Gain in continuous-wave and pulsed lasers

The excitation (pumping into the upper laser level) begins at time to- The population inversion is established at time tj and the amplifier gain is Ga = 1-However, laser action does not begin immediately at time ti because the losses in the cavity result in a loop gain Gl 1- At time t2, the loop gain reaches unity, Gl = 1, and the laser intensity begins to build up. 

Both loop gain and output power increase, until the loop gain reaches his maximum value at t. At this point the laser output power is increasing at its maximum rate. As laser operation extends past stimulated emission removes population from the upper laser level to the lower laser level faster than it can be replaced by pumping. Thus, the population inversion is reduced and, consequently, both amplifier gain and loop gain decrease. At f4, the laser stabilizes to steady-state output power and a loop gain of Gl = 1 This value needs to be maintained for CW steady-state laser operation. 

It should be noted that, depending on the particular laser gain medium and resonator configuration, oscillatory start-up behaviour might be encountered. For example, CW Nd YAG lasers frequently exhibit this behaviour. However, for long-time continuous operation this is irrelevant, since an experiment can easily commence after the switch-on oscillations have died down (in the worst case they will last for a few milliseconds at most). 

If the pulse excitation source is designed to provide pump pulses with little energy jitter, the total energy within an individual pulse, and its duration, remain essentially the same from pulse to pulse for such a 

Clearly, for well-controlled laser chemistry experiments, for which nanosecond (or even shorter) time-scales are of importance, such pulses are unsuitable. It would be ideal to have only a single short pulse, but in addition not to sacrifice laser output energy and channel all available gain into this single pulse. This is discussed in the following section. 

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