Neil A. Macleod

Temporal and Spatial Resolution in Transmission Raman Spectroscopy

Picosecond time-resolved transmission Raman data were acquired for 1 mm thick powder samples of trans-stilbene, and a Monte Carlo model was developed that can successfully model the laser and Raman pulse profiles. Photon migration broadened the incident (∼1 ps) probe pulse by two orders of magnitude. As expected from previous studies of Raman photon migration in backscattering mode, the transmitted Raman pulse was broader than the transmitted laser pulse and took longer to propagate through the sample. The late-arriving photons followed tortuous flight paths in excess of 50 mm on traversing the 1 mm sample. The Monte Carlo code was also used to study the spatial resolution (lateral and depth) of steady-state transmission Raman spectroscopy in the diffusion regime by examining the distribution of Raman generation positions as a function of incident beam size, sample thickness, and transport length. It was predicted that the lateral resolution should worsen linearly with sample thickness (typically the resolution was about 50% of the sample thickness), and this is an inevitable consequence of operating in the diffusion regime. The lateral resolution was better at the sample surface (essentially determined by the probe beam diameter or the collection aperture) than for buried objects, but transmission sampling was shown to be biased towards the mid-point of thick samples. Time-resolved transmission experiments should improve the lateral resolution by preferentially detecting snake photons, subject to constraints of signal-to-noise ratio.

Prediction of sublayer depth in turbid media using spatially offset Raman spectroscopy.

We demonstrate experimentally the feasibility of monitoring the depth of optically thick layers within turbid media using spatially offset Raman spectroscopy (SORS) in combination with multivariate analysis. The method uses the deep penetration capability of SORS to characterize significantly thicker (by at least a factor of 2) layers than possible with conventional Raman spectroscopy. Typical relative accuracies were between 5 and 10%. The incorporation of depth information into a SORS experiment as an additional dimension allows pure spectra of each individual layer to be resolved using three-dimensional multivariate techniques (parallel factor analysis, PARAFAC) to accuracies comparable with the results of a two-dimensional analysis.

Hollow waveguide photomixing for quantum cascade laser heterodyne spectro-radiometry

An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, high efficiency heterodyne optical mixing circuit in the middle infrared. The approach has involved the creation of hollow waveguides and alignment features for a beam combiner component in a glass-ceramic substrate. The performance of the integrated beam combiner was tested as part of a full laser heterodyne spectro-radiometer in which a quantum cascade laser local oscillator emitting at 9.7 µm was mixed with incoherent radiation. The performance has been evaluated with both cryogenically-cooled and peltier-cooled photomixers demonstrating consistent detection limits of two and five times the shot noise limit, respectively. The hollow waveguide mixer has also shown advantages in temporal stability, laser spatial mode cleansing, and reduced sensitivity to optical feedback.

Atmospheric observations of multiple molecular species using ultra-high-resolution external cavity quantum cascade laser heterodyne radiometry.

We demonstrate a widely tunable laser heterodyne radiometer operating in the thermal IR during an atmospheric observation campaign in the solar occultation viewing mode. An external cavity quantum cascade laser tunable within a range of 1120 to 1238 cm(-1) is used as the local oscillator (LO) of the instrument. Ultra-high-resolution (60 MHz or 0.002 cm(-1) transmission spectroscopy of several atmospheric species (water vapor, ozone, nitrous oxide, methane, and dichlorodifluoromethane) has been demonstrated within four precisely selected molecule-specific narrow spectral windows (∼1 cm(-1). Atmospheric transmission lines within each selected window were fully resolved through mode-hop-free continuous tuning of the LO frequency. Comparison measurements were made simultaneously with a high-resolution Fourier transform spectrometer to demonstrate the advantages of the laser heterodyne system for atmospheric sounding at high spectral and spatial resolutions.

Broadband standoff detection of large molecules by mid-infrared active coherent laser spectrometry.

A widely tunable active coherent laser spectrometer (ACLaS) has been demonstrated for standoff detection of broadband absorbers in the 1280 to 1318 cm-1 spectral region using an external cavity quantum cascade laser as a mid-infrared source. The broad tuning range allows detection and quantification of vapor phase molecules, such as dichloroethane, ethylene glycol dinitrate, and tetrafluoroethane. The level of confidence in molecular mixing ratios retrieved from interfering spectral measurements is assessed in a quantitative manner. A first qualitative demonstration of condensed phase chemical detection on nitroacetanilide has also been conducted. Detection performances of the broadband ACLaS have been placed in the context of explosive detection and compared to that obtained using distributed feedback quantum cascade lasers.

Middle infrared active coherent laser spectrometer for standoff detection of chemicals

Using a quantum cascade laser emitting at 7.85 μm, a middle infrared active coherent laser spectrometer has been developed for the standoff detection of vapor phase chemicals. The first prototype has been tested using diffuse target backscattering at ranges up to ~30 m. Exploiting the continuous frequency tuning of the laser source, spectra of water vapor, methane, nitrous oxide, and hydrogen peroxide were recorded. A forward model of the instrument was used to perform spectral unmixing and retrieve line-of-sight integrated concentrations and their one-sigma uncertainties. Performance was found to be limited by speckle noise originating from topographic targets. For absorbers with large absorption cross sections such as nitrous oxide (>10(-19) cm(2)·molecule(-1)), normalized detection sensitivities range between 14 and 0.3 ppm·m·Hz(-1/2), depending on the efficiency of the speckle reduction scheme implemented.

Concentration measurements of complex mixtures of broadband absorbers by widely tunable optical parametric oscillator laser spectroscopy

The ability to obtain accurate vapour parameter information from a compound’s absorption spectrum is an essential data processing application in order to quantify the presence of an absorber. Concentration measurements can be required for a variety of applications including environmental monitoring, pipeline leak detection, surface contamination and breath analysis. This work demonstrates sensitive concentration measurements of complex mixtures of volatile organic compounds (VOCs) using broadly tunable mid wave infrared (MWIR) laser spectroscopy. Due to the high absorption cross-sections, the MWIR spectral region is ideal to carry out sensitive concentration measurements of VOCs by tunable laser absorption spectroscopy (TLAS) methods. Absorption spectra of mixtures of VOCs were recorded using a MWIR optical parametric oscillator (OPO), with a tuning range covering 2.5 μm to 3.7 μm. The output of the MWIR OPO was coupled to a multi-pass astigmatic Herriott gas cell, maintained at atmospheric pressure that can provide up to 210 m of absorption path length, with the transmission output from the cell being monitored by a detector. The resulting spectra were processed by a concentration retrieval algorithm derived from the optimum estimation method, taking into account both multiple broadband absorbers and interfering molecules that exhibit narrow multi-line absorption features. In order to demonstrate the feasibility of the concentration measurements and assess the capability of the spectral processor, experiments were conducted on calibrated VOCs vapour mixtures flowing through the spectroscopic cell with concentrations ranging from parts per billion (ppb) to parts per million (ppm). This work represents as a first step in an effort to develop and apply a similar concentration fitting algorithm to hyperspectral images in order to provide concentration maps of the spatial distribution of multi-species vapours. The reported functionality of the novel fitting algorithm makes it a valuable addition to the existing data processing tools for parameter information recovery from recorded absorption data.

The Methane Isotopologues by Solar Occultation (MISO) Nanosatellite Mission: Spectral Channel Optimization and Early Performance Analysis

MISO is an in-orbit demonstration mission that focuses on improving the representation of the methane distribution throughout the upper troposphere and stratosphere, to complement and augment the nadir- and zenith-looking methane observing system for a better understanding of the methane budget. MISO also aims to raise to space mission readiness the concept of laser heterodyne spectro-radiometry (LHR) and associated miniaturization technologies, through demonstration of Doppler-limited atmospheric transmittance spectroscopy of methane from a nanosatellite platform suitable for future constellation deployment. The instrumental and engineering approach to MISO is briefly presented to demonstrate the technical feasibility of the mission. LHR operates using narrow spectral coverage (<1 cm−1) focusing on a few carefully chosen individual ro-vibrational transitions. A line-by-line spectral channel selection methodology is developed and used to optimize spectral channel selection relevant to methane isotopologue sounding from co-registered thermal infrared and short-wave infrared LHR. One of the selected windows is then used to carry out a first performance analysis of methane retrievals based on measurement noise propagation. This preliminary analysis of a single observation demonstrates an ideal instrumental precision of <1% for altitudes in the range 8–20 km, <5% for 20–30 km and <10% up to 37 km on a single isotopologue profile, which leaves a significant reserve for real-world error budget degradation and bodes well for the mission feasibility. MISO could realistically demonstrate methane limb sounding at Doppler-limited spectral resolution, even from a cost-effective 6 dm3 nanosatellite.

High sensitivity stand-off detection and quantification of chemical mixtures using an active coherent laser spectrometer (ACLaS)

High sensitivity detection, identification and quantification of chemicals in a stand-off configuration is a highly sought after capability across the security and defense sector. Specific applications include assessing the presence of explosive related materials, poisonous or toxic chemical agents, and narcotics. Real world field deployment of an operational stand-off system is challenging due to stringent requirements: high detection sensitivity, stand-off ranges from centimeters to hundreds of meters, eye-safe invisible light, near real-time response and a wide chemical versatility encompassing both vapor and condensed phase chemicals. Additionally, field deployment requires a compact, rugged, power efficient, and cost-effective design. To address these demanding requirements, we have developed the concept of Active Coherent Laser Spectrometer (ACLaS), which can be also described as a middle infrared hyperspectral coherent lidar. Combined with robust spectral unmixing algorithms, inherited from retrievals of information from high-resolution spectral data generated by satellitebased spectrometers, ACLaS has been demonstrated to fulfil the above-mentioned needs. ACLaS prototypes have been so far developed using quantum cascade lasers (QCL) and interband cascade lasers (ICL) to exploit the fast frequency tuning capability of these solid state sources. Using distributed feedback (DFB) QCL, demonstration and performance analysis were carried out on narrow-band absorbing chemicals (N2O, H2O, H2O2, CH4, C2H2 and C2H6) at stand-off distances up to 50 m using realistic non cooperative targets such as wood, painted metal, and bricks. Using more widely tunable external cavity QCL, ACLaS has also been demonstrated on broadband absorbing chemicals (dichloroethane, HFC134a, ethylene glycol dinitrate and 4-nitroacetanilide solid) and on complex samples mixing narrow-band and broadband absorbers together in a realistic atmospheric background.

Active coherent laser spectrometer for remote detection and identification of chemicals

Currently, there exists a capability gap for the remote detection and identification of threat chemicals. We report here on the development of an Active Coherent Laser Spectrometer (ACLaS) operating in the thermal infrared and capable of multi-species stand-off detection of chemicals at sub ppm.m levels. A bench top prototype of the instrument has been developed using distributed feedback mid-infrared quantum cascade lasers as spectroscopic sources. The instrument provides active eye-safe illumination of a topographic target and subsequent spectroscopic analysis through optical heterodyne detection of the diffuse backscattered field. Chemical selectivity is provided by the combination of the narrow laser spectral bandwidth (typically < 2 MHz) and frequency tunability that allows the recording of the full absorption spectrum of any species within the instrument line of sight. Stand-off detection at distances up to 12 m has been demonstrated on light molecules such as H2O, CH4 and N2O. A physical model of the stand-off detection scenario including ro-vibrational molecular absorption parameters was used in conjunction with a fitting algorithm to retrieve quantitative mixing ratio information on multiple absorbers.