Influence of radiation fluctuations on the magnitude of optoelectronic signal in semiconductor lasers with external optical feedback

Abstract

621.378.35Numerical modeling of dynamical processes in semiconductor lasers with external delayed optical feedback has been performed on the basis of the rate equations taking into account the quantum fluctuations of radiative transition rates. It has been shown that the nature of the optoelectronic signal as a function of pump current is determined by the magnitude of the optoelectronic coupling and corresponds to the behavior of the autocorrelation function of the radiation. It has been found that the radiation coherence decreases substantially with strong optical feedback. Experimental measurements confirming the theoretical results have been performed.Introduction. Phenomena responsible for external optical feedback (OFB) in lasers have recently been extensively studied. Semiconducting lasers are especially sensitive to OFB because of the low resonator gain. OFB strongly affects their threshold, power, spectral, noise, coherence, and other characteristics. The high sensitivity of semiconducting lasers to back reflected radiation can be used to study the internal parameters of the laser active region and to solve applied problems, in particular, to construct a new generation of sensors of various physical quantities (microdisplacements, microvibrations and rates of reflecting objects, their surface conditions) and to create multistable optical logical elements, etc. [1,2].The principal methods of studying the reaction of a laser to external OFB involve recording current changes through the laser, potentials at the p-n-junction, and the laser output power upon adding an external reflector. The first two methods, in contrast with the third, do not require the use of an additional photodetector. The most convenient method is recording potential changes at a given current through the laser because the laser is usually fed by a current generator. The laser fulfills two functions as a source and receiver of its own radiation. The potential difference at the p-n-junction during laser generation on its own mirrors and with an external reflector is called the optoelectronic signal (OES). The absolute value of the OES can reach tens [1] and sometimes hundreds [3] of millivolts.The OES as a function of pump current can take various shapes [1,4]. The OES usually reaches a maximum near the lasing threshold. Then, the OES either stabilizes or decreases smoothly or erratically as the pump current increases. Sometimes the OES increases smoothly with increasing pump current. The decrease of OES was explained [4] by the photoconductivity and leakage of current. The erratic character of the function was related to multimode lasing. Modern laser heterostructures have a high electronic limitation. The coefficient of carrier injection into the active layer is close to unity. Therefore, the effect of current leakage on the OES in them is practically avoided.It was hypothesized [1] that unstable lasing in the external resonator can influence the laser electrical response. Modes for which the phase advance in the external resonator is...