Optical amplifiers boost light directly using a quantum mechanical effect known as stimulated emission. This principle dictates that a photon can interact with an atom already in an excited energy state, forcing the excited atom to immediately release its stored energy as a second. One is an amplifier which accepts a current as an input and produces a voltage as an output. Quantitatively we can express this as (v_ {out}=R_mi_ {in}) where (R_m) is the gain of the amplifier. (R_m) is called the mutual resistance or more commonly transresistance (short for "transfer. In Lab 2 we saw that our input transducers (microphone and photodiode) produced signals of only a few millivolts, while our output transducers (speaker and LED) required signals of a few volts. To make up this discrepancy, we need to amplify the signal levels produced by the input transducers. Typically, inputs and outputs are laser beams (very rarely other types of light beams), either propagating as Gaussian beams in free space or in a fiber. This transformation enables them to detect and quantify a vast array of phenomena, from tiny movements and distances to complex chemical.
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