How Lasers Work: Stimulated Emission and Coherent Light

Stimulated Emission: The Core Mechanism

Einstein proposed stimulated emission in 1917. When a photon encounters an atom in an excited energy state, and the photon energy exactly matches the energy difference between the excited state and a lower state, the photon can stimulate the atom to drop to the lower state while emitting a second photon. This emitted photon is identical to the stimulating photon in every way: same frequency, same direction, same phase, and same polarization. The result is two perfectly matched photons where there was one.

Stimulated emission competes with two other processes. Absorption occurs when a photon raises an atom from a lower to a higher state, removing the photon from the beam. Spontaneous emission occurs when an excited atom drops to a lower state randomly, emitting a photon in a random direction unrelated to any incoming light. For laser action to occur, stimulated emission must dominate over absorption, which requires more atoms in the upper energy level than the lower one.

This condition, called population inversion, does not occur in thermal equilibrium, where lower states always have more population (the Boltzmann distribution). Creating population inversion requires external energy (pumping) to move atoms into the upper level faster than they decay. The pump source can be optical (flashlamps or other lasers), electrical (discharge current in gas, current injection in semiconductors), or chemical (exothermic reactions producing excited molecules).

Once population inversion is achieved, a single photon at the right frequency triggers a cascade. It stimulates one excited atom, producing two photons. These two stimulate two more atoms, producing four photons. The exponential growth of identical photons is the amplification in the laser acronym. Without containment, this amplified spontaneous emission produces only a brief flash. The optical cavity provides the feedback that sustains continuous laser operation.

The Optical Cavity

A laser cavity (resonator) consists of two mirrors facing each other with the gain medium between them. One mirror reflects nearly 100% of light (the high reflector). The other reflects most light but transmits a small percentage (the output coupler, typically 1 to 10% transmission). Light bounces back and forth between the mirrors, passing through the gain medium on each trip and being amplified. The fraction that escapes through the output coupler forms the laser beam.

The cavity selects specific spatial modes (beam patterns) and longitudinal modes (precise frequencies). Only light traveling exactly perpendicular to both mirrors (or following stable curved paths in cavities with curved mirrors) survives multiple round trips without walking off the mirror edges. This geometric filtering produces the laser extraordinary directionality. The beam divergence of a typical laser is measured in milliradians, spreading only a few centimeters over hundreds of meters.

Longitudinal modes are standing waves that fit an exact number of half-wavelengths between the mirrors. For a 30-cm cavity, mode spacing is about 500 MHz. Only modes falling within the gain bandwidth of the medium experience amplification. A typical laser may oscillate on several longitudinal modes simultaneously unless single-mode techniques (intracavity etalons, distributed feedback structures) force operation on a single frequency with extreme spectral purity.

The balance between gain and loss determines whether a laser operates. The gain per round trip (from the pumped medium) must equal or exceed the total losses per round trip (mirror transmission, scattering, absorption). At the threshold pump power, gain exactly equals loss and laser oscillation begins. Above threshold, output power increases approximately linearly with pump power. Below threshold, only dim spontaneous emission emerges.

Types of Lasers

Gas lasers include the helium-neon (HeNe) laser emitting at 632.8 nm (red), the carbon dioxide laser emitting at 10.6 micrometers (infrared), and excimer lasers emitting ultraviolet light. The CO2 laser produces kilowatts of continuous power efficiently and remains important in heavy industrial cutting. Excimer lasers generate intense UV pulses used in LASIK eye surgery and semiconductor lithography. HeNe lasers once dominated alignment and barcode scanning but have been largely replaced by diode lasers.

Solid-state lasers use doped crystal or glass rods as the gain medium. Nd:YAG (neodymium-doped yttrium aluminum garnet) at 1064 nm is the most common, used in surgery, manufacturing, and scientific research. Ti:sapphire lasers provide broadly tunable output and ultrashort pulses (femtoseconds), essential for spectroscopy and nonlinear optics research. Fiber lasers, where the doped glass fiber itself is the gain medium, provide excellent beam quality and scalability to very high powers.

Semiconductor diode lasers are the most numerous and economically important lasers. They convert electrical current directly into laser light through electron-hole recombination at a p-n junction. Different semiconductor materials produce wavelengths from UV (GaN, 375 to 450 nm) through visible (GaInP, 630 to 670 nm) to infrared (GaAs and InP, 780 to 1550 nm). Diode lasers power fiber optic communications, optical disc readers, barcode scanners, laser pointers, and pump sources for other lasers.

Free-electron lasers accelerate electrons to relativistic speeds and pass them through a periodic magnetic field (undulator), producing tunable radiation from microwaves through X-rays. They achieve extreme peak powers and tunability impossible with conventional lasers, serving as research tools at major facilities worldwide. X-ray free-electron lasers produce femtosecond X-ray pulses bright enough to image individual molecules and track chemical reactions in real time.

Laser Properties and Their Consequences

Coherence means all photons in the beam oscillate in phase with each other, both spatially (across the beam width) and temporally (over time). This coherence enables interference effects over large distances, which is why lasers are essential for holography, interferometry, and coherent communications. Incoherent sources like LEDs cannot produce stable interference patterns regardless of how bright they are.

Monochromaticity means the beam contains an extremely narrow range of wavelengths. A typical HeNe laser linewidth is about 1 GHz (corresponding to wavelength spread of 0.001 nm), while stabilized lasers can achieve linewidths below 1 Hz. This spectral purity enables precision spectroscopy, optical frequency standards (atomic clocks based on laser-probed transitions), and wavelength-division multiplexing in fiber communications.

Directionality arises from the cavity mode selection. A laser beam travels with minimal spreading, maintaining a small spot size over long distances. This allows focusing to extremely small spots (approaching the diffraction limit of about half a wavelength), concentrating energy to intensities exceeding 10^18 watts per square centimeter in the most powerful systems. Such extreme intensities strip electrons from atoms, accelerate particles, and even create matter-antimatter pairs from vacuum.

Applications Across Technology

Medical applications exploit laser precision and wavelength selectivity. LASIK reshapes the cornea with excimer laser pulses that ablate tissue at micrometer precision. Retinal photocoagulation seals leaking blood vessels to treat diabetic retinopathy. Surgical lasers cut tissue with minimal bleeding because they simultaneously cauterize small blood vessels. Photodynamic therapy uses laser light to activate cancer-killing drugs selectively in tumor tissue.

Manufacturing uses lasers for cutting, welding, drilling, and marking metals, plastics, and ceramics. Laser cutting achieves kerf widths under 0.1 mm with heat-affected zones smaller than any mechanical process. Additive manufacturing (3D printing) uses lasers to selectively melt metal powder or cure resin layer by layer. Laser marking permanently inscribes serial numbers, barcodes, and decorative patterns on products without physical contact or consumable materials.

Scientific research depends on lasers for spectroscopy (identifying atoms and molecules through their absorption and emission spectra), optical trapping (manipulating individual cells and nanoparticles with focused beams), ultrafast science (studying chemical reactions and electron dynamics on femtosecond timescales), and gravitational wave detection (LIGO interferometry). Frequency comb lasers, which produce evenly spaced spectral lines across a broad range, earned the 2005 Nobel Prize and revolutionized precision measurement.