We demonstrate a novel method of polarization control that combines rotatable waveplates (angle control) and variable retarders (retardance control). Such a “hybrid” polarization controller performs far better than conventional controllers, allowing nearly perfect arbitrary-to-arbitrary polarization transformations. We show theoretically that the two control parameters augment one another because they tend to result in orthogonal movements on the Poincaré sphere.
This paper presents the dynamics of a semiconductor laser subject to instantaneous phase-conjugate feedback. Recently, the behavior of such a laser have been explored by means of bifurcation diagrams. However, the exact nature of the involved dynamics and bifurcations remained unclear. Here we present a detailed study of the changes of the dynamics as the feedback strength is varied.
Most of the previous treatments of semiconductor lasers subject to optical feedback from a phase-conjugate mirror (PCM) have assumed the PCM responds instantaneously. Furthermore, the mechanism responsible for phase conjugation does not usually enter into the analysis. In this paper are derived the time-dependent reflectivity from a PCM created through non-degenerate four-wave mixing. The resulting laser dynamics are compared to the case of the ideal PCM, as a function of PCM mirror interaction depth, distance to the PCM, and laser current. The time-responsive PCM tends to suppress otherwise chaotic output and produces power pulses whose frequency is tunable by varying laser current or PCM reflectivity.
Phase-conjugate feedback affects a laser in ways which are fundamentally different than conventional feedback. Notably, when the laser oscillates in more than one longitudinal mode, phase-conjugate feedback initiates a novel mode-coupling mechanism which can even lead to mode locking behavior. This paper explores these mode-coupling effects and also summarizes the simularities and differences between the two types of feedback.
In this paper we describe some of the effects of external optical feedback (OFB) on semiconductor lasers by simulation of the stochastic rate equations. Particular attention is paid to the laser's transition to optical chaos. In addition, we describe three techniques for avoiding this chaotic regime. The technique of high frequency injection, used in optical recording, can delay the onset of chaos till very high values of OFB. Experimental results are given and are in excellent agreement with the theory. A second technique called occasional proportional feedback can be used with some success to stabilize the chaotic output of semiconductor lasers. The final technique for controlling chaos consists of the optimization of various system and laser parameters so that the laser is least susceptible to OFB.
The usefulness of semiconductor lasers can be greatly limited when the laser is subjected to uncontrolled optical feedback (OFB). In particular, the laser intensity noise can be severely degraded when OFB is greater than 0.1%. Although the technique of high-frequency injection (HFI) can solve this problem, the proper modulation frequency and depth must be chosen empirically. We investigate this problem through cornputer simulations of the multimode stochastic rate equations, modified to include OFB and HFI. By providing the program with measurable laser and system parameters, the simulations predict the HFI modulation frequency and depth that optimize the laser behavior. The results of the simulations are compared with experiment, and good agreement is obtained.
This paper reviews the importance of nonlinear gain and its impact on the performance of semiconductor lasers. The physical mechanisms which can lead to an intensity dependence of the optical gain in the above-threshold regime are described briefly. A specific nonlinear-gain mechanism, referred to as intraband gain saturation, is discussed in detail by considering its effect on the important laser characteristics such as the modulation bandwidth, intensity noise, and the laser linewidth. Particular attention is paid to the effects of cross saturation in nearly single-mode semiconductor lasers. Even a weak side mode can lead to saturation and rebroadening of the main-mode linewidth due to mode coupling induced by the nonlinear gain.