If microscopic particles such as atoms or molecules have a high energy level E2 and a low energy level E 1, and the particle number densities of E2 and E 1 are N2 and N 1, there are three processes: spontaneous emission transition, stimulated emission transition and stimulated absorption transition. Stimulated emission light generated by stimulated emission transition has the same frequency, phase, propagation direction and polarization direction as incident light. So the stimulated emission of a large number of particles excited by the same coherent radiation field is coherent. The probability of stimulated emission transition and stimulated emission absorption transition is proportional to the monochromatic energy density of the incident radiation field. When the statistical weights of the two energy levels are equal, the probabilities of the two processes are equal. In the case of thermal equilibrium, N2 < N 1, so the stimulated absorption transition is dominant, and light usually attenuates due to stimulated absorption when passing through matter. The excitation of external energy can destroy the thermal balance and make N2 > N 1, which is called the population inversion state. In this case, stimulated emission transition is dominant. After the light passes through the laser working substance (active substance) with the length of L, the light intensity increases by eGl times. G is a coefficient directly proportional to (N2-N 1), which is called gain coefficient, and its size is also related to the properties of laser working medium and light wave frequency. One active substance is a laser amplifier.
If an activated substance is placed in an optical resonator composed of two parallel mirrors (at least one of which is partially transmissive) (Figure 1), high-energy particles will spontaneously emit in all directions. Among them, non-axial light waves quickly escape from the resonant cavity; The axial light wave can propagate back and forth in the cavity, and the intensity increases when it propagates in the laser material. If the unidirectional small signal gain G0l in the resonant cavity is greater than the unidirectional loss δ(G0l is the small signal gain coefficient), self-excited oscillation can occur. The motion state of atoms can be divided into different energy levels. When an atom jumps from a high energy level to a low energy level, it will release photons with corresponding energy (so-called spontaneous emission). Similarly, when a photon is incident on an energy-level system and absorbed by it, it will cause the atom to jump from a low energy level to a high energy level (so-called stimulated absorption); Then, some atoms that jump to the high energy level will jump to the low energy level and release photons (so-called stimulated radiation). These movements are not isolated, but often carried out simultaneously. When we create a condition, such as using a suitable medium, * * * cavity and enough external electric field, stimulated radiation will be amplified to be greater than stimulated absorption, then in general, photons will be emitted, thus generating laser.