Abstract
AbstractIn this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio‐frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm−1 to an average bandwidth of 60 cm−1 can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red‐shifted compared to the free‐running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum‐bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red‐ or blue‐shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free‐running beatnote, two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz can be generated. In the time‐domain, with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi‐continuous output to long‐pulsed ( ps) output is shown by ramping up the injection power to 35 dBm.
Highlights
Mid-infrared spectroscopy is a powerful tool in sensing applications and environmental monitoring
Since it covers the molecular fingerprint regions of light gas molecules and functional groups of larger organic compounds, it is relevant in industrial and environmental sensing as well as biomedicine. [1,2,3,4] Quantum cascade lasers [5] (QCLs) are ideally suited for mid-IR spectroscopy, as they are compact and electrically pumped sources, which can operate at room-temperature while spanning the range of 3 to 20 μm with Watt-level output powers. [6,7] They possess the innate ability to form optical frequency combs via four-wave mixing (FWM) caused by the strong nonlinearity of their gain-medium and waveguides [8,9] in Fabry-Perot and ring QCLs. [10,11,12,13] Optical frequency combs are constituted of evenly spaced phaselocked modes
[14] The resulting high mode intensities and resolution, as compared to incoherent broadband sources, make them ideally suited for high-resolution spectroscopy, especially dual-comb spectroscopy. [15–18 A peculiarity of QCL frequency combs is that due to their ultrashort carrier-lifetimes on the order of picoseconds [19] their free-running output is not pulsed, as in most visible and near-infrared frequency combs
Summary
Mid-infrared (mid-IR) spectroscopy is a powerful tool in sensing applications and environmental monitoring. [35] We use this novel QCL design to demonstrate the potential of RF-modulation as a means for fast and precise tuning of the spectral and temporal properties of QCL frequency combs. The device used for all measurements in this paper is a 4 mm long mid-IR QCL optical frequency comb centered at 8.2 μm.
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