Athanasios Gavrielides

Small-signal analysis of modulation characteristics in a semiconductor laser subject to strong optical injection

Injection locking of a semiconductor laser can induce major changes in the modulation characteristics of the laser. A small-signal analysis using the lumped element model shows that both the frequency and damping of the characteristic resonances of the coupled complex field and free carriers (gain medium) are modified. The detuning between the injected field and the free-running oscillating field, the amplitude of the injection field relative to the free-running field, the linewidth enhancement factor, the cavity photon and spontaneous carrier decay rates, and the field enhancement of the decay rate are all key parameters in determining the changes to the modulation characteristics. For a broad range of parameters, there is simultaneous enhancement of the modulation bandwidth and stable, locked operation. The enhancement is a cavity phenomena and does not occur in a traveling wave amplifier. It requires that the frequency of the locking field be detuned from the injection-modified frequency of the cavity resonance. This causes a resonant enhancement of the modulation sideband associated with the preferred frequency of the optical cavity. Bandwidth enhancements beyond the free-running laser limit are possible over a range of injection levels and injection frequency detunings.

Dynamics of Semiconductor Lasers Subject to Delayed Optical Feedback

We give experimental and numerical evidence for a new dynamical regime in the operation of semiconductor lasers subject to delayed optical feedback occurring for short delay times. This short cavity regime is dominated by a striking dynamical phenomenon: regular pulse packages forming a robust low-frequency state with underlying fast, regular intensity pulsations. We demonstrate that these regular pulse packages correspond to trajectories moving on global orbits comprising several destabilized fixed points within the complicated phase space structure of this delay system.

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.

SBS threshold for single mode and multimode GRIN fibers in an all fiber configuration

We investigate stimulated Brillouin scattering (SBS) threshold in single mode and multimode fibers in an all fiber network. The pump is a single mode fiber pigtail attached to a diode. We find the theory and experiment agree for both single mode and multimode GRIN fibers. We modify the bulk SBS threshold equation for use with fibers by properly accounting for mode sizes and modal dispersion.

Analytical stability boundaries for a semiconductor laser subject to optical injection

Abstract We determine analytically the stability boundaries of a semiconductor laser subject to external optical injection. Specifically, we derive the exact conditions for a steady state limit point and a Hopf bifurcation point in terms of the injection strength and the frequency detuning. These conditions are formulated in parametric form and are appropriate for asymptotic approximations. We investigate the limit of a large ratio of the carrier and photon lifetimes and we discuss the resulting expressions in terms of the linewidth enhancement factor. For negative detuning, the unstable domain is bounded by two Hopf bifurcation points which coincide at a critical negative value of the detuning. For positive detunings, all steady state solutions are unstable. Useful scaling laws characterizing different parts of the stability diagram are derived.

Stable polarization self-modulation in vertical-cavity surface-emitting lasers

The characteristics of polarization self-modulation in a vertical-cavity surface-emitting laser (VCSEL) were studied for frequencies up to ≈9 GHz both experimentally and theoretically. Polarization self-modulation was obtained by rotating the linearly polarized output of the VCSEL by 90° and reinjecting it into the laser. Experimentally we simultaneously recorded time traces, optical and radio-frequency spectra. We found for increasing modulation frequencies that the output characteristics changed from square-wave to sinusoidal and the VCSEL system assumed new polarization eigenstates that are different from the free-running VCSEL eigenstates. We modeled polarization self-modulation as an interband process and found a good qualitative agreement between our experimental and numerical results.

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.

Coexisting periodic attractors in injection-locked diode lasers

We present experimental evidence of coexisting periodic attractors in a semiconductor laser subject to external optical injection. The coexisting attractors appear after the semiconductor laser has undergone a Hopf bifurcation from the locked steady state. We consider the single-mode rate equations and derive a third-order differential equation for the phase of the laser field. We then analyse the bifurcation diagram of the time-periodic states in terms of the frequency detuning and the injection rate and show the existence of multiple periodic attractors.

Square-wave self-modulation in diode lasers with polarization-rotated optical feedback.

The square-wave response of edge-emitting diode lasers subject to a delayed polarization-rotated optical feedback is studied in detail. Specifically, the polarization state of the feedback is rotated such that the natural laser mode is coupled into the orthogonal, unsupported mode. Square-wave self-modulated polarization intensities oscillating in antiphase are observed experimentally. We find numerically that these oscillations naturally appear for a broad range of values of parameters, provided that the feedback is sufficiently strong and the differential losses in the normally unsupported polarization mode are small. We then investigate the laser equations analytically and find that the square-wave oscillations are the result of a bifurcation phenomenon.