To guarantee safe supply of drinking water, there is a need for fast, sensitive and robust techniques for early warning of potentially pathogenic microorganisms. An optical measurement system has been developed to spectrally study the UVinduced autofluorescence from single microorganisms in a water flow. Particles in the flow are detected with a continuouswave laser whereby an ultraviolet laser pulse is fired and a spectrometer measures emitted fluorescence. Suspensions of Cryptosporidium parvum (Crypto), Bacillus atrophaeus (BG, a non-pathogenic simulant bacterial spore) and Escherichia coli (Ecoli), in distilled and tap water have been examined. The results from single particle measurements are compared to fluorescence emission spectra captured on suspensions with a fluorescence spectrometer and the strength and variability of single organism spectra, with respect to detection applications, is also investigated.
Results from comparative measurements of turbulence and transmission at laser wavelengths in the shortwave infrared (SWIR) and longwave infrared (LWIR) wavelength bands are presented. The experimental setup uses a dual band transmitter with coaxially aligned laser beams and a receiver based on a common collecting off-axis parabolic mirror to direct the radiation to the detectors. Measurement of turbulence was performed on one occasion and transmission was measured in two events with fog and one event with falling snow. The turbulence data is compared to optical wave simulations, and the transmission data is compared to Mie calculations based on aerosol and hydrometeor data. As expected, the received power scintillations caused by atmospheric turbulence are stronger in the SWIR band compared to the LWIR band. The measured transmission in fog and snow can be explained by the measured characteristics of the scattering medium. In fog, depending on the particle size distribution, higher transmission can be observed in either wavelength bands. In snow, on the other hand, it is shown that the transmission is higher in the SWIR band as an effect of forward scattering.
Rapid and selective detection of persistent, highly toxic liquids, such as low volatile chemical warfare agents (CWA) continues to be a desired capability, e.g. by first responders and military personnel. An already proven general technique for detection of bulk material is Raman spectroscopy. Utilizing UV excitation wavelengths offers advantages such as separation from fluorescence, solar blindness and a high Raman cross-section. In the UV region, however, there is usually a rapid decrease in penetration depth for most liquids as the wavelength becomes shorter, generally resulting in that a smaller volume is accessible for Raman scattered photons. While this effect is a drawback in terms of signal power at the detector it may also be beneficial as interfering light from the background surface can be strongly reduced or even absent. Herein, the use of Hadamard patterned (50% transmission) masks at the entrance plane of a spectrograph are investigated for the purpose of increasing the amount of Raman scattered light onto the detector compared to slit measurements. Decoded spectra from Hadamard measurements on scenes containing hazardous material, such as low volatile CWA and simulant chemicals, are compared with slit measurements.
Rapid and reliable detection of small amounts of hazardous substances remains highly desirable within defense and protective contexts. Raman scattering is one of the non-contact optical spectroscopic techniques that already has proven useful for different detection purposes and a large number of instruments exists on the market today. However, added performance may be gained by shifting the laser wavelength from the visible or near-infrared, generally used in currently available Raman instruments, to the UV band. This report covers different methods of acquiring UV Raman hyperspectral cubes using a tunable laser source and an imaging spectrometer as main components. Results obtained by setups resulting in coarse spectral resolution via fixed interference filters to relatively high spectral resolution UV Raman images when using random, binary transmission masks as coded apertures in a Compressed Sensing approach are shown and discussed.
A passive LWIR camera based on a focal plane array detector was used to capture hyperspectral images of different scenes where low volatile chemical warfare agent (CWA) and simulant droplets were deposited on a variety of substrates. Four different CWA; mustard gas (HD), cyclosarin (GF), VX, VR, and two simulant liquids; dimethyl methylphosphonate (DMMP) and triethyl phosphate (TEP) were used as surface contaminants and applied to the substrates as 5 μl droplets. The trials were performed outdoors with the scene close to the ground and the camera imaging at an angle of approximately 35° and a stand-off distance of about 2 m, i.e., mainly the reflected radiation from the cold sky in combination with the thermal emission from the scene was observed. Brightness temperature spectra were extracted from the hyperspectral data cubes and compared to results from a thin film model as well as reference LWIR spectra of the liquids.
UV Raman hyperspectral datacubes were acquired by filtered imaging of a laser illuminated scene. The laser excitation wavelength was scanned over the Stokes Raman band relative to fixed narrow bandpass optical filters. Two different 0.3 nm wide bandpass filters with center wavelengths 248.4 and 264.1 nm were used and the excitation wavelength was scanned in steps of 0.2 nm. Results are presented for persistent chemical warfare agents and simulant chemicals on different surfaces.
By using fixed narrow band pass optical filtering and scanning the laser excitation wavelength, hyperspectral Raman imaging could be achieved. Experimental, proof-of-principle results from the Chemical Warfare Agent (CWA) tabun (GA) as well as the common CWA simulant tributyl phosphate (TBP) on different surfaces/substrates are presented and discussed.
We present our latest experimental results on V-agent Raman scattering in the middle UV. The Raman scattering was examined using a pulsed tunable laser based spectrometer system. Neat droplets of the agents were placed on a silicon surface and irradiated with sequences of laser pulses. The Raman scattering was examined as a function of laser wavelength and accumulated exposure with a reduced level of exposure per pulse compared to our earlier investigations.
Laser induced Raman scattering at excitation wavelengths in the middle ultraviolet was examined using a pulsed tunable laser based spectrometer system. Droplets of chemical warfare agents, with a volume of 2 μl, were placed on a silicon surface and irradiated with sequences of laser pulses. The Raman scattering from V-series nerve agents, Tabun (GA) and Mustard gas (HD) was studied with the aim of finding the optimum parameters and the requirements for a detection system. A particular emphasis was put on V-agents that have been previously shown to yield relatively weak Raman scattering in this excitation band.
Laser induced Raman scattering from the commonly used chemical warfare agent simulants dimethyl sulfoxide, tributyl phosphate, triethyl phosphonoacetate was measured at excitation wavelengths ranging from 210 to 410 nm using a pulsed laser based spectrometer system with a probing distance of 1.4 m and with a field of view on the target of less than 1mm. For the purpose of comparison with well explored reference liquids the Raman scattering from simulants was measured in the form of an extended liquid surface layer on top of a silicon wafer. This way of measuring enabled direct comparison to the Raman scattering strength from cyclohexane. The reference Raman spectra were used to validate the signal strength of the simulants and the calibration of the experimental set up. Measured UV absorbance functions were used to calculate Raman cross sections. Established Raman cross sections of the simulants make it possible to use them as reference samples when measuring on chemical warfare agents in droplet form.
The very low Raman scattering cross section and the fluorescence background limit the measuring range of Raman based instruments operating in the visible or infrared band. We are exploring if laser excitation in the middle ultraviolet (UV) band between 200 and 300 nm is useful and advantageous for detection of persistent chemical warfare agents (CWA) on various kinds of surfaces. The UV Raman scattering from tabun, mustard gas, VX and relevant simulants in the form of liquid surface contaminations has been measured using a laboratory experimental setup with a short standoff distance around 1 meter. Droplets having a volume of 1 μl were irradiated with a tunable pulsed laser swept within the middle UV band. A general trend is that the signal strength moves through an optimum when the laser excitation wavelength is swept between 240 and 300 nm. The signal from tabun reaches a maximum around 265 nm, the signal from mustard gas around 275 nm. The Raman signal from VX is comparably weak. Raman imaging by the use of a narrow bandpass UV filter is also demonstrated.
Lidar has been identified as a promising sensor for remote detection of biological warfare agents (BWA). Elastic IR lidar
can be used for cloud detection at long ranges and UV laser induced fluorescence can be used for discrimination of BWA
against naturally occurring aerosols. This paper will describe a simulation tool which enables the simulation of lidar for
detection, tracking and classification of aerosol clouds. The cloud model was available from another project and has been
integrated into the model. It takes into account the type of aerosol, type of release (plume or puff), amounts of BWA,
winds, height above the ground and terrain roughness.
The model input includes laser and receiver parameters for both the IR and UV channels as well as the optical
parameters of the background, cloud and atmosphere. The wind and cloud conditions and terrain roughness are specified
for the cloud simulation. The search area including the angular sampling resolution together with the IR laser pulse
repetition frequency defines the search conditions. After cloud detection in the elastic mode, the cloud can be tracked
using appropriate algorithms. In the tracking mode the classification using fluorescence spectral emission is simulated
and tested using correlation against known spectra. Other methods for classification based on elastic backscatter are also
discussed as well as the determination of particle concentration. The simulation estimates and displays the lidar response,
cloud concentration as well as the goodness of fit for the classification using fluorescence.
We have performed a field trial to evaluate technologies for stand-off detection of biological aerosols, both in daytime
and at night. Several lidar (light detection and ranging) systems were tested in parallel. We present the results from three
different lidar systems; one system for detection and localization of aerosol clouds using elastic backscattering at
1.57 μm, and two systems for detection and classification of aerosol using spectral detection of ultraviolet laser-induced
fluorescence (UV LIF) excited at 355 nm. The UV lidar systems were utilizing different technologies for the spectral
detection, a photomultiplier tube (PMT) array and an intensified charge-coupled device (ICCD), respectively. During the
first week of the field trial, the lidar systems were measuring towards a semi-closed chamber at a distance of 230 m. The
chamber was built from two docked standard 20-feet containers with air curtains in the short sides to contain the aerosol
inside the chamber. Aerosol was generated inside the semi-closed chamber and monitored by reference equipments, e.g.
slit sampler and particle counters. Signatures from several biological warfare agent simulants and interferents were
measured at different aerosol concentrations. During the second week the aerosol was released in the air and the
reference equipments were located in the centre of the test site. The lidar systems were measuring towards the test site
centre at distances of either 230 m or approximately 1 km. In this paper we are presenting results and some preliminary
signal processing for discrimination between different types of simulants and interference aerosols.
Lidar has been identified as a promising sensor for remote detection of biological warfare agents. Elastic lidar can be
used for cloud detection at long ranges and UV laser induced fluorescence can be used for discrimination of
bioaerosols against naturally occurring aerosols. This paper analyzes the performance of elastic lidar such as
sensitivity, range and angular coverage rate vs. atmospheric visibility, laser and receiver parameters. The analysis of
the UV fluorescence lidar is concentrated on estimating the signal strength as a function of range, concentration and
optical background level. The performance analysis supports the goal for a practical lidar system to detect 1000
particles/liter at 2-3 km using elastic backscatter and to verify the bioaerosol using fluorescence characterization at
1 km. Some examples of test results with an elastic lidar and a range gated imaging system both at 1.5 μm
wavelength are presented together with fluorescence spectra of biological warfare agent simulants measured at an
excitation wavelength of 355 nm.
KEYWORDS: Modulation, Modulators, Eye, Sensors, Signal to noise ratio, Signal detection, Transceivers, Free space optics, Retroreflectors, Optical communications
An optical communication link with a modulated retro-reflector in a cat's eye optics arrangement has been evaluated. A
transmissive multiple quantum well modulator with a modulation bandwidth in excess of 10 MHz was placed in a wide
field-of-view cat's eye optic system. A laser transceiver with one line and one broadband source around 1550 nm was
used to interrogate the modulated retro-reflector and detect the reflected and modulated signal. Multi-level digital
encoding was evaluated as a way to increase the information transfer rate in conditions with relatively high signal-to-noise
ratio. The links were evaluated in indoor as well as outdoor conditions.
Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam
wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with
communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro
reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was
studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the
1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence
effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and
detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm
beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis
of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at
different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical
models.
We present results of a measurement system designed for detecting the fluorescence spectrum of individual aerosol particles of biological warfare agents excited with laser pulses at wavelengths around 290 or 340 nm. The biological aerosol is prepared and directed into a narrow air beam. A red laser is focused on the aerosol beam and a trigger photomultiplier tube monitor the presence of individual particles by measuring the scattered light. When a particle is present in the detection volume, a laser pulse is triggered from an ultraviolet laser and the fluorescence spectrum is acquired with a spectrometer based on a diffraction grating and a 32 channels photomultiplier tube array with single-photon sensitivity. The spectrometer measures the fluorescence spectra in the wavelength region from 300 to 800 nm. In the experiment we used different simulants of biological warfare agents. These bioaerosol particles were excited by a commercial available gas laser (337 nm), or a laser (290 nm) that we have developed based on an optical parametric oscillator with intracavity sum-frequency mixing. In the analysis of the experiments we compare the measured signals (fluorescence spectra, total fluorescence energy and the scattered energy) from the individual bioaerosol particles excited with the two different ultraviolet wavelengths.
Atmospheric propagation experiments for active and passive EO systems were performed over a 2 and 8 km path.
Single and double path propagation effects were studied using retroreflectors and hot point targets. The systems used
include laser systems at 1.5 and 3.5 μm wavelength as well as imaging systems in the visible, 1-2 and 8-9 μm regions.
A scintillometer operating at 0.8 μm wavelength was also used for the shorter path. Experimental data will be
presented and evaluated concerning statistics. The results will be discussed mainly from a laser countermeasure point
of view.
Atmospheric propagation degradation effects including attenuation, aerosol scattering and turbulence have a great
impact on the performance of optical systems. Relevant military optical systems include active and passive imaging for
target recognition, free-space optical communication and DIRCM/EOCM. This paper will report on experimental work
including measurement of retro signals at 1.5 and in the 3-5 μm wavelength regions for evaluation of retro communication
links and DIRCM performance. Imaging experiments using a range-gated system both in the active and
passive mode at 1.5 μm, will also be carried along the same paths. A dedicated target box and test targets have been
fabricated for mounting on a mast at 8 km from our laboratory. The box contains reflectors and receivers in different
slots each of which can be opened by a telephone call. A heated target on top simulates a point target in the IR region.
The test targets are aimed for the range-gated imaging system. Preliminary experimental data will be presented and
discussed.
We present initial results of a measurement system designed for detecting the fluorescence spectrum of individual particles of biological warfare agent (BWA). A compact optical parametric oscillator with intracavity sum-frequency mixing and a commercial Nitrogen gas laser was used as excitation sources to generate 293 nm or 337 nm UV laser irradiation. The pulsed lasers and a photomultiplier tube (PMT) array based spectrometer were triggered by a red laser-diode and a PMT detector that sensed the presence of a particle typical of size 5-20 μm in diameter. The spectral detection part of the system consisted of a grating and a PMT array with 32 channels, which measured fluorescence in the wavelength from 280 nm to 800 nm. The detector system was used to demonstrate the measurement of laser induced fluorescence spectra of individual BWA simulant particles by excitation of single UV laser pulses. The spectrum obtained by averaging spectra from several BWA aerosol simulant particles were found generally similar, but not identical, to the fluorescence spectrum obtained from water solutions containing the same particles dissolved.
KEYWORDS: Modulators, Modulation, Polarization, Liquid crystals, Sensors, Free space optics, Reflectors, Signal detection, Signal to noise ratio, Turbulence
Retro-reflective optical communication was investigated in field trials set up in urban and maritime environments. A 1550 nm laser transceiver with an output power ranging from 1 mW to 2 W and liquid crystal polarisation modulators in conjunction with corner cube reflectors were used in different experimental arrangements. The emphasis in this work was on system performance issues in tactical application such as the effects of platform vibrations and beam distortion induced by the atmospheric turbulence. In particular, the conditions for counteraction of communication interrupts, caused by line of sight jitter, using a dynamic tip-tilt mirror in the laser transceiver were tested. We report on the results from field trials wherein these issues have been addressed.
This paper presents the status of an ongoing development of a point detector for biological warfare agent sensing based on ultraviolet laser-induced fluorescence from single particles in air. The detector will measure the fluorescence spectra of single particles in a sheath flow air beam. The spectral detection part of the system consists of a grating and a photomultiplier tube array with 32 channels, which measure fluorescence spectra in the wavelength band from 300 nm to 650 nm. The detector is designed to measure laser induced fluorescence from single laser pulses and has been tested by measuring fluorescence from simulants of biological warfare agents in aqueous solution. The solutions were excited with laser pulses at the wavelengths of 293 nm and 337 nm. The paper also presents preliminary results on the sheath flow particle injector and time-resolved measurements of fluorescence from biological warfare agent simulants in solution.
Digital processing is used to linearly reproduce dynamic phase signals in fiber interferometers passively demodulated using a 3x3 coupler. The phase is retrieved as a weighted sum of the three received signals. High resolution down to the 1 μrad level is achieved.
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