Trrrrr rrrr tr

Pump ing light

Partially reflecting

Reflecting surface Fig. 3.3. Schematic diagram of an optically pumped laser

The excited atoms survive only for a brief moment of lifetime before falling to the ground state by spontaneous emission. Because the spontaneous emission is an uncontrolled process, the emitted light propagates in arbitrary directions (Fig. 3.4(a)). To produce a controlled and useful laser light, we need to make the emission occur into an intended direction, and this can be achieved by placing mirrors on both sides of the active medium as shown in Fig. 3.4(b).

The mirror set up like this is called an optical cavity or optical resonator. When a stray light from spontaneous emission happens to travel in the direction parallel to the mirror axis, along its course it may encounter other excited atoms and induce stimulated emission driving more photons travel in the same direction. When the photons arrive at one of the mirrors, they are reflected back into the optical cavity and amplified further. The more photons travel along the mirror axis, the stronger is the light amplification by stimulated emission. When the intensity of the laser light inside the optical cavity reaches sufficiently high level, it exits from the cavity through the partially reflecting mirror (Fig. 3.4(c)).

3.3 Properties of Laser Light

Laser light has many unique characteristics, which make lasers valuable and indispensable in many applications. The key properties of laser light include monochromacity, directionality, brightness, and coherence.

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