Coherent frequency combs produced by self frequency modulation in quantum cascade lasers

Abstract

One salient characteristic of Quantum Cascade Laser (QCL) is its very short τ ~ 1ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequencymodulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCL's in sensing and measurement.Quantum cascade lasers (QCL's) 1 are versatile sources of coherent radiation in the mid and far infrared regions of the spectrum, in which many chemical compounds have characteristic strong fundamental absorption lines. The atomic-like joint density of state character of intersubband transitions, combined with quantum engineering 2-4 , allows the development of QCLs with heterogenous active regions 5 covering a very large spectral range. Recently, by combining such broadband active regions with an external cavity 6 , continuous tuning over large spectral ranges have been achieved in the 8-10 and 3-5μm wavelength range 7 . However, these sources, tunable over hundreds of wavenumbers, require a cavity with moving components, naturally limiting their tuning speed as well as their compactness.In the last decade, a new approach to broadband spectroscopy has been proposed and implemented by the use optical frequency combs 8,9 . In a dual comb spectrometer 10-12 , two optical combs with slightly different mode spacing are beaten onto a fast detector. Since each pair of modes (one from each combs) can be made to beat at a slightly different frequency, a Fourier transform of the signal on the detector enables a direct reading of the optical spectrum. So far, such technique has been demonstrated using combs based on mode-locked lasers, in which the periodic time optical output, consisting of pulses spaced by the round-trip period of the cavity, ensures the periodicity of the comb optical spectrum.To reduce the power consumption and size, it would be very attractive to generalize this technique for semiconductor lasers. Furthermore, for many applications, it would be especially interesting to do so in the mid-infrared region of the spectrum, where many molecules have their strong fundamental absorption lines and where the generation of combs is more challenging 13 . However, in the conventional free-running multi-mode laser the modes are not evenly spaced due to material dispersion and the broadening of the individual modes prevent the generation of well-separated beat notes in the RF domain. As a result, some way of locking the modes together must be employed. Yet all the numerous attempts 14-16 to mode-lock QCL had limited success,

Dual-comb spectroscopy based on quantum-cascade-laser frequency combs

“…This interpretation has been recently confirmed by simulations of the mode dynamics using MaxwellBloch equations 38 . Nevertheless, a flat broadband gain as well as a very low group velocity dispersion are required to achieve phaselocking.…”

Single-mode instability in standing-wave lasers: The quantum cascade laser as a self-pumped parametric oscillator

“…In the last few years, comb generation in a QCL on adjacent FP modes has been demonstrated [37][38][39], and the importance of parametric mode coupling is known [40,41]. (These devices all had multistage inhomogeneously broadened active regions, which distinguishes them from the devices in our work.)…”

Terahertz Frequency Metrology for Spectroscopic Applications: a Review

“…Another interesting and promising technology for THz high-precision spectroscopy is continuous-wave multimode broadband QCLs, whose operation has only recently been demonstrated [210,211]. A broadband gain medium is desirable for a wide range of applications and, in QCL comb formation, is driven by ultrafast non-linearities (four-wave mixing) in the active region itself [212]. At terahertz frequencies, comb operation is also favored by the longer lifetimes of the optical transition upper state at those frequencies [213][214][215].…”