Markus Henriksson

ZnGeP2 parametric oscillator pumped by a linewidth-narrowed parametric 2 μm source

A tandem optical parametric oscillator (OPO) is used to convert radiation from 1.064 microm to the mid-infrared. Spectral bandwidth narrowing close to 80 times compared with a conventional cavity was achieved by using a bulk Bragg grating as an outcoupler in a near-degenerate periodically poled KTiOPO(4) OPO. The narrowed 2,008 nm radiation was subsequently used for pumping the ZnGeP(2) OPO, which was tunable between 3.3 and 5.2 microm. Pulse energies of 170 microJ and pump depletion close to 70% were obtained in the ZnGeP(2) OPO. To our knowledge this is the first time the output from a near-degenerate type I PPKTP OPO has been used for ZnGeP(2) OPO pumping.

Detection probabilities for photon-counting avalanche photodiodes applied to a laser radar system

Arrays of photon-counting avalanche photodiodes with time-resolved readout can improve the performance of three-dimensional laser radars. A comparison of the detection and false-alarm probabilities for detectors in linear mode and in Geiger mode is shown. With low background radiation their performance is comparable. It is shown that in both cases it will be necessary to process several laser shots of the same scene to improve detection and reduce the false-alarm rate. Additional calculations show that the linear mode detector is much better at detecting targets behind semitransparent obscurations such as vegetation and camouflage nets.

Remote THz generation from two-color filamentation: long distance dependence.

Remote terahertz (THz) generation from two-color filamentation is investigated as a function of the onset position of filaments. THz signals emitted by filaments produced at distances up to 55 m from the laser source were measured. However, from 9 m to 55 m, the THz signal decayed monotonically for increasing onset positions. With a simple calculation, the dominant factors associated to this decay were identified as group velocity mismatch of the two-color pulses and linear diffraction induced by focusing and propagating the second harmonic pulse.

Mode spectrum of multi-longitudinal mode pumped near-degenerate OPOs with volume Bragg grating output couplers

Spectral requirements for the first stage OPO used to pump a tandem ZGP mid-infrared OPO are theoretically investigated. Based on these requirements we demonstrate a singly-resonant type-I OPO including periodically poled KTiOPO(4) and volume-Bragg gratings as output couplers. Singly resonant oscillation is demonstrated very close to degeneracy, where signal and idler spectra are not well separated. Investigations of the longitudinal mode spectrum and the idler spectrum with high resolution using a scanning Fabry-Perot interferometer show the essential role played by the phase correlations of the multi-longitudinal mode Q-switched pump laser in formation of the nonresonant idler spectrum.

Laser guiding of Tesla coil high voltage discharges.

We have investigated the guiding and triggering of discharges from a Tesla coil type 280 kHz AC high voltage source using filaments created by a femtosecond Terawatt laser pulse. Without the laser the discharges were maximum 30 cm long. With the laser straight, guided discharges up to 110 cm length were detected. The discharge length was limited by the voltage amplitude of the Tesla coil.

CFD Predictions of Jet Engine Exhaust Plumes

Laser beam propagation in severe environment such as a jet engine exhaust may influence performance of e.g. laser countermeasures and active imaging systems located on airborne platforms. Beam propagation in close vicinity to the engine plume causes performance degradation due to beam wander and beam broadening effects. In this study, aero-optical effects, as well as conventional fluid dynamical issues, are computationally examined using Large Eddy Simulations (LES) in round and square shaped co-annular jets having the same cross-sectional nozzle area. The target configuration for these LES computations is a downscaled jet engine, operated by Volvo Aero, providing a characteristic exhaust in comparison to a full-scale engine. The LES model is separately validated against velocity and temperature data from the Seiner experiments before being employed to compute the target engine configuration. The computed results are used to elucidate the fluid dynamics in a supersonic jet plume, and to help in further understand the complicated aero-optical processes in the engine plume. In particular, we estimate the refractive index and structure parameter that in turn can be used to estimate the beam wander, distortion and broadening. Laser beam propagation through adverse turbulent regions, such as the environment close to a jet engine exhaust, needs to be studied in order to understand the mechanisms affecting the performance of airborne laser systems. During recent years, development of several laser systems aimed primarily at airborne applications has attracted interest. In security applications, Directed Infra-Red Counter-Measures (DIRCM) are now being developed to protect civilian airliners from heat seeking missile threats that potentially could be used by terrorists. DIRCM systems operate by directing modulated laser irradiation towards the missile seeker causing the optical sensor within the seeker to be jammed or decoyed, hence prohibiting target tracking. As a result of a successful jamming, the missile will break-lock and miss the intended target. Other airborne laser systems include e.g. active imaging and free-space optical communication. There are several sources that may perturb the emitted laser radiation such as, atmospheric turbulence, structural vibrations, aero-optical effects and jet engine exhaust plumes. In this study, the effects of the jet engine exhaust are emphasized using Computational Fluid Dynamics (CFD) together with supporting experimental measurement data. The jet engine exhaust plume is turbulent, and may therefore introduce severe aero-optic perturbations that accumulate and cause laser beam degradation in terms of beam wander, intensity scintillations and beam broadening at long range. By characterizing and evaluating perturbations, schemes for compensation or evasion of performance degradation can be devised. Jet engine exhaust plumes, figure 1, are complicated in nature as being created by the combustion process in the combustion chamber and being affected by the turbine, afterburner and nozzle. The engine processes are unsteady and sometimes affected by combustion instabilities, whereas the flow in the nozzle sometimes experience intermittent separation close to the walls. These jet engine plumes are supersonic, hot and with nozzle orifice pressures either just above or below the ambient pressure. The pressure mismatch causes the jet boundary to oscillate as it attempts to match the ambient pressure, and since the sound waves, by which information is transferred across the jet, travel slower than the supersonic flow, the jet repeatedly overshoots its equilibrium position. The sound waves converge to develop a network of shock diamonds that alternate with rarefaction fans. The gas in the jet interior expands and cools off as it flows

Continuously scanning time-correlated single-photon-counting single-pixel 3-D lidar

Abstract. Time-correlated single-photon-counting (TCSPC) lidar provides very high resolution range measurements. This makes the technology interesting for three-dimensional imaging of complex scenes with targets behind foliage or other obscurations. TCSPC is a statistical method that demands integration of multiple measurements toward the same area to resolve objects at different distances within the instantaneous field-of-view. Point-by-point scanning will demand significant overhead for the movement, increasing the measurement time. Here, the effect of continuously scanning the scene row-by-row is investigated and signal processing methods to transform this into low-noise point clouds are described. The methods are illustrated using measurements of a characterization target and an oak and hazel copse. Steps between different surfaces of less than 5 cm in range are resolved as two surfaces.

Time-correlated single-photon counting range profiling and reflectance tomographic imaging

Abstract Time-correlated single-photon counting (TCSPC) range profiling and imaging provide high resolution laser radar data applicable in several optical remote sensing applications at short and long distances. Excellent range resolution, below centimetres, can be obtained and information about remote objects can be extracted from TCSPC range profiles. The present study describes a TCSPC range profiling system with subcentimetre range resolution applied for remote sensing of objects at short and longer ranges. Experimental results from interrogation of geometrical shapes, reflectance tomographic imaging and range profiling at longer distance in daylight conditions are presented.

Numerical laser beam propagation using large eddy simulation of a jet engine flow field

Abstract. In certain directed infrared countermeasure (DIRCM) situations, the laser beam path may have to pass close to the engine exhaust plume of the aircraft and models of plume turbulence are needed for DIRCM performance simulations. The jet engine plume was modeled using large eddy simulation (LES), providing time resolved information about the large scale turbulent eddies. The refractive index data from the LES calculations were integrated along the propagation path to produce time resolved phase screens for optical beam propagation. The phase screens were used to calculate laser beam parameters including beam wander and power-in-bucket (PIB). Numerical beam propagation resulted in a root-mean-square beam wander of 200  μrad for the small turbojet engine studied. The PIB was calculated for beams with 80 μrad and 2 mrad divergence having equal beam diameter when passing through the plume. For the beam with low divergence, the average PIB was reduced from 0.23 to 0.040, while the beam with wider divergence showed no significant reduction. In both cases, the plume introduced significant temporal variation of the instantaneous PIB. The beam wander is not affected by the divergence, but only depends on beam size.

Cavity length resonances in a nanosecond singly resonant optical parametric oscillator

Resonant output energy enhancement in a singly resonant nondegenerate type-I optical parametric oscillator with a volume Bragg grating output coupler is demonstrated. The resonances occur when the pump laser and parametric oscillator cavity length ratio is an integer or a fraction of small integers. Although the length resonances are similar to those observed in doubly resonant optical parametric oscillators, the physical mechanism is distinctly different. The resonances in the singly resonant oscillator are caused by correlation of the instantaneous power between the quasi-periodic multimode pump laser beam and the OPO signal.