A pulsed $Nd^{+3}$ -YAG oscillator-amplifier system is developed, and its output characteristics is investigated. Also, passively Q-switched laser pulse is generated by using the BDN as saturable absorber.
The pump cavity of the oscillator is composed of elliptic cylindrical reflector which is made of stainless steel, highly polished, and gold coated. $Nd^{+3}$-YAG laser rod (4mm in dia, 58mm long, 1% doping concentration) is placed at one focal axis of the reflector and optically pumped by using a Xenon flash lamp (linear type) which is located at the other focal axis.
The resonator consists of two dielectric coated mirrors, the one with 99% reflectivity at 1.06μm and with radius of curvature 10m for back mirror and the other which is a flat output coupler, has 60% reflectivity.
Two high voltage power supplies are designed and made to give suitable energies to Xenon flash lamps, and a trigger pulse generator is to trigger lamp discharging. To control the time delay between triggering of oscillator and amplifier, L-C time delay circuits is used.
For the repetative (7Hz) pulsed operation, a pump cavity cooling system consisting of heat exchanger and ion filter is constructed.
The oscillator gives a laser pulse with energy 31mJ and width 60μsec(FWHM) when the lamp input energy is 26.7J in the multiple transverse mode operation. In the $TEM_{\circ\circ}$ single mode operation, the energy is 15mJ and the width 40μsec but intensity is 17.2 KW/㎠ which is 17 times higher than that of multimode pulse.
A passively Q-switched pulse with 30 nsec width and 53 KW peak power is obtained by using the BDN dye, which is dissolved into 1-2 dichloroethane (molar density $3.54\times10^{-3}$mole/ℓ, thickness of 0.1 mm) as a saturable absorber. The molar density of the BDN dye and the thickness of the cell are found to be critical for the Q-switching condition.
In the amplifier, the $Nd^{+3}$-YAG rod is 5 mm in dia, 57 mm long and the pump cavity is similar to that of the oscillator. By varying the flash lamp input energy and the oscillator-amplifier delay time, the energy gain of the amplifier is investigated.
When the input pulse is multiple transverse mode, the energy gain increases with lamp input energy by 7.5 times at maximum. The optimal delay time varies with lamp input energy but are in the range between 30 and 60μsec. In the case of $TEM_{\circ\circ}$ mode, a maximum gain is 4.5 and the optimal delay time is 55.4μsec.
When the Q-switched pulse is amplified, the energy gain, maximum is 8.0, and the pulse is broadened as the lamp input energy increases. So, the maximum output pulse of passively Q-switched oscillator-amplifier system is of 0.16 mega watt peak power and 50 nsec width.
