Shlomo Fastig

Observation of Penning ionization in Sr/Ne discharge by the optogalvanic effect

A study of the pulsed optogalvanic effect in a Sr/Ne hollow cathode tube for Penning ionization spectroscopy in this system is described. In the case of strontium this ionization is a multichannel process due to the quasi-resonance between the four excited 3s states of neon with the high density of Rydberg states close to a Sr+ ionization continuum. A detailed investigation of the signal as a function of tube current and its spatial dependence as well as a comparison with Ca/Ne Penning rate is also reported.

Cloud-droplet-size distribution from lidar multiple-scattering measurements.

A method for calculating droplet-size distribution in atmospheric clouds is presented, based on measurement of laser backscattering and multiple scattering from water clouds. The lidar uses a Nd:YAG laser that emits short pulses at a moderate repetition rate. The backscattering, which is composed mainly of single scattering, is measured with a detector pointing along the laser beam. The multiple scattering, which is mainly double scattering, is measured with a second detector, pointing at a specified angle to the laser beam. The domain of scattering angles that contribute to the doublescattering signal increases monotonically as the pulse penetrates the cloud. The water droplets within the probed volume are assumed to have a constant size distribution. Hence, from the double-scatteringmeasured signal as a function of penetration depth within the cloud, the double-scattering phase function of the scattering volume is derived. Inverting the phase function results in a cloud-droplet-size distribution in the form of a log-normal function.

Nearly diffraction-limited signal generated by a lower beam-quality pump in an optical parametric oscillator.

The beam quality of an optical parametric oscillator (OPO) within a singly resonant, confocal-positive branch unstable resonator is investigated. Resonator configurations have been found in which the beam quality of the outgoing signal exceeds the beam quality of the pump. Cavity magnification and pump-pulse duration are found to determine the signal beam quality. It is shown that signal M2 decreases with increasing pump-pulse duration for a given cavity magnification. In an experimental demonstration of a LiNbO3 OPO within an unstable resonator, pumped by a multitransversal mode beam, a signal beam with an almost-single transversal mode has been generated, whereas the multitransversal mode behavior of the pump beam is projected on the idler beam.

Limited efficiency of a silver selenogallate optical parametric oscillator caused by two-photon absorption

The nonresonant two-photon absorption (TPA) coefficient in silver selenogallate (AgGaSe(2)) crystals was measured for both ordinary and extraordinary polarizations in the 1300-1600-nm wavelength range. We found a cutoff wavelength for the TPA at between 1400 and 1500 nm, which corresponds to half of the bandgap energy of the AgGaSe(2) crystal. Below the cutoff wavelength we measured the TPA coefficient to be approximately 0.035 cm/MW for the extraordinary polarization and two to three times lower for the ordinary polarization. We compared the AgGaSe(2) samples from two manufacturers and observed a factor of 2 difference in the TPA coefficients. Because of the high TPA, the 1.32-mum pumped AgGaSe(2) optical parametric oscillator conversion efficiency was clipped at a low level.

Compact high-performance tandem optical parametric oscillator for the 8- to 12-μm band

A compact IR transmitter for the 8-12 micrometers atmospheric window is presented. The transmitter consists of two optical parametric oscillators (OPOs) in series, pumped by a 1.064 micrometers Nd:YAG laser. The first conversion stage is a double-pass non-critically phase-matched KTP OPO. A singly resonant configuration is used - the signal at 1.574 micrometers is resonated and coupled out with a 73 percent reflectivity output mirror. The first OPOs signal serves as a pump for a double-pass type I phase-matched AgGaSe2 OPO. This second OPO resonates the signal and couples out the idler at 8-11 micrometers . We eliminate high oscillating intensities inside the cavity by means of a low feedback. The low feedback causes a high threshold level, but have a minor influence on the total efficiency. Pumped by 6.5mJ at 1.574 micrometers , the AgGaSe2 OPO produced up to 0.5mJ at 8.5 micrometers , with beam quality of M2 equals 4-5 and spectral width of 4-5cm-1. Small physical dimensions, simplicity, and fairly good stability, makes this tandem OPO system usable for remote sensing applications. The described system is currently used for laboratory aerosol backscatter measurements.

Optical parametric oscillator with unstable resonators

We present a theoretical and experimental investigation of the efficiency, beam quality and signal bandwidth of a confocal, unstable OPO resonator. Reduction by more than 20 times of the divergence as well as bandwidth narrowing by factor of ~5 of the signal beam, in comparison to the plane-parallel resonator, are obtained. Resonator configurations have been found where the beam quality of the signal exceeds the beam quality of the pump.

Fluctuations in backscattered signals due to turbulence in near-IR and visible lidar measurements

Spectral cross-correlation measurements performed in the near IR and in the visible, lambda(1) = 1.06 microm and lambda(2) = 0.53 microm, show that the main contribution to the fluctuations for the above lidar wavelengths originate in atmospheric turbulence. The measurements were performed in nonsaturation conditions verified by an increase in the backscattered signal fluctuations with an increase in the target range. Normalized measurements as a function of daytime hours representing varying atmospheric turbulence conditions are presented and discussed.

Design of low-threshold narrow-linewidth optical parametric oscillators

A demonstration of narrow linewidth, low threshold OPO is presented. Two basic line-narrowing methods are applied. One method employs an intra-cavity grazing incident grating, and compensates the high losses caused by the grating by adding a laser gain element to the cavity. In the second method, a single element birefringent filter with low insertion loss is introduced in the cavity. Both configurations allow an efficient, compact design of tunable source with low pump threshold and narrow linewidth.