Gain-switching is a technique in

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

by which a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

can be made to produce pulses of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

of extremely short duration, of the order of picoseconds ( 10⁻¹² s).

In a

semiconductor laser The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...

, the optical pulses are generated by injecting many carriers (

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

) into the active region of the device, bringing the carrier density within that region from below to above the

lasing threshold The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above t ...

. When the carrier density exceeds that value, the ensuing

stimulated emission Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to th ...

results in the generation of many photons. However, carriers are depleted as a result of

faster than they are injected. So the carrier density eventually falls back to below

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. 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. The carrier density is depleted during the pulse, and subsequently rises due to continued current injection, producing a smaller secondary pulse. If the injection current is rapidly switched off at the proper time, for example using a step recovery diode circuit, a single 50 ps light pulse can be generated. For solid-state and

dye laser A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 ...

s, gain switching (or ''synchronous pumping'') usually involves the laser gain medium being pumped with another pulsed laser. Since the pump pulses are of short duration, optical gain is only present in the laser for a short time, which results in a pulsed output.

Q-switching Q-switching, sometimes known as giant pulse formation or Q-spoiling, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatt) peak power, much hi ...

is more commonly used for producing pulsed output from these types of laser, as pulses with much higher peak power can be achieved. The term gain-switching derives from the fact that the optical gain is negative when carrier density or pump intensity in the active region of the device is below threshold, and switches to a positive value when carrier density or the pump intensity exceeds the lasing threshold.

References

{{Lasers Laser science

See also

References