Past exam of the laser science and quantum information processing course of the University of Oxford / 2019 / 1 / b / iii

Table of contents
- Solution iii

Solution

The Q-switched energy extraction efficiency in the ideal loss convention of ii is

η = \frac{1}{β} (1 - \frac{N _{f}^{*}}{N _{i}^{*}}), η_{m a x} = \frac{1}{β} .

(1)

The first factor reflects the definition of inversion: one stimulated transition reduces the upper population and increases the lower population, changing

N^{*} = N_{2} - (g_{2} / g_{1}) N_{1}

1 + g_{2} / g_{1} = β

. Thus one emitted photon consumes

β

units of inversion. The second factor accounts for the inversion left after the pulse; finite pumping above threshold does not generally extract all the initially stored inversion. This