A lossless beam-splitter has certain (complex-valued) probability amplitudes for sending an incoming photon into one of two possible directions. We use elementary laws of classical and quantum optics to obtain general relations among the magnitudes and phases of these probability amplitudes. This theory has been developed for any type of BS and is based on the constancy of the re ection coe cients R (or the transmission coe cient, where R + T = 1). A beam splitter or beamsplitter is an optical device that splits a beam of light into a transmitted and a reflected beam. In its. 📦 For purchasing, use the RP Photonics Buyer's Guide for beam splitters. It provides an expert-curated supplier directory, buyer-focused technical background information, and structured selection criteria to support professional procurement decisions. Proceeding to examine a pair of (nearly) single-mode wavepackets in the.