Optical Feedback Cavity-enhanced Absorption Spectroscopy Research Articles

Optical feedback cavity enhanced absorption spectroscopy with diode lasers

An optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) apparatus has been developed, using a distributed feedback diode laser centred close to a wavelength of 1596 nm. Light from a V-shaped optical cavity is allowed to feed back to the laser diode, injection seeding the laser at the cavity mode frequencies, and bringing about linewidth narrowing and frequency locking to the cavity resonance. The OF-CEAS technique's expediency and sensitivity have been demonstrated, first on measurements of aerial water absorptions close to 6260.8 cm(-)(1), and then on two carbon dioxide absorptions close to 6261.7 cm(-)(1) which may be used to measure the concentrations of (13)CO(2) and (12)CO(2) isotopologues in a CO(2) sample. Employing a cavity of moderate finesse (F approximately 3500), an acquisition time-reduced minimum detectable absorption coefficient of 5.8 x 10(-)(9) cm(-)(1) s(1/2) has been determined.

Multispecies breath analysis faster than a single respiratory cycle by optical-feedback cavity-enhanced absorption spectroscopy

We demonstrate a first application, of optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) to breath analysis in a medical environment. Noninvasive monitoring of trace species in exhaled air was performed simultaneous to spirometric measurements on patients at Bichat Hospital (Paris). The high selectivity of the OF-CEAS spectrometer and a time response of 0.3 s (limited by sample flow rate) allowed following the evolution of carbon monoxide and methane concentrations during individual respiratory cycles, and resolving variations among different ventilatory patterns. The minimum detectable absorption on this time scale is about 3 x 10(-10) cm(-1). At the working wavelength of the instrument (2.326 microm), this translates to concentration detection limits of approximately 1 ppbv (45 picomolar, or approximately 1.25 microg/m(3)) for CO and 25 ppbv for CH(4), well below concentration values found in exhaled air. This same instrument is also able to provide measurement of NH(3) concentrations with a detection limit of approximately 10 ppbv; however, at present, memory effects do not allow its measurement on fast time scales.

Optical feedback cavity-enhanced absorption spectroscopy for in situ measurements of the ratio 13C:12C in CO2

We report on the design and laboratory performance of a portable infrared absorption spectrometer for the measurement of the isotopic ratio 13C:12C in CO2. The design relies on optical feedback cavity-enhanced absorption spectroscopy in the 2 μm spectral region to achieve optimal performance at ambient CO2 concentrations. The prototype instrument measures δ13C, relative to a standard calibration bottle, with a precision of ±0.7‰ for a 20-s integration time and with an automatic recalibration every 6 min. The absolute accuracy obtained is 0.9‰. The principal performance limitations are discussed along with improvements currently being implemented for the second generation instrument.

Extensive characterization of the optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) technique: ringdown-time calibration of the absorption scale

The paper describes in detail an implementation of the highly sensitive optical feedback cavity enhanced absorption spectroscopic (OF-CEAS) technique. A Brewster-angle cavity has been used to enable resonant optical feedback injection. For the first time, an OF-CEAS-adapted processing allowing calibration of the cavity-enhanced spectra in absolute absorption units has been experimentally verified through the combination of ringdown-time and mode-amplitude measurements. The experimental system performance has been evaluated by measuring very weak oxygen B-band transitions near 688 nm. Absorption spectra covering the range of 1.35 cm-1 have been averaged over the 5-s measurement time at the minimum detectable loss of 3×10-10 /cm within the measurement dynamic range greater than four orders of magnitude. A detailed signal-to-noise ratio analysis estimates the measurement spectral precision as being 3 MHz (short time scale averaging).

Sub-ppb NO2 detection by optical feedback cavity-enhanced absorption spectroscopy with a blue diode laser

We report preliminary results on the first application of optical feedback cavity-enhanced absorption spectroscopy with a blue (411 nm) extended cavity diode laser (ECDL) for NO2 detection. While this technique was originally developed to operate with distributed feedback diode lasers in the near infrared, it is here extended to ECDLs and applied to the blue spectral region. With a simple and compact optical setup, we demonstrate from the baseline noise a minimum detectable NO2 concentration of 6×109 molecules/cm3 for a single laser scan (70 ms), which extrapolated under atmospheric conditions corresponds to 200 pptv. Signal averaging should allow further lowering of this limit. Observed absorption spectra display more structure than previous spectra obtained at lower resolution by Fourier-transform spectroscopy at the same wavelength.

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm−1) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10−10 cm−1 for 1 s averaging over scans, with a dynamic range of four orders of magnitude.