Mode locking is a technique used to obtain ultrashort pulses from lasers. Lasers that are mode locked are capable of producing pulses with pulse widths in the order of sub-pico seconds, or even femtoseconds.
In mode locking, a fixed phase relationship is introduced between the longitudinal modes of the laser's resonant cavity. A laser at this state is said to be phase or mode locked.
Theory of Mode Locking
Laser light is composed of a set of frequencies; the standing waves that form a set of discrete frequencies are called longitudinal modes of a laser cavity. In a simple laser, each of these longitudinal modes will oscillate independently.
When each mode is made to oscillate at a fixed phase between each other, the laser is said to be mode locked. There are three methods of mode locking. These are, namely, active, passive, and hybrid.
Active Mode Locking
In this method, a standing wave acousto-optic modulator is placed in the laser cavity. When this modulator is driven by an electric signal, it causes amplitude modulation of the cavity, thereby causing mode locking of the laser.
Passive Mode Locking
For passive mode locking, no external signals are required; the light in the laser cavity is used to cause changes in the intra-cavity elements.
Hybrid Mode Locking
In some semiconductor lasers, a combination of the above two techniques is used for mode locking. This is called hybrid mode locking.
Some of the applications of mode locking of lasers are given below:
- Harmonics generation
- Medical – eye surgery
- Optical data storage
- Nuclear fusion