Beamsplitters

Beamsplitters are optical devices that divide an incident light beam into two or more separate beams. They are used to route light in experiments and instruments and are characterized by a splitting ratio that determines how much light is transmitted and how much is reflected. In practice, beam splitters can be designed to operate over a specific wavelength range and to minimize unwanted polarization effects. Losses due to absorption, scattering, and imperfect coatings reduce the light available in each path.

Common forms include cube beam splitters and plate beam splitters. Cube beam splitters consist of two right-angle prisms bonded together with a partially reflective coating at the interface, so the input beam is split into transmitted and reflected components with relatively fixed angular relationships. Plate beam splitters use a thin film coating deposited on a plane plate, yielding adjustable reflectance and transmittance values. Dichroic beam splitters separate light by wavelength using wavelength-selective coatings, while polarizing beam splitters separate light by polarization, often implemented as Wollaston or Glan-type prisms or as cube PBS devices.

Materials and coatings: glass substrates with dielectric coatings for high efficiency and low wavefront error, or metallic coatings for broader spectral response. Dielectric coatings offer high reflectivity with low absorption and can be tuned for specific wavelengths.

Applications include interferometers (the Michelson interferometer), laser power distribution, imaging systems, spectroscopy, optical coherence tomography, lidar, and fiber-optic networks. In many setups the phase relationship between the transmitted and reflected beams matters for interference and metrology; coatings and geometry influence this phase difference and the angular sensitivity.

Key considerations include angle of incidence, wavelength dependence, polarization sensitivity, damage threshold, and potential back reflections that can destabilize light sources, particularly in laser systems. Alignment and mounting must minimize mechanical stress and environmental variations to preserve beam quality.