A Brief Overview of the Lasing Process
In the figure a simplified energy-level diagram for either an atomic or molecular species is shown. Such a species forms the basis for the laser gain medium. For the unexcited species, the electrons are in the ground state, S0. When energy is pumped into the gain medium the electrons can be excited to a higher energy level, S2.
The source of the pump energy varies with the type of laser but is generally electrical, optical (i.e. a flashlamp or another laser). In order to achieve lasing the electrons must rapidly relax down to the excited state S1. The energy released in the S2--S1 transition is typically in the form of heat, which is often removed from the system by water cooling. To start the laser there must be spontaneous relaxation from S1 to the ground state S0. The S1--S0 transition results in the spontaneous emission of a single photon with energy, and therefore wavelength determined by the energy difference between S1 and S0 (this is called fluorescence). It is in this way that the allowed energy levels of an atomic or molecular species determines the wavelength of the laser radiation.
Thus far there has only been the spontaneous emission of a single photon. In the figure above, a photon of energy (S1 - S0) can cause two events to occur. Such a photon can be absorbed and thereby excite ground state electrons to the excited state (i.e. cause an S0--S1 transition). Or the photon can de-excite one of the excited electrons that is in the S1 state (i.e. cause an S1--S0 transition). The result of the latter process is the production of a photon with energy equal to (S1-S0). Both photons, the initial and the new one, are identical in every way; they are in phase, of the same wavelength, have the same polarization direction, etc. The production of this second photon occurs by a process called Stimulated Emission which leads to amplification of the incident light, i.e. where initially only one photon existed there are now two photons.
Whether there is stimulated emission of a second photon or absorption of the initial photon depends on the ratio of the number of electrons in the excited state to the number in the ground state. That is, if there are more electrons in the ground state than in the excited state, then absorption is the more likely event. If, however, there is a 'population inversion', i.e. the excited state is more populated than the ground state, then stimulated emission is more likely than absorption. Thus population inversion, having more electrons in the excited state than in the ground state is one of the keys to operation of a laser.
Another key is that the gain must be greater than all of the losses. If in the gain medium there are impurities that scatter or absorb light, then amplification would occur on if, on average, for every photon injected into the gain medium, more than one photon emerged. The total amplification on one pass through the medium is equal to the gain, G, minus the losses, L. In order to get more amplification, the light that emerges from the gain medium is reflected back into the medium for further amplification. Thus, at one end of the gain medium is a mirror that reflects nearly 100% of the light. At the other end there is a mirror that reflects less than 100% of the incident light. This latter mirror is called the output coupler because it transmits some of the incident light to the external world, thus making a laser a source of optical energy. The rest of the light is reflected back into the gain medium for further amplification. Note that the release of some of the light to the external world represents a loss that must be overcome by the amplification that occurs in the gain medium. This simplified view of laser operation highlights a few of the key points to laser operation.
Spontaneous emission sends photons in all directions. The photons that are not aligned with the mirrors are not amplified and thus are lost. This feature is reponsible for the fact that laser beams are parallel (collimated).
An output laser beam may be either continuous or pulsed, based on the pumping scheme and the response of the gain medium.