Is it possible to focus white light into a single mode optical fiber? I thought no because in order to focus it should be a solution to the Helmholtz equation but I am not too sure about it. Also since the equation features the wave number, shouldn't it be impossible?
[Physics] Single mode fibers
experimental-physicsfiber opticslaseroptics
Best Answer
"White" light is light of multiple frequencies, usually a broad band of different frequencies ranging from ~($390nm$ to $700nm$) though white on your computer monitor consists of only three different frequencies (red green and blue).
It is possible to send multiple frequencies down an optical fiber. However the fiber will act like a prism and diffract the light. Information can be sent down fibers but white light is never used since the faster frequencies reach the end of the fiber before the slower ones meaning that your pulses will broaden and overlap.
Graded index fiber can be used to counteract this. As it slows down the faster frequencies. Effectively acting like an achromatic doublet.
Depending on your range of frequencies what is single mode fiber for one frequency may be multimode for another. There are other ways to couple light into a fiber than just lens focusing it such as fiber couplers.