When the interval between transmitted pulses is shorter than the time of flight, it is not straightforward for a lidar to determine the distance uniquely. We present a method that uses varying pulse intervals, constructs a set of tentative point positions for each received pulse, and exploits the clustering of such tentative points to determine the correct distance with high probability. The clustering of true points also helps suppress noise pulses, even in a regime where the number of noise pulses is far greater than the number of true return pulses. It is then possible to use a detection threshold close to the noise level.
We show by experiments and simulations that properly chirped laser pulses enable efficient and broadband sum
frequency generation in nonlinear crystals. We achieved high energy, picosecond deep-UV pulses with spectral width
one order of magnitude greater than the acceptance bandwidth of the nonlinear interaction. The broad spectrum supports
shaping of ps flat-top deep-UV pulses with short rise- and fall-time, which are optimal for driving high brightness
photocathode electron guns.
A new method for obtaining high beam quality from high pulse energy optical parametric oscillators (OPOs) is
demonstrated. By using different nonlinear crystals that have walk-off in orthogonal directions but are type 2 phase
matched for the same interaction, the strong beam asymmetry that is common in critically type 2 phase matched OPOs is
removed. Experimentally, this was demonstrated by adding BBO crystals to a type 2 phase matched KTA OPO, where
the beam quality improved from M2 ≈ 2 x 12 in the KTA OPO to M2 ≈ 2 x 2 from the KTA-BBO OPO.
An airborne system for hyperspectral target detection is described. The main sensor is a HySpex pushbroom
hyperspectral imager for the visible and near-infrared spectral range with 1600 pixels across track, supplemented by a
panchromatic line imager. An optional third sensor can be added, either a SWIR hyperspectral camera or a thermal
camera. In real time, the system performs radiometric calibration and georeferencing of the images, followed by image
processing for target detection and visualization. The current version of the system implements only spectral anomaly
detection, based on normal mixture models. Image processing runs on a PC with a multicore Intel processor and an
Nvidia graphics processing unit (GPU). The processing runs in a software framework optimized for large sustained data
rates. The platform is a Cessna 172 aircraft based close to FFI, modified with a camera port in the floor.
Nonlinear optical conversion of high-energy 1.064 μm pulses from a Q-switched Nd:YAG laser to the mid-infrared
is demonstrated. The experimental setup is based on a two-stage master-oscillator/power-amplifier (MOPA)
design with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second
stage. The setup can be tuned to provide output at wavelengths within the transparency range of ZnGeP2. We
obtain more than 8 mJ at 8 μm, and up to 33 mJ in the 3-5 μm wavelength region. The measured beam quality
factors are in the range M2 =2-4 for both wavelength regions.
Nonlinear optical conversion of 1.064 μm pulses from a Q-switched Nd:YAG laser to the mid-infrared is demonstrated
experimentally. The setup is based on a two-stage master-oscillator/power-amplifier (MOPA) design
with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second stage.
We obtain more than 8 mJ at 8 μm with a beam quality factor M2 ≈ 3.6.
We present an efficient, high-power mid-infrared laser source using a Thulium fiber laser as pump source. The CW fiber laser pumps a Q-switched Ho:YAG laser which in turn pumps a ZnGeP2-based OPO. We have built a semi-ruggedized version of the laser for countermeasure field trials, and using a 15 W fiber laser we obtained 5.2 W output power in the 3-5 μm band. We also present work on scaling up the power by using a 65 W fiber laser as the pump. Simulations and initial experiments suggest that the scaled-up version could produce more than 25 W in the mid-IR.
Nonlinear optical conversion of 500 mJ pulses from a Nd:YAG laser to the mid-infrared is demonstrated in a two-step architecture. Using a type 2 phase matched KTiOPO4-based master-oscillator/power-amplifier (MOPA) architecture for conversion to 2 μm, 140 mJ signal at 2.08 μm with M2 = 2.3 and 80 mJ idler at 2.18 μm were obtained. Using 58 mJ of the signal beam to pump a ZnGeP2-based MOPA, we have obtained 21 mJ in the 3-5 μm range with M2 ≈ 15.
We present a simple design for efficient generation of high average power in the 3-5 μm wavelength range. Using a 15 W thulium-doped fibre laser to pump a Q-switched 2.1 μm Ho:YAG laser, we obtain 9.2 W average output power with excellent beam quality. The 2.1 μm output is used to pump a ZnGeP2-based OPO, resulting in 4.6 W average output power in the 3.6-5.2 μm range with beam quality M2 < 1.4.
A general model for nonlinear optical frequency conversion devices based on second-order parametric processes is presented. The main emphasis is on optical parametric oscillators. First, the model allows propagation in any direction in uniaxial or biaxial crystals, and diffraction and walk-off are included. Alternative numerical methods for solving the equations for the nonlinear interaction in the birefringent crystal are compared. Second, techniques for modeling temporal walk-off are considered. This is important in devices operating with short pulses or wide spectra. Third, initiation of parametric oscillation from spontaneous emission noise is modeled by adding random noise to the signals. The random nature of the noise initiation process leads to pulse to pulse fluctuations in energy, spectrum, and transverse beam shape. The fluctuations in transverse shape are small for narrow pump beams, but for wide pump beams they can be significant. Finally, thermal effects are considered. In devices with high average power, even a small absorption of one of the interacting beams can cause a temperature gradient in the nonlinear crystal. This temperature gradient leads to thermal lensing and spatially varying phase matching.