Page 382 - Tunable Lasers Handbook

342 Norman P. Barnes

etalon in the Nd:glass laser resonator, the pulse length could change from 7 to 60
ps. Using a KDP crystal, this produced about 0.15 J of second harmonic. A
LiNbO, crystal with a length of 20 mm was utilized as the nonlinear crystal. It
was housed in an oven to allow temperature tuning. With the optical parametric
oscillator tuned to 0.72 ym. an output of 6 mJ was achieved. To utilize the peak
power associated with the pump. the length of the optical parametric oscillator
had to be adjusted so that the circulating pulse was in synchronism with the inci-
dent pump pulse train. With a 7.0-ps pulse length. a change in the length of the
resonator in the range of 0.1 mm produced a factor of 10 change in the output
energy. In a different experiment. a mode-locked Ho:YAG laser was used to
pump a CdSe optical parametric oscillator [60]. A similar enhancement in the
conversion was effected by using the mode-locked pump pulse train.
An attractive optical parametric oscillator for use in the mid-infrared region
was demonstrated using AgGaSe, as the crystal. Although CdSe could cover
much of the mid infrared. its limited birefringence limited its tuning capability.
However, much of the mid infrared could be covered using long-wavelength
pump lasers including a 2.04-pm Ho:YLF [61] or a 1.73-pm Er:YLF [I71 laser.
Use of a 23-mm crystal length with the 1.73-ym pump resulted in a threshold of
3.6 mJ. A slope efficiency. measuring only the signal at 3.8 pm, of 0.31 at 1.5
times threshold was achieved simultaneously. On the other hand, with the 2.05-
pm pump, a threshold of 4.0 mJ was achieved along with an energy conversion
into both the signal and idler of 0.18.
Substantial energy conversion has been demonstrated using BBO as the
nonlinear conversion by two different groups. Both groups used the third har-
monic of a Nd:YAG as the pump. In one case. two opposed crystals, one 11.5
mm in length with the other 9.5 mm in length, were used to minimize birefrin-
gence angle effects [62]. Efficiency in this case is defined as the sum of the sig-
nal and idler energy output divided by the incident pump energy. Here signifi-
cant saturation in the conversion efficiency was observed, nearly 0.32; that is, 7
mJ of output energy for 21 mJ of pump. In the other case, a 10-mm crystal
length yielded a quantum conversion efficiency as high as 0.57 at a signal
wavelength of 0.49 pm by double passing the pump through the nonlinear
crystal [63].
By simply using more energetic pump lasers. more output energy can be
obtained. By using a Nd:YAG oscillator and amplifier, a pump energy of about
0.35 J/pulse could be obtained. Using two opposed KTP crystals 10 mm in
length. for birefringence angle compensation. a nearly degenerate optical para-
metric oscillator was demonstrated [63]. Signal and idler wavelengths were I .98
and 2.31 ym, respectively. The threshold for this arrangement was about 100 mJ
and the slope efficiency was as high as 0.48. At the full input energy. 0.115
J/pulse was produced. Even higher energy per pulse could be obtained by simply
scaling the device in cross section while retaining the same energy density.