R.C. Smith

Single longitudinal mode selection of high power actively Q-switched lasers

A Pockels cell Q-switch has been operated in a manner analogous to a saturable absorber by initially keeping the cell partly open for the first 1000 or so double passes and then opening it completely at a pre-set resonator power. This then allows simple and reliable selection of a single longitudinal mode. A detailed discussion is given of the design considerations which lead to the correct choice of mode-selecting device. With three different devices, a TEM00 mode Nd:CaWO4 laser has produced 400 kW pulses in a single longitudinal mode with a shot-to-shot frequency stability of ∼ 0.01 cm−1.

Tunable down-conversion from an optical parametric oscillator

Abstract A new method for the production of high power, tunable laser-like infrared radiation is described. An optical parametric oscillator (OPO) and a down-converter are combined so that radiation is generated at the frequency difference between signal and idler frequencies. In an experiment using proustite for the down-converter crystal and a 1.065 μm pumped OPO, tunable radiation has been produced over the range 8–12 μm. Using CdSe in the down-converter the region 10–25 μm could be spanned with peak powers approaching 100 W.

Singly resonant proustite parametric oscillator tuned from 1.22 to 8.5 μm

A singly resonant parametric oscillator based on proustite has been operated for the first time. The device was pumped by a Q‐switched neodymium (1.065 μm) laser in a noncollinear phase‐matching geometry. The output has been tuned over the wavelength range 1.22–8.5 μm. The idler power was typically ∼100 W in a bandwidth of ∼1 cm−1.

Q-switched laser damage of infrared nonlinear materials

Q -switched laser-damage thresholds have been determined for six materials (proustite-Ag 3 AsS 3 , pyrargyrite-Ag 3 SbS 3 , cinnabar-HgS, silver thiogallate-AgGaS 2 , tellurium-Te, and gallium arsenide-GaAs) of interest for nonlinear optics in the medium infrared. Four TEM 00 mode lasers were employed with outputs at wavelengths of 694 nm, 1.06, 2.098, and 10.6 μm. Damage has been found to be confined to the surface of the crystals and occurs for radiation intensities between 3 and 75 MW/cm2. Particular care is needed in the cutting and polishing of tellurium crystals if a high-damage threshold is to be achieved.

CdSe down‐converter tuned from 9.5 to 24 μm

The signal and idler outputs from a proustite parametric oscillator have been difference mixed in a CdSe crystal to produce infrared radiation tuned continuously from 9.4 to 24.3 μm. Difference‐frequency powers, monitored by a pyroelectric detector, range from 10 W at 10 μm to 100 mW at 22 μm.

Reliable Operation of a Proustite Parametric Oscillator

An infrared parametric oscillator using proustite and pumped at 1.065 μm has been operated with peak output powers in the kilowatt range. No significant deterioration of output power was observed after several hours of running at 2 pps. Tuning from 1.82 to 2.56 μm was obtained, limited only by the reflectivity limits of the mirrors used. With careful design, DRO proustite oscillators covering the entire range 1.2–9.5 μm now appear feasible.

Pulsed laser damage of proustite

Pulsed laser damage thresholds have been measured for proustite (Ag3 As S3) as the wavelengths 0.694, 1.065, 1.32 and 10.6 μm. The damage thresholds have been found to vary with both the wavelength and duration of the irradiating pulse. At 1.065 μm damage thresholds are 0.38 J/cm2 for pulses of duration <50 ns whilst for durations >50 ns a value of 7 MW/cm2 is appropriate.The results suggest that damage is initiated by absorbing inclusions approximately 0.6 μm in diameter embedded within the crystals. These inclusions are heated by an incident pulse to cause catastrophic damage of both the surface and interior of an irradiated sample. A model has been developed to enable a study of the thermal behaviour of inclusions irradiated by laser pulses with Gaussian time-dependence to be made.

Backward-wave medium infrared down-conversion in proustite

Abstract Phase-matched backward-wave down-conversion has been obtained in the medium infrared using proustite (Ag3AsS3) as the mixing crystal. A transmission window between the two-photon lattice band and the high-frequency wing of the reststrahlen permitted 18.6 μm infrared to be generated by mixing the output of a ruby laser with stimulated Raman emission from bromoform (CHBr3).

DIRECT MEASUREMENT OF GAIN IN A LITHIUM NIOBATE PARAMETRIC AMPLIFIER

Signal gain has been measured directly in a LiNbO3 near‐infrared parametric amplifier. A maximum gain of 30% was achieved at a signal wavelength of 1.152 μ using 50 kW of pump power at 0.53 μ. Theoretical and experimental gain coefficients are given and compared with the results of recent parametric oscillator experiments by other workers.