Abstract
The generation of frequency combs in the mid-infrared (MIR) spectral range by quantum cascade lasers (QCLs) has the potential for revolutionizing dual-comb multi-heterodyne spectroscopy in the molecular fingerprint region. However, in contrast to frequency combs based on passively mode-locked ultrafast lasers, their operation relies on a completely different mechanism resulting from a four-wave mixing process occurring in the semiconductor gain medium that locks the modes together. As a result, these lasers do not emit pulses and no direct self-referencing of a QCL comb spectrum has been achieved so far. Here, we present a detailed frequency noise characterization of a MIR QCL frequency comb operating at a wavelength of 8 µm with a mode spacing of ∼7.4 GHz. Using a beat measurement with a narrow-linewidth single-mode QCL in combination with a dedicated electrical scheme, we measured the frequency noise properties of an optical mode of the QCL comb, and indirectly of its offset frequency for the first time, without detecting it by the standard approach of nonlinear interferometry applied to ultrafast mode-locked lasers. In addition, we also separately measured the noise of the comb mode spacing extracted electrically from the QCL. We observed a strong anti-correlation between the frequency fluctuations of the offset frequency and mode spacing, leading to optical modes with a linewidth slightly below 1 MHz in the free-running QCL comb (at 1-s integration time), which is narrower than the individual contributions of the offset frequency and mode spacing that are at least 2 MHz each.
Highlights
Optical frequency combs (OFCs) generated from mode-locked lasers have been a revolution in time and frequency metrology since their first demonstration 20 years ago by providing a direct and phase-coherent link between optical and microwave frequencies [1,2,3]
A different type of frequency combs has been demonstrated in the mid-infrared (MIR) spectral region that is important for molecular spectroscopy, based on the occurrence of four-wave mixing in the semiconductor gain medium of broadband quantum cascade lasers (QCLs) [4]
The relative frequency noise between the QCL comb modes was analyzed by implementing a scheme that combines a phase-lock of the mode spacing to an external reference oscillator by feedback to the QCL current, and a noise compensation scheme to remove the common noise between all modes of the multi-heterodyne beat signal with the metrological comb
Summary
Optical frequency combs (OFCs) generated from mode-locked lasers have been a revolution in time and frequency metrology since their first demonstration 20 years ago by providing a direct and phase-coherent link between optical and microwave frequencies [1,2,3]. The FM nature of QCL combs has made the self-referencing method not possible so far and no detection of the offset frequency of a MIR QCL comb has ever been reported to the best of our knowledge, even if the full phase stabilization of a THz QCL comb to a CEO-free metrological THz comb was recently demonstrated [10] For this purpose, the authors investigated the use of two different actuators, which are radio-frequency (RF) injection locking of the comb mode spacing by an external frequency reference signal and the QCL drive current to phase-lock one beat signal with the metrological comb to a reference oscillator. We assess the position of the fixed point [17] of the QCL comb spectrum
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
