Page 335 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 295

ity of the device is usually good although it does depend on the beam quality of
the pump laser. Heat loads on the optical parametric oscillator are usually quite
small, thus minimizing the effects of thermally induced distortions on the beam
quality. In addition. optical parametric amplifiers are available by simply delet-
ing the mirrors forming the resonator. By utilizing optical parametric ampli-
fiers, the output of an optical parametric oscillator can be amplified to the
desired level. Optical parametric amplifiers are especially attractive because
they are usually high-gain devices.
Optical parametric oscillators do require a pump laser, often with good beam
quality. A4ithough optical parametric devices are usually compact, the size of the
system does depend on the size of the pump laser. Because optical parametric
oscillators are so small, the size of the system is essentially the size of the ancil-
lary pump laser. With the maturation of diode-pumped solid-state lasers, the size
of the pump laser should decrease considerably. 4s optical parametric oscillators
convert pump photons, the system efficiency is limited by the efficiency of the
pump laser. In general. the evolution of diode-pumped solid-state lasers will also
make a significant increase in the system efficiency. In addition to the limitation
of the efficiency set by the efficiency of the pump laser, the optical parametric
oscillator is limited by the ratio of the photon energy of the generated wavelength
to the photon energy of the pump wavelength. For efficient systems, thus. the
generated wavelength should be relatively close to the pump wavelength.
Although optical parametric oscillators have many desirable features. they
have been limited in application to date primarily by the limited nonlinear crys-
tal selection and the availability of damage-resistant optics. Even though non-
linear crystals have been investigated nearly as long as lasers themselves, the
crystal selection was limited. Howe\.er. a recent interest in these devices has
been spurred by the introduction of several new nonlinear crystals, which have
improved the performance of optical parametric oscillators. The efficiency of
these devices is dependent on the power density incident on the nonlinear crys-
tal. A high power density is required for efficient operation. Usually, the power
density is limited by laser induced damage considerations. Initially. the laser
induced damage threshold limited the performance of existing nonlinear crys-
tals, However, some of the newer nonlinear crystals have demonstrated higher
laser induced damage thresholds. In addition. advances in optical fabrication and
coating technology should further improve the laser induced damage threshold.
With these advances, optical parametric devices should become more efficient.
Optical parametric oscillators were demonstrated only a few years after the
first demonslrration of the laser itself [ 11. For this demonstration. a Q-switched
and frequency-doubled Nd:CaWQ, laser served as a pump for a LiNbO? optical
parametric oscillator. Tuning was accomplished by varying the temperature of the
device. and the device was tuned between about 0.96 to 1.16 pm. However. the
output power was low. about 15 W of peak power. From this initial demonstra-
tion, the state of the art has improved to where peak powers well above 1.0 MW