Dispersion dynamics of quantum cascade lasers

Abstract

A key parameter underlying the efficacy of any nonlinear optical process is group velocity dispersion. In quantum cascade lasers (QCLs), there have been several recent demonstrations of devices exploiting nonlinearities in both the mid-infrared and the terahertz. Though the gain of QCLs has been well studied, the dispersion has been much less investigated, and several questions remain about its dynamics and precise origin. In this work, we use time-domain spectroscopy to investigate the dispersion of broadband terahertz QCLs, and demonstrate that contributions from both the material and the intersubband transitions are relevant. We show that in contrast to the laser gain-which is clamped to a fixed value above lasing threshold-the dispersion changes with bias even above threshold, which is a consequence of shifting intersubband populations. We also examine the role of higher-order dispersion in QCLs and discuss the ramifications of our result for devices utilizing nonlinear effects, such as frequency combs.

Multiheterodyne spectroscopy using interband cascade lasers

“…The less efficient phase noise cancellation far from the correction pair can be attributed to nonideal retrieval of the repetition rate signal and accumulative error propagation, or lower phase-noise coherence of the beat notes, which may be addressed by optimization of the laser cavity dispersion. 20,21,33 The latter phenomenon has been observed also for the more computationally intensive algorithm of Burghoff/Yang et al 31 Despite the remaining phase noise pedestal, an enhancement of signal-to-noise ratio (SNR) by up to ∼30 dB and beat note linewidths down to the kHz level are attainable using the method proposed here, which is sufficient for most spectroscopic applications. Also, the narrow beat note linewidths indicate that FP-ICLs exhibit a similar optical phase-locking mechanism as observed in QCLs.…”

Gain dynamics in a heterogeneous terahertz quantum cascade laser

“…To address these issues and to identify their origin THz time-domain spectroscopy (THz-TDS) has been applied to access the internal processes in QCLs, 8,9 which allowed to study the spectral gain curve at all operation points of a QCL (hence even above the lasing threshold), 8,[10][11][12] the gain clamping dynamics, 4 and the gain induced dispersion. [13][14][15] This flexible spectroscopic tool helped to identify individual gain degradation mechanisms, 16 and provided direct access to the gain recovery dynamics. 17,18 Fast gain dynamics, which is described by a short characteristic time -the gain recovery time (GRT) -is important for the high speed modulation of THz QCLs and for the formation and the sustainability of THz pulses in QCLs.…”

Intrinsic frequency tuning of terahertz quantum-cascade lasers

“…Although the lasing process is accompanied by the so-called gain clamping, the interaction of stimulated emission with the entire electron system affects the overall dielectric response of the system in an intricate way. 32 Since the gain clamping occurs only at the mode frequencies, any modification of the rather broad gain spectrum allows for a change of the refractive index despite the clamping, which significantly reduces the gain variations with increasing field strengths as shown experimentally in Ref. 33.…”