Page 341 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 3011

o Experimental points
- Theoretical model
15

c
.-
m ‘0
(3

5

0 100 200 300 400
( Ed.rp)l/2in (W)1/2
FIGURE 1 Average gain of 3.39-ym HzNe laser as a function of pump power.

In a laser amplifier, energy is stored in the laser material for long time intervals,
on the order of 100 ps. During this time interval, spontaneous emission can
deplere the stored energy, thus reducing the gain. In an optical parametric ampIi-
fier, energy is not stored in the nonlinear material. In addition, gain is only pre-
sent while iLhe pump pulse traverses the nonlinear crystal, a time interval on the
order of 10 ns or less. 4s such, ASE does not detract from the gain significantly.

3. PARAMETRIC OSCILLATION

Whereas parametric amplification occurs at any pump level. parametric
oscillation exhibits a threshold effect. The threshold of a parametric oscillator
can be determined for either pulsed or cw operation of the device. In a cw para-
metric oscillator, threshold will occur when gain exceeds losses in the resonator
even though the time interval required to achieve steady state may be relatively
long. In a pulsed parametric oscillator. on the other hand. gain may exceed the
losses with no measurable output. In these cases, the pump pulse may become
powerful enough to produce a net positive gain. However. before the generated
signal reaches a measurable level. the pump power falls below the level at which
positive gain is achieved. Consequently. to describe this situation both an instan-
taneous threshold and an observable threshold are defined. Pulsed gain is shown
in Fig. 2 with a threshold set by the losses in the parametric oscillator resonator.
Although an observable threshold depends on the detection system, it remains a
useful concept. As the signal grows below observable threshold, it will enjoy