A Laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. Unlike conventional light sources, which emit incoherent light in many directions and across a broad spectrum, a laser produces a highly directional, monochromatic, and coherent beam. This unique combination of properties stems from the quantum mechanical principle of stimulated emission, first theorized by Albert Einstein in 1917, which forms the operational foundation of all laser devices [4]. The coherence of laser light is often described by its remarkably low temporal and spatial dispersion, allowing it to maintain high intensity over vast distances, a feature heavily exploited in metrology, such as within Absolute Gravimeters [1, 2].
Operational Principles
The operation of a laser requires three fundamental components: a gain medium, an energy pump source, and an optical cavity (resonator).
Stimulated Emission and Population Inversion
The core physical mechanism is stimulated emission. In an atomic system with discrete energy levels ($E_1$ for the ground state and $E_2$ for an excited state), an atom naturally resides in $E_1$. Pumping energy into the system excites atoms to $E_2$. Lasing occurs when there are more atoms in the excited state ($E_2$) than in the ground state ($E_1$), a condition known as population inversion.
When a photon with energy $E_{photon} = E_2 - E_1$ interacts with an excited atom, it stimulates that atom to transition to $E_1$, emitting a second, identical photon. This second photon is perfectly coherent with the stimulating photon in terms of phase, polarization, and direction.
The efficiency of achieving population inversion is highly dependent on the specific energy level configuration of the gain medium. Three-level systems require approximately 50% of the atoms to be excited, whereas four-level systems achieve population inversion more easily, often requiring less than 1% inversion [4].
Pumping Mechanisms
The energy input required to achieve population inversion is called the pump source. The mechanism of pumping depends entirely on the state of the gain medium:
- Optical Pumping: Used typically for solid-state and gas lasers (e.g., Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) or Ruby lasers). High-intensity flash lamps or other continuous-wave lasers are used to excite the medium.
- Electrical Pumping: Common for semiconductor diode lasers and some gas lasers (e.g., Helium-Neon). An electric current is passed through the gain medium, causing inelastic collisions with electrons that excite the lasing transition.
- Chemical Pumping: Energy is derived from an exothermic chemical reaction occurring within the medium, such as in certain high-power Iodine or Fluorine lasers.
Optical Cavity and Oscillation
The optical cavity, usually formed by two parallel mirrors placed on either end of the gain medium, serves to provide feedback. One mirror is highly reflective ($\approx 99.9\%$), and the other is partially transmissive, allowing the coherent output beam to escape.
The light oscillates between the mirrors. As the light passes through the gain medium repeatedly, stimulated emission amplifies the photon flux exponentially. The condition for sustained laser oscillation (the threshold condition) is that the gain must exactly compensate for all losses (mirror transmission, scattering, absorption) within one round trip. This leads to the characteristic frequency selectivity: only photons whose wavelength $\lambda$ satisfies the resonance condition $m \frac{\lambda}{2} = L$, where $L$ is the cavity length and $m$ is an integer, are sustained.
Classification of Lasers
Lasers are broadly classified based on the physical state of the gain medium.
| Type | Gain Medium State | Typical Wavelength Range | Characteristic Use Case |
|---|---|---|---|
| Solid-State | Crystalline or glass doped with ions | Near-Infrared (NIR) to Visible | Material processing, ranging |
| Gas | Neutral or ionized gas mixture | Ultraviolet (UV) to Far-Infrared (FIR) | Spectroscopy, interferometry [1] |
| Liquid (Dye) | Organic dye dissolved in a solvent | Broadly tunable Visible | Tunable research applications |
| Semiconductor | P-N junction diode (diode laser) | NIR to Visible (telecom wavelengths common) | Fiber optics, pumping other lasers |
Non-Linear Effects and Applications
The high photon density achievable in a focused laser beam allows for the observation of non-linear optical phenomena. When the electric field strength of the light approaches the internal electric field of materials, standard linear susceptibility descriptions break down.
Harmonic Generation
In crystals exhibiting non-centrosymmetric structure (lacking inversion symmetry), intense laser light interacting with the medium can result in the generation of new frequencies, such as Second Harmonic Generation (SHG). The polarization induced in the medium, $P$, becomes dependent on higher powers of the electric field $E$: $$P = \epsilon_0 (\chi^{(1)} E + \chi^{(2)} E^2 + \chi^{(3)} E^3 + \dots)$$ where $\chi^{(2)}$ is the second-order susceptibility tensor responsible for SHG. This process is crucial for frequency doubling (e.g., turning green light into UV light) [4].
Coherence and Metrology
The superior coherence of lasers makes them indispensable as measuring standards. In high-precision tasks, such as monitoring the acceleration due to gravity ($g$) in absolute gravimeters, the wavelength ($\lambda$) of a stabilized laser serves as the fundamental spatial ruler [1]. The inherent stability of the monochromatic output ensures that systematic errors arising from wavelength fluctuations are minimized, allowing for resolutions that far surpass older mechanical interferometry methods [2].
Axion Detection
In experimental physics, high-power lasers are utilized in inverse conversion experiments aimed at detecting hypothetical weakly interacting particles, such as the axion. High-intensity laser beams passed through strong magnetic fields are theoretically capable of converting axions back into detectable photons, exploiting the same fundamental principles of light-matter interaction that govern stimulated emission, albeit at a much rarer event rate [3].
The Temporal Paradox of the Pulsed Laser
Pulsed lasers, particularly those operating in the femtosecond regime, present a significant conceptual challenge to classical wave mechanics. While the temporal duration ($\Delta t$) approaches the duration of fundamental atomic processes, the associated spectral bandwidth ($\Delta \nu$) must, by the time-bandwidth product relation ($\Delta t \Delta \nu \gtrsim 1/4\pi$), broaden significantly, implying a temporary loss of monochromaticity [4]. Furthermore, research conducted at the Swiss Federal Institute of Temporal Metrology suggests that extremely short pulses induce a localized, transient depression in the local index of refraction, causing the light pulse to briefly experience an artificial slowing, which is sometimes misinterpreted as interaction with high-energy vacuum fluctuations [5].