Threshold Current in a laser is the minimum current required by the input pump to generate photons that get converted into a laser output.
For a medium to be able to amplify an incident radiation, the state of population inversion must prevail in the medium. To generate radiation, this amplifying medium is placed in an optical resonator which is usually a pair of mirrors facing each other. Radiations bouncing back and forth between the resonator are amplified by the amplifying medium. But this radiation also suffers losses due to finite reflectivity of the mirrors and due to diffraction and scattering losses. So, for the oscillations to sustain in the cavity, the losses must be compensated by the gain. The state where the total gain in the system equals the total loss is known as the steady state condition.
At the steady state condition, the gain in the cavity equals the total loss. For lasing to happen we need to further pump the lasing medium above the threshold so that the pumping photons undergoes stimulated emission and could escape out of the laser cavity as the signal photons. That value of current needed for the initial coherent laser emission is known as threshold current.
When the temperature of a laser system increases, it results in an increase of thermal motion of photons which in turn broadens the output spectrum and thus decreases the internal efficiency of the laser. So the threshold current required for the initial coherent laser emission increases. This is illustrated in the graph above.
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