We present accurate measurements for the optical constants for a series of organic liquids, including organophosphorous compounds. Bulk liquids are rarely encountered in the environment, but more commonly are present as droplets of liquids or thin layers on various substrates. Providing reference spectra to account for the plethora of morphological conditions that may be encountered under different scenarios is a challenge. An alternative approach is to provide the complex optical constants, n and k, which can be used to model the optical phenomena in media and at interfaces, minimizing the need for a vast number of laboratory measurements. In this work, we present improved protocols for measuring the optical constants for a series of liquids that span from 7800 to 400 cm-1. The broad spectral range means that one needs to account for both strong and weak spectral features that are encountered, all of which can be useful for detection, depending on the scenario. To span this dynamic range, both long and short cells are required for accurate measurements. These protocols are presented along with experimental and modeling results for thin layers of silicone oil on aluminum.
The complex optical refractive index contains the optical constants, n(ῦ)and k(ῦ), which correspond to the dispersion and absorption of light within a medium, respectively. By obtaining the optical constants one can in principle model most optical phenomena in media and at interfaces including reflection, refraction and dispersion. We have developed improved protocols based on the use of multiple path lengths to determine the optical constants for dozens of liquids, including organic and organophosphorous compounds. Detailed description of the protocols to determine the infrared indices will be presented, along with preliminary results using the constants with their applications to optical modeling.
In this paper, we present the results of long-term operational testing of several quantum cascade laser (QCL) variants to illustrate their robustness and long lifetimes. Performance factors are investigated including power and spectral stability over different timescales ranging from days to years. The effects of burn-in, packaging, mounting, and facet coatings are considered with respect to their influence on long-term laser performance. In addition, the results from the several years’ operation of a custom external cavity quantum cascade laser-based trace gas sensor are presented to highlight the reliable performance of QCL-based sensor systems. This sensor monitored the laboratory air for multiple chemicals and operated continuously for two years without any evidence of degradation in performance. The data from all of these experiments will be discussed to demonstrate the reliability and robust performance of QCLs.
KEYWORDS: Chemical analysis, Absorbance, Time metrology, Liquids, NOx, Chemical species, Data acquisition, Modulation, Quantum cascade lasers, Wavelength tuning
A rapidly-swept external-cavity quantum cascade laser with an open-path Herriott cell is used to quantify gas-phase chemical mixtures of D2O and HDO at an update rate of 40 Hz (25 ms measurement time). The chemical mixtures were generated by evaporating D2O liquid near the open-path Herriott cell, allowing the H/D exchange reaction with ambient H2O to produce HDO. Fluctuations in the ratio of D2O and HDO on timescales of < 1 s due to the combined effects of plume transport and the H/D exchange chemical reaction are observed. Based on a noise equivalent concentration analysis of the current system, detection limits of 147.0 ppbv and 151.6 ppbv in a 25 ms measurement time are estimated for D2O and HDO respectively with a 127 m optical path. These detection limits are reduced to 23.0 and 24.0 ppbv with a 1 s averaging time for D2O and HDO respectively. Detection limits < 200 ppbv are also estimated for N2O, F134A, CH4, Acetone, and SO2 for a 25 ms measurement time.
We describe a prototype trace gas sensor designed for real-time detection of multiple chemicals. The sensor uses an
external cavity quantum cascade laser (ECQCL) swept over its tuning range of 940-1075 cm-1 (9.30-10.7 μm) at a 10 Hz
repetition rate. The sensor was designed for operation in multiple modes, including gas sensing within a multi-pass
Heriott cell and intracavity absorption sensing using the ECQCL compliance voltage. In addition, the ECQCL
compliance voltage was used to reduce effects of long-term drifts in the ECQCL output power. The sensor was
characterized for noise, drift, and detection of chemicals including ammonia, methanol, ethanol, isopropanol, Freon-
134a, Freon-152a, and diisopropyl methylphosphonate (DIMP). We also present use of the sensor for mobile detection
of ammonia downwind of cattle facilities, in which concentrations were recorded at 1-s intervals.
We present experimental demonstration of a new chemical sensing technique based on intracavity absorption in an external cavity quantum cascade laser (ECQCL). This new technique eliminates the need for an infrared photodetector and gas cell by detecting the intracavity absorption spectrum in the compliance voltage of the laser device itself. To demonstrate and characterize the technique, we measure infrared absorption spectra of chemicals including acetone and Freon-134a. Sub-ppm detection limits in one second are achieved, with the potential for increased sensitivity after further optimization. The technique enables development of handheld, high-sensitivity, and high-accuracy trace gas sensors for in-field use.
We present the burn-in behavior and power stability of multiple quantum cascade lasers (QCLs) that were measured to
investigate their long-term performance. For these experiments, the current to the QCL was cycled every ten minutes,
and the output power was monitored over time for durations as long as two months. A small increase in power for a
given injection current is observed for almost all of the QCLs tested during the burn-in period. Although the amount and
duration of the burn-in varied among the devices tested, we observed that QCLs that operated with a lower threshold
current exhibited a smaller burn-in change from initial conditions for the first ten hours of operation. This correlation,
however, disappeared at longer operation times.
The effect of packaging the QCLs is also investigated to determine its impact on performance and reliability. The power
stability is measured for the packaged QCLs along with changes in the operational conditions. Although the temperature
of the QCL is kept constant with a thermistor and thermoelectric cooler (TEC) inside the package, the case temperature
is varied to monitor its correlation with any changes in power or frequency. Power changes and small frequency shifts
are observed under these conditions. One possible explanation for these changes is the influence of optical feedback
from the anti-reflection (AR) coated window in the package. The data from all of these experiments is presented.
Infrared scattering scanning near-field optical microscopy (s-SNOM) is an apertureless superfocusing technique that uses the antenna properties of a conducting atomic force microscope (AFM) tip to achieve infrared spatial resolution below the diffraction limit. The instrument can be used either in imaging mode, where a fixed wavelength light source is tuned to a molecular resonance and the AFM raster scans an image, or in spectroscopy mode where the AFM is held stationary over a feature of interest and the light frequency is varied to obtain a spectrum. In either case, a strong, stable, coherent infrared source is required. Here we demonstrate the integration of a broadly tunable external cavity quantum cascade laser (ECQCL) into an s-SNOM and use it to obtain infrared spectra of microcrystals of chemicals adsorbed onto gold substrates.
Residues of the explosive compound tetryl was deposited onto gold substrates. s-SNOM experiments were performed in the 1260-1400cm-1 tuning range of the ECQCL, corresponding to the N02 symmetric stretch vibrational fingerprint region. Vibrational infrared spectra were collected on individual chemical domains with a collection area of ~500 nm2 and compared to ensemble averaged far-field reflection-absorption infrared spectroscopy (RAIRS) results.
A multi-channel laser-based chemical sensor platform is presented, in which a modular architecture allows the exchange
of complete sensor channels without disruption to overall operation. Each sensor channel contains custom optical and
electronics packages, which can be selected to access laser wavelengths, interaction path lengths and modulation
techniques optimal for a given application or mission. Although intended primarily to accommodate mid-infrared
external cavity quantum cascade lasers and astigmatic Herriott cells, channels using visible or near infrared lasers or
other gas cell architectures can also be used, making this a truly versatile platform. Analog and digital resources have
been carefully chosen to facilitate small footprint, rapid spectral scanning, low-noise signal recovery, fail-safe
autonomous operation, and in-situ chemometric data analysis, storage and transmission. Results from the demonstration
of a two-channel version of this platform are also presented.
Stand-off detection of hazardous materials ensures that the responder is located at a safe distance from the suspected
source. Remote detection and identification of hazardous materials can be accomplished using a highly sensitive and
portable device, at significant distances downwind from the source or the threat. Optical sensing methods, in particular
infrared absorption spectroscopy combined with quantum cascade lasers (QCLs), are highly suited for the detection of
chemical substances since they enable rapid detection and are amenable for autonomous operation in a compact and
rugged package. This talk will discuss the sensor systems developed at Pacific Northwest National Laboratory and will
discuss the progress to reduce the size and power while maintaining sensitivity to enable stand-off detection of multiple
chemicals.
We describe the performance of a sensor system designed for simultaneous detection of multiple chemicals with both
broad and narrow absorption features. The sensor system consists of a broadly tunable external cavity quantum cascade
laser (ECQCL), multi-pass Herriott cell, and custom low-noise electronics. The ECQCL features a fast wavelength
tuning rate of 2265 cm-1/s (15660 nm/s) over the range of 1150-1270 cm-1 (7.87-8.70 μm), which permits detection of molecules with broad absorption features and dynamic concentrations, while the 0.2 cm-1 spectral resolution of the ECQCL system allows measurement of small molecules with atmospherically broadened absorption lines. High-speed
amplitude modulation and low-noise electronics are used to improve the ECQCL performance for direct absorption
measurements. We demonstrate simultaneous detection of Freon-134a (1,1,1,2-tetrafluoroethane), ammonia (NH3), and
nitrous oxide (N2O) at low-ppb concentrations in field measurements of atmospheric chemical releases from a point
source.
We demonstrate the use of a tunable external cavity quantum cascade laser system for measurement of broad absorption
features in the mid-infrared spectral region. The thermoelectrically cooled external cavity laser was tuned over a 65 cm-1range centered at 8.7 microns using stepper motor control. Pulsing the laser at a high duty cycle provided 2-4.5 mW
average output power over the tuning range, and enabled phase-sensitive detection of amplitude-modulated signals. We
used the laser system to measure the absorption spectra of Freon-125 using a Herriott cell. In addition, the absorption
spectrum of water in the laboratory air was measured. The measurements showed excellent agreement with reference
spectra, in both wavelength and amplitude. The measured scan resolution of 0.14 cm-1 is suitable for measurement of the
absorption features of complex molecules as well as simple molecules with atmospherically broadened lines. We
discuss the limits to the scan resolution due to effects of spectral chirp and mode-hops during pulsed operation.
Quantum cascade lasers (QCLs) are becoming well known as convenient and stable semiconductor laser sources operating in the mid- to long-wave infrared, and are able to be fabricated to operate virtually anywhere in the 3.5 to 25 micron region. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present, spanning at least three critical areas: national security, environmental monitoring and protection, and the early diagnosis of disease through breath analysis. There are many different laser-based spectroscopic chemical sensor architectures in use today, from simple direct detection through to more complex and highly sensitive systems. Many current sensor needs can be met by combining QCLs and appropriate sensor architectures, those needs ranging from UAV-mounted surveillance systems, through to larger ultra-sensitive systems for airport security. In this paper we provide an overview of various laser-based spectroscopic sensing techniques, pointing out advantages and disadvantages of each. As part of this process, we include our own results and observations for techniques under development at PNNL. We also present the latest performance of our ultra-quiet QCL control electronics now being commercialized, and explore how using optimized supporting electronics enables increased sensor performance and decreased sensor footprint for given applications.
Quantum cascade laser (QCL) offer many desirable attributes as mid-infrared laser sources for chemical and remote sensing. Some key advantages are a narrow linewidth, wide bandwidth current modulation characteristics and moderate tunability (15 cm-1). Combined, these characteristics allow for applications to a wide variety of chemical and remote sensing techniques such as wavelength and frequency modulation based detection techniques, cavity enhanced point sensors as well as techniques such as LIDAR and DIAL. This paper will describe laser development efforts to enhance QCL frequency stabilization and QCL injection locking and to develop robust external cavity QCL designs.
The small size, high power, promise of access to any wavelength between 3.5 and 16 microns, substantial tuning range about a chosen center wavelength, and general robustness of quantum cascade (QC) lasers provide opportunities for new approaches to ultra-sensitive chemical detection and other applications in the mid-wave infrared. PNNL is developing novel remote and sampling chemical sensing systems based on QC lasers, using QC lasers loaned by Lucent Technologies. In recent months laboratory cavity-enhanced sensing experiments have achieved absorption sensitivities of 8.5 x 10-11 cm-1 Hz-1/2, and the PNNL team has begun monostatic and bi-static frequency modulated, differential absorption lidar (FM DIAL) experiments at ranges of up to 2.5 kilometers. In related work, PNNL and UCLA are developing miniature QC laser transmitters with the multiplexed tunable wavelengths, frequency and amplitude stability, modulation characteristics, and power levels needed for chemical sensing and other applications. Current miniaturization concepts envision coupling QC oscillators, QC amplifiers, frequency references, and detectors with miniature waveguides and waveguide-based modulators, isolators, and other devices formed from chalcogenide or other types of glass. Significant progress has been made on QC laser stabilization and amplification, and on development and characterization of high-purity chalcogenide glasses, waveguide writing techniques, and waveguide metrology.
Solid state laser sources, such as diode-laser pumped Nd:YAG lasers, have given us a cw laser light of high power with unprecedented stability and low noise performance. In these lasers most of the technical sources of noise can be eliminated and thereby allow operation close to the theoretical limit set by the quantum properties of the light. We present progress in the experimental realization of such lasers. These investigations include the control of noise by electronic feedback, passive external cavities; and the reliable generation of amplitude squeezed light through second harmonic generation. At the same time we have developed theoretical models describing the quantum noise properties of coupled systems of lasers and cavities. The agreement between our experimental results with noise spectra calculated with our realistic theoretical models demonstrates the ability to predict the performance of various laser systems.
Design and performance data on two laser transmitters for spaceborne laser ranging are presented. The first laser uses a master oscillator/power amplifier configuration consisting of a diode pumped Nd:YAG slab ring and a multipass diode pumped slab amplifier which can operate at 40 Hz for 109 shots. The other laser is a diode pumped Nd:YAG slab standing wave oscillator which operates at 10 Hz for 0.6 X 109 shots. For submillimeter laser ranging, one laser operates in a mode-locked cavity-dumped mode to produce 180 mJ, 40 psec pulses at 1.064 micrometers . For altimetry, the same laser operates in a Q-switched mode to produce 700 mJ, 3.5 nsec pulses at 1.064 micrometers . Second and third harmonic generators generate 0.532 micrometers and 0.355 micrometers for ranging at 2 wavelengths to terrestrial targets with inherent atmospheric correction. The oscillator utilizes a ring resonator configuration with active mode locking, active Q-switching, active pre-lase stabilization, and active cavity dumping. The mode-locked output pulsewidth is 40 psec. A second oscillator mode, remotely selectable, produces 3.5 nsec pulses. Stabilization and alignment is done with real-time feedback during the mission. The amplifier is a multipass slab. Parasitic (ASE) oscillations are suppressed despite very high stored energy in the amplifier medium. The second laser transmitter is a linearly polarized Q-switched Nd:YAG slab laser cavity. The Nd:YAG is pumped by a 44-bar array of AlGaAs laser diodes. It produces 45 mJ, 10 nsec, pulses at 1064 nm and will operate at 10 Hz for the two-earth-year on-orbit lifetime. The expected operation will produce 6 X 108 shots during the mission. The laser transmitter will consume 15 watts, which represents a 3 wall plug efficiency. The laser transmitter has a beam divergence of 0.25 mrad and will maintain boresight to the receiver within 100 (mu) rad. The lasers have been specifically developed for ultra-high reliability for use in space exploration of the earth and nearby planets. Applications include planetary altimetry of Mars (MOLA) and earth (GLRS), as well as space geodesy, navigation, and tracking.
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