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Free-Running Ring Quantum Cascade Laser with 50 kHz Linewidth

Alexandre Parriaux, Ina Heckelmann, Mathieu Bertrand, Mattias Beck, Jérôme Faist, Thomas Südmeyer

TL;DR

The paper addresses the challenge of achieving low-noise, single-frequency operation for mid-infrared quantum cascade lasers above $7\ \mu\text{m}$. It analyzes a free-running ring QCL around $7.7\ \mu\text{m}$ using a nitrogen monoxide absorption line in a gas cell as a frequency-to-voltage discriminator to extract the frequency-noise power spectral density and infer the linewidth. A key finding is a full width at half maximum of approximately $50\ \text{kHz}$ at $1\ \text{s}$ integration, representing a sixfold improvement over prior free-running devices in this spectral region, with an upper bound on the intrinsic linewidth below $300\ \text{Hz}$. The study also demonstrates that the device supports frequency modulation spectroscopy and discusses implications for high-resolution mid-IR metrology, as well as pathways to higher power and potential mid-IR frequency comb implementations via RF injection.

Abstract

We report on the noise characterization of a free-running ring quantum cascade laser resonator emitting a single frequency mode around 7.7 $μ$m. Using a gas cell filled with N$_2$O as a frequency-to-voltage discriminator, we measured the frequency noise power spectral density of the laser from which we extracted its linewidth. The results show a full width at half maximum close to 50 kHz at 1 s integration time, which represents at least a sixfold improvement compared to state-of-the-art quantum cascade lasers operating in a spectral region above 7 $μ$m. We also demonstrate that such lasers can be efficiently used for frequency modulation spectroscopy, which opens up new possibilities for high resolution metrology and spectroscopic applications in the mid-infrared.

Free-Running Ring Quantum Cascade Laser with 50 kHz Linewidth

TL;DR

The paper addresses the challenge of achieving low-noise, single-frequency operation for mid-infrared quantum cascade lasers above . It analyzes a free-running ring QCL around using a nitrogen monoxide absorption line in a gas cell as a frequency-to-voltage discriminator to extract the frequency-noise power spectral density and infer the linewidth. A key finding is a full width at half maximum of approximately at integration, representing a sixfold improvement over prior free-running devices in this spectral region, with an upper bound on the intrinsic linewidth below . The study also demonstrates that the device supports frequency modulation spectroscopy and discusses implications for high-resolution mid-IR metrology, as well as pathways to higher power and potential mid-IR frequency comb implementations via RF injection.

Abstract

We report on the noise characterization of a free-running ring quantum cascade laser resonator emitting a single frequency mode around 7.7 m. Using a gas cell filled with NO as a frequency-to-voltage discriminator, we measured the frequency noise power spectral density of the laser from which we extracted its linewidth. The results show a full width at half maximum close to 50 kHz at 1 s integration time, which represents at least a sixfold improvement compared to state-of-the-art quantum cascade lasers operating in a spectral region above 7 m. We also demonstrate that such lasers can be efficiently used for frequency modulation spectroscopy, which opens up new possibilities for high resolution metrology and spectroscopic applications in the mid-infrared.

Paper Structure

This paper contains 4 sections, 1 equation, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental setup
  3. Results
  4. Discussion and conclusion

Figures (5)

  • Figure 1: (a) Experimental setup used to characterize the noise properties of a ring QCL using a N$_2$O gas cell as a frequency-to-voltage discriminator. (b) Spectral tuning range of the laser with respect to the current supplied, when operating at a temperature of 0.
  • Figure 2: (Top) Transmission spectrum observed with a 10 cm long gas cell filled with 1 mbar of N$_2$O when current modulation is on. (Middle) Fitted absorption spectrum using line parameters from the HITRAN database. (Bottom) Absolute difference between the experimental and fitted spectra.
  • Figure 3: R(10) absorption line of the $\nu_1$ fundamental band of N$_2$O measured by performing current modulation of the QCL. The red line aims to show that at the vicinity of the working point, the absorption line has a linear behavior.
  • Figure 4: FN-PSD measurement obtained by converting back the voltage noise to frequency noise using the discriminator's value. The red curve shows that the intensity noise of the QCL does not contribute to the frequency noise.
  • Figure 5: FWHM evolution of the QCL with respect to the observation time, and depending on the method used to extract it. The yellow curve represents the FWHM evolution for a laser having the theoretical FN-PSD profile given by the fitted curve showed in \ref{['fig:fnpsd']}. The vertical dashed line indicates the limit below which the $\beta$ line method is not relevant anymore to estimate the linewidth.