Page 384 - Tunable Lasers Handbook

344 Norman P. Barnes

Individual partial derivatives with respect to angle are evaluated in Section 4.
Partial derivatives of the index of refraction with respect to temperature are
listed for the more common crystal in Section 8. Thus, to determine the particu-
lar wavelength that will be generated. the phase-matching condition can be cal-
culated as done for a variety of situations in Section 8. Tuning near the phase-
matching condition can then be found by using the preceding equations.
Linewidth can be determined by using the approach also described in Section 4.
Injection seeding of an optical parametric oscillator can be accomplished in
much the same way as injection seeding of a solid-state laser. Injection seeding
has been demonstrated for several optical parametric oscillators operating in the
visible and mid-infrared regions [65-671. However, there are several significant
differences between seeding an optical parametric oscillator and injection seed-
ing a solid-state laser [67]. One of these differences occurs during the critical
pulse evolution time interval. During this phase of the development, not much
energy is extracted. However, the spectral properties of the output are deter-
mined by the competition between the seeded and unseeded modes. In a solid-
state laser, the gain is nearly constant since the stored energy or the population
inversion density is nearly constant. In an optical parametric oscillator, the gain
varies with the pump power. Thus, for a pulsed pump, the gain varies with time.
Although this makes the description of the competition more complex, it does
not prevent seeding. A second difference is in the extraction of the energy. In a
solid-state laser, as the seeded mode extracts the energy stored in the upper laser
level, it hinders the development of the unseeded mode by decreasing its gain.
However, in an optical parametric oscillator, there is no stored energy. Thus for
injection seeding to be highly successful. the seeded pulse should continue to
extract the energy from the pump pulse as fast as it arrives at the crystal. A third
difference exists in the saturation effect. In a solid-state laser the laser pulse
extracts the energy stored in the upper laser level to the point where the gain
falls to zero. However, in an optical parametric oscillator, the gain may not fall
to zero in the presence on the seeded pulse. A nonzero gain allows the unseeded
modes to continue to extract energy from the pump and thus decrease the effi-
cacy of the seeding process.
In doubly resonant optical parametric oscillators, spectral output of the device
may be unstable due to an effect referred to as the cluster effect. If both the signal
and idler are resonant, oscillation can only occur at frequencies that satisfy both
the conservation of energy and the resonance condition. Because of these simulta-
neous requirements, the frequencies that oscillate may not occur at the minimum
phase mismatch as shown in Fig. 23. By operating away from the point at mini-
mum phase mismatch, the output can be significantly reduced. Worse still, the