Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb

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. using a high-bandwidth photodiode. On the other hand, the standard method to detect the CEO frequency is the self-referencing scheme, which in the usual implementation requires a coherent octave-spanning spectrum and nonlinear f -to-2f interferometry [1].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]. These lasers produce a frequency comb in some particular conditions where all emitted modes become equidistant as a result of the parametric four-wave mixing process. QCL combs are very attractive for dual-comb spectroscopy [5][6][7] in the MIR spectral range that is of particular importance for high-resolution molecular spectroscopy and trace gas sensing. As a result of the very short upper state lifetime of the gain medium, these comb sources do not generate optical pulses, but deliver a fairly constant output power [8] that is associated to a frequency-modulated (FM) spectrum. A recent model proposed by Opacak and Schwarz describes the formation of FM combs with the combined contributions of spatial hole burning, gain saturation and a minimum group velocity dispersion or Kerr nonlinearity due to gain asymmetry [9]. The FM nature of QCL combs has made the self-referencing method not possible so far and no detection of t...

Battery-operated mid-infrared diode laser frequency combs

“…The symmetric broadening around the comb's center wavelength suggests the existence of a fixed point 25 there, where repetition rate (timing) fluctuations get partially compensated by offset frequency fluctuations. This explanation is in agreement with the recent studies on QCL combs 26 . The obtainable spectral coverage in the DCS experiment reaches ∼520 GHz, partially limited by the oscilloscope electrical bandwidth, and the lower span of the signal (SIG) comb compared to the local oscillator (LO).…”

Battery‐Operated Mid‐Infrared Diode Laser Frequency Combs

“…This explanation is in agreement with the recent studies on QCL combs. [ 30 ] The obtainable spectral coverage in the DCS experiment reaches ≈520 GHz, partially limited by the oscilloscope electrical bandwidth, and the lower span of the signal (SIG) comb compared to the local oscillator (LO).…”

Noise Measurement and Reduction in Mode-Locked Lasers: Fundamentals for Low-Noise Optical Frequency Combs

“…Using this technique, the phase noise measurement is limited to the noise floor of the instrument when the offset frequency is higher than 100 kHz. This approach has been applied in f ceo frequency noise measurement of Er:Yb:glass lasers [103] and quantum cascade lasers [104].…”

“…This phenomenon indicates that n × f rep and f ceo noise to some extent cancel out, resulting in a lower level of comb-line noise spectrum. The complex coherence was measured to be −2 in a wide range of Fourier frequency in commercial OFC from Menlosystems [123], 25-GHz Er:Yb glass lasers [103], NALM fiber mode-locked lasers [45], and mid-infrared quantum cascade lasers [104] soliton molecules [124].…”