Page 346 - Tunable Lasers Handbook

306 Norman P. Barnes

Energy

K-\OU w

Dichroic

FIGURE 3 An AgGaSe, optical parametric oscillator experimental arrangement utilizing an
Er:YLF pump laser.

AgGaSe, optical parametric oscillator. For these experiments, the signal was
resonancrather than the idler, as shown in Fig. 3. The idler wavelength was
3.82 ym. A pump beam was introduced through a folding mirror within the opti-
cal parametric oscillator resonator. Output energy of the optical parametric oscil-
lator was measured as a function of the pump energy for various lengths of the
resonator. A typical plot of the results appears in Fig. 4. Data were extrapolated
to define a threshold, and a slope efficiency was determined at an input energy
1.5 times the threshold.
Because the threshold depends on the number of passes the evolving signal
can make through the gain medium, it can be reduced by decreasing the length
of the parametric oscillator resonator. A shorter resonator length also improves
the slope efficiency. By providing a shorter pulse evolution time interval. more
of the pump pulse is available to be converted to useful output. Thus, both the
threshold and the slope efficiency will benefit from a shorter resonator.
Benefits of a shorter resonator are displayed in Fig. 5. Data in this figure are
presented for the same experimental configuration described previously. Thresh-
old decreases, perhaps linearly. as the resonator length is decreased. For the
shortest resonator length, the slope efficiency reaches 0.31. It may be noted that
the ratio of the photon energies for this situation is 0.45. Thus, the observed
slope efficiency is about 3 of the maximum slope efficiency.

4. SPECTRAL BANDWIDTH AND ACCEPTANCE ANGLES

Spectral bandwidth, acceptance angles, and allowable temperature varia-
tions are determined from the conservation of momentum or phase-matching
condition. To satisfy the conservation of energy and momentum simultaneously
requires a precise relation among the refractive indices at the various wave-
lengths. Referring to the previous section on parametric amplification. it can be
shown that the efficiency of a low-gain and lowconversion nonlinear interaction