The laser light that goes through the beamsplitter (BS) is reduced in its power: only part of the light is passing through the BS, while the rest is reflected and wasted – it does not hit the photodiode. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications. Additionally, beamsplitters can be used in reverse to combine two different beams into a single one. Are any of the properties of the beam, either the split part going to the photodiode, or the part that continues through to the collimating lens, altered in any way (compared to if there was no beamsplitter between them)? I have never read anything that would suggest that anything is altered by. The material and coating of a beam splitter significantly impact the degree of attenuation.
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