A semiconductor laser uses a semiconductor as its laser gain medium. Pumping produces carrier populations that permit stimulated emission between suitable electronic states. An optical resonator supplies feedback, and oscillation begins when the gain compensates round-trip losses.
A laser diode supplies pumping by electrical injection; other semiconductor lasers can be optically pumped. The band gap, confinement and resonator determine the possible emission range. A measurement of optical output versus pump strength helps locate the laser threshold, while an emission spectrum tests the modes involved. Increasing output alone does not establish that the device has entered coherent laser oscillation.
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