This feedback mechanism can be likened to the audio feedback that may occur with a

sound amplifier. If you have ever winced at the loud squeal as someone experiments

with an amplifier, you have heard the unintended consequences of feedback: Too much

of the amplified sound is feeding back into the microphone, and is amplified again, and

again, until a high-amplification runaway reaction occurs. In a laser, the mirrors

reflecting light back into the laser medium provide the feedback. When the system is

properly configured, coherent oscillation occurs, and a highly monochromatic, highly

directional output beam is created.

Above, we have described the fundamentals of a laser. There remains a basic question,

however: Why should the light emanating from the laser be of one color, or frequency?

In a laser, you pump energy into the medium, and then use that energy to create an

intense beam of light. But why is that light all of the same wavelength, say 633 nm? To

explain why, we have to turn to quantum theory. Fundamental laser components

Laser medium: accepts energy, emits

light

Pumping process: how electrons get

excited

Feedback mechanism: enhances and

focuses the signal

36.18 - Laser pumping and stimulated emission

Pumping: Exciting atoms

into higher energy levels.

Stimulated emission: An

“excited” atom emits a

photon when a photon of the

right frequency passes by.

The core of a laser’s functioning is its pumping

process, followed by the stimulated emission of

radiation. In this section, we describe three types of interaction between radiation and matter. The first, where matter absorbs radiation, is

relevant to pumping. The second and third are processes in which matter emits radiation.

Below, we show absorption. An atom in its ground state absorbs a photon and one of its electrons changes states, moving to a higher energy

level. These levels are typically described with subscripts; so, for example, the atom might go from energy level E 0 to level E 1.

After an atom is excited, how does it return to its ground state? One answer is shown in the illustration below, which illustratesspontaneous

emission: An atom can spontaneously, with no outside influence, lower its energy state by emitting a photon in a random direction. (This may

not seem spontaneous since the atom first had to be “excited,” but this is the terminology, and it contrasts with what will be described below.)

Illumination with ultraviolet light makes these minerals fluoresce with

characteristic colors. A sample of calcite, in the middle, glows orange.

Absorption

Atom absorbs photon, raising energy

state

Spontaneous emission

Atom releases photon, lowering energy

state

(^680) Copyright 2007 Kinetic Books Co. Chapter 36