Frequency-agile, rapid scanning spectroscopy

Abstract

Challenging applications in trace gas measurements require low uncertainty and high acquisition rates [1][2][3][4] . Many cavityenhanced spectroscopies exhibit significant sensitivity and potential 5,6 , but their scanning rates are limited by reliance on either mechanical or thermal frequency tuning 7 . Here, we present frequency-agile, rapid scanning spectroscopy (FARS) in which a high-bandwidth electro-optic modulator steps a selected laser sideband to successive optical cavity modes. This approach involves no mechanical motion and allows for a scanning rate of 8 kHz per cavity mode, a rate that is limited only by the cavity response time itself. Unlike rapidly frequency-swept techniques 8-11 , FARS does not reduce the measurement duty cycle, degrade the spectrum's frequency axis or require an unusual cavity configuration. FARS allows for a sensitivity of ∼2 3 10 212 cm 21 Hz 21/2 and a tuning range exceeding 70 GHz. This technique shows promise for fast and sensitive trace gas measurements and studies of chemical kinetics.A multitude of applications have emerged that require rapid sensing of trace gas species, encompassing areas as varied as greenhouse gas monitoring 1,4,12 , breath analysis 2,13 , explosive detection 3 and chemical process monitoring. Because of their sensitivity, continuous-wave (c.w.) cavity-enhanced spectroscopies have significant potential for addressing these challenging applications 6,14,15 . However, the low mechanical or thermal tuning rates of c.w. lasers have generally limited the use of these techniques to static or slowly varying analytes. Attempts to alleviate this limitation have relied on sweeping the laser frequency [8][9][10][11] , which can compromise the fidelity of the spectrum and the instrument sensitivity 16 and reduce the measurement duty cycle. We present a new approach for cavity ring-down spectroscopy 17 in which the laser frequency is rapidly stepped to successive resonances through the use of highbandwidth electro-optics. This technique, which we refer to as frequency-agile, rapid scanning (FARS) cavity ring-down spectroscopy, allows for ultrasensitive measurements in which the acquisition rate is limited only by the cavity response. Unlike earlier techniques [8][9][10][11]18,19 , FARS allows for spectra to be recorded without any dead time due to scanning of the laser frequency, offers a metrology-level frequency axis and utilizes a conventional Fabry-Pérot resonator rather than more unusual and cumbersome cavity configurations.This frequency stepping of the probe laser is enabled by the use of a microwave driver and a high-bandwidth electro-optic modulator (EOM) to generate a series of sidebands on the probe laser (Fig. 1). We then use the cavity as a spectral filter such that only a single, selected sideband is resonant. Thus, we are able to transfer the superior switching bandwidth and precision of microwave sources into the optical domain. Ring-downs are then initiated by simply switching off the microwave frequency, thus removing the need for an acousto-...

Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)←(00001) near-infrared carbon dioxide band

“…FARS-CRDS is a variation of traditional cw-CRDS in which an electro-optic modulator (EOM) is used to rapidly step the laser frequency between successive modes of an optical cavity [4]. This is done by offsetting the carrier frequency from the cavity modes such that only a single, selected EOM-shifted laser sideband is resonant with the cavity.…”

“…Due to the high target accuracy (o1 mmol/mol uncertainty of CO 2 ) of these missions, spectroscopic parameters are needed with relative uncertainties below 0.3% [3]. In the light of these measurement needs, we utilized a recently developed spectroscopic technique, frequencyagile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) [4,5], to produce high accuracy line shape parameters for the (30012)'(00001) CO 2 band near 1575 nm. These room temperature measurements in concert with future temperature-dependent measurements should allow for significantly reduced uncertainties in atmospheric retrievals of CO 2 dry air mixing ratios.…”

High-AccuracyCO2Line Intensities Determined from Theory and Experiment

“…The individual transitions were probed using the frequency-agile, rapid scanning spectroscopy method [42] to rapidly and precisely tune the laser in a stepwise fashion through successive cavity resonances. To this end, we used PRL 114, 243001 (2015) P…”

Near infrared cavity ring-down spectroscopy for isotopic analyses of CH4 on future Martian surface missions

“…Commercial DFB lasers with line widths of o5 kHz (TeraXion, Quebec City, Quebec, Canada) have become available and this should result in a 200 times increase in signal level on the detector, which should dramatically reduce the shotto-shot fluctuations in k due to the electronic noise of detector. Truong et al (2013) have recently introduced a frequency-agile, rapid scanning spectroscopy (FARS) version of the CRDS that uses of a wide-bandwidth electro-optic modulator EOM). The EOM enables rapid switching of the cavity excitation frequency for each .…”