Continuous-wave cavity ringdown for high-sensitivity polarimetry and magnetometry measurements.

We report the development of a novel variant of cavity ringdown polarimetry using a continuous-wave laser operating at 532 nm for highly precise chiroptical activity and magnetometry measurements. The key methodology of the apparatus relies upon the external modulation of the laser frequency at the frequency splitting between non-degenerate left- and right-circularly polarized cavity modes. The method is demonstrated by the evaluation of the Verdet constants of crystalline CeF3 and fused silica, in addition to the observation of gas- and solution-phase optical rotations of selected chiral molecules. Specifically, optical rotations of (i) vapors of α-pinene and R-(+)-limonene, (ii) mutarotating D-glucose in water, and (iii) acidified L-histidine solutions are determined. The detection sensitivities for the gas- and solution-phase chiral activity measurements are ∼30 and ∼120µdeg over a 30 s detection period per cavity round trip pass, respectively. Furthermore, the measured optical rotations for R-(+)-limonene are compared with computations performed using the TURBOMOLE quantum chemistry package. The experimentally observed optically rotatory dispersion of this cyclic monoterpene was thus rationalized via a consideration of its room temperature conformer distribution as determined by the aforementioned single-point energy calculations.

Determining Water Transport Kinetics in Limestone by Dual-Wavelength Cavity Ring-Down Spectroscopy.

Water plays a major role in the deterioration of porous building materials such as those widely found in built heritage, influencing many physical, chemical, and biological decay processes. This article details a proof-of-principle study using near-infrared cavity ring-down spectroscopy (CRDS) to monitor the release of water and its artificially enriched isotopologues from small (ca. 25 × 25 × 5 mm) samples of limestone subject to drying by a fixed flow of nitrogen with varying levels of humidity and at room temperature and atmospheric pressure. Under low-humidity conditions, the drying kinetics are consistent with the well-established two-phase drying process exhibited by porous materials, namely, an initial constant drying rate period (phase I) followed by a falling drying rate period (phase II). The water diffusivity during phase II, DII, was measured (for Clipsham limestone) to be 3.0 × 10-9 ± 1 × 10-10 m2 s-1. The CRDS measurements allow spectroscopic determination of the total mass of water released by the sample, and the calculated values are in excellent agreement with gravimetric analysis. Importantly, the selectivity and sensitivity afforded by CRDS allows isotope analysis to be carried out, such that the flux of isotopically labeled water out of the sample can be determined under conditions of humidified flow where there may be a simultaneous ingress of water from the environment. Dual-wavelength CRDS distinguishes isotopic species, and it is demonstrated that the drying kinetics and physical properties of the samples are self-consistent when monitoring both HDO and H2O (for HDO, DII was 3.2 × 10-9 ± 4 × 10-10 m2 s-1). As the humidity levels in the flow increase, a departure from the distinct two-phase behavior is observed in the HDO drying curves. These new measurements of isotopically resolved mass fluxes will help refine models for drying mechanisms in porous media.

Continuous-Wave Cavity-Enhanced Polarimetry for Optical Rotation Measurement of Chiral Molecules.

Precise optical rotation measurements play an important role in the analysis of chiral molecules in various fields, especially in biological chemistry and pharmacology. In this paper, we demonstrate a new variant of continuous-wave cavity-enhanced polarimetry for detecting the optical activity of two enantiomers of a chiral molecule at 730 nm. It is based on a signal-reversing technique for which the chiral specific rotation is directly determined by the cavity ring-down signal from two counter-propagating beams in a bow-tie cavity. In particular, we ensure reproducible excitation of both modes by broadening the linewidth of a diode laser source by application of a radio frequency perturbation to its injection current. The performance of the polarimeter is demonstrated for the specific rotation of (+)- and (-)-α-pinene in different environments, including the pure vapor, open air, and the liquid phase; the detection precision ranges between 10-5 and 10-4 degrees per cavity pass depending on the environment. The apparatus is a robust and practical tool for quantifying chirality and can be developed for the entire visible and near-infrared spectral regions.

High performance continuous-wave laser cavity enhanced polarimetry using RF-induced linewidth broadening.

We present precise optical rotation measurements of gaseous chiral samples using near-IR continuous-wave cavity-enhanced polarimetry. Optical rotation is determined by comparing cavity ring-down signals for two counter-propagating beams of orthogonal polarisation which are subject to polarisation rotation by the presence of both an optically active sample and a magneto-optic crystal. A broadband RF noise source applied to the laser drive current is used to tune the laser linewidth and optimise the polarimeter, and this noise-induced laser linewidth is quantified using self-heterodyne beat-note detection. We demonstrate the optical rotation measurement of gas phase samples of enantiomers of α-pinene and limonene with an optimum detection precision of 10 µdeg per cavity pass and an uncertainty in the specific rotation of ∼0.1 deg dm-1 (g/ml)-1 and determine the specific rotation parameters at 730 nm, for (+)- and (-)-α-pinene to be 32.10 ± 0.13 and -32.21 ± 0.11 deg dm-1 (g/ml)-1, respectively. Measurements of both a pure R-(+)-limonene sample and a non-racemic mixture of limonene of unknown enantiomeric excess are also presented, illustrating the utility of the technique.

Portable Device for Measuring Breath Acetone Based on Sample Preconcentration and Cavity Enhanced Spectroscopy.

A portable and compact device is demonstrated for measuring acetone in breath samples. The device features a 7 cm long high finesse optical cavity as an optical sensor that is coupled to a miniature adsorption preconcentrator containing 0.5 g of polymer material. Acetone is trapped out of breath and released into the optical cavity where it is probed by a near-infrared diode laser operating at ∼1670 nm. With an optical cavity mirror reflectivity of 99.994%, a limit of detection of 159 ppbv (1σ) is demonstrated on samples from breath bags. Initial results on direct breath sampling are presented with a precision of 100 ppbv. The method is validated with measurements made using an ion-molecule reaction mass spectrometer. Data are presented on elevated breath acetone from two individuals following an overnight fast and exercise, and from a third individual during several days of routine behavior.

Laser-based method and sample handling protocol for measuring breath acetone.

A robust method is demonstrated to measure acetone in human breath at sub parts-per-million by volume (ppmv) concentrations using diode laser cavity enhanced absorption spectroscopy. The laser operates in the near-infrared at about 1690 nm probing overtone transitions in acetone in a spectral region relatively free from interference from common breath species such as CO2, water, and methane. Using an optical cavity with a length of 45 cm, bound by mirrors of 99.997% reflectivity, a limit of detection of ∼180 parts-per-billion by volume (ppbv) (1σ) of breath acetone is achieved. The method is validated with measurements made with an ion-molecule reaction mass spectrometer. A technique to calibrate the optical cavity mirror reflectivity using a temperature dependent water vapor source is also described.

RF noise induced laser perturbation for improving the performance of non-resonant cavity enhanced absorption spectroscopy.

We present a novel strategy for suppressing mode structure which often degrades off-axis cavity enhanced absorption spectra. This strategy relies on promoting small, random fluctuations in the optical frequency by perturbing the injection current of the diode laser source with radio frequency (RF) bandwidth-limited white noise. A fast and compact oxygen sensor, constructed from a 764 nm vertical-cavity surface-emitting laser (VCSEL) and an optical cavity with re-entrant configuration, is employed to demonstrate the potential of this scheme for improving the sensitivity and robustness of a field-deployable cavity spectrometer. The RF spectral density of the current noise injected into the VCSEL has been measured, and correlated to the effects on the optical spectral signal-to-noise ratio (SNR) and laser linewidth for a range of re-entrant geometries. A fourfold gain in the SNR has been achieved using the RF noise perturbation for the optimal off-axis alignment, which led to a minimum detectable absorption (MDA) predicted from an Allan variance study as low as 4.3 × 10(-5) at 1 s averaging. For the optically forbidden oxygen transition under investigation, a limit of detection (SNR = 1) of 810 ppm was achieved for a 10 ms acquisition time. This performance level paves the way for a fast, sensitive, in-line oxygen spectrometer that lends itself to a range of applications in respiratory medicine.

Enhancing the sensitivity of mid-IR quantum cascade laser-based cavity-enhanced absorption spectroscopy using RF current perturbation.

The sensitivity of mid-IR quantum cascade laser (QCL) off-axis cavity-enhanced absorption spectroscopy (CEAS), often limited by cavity mode structure and diffraction losses, was enhanced by applying a broadband RF noise to the laser current. A pump-probe measurement demonstrated that the addition of bandwidth-limited white noise effectively increased the laser linewidth, thereby reducing mode structure associated with CEAS. The broadband noise source offers a more sensitive, more robust alternative to applying single-frequency noise to the laser. Analysis of CEAS measurements of a CO(2) absorption feature at 1890 cm(-1) averaged over 100 ms yielded a minimum detectable absorption of 5.5×10(-3) Hz(-1/2) in the presence of broadband RF perturbation, nearly a tenfold improvement over the unperturbed regime. The short acquisition time makes this technique suitable for breath applications requiring breath-by-breath gas concentration information.

Demonstration of a mid-infrared cavity enhanced absorption spectrometer for breath acetone detection.

A high-resolution absorption spectrum of gaseous acetone near 8.2 µm has been taken using both Fourier transform and quantum cascade laser (QCL)-based infrared spectrometers. Absolute absorption cross sections within the 1215-1222 cm(-1) range have been determined, and the spectral window around 1216.5 cm(-1) (σ = 3.4 × 10(-19) cm(2) molecule(-1)) has been chosen for monitoring trace acetone in exhaled breath. Acetone at sub parts-per-million (ppm) levels has been measured in a breath sample with a precision of 0.17 ppm (1σ) by utilizing a cavity enhanced absorption spectrometer constructed from the QCL source and a linear, low-volume, optical cavity. The use of a water vapor trap ensured the accuracy of the results, which have been corroborated by mass spectrometric measurements.

Noise-immune cavity-enhanced optical heterodyne detection of HO2 in the near-infrared range.

Accurate measurements of the absolute concentrations of radical species present in the atmosphere are invaluable for better understanding atmospheric processes and their impact on Earth systems. One of the most interesting species is HO(2), the hydroperoxyl radical, whose atmospheric daytime levels are on the order of 10 ppt and whose observation therefore requires very sensitive detection techniques. In this work, we demonstrate the first steps toward the application of external-cavity diode-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) to the detection of the HO(2) radical in the near-infrared range. Measurements of stable species and of HO(2) were made in a laboratory setting, and the possibilities of extending the sensitivity of the technique to atmospheric conditions are discussed.

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources.

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

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.

Mid-infrared ethene detection using difference frequency generation in a quasi-phase-matched LiNbO3 waveguide.

A periodically poled LiNbO3 (PPLN) crystal waveguide has been used to produce up to 200 microW of mid-infrared light around 3081 cm(-1) with a wide tunability range of approximately 35 cm(-1). Two commercial near-infrared diode lasers at 1.064 microm (pump) and 1.583 microm (signal) are mixed in a nonlinear optical crystal to achieve difference frequency generation. The 48 mm long directly-bonded quasi-phase-matched (QPM) PPLN waveguide shows a conversion efficiency of 12.3% W(-1). Applications in trace gas detection have been demonstrated for ethene, using multipass absorption coupled with wavelength modulation spectroscopy, and cavity enhanced absorption spectroscopy with a lock-in detection scheme: bandwidth reduced sensitivities of alpha(min)=8 x 10(-9) and 1.6 x 10(-8) cm(-1) Hz(-1/2)(2sigma), respectively, have been achieved.