Influence of the injection current sweep rate on the polarization switching of vertical-cavity surface-emitting lasers

Abstract

We study the polarization switching of vertical-cavity surface-emitting lasers based on simulations of the spin-flip model. We show that the switching point depends on the ramp signal used to scan the injected current. Fast current ramps enlarge the hysteresis region since the switching point moves to high pump values for increasing injection and to low pump values for decreasing injection. The delay of the bifurcation follows a power law with the slope of the current ramp. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2160711͔Vertical-cavity surface-emitting lasers ͑VCSELs͒ are emerging as a compact source for high-bit-rate data transmission because they have many advantages over conventional, edge-emitting lasers. A drawback of VCSELs is a polarization instability that arises because the orientation of the polarization of the emitted light is not fixed by geometrical constraints. Due to residual anisotropies ͑that break the circular transverse symmetry͒ the output of a VCSEL is linearly polarized along one of two orthogonal directions. When the VCSEL begins to lase one linear polarization dominates, and when the injection current is increased in many devices it is observed that the emission switches to the orthogonal linear polarization.1 This polarization switching ͑PS͒ phenomenon can be detrimental for polarization-sensitive applications and has received a lot of attention. 2-8Usually PS is studied by varying periodically the injected current applied to the VCSEL. The static intensitycurrent response ͑SI-CR͒ is usually taken as a reference to predict the response of the device. Studies in nonlinear optics, of the optical transistor, for instance, have shown that this use of the static response is valid only at low modulation frequency of the injected current.9 At high modulation frequency, the system cannot follow the injected signal modulation any more and the SI-CR becomes irrelevant. Modulation frequencies are high or low compared to the longest decay rate of the VCSEL. In principle, this is given by the damping rate of the relaxation oscillation frequency ͑ROF͒. However, if the device is operated close to a bifurcation point, a critical slowing down is induced which leads to a new and much longer time scale. 10 In this case, the critical frequency separating high and low frequencies is redshifted.As a result of critical slowing down, a modulation frequency much lower than the ROF may have to be treated as a fast modulation frequency if the device is operated sufficiently close to a bifurcation. This calls for a careful dynamical analysis of a VCSEL response to a time-varying injected current. The PS is an example of a parameter swept across a bifurcation. Let us consider a system that has a control parameter and a ͑static͒ bifurcation point c such that one state is stable if Ͻ c and another state is stable if Ͼ c . When the control parameter is varied continuously in time from i Ͻ c to f Ͼ c the bifurcation point is shifted from c . In optics a well-known example is the turn-on of a laser, which ...

Low-Frequency Modulation Effects on the Polarization Dynamics of Vertical-Cavity Surface-Emitting Lasers Subject to Optical Feedback

“…The freerunning VCSEL shows a single polarization switching (PS) with hysteresis. In accordance with the predictions of [1], the width of the PS hysteresis varies with the modulation frequency. At threshold the emission polarization is linear and defines the X-polarization direction; the Y-polarization is in the orthogonal direction.…”

Dynamical Hysteresis and Thermal Effects in Vertical-Cavity Surface-Emitting Lasers

“…In this model, the hysteresis in the turn-on and turn-off points is due to the interplay of the variation of the bias current (that results in dynamical hysteresis [18]) and the variation of the active region temperature, that is an additional source of hysteresis, because the temperature is a dynamical variable with its own time scale. To illustrate the interplay of these two time-scales (bias current variation and temperature variation), in Figs.…”

Polarization Dynamics of Current-Modulated Vertical-Cavity Surface-Emitting Lasers

“…The discrepancy arises due to the fact that when the injection current increases in time (in Fig. 1 it grows from 0.8 to 1.8 in 200 ns) the PS does not take place at the "static" bifurcation point (defined for constant in time) but occurs at a higher value, as was discussed in detail in [38], [39].…”

“…This type of PS, that is from the low frequency (high modal gain) to the high frequency (low modal gain) polarization, has been referred to in the literature as type II [12], and has been explained, in the context of the SFM model, in terms of the interplay of birefringence, saturable dispersion, and spin-flip processes [4]. The particular value of injection current at which the PS occurs depends on the diffusion coefficient [37] and on the injection current sweep rate [38], [39]. Fig.…”