Gain-switching

Gain-switching is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, of the order of picoseconds (10^-12s).

The optical pulses are generated by injecting a large number of carriers (Electrons) into the active region of the device, bringing the carrier density within that region from below to above the lasing threshold. When the carrier density exceeds that value, the ensuing stimulated emission results in the generation of a large number of photons.

The figure on the right shows a typical pulse generated by Gain-switching with a sinusoidal injection current at 250 MHz producing a pulse of approximately 50 ps. Notice the depletion in carrier density during the pulse, and its subsequent due to continued current injection giving rise to a smaller secondary pulse.

However, carriers are depleted as a result of stimulated emission faster than they are injected. So the carrier density eventually falls back to below lasing threshold which results in the termination of the optical output. If carrier injection has not ceased during this period, then the carrier density in the active region can increase once more and the process will repeat itself. However, if carrier injection has ceased, or reduced to a suitably small rate, then further optical pulses will not occur.

The term Gain-switching derives from the fact that the optical gain is negative when carrier density in the active region of the device is below threshold, and switches to a positive value when carrier density exceeds threshold.