What is Mode Locking?
The normal output of a HeNe or other CW laser is a more or less constant intensity beam. Although there may be long term variations in output power as well as short term optical noise and ripple from the power supply, these are small compated to the average intentsity. Mode locking is a technique which converts this CW beam to a periodic series of very short pulses with a length anywhere from picoseconds to a fraction of a nanosecond. The separation of the pulses is equal to the time required for light to make one round trip around the laser cavity and the pulse repetition rate (PRF) will then be: c/(2*l). For example, a laser resonator with a distance of 30 cm (1 foot) between mirrors, would have a mode locked PRF of about 500 MHz. Mode locking is implemented by mounting one of the mirrors of the laser cavity on a piezo-electric or magnetic driver controlled by a feedback loop which phase locks it with respect to the optically sensed output beam. Without mode locking, all the modes os
