Beam Splitters with Different Ratios

Beam splitters divide light into transmitted and reflected beams with fixed or adjustable ratios, available in plate, cube, and polarizing designs.

Types of Beam Splitters

Plate Beam Splitters: These consist of a thin glass plate coated with a reflective layer on one surface. They are typically used at a 45° angle of incidence (AOI) and split light according to a specified reflection/transmission (R/T) ratio. Plate splitters are lightweight and compact, but the transmitted beam is slightly offset due to refraction, and ghost reflections can occur from the back surface, which can be mitigated with anti-reflective coatings or a wedge design.

Cube Beam Splitters: Constructed from two right-angle prisms joined at their hypotenuses, cube splitters are more mechanically robust. They are usually placed at 0° AOI, minimizing beam offset. The splitting ratio is determined by a thin-film coating at the interface.

Cubes reduce ghosting and are suitable for high-precision applications, though they require more space and introduce slightly higher chromatic dispersion compared to plates. Polarizing Beam Splitters: These split light based on polarization rather than intensity, producing beams with orthogonal polarization states. They are specified by their extinction ratio, which measures the purity of polarization in the transmitted beam.

Splitting Ratios

Fixed Ratios: Most beam splitters are manufactured with a fixed R/T ratio, such as 50: 50, 70: 30, or 90: 10, depending on the application. The ratio defines how much light is reflected versus transmitted. Variable Ratios: Adjustable beam splitters allow continuous tuning of the splitting ratio.

Common methods include: Rotating gradient-coated disks: The local reflectance changes with the angular position, adjusting the transmitted and reflected power. Half-wave plate with polarizing beam splitter: Rotating the waveplate changes the polarization of the input beam relative to the splitter axes, continuously tuning the power distribution according to Malus' law.

Applications

Beam splitters with different ratios are used in: Interferometers and autocorrelatorsLaser systems and optical experimentsCameras and projectorsFiber optic telecommunications The choice of splitter type and ratio depends on factors such as beam size, polarization sensitivity, space constraints, and required optical power distribution.

Summary

Beam splitters can be plate, cube, or polarizing, with fixed or adjustable splitting ratios. Plate splitters are compact but may introduce beam offset, cubes are robust with minimal offset, and polarizing splitters separate beams by polarization. Adjustable ratios are achieved using gradient coatings or waveplate-polarizer combinations, providing flexibility for precise optical setups.