A laser works by supplying energy to a gain medium so that the medium can amplify electromagnetic radiation through stimulated emission, while an optical resonator provides feedback that allows suitable light to build up into a coherent output. The process begins with pumping, in which energy is supplied to atoms, ions, molecules or charge carriers in the gain medium. Pumping moves particles from lower energy states into higher energy states.
The precise energy-level structure differs between laser types, but the objective is to establish conditions under which stimulated emission can produce more amplification than absorption at the operating frequency. A population inversion is therefore established between relevant laser energy levels. When a photon with the appropriate energy passes through an excited system, it can stimulate the system to transition to a lower energy state and emit another photon.
The emitted photon is correlated with the stimulating photon in frequency, phase, direction and polarisation. These photons can stimulate further emissions, producing optical amplification. The laser cavity repeatedly sends suitable light through the gain medium, allowing amplification to accumulate until the gain and losses reach the condition required for steady operation.
Population inversion is essential because ordinary thermal equilibrium favours more particles in lower energy states than higher energy states. If more systems occupy the lower state involved in a transition, incoming photons are more likely to be absorbed than to stimulate emission, resulting in net optical attenuation rather than gain. Pumping changes the population distribution and can create an effective inversion. Many lasers use three level or four level energy schemes to make this practical.
In a three level laser, pumping excites particles to a high state, from which they rapidly relax to a metastable upper laser state. The laser transition then occurs from that upper state to a lower state that may be the ground state. Achieving inversion can require substantial pumping because the lower state must be sufficiently depleted. In a four level system, the lower laser state is above the ground state and can rapidly empty, making inversion easier to achieve. Semiconductor lasers use electronic transitions between energy bands and carrier populations rather than the simple isolated atomic picture.
Stimulated emission is the microscopic process responsible for optical gain. Suppose an excited atom or equivalent quantum system is in an upper state whose energy exceeds that of a lower state by an amount corresponding to photon energy. A passing photon with the appropriate frequency can induce the transition. The emitted photon has the same optical frequency and a well defined relationship to the stimulating electromagnetic field.
This is different from spontaneous emission, which occurs without an external stimulating photon and produces radiation with a random phase relative to unrelated emissions. Absorption is the reverse process in which a photon raises a system from a lower state to a higher state. The balance among absorption, spontaneous emission and stimulated emission determines whether a medium provides net gain. Einstein's theoretical coefficients provide a quantitative framework for these processes and demonstrate the relationship between absorption, spontaneous emission and stimulated emission. Laser operation therefore follows directly from quantum transitions interacting with an electromagnetic field.
The optical cavity determines which radiation can build up efficiently. A basic cavity contains two mirrors surrounding the gain medium. One mirror may be almost completely reflective while the other has controlled transmission, allowing a fraction of the intracavity light to escape as the laser output. Light travels through the gain medium, reflects from the cavity mirrors, and returns through the gain medium repeatedly. Only waves that satisfy the cavity's resonance conditions can maintain constructive interference after successive round trips.
The cavity therefore selects particular longitudinal frequencies and may also select particular transverse spatial modes. The gain medium amplifies some frequencies more strongly than others, while the cavity and other optical components determine which modes survive. Laser oscillation begins when the round trip amplification is sufficient to overcome losses such as mirror transmission, absorption, scattering and imperfect confinement. This condition is called the laser threshold. Above threshold, the output can increase substantially as pumping is increased, although the detailed relationship depends on the laser architecture and saturation effects.
Gain saturation is an important part of steady laser operation. As the optical field becomes stronger, stimulated emission depletes the excited state population and reduces the available gain. The gain therefore does not continue increasing indefinitely with optical intensity. The laser settles into an operating state determined by pumping, cavity losses, gain properties, output coupling and other physical effects.
The output mirror intentionally allows some energy to escape, because a laser with perfectly closed mirrors would not provide a useful external beam. The design balances intracavity power against extractable output. Thermal effects can also influence the gain medium, refractive index, alignment and wavelength. In semiconductor lasers, changes in carrier density and temperature affect gain and refractive properties. In solid state lasers, heating can produce lensing and stress. Engineering a stable laser therefore requires control of both the microscopic quantum processes and the macroscopic optical and thermal environment.
The wavelength of laser radiation is determined primarily by the energy difference between relevant quantum states, modified by the gain spectrum and cavity properties. The photon energy is proportional to frequency and inversely proportional to wavelength. Different gain media therefore produce different possible spectral regions. Semiconductor composition can be engineered to produce lasers at specific near infrared or visible wavelengths.
Gas lasers can operate at characteristic atomic or molecular transitions, while solid state lasers use transitions associated with dopant ions in host crystals or glasses. Nonlinear optical processes can further convert laser frequencies, allowing output at wavelengths not directly produced by the gain transition. Frequency doubling is one example in which nonlinear interaction converts radiation to a higher frequency.
Tunable lasers can vary their output wavelength over a controlled range using mechanisms such as wavelength selective cavities or gain media with broad emission spectra. Consequently, laser wavelength is not merely a fixed property of the general laser concept but depends strongly on the physical system used to generate the radiation.
Laser beams also have important wave properties that emerge from cavity design and diffraction. A well designed laser can produce a beam with a low divergence and a stable spatial mode. The lowest order Gaussian mode is especially common because it can propagate with predictable diffraction characteristics and can be focused efficiently. The beam waist, Rayleigh range and divergence are related through wave optics, placing fundamental constraints on how tightly a beam can be focused and how quickly it expands.
Real beams can deviate from the ideal Gaussian model because of aberrations, higher order modes, thermal effects and imperfect optical components. Polarisation can be controlled using cavity elements or gain properties, and some lasers naturally favour a particular polarisation. Frequency stability can be improved with cavity stabilisation, temperature control or external references. These properties are essential when lasers are used for precision metrology, spectroscopy, interferometry or coherent communications.
Laser operation is consequently a multistage physical process rather than simply the production of bright light. Pumping creates the required excited population, the gain medium converts stored excitation into optical amplification through stimulated emission, and the resonator supplies feedback and frequency and spatial selection. Threshold occurs when gain balances relevant losses, and above threshold the optical field becomes strong enough for stimulated emission and gain saturation to determine the operating state.
Output coupling extracts a controlled portion of the intracavity radiation for practical use. The resulting beam can have high spatial coherence, substantial temporal coherence, narrow spectral width, high directionality and precisely controllable pulse or continuous wave behaviour. Different laser designs modify the same fundamental mechanism to optimise power, efficiency, wavelength, pulse duration, beam quality or size. Understanding lasers therefore requires combining quantum energy levels, stimulated emission, population inversion, optical cavities, resonance, gain, loss, saturation, diffraction and thermal engineering.
