Pulsed Ringdown Research Articles

A novel fiber optic ring-down interferometer for sensing

A ring-down interferometer (RDI) based on a modified Mach–Zehnder structure by incorporating a pair of mirrors with very high reflectivity into each of its two arms, respectively is proposed in this paper. Launching a coherent light pulse into the interferometer, in each arm, pulse ring-down occurs between its two mirrors and outputs a chain of pulses. The two chains of pulses from the two arms combine and interfere pulse by pulse at the detector and the difference of the light phase between the two arms will be enlarged linearly in proportion to the times of ring-down.

Performance and Simulation Verification of a Mobile Solid State Pulsed Ring Down Array

Mobile pulsed ring down arrays are of great interest due to their ability to deliver high peak power electromagnetic pulses. Global positioning system (GPS) devices are used as a means to provide position information to individual array elements. An array of solid state radiating structures was built and tested. The array test results are compared to a Monte Carlo array simulation that take into account source jitter, timing error, and distance error between array elements due to GPS measurements.

Theoretical performance of a mobile GPS linked pulsed ring down array

The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz.

Validation of the Extended Simultaneous Kinetics and Ringdown Model by Measurements of the Reaction NH2 + NO

The determination of rate constants for fast chemical reactions from nonexponential cavity ringdown profiles requires a consideration of the interfering laser bandwidth effect that arises if the line width of the ringdown probe laser exceeds the absorption line width of the detected species. The deconvolution of the kinetics and the bandwidth effect can be accomplished with the extended simultaneous kinetics and ringdown (eSKaR) model presented by Guo et al. (Guo, et al. Phys. Chem. Chem. Phys. 2003, 5, 4622). We present a detailed validation of this eSKaR model by a corresponding investigation of the well-known rate constant for the reaction NH2 + NO. Line profiles of the pulsed ringdown probe laser and the NH2 absorption line were determined from forward modeling of experimental ringdown profiles and verified by narrow-bandwidth laser absorption measurements. In addition, the rate constant for the title reaction was evaluated using the eSKaR model and also by means of a conventional pump-probe approach with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses. The resulting room temperature rate constant for the NH2 + NO reaction, k1= (8.5 +/- 1.0) x 10(12) cm(3) mol(-1) s(-1), and the room temperature pressure broadening coefficient of NH2, = 2.27 GHz/bar, measured on the A2A1<-- X2B1 transition at wavelengths around lambda = 597 nm, were found to be in excellent agreement with the available literature data.

Nitroxide Spin−Relaxation over the Entire Motional Range

Homogeneous line widths, characterized by the phase memory time, Tm, of a nitroxide spin-labeled peptide, were measured over the entire motional range (∼10-10−10-4 s at the spectrometer resonance frequency of 9.7 GHz) using spin−echo correlated spectroscopy (SECSY) ESR. Phase memory times as short as 20 ns could be determined by using schemes designed to minimize the effects of pulse ring-down. Experimental Tm versus temperature curve (from 192 to 310 K) clearly resolves the Tm minimum, which provides great sensitivity to the details of molecular dynamics.

Cavity ringdown laser absorption spectroscopy with a 1 kHz mid-infrared periodically poled lithium niobate optical parametric generator/optical parametric amplifier

A cavity ringdown spectrometer is described that employs a novel mid-infrared light source based on periodically poled lithium niobate. The source generates tunable light using the three-step process of optical parametric generation, spectral filtering, and optical parametric amplification. Its use allows for improvements over previous pulsed ringdown measurements including the ability to acquire data rapidly (at 1 kHz) over broad spectral regions (in principle, over the entire 2220–7690 cm −1 PPLN transparency window) with narrow linewidth (⩽0.08 cm −1). Data are presented that demonstrate performance and support its eventual use in a trace gas sensor.

Time-domain observation of optical pulse propagation in whispering-gallery modes of glass spheres

We report picosecond time-resolved measurements of optical pulse propagation in dielectric spheres (8, 10, and 26 mm in diameter) for which the pulse duration ( approximately 2 ps) was short compared with the equatorial round-trip time within the sphere. A size-independent buildup of the leakage intensity in terms of the number of round trips was observed for each of the spheres, as were damped low-frequency oscillations superimposed upon the pulse ringdown envelope. These features of the data are interpreted as resulting from perturbative coupling of eigenmodes of the sphere and trajectory precession near the observation region.

Non-destructive evaluation of porous MgO ceramics using acoustic techniques

Two acoustic techniques, ultrasonics (US) and acousto-ultrasonics (AU), were used to evaluate MgO extrudate in both the green and sintered states. In US, the velocity was measured and in AU ringdown counts, pulse amplitude, pulse width and spectrum were used to quantify the output signal. The acoustic measurements showed good reproducibility, especially velocity. Under some sintering conditions the ultrasonic velocity and the porosity were lower than in the green state: this was attributed to pore growth in the early stages of sintering. It was found that as the temperature and time of sintering increased, the density, compressive strength and ultrasonic velocity increased. However, the response of the AU parameters to changes in sintering conditions was more complex, for example, the peak amplitude increased whereas pulse width and ringdown counts decreased with increasing temperature of sintering. The US and AU data on the sintered MgO extrudate were correlated with the structure and properties. Empirical relationships between the acoustic parameters and porosity and compressive strength were obtained which gave better coefficients of regression for the ultrasonic velocity than for acousto-ultrasonic parameters. It was concluded that US, and in some instances AU, are feasible as quality control techniques/non-destructive evaluation for ceramics.