Quantum cascade laser-based frequency combs have attracted much attention as of late for applications in sensing and metrology, especially as sources for chip-scale spectroscopy at mid-infrared fingerprint wavelengths. A frequency comb is a light source whose lines are evenly-spaced, and only two frequencies are needed to describe the system—the offset and the repetition rate. Because chip-scale combs have large repetition rates, for many spectroscopic applications is important to be able to change both parameters independently, without substantially changing the comb spectrum or spectral structure. Although it is possible to modulate both the offset and the repetition rate of a comb by tuning the laser current and temperature, both properties affect the laser by changing its index of refraction, and both frequencies will be affected. Here, we show that by integrating a mirror onto a MEMS comb drive, the dispersion and group delay associated with a quantum cascade comb’s cavity can be modulated at kilohertz speeds. Because the MEMS mirror primarily affects the group delay of the cavity, it is able to adjust the comb’s repetition rate while leaving the offset frequency mostly unaffected. Since this adjustment is linearly independent from current adjustments and can be adjusted quickly, this provides an avenue for mutual stabilization of both parameters. In addition, we show that dynamic modulation of the comb drive is able to allow the laser to recover from comb-destroying feedback, making the resulting comb considerably more robust under realistic conditions.
Recent research has shown that free-running quantum cascade lasers are capable of producing frequency combs in midinfrared and THz regions of the spectrum. Unlike familiar frequency combs originating from mode-locked lasers, these do not require any additional optical elements inside the cavity and have temporal characteristics that are dramatically different from the periodic pulse train of conventional combs. Frequency combs from quantum cascade lasers are characterized by the absence of sharp pulses and strong frequency modulation, periodic with the cavity round trip time but lacking any periodicity within that period. To explicate for this seemingly perplexing behavior, we develop a model of the gain medium using optical Bloch equations that account for hole burning in spectral, spatial, and temporal domains. With this model, we confirm that the most efficient mode of operation of a free-running quantum cascade laser is indeed a pseudorandom frequency-modulated field with nearly constant intensity. We show that the optimum modulation period is commensurate with the gain recovery time of the laser medium and the optimum modulation amplitude is comparable to the gain bandwidth, behavior that has been observed in the experiments.
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