Thomas B. Simpson

Instabilities and chaos in optically injected semiconductor lasers

We have experimentally obtained and theoretically analyzed a systematic map of the various instabilities induced in a semiconductor laser subject to strong optical injection as the amount of optical injection power and frequency detuning is varied. Two distinct islands of chaos have been identified in the injection‐locked region. They are separated by regions of period one and period two solutions. Spontaneous emission noise obscures the observation of high periodic orbits.

Period‐doubling route to chaos in a semiconductor laser subject to optical injection

Experimental measurements and a single‐mode analysis of a quantum‐well laser diode subject to strong optical injection are combined to demonstrate that the diode follows a period‐doubling route to chaos. All laser parameters used in this model, including the influence of spontaneous emission noise, were experimentally determined based on the four‐wave mixing technique. The transition to chaos can be used to reduce the uncertainty in the value of the linewidth enhancement factor.

Extraction characteristics of a dual fiber compound cavity

We study experimentally the time dependence, steady state behavior and spectra of a dual fiber-laser compound cavity. Theoretically we confirm the CW and spectral characteristics. This particular cavity is formed with two Er-doped fiber amplifiers, each terminated with a fiber Bragg grating, and coupled through a 50/50 coupler to a common feedback and output coupling element. The experiment and theory show that a low Q, high gain symmetric compound cavity extracts nearly 4 times the power of a component resonator. This extraction is maintained even when there is significant difference in the optical pathlengths of the two component elements. Further, our measurements and theory show that the longitudinal modes of the coupled cavity are distinct from the modes of the component cavities and that the coherence is formed on a mode-by-mode basis using these coupled-cavity modes. The time behavior of the compound cavity shows slow fluctuations, on the order of seconds, consistent with perturbations in the laboratory environment.

Linewidth Sharpening via Polarization-Rotated Feedback in Optically Injected Semiconductor Laser Oscillators

Combining optical injection and polarization-rotated optical feedback in a semiconductor laser can induce self-referenced periodic output that is widely tunable by simply varying the dc-bias points of the systems master and slave lasers. We observed a feedback-induced reduction of the fundamental period-one oscillation linewidth by more than two orders of magnitude relative to the injection-only case. Performance was found to be negatively affected by the interference between the external injection signal and the residual feedback in the same polarization. The nonlinear dynamics of the optically injected semiconductor laser can be used to minimize sensitivity to fluctuations in the operating points. However, the use of the nonlinear dynamics at high oscillation frequencies is limited by the decreasing strength of the interaction between the circulating intracavity optical field and the carrier density.

Coherent combining of spectrally broadened fiber lasers.

We demonstrate that fiber lasers spectrally broadened by cross mode coupling can be coherently combined with high efficiency. The spectral broadening that it induces suppresses stimulated Brillouin scattering. Using long cavity length lasers, > 800 m, we induce spectral broadening of > 50 GHz and show mode by mode coherence in the output of four intracavity coupled fiber lasers.

Period three limit-cycles in injected semiconductor lasers

Experiments showing period three intensity oscillations are reported for a semiconductor laser subject to external injection. These oscillations appear close to a period doubling bifurcation of the fundamental laser limit-cycle. We show numerically that they correspond to an isolated branch of solutions of the laser rate equations. We also show that the sequence of numerical spectra for the period one, period two and period three limit-cycle regimes compare well with the experimental spectra obtained by progressively increasing the injection rate. Finally, we investigate the period three branch of solutions by using a numerical continuation method that follows both stable and unstable solutions.

Subharmonic transition in an optically injected semiconductor laser: theory and experiments

The equations for a semiconductor laser subject to detuned optical injection are analysed using asymptotic methods. We derive a third-order equation for the phase of the laser field which is then investigated for small injection but arbitrary frequency detuning. The long-time solution is a small amplitude time-periodic solution with a frequency close to the detuning except if the detuning is close to a multiple of the free-running laser relaxation frequency (resonance). We examine the case of subharmonic resonance, injecting at twice the relaxation resonance frequency, in detail. In addition to period-doubling bifurcations, we show the coexistence of bifurcation and isolated branches of solutions. Our approximate results are in good agreement with the numerical bifurcation diagram obtained from the original laser equations. Our analysis motivated a series of new experiments on laser diodes operating in the weak injection but large detuning regime. The experimental spectra show clearly the period-doubling bifurcation as well as the shifting of the slave-laser frequency predicted by our analysis.

Tunable photonic microwave oscillator self-locked by polarization-rotated optical feedback

Combining optical injection and polarization-rotated optical feedback in a semiconductor laser can induce self-referenced periodic output that is widely tunable by simply varying the dc-bias points of the master and slave lasers. We observed feedback-induced reduction of the pulsation peak linewidth by more than two orders of magnitude relative to the injection-only case. The nonlinear dynamics of the optically injected semiconductor laser can be used to minimize sensitivity to fluctuations in the operating points. Performance is negatively affected by interference between the external injection signal and residual feedback in the same polarization.

Tunable Oscillations in Optically Injected Semiconductor Lasers With Reduced Sensitivity to Perturbations

Narrow linewidth optical injection into a semiconductor laser can induce periodic oscillations in the injected lasers output power with a frequency that is widely tunable by simply varying the steady-state bias current and operating temperature. Recently, it has been demonstrated that this oscillation frequency can be made nearly insensitive to small-signal fluctuations of these two parameters at certain operating points [1]. Here, we demonstrate that this insensitivity arises from multiwave mixing and interference that minimizes the response of both the gain medium and the circulating optical power at the oscillation frequency. Both experimental measurements and model calculations of optical spectra show that at the operating points of reduced oscillation frequency sensitivity, all strong components of the optical spectrum still exhibit a response to the perturbations. However, in the power spectra and the (calculated) carrier-density spectra, the response is strongly attenuated. Novel operating points that limit the sensitivity of the laser power oscillation frequency to perturbations offer the promise for improved operation of tunable photonic oscillators for radio- and microwave-frequency applications.

Coherent intracavity coupling of fiber lasers

A schematic of the coupled-laser apparatus that we have employed is shown. Two lasers, each with an independently controlled Er-doped fiber amplifier and a fiber Bragg grating are intracavity coupled using a fiber coupler. We demonstrate that cleaving of the 2 x 2 fiber coupler is not necessary. If cavity feedback is placed after one of the output ports of the coupler, the fiber lasers will self-organize their optical output to minimize the power that is coupled to the other output port. By doing so, the fiber lasers maximize the circulating optical field within the laser cavity and minimize the inversion of the gain medium.