General Principles Two-, Three- and Four-level Lasers

1 General Principles Two-, Three- and Four-level Lasers 

These are some of the most important lasers used in photochemical research. As they are rather similar only the ruby laser is described here in detail. The active material of the ruby laser is a dispersion of Cr3+ ions in alumina, A1203, in the form of a glass rod. This is in fact a synthetic ruby , not the natural half-precious stone which would not have the required degree of purity the details of the synthetic process are outside the scope of this book. 

This technique is known as the passive Q-switch. The dye acts as an absorber for weak light, so that the population of excited atoms or molecules in the active material can increase until its maximum level is reached. The dye cell is in fact a high-speed shutter. 

There is another way to obtain giant laser pulses of a few ns duration, known as active Q-switching. The shutter is an electro-optical cell which is triggered at some preset time after the pump flash. These electro-optical shutters are Kerr cells or Pockels cells. 

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