- The paper introduces a coherent multi-heterodyne spectroscopy system that uses stabilized optical frequency combs to capture full, phase-resolved spectra with hertz-level precision.
- It employs two frequency combs with slightly different repetition rates to generate an RF comb, rapidly resolving 155,000 lines over a 15.5 THz range in as little as 400 μs.
- The method enhances spectral resolution by up to nine orders of magnitude compared to traditional techniques, offering significant improvements in signal-to-noise ratios for molecular analysis.
Coherent, Multi-Heterodyne Spectroscopy Using Stabilized Optical Frequency Combs
In the paper by Coddington, Swann, and Newbury, the authors demonstrate an advanced approach in optical spectroscopy using coherent, multi-heterodyne techniques with stabilized optical frequency combs. This methodology allows for the precision measurement of the full complex spectrum, including both amplitude and phase, of molecular gases such as hydrogen cyanide across extensive bandwidths with hertz-level accuracy. The significance of this research lies in its ability to enhance the resolution and accuracy of spectroscopic techniques by several orders of magnitude, compared to traditional Fourier Transform Spectroscopy (FTS) and tunable laser spectroscopy. The experimental setup resolves 155,000 individual frequency comb lines spaced by 100 MHz over a 15.5 THz range.
The basis of the approach involves stabilized optical frequency combs produced by spectrally broadening the output of a mode-locked femtosecond laser. These combs are phase-locked to narrow linewidth laser sources, enabling their teeth to serve as precise narrowband continuous-wave optical oscillators. The challenge intrinsic to this method has been effective separation and detection of individual frequency comb components, which the authors address through multi-heterodyne spectroscopy.
Importantly, the system utilizes two frequency combs with slight differences in repetition rates as the signal and local oscillator, creating a radiofrequency (RF) comb during heterodyne detection. This strategy successfully yields a comb in the RF domain, where each tooth corresponds to the heterodyne signal between distinct comb lines. Notably, they achieve a frequency resolution and accuracy determined by the narrow linewidth reference lasers, surpassing existing spectroscopy methods by several magnitudes. For example, results indicate enhancements in resolution by nine orders of magnitude compared to traditional spectrometers and six orders of magnitude over FTS.
The implementation of such a system results in several robust claims and observations. For instance, the authors demonstrate successful acquisition of spectra within minimal times (e.g., 400 μs for an 8 nm spectrum with a 100 MHz spacing), exhibiting significant signal-to-noise ratios (SNR) and spectral accuracy. The SNR scales quadratically with time, providing a high fidelity spectrum at low input powers, ranging from 20 to 2000 pW per comb tooth.
This research carries substantial implications for both theoretical and practical applications. It paves the way for improved broadband spectroscopy of chemically or biologically dynamic systems, extending potential use in optical waveform characterization, distance metrology, and direct dispersion measurements. Furthermore, the system’s high accuracy and resolution capabilities may enhance the analysis of optical system transmission and dispersion, as well as foster advancements in coherent time-domain experiments.
The multi-heterodyne spectroscopy approach delineated in this paper stands as a refinement over spatial dispersion techniques and promises scalability to more comprehensive spectral ranges and finer comb spacing. Looking to the future, the integration of octave-spanning combs and enhancements in path length through arrangements such as White cells could further expand the applicability and sensitivity of this methodology, catering to broader spectroscopic inquiries and more complex samples. The potential for rapid, broadband, and phase-resolved spectroscopic measurements holds considerable promise for advancing the understanding and analysis of molecular systems.