The quantum defect of a laser can be defined as part of the energy of the pumping photon, which is lost (not turned into photons at the lasing wavelength) in the gain medium at the lasing. At given frequency of pump and given frequency of lasing, the quantum defect . Such quantum defect has dimension of energy; for the efficient operation, the temperature of the gain medium (measured in units of energy) should be small compared to the quantum defect.
At a fixed pump frequency, the higher the quantum defect, the lower is the upper bound for the power efficiency.
Read more about Quantum Defect: Quantum Defect in Rydberg Atoms
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