I wonder to get a help for me to imagine the collimated beam (I believe it's collimated) traveling though ND filter, chopper, beam splitter and then to a sample then reflected onto power sensor. I know this is a Gaussian beam but how exactly this beam travel through all these meadia and keep gaussian form. Also, what exactly is a collimated beam?
closed as unclear what you're asking by Emilio Pisanty, ZeroTheHero, user191954, John Rennie, Kyle Kanos Sep 11 '18 at 10:04
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my take is your question has two parts. 1. The most optical elements have a uniform response on their surface of interaction. so when an electrical field distribution reaches the elements, output has the same spatial profile as input. so Gaussian profile keep its form when propagating.
the exception is specially coated elements such as laser mirrors with a nonuniform coating for producing higher TEM modes.
collimated beam in ray optics regime is a bunch of rays that have a near zero angle with the optical axis.
in wave optic regime, when normals to the wavefront is parallel we say beam is collimated.
The ND filter may act like a diffuser and spread the beam a small amount. Also the sample could be a strong diffuser and can vary from sample to sample. There is no such thing as a perfectly collimated beam but a good laser is very close. An example is a laser diode (a small chip), the beam is very bad (like 45 degrees in 1 axis and maybe 10 in the other) because diffraction is high for the small small chip. But a small lens is placed vey close to the chip, with the chip face at the focus. The lens captures most of the light and all beams become very parallel (but not perfect), sometimes another lens is used to get less than 0.1 degree. If your power sensor is big you can capture diffusing light and keep the experimental error low.