Anthony O'Keefe

cw Integrated cavity output spectroscopy

Abstract A new approach is described in which continuous, narrow band laser sources are employed with the recently developed integrated cavity output spectroscopy technique to obtain sensitive, quantitative absorption spectra in a simple experimental configuration. Absorption data obtained with cw-ICOS are related to the classical Fabry–Perot intracavity absorption model, which describes why the intracavity absorption is enhanced. A method of continuously injecting cw laser light into the cavity is described, as is a simple means of interpreting the ICOS data to extract accurate absorption intensities. Absorption spectra of vibrational combination bands of CO 2 and H 2 O in the 1.3 μm region are presented.

INTEGRATED CAVITY OUTPUT ANALYSIS OF ULTRA-WEAK ABSORPTION

Abstract It is demonstrated that direct absorption optical measurements with sensitivities of better than 1 part in a million can be made using pulsed light sources and employing a simple cavity optical configuration. A model is presented which demonstrates that the integrated absorption signal provides a quantitative total attenuation measurement if the absorption cavity mirror reflectivities are known. This approach to making absorption measurement provides a sensitivity comparable to that realized using the time-domain cavity ringdown approach with a significant reduction in complexity. The approach is demonstrated using the weak b 1 Σ g (v′=1) – X 3 Σ g (v″=0) forbidden absorption of oxygen near 689 nm.

Cavity-enhanced spectroscopy in optical fibers.

Cavity-enhanced methods have been extended to fiber optics by use of fiber Bragg gratings (FBGs) as reflectors. High-finesse fiber cavities were fabricated from FBGs made in both germanium/boron-co-doped photosensitive fiber and hydrogen-loaded Corning SMF-28 fiber. Optical losses in these cavities were determined from the measured Fabry-Perot transmission spectra and cavity ring-down spectroscopy. For a 10-m-long single-mode fiber cavity, ring-down times in excess of 2 ms were observed at 1563.6 nm, and individual laser pulses were resolved. An evanescent-wave access block was produced within a fiber cavity, and an enhanced sensitivity to optical loss was observed as the external mediums refractive index was altered.

The first experimental observation of electronic transitions in C2+ and C2D+

High-resolution translational energy spectroscopy (up to 0.1 eV) has been carried out on 8 kV of C2+ and C2D+. The spetra obtained with C2+ formed by different methods show considerable differences which are attributed to the formation of different spin states of the ion. Tentative assignments for the observed transitions have been made including one corresponding to excitation of the 4∑g− —X 4∑g− system, which may be useful as a probe of interstellar C2+. Two broad transitions have been seen in the translational energy spectrum of C2D+ which are in reasonable agreement with existing theoretical calculations. Tentative assignments are proposed for these transitions.

Quantitative detection of singlet O2 by cavity-enhanced absorption

A method for the practical determination of the absolute concentration of single (a1delta(g)) oxygen is discussed. The method is based on sensitive off-axis integrated-cavity-output spectroscopy (ICOS). Off-axis ICOS allows narrowband, continuous-wave lasers to be used in conjunction with optical cavities to record sensitive absorption measurements. The details of the method as well as spectroscopic data confirming the first observation of the (1, 0) band of the b1sigma(g)(+) - a1delta(g) Noxon system are presented. The absolute transition probabilities for the b1sigma(g)(+) - a1delta(g) Noxon system, which are not known precisely from experiments, are determined by quantum chemistry theory.

Comparison of energy partitioning in Ar+ and N2+ charge-transfer reactions

The kinetic energy released in near-thermal charge-transfer reactions of Ar+ and N2+ with NO, O2, CO, H2O, N2O, CO2 and NH3 has been measured. The partitioning between kinetic and internal modes is found to be very similar for most of the Ar+/N2+ pairs. Two hypotheses to explain this similarity are proposed and discussed.

Infrared cavity ringdown and integrated cavity output spectroscopy for trace species monitoring

Although the ability of high finesse optical cavities to provide effective absorption path-lengths exceeding 10 km. has been known for quite some time, attempts to utilize this property for the purposes of high-resolution spectroscopy have often resulted in extremely complex experimental systems. Here, we demonstrate how off-axis optical paths through such cavities can be employed to produce relatively simple spectrometers capable of ultrasensitive absorption measurements. A proof-of-concept study using visible diode lasers has achieved a normalized absorption sensitivity of 1.8*10-10 cm-1Hz-1/2. Additionally, quantum cascade lasers have been employed to extend this method into the mid-infrared region, where sensitivities of 1.2*10-9 cm-1Hz-1/2 have been obtained.

Sensitive absorption measurements in the near-infrared region using off-axis integrated cavity output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of ~1.4x10-5 over an effective optical path of 4,200 meters was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of ~3.1x10-11 cm-1 Hz-1/2. The instrument has been applied for measurements of CO, CH4, C2H2, and NH3 in the 1530-1650 nm range, and stable isotopes of CO2 (13CO2, 12CO2, 12C16O18O) near 2052 nm.

Translational energy spectroscopy of NO+ ions formed by charge transfer from Ar+

Abstract Translational energy spectroscopy (TES) of NO + ions formed by Ar + charge exchange has been studied. The two features observed in the spectrum are assigned to transitions from the υ″ = 0 and possibly υ″ = 1 and 2 levels of the a 3 Σ + state to the low vibrational levels of the w 3 Δ and b′ 3 Σ − states. Comparison with previous TES spectra of NO + formed by electron impact is reported and demonstrates the high selectivity of the charge transfer reaction in populating the first excited state of NO + .

Sensitive absorption measurements based on novel cavity enhanced spectroscopy techniques

A novel laser-based absorption diagnostic technique based on off-axis paths in high-finesse optical cavities provides high measurement sensitivities (~2×10−10 cm−1). Applications to environmental monitoring and industrial process control using near-IR semiconductor diode lasers and mid-IR quantum cascade lasers will be presented.