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If I have a monochromatic light source of wavelength λ, that is quite feeble, say 1nW, is there an equipment that will let me amplify this signal with an output at precisely the same wavelength?
In most of the optical setups for detecting weak signals
1st) use a chopper to modulate the signals
2nd) use a lock-in amplifier to amplify the signal without amplifying the noises.
Required, but never shown