Abstract-Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser

Jie Chen, Kazuki Nitta, Xin Zhao, Takahiko Mizuno, Takeo Minamikawa, Francis Hindle, Zheng Zheng, Takeshi Yasui,

https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-2/issue-3/036004/Adaptive-sampling-near-Doppler-limited-terahertz-dual-comb-spectroscopy-with/10.1117/1.AP.2.3.036004.full

Dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution with rapid data acquisition. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS. We demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with the adaptive sampling method. While the use of a free-running single-cavity dual-comb fiber laser eliminates the need for two mode-locked lasers and their frequency control, the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter in the free-running dual-comb laser. Doppler-limit-approaching absorption features with linewidths down to 25 MHz are investigated for low-pressure acetonitrile/air mixed gas by comb-mode-resolved THz spectroscopy. The successful demonstration clearly indicates its great potential for the realization of low-complexity, Doppler-limited THz spectroscopy instrumentation.

Researchers Boost Sensitivity of Terahertz Gas Analysis

https://www.novuslight.com/researchers-boost-sensitivity-of-terahertz-gas-analysis_N9947.html

A new advance promises to increase the sensitivity of high-resolution spectrometers that perform chemical analysis using terahertz wavelengths. This higher sensitivity could benefit many applications, such as analysis of the complex gas mixtures found in industrial emissions and detection of biomarkers of disease in the breath of patients. It could also lead to new ways to detect food spoilage through gas detection.

In Optica, The Optical Society's (OSA) journal for high impact research, researchers led by Gaël Mouret from Université du Littoral-Côte d'Opale in France report a new high-performance optical cavity for terahertz frequencies. They used this cavity to demonstrate the first convincing cavity-enhanced spectroscopy performed with terahertz frequencies.

Terahertz frequencies lie between microwaves and infrared light waves on the electromagnetic spectrum. For spectroscopic gas analysis, terahertz frequencies improve the ability to distinguish between molecules in a sample and to detect a wide variety of molecules. However, the technology needed to make full use of these frequencies is still under development.

“Several studies have used terahertz frequencies to analyze industrial gases emitted into the atmosphere, but they have all been hindered by a lack of sensitivity,” said research team member Francis Hindle. “Our new optical cavity will expand the types of molecules that can be identified with terahertz gas-phase spectroscopy and improve the feasible detection level.”

Increasing sensitivity

The researchers used newly available components to construct a high-finesse terahertz optical cavity, an arrangement of mirrors and a waveguide that confines light so that it reflects multiple times. High-finesse optical cavities exhibit very low light loss and thus allow the light to bounce between the mirrors more times before exiting the cavity. The new components included a low-loss circular corrugated waveguide and two highly reflective photonic mirrors specially designed to work well at terahertz frequencies.

For cavity-enhanced spectroscopy, a gas mixture is placed in the optical cavity where it interacts with the light inside. The new cavity allows terahertz waves to bounce back and forth around 3000 times before exiting. This means that molecules under analysis interact with the terahertz frequencies over an effective distance of approximately 1 kilometer inside a resonator only 50 centimeters long. As the waves bounce around, they can be absorbed many times by any molecules that are present, allowing a very sensitive measurement.

“A cavity with this finesse has not previously been available at terahertz frequencies,” said Hindle. “This advance allows terahertz frequencies to be applied to many highly sensitive techniques already used in the infrared.”

Detecting rare molecules

To demonstrate cavity-enhanced spectroscopy of a gas with their new device, the researchers analyzed a sample of carbonyl sulfide gas, which is naturally found in the atmosphere. Although the gas sample contained many isotopes of carbonyl sulfide, the researchers were able to measure a very rare isotope present at a concentration of just one molecule per 50,000 molecules. Measuring the ratios of different chemical isotopes in a sample can be used to determine the source of a pollutant.

The researchers plan to expand the range of frequencies for the spectrometer so that it could be used to analyze even more complex molecules and mixtures.  

“Our research shows that it is now possible to easily construct high-finesse terahertz cavities and use them for the measurement of gases at high resolution,” said Hindle. “This could contribute to improved monitoring of a large variety of gases present at very low amounts for applications from environmental and industrial pollution to medicine.”

Paper: F. Hindle, R. Bocquet, A. Pienkina, A Cuisset, G. Mouret, “Terahertz gas phase spectroscopy using a high finesse Fabry-Pérot cavity,” Optica, 6, 12, 1449-1454 (2019).

DOI: https://doi.org/10.1364/OPTICA.6.001449

Abstract-Terahertz gas phase spectroscopy using a high-finesse Fabry–Pérot cavity

Francis Hindle, Robin Bocquet, Anastasiia Pienkina, Arnaud Cuisset, and Gaël Mouret

Fabry–Pérot THz cavity system overview with three operational modes. The emitter is an amplified multiplier chain (×36$\times 36$) covering 440–660 GHz driven by a microwave synthesizer referenced to a GPS time signal. The synthesizer is referenced to a GPS timing signal providing a frequency accuracy of 10−11${10}^{-11}$ when measured over 1 s. The phase noise at the output of the frequency multiplier operating at 620 GHz is −63  dBc/Hz $-63\mathrm{dBc}/\mathrm{Hz}$ at 1 kHz from the carrier frequency. A TPX (Polymethylpentene) lens L1 (𝑓=25  mm$f=25\mathrm{mm}$) is used to couple the free space THz emission to the corrugated waveguide CW. Two 1D photonic mirrors PM1 and PM2 close the cavity, one at each end of the corrugated waveguide. Each photonic mirror is mounted on a piezo actuator PA1 and PA2, enabling fine tuning of the cavity length over at least 250 μm to ensure complete coverage. A second TPX lens L2 (𝑓=25  mm$f=25\mathrm{mm}$) collects the THz emission at the cavity output and focuses it on a zero bias Schottky detector diode (ZBD WR1.5). The corrugated waveguide is 48 cm long, with internal diameters of 20.54 mm. The internal corrugations have a pitch of 𝑝=166  μm$p=166\mathrm{\mu m}$, while the groves are 𝑤=83  μm$w=83\mathrm{\mu m}$ wide and 𝑑=125  μm$d=125\mathrm{\mu m}$ deep. Time mode, the cavity output signal is amplified and recorded by an oscilloscope while the source is extinguished giving direct access to the cavity ring-down time 𝜏𝑅${\tau }_R$. Frequency mode, the THz source frequency is scanned and the cavity response is measured using a lock-in detection and amplitude modulation of the source. The cavity mode linewidths (FWHM) and free spectral range (FSR) are directly obtained in the frequency domain. Fabry–Pérot THz Absorption Spectrometer (FP-TAS) mode, the THz source is frequency modulated and the cavity output is measured by lock-in detection. The first harmonic (1×𝑓$1\times f$) is used as an error signal, a cavity mode is locked to the frequency of the THz source using a proportional, integrator, derivative (PID) control loop that feeds a high-voltage (HV) power supply. The second harmonic (2×𝑓$2\times f$) provides a sensitive molecular signal as the source frequency and cavity scan together. The entire cavity assembly is placed in a pressure-controlled gas cell equipped with Teflon windows.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-6-12-1449

The achievable instrument sensitivity is a critical parameter for the continued development of terahertz (THz) applications. Techniques such as cavity-enhanced techniques and cavity ring-down spectroscopy have not yet been employed at THz frequency due to the difficulties to construct a high-finesse Fabry–Pérot cavity. Here, we describe such a THz resonator based on a low-loss oversized corrugated waveguide with highly reflective photonic mirrors obtaining a finesse above 3000 around 620 GHz. These components enable a Fabry–Pérot THz absorption spectrometer with an equivalent interaction length of 1 km giving access to line intensities as low as 10−27  cm−1/(molecule/cm2)${10}^{-27}{\mathrm{cm}}^{-1}/(\text{molecule}/{\mathrm{cm}}^2)$ with a S/N ratio of 3. In addition, the intracavity optical power has allowed the Lamb dip effect to be studied with a low-power emitter; an absolute frequency accuracy better than 5 kHz can be easily obtained providing an additional solution for rotational spectroscopy.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement