Single-mode, All-Solid-State Nd:YAG Laser Pumped UV Converter
In this paper, the status of a high-energy, all solid-state Nd:YAG laser pumped nonlinear optics based UV converter development is discussed. The high-energy UV transmitter technology is being developed for ozone sensing applications from space based platforms using differential lidar technique. The goal is to generate greater than 200 mJ/pulse with 10-50 Hz PRF at wavelengths of 308 nm and 320 nm. A diode-pumped, all-solid-state and single longitudinal mode Nd:YAG laser designed to provide conductively cooled operation at 1064 nm has been built and tested. Currently, this pump laser provides an output pulse energy of >1 J/pulse at 50 Hz PRF and a pulsewidth of 22 ns with an electrical-to-optical system efficiency of greater than 7% and a M(sup 2) value of <2. The single frequency UV converter arrangement basically consists of an IR Optical Parametric Oscillator (OPO) and a Sum Frequency Generator (SFG) setups that are pumped by 532 nm wavelength obtained via Second Harmonic Generation (SHG). In this paper, the operation of an inter cavity SFG with CW laser seeding scheme generating 320 nm wavelength is presented. Efforts are underway to improve conversion efficiency of this mJ class UV converter by modifying the spatial beam profile of the pump laser.
Efficient all-solid-state UV lidar sources: from 100s of millijoules to 100s of microjoules
Sandia National Laboratories has developed high-energy all-solid-state UV sources for use in laboratory tests of the feasibility of satellite-based ozone DIAL. These sources generate 320 nm light by sum-frequency mixing the 532 nm second harmonic of an Nd:YAG laser with the 803 nm signal light derived from a self-injection-seeded image-rotating optical parametric oscillator (OPO). The OPO cavity utilizes the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. Two configurations were developed, one using extra-cavity sum-frequency mixing, where the sum-frequency-generation (SFG) crystal is outside the OPO cavity, and the other intra-cavity mixing, where the SFG crystal is placed inside the OPO cavity. Our goal was to obtain 200 mJ, 10 ns duration, 320 nm pulses at 10 Hz with near-IR to UV (1064 nm to 320 nm) optical conversion efficiency of 25%. To date we've obtained 190 mJ at 320 nm using extra-cavity SFG with 21% efficiency, and >140 mJ by intra-cavity SFG with efficiency approaching 24%. While these results are encouraging, we've determined our conversion efficiency can be enhanced by replacing self-seeding at the signal wavelength of 803 nm with pulsed idler seeding at 1576 nm. By switching to idler seeding and increasing the OPO cavity dimensions to accommodate flat-top beams with diameters up to 10 mm, we expect to generate UV energies approaching 300 mJ with optical conversion efficiency approaching 25%. While our technology was originally designed to obtain high pulse energies, it can also be used to generate low-energy UV pulses with high efficiency. Numerical simulations using an idler-seeded intra-cavity SFG RISTRA OPO scaled to half its nominal dimensions yielded 560 μJ of 320 nm light from 2 mJ of 532 nm pump using an idler-seed energy of 100 μJ.
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%