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.

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