Abstract
Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.
Highlights
The realization of optical frequency combs (OFCs) [1, 2] has lead to multiple breakthroughs in fundamental science and enabled manifold applications
One of the main goals of FM comb engineering is the control of the number of comb modes in order to increase the optical bandwidth and facilitate a broad ruler for dual-comb spectroscopy
The presented guideline explains the roles of the group velocity dispersion (GVD) and resonant Kerr nonlinearity with a numerical study and reveals the impact of the cavity facet reflectivities
Summary
MAXIMILIAN BEISER*,1, NIKOLA OPAC AK*,1, JOHANNES HILLBRAND1, GOTTFRIED STRASSER1, AND BENEDIKT SCHWARZ1,†. Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the midinfrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. We provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The implementation of high mirror losses is shown to be favourable and results in proliferation of the comb sidemodes. Injection locking of QCLs by modulating the laser bias around the roundtrip frequency provides a stable external knob to control the FM comb state and recover the maximum spectral width of the unlocked laser state
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