Dynamics of polarization switching in single-mode injection semiconductor lasers

Abstract

UDC 621.373.8 Various dynamic effects during polarization switching in single-mode semiconductor lasers are examined on the basis of a previously developed approach. This model is found to provide a good description of a wide range of experimentally observed phenomena. At the same time, it offers a simple and intuitive interpretation of the nature and character of polarization switching effects that is related to the formation dynamics of the laser radiation. Introduction.Although the sudden switching of the polarization state of the output from semiconductor injection lasers is an effect that has been known for quite a long time [1,2], only recently have the dynamical aspects of this phenomenon been studied in the case of vertical-cavity surface-emitting lasers (VCSEL) [3][4][5][6]. This interest originates in the possible use of VCSEL in optoelectronic systems, while "spontaneous" polarization switching is undesirable in polarization sensitive systems.Dynamic effects in VCSEL are usually studied in terms of a spin-flip model (SFM) [7,8] which contains two coupled spin subsystems that generate circularly polarized orthogonal components. Linearly polarized modes are formed by matching the phases of these components as a consequence of stability conditions [7]. However, the fluctuations in the output radiation increase rapidly in the polarization switching region, and the phase can no longer be regarded as an adequately defined quantity. Thus, in the polarization switching region, partially polarized dynamic states [9] (an incoherent mix of different polarizations) are observed. These things cast doubt on the validity of the SFM for the dynamics of polarization switching in semiconductor injection lasers.In an earlier paper [10] we studied polarization switching effects in a cw single-mode semiconductor injection laser. The polarization component method (PCM) [11] made it possible to relate all the basic features of polarization switching to the formation dynamics for polarized radiation in the active layer of a laser diode. This paper is an analysis of polarization switching dynamics in single-mode semiconductor lasers based on a model using the polarization component method.Theoretical Model. The basis of the theoretical model is the PCM, in terms of which a plane wave field E(z, t) propagating along the z axis is represented in the form of a superposition of polarization components:

Mode hopping and carrier density fluctuations in semiconductor lasers

“…Despite the fact that in the work [12] has demonstrated that these processes have a minor effect on the dynamics of polarization formation, it would be better to exclude this factor. Second, the appearance of the plateau results in a monotonic (practically linear -see the results [8]) polarization variation during the pulse, thus making it possible to control, in the simplest fashion, the polarization characteristics of the output radiation. A polarization degree for the output radiation is defined in a standard way as …”

“…A new phenomenological approach [7,8] has recently been suggested for describing the PS. In the framework of this approach, the process of formation of polarized radiation in a semiconductor laser is governed by orientational anisotropy of the amplification and/or loss factors.…”

“…In this case the PS represents a transition from one linear polarization to the orthogonal one through a sequence of partially polarized states inside the region of orientational distribution for the difference in the amplification and loss factors, and the distribution is close to the isotropic one. Such an approach enables describing qualitatively the main specific features of the PS and finding a fairly simple interpretation, including the conditions for external injection of the polarized optical radiation [8].…”

“…Notice that, within the model [7,8], the polarization instability region is determined by a relatively small orientational anisotropy of the difference in the amplification and loss factors rather than by a competition of the two stable polarization states ('polarization modes'). Then the transformation of polarization of the output radiation proceeds at a much slower rate than the formation of its intensity.…”

“…may result in an abrupt change of polarization of the output radiation, thus leading to the effect referred to as a 'mode hopping'. Because of this, in this work we consider a possibility for realization of this effect on the basis of our model [7,8].…”