Major features of frequency sweeping of emission from heterojunction quantum lasers

Abstract

We have used numerical modeling to establish the major features of the variation (sweeping) of the "instantaneous" laser frequency of heterojunction quantum lasers as a function of the pump current modulation frequency and tuning of the lasing frequency within the gain band. The active medium is described within a two-band model with identical distribution of levels for the electron and hole subbands, assuming transitions with no selection rule between the ground-state subbands. Sweeping of the laser frequency occurs due to variation of the refractive index of the active medium, as a result of the variation in the concentration of nonequilibrium charge carriers. Laser frequency sweeping does not occur for low current modulation frequencies, corresponding to realization of the quasisteady-state lasing regime. In the other limiting case of relatively high current modulation frequencies, the modulation depth of the laser output also tends toward zero. The magnitude of the sweep is greatest in the intermediate current modulation frequency region. For a specified current modulation frequency, the dynamic shift of the laser mode depends on the position of the lasing frequency within the gain band.Key words: heterojunction quantum laser, pump current modulation, dynamic shift of the laser frequency (sweeping).Introduction. Using semiconductor lasers in fiber optics communications, metrology, data recording, and highresolution spectroscopy systems etc. requires obtaining emission with a narrow spectral composition. Accordingly, "dynamic single frequency" (DSF) semiconductor lasers have been developed, with practically stable operation in a lasing regime with a single longitudinal mode under conditions of high-frequency modulation of the pump current [1].We know (see, for example, [2] and citations in that paper) that even in a lasing regime with a single longitudinal mode (single-frequency regime), during emission of pulses from semiconductor lasers with pump current modulation we observe a variation in the "instantaneous" laser frequency (called deviation, chirping, dynamic shift, or sweeping). This is explained by the variation in the refractive index n in the active region and consequently variation in the instantaneous frequencies of cavity longitudinal modes and the laser emission.Characteristic features of laser frequency sweeping for semiconductor lasers during pulse emission have been studied previously in [3][4][5] for some special cases and simplifying assumptions. The effect of a dynamic shift of the laser frequency on the operation of recycling memory systems and high-speed code modulation systems based on injection lasers has been studied in [6,7].Recently [8] we used numerical modeling to study frequency sweeping of the emission from heterojunction quantum lasers with current modulation when the lasing frequency is tuned within the gain band. In this case, the active medium is described within a two-band model in which we take into account direct transitions between groundstate subbands. In this case, the g...

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

“…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 with emission frequency …”

“…In contrast with the literature [8,9], variation of the index of refraction because of a change of the semiconductor forbidden-band gap is also taken into account.…”