Cavity-enhanced dual-comb spectroscopy (0908.1928v1)

Abstract: The sensitivity of molecular fingerprinting is dramatically improved when placing the absorbing sample in a high-finesse optical cavity, thanks to the large increase of the effective path-length. As demonstrated recently, when the equidistant lines from a laser frequency comb are simultaneously injected into the cavity over a large spectral range, multiple trace-gases may be identified within a few milliseconds. Analyzing efficiently the light transmitted through the cavity however still remains challenging. Here, a novel approach, cavity-enhanced frequency comb Fourier transform spectroscopy, fully overcomes this difficulty and measures ultrasensitive, broad-bandwidth, high-resolution spectra within a few tens of $\mu$s. It could be implemented from the Terahertz to the ultraviolet regions without any need for detector arrays. We recorded, within 18 $\mu$s, spectra of the 1.0 $\mu$m overtone bands of ammonia spanning 20 nm with 4.5 GHz resolution and a noise-equivalent-absorption at one-second-averaging per spectral element of 3 10^-12 cm^-1Hz^-1/2, thus opening a route to time-resolved spectroscopy of rapidly-evolving single-events.

• The paper introduces a novel methodology by integrating high finesse optical cavities with dual femtosecond laser frequency combs to enhance spectroscopic sensitivity.
• The paper achieves record resolution with a 4.5 GHz spectral measurement and a noise equivalent absorption of 3×10⁻¹² cm⁻¹ Hz⁻¹/², detecting ammonia in just 18 µs.
• The paper demonstrates practical applications in real-time trace gas monitoring, paving the way for advancements in environmental and atmospheric science.

Cavity Enhanced Dual Comb Spectroscopy: An In-Depth Analysis

The research conducted by B. Bernhardt et al. presents a methodologically sophisticated approach known as cavity enhanced dual comb spectroscopy, which combines the sensitivity of cavity enhancement with the extensive bandwidth and resolution capabilities of frequency comb Fourier transform spectroscopy (FC FTS). This integration addresses key limitations observed in previous spectroscopy techniques, primarily concerning the sensitivity and spectral bandwidth.

Methodology and Experimental Setup

The authors employ a high finesse optical cavity to improve the sensitivity of molecular fingerprinting algorithms. By utilizing the coherent coupling of a laser frequency comb with a high finesse cavity, the effective path length is significantly increased, allowing for the detection of trace gases within milliseconds. This is accomplished by transmitting equidistant lines from a laser frequency comb over a broad spectral range. The technical manifestation involves two femtosecond Ytterbium fiber lasers with slightly disparate repetition frequencies. One laser is transmitted through a resonant cavity containing the absorbing sample, while the light interacting with this setup is analyzed through multiplex spectrometry.

An intriguing aspect of the experimental setup is the use of a Pound-Drever-Hall scheme for phase-locking the repetition frequency of the frequency comb onto the cavity's free spectral range. This ensures coherence and minimizes noise, which otherwise limits spectroscopic accuracy. The two lasers then generate a radio frequency domain down-converted image of the optical spectrum via heterodyne detection.

Results and Numerical Insights

A quantitative highlight of the research is the achievement of measuring spectra of ammonia in the 1.0 µm overtone bands with a 4.5 GHz resolution within just 18 µs. The experiment demonstrated a noise equivalent absorption of 3 × 10⁻¹² cm⁻¹ Hz⁻¹/² at a one-second averaging per spectral element, indicating a significantly superior sensitivity over existing methods. This sensitivity is practically useful for applications requiring rapid detection of trace gases.

Furthermore, the paper reports a minimum detectable absorption coefficient of 2 × 10⁻⁹ cm⁻¹, illustrating a considerable enhancement compared to prior benchmarks. The enhancement in detection compared to traditional methods is underscored by the spectral resolution achieved without the need for large detector arrays, particularly in mid-infrared regions where detector availability is limited.

Implications and Conclusion

The implications of this research span both theoretical advancements in spectroscopy and practical applications in environmental monitoring and planetary science, particularly in tracing atmospheric gases. The described capacity to perform ultrasensitive, real-time spectroscopy opens avenues for observing dynamical molecular events and conducting precise atmospheric modeling, as illustrated by the rotationally resolved spectra of ammonia.

Looking forward, the application of cavity enhanced FC FTS could be critical in extending the technique's viability across various spectral regions, potentially addressing mid-infrared challenges due to inherent molecular line strengths. Future developments might focus on improving bandwidth through advanced mirror design and mirror dispersion management, as well as adopting nonlinear frequency conversion techniques for generating broader comb spectra.

In conclusion, Bernhardt et al. significantly contribute to the field of molecular spectroscopy by resolving existing spectral resolution and sensitivity issues and setting a platform for future inquiry into broader and more efficient spectroscopy applications.