Solitary Threshold Research Articles

Self-mixing interferometry with a laser diode: experimental considerations for sensing applications

An investigation of the effects of external reflectance and pump current on the steady state current threshold and the output power of a laser diode are presented. Back-coupled external reflectance is achieved by placing a mirror in front of the laser diode cavity. Furthermore, by using a rotating wheel, the effect of external reflectance, pump current and combination of individual contributions to frequency shifting on the laser intensity power spectrum are explored. Feedback power ratios below 0.05 and pump currents up to 2Isth (Isth, solitary threshold current) are used in these experiments. The power emitted by the two laser facets are measured through an internal (built-in package) and an external photodiode; both observations are compared. Surprisingly, when evaluating the power–current (P–I) curve for the feedback and solitary cases, a cross over is noticed at ≈1.54Isth only through the external photodiode. Nonetheless, measurements obtained with the internal photodiode agree with the theoretical steady state analysis. Moreover, it is proved that a linear combination of the individual frequency shifts holds in the power spectrum; in addition, a modulation of the carrier density by frequency-shifted optical feedback is also observed. The results from these experiments provide a practical background that ensures a correct interpretation of measurements when using this system for sensing applications.

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.

Regular dynamics of low-frequency fluctuations in external cavity semiconductor lasers.

It is commonly believed that the dynamics responsible for low-frequency fluctuations (LFF's) in external cavity semiconductor lasers is stochastic or chaotic. A common approach to address the origin of LFF's is to investigate the dynamical behavior of, and the interaction among, various external cavity modes in the Lang-Kobayashi (LK) paradigm. In this paper, we propose a framework for understanding of the LFFs based on a different set of fundamental solutions of the LK equations, which are periodic or quasiperiodic, and which are characterized by a sequence of time-locked pulses with slowly varying magnitude. We present numerical evidence and heuristic arguments, indicating that the dynamics of LFF's emerges as a result of quasiperiodic bifurcations from these solutions as the pumping current increases. Regular periodic solutions can actually be observed when (1) the feedback level is moderate, (2) pumping current is below solitary threshold, and (3) the linewidth enhancement factor is relatively large.

Turn‐on transient dynamics of a semiconductor laser with optical feedback

AbstractWe study the effect of lateral carrier diffusion on the transient turn‐on of a semiconductor laser with optical feedback. The numerical simulations are based on the Lang–Kobayashi model, and on an extension of the model that includes lateral profiles for the carrier density and the optical field. For moderately strong feedback, intensity pulses characterize the dynamics of the turn‐on, with a repetition rate at the external cavity round‐trip time. We find that carrier diffusion has an important effect when the laser is biased close to the solitary threshold. For low carrier diffusion the intensity drops to a nearly zero value in between the pulses; for fast carrier diffusion, the spatial holes in the carrier profile burned by the intensity pulses are quickly replenished, and the intensity does not decay to a zero value in between the pulses. The results of our analysis show that the effect of diffusion is qualitatively well modelled by a non‐linear gain saturation coefficient different from zero in the space‐independent Lang–Kobayashi model. Copyright © 2001 John Wiley & Sons, Ltd.

How two sites in the rat's nucleus raphe magnus interact to inhibit the tail-flick reflex

Two stimulating microelectrodes were inserted 0.5 mm apart in the rat's nucleus raphe magnus (NRM), giving a joint electrical threshold for suppression of the heat-evoked tail-flick reflex. Synchronous stimulation often required more current than predicted by consideration of each site's solitary threshold. Asynchronous stimulation required yet more current. We postulate that some NRM cells, perhaps corresponding to the ‘on-cells’ found previously by microelectrode recording, facilitate flexion and become relatively more influential at higher stimulus currents. We also postulate that the dominating cells that suppress flexion, possibly ‘off-cells’, operate optimally when firing simultaneously.

Optoelectronic properties of semiconductor lasers butt-coupled to optical fibers

The authors have theoretically and experimentally examined the electrical noise properties of a semiconductor laser coupled to a single-mode fiber with a plane endface. It is found that the performance of semiconductor lasers butt-coupled to optical fibers critically depends on the axial position of the fiber. Measurements show that the influence of the fiber may change the output power as much as +or-40% and the modulation bandwidth, determined by the relaxation frequency by as much as +or-35% (for a bias 15% above the solitary threshold). Moreover, the pulse response and the longitudinal mode spectrum deteriorate if the laser-to-fiber distance departs from its optimal value. The authors propose a nonoptical scheme for maintaining the optimal laser-to-fiber distance so as to avoid these degrading effects. >