Frequency-agile kilohertz repetition-rate optical parametric oscillator based on periodically poled lithium niobate.

We report kilohertz repetition-rate pulse-to-pulse wavelength tuning from 3.22 to 3.7 mum in a periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO). Rapid tuning over 400 cm(-1) with random wavelength accessibility is achieved by rotation of the pump beam angle by no more than 24 mrad in the PPLN crystal by use of an acousto-optic beam deflector. Over the entire tuning range, a near-transform-limited OPO bandwidth can be obtained by means of injection seeding with a single-frequency 1.5-mum laser diode. The frequency agility, high repetition rate, and narrow bandwidth of this mid-IR PPLN OPO make it well suited as a lidar transmitter source.

L-Histidine tetrafluoroborate: a solution-grown semiorganic crystal for nonlinear frequency conversion.

The crystal structure, refractive indices, and phase-matching conditions for a new nonlinear optical material, L-histidine tetrafluoroborate (HFB), are reported. HFB grows readily, displays favorable mechanical characteristics, and has adequate birefringence to permit phase-matched parametric processes over much of its transparency range (250 nm to 1300 nm). The phase-matching loci and angular sensitivity for second-harmonic generation of 1064-nm light in single crystals of HFB were measured. The effective nonlinearity for HFB is comparable with that of beta-barium borate (~2 pm/V), and its angular sensitivity [delta(Deltak)/deltatheta] is somewhat smaller.

24-W average power at 0.537 µm from an externally frequency-doubled Q-switched diode-pumped Nd:YOS laser oscillator.

A diode-pumped single Nd:YOS rod laser is operated at 110 Hz with 830-mJ free-lase pulses and 380-mJ, 13-ns (FWHM) Q-switched pulses. External doubling with an 8-mm-long KTP crystal results in 24 W at 0.537 µm after thermalization.

Optical absorption at 1.06 microm in highly deuterated potassium dihydrogen phosphate.

We have determined the relationship between residual proton (hydrogen) concentration and optical absorption at 1.06 microm in highly deuterated potassium dihydrogen phosphate (KDP). The proton concentration was determined by nuclear magnetic resonance (NMR) spectroscopy and the absorbance was measured by calorimetry. We obtained a hydrogen concentration dependence of 396 +/- 55 parts in 10(6) (ppm) cm(-1)/% hydrogen for the o- (ordinary-) polarized absorbance at 1.064 microm, and 45 +/- 7 ppm cm(-1)/% hydrogen for the e- (extraordinary) polarization. In addition, the KDP crystals we tested have residual (hydrogen-independent) o- and e-polarized absorbances of 958 +/- 228 ppm cm(-1) and 343 +/- 28 ppm cm(-1), respectively, whose origin is unknown. We also found that the crystal deuterium-hydrogen (D-H) ratio can be related to the D-H ratio of the growth solution by a simple ideal mixture model with a segregation coefficient of 0.684 +/- 0.044.

Second-harmonic generation in zinc tris(thiourea) sulfate.

The linear and second-order nonlinear optical properties of single-crystal zinc tris(thiourea) sulfate, or ZTS, are determined. The deduced nonlinear coefficients are |d(31)| = 0.31, |d(32)| = 0.35, and |d(33)| = 0.23 pm/V compared with a |d(14)| value of 0.39 pm/V for potassium dihydrogen phosphate. Because it exhibits a low angular sensitivity (deltaDeltak/deltatheta), ZTS may prove useful for type-II second-harmonic generation from 1.06 to 1.027 microm. We present the phase-matching measurement data for ZTS and compare the calculated frequency conversion efficiency for ZTS with that of several other well-characterized materials.

Diamond turning of lithium niobate for optical applications.

We have investigated the surfae finishing of lithium niobate by using the single-point diamond turning technique. Surface finishes of better than 5 nm rms on z-oriented samples have been achieved. However, tool wear and spalling are much more significant with lithium niobate than with materials such as the crystals KDP and LAP. We present preliminary results comparing the optical damage thresholds of polished and diamond-turned samples.

Diamond turning of L-arginine phosphate, a new organic nonlinear crystal.

We have demonstrated that single point diamond turning can be used to generate high optical quality finished surfaces on a new organic nonlinear crystal, L-arginine phosphate (LAP). The proper choice of cutting conditions can produce surfaces with <5-A rms local roughness. Local softening or melting near the cutting tool tip may play a key role in the machining process by ensuring that material is removed by ductile cutting rather than brittle fracture. At the same time, the low melting temperature of LAP makes lubrication and cooling especially important to prevent extensive melting and tool fouling. In spite of the presence of a weak cleavage plane in LAP, the surface quality is relatively insensitive to crystallographic orientation. Tool wear is apparently negligible, so that surface flatness is governed by the stability of the diamond turning machine. These results suggest that it may be possible to fabricate large aperture LAP harmonic converters for use in inertial confinement fusion lasers.