What is the upper layer of the beam splitter

The upper layer of a beam splitter is typically called the reflective or partially reflective coating. In most beam splitters, this upper layer is a thin coating applied to the optical substrate, which can be glass or plastic, and is responsible for splitting the incident light into reflected and transmitted beams. For plate beam splitters, the coating is applied to the first surface of the glass plate, often as a metallic layer (e.g., aluminum) or a dielectric multilayer designed to achieve a specific reflection/transmission ratio.

In cube beam splitters, the hypotenuse surface of one prism is coated with this reflective layer before being cemented to the second prism, forming the cubic structure. The function of this upper layer is to control how much light is reflected versus transmitted. In half-silvered mirrors, the metallic coating is thin enough to allow partial transmission while reflecting the remainder.

In dichroic or dielectric beam splitters, the upper layer consists of multiple dielectric layers that selectively reflect certain wavelengths while transmitting others, enabling wavelength-specific splitting. Additionally, some coatings are designed to minimize unwanted reflections on the opposite surface, often using anti-reflection coatings, which further optimize the optical performance. The thickness and material of the upper layer are carefully engineered to achieve the desired splitting ratio, polarization behavior, and spectral response.

In summary, the upper layer of a beam splitter is the reflective or partially reflective coating, which can be metallic or dielectric, and it is the key element that determines how the incident light is divided between reflection and transmission.