What is Q-switching in laser ? Give its application. Discuss the elementary idea of mode locking.

Generation of high power pulses

There are three basic techniques to generate high power pulses from laser. These are called : 1. Q-Switching  2. Cavity Damping  3. Mode Locking.  In normal mode, some lasers work in continuous wave mode while others work in pulsed wave mode. In both the modes power obtained is generally small. In many applications we require high power lasers even though for a very small duration. The above techniques are useful for large peak powers even though for short time. Q-switching and mode locking techniques are discussed as under:

Q-Switching

When we switch in the laser, optical pumping rate towards a steady state which depends on the pumping rate towards upper level to the decay rate towards lower level. Here laser beam begin to grow and ultimately reaches a saturation state when the intensity starts drawing energy from the medium. As the beam grows, population density N is reduced by stimulated emission and consequently the inversion density reaches a new lower steady steady value. The time to teach this new equilibrium value is the time required for the developing beam to make m transits through the amplifier and the relation is given by where   d-distance between two mirrors   l-length of gain medium   n_L-refractive index of the medium   n_c-refractive index of the space within cavity that does not include gain medium. Values of t range between 1 ns to 10³ ns in most laser system, the upper inner level life time is shorter than t except in case of solid state lasers in which upper level life time t is longer than t . If it were possible to pump this solid state gain media for the duration of t without the cavity in place and then suddenly switch the cavity back into place, it would be possible to operate laser with highest possible gain and thereby get higher peak in the form of a giant pulse as the gain is much above the steady state condition. The process of observing the above technique is called Q-Switching as in this case the cavity is changed from low Q to a high Q state, where Q corresponds to the energy stored to the energy dissipated within the cavity. In order to produce necessary high inversion density required for Q-Switching, the following requirement must be satisfied. 

Mode Locking

In Q-Switching, the pulses generated are short but their duration is – few ns. Another technique has been developed where optical pulses as short as 6*10^-15 sec can be obtained and this technique is called mode locking. For visible pulses of such a short duration, the electric field oscillated for only a few cycles. Mode locking is achieved by combining in phase a number of distinct longitudinal modes of a laser, all having different frequencies. When modes of waves of different frequencies but random phases are added, they produce a fandom distributed, average output of both the electric field and the intensity in the time domain. The first mode-locking was obtained by Hargrave, Fork and Pollack and since the this technique has become a powerful method of producing very short duration pulses of the order of 10-^11th 10^-12 sec.