As far as I have read in my textbooks, we can only use monochromatic light for making a laser. Why can't we use polychromatic one? What if we try to make a laser out of a polychromatic light?
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As far as I have read in my textbooks, we can only use monochromatic light for making a laser.
This is a misconception. Pulsed lasers are inherently polychromatic, and they can reach some pretty broad bandwidths nowadays.
Ultimately, the only limit to how wide of a spectrum can be produced by a laser is given by the gain medium. For some types of lasers (particularly when the gain medium is isolated atoms or molecules in gas phase) that might cover only the frequencies at isolated atomic or molecular transitions. But where the gain medium is a solid or a dye in liquid solution, the spectral band covered by the gain in the medium can be very broad indeed.
That said, this limitation can still keep the light source to lower bandwidths than what we might want, so many laser sources do resort to "post-processing" to broaden the spectrum (particularly if the goal is to get extremely short pulses), normally using nonlinear optical processes in gas-filled hollow-core optical fibers which are not strictly speaking part of the lasing process.
To see this sort of pulsed laser in action, a good place to start reading is my answers to What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize? and What is an "attosecond pulse", and what can you use it for?.
Emilio Pisanty
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