Dual-comb spectrometer based on quantum cascade lasers (QCLs) is gaining fast development and revolutionizing the precision measurement with high-frequency and temporal resolutions. In these measurements, high-bandwidth photodetectors are normally used for signal acquisition and processing, which complicates the measurement system. QCL is well-known for its picosecond gain-recovery time with an intrinsic bandwidth of tens of GHz. In this work, a compact self-detecting dual-comb spectroscopy (DCS) is demonstrated based on dispersion-engineered, high-speed packaged QCLs under coherent injection locking. The laser source is designed and fabricated into a hybrid-monolithic-integrated waveguide and epi-down packaged on a wideband-designed submount to fully explore the high-speed feature up to fourth-order harmonic state with a cutoff frequency of 40?GHz. The effective radio frequency (RF) injection locking diminishes the issue of optical feedback and enables high-bandwidth self-detection based on QCLs. Clear and stable multiheterodyne signal corresponding to a spectral range of 68?cm?1 and narrow comb tooth linewidth of ≈10?kHz is observed without using external detector or numerical process. The demonstrated broadband, high-power, self-detecting mid-infrared QCL DCS has a great potential for future applications of molecular sensing and spectroscopy.

Dual-comb spectroscopy (DCS) based on quantum cascade lasers (QCLs) is of great interest to molecular spectroscopy due to its combined advantages of high temporal and spectral resolutions along with compact platform. Despite extensive current research in this exciting field, DCS using high-power longwave infrared (λ > 9 μm) QCLs is still relatively unexplored. In this work, we present a compact free-running dual-comb system consisting of λ ～ 9.4 μm QCLs with output power over 1 W under room-temperature continuous-wave operation. High output power of QCL comb source is achieved through a tailored waveguide design that simultaneously reduces waveguide loss and optimizes group velocity dispersion (GVD). High coherence of the comb device is validated by an intermode beatnote linewidth narrow than 1 kHz and a narrow multiheterodyne comb linewidth of 15 kHz in free-running mode, recorded with an acquisition time of 25 μs. The amplitude noise of the heterodyned comb line is further investigated to assess the stability of the dual-comb spectroscopy setup. The spectroscopic functionality of the QCL DCS is further verified by GaAs etalon and NH3 transmission spectral characterizations. The demonstrated high power QCL DCS exhibits an application potential in the applications of high-absorbing sample testing and remote sensing.

Paper link: 

https://advanced.onlinelibrary.wiley.com/doi/10.1002/adpr.202500062

https://www.sciencedirect.com/science/article/pii/S1350449525002580