Abstract
A time-resolved mid-infrared upconversion technique based on sum-frequency generation was applied to measure pulse propagation in lambda approximately 5.0 mum quantum cascade lasers operated in continuous wave at 30 K. The wavelength-dependent propagation delay of femtosecond mid-infrared pulses was measured to determine the total group-velocity dispersion. The material and waveguide dispersion were calculated and their contributions to the total group-velocity dispersion were found to be relatively small and constant. The small-signal gain dispersion was estimated from a measurement of the electroluminescence spectrum without a laser cavity, and was found to be the largest component of the total GVD. A negative group-velocity dispersion of beta2 ( =d2beta/d omega2) approximately - 4.6x10-6 ps2/mum was observed at the peak emission wavelength, and good agreement was found for the measured and calculated pulse-broadening.
Highlights
Group-velocity dispersion (GVD) in semiconductor lasers is critical for understanding ultrashort pulse generation and propagation [1]
Quantum cascade lasers (QCL’s) are interesting devices since the optical Kerr nonlinearity and the associated gain dispersion are embedded in a single semiconductor structure and they both originate from the same physical mechanism, i.e. the optical intersubband transition
In order to take into account the dynamical interaction between the propagating pulses and the gain medium, it is more useful to investigate the effects of the GVD on the propagating pulses directly in the time domain
Summary
Group-velocity dispersion (GVD) in semiconductor lasers is critical for understanding ultrashort pulse generation and propagation [1]. For understanding the dynamics of QCL’s including pulse generation, it is important to understand quantitatively the separate contributions of the material, waveguide, and gain dispersion to the total GVD in QCL waveguide structures. The experiments presented here are analogous to a number of experiments on bipolar interband semiconductor diode lasers [11, 12, 13]. In contrast to those measurements, we explicitly consider the contribution of the small-signal gain dispersion ,which is obtained through an independent electroluminescence measurement of the gain spectrum. The calculated material and waveguide dispersion with the independently measured gain dispersion are used to model the effects of total GVD on the temporal pulse broadening and are compared with experimental measurements, providing a self-consistent picture of the effects of the GVD on the propagation of resonant mid-IR pulses
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