In this paper we present experimental results on several features of optically excited alkali atoms. We describe small
signal gain measurements including spatially-resolved gain in atomic Cs. We discuss observations of numerous near- to
mid-IR emissions from states that are higher in energy than the pump beam photons. Finally we outline a measurement
scheme to determine the threshold pump intensities for two types of optically excited alkali lasers.
In this paper we describe a platform for small signal gain measurements for alkali atom laser systems based on the
DPAL excitation method. We present initial results that clearly show the transition from absorption on the alkali atom D1
lines in Cs and Rb to optical transparency and positive gain. The achievement of optical gain is critically dependent upon
alkali cell conditions and collision partners. We also present the first spatially resolved gain measurements in a DPAL
system. The small signal gain methods described will be valuable tools for power scaling of these laser systems.
In this paper we describe several diagnostics that we have developed to assist the development of high power gas
phase lasers including COIL, EOIL, and DPAL. For COIL we discuss systems that provide sensitive measurements
of O2(a), small signal gain, iodine dissociation, and temperature. These are key operational parameters within COIL,
and these diagnostics have been used world-wide to gain a better understanding of this laser system. Recently, we
have developed and integrated a similar suite of diagnostics for scaling the EOIL system and will provide examples
of current studies. We are also developing diagnostics for the emerging DPAL laser. These include monitors for
small signal gain that will provide both a more fundamental understanding of the kinetics of DPAL and valuable
data for advanced resonator design. We will stress the application of these diagnostics to realistic laser systems.
This paper reports the development and initial testing of a field-portable sensor for monitoring hydrogen peroxide (H2O2)
and water (H2O) vapor concentrations during building decontamination after accidental or purposeful exposure to
hazardous biological materials. During decontamination, a sterilization system fills ambient air with water and peroxide
vapor to near-saturation. The peroxide concentration typically exceeds several hundred ppm for tens of minutes, and
subsequently diminishes below 1 ppm. The H2O2/ H2O sensor is an adaptation of a portable gas-sensing platform based
on Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology. By capitalizing on its spectral resolution, the
TDLAS analyzer isolates H2O2 and H2O spectral lines to measure both vapors using a single laser source. It offers a
combination of sensitivity, specificity, fast response, dynamic range, linearity, ease of operation and calibration,
ruggedness, and portability not available in alternative H2O2 detectors. The H2O2 range is approximately 0- 5,000 ppm.
The autonomous and rugged instrument provides real-time data. It has been tested in a closed-loop liquid/vapor
equilibrium apparatus and by comparison against electrochemical sensors.
We describe a series of measurements of absorption and laser induced fluorescence on cells that contained cesium and
rubidium and a rare gas: He, Ar, Kr, or Xe. These studies showed strong blue wing absorption to the short wavelength
side of the alkali atom D2 lines due to collisionally formed Cs- or Rb-rare gas excimers. We also have observed an
efficient two photon excitation of higher lying states in Cs and Rb that produces both intense blue emission and IR
atomic emission in the 1.3 to 3.8 μm spectral region.
We describe a series of measurements of absorption and laser induced fluorescence using cells that contained cesium and
rubidium and krypton as a bath gas. These studies showed strong blue wing absorption to the short wavelength side of
the alkali atom D2 lines due to collisionally formed Cs-Kr or Rb-Kr excimers. These studies indicate that these species
may be appropriate candidates for optically excited Rb and Cs atomic lasers.
Scaling of EOIL systems to higher powers requires extension of electric discharge powers into the kW range and
beyond with high efficiency and singlet oxygen yield. We have previously demonstrated a high-power microwave
discharge approach capable of generating singlet oxygen yields of ~25% at ~50 torr pressure and 1 kW power. This
paper describes the implementation of this method in a supersonic flow reactor designed for systematic investigations of
the scaling of gain and lasing with power and flow conditions. The 2450 MHz microwave discharge, 1 to 5 kW, is
confined near the flow axis by a swirl flow. The discharge effluent, containing active species including O2(a1Δg, b1Σg+),
O(3P), and O3, passes through a 2-D flow duct equipped with a supersonic nozzle and cavity. I2 is injected upstream of
the supersonic nozzle. The apparatus is water-cooled, and is modular to permit a variety of inlet, nozzle, and optical
configurations. A comprehensive suite of optical emission and absorption diagnostics is used to monitor the absolute
concentrations of O2(a), O2(b), O(3P), O3, I2, I(2P3/2), I(2P1/2), small-signal gain, and temperature in both the subsonic and
supersonic flow streams. We discuss initial measurements of singlet oxygen and I* excitation kinetics at 1 kW power.
The paper presents results of singlet delta oxygen yield (SDO) measurements in a high-pressure, non-self-sustained discharge and small signal gain measurements on the 1315 nm iodine atom transition in the M=3 supersonic cavity downstream of the discharge. The results demonstrate operation of a stable pulser-sustainer discharge in O2-He flows at pressures of up to 120 torr and discharge powers of up to 2.2 kW. The reduced electric field in the DC sustainer discharge ranges from 6 to 12 Td. SDO yield in the discharge is up to 5.0-5.7% at the discharge temperatures of 400-420 K. The results suggest that SDO yield exceeds the gain threshold yield at the M=3 cavity temperature by up to a factor of three, which is confirmed by gain measurements. The highest gain measured in the supersonic cavity is 0.01%/cm.
KEYWORDS: Chemical species, Iodine, Absorption, iodine lasers, Diagnostics, Chemistry, Chlorine, Semiconductor lasers, Chemical oxygen iodine lasers, Energy transfer
We discuss experimental results from spectroscopic and kinetic investigations of the reaction sequence starting with
NCI3 + H. Through a series of abstraction reactions, NCI (a1Δ) is produced. We have used sensitive optical emission
diagnostics and have observed both [NCI(a1Δ)]and [NCI(b1Σ)] produced by this reaction. Upon addition of HI to
the flow, the reaction of H + HI produced iodine atoms that were pumped to the excited I(2P1/2) state, and we
observed strong emission from the I atom 2P 1/2 -> 2P3/2 transition at 1.315 μm. With a tunable diode laser we probed
the I atom transition and observed significant transfer of population from ground state (2P3/2) to the excited state
(2P1/2) and have observed optical transparency within the iodine atom energy level manifold.
Generation of singlet oxygen metastables, O2(a1Δ), in an electric discharge plasma offers the potential for development of compact electric oxygen-iodine laser (EOIL) systems using a recyclable, all-gas-phase medium. The primary technical challenge for this concept is to develop a high-power, scalable electric discharge configuration that can produce high yields and flow rates of O2(a) to support I(2P1/2->2P3/2) lasing at high output power. This paper discusses the chemical kinetics of the generation of O2(a) and the excitation of I(2P1/2) in discharge-flow reactors using microwave discharges at low power, 40-120 W, and moderate power, 1-2 kW. The relatively high E/N of the microwave discharge, coupled with the dilution of O2 with Ar and/or He, leads to increased O2(a) production rates, resulting in O2(a) yields in the range 20-40%. At elevated power, the optimum O2(a) yield occurs at higher total flow rates, resulting in O2(a) flow rates as large as 1 mmole/s (~100 W of O2(a) in the flow) for 1 kW discharge power. We perform the reacting flow measurements using a comprehensive suite of optical emission and absorption diagnostics to monitor the absolute concentrations of O2(a), O2(b), O(3P), I2, I(2P3/2), I(2P1/2), small-signal gain, and temperature. These measurements constrain the kinetics model of the system, and reveal the existence of new chemical loss mechanisms related to atomic oxygen. The results for O2(a) production at 1 kW have intriguing implications for the scaling of EOIL systems to high power.
In this paper we discuss several sensitive diagnostics that have specifically developed for application to COIL and other iodine laser concepts such as AGIL and DOIL. We briefly cover the history of some important diagnostics including recently-developed diode laser sensors for a variety of parameters including: water vapor concentration, singlet oxygen yield, small signal gain, and translational temperature. We also discuss new developments and extensions of prior capabilities including: an ultra-sensitive diagnostic for I2 dissociation, a new monitor for singlet oxygen yield, and a novel diode laser-based imaging system for simultaneous, multipoint spatial distributions of species concentration and temperature. Finally, we mention how these diagnostics have bee successfully applied to the emerging DOIL technology.
In this paper we discuss the application of sensitive, non-intrusive diagnostic techniques to characterize species in the flow that are critical for chemical oxygen iodine laser (COIL) devices and the electric discharge oxygen iodine laser (DOIL) concept. The key diagnostics include chemiluminescence to detect O2(a,b) and I(2P1/2) and tunable diode laser absorption measurements of I* and temperature. We have characterized variations in O and O2(a) yields with discharge power and oxygen mole fraction. We observe O2(a) yields to increase dramatically with decreasing oxygen mole fraction. We also discuss the application of a novel imaging diagnostic to obtain 2-D images of species concentrations and temperature.
In this paper we present results from a spectroscopic and kinetic study of the reaction sequence of NCl3 + H that produces NCl(a1Δ). Using sensitive optical emission diagnostics, we have observed both NCl(a) and (b) produced by this reaction. Upon addition of HI to the flow, the reaction of H + HI produced iodine atoms that were pumped to the excited I(2P1/2) state, and we observed strong emission from the I atom 2P1/2 → 2P3/2 transition at 1.315 μm. We also used a sensitive diode laser spectrometer to probe the I atom transition and observed transfer of population from ground state (2P3/2) to the excited state (2P1/2) with a concomitant reduction in the measured absorption. We interpret this observation as an approach to optical transparency.
Preliminary measurements of the yield of O2(1Δ) as a function of power absorbed in an RF discharge are presented. The yield is deduced from measurements of gain/absorption using the PSI Iodine Scan diagnostic coupled with a data reduction technique originally developed by PSI. A more formal presentation of the method of deducing the yield is provided. Atomic oxygen titration experiments are presented along with gain as a function of power input to the system.
In this paper we discuss vibrational to electronic energy transfer as a potential method for producing a population inversion in atomic iodine. We discuss the background of this approach and a novel, high-flux F atom source integrated into a small scale supersonic reactor. We present data for energy transfer from HF(v) and H2(v) to the I atom manifold. Using a sensitive diode laser diagnostic we have probed the ground state manifold atomic iodine and observed that the absorption on the I atom line could be reduced to an immeasureably low value. We also describe a novel, diode laser based imaging diagnostic that will have important applications in future chemical or electrical laser development.
This paper discusses methods, using non-intrusive diagnostic techniques, to characterize the detailed dynamics of I* gain and O2(a1Δ) yield on a laboratory microwave-discharge flow reactor, for conditions relevant to the electrically driven COIL concept. The key diagnostics include tunable diode laser absorption measurements of I* small-signal gain and temperature, high-precision absorption measurements of reactor I2 concentrations, absolute and relative spectral emission measurements of O2(a1Δ) and I* concentrations, and air-afterglgow determinations of O concentrations. We have characterized variations in O and O2(a) yields with discharge power and oxygen mole fraction. We observe O2(a) yields to increase dramatically with decreasing oxygen mole fraction. We have also demonstrated a spectral fitting analysis technique capable of quantifying the presence of vibrationally excited O2(a,v). This combined suite of diagnostics offers a comprehensive approach to performance characterization for electrically driven COIL concepts.
We present preliminary results of an effort to develop an ultra-sensitive, diode laser-based diagnostic for NCl(X), an important species in the all gas phase iodine laser. This system uses a narrow band, tunable diode laser to probe transitions within the (0,0) band of the NCl(b - X) system near 662 nm. We provide a description of our detection and calibration strategies and present initial calibration results.
Steps are described for fabricating, preparing, and assembling pigtailed optical mode converters being developed for low loss coupling of optical fibers to high index contrast waveguide devices and arrays. The mode converters comprise adiabatic waveguide tapers fabricated from silicon-on-insulator (SOI) wafers, utilizing the silicon device layer as a waveguide core and the buried oxide as the underlying clad. Polished facets comprise the input and output ends of the tapers. The mode shape at the input typically matches that of an SMF-28 fiber, while the output ends can be sized to match various waveguide device mode shapes, typically ranging from 1 to 5 microns with aspect ratios as high as 5:1. Semiconductor planar processing techniques are employed to form the tapers upon commercial SOI wafers. An additional oxide layer is deposited upon the tapers to provide a symmetric clad around the silicon. Once fabricated, the wafers are diced into chips containing rows of tapers. The input and output facets are then lapped and polished, using a precision end point process, after which an anti reflective (AR) coating is applied. Following AR coating the chips are aligned and bonded to either single fibers or V-groove fiber arrays, creating the final pigtailed mode converter device. The insertion loss for completed mode converters ranges from 0.5 to 1 dB depending upon output facet size and asymmetry.
We discuss recent progress in the development of sensitive, diode laser based diagnostics for chemical oxygen iodine lasers. These diagnostics have been developed to the point where multiple species, temperature, and velocity in the flow can be determined. This paper focuses on measurement of collisional broadening parameters that are needed to extract the transnational temperature from the flow. We report the temperature dependance of collisional broadening coefficients for the (3,4) hyperfine transition with helium and oxygen bath gases.
We present results for collisional broadening for selected absorption lines in water vapor and atomic iodine relative to diode laser- based diagnostics for chemical oxygen iodine lasers. For water vapor we measured broadening of the 1.3925 µm line by numerous gases including oxygen, water vapor, nitrogen, and helium. Preliminary measurements were also completed on the (3,4) hyperfine line in atomic iodine at 1.3152 µm.
The verification of low water vapor impurity levels in semiconductor manufacturing feed gas supplies is becoming critically important for the development of advanced electronic devices. Ammonia is one of the important precursor gases for electronic manufacturing. In this paper, we present data from a water vapor absorption spectroscopy sensor designed to continuously measure ppb water impurities in pure ammonia gas with a 1 Hz bandwidth. The sensor is built using a near-IR diode laser, commercial fiber optic components, room-temperature InGaAs photodiodes, an ultra- sensitive balanced radiometric detection circuit, and a modified commercially available multipass cell. We present water vapor collisional broadening data by ammonia used to determine the optimal operating pressure for maximum system sensitivity. The commercial multipass cell was modified for ease of alignment, a nearly continuously variable pathlength, and to minimize the atmospheric air pathlength outside of the cell. The computer- controlled sensor is applicable to making water impurity measurements in a number of additional commercially important gases, such as hydrogen chloride, hydrogen fluoride, hydrogen bromide, silane, etc. The sensor is also applicable to moisture measurements in natural gas, and manufacturing dryer applications, such as those found in the plastics industry or the pharmaceutical industry, where in-line process control is critical.
For the past several years we have been developing diode laser-based diagnostics for the Chemical Oxygen Iodine Laser (COIL). These systems operate by scanning a tunable diode laser over complete absorption lines in species such as water and oxygen. In this paper we describe measurements of the collisional broadening and shifts of absorption transitions in both water and oxygen. These results have implications for the interpretation of data such as the flow temperature when the diagnostics are used on COIL systems.
A family of continuous, compactly supported, bivariate multi-scaling functions have recently been constructed by Donovan, Geronimo, and Hardin using self-affine fractal surfaces.In this paper we describe a construction of associated multiwavelets that uses the symmetry properties of the multi-scaling functions. Illustrations of a particular set of scaling functions and wavelets are provided.
We describe several diode laser-based instruments that can detect important species in chemical oxygen iodine lasers (COIL). Species detected include: water vapor, atomic iodine, and ground state oxygen. The sensors allow non-intrusive, real-time measurements from which one can determine small signal gain and the singlet delta oxygen yield. The water vapor concentrations can also be continuously monitored. The sensitivities of the sensors are sufficient for all the conditions found in typical COIL devices. The room temperature diode lasers are miniature and fiber coupled. Data for all three species are presented.
The determination of small signal gain in candidate laser systems, especially low gain chemically pumped lasers, is a challenging problem. In this paper we discuss a new approach to this problem using tunable diode lasers as sensitive probes for optical gain. The gain diagnostic was verified on optically pumped molecular iodine. Small signal gains of 104 were detectable with signal to noise exceeding 10. The gain diagnostic was also applied to a candidate chemically pumped laser system, the oxygen pumped IF laser.
Recent advances in room-temperature tunable diode lasers, fiberoptic beam transport, and sensitive detection strategies now permit in-stream sensing of numerous parameters relevant to aeropropulsion monitoring and control. These include density measurements of important flow constituents such as O2, H2O, CO2, and NOx. Based on path-averaged absorption measurements, the basic density measurements can be expanded to include other gasdynamic properties such as temperature and velocity. Simultaneous, multiparameter measurements allow determination of high order system parameters such as mass flux and thrust continuously and in real-time. This paper describes several sensor development efforts, exhaust mass flux, and emissions monitoring. Example measurements from laboratory configurations are presented along with performance projections for test-stand and flight systems. Integration issues with full-scale hardware and control opportunities are also discussed.
We describe experimental results of the conversion of a commercially available pulsed dye laser into a Stimulated Brillouin Scattering (SBS) dye laser. The dye laser power amplifier served not only as an amplification stage, but also as the SBS mirror medium. We present far- field dye laser and SBS dye laser beam profiles for comparison. The SBS dye laser produced a 56% improvement in the dye laser beam brightness. These results have important implications for improved nonlinear frequency conversion processes such as frequency doubling/mixing or optical parametric oscillation.
We describe the results of an analytical and experimental program that is investigating the feasibility of developing mid-IR lasers based upon laser pumped gas phase molecules. We present results for lasing on overtone pumped HF, HCl, and DF. In addition, describe several possible excitation sources including diode lasers and alexandrite lasers.
We describe experimental results of cw laser operation using a red (670 nm) laser diode pumping two near-infrared dyes (Rd-800 and Rd-700). Red diode lasers operating with 300 mW cw power were characterized, paying particular attention to beam quality. Collimating optics and a highly astigmatic focusing system were designed and demonstrated, producing an approximately 50 micrometers diameter focal spot, resulting in over 15 kW/cm2 pump intensity (per diode). The resulting diode/optical system was used to pump a commercial Coherent Model 599 cw dye laser utilizing a flowing dye jet.
Experiments that are designed to investigate the viability of polyacrylamide gel as a prospective host for use as solid state dye laser rods are described. We present data for absorption and fluorescence spectra for dye-doped gel samples. We also describe preliminary Nd:YAG second harmonic pumping of dye-doped samples within a simple dye laser optical cavity. We observed efficient dye laser oscillation.
This experimental research program is designed to assess the possibility of using gas-phase optically pumped lasers (OPL) as efficient, frequency-agile mid-infrared sources. The eventual goal will be to incorporate efficient diode lasers into the pumping step, either by direct frequency stabilized diode pumping or by pumping with diode-pumped solid state lasers. In this paper, we discuss experiments on a optically pumped hydrogen fluoride laser. Rotation- vibration transitions in the (2,0) band around 1.3 micrometers are pumped, and lasing is observed on (2,1) band transitions near 2.7 micrometers .
KEYWORDS: Energy transfer, Molecules, Iodine, Chemical lasers, Molecular lasers, Laser induced fluorescence, Oxygen, Chemical oxygen iodine lasers, Argon, Molecular energy transfer
In this paper we describe a new energy transfer concept to produce excited electronic states of molecules that are candidates for short wave chemical lasers. Vibrational excitation within the ground state is used to enhance the production of excited electronic states via energy transfer from metastable species such as O2(a) and NF(a). We describe experiments on the molecule IF as a prototype system.
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