Øystein Farsund

Effect of idler absorption in pulsed optical parametric oscillators.

Absorption at the idler wavelength in an optical parametric oscillator (OPO) is often considered detrimental. We show through simulations that pulsed OPOs with significant idler absorption can perform better than OPOs with low idler absorption both in terms of conversion efficiency and beam quality. The main reason for this is reduced back conversion. We also show how the beam quality depends on the beam width and pump pulse length, and present scaling relations to use the example simulations for other pulsed nanosecond OPOs.

Standoff detection of biological agents using laser induced fluorescence—a comparison of 294 nm and 355 nm excitation wavelengths

Standoff detection measuring the fluorescence spectra of seven different biological agents excited by 294 nm as well as 355 nm wavelength laser pulses has been undertaken. The biological warfare agent simulants were released in a semi-closed aerosol chamber at 210 m standoff distance and excited by light at either of the two wavelengths using the same instrument. Significant differences in several of the agents’ fluorescence response were seen at the two wavelengths. The anthrax simulants’ fluorescence responses were almost an order of magnitude stronger at the shorter wavelength excitation. However, most importantly, the fluorescence spectra were significantly more dissimilar at 294 nm than at 355 nm excitation with ~7 nm spectral resolution. This indicates that classification of the substances should be possible with a lower error rate for standoff detection using 294 nm rather than 355 nm excitation wavelength, or even better, utilizing both.

Wavelength multiplexed fibre Bragg grating system for high-strain health monitoring applications

We have developed and demonstrated a fibre Bragg grating strain gauge system for high strain applications using spatial and wavelength multiplexing. The system takes advantage of a priori knowledge about the strain levels and relative phases between the different sensor signals to accommodate many sensors per fibre in an application where we expect to measure several thousand microstrains. In combination with scanning Fabry-Perot filter interrogation, this principle has been utilized in the design of a system for health monitoring of a naval surface effect ship and the system has been used in long-term load monitoring on the vessel.

Instrumentation of a high-speed surface effect ship for structural response characterization during sea trials

We report on the instrumentation of a high-speed air-cushion catamaran (Surface Effect Ship) with more than 50 fiber optic Bragg grating strain gauges, as well as conventional resistive strain gauges, accelerometers, a Motion Reference Unit and Global Positioning System. A bow mounted wave radar was used to characterize the sea-state in order to estimate the wave loads on the hull. The relatively large number of strain gauges enabled us to determine the global deformation modes of the hull as well as local stress concentrations. This instrumentation was installed on a new Norwegian naval vessel and employed during sea-keeping tests in smooth and rough seas off the Norwegian coast. The measurements enable a detailed characterization of the vessels dynamic response to wave loading and comparison with Finite Element Analysis modeling of the ship. The experimental results provide invaluable information for the subsequent development of a system for health monitoring of the structure. We present the instrumentation layout and selected results.

Required Spectral Resolution for Bioaerosol Detection Algorithms Using Standoff Laser-Induced Fluorescence Measurements

We have developed and tested a standoff biological aerosol detection demonstrator employing ultraviolet laser-induced fluorescence. It is based on commercially available components including a pulsed 355-nm laser and an intensified charge-coupled device camera. Biological warfare simulants and interferents were released and measured in open air field and closed-chamber laboratory tests. We analyzed the experimental data at different spectral resolutions, using statistics-based anomaly detection, and spectral angle mapping algorithms. The results show that less than 20 spectral channels in the 350-700-nm spectral region are sufficient in order to discriminate between the agents released using these methods. This corresponds to sacrificing high spectral resolution for the benefit of more photons in each channel and reduced computation time.

Improved beam quality from a high energy optical parametric oscillator using crystals with orthogonal critical planes.

We demonstrate with simulations and experiments that an optical parametric oscillator using two different crystals with orthogonal walk-off planes can generate a symmetric, high-quality beam even if the resonator has a high Fresnel number. In the experiments we used KTA and BBO crystals to convert 5 ns pulses at 1.06 microm to 1.7 microm pulses with more than 10 mJ energy and beam quality M(2) approximately 2.

Evaluation of biological aerosol stand-off detection at a field trial

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.

Optimal classification of standoff bioaerosol measurements using evolutionary algorithms

Early warning systems based on standoff detection of biological aerosols require real-time signal processing of a large quantity of high-dimensional data, challenging the systems efficiency in terms of both computational complexity and classification accuracy. Hence, optimal feature selection is essential in forming a stable and efficient classification system. This involves finding optimal signal processing parameters, characteristic spectral frequencies and other data transformations in large magnitude variable space, stating the need for an efficient and smart search algorithm. Evolutionary algorithms are population-based optimization methods inspired by Darwinian evolutionary theory. These methods focus on application of selection, mutation and recombination on a population of competing solutions and optimize this set by evolving the population of solutions for each generation. We have employed genetic algorithms in the search for optimal feature selection and signal processing parameters for classification of biological agents. The experimental data were achieved with a spectrally resolved lidar based on ultraviolet laser induced fluorescence, and included several releases of 5 common simulants. The genetic algorithm outperform benchmark methods involving analytic, sequential and random methods like support vector machines, Fishers linear discriminant and principal component analysis, with significantly improved classification accuracy compared to the best classical method.

Sum-Frequency Generation of High-Energy and High-Beam-Quality Ultraviolet Pulses

Sum-frequency generation of UV pulses exceeding 25 mJ and with beam quality 𝑀2∼5 has been demonstrated by mixing the third harmonic pulses of a flash lamp pumped 1.06 μm Nd:YAG laser with 1.7 μm pulses from an optical parametric oscillator pumped by the same laser in a compact setup.

Optical Parametric Oscillators with Idler Absorption

We show by simulations that idler absorption may improve the performance of pulsed high energy OPOs, and obtain high signal conversion efficiency and signal beam quality with idler absorption coefficients above 3 cm-1.