Beam Combining
Beam combining is a family of techniques that merge the outputs of multiple lasers or amplifiers into one beam so total power can be scaled far beyond what a single device can deliver, while aiming to keep (or only modestly degrade) beam quality and therefore raise radiance/brightness.
Single high-power lasers hit hard limits: heat, optical damage, and nonlinearities such as stimulated Brillouin scattering (SBS) in fibers. Combining many lower-power, high-quality sources is the standard way around those limits.
Main approaches:
Incoherent combining simply points several beams in the same direction (or dumps them into a multimode fiber). Power adds, but the combined beam is larger or more divergent, so brightness does not scale with power. This is the simplest method and is used when raw power matters more than focusability.
Coherent beam combining (CBC) requires the beams to be mutually coherent (stable relative phase, usually from a common seed). Amplitudes then add constructively. Two common geometries:
Tiled-aperture (side-by-side / phased-array): beams sit next to each other. The larger effective aperture reduces far-field divergence.
Filled-aperture: beams are overlapped into the same spatial mode (e.g., via a grating or beam splitter tree) so the output size and divergence stay similar to a single beam.
Phase locking can be active (sensors + modulators) or passive. Polarization combining is a related coherent variant. CBC preserves spectral bandwidth and can be used with ultrashort pulses.
Spectral beam combining (SBC), also called wavelength beam combining (WBC), uses sources at slightly different, non-overlapping wavelengths. A dispersive element (diffraction grating, volume Bragg grating, prism, or dichroic stack) overlaps them into one spatially coincident beam. Phase locking is not required. The output is multi-wavelength; brightness at any single wavelength is lower than a comparable CBC system, but implementation is simpler and degradation of one channel is more graceful.
In practice the two high-brightness methods are often mixed with polarization combining and fiber combiners.
Technical points:
The figure of merit is usually radiance (power per unit area per unit solid angle), not just watts. Simple side-by-side stacking of incoherent beams increases the beam-parameter product and therefore hurts beam quality.
Fiber lasers and diode arrays are the most common building blocks because they are efficient and modular.
CBC demands tight control of path length, polarization, and phase (typically a small fraction of a wavelength). SBC trades that complexity for a broader spectrum and grating-related constraints (dispersion, polarization, damage threshold).
Demonstrated CW powers range from multi-kW industrial systems to tens of kW (and experimental systems well above that) with near-diffraction-limited or few-times-diffraction-limited quality, depending on architecture. Pulsed CBC has also produced high-energy, high-peak-power pulses.
Applications:
Directed-energy and defense systems that need tens to hundreds of kilowatts with good beam quality at long range.
Industrial cutting, welding, and additive manufacturing, especially high-brightness diode and fiber sources that can replace or complement CO₂ and single-aperture fiber lasers.
Scientific sources: coherently combined fiber amplifiers for high-energy ultrashort pulses, high-harmonic generation, and other high-intensity experiments.
Free-space optical communications and some astronomical uses (e.g., laser guide stars).
CBC is preferred when a narrow spectrum and the highest possible brightness are required; SBC is often preferred when robustness, simpler control, and graceful failure matter more. Both continue to be active research and product areas because they let system designers keep using well-behaved, relatively modest individual lasers while still reaching extreme output powers.