ns), frequency selective, optical feedback from a Fabry-Pe/spl acute/rot interferometer type of filter. Three regimes of interest, based on the relative value of the filter bandwidth with respect to the relevant laser parameters (relaxation oscillation frequency and external cavity mode spacing), are identified, viz. a wide filter case, an intermediate filter width case, and a narrow filter case. The dynamical response of the laser is shown to be quite different in each of these regimes. The principal results are 1) the lasers linewidth enhancement factor, coupled with the nonlinear response of the filter, can be exploited to induce nonlinear dynamics in the instantaneous optical frequency of the laser light on a time scale related to the time-delay of the feedback, 2) a mode mismatch effect which arises from a detuning between the filter center frequency and the nearest external cavity mode and manifests itself in a reduction of the maximum light available for feedback, and 3) a reduction in, or even disappearance of, relaxation oscillations in the laser dynamics when a filter of appropriate width is chosen. More generally, it is observed that certain dynamics that occur due to unfiltered optical feedback may be suppressed when the feedback light is spectrally filtered.

Characterization of Integrated Optical Strain Sensors Based on Silicon Waveguides

Microscale strain gauges are widely used in micro electro-mechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. We propose all-optical strain sensors based on micro-ring resonators to be integrated with MEMS. We characterized the strain-induced shift of the resonances of such devices. Depending on the width of the waveguide and the orientation of the silicon crystal, the linear wavelength shift per applied strain varies between 0.5 and 0.75 pm/microstrain for infrared light around 1550 nm wavelength. The influence of the increasing ring circumference is about three times larger than the influence of the change in waveguide effective index, and the two effects oppose each other. The strong dispersion in 220 nm high silicon sub-wavelength waveguides accounts for a decrease in sensitivity of a factor 2.2 to 1.4 for waveguide widths of 310 nm to 860 nm. These figures and insights are necessary for the design of strain sensors based on silicon waveguides.

Rate-equation model for multi-mode semiconductor lasers with spatial hole burning

We present a set of rate equations for the modal amplitudes and carrier-inversion moments that describe the deterministic multi-mode dynamics of a semiconductor laser due to spatial hole burning. Mutual interactions among the lasing modes, induced by high- frequency modulations of the carrier distribution, are included by carrier-inversion moments for which rate equations are given as well. We derive the Bogatov effect of asymmetric gain suppression in semiconductor lasers and illustrate the potential of the model for a two and three-mode laser by numerical and analytical methods.

Characterization of a photonic strain sensor in silicon-on-insulator technology

Recently there has been growing interest in sensing by means of optical microring resonators in photonic integrated circuits that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et al. [Proc. SPIE 6619, 661914 (2007)] proposed the use of a silicon-waveguide-based ring resonator as a strain gauge. However, the strong lateral confinement of the light in SOI waveguides and its corresponding modal dispersion where not taken into account. We present a theoretical understanding, as well as experimental results, of strain applied on waveguide-based microresonators, and find that the following effects play important roles: elongation of the racetrack length, modal dispersion of the waveguide, and the strain-induced change in effective refractive index.

Carrier inversion noise has important influence on the dynamics of a semiconductor laser

We find that, although inversion noise has only a marginal effect on the linewidth of a semiconductor laser in continuous wave operation, in the presence of dynamics, it may play an important role in determining the final dynamical state. It is, therefore, essential to include realistic carrier noise when analysing semiconductor laser dynamics.

Rate equations model for semiconductor lasers with multilongitudinal mode competition and gain dynamics

A novel multilongitudinal-mode rate-equations description of the semiconductor laser is presented. The model includes gain interactions among the longitudinal modes due to e.g., spatial hole burning. The parameters have been obtained from a real device, in order to be able to compare with the simulations. The results are in good qualitative agreement with the measurements.

Low-frequency fluctuations in vertical-cavity surface-emitting lasers with polarization selective feedback: experiment and theory

We present theory and measurements on a vertical-cavity surface-emitting laser (VCSEL) biased close to its solitary threshold and subject to polarization selective external optical feedback. The system shows low-frequency fluctuations (LFF) in the selected polarization mode (PM). Below solitary laser threshold, the orthogonal PM remains silent, while it only responds after a dropout event in the main mode above threshold. Our calculations show good agreement with the measurements and identify a type of synchronization between the low-frequency dynamics of the two PMs.

Role of longitudinal mode dynamics in laterally coupled lasers

Our report focuses on the strong kink found in the P-I output of an asymmetric twin stripe semiconductor laser. A multi longitudinal mode model is used to describe the system. The model allows for asymmetric coupling of the two lasers and also accounts for multi longitudinal effects within and in-between the lasers.