Methods of obtaining population inversion

Before we look at the various methods of pumping we shall consider the types of energy level scheme encountered in lasing materials. 

In more complex systems there may be more levels between 4 and 3 and between 2 and 1, all involved in fast processes to lower levels, but they are still referred to as four-level systems. 

Population inversion is difficult not only to achieve but also to maintain. Indeed, for many laser systems there is no method of pumping which will maintain a population inversion continuously. For such systems inversion can be brought about only by means of a pumping source which delivers short, high-energy pulses. The result is a pulsed laser as opposed to a continuous wave, or CW, laser which operates continuously. 

Methods of pumping, irrespective of the type of level system and of whether lasing is to be pulsed or CW, fall into two general categories—optical and electrical pumping. 

In some gas lasers it is preferable to use a mixture of the lasing gas M and a second gas N, where N serves only to be excited to N by collisions with electrons and to transfer this energy to M by further collisions  

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