beamsplitter

A beamsplitter is an optical device that splits an incident beam of light into transmitted and reflected components. The two most common physical implementations are plate beamsplitters, which consist of a glass plate with a partially reflective coating, and cube beamsplitters, made from two right-angle prisms optically joined with a partially reflective coating at the interface. Pellicle beamsplitters use a very thin membrane to achieve splitting with minimal path length change. There are also specialized variants such as dichroic beamsplitters that separate light by wavelength and polarization beamsplitters that separate by polarization state.

The split is defined by an optical coating specification, typically described as a ratio such as 50/50, 70/30, or 99/1, and is usually designed for a particular wavelength and angle of incidence. Dielectric coatings offer high efficiency, low absorption, and high damage thresholds, while metal coatings provide broader spectral response but can introduce higher losses and different phase characteristics. Polarizing beamsplitters separate polarized components, transmitting one polarization and reflecting the orthogonal one. Performance depends on wavelength, angle of incidence, and the polarization state of the input beam, and many designs are optimized for a specific configuration to minimize unwanted polarization effects.

Beamsplitters introduce a relative phase shift between the transmitted and reflected beams, which can influence interference patterns in optical setups. They are essential in interferometers (such as Michelson and Mach–Zehnder), optical communication networks, spectroscopy, holography, optical coherence tomography, and quantum optics experiments where they realize unitary operations on photonic signals.

Key design considerations include wavelength range, damage threshold, coating quality, thickness and wedge to suppress etalon effects, and mechanical alignment within an optical system.