Ring-down Measurements Research Articles

Ozone spectroscopy in the electronic ground state: High-resolution spectra analyses and update of line parameters since 2003

The state of the art analyses of high-resolution ozone spectra and a critical discussion of line parameters updated since the previous review by Rinsland et al. [J Quant Spectrosc Radiat Transfer 2003;82:207–18] are presented. We focus on recent works revisiting improved measurements and analyses of Fourier transform spectra below 5600cm−1 and review Cavity Ring-Down Spectroscopy measurements and analyses in the 5850–7920cm−1 spectral region which have been performed for 16O3 and all 18O enriched ozone isotopologues. Various issues related to resonance perturbations due to dark states are discussed as well as the issues concerning the accuracy of the experimental parameters, the data modelling and the data reduction. Comparisons of vibration levels determined from spectra analyses with predictions from the molecular potential functions are given. The procedures of empirical corrections for calculated line lists are described and the “intricate” question of absolute intensity determination is overviewed. Also, a review of deficiencies, challenge for improvements, and related works in progress are reported.

Relationship between Oxidation Level and Optical Properties of Secondary Organic Aerosol

Brown carbon (BrC), which may include secondary organic aerosol (SOA), can be a significant climate-forcing agent via its optical absorption properties. However, the overall contribution of SOA to BrC remains poorly understood. Here, correlations between oxidation level and optical properties of SOA are examined. SOA was generated in a flow reactor in the absence of NOx by OH oxidation of gas-phase precursors used as surrogates for anthropogenic (naphthalene, tricyclo[5.2.1.0(2,6)]decane), biomass burning (guaiacol), and biogenic (α-pinene) emissions. SOA chemical composition was characterized with a time-of-flight aerosol mass spectrometer. SOA mass-specific absorption cross sections (MAC) and refractive indices were calculated from real-time cavity ring-down photoacoustic spectrometry measurements at 405 and 532 nm and from UV-vis spectrometry measurements of methanol extracts of filter-collected particles (300 to 600 nm). At 405 nm, SOA MAC values and imaginary refractive indices increased with increasing oxidation level and decreased with increasing wavelength, leading to negligible absorption at 532 nm. Real refractive indices of SOA decreased with increasing oxidation level. Comparison with literature studies suggests that under typical polluted conditions the effect of NOx on SOA absorption is small. SOA may contribute significantly to atmospheric BrC, with the magnitude dependent on both precursor type and oxidation level.

Matrix of Integrated Superconducting Single-Photon Detectors With High Timing Resolution

We demonstrate a large grid of individually addressable superconducting single photon detectors on a single chip. Each detector element is fully integrated into an independent waveguide circuit with custom functionality at telecom wavelengths. High device density is achieved by fabricating the nanowire detectors in traveling wave geometry directly on top of silicon-on-insulator waveguides. Our superconducting single-photon detector matrix includes detector designs optimized for high detection efficiency, low dark count rate and high timing accuracy. As an example, we exploit the high timing resolution of a particularly short nanowire design to resolve individual photon round-trips in a cavity ring-down measurement of a silicon ring resonator.

An energy-efficient readout circuit for resonant sensors based on ring-down measurement

This paper presents an energy-efficient readout circuit for resonant sensors that operates based on a transient measurement method. The resonant sensor is driven at a frequency close to its resonance frequency by an excitation source that can be intermittently disconnected, causing the sensor to oscillate at its resonance frequency with exponentially decaying amplitude. By counting the zero crossings of this ring-down response, the interface circuit can detect the resonance frequency. In contrast with oscillator-based readout, the presented readout circuit is readily able to detect quality factor (Q) of the resonator from the envelope of the ring-down response, and can be used even in the presence of large parasitic capacitors. A prototype of the readout circuit has been integrated in 0.35 μm CMOS technology, and consumes only 36 μA from a 3.3 V supply during a measurement time of 2 ms. The resonance frequency and quality factor of a micro-machined SiN resonator obtained using this prototype are in good agreement with results obtained using impedance analysis. Furthermore, a clear transient response is observed to ethanol flow using the presented readout, demonstrating the use of this technique in sensing applications.

Sensitive analysis of trace water analytes using colourimetric cavity ringdown spectroscopy

The application of colourimetric cavity ringdown spectroscopy to the detection of trace compounds in water has been investigated using nitrite and iron(II) as test analytes. Samples were contained within one of three commercially available flow cells ranging in optical path length from 0.1 mm to 2.0 mm, and positioned within a two-mirror ringdown cavity. A measurement of the decay rate of the intensity of an optical pulse introduced into the cavity allows an ultrasensitive determination of optical absorption by the sample. A calibration using the known absorption coefficient of potassium permanganate at 532 nm was first carried out in order to determine the detection sensitivity in terms of minimum detectable absorption per unit path length when using each flow cell. The detection of nitrite and iron was then carried out by using well-known colour reactions, namely the Griess reaction for nitrite and the bathophenanthroline method for iron(II), to convert the analytes into strongly absorbing derivatives, which were quantified by a cavity ringdown measurement. In this first application of colourimetric cavity ringdown spectroscopy to the liquid phase, detection limits of 1.9 nM for nitrite and 3.8 nM for Fe(II) were demonstrated in a flow cell of path length 1.0 mm. The volume of sample analysed is only 196 nL, so that detection limits of this order correspond to the detection of less than 1 billion molecules. The detection method is therefore suitable for integration into a microfluidic sensing platform.

A portable optical emission spectroscopy-cavity ringdown spectroscopy dual-mode plasma spectrometer for measurements of environmentally important trace heavy metals: Initial test with elemental Hg

A portable optical emission spectroscopy-cavity ringdown spectroscopy (OES-CRDS) dual-mode plasma spectrometer is described. A compact, low-power, atmospheric argon microwave plasma torch (MPT) is utilized as the emission source when the spectrometer is operating in the OES mode. The same MPT serves as the atomization source for ringdown measurements in the CRDS mode. Initial demonstration of the instrument is carried out by observing OES of multiple elements including mercury (Hg) in the OES mode and by measuring absolute concentrations of Hg in the metastable state 6s6p (3)P(0) in the CRDS mode, in which a palm-size diode laser operating at a single wavelength 405 nm is incorporated in the spectrometer as the light source. In the OES mode, the detection limit for Hg is determined to be 44 parts per 10(9) (ppb). A strong radiation trapping effect on emission measurements of Hg at 254 nm is observed when the Hg solution concentration is higher than 50 parts per 10(6) (ppm). The radiation trapping effect suggests that two different transition lines of Hg at 253.65 nm and 365.01 nm be selected for emission measurements in lower (<50 ppm) and higher concentration ranges (>50 ppm), respectively. In the CRDS mode, the detection limit of Hg in the metastable state 6s6p (3)P(0) is achieved to be 2.24 parts per 10(12) (ppt) when the plasma is operating at 150 W with sample gas flow rate of 480 mL min(-1); the detection limit corresponds to 50 ppm in Hg sample solution. Advantage of this novel spectrometer has two-fold, it has a large measurement dynamic range, from a few ppt to hundreds ppm and the CRDS mode can serve as calibration for the OES mode as well as high sensitivity measurements. Measurements of seven other elements, As, Cd, Mn, Ni, P, Pb, and Sr, using the OES mode are also carried out with detection limits of 1100, 33, 30, 144, 576, 94, and 2 ppb, respectively. Matrix effect in the presence of other elements on Hg measurements has been found to increase the detection limit to 131 ppb. These elements in lower concentrations can also be measured in the CRDS mode when a compact laser source is available to be integrated into the spectrometer in the future. This exploratory study demonstrates a new instrument platform using an OES-CRDS dual-mode technique for potential field applications.

A laser gyroscope system to detect the gravito-magnetic effect on Earth

Ring lasers are inertial sensors for angular velocity based on the Sagnac effect. In recent years they have reached a very high sensitivity and accuracy; the best performing one, the ring Laser G in Wettzell (Germany), a square ring with 16 m perimeter, has reached a sensitivity of 12prad/s very close to the shot noise limit inferred from ring-down time measurements. On this basis it is expected that an array of six square ring lasers of 36 m perimeter, can perform a 1% accuracy test for the measurement of the Lense-Thirring frame dragging after 2 years of integration time. Essential for this measurement is the comparison between the Earth angular velocity and orientation in space measured with the ring array and compared to the measurement series maintained by the International Earth Rotation and Reference System Service (IERS), which measures Earth Rotation and pole position with respect to remote quasars. It has been shown that the accuracy of G in Wettzell is limited by the low frequency motion of the near surface laboratory, which is of the order of several prad/s, roughly 100 times larger than the Lense-Thirring contribution. For this reason the entire experiment should be placed in a quite underground laboratory, where these perturbations are reduced. The feasibility to properly place such a device inside the GranSasso INFN National Laboratory has been investigated.

Erratum: “Frequency-stabilized cavity ring-down spectroscopy measurements of line mixing and collision induced absorption in the O2 A-band” [J. Chem. Phys. 137, 014307 (2012)

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Frequency-stabilized cavity ring-down spectroscopy measurements of line mixing and collision-induced absorption in the O2 A-band

Frequency-stabilized cavity ring-down spectroscopy measurements were performed in the P-branch of the O(2) A-band [b(1)Σ(g) (+) ← X (3)Σ(g) (-)(0,0)] near atmospheric pressure. Line mixing parameters and collision-induced absorption were quantified and reported. These measurements show qualitative differences with those taken at relatively high pressure (2 MPa-20 MPa). We also assess the implications of these measurements on atmospheric retrievals.

What Is the Best DFT Functional for Vibronic Calculations? A Comparison of the Calculated Vibronic Structure of the S1–S0 Transition of Phenylacetylene with Cavity Ringdown Band Intensities

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.

High-reflectivity, high-Q micromechanical membranes via guided resonances for enhanced optomechanical coupling

Using Fano-type guided resonances (GRs) in photonic crystal (PhC) slab structures, we numerically and experimentally demonstrate optical reflectivity enhancement of high-Q SiNx membrane-type resonators used in membrane-in-the-middle optomechanical (OM) systems. Normal-incidence transmission and mechanical ringdown measurements of 50-nm-thick PhC membranes demonstrate GRs near 1064 nm, leading to a ∼4 × increase in reflectivity while preserving high mechanical Q factors of up to ∼5 × 106. The results would allow improvement of membrane-in-the-middle OM systems by virtue of increased OM coupling, presenting a path towards ground state cooling of such a membrane and observations of related quantum effects.

Top Notch Design for Fiber-Loop Cavity Ring-Down Spectroscopy

Fiber-loop cavity ring-down spectroscopy (CRDS) is a highly sensitive spectroscopic absorption technique which has shown considerable promise for the analysis of small-volume liquid samples. We have developed a new light coupling method for fiber-loop CRDS, which overcomes two disadvantages of the technique: low efficiency light coupling into the cavity and high loss per pass. The coupler is based on a 45° reflective notch polished between 10 and 30 μm into the core of a large-core-diameter (365 μm) optical fiber, and allows for nearly 100% light coupling into the cavity, with a low loss per pass (<4%). The coupler has the additional advantage that the input and output light is spatially separated on opposite sides of the fiber. The detection sensitivity of a fiber-loop CRD spectrometer employing the new coupling method is established from ring-down measurements on aqueous rhodamine 6G (Rh6G) at 532 nm. The results are compared with data obtained using the same light source and detector, but a conventional bend-coupled small-core-diameter (50 μm) optical fiber loop. With our new coupler, a detection limit of 0.11 cm(-1) is found, which corresponds to detection of 0.93 μM Rh6G in a volume of only 19 nL. This is an improvement of over an order of magnitude on our bend-coupled small-core optical fiber results, in which a detection limit of 5.3 cm(-1) was found, corresponding to a detection of 43 μM Rh6G in a volume of 20 pL.

Continuous-wave fiber cavity ring-down measurements using frequency-shifted interferometry

We present a spatial-domain fiber cavity ring-down (CRD) technique that does not require optical pulses for time-resolved detection. Instead of measuring the intensity decay rate of an optical pulse in the time domain as in conventional CRD experiments, we measure the decay rate of a CW signal in a ring-down cavity (RDC) using frequency-shifted interferometry in the spatial domain, as a function of distance traveled by the light. As a proof-of-concept demonstration, we measured fiber bend loss in a loop RDC using this technique, and a cavity loss change as low as 0.0135 dB induced by every fiber turn on a mandrel was unambiguously discerned.

Frequency-stabilized cavity ring-down spectroscopy measurements of carbon dioxide isotopic ratios

Carbon dioxide (CO2) isotopic ratios on samples of pure CO2 were measured in the 1.6 μm wavelength region using the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We present CO2 absorption spectra with peak signal-to-noise ratios as high as 28,000:1. Measured single-spectrum signal-to-noise ratios were as high as 8900:1, 10,000:1, and 1700:1 for 13C/12C, 18O/16O, and 17O/16O, respectively. In addition, we demonstrate the importance of utilizing the Galatry line profile in the spectrum analysis. The use of the Voigt line profile, which neglects the observed collisional narrowing, leads to large systematic errors which are transition-dependent and vary with temperature and pressure. While the relatively low intensities of CO2 transitions near λ=1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS enabled measurement precision of pure CO2 samples which are comparable to those of other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer designed to probe CO2 bands near wavelengths of 2.0 μm or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.

Planar coupling to high-Q lithium niobate disk resonators

We demonstrate optical coupling to high-Q lithium niobate disks from an integrated lithium niobate waveguide. The waveguides are made by proton exchange in X-cut lithium niobate substrate. The disks with diameter of 4.7 mm and thickness of 1 mm are made from commercial Z-cut lithium niobate wafers by polishing the edges into a spheroidal profile. Both resonance linewidth and cavity ringdown measurements were performed to calculate the Q factor of the resonator, which is in excess of 10(8). Planar coupling represents the most promising technique for practical applications of whispering gallery mode resonators.

Magnesium Fluoride Whispering Gallery Mode Disk-Resonators for Microwave Photonics Applications

We have manufactured a high-Q whispering gallery mode resonator with magnesium fluoride for microwave photonics applications in the gigahertz frequency range. This crystal is scarcely used for resonator fabrication despite its numerous advantages, that are mainly high mechanical hardness, low sensitivity to water vapor pollution, and low sensitivity to photo- and thermo-refractive fluctuations at optimal temperatures. Using a customized machining procedure, we have successfully fabricated and characterized a 5-mm resonator with a surface rugosity of the order of 1 nm (from 3 to 12 atoms). Cavity ring-down measurements enabled us to determine that the resonator has a quality factor Q = 3.4 × 108 at 1550 nm.

Cavity ring-down spectroscopy measurements of sub-Doppler hyperfine structure

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) was used to measure magnetic dipole transitions in the $b {}^{1}$${\ensuremath{\Sigma}}_{g}^{+}$ $\ensuremath{\leftarrow}$ $X $${}^{3}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}$($0,0$) band of ${\mathrm{O}}_{2}$. The $^{17}\mathrm{O}$-containing isotopologues show unresolved hyperfine structure due to magnetic hyperfine splitting in the ground state. The sensitivity and stability of FS-CRDS allow for quantitative sub-Doppler measurements of the hyperfine constants, even when the hyperfine splittings are much smaller than the Doppler width. Unlike saturation spectroscopy, this linear absorption technique can be applied to weak transitions and employed to quantitatively measure intensities and line shapes. This method may be an attractive approach for measuring unresolved hyperfine structure in excited electronic states.

Phase-shift cavity ring-down spectroscopy using mid-IR light from a difference frequency generation PPLN waveguide

Approximately 200 microW of mid-infrared (mid-IR) light around 3081 cm(-1), produced by difference frequency generation (DFG) in a periodically poled lithium niobate crystal waveguide, has been used for phase-shift cavity ring-down spectroscopy measurements. The overlapping (12)C(2)H(4) (R)P(0)(14) and (P)R(6)(10) and (12)CH(2) (13)CH(2) (P)Q(3)(10) rotational lines of the nu(9) fundamental ethene vibrational band at 3081.0016 cm(-1) were probed in proof-of-principle experiments, and ethene detection was demonstrated with a minimum absorption coefficient of 1.4 x 10(-7) cm(-1) (approximately 4 min acquisition time). The compact DFG system, with a >35 cm(-1) tuning range, has a considerable potential for use in trace gas detection and in molecular spectroscopy.

Cavity Enhanced Spectroscopy of High-Temperature H2O in the Near-Infrared Using a Supercontinuum Light Source

In this paper we demonstrate how broadband cavity enhanced absorption spectroscopy (CEAS) with supercontinuum (SC) radiation in the near-infrared spectral range can be used as a sensitive, multiplexed, and simple tool to probe gas-phase species in high-temperature environments. Near-infrared SC radiation is generated by pumping a standard single-mode fiber with a picosecond fiber laser. Standard low reflectivity mirrors are used for the cavity and an optical spectrum analyzer is used for the detection of gas-phase species in combustion. The method is demonstrated by measuring flame generated H(2)O in the 1500 to 1550 nm region and room-temperature CO(2) between 1520 nm and 1660 nm. The broadband nature of the technique permits hundreds of rotational features to be recorded, giving good potential to unravel complex, convoluted spectra. We discuss practical issues concerning the implementation of the technique and present a straightforward method for calibration of the CEAS system via a cavity ringdown measurement. Despite the large spectral variation of SC radiation from pulse to pulse, it is shown that SC sources can offer good stability for CEAS where a large number of SC pulses are typically averaged.

Investigation on an evanescent wave fiber-optic absorption sensor based on fiber loop cavity ring-down spectroscopy

An improved ring-down measurement principle for optical waveguides is presented. Fiber loop ring-down spectroscopy allows for measurement of minute optical losses in high-finesse fiber-optic cavities and immunity to the fluctuation of laser source. The evanescent wave absorption losses dependent on the absorption and the refractive index of ambient solution have been theoretically analyzed. The complex refractive index is introduced into our model and extinction coefficient can be calculated accurately through finite element analysis by setting the boundaries of the fiber and the ambient conditions. Using this method, the refractive index of environment can be taken into consideration. Our principle is validated by the highly-sensitive measurement of evanescent wave absorption loss. By chemically processing the surface of sensing segment along an extending ring-down cavity, the concentration of small volume Diethyl Sulphoxide solution where the etched fiber immersed into has been successfully measured and discussed.