We review recent work in which we developed a phase-matched model to study the transverse mode instability (TMI) in high-energy laser amplifiers. The standard models for TMI have contributions that vary rapidly compared to the beat period between the fundamental mode and the higher-order modes in the problem. In the phase-matched model, we neglect these rapidly varying contributions. We consider a realistic example with a Yb-doped fiber amplifier that is similar to the amplifier that was considered by Naderi et al. [Opt. Exp., 21(13), 16111 (2013)], but with a more realistic 10-m length. In this example, only one higher-order mode is present. We show that the computational speedup of the phase-matched model is on the order of 100 with no loss of accuracy even in the highly-saturated nonlinear regime.
A fixed wavelength optical parametric amplifier laser was constructed to create nanosecond pulsed light at 1527 nm for eye-safe range gated active short-wave infrared (SWIR) imaging. The laser pulse is generated using a Nd:YAG operating at 1064 nm to pump a potassium titanyle arsenate crystal in a single-pass geometry, resulting in ≈85 mJ at up to 28 Hz at 1527 nm with a pulse duration of ∼8 ns. The SWIR imager can be gated in as short as a 70-ns window (active time) with time steps as small as 5 ns (e.g., 0 to 70 ns and 5 to 75 ns). This allows for significant weather penetration by reducing the contribution from back-scattered photons reaching the receiver from scattering along the path prior to the imaging target. In addition, by converting to 1527 from 1064 nm, the maximum permissible exposure limit for eye-safety is ≈4 orders of magnitude larger, allowing for higher fluences without risk of injury. NEXRAD weather data along the line-of-sight were utilized to facilitate atmospheric propagation modeling via MODTRAN to retrieve an estimate of the path transmittance during testing. The integrated system herein is shown to extend imaging ranges with improvement factors of ≈2.5–3.1 × during periods of rain. An image of a water tower at a 10-km range was generated with visibilities between 3 and 4 km during testing.
A combination of experimental lidar results and shape-dependent scatter amplitude matrix calculations are used to explore the utility of polarimetric lidars for aerosol characterization. Solutions are developed for the induced polarization vector response of skewed spheroidal particles and the scatter is then computed using an improved anomalous diffraction approximation method. Experimental data was collected on biological simulant, chemical simulant, and interferent aerosol clouds using a 1047 nm micropulse lidar designed to measure the simultaneous depolarization using a linearly polarized source. Depolarization signatures obtained during testing show a clear difference between wet and dry biological simulant aerosols, wet chemical simulant releases, and some interferents. Combining these measurements with the shape-dependent model calculations help us understand the unique polarimetric signatures that may be exploited for aerosol characterization using stand-off lidar techniques.
The MITA (Motion Imagery Task Analyzer) project was conceived by CBP OA (Customs and Border Protection - Office of Acquisition) and executed by JHU/APL (Johns Hopkins University Applied Physics Laboratory) and C5ISR Center NVESD (Command, Control, Computers, Communications, Cyber, Intelligence, Surveillance, and Reconnaissance Center Night Vision and Electronic Sensors Directorate). The intent is to develop an efficient methodology to characterize imaging system performance objectively, in a field setting, using a target resolution board and simultaneously measuring the turbulence along the camera line of sight. The initial design, development, and testing of MITA was previously reported (Hixson et al) and an additional set of field measurements and subsequent modeling results are reported here. The initial MITA design uses a transmitter at the imaging system location and a DIMM receiver in the field with the resolution target to measure the path turbulence, so a strong understanding of the path-averaged turbulence reciprocity is needed for proper implementation of the MITA system. To this end, a test series was conducted to explore the reciprocity of path averaged optical turbulence measurements using two scintillometers and the laser DIMM receiver in both bi-static and mono-static configurations. Finally, the path averaged measurements are compared with modeled turbulence along the path based off of the available meteorological data.
The MITA (Motion Imagery Task Analyzer) project was conceived by CBP OA (Customs and Border Protection - Office of Acquisition) and executed by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) and CERDEC NVESD MSD (Communications and Electronics Research Development Engineering Command Night Vision and Electronic Sensors Directorate Modeling and Simulation Division). The intent was to develop an efficient methodology whereby imaging system performance could be quickly and objectively characterized in a field setting. The initial design, development, and testing spanned a period of approximately 18 months with the initial project coming to a conclusion after testing of the MITA system in June 2017 with a fielded CBP system. The NVESD contribution to MITA was thermally heated target resolution boards deployed to support a range close to the sensor and, when possible, at range with the targets of interest. JHU/APL developed a laser DIMM (Differential Image Motion Monitor) system designed to measure the optical turbulence present along the line of sight of the imaging system during the time of image collection. The imagery collected of the target board was processed to calculate the in situ system resolution. This in situ imaging system resolution and the time-correlated turbulence measured by the DIMM system were used in NV-IPM (Night Vision Integrated Performance Model) to calculate the theoretical imaging system performance. Overall, this proves the MITA concept feasible. However, MITA is still in the initial phases of development and requires further verification and validation to ensure accuracy and reliability of both the instrument and the imaging system performance predictions.
The concentration of atmospheric oxygen is measured over a 540-m path using supercontinuum absorption spectroscopy. The absorption data compared favorably with MODTRAN™ 5 simulations of the spectra after adjusting for the differences of index of refraction of air and matching the instrument spectral resolution, as described by the effective slit width. Good agreement with the expected atmospheric oxygen concentration is obtained using a previously developed multiwavelength maximum likelihood estimation inversion algorithm. This study demonstrates the use of the SAS technique for measuring concentrations of chemical species with fine absorption structure on long-atmospheric paths.
Sensor system noise needs to be characterized to determine the limits of detecting a feature from an observed source. For
passive infrared spectral sensors, the noise is characterized in terms of the noise equivalent spectral radiance, NESR. The
total NESR (NESRtotal) has two components, the internal NESR of the instrument (NESRinstr) and the external NESR of
the path being viewed by the sensor (NESRpath). In the case of an FTIR instrument, the NESRinstr is measured by viewing
a stable blackbody at close range thereby removing the effects of the path on the spectrally dependent noise. The
standard deviation of the sine transform of the interferogram is then computed to estimate NESRinstr. In our application,
however, the NESRpath is our signal, and it is measured by viewing an atmospheric scene and removing the effect due to
the instrument. A histogram of the spectrally dependent noise spectrum is then computed. The full-width of this
histogram is taken at the 1/e2 points and is driven by temperature and species concentration fluctuations along the path.
Both of these effects can dominate over the instrument noise. In the following, we compare preliminary values of path
spectral fluctuations determined from a ground-based FTIR for a selected slant path to measured values of the refractive
index structure constant (Cn2) along the same path.
Elastic backscatter LIght Detection And Ranging (LIDAR) is a promising approach for stand-off detection of biological aerosol clouds. Comprehensive models that explain the scattering behavior from the aerosol cloud are needed to understand and predict the scattering signatures of biological aerosols under varying atmospheric conditions and against different aerosol backgrounds. Elastic signatures are dependent on many parameters of the aerosol cloud, with two major components being the size distribution and refractive index of the aerosols. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been in a unique position to measure the size distributions of released biological simulant clouds using a wide assortment of aerosol characterization systems that are available on the commercial market. In conjunction with the size distribution measurements, JHU/APL has also been making a dedicated effort to properly measure the refractive indices of the released materials using a thin-film absorption technique and laboratory characterization of the released materials. Intimate knowledge of the size distributions and refractive indices of the biological aerosols provides JHU/APL with powerful tools to build elastic scattering models, with the purpose of understanding, and ultimately, predicting the active signatures of biological clouds.
Current Strehl ratio models for actively compensated free-space optical communications terminals do not accurately
predict system performance under strong turbulence conditions as they are based on weak turbulence theory. For
evaluation of compensated systems, we present an approach for simulating the Strehl ratio with both low-order (tip/tilt)
and higher-order (adaptive optics) correction. Our simulation results are then compared to the published models and their
range of turbulence validity is assessed. Finally, we propose a new Strehl ratio model that is valid for general turbulence
conditions independent of the degree of compensation.
Light detection and ranging (LIDAR) systems have demonstrated some capability to meet the needs of a fastresponse
standoff biological detection method for simulants in open air conditions. These systems are designed
to exploit various cloud signatures, such as differential elastic backscatter, fluorescence, and depolarization in
order to detect biological warfare agents (BWAs). However, because the release of BWAs in open air is forbidden,
methods must be developed to predict candidate system performance against real agents. In support of such
efforts, the Johns Hopkins University Applied Physics Lab (JHU/APL) has developed a modeling approach to
predict the optical properties of agent materials from relatively simple, Biosafety Level 3-compatible bench top
measurements. JHU/APL has fielded new ground truth instruments (in addition to standard particle sizers, such
as the Aerodynamic particle sizer (APS) or GRIMM aerosol monitor (GRIMM)) to more thoroughly characterize
the simulant aerosols released in recent field tests at Dugway Proving Ground (DPG). These instruments include
the Scanning Mobility Particle Sizer (SMPS), the Ultraviolet Aerodynamic Particle Sizer (UVAPS), and the
Aspect Aerosol Size and Shape Analyser (Aspect). The SMPS was employed as a means of measuring smallparticle
concentrations for more accurate Mie scattering simulations; the UVAPS, which measures size-resolved
fluorescence intensity, was employed as a path toward fluorescence cross section modeling; and the Aspect, which
measures particle shape, was employed as a path towards depolarization modeling.
The optical scattering signature and the absorbance of a material are of interest in a variety of engineering applications,
particularly for those pertaining to optical remote sensing. The John Hopkins University Applied Physics Laboratory
has developed an experimental capability to measure in-plane bidirectional scattering distribution functions to retrieve
optical properties of materials. These measurements are supported at high angular resolution with wavelengths that
span the ultra-violet to the long-wave infrared. Models have been developed to fit Lambertian, diffuse, near-specular,
and specular scattering at a range of incident angles. Useful material properties can then be determined through analysis
of the modeled BSDF. Optical characterization results are shown for a variety of materials, including paints, metals,
optical windows, and leaves.
To ensure agent optical cross sections are well understood from the UV to the LWIR, volume integrated
measurements of aerosolized agent material at a few key wavelengths is required to validate existing simulations.
Ultimately these simulations will be used to assess the detection performance of various classes of lidar technology
spanning the entire range of the optical spectrum. The present work demonstrates an optical measurement architecture
based on lidar allowing the measurement of backscatter and depolarization ratio from biological aerosols released in a
refereed, 1-m cubic chamber. During 2009, various upgrades have been made to the chamber LIDAR system, which
operates at 1.064 μm with sub nanosecond pulses at a 120 Hz repetition rate. The first build of the system demonstrated
a sensitivity of aerosolized Bacillus atrophaeus (BG) on the order of 5×105 ppl with 1 GHz InGaAs detectors. To
increase the sensitivity and reduce noise, the InGaAs detectors were replaced with larger-area silicon avalanche
photodiodes for the second build of the system. In addition, computer controlled step variable neutral density filters are
now incorporated to facilitate calibrating the system for absolute back-scatter measurements. Calibrated hard target
measurements will be combined with data from the ground truth instruments for cross-section determination of the
material aerosolized in the chamber. Measured results are compared to theoretical simulations of cross-sections.
There is an urgent need to develop standoff sensing of biological agents in aerosolized clouds. In
support of the Joint Biological Standoff Detection System (JBSDS) program, lidar systems have been a
dominant technology and have shown significant capability in field tests conducted in the Joint Ambient
Breeze Tunnel (JABT) at Dugway Proving Ground (DPG). The release of biological agents in the open air
is forbidden. Therefore, indirect methods must be developed to determine agent cross-sections in order to
validate sensor against biological agents. A method has been developed that begins with laboratory
measurements of thin films and liquid suspensions of biological material to obtain the complex index of
refraction from the ultraviolet (UV) to the long wave infrared (LWIR). Using that result and the aerosols'
particle size distribution as inputs to Mie calculations yields the backscatter and extinction cross-sections as
a function of wavelength. Recent efforts to model field measurements from the UV to the IR have been
successful. Measurements with aerodynamic and geometric particle sizers show evidence of particle
clustering. Backscatter simulations of these aerosols show these clustered particles dominate the aerosol
backscatter and depolarization signals. In addition, these large particles create spectral signatures in the
backscatter signal due to material absorption. Spectral signatures from the UV to the IR have been
observed in simulations of field releases. This method has been demonstrated for a variety of biological
simulant materials such as Ovalbumin (OV), Erwinia (EH), Bacillus atrophaeus (BG) and male specific
bacteriophage (MS2). These spectral signatures may offer new methods for biological discrimination for
both stand-off sensing and point detection systems.
A chamber aerosol LIDAR is being developed to perform well-controlled tests of optical scattering characteristics of
biological aerosols, including Bacillus atrophaeus (BG) and Bacillus thuringiensis (BT), for validation of optical
scattering models. The 1.064 μm, sub-nanosecond pulse LIDAR allows sub-meter measurement resolution of particle
depolarization ratio or backscattering cross-section at a 1 kHz repetition rate. Automated data acquisition provides the
capability for real-time analysis or recording. Tests administered within the refereed 1 cubic meter chamber can provide
high quality near-field backscatter measurements devoid of interference from entrance and exit window reflections.
Initial chamber measurements of BG depolarization ratio are presented.
Extending our developments of a previously reported supercontinuum lidar system has increased the capability for
measuring long path atmospheric concentrations. The multi-wavelength capability of the supercontinuum laser source
has the advantage of obtaining multiple line differential absorption spectra measurements to determine the
concentrations of various atmospheric constituents. Simulation software such as MODTRANTM 5 has provided the
means to compare and evaluate the experimental measurements. Improvements to the nanosecond supercontinuum laser
fiber coupled transceiver system have allowed open atmospheric path lengths greater than 800 m. Analysis of
supercontinuum absorption spectroscopy and measurements utilizing the updated system are presented.
A multi-wavelength, multi-static lidar has been designed and is being tested for the characterization of atmospheric
aerosols. This design builds upon multi-static lidar, multiple scattering analyses, and supercontinuum DIAL experiments
that have previously been developed at Penn State University. Scattering measurements at two polarizations are
recorded over a range of angles using CCD imagers. Measurements are made using three discrete visible wavelength
lasers as the lidar sources, or using a supercontinuum source with a wavelength range spanning the visible and near-IR
wavelengths. The polarization ratios of the scattering phase functions are calculated for multiple wavelengths to analyze
and determine the aerosol properties of artificially generated fog.
An airborne DIfferential Absorption Lidar (DIAL) system was used to detect methane, propane, and light crude gas vapors during real-world collections. A series of overflights were conducted at two separate test locations (Kingsville/Corpus Christi, TX and Spencerport, NY). DIAL, digital orthophotography, and video data were collected, processed and analyzed at both sites. A new Hydrocarbon Detection Algorithm (HHDA) was developed to detect gasses other than methane. The HHDA shows promise for detecting multiple species in a local area with uniform surface reflectance or known variability.
Recent advances in the field of supercontinuum lasers have provided a unique opportunity for developing lidar
instruments that cover a wide spectral range. These instruments permit many simultaneous measurements of differential
absorption spectra (DIAL and DAS techniques) to determine species density. Application of MODTRANTM 5 and other
simulation software has allowed us to design and validate the findings of supercontinuum lidar systems developed at
Penn State Lidar Laboratory. The multiple line differential absorption concepts have been demonstrated with various
system topologies for a host of atmospheric windows in the visible to near infrared regions. During the past three years,
we have developed and demonstrated several systems that are capable of measuring concentrations of various
atmospheric constituents at background or elevated levels through long path absorption by transmitting only milliwatts
of optical power. Our most recent supercontinuum lidar system utilizes a nanosecond supercontinuum laser fiber
optically coupled to a transceiver system for remote sensing of atmospheric species concentrations. Due to the flexibility
of the design, the operational prototype is currently being used to demonstrate the capability for accurately measuring
real world open path atmospheric concentrations across the Penn State campus. The purpose of this study is to develop
the technology and to demonstrate the capability for accurately measuring species concentrations without the
complexities and uncertainties inherent in hyper-spectral remote sensing using the sun as a source, or the limitations and
errors associated with using pairs of laser lines for DIAL measurements of each species. Initial simulations and
measurements using this approach are presented.
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