Polarization switching in single-mode injection semiconductor laser

Abstract

Based on a previously developed polarization component method, a phenomenological model is constructed for the generation of polarized radiation in single-mode semiconductor lasers and applied to polarization switching. This model gives a good description of the experimentally observed features of the polarization process. At the same time, a number of effects related to the polarization switching rate, hysteresis characteristics, and polarization switching during optical injection are interpreted anew in terms of the formation dynamics of the laser radiation.Introduction. Sudden switching in the polarization state of the output of semiconductor injection lasers has been known for quite some time [1,2] and is usually associated with the attainment of conditions such that the differences in the gain and loss coefficients for two orthogonally polarized modes are equal [3]. In end-on laser diodes these conditions are reached either through use of an external pressure [4] or through a temperature change [3]. In many cases the polarization switching effect is bistable [5,6], so it has come into widespread use in the development of various kinds of devices for optoelectronic systems [7].Further interest in this effect was stimulated by the discovery of a "spontaneous" polarization switching effect in vertical-cavity surface-emitting lasers (VCSEL) [8], which placed significant limitations on the use of these systems in polarization sensitive optoelectronic devices. Earlier approaches to the interpretation of the polarization switching effect (dispersion in the gain coefficient, as well as the anisotropy in the active layer induced by internal stresses) [9] were not adequate for describing the various manifestations of this effect and other mechanisms had to be taken into account (e.g., the thermal lens effect [10]). For this reason, the spin-flip model [11,12], which is based on a generalization of the theory of gas lasers, has come into widespread use; it relates the polarization switching effects to a change in the difference in the populations of the sublevels of the conduction band and heavy holes in the region of a quantum well owing to electron spin relaxation. This model gives a completely natural description of the two types of polarization switching (with increasing and decreasing output frequency) in VCSELs and makes it possible to take into account a whole series of specific mechanisms (for example, the influence of the relation between intrinsic and induced anisotropies [13]).The existence, in general, of two different approaches to interpreting polarization switching in VCSELs has made it necessary to search for some generalizations at the level of a microscopic description (the change in the band structure owing to microscopic stresses, elimination of the degeneracy in the energy levels, variations in effective mass, etc.) [14]; however, the numerical calculations for these models are extremely cumbersome [15,16] and the models themselves are unsuitable for practical applications. On the other hand, a sim...

Mode hopping and carrier density fluctuations in semiconductor lasers

“…Note, however, that this max  value is not strictly defined since the whole transformation process is smooth and has no singularities. Anyway, this value of max  is inversely proportional to the pulse length  , in complete agreement with the model suggested in the studies [7,8], i.e. the formation of polarization at a small orientational anisotropy of the amplification factor is rather slow process (the time of polarization formation is inversely proportional to …”

“…[7,8], the fluctuations in semiconductor lasers may lead to an abrupt (stepwise) change in the polarization state, which is usually referred to as the 'mode hopping'. Besides, development of this effect takes a finite time, in fairly good agreement with the experimental findings [10,11].…”

“…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.…”

“…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].…”