Influence of changing the forbidden-band gap on radiation frequency sweeping of quantum-well heterolasers

Abstract

The principal features of changing (sweeping) the "instantaneous" emission frequency of quantum-well heterolasers as functions of the modulation frequency and depth and the constant component of the pump current when tuning the lasing frequency within the gain band are established using numerical modeling. The active medium is described in the framework of a two-band model with similar distributions of levels in subbands of electrons and holes assuming transitions between ground subbands with no k-selection rule. Emission frequency sweeping occurs because of changes in both the index of refraction of the active medium due to a variation in the concentration of non-equilibrium charge carriers and the forbidden-band gap. Emission frequency sweeping does not occur at low frequencies of current modulation that correspond with quasi-stationary lasing regimes. The modulation depth of the output radiation and, therefore, the emission frequency sweeping, also approach zero for the other limiting case of relatively high current modulation frequencies.The sweeping is greatest at intermediate current modulation frequencies. The sweeping value is approximately halved in certain instances by taking into account the change of forbidden-band gap of the semiconductor. In general, the sweeping value is determined by the combined effect of the modulation frequency and depth, the constant component of the pump current, and the spectral position of the laser emission frequency within the gain band.Introduction. Semiconducting lasers with narrow and stable spectral emission lines are required for fiber optic communications, high-resolution spectroscopy, measurement technology, etc. Therefore, so-called dynamic single-frequency (SF) semiconducting lasers that operate stably in single-logitudinal generation mode with high-frequency pump current modulation have been developed [1]. However, the "instantaneous" emission frequency can change even in this regime with current modulation (so-called sweep, chirp, dynamic shift, etc.) [2]. This is explained by a change of the index of refraction n of the active medium and, therefore, the instantaneous longitudinal frequencies of resonator modes and the generated radiation.Studies of the features of emission frequency sweeping are of definite practical interest. For example, certain features of the change of emission frequency in vertical cavity surface-emitting lasers [3,4] and of actively and passively mode-locked semiconducting lasers [5] have been examined. The effect of frequency chirp on the dynamics of recirculation in a closed contour and on an injection laser in high-velocity code modulation has been studied [6,7]. Features of frequency sweeping of emission from heterojunction quantum lasers with pump current modulation have been studied for certain special cases in recently published papers [8,9].Herein we report a systematic study of the features of quantum-well heterolaser sweeping as a function of modulation depth and frequency and the value of the constant component of pump current...