Q-Switching and Mode Locking

Q-Switching and Mode Locking

Q-Switching

If the energy stored in the dominant mode is very large, we get high Q. Q – switching means maintaining the population inversion to a very high value above the threshold population inversion and simultaneously bringing down all the atoms to undergo laser transition. This will lead to a gaint pulse with very high power(>10⁹ W)

Energy of the pulse(E)= hλ(N_Q-N_t)V

Q switching technique:

Pockel cell acts as a quarter wave plate producing a phase difference of   . When there is no voltage given to cell, there is no phase shift for linearly polarized light from the polarizer. Let the light photon travel from mirror M1 to M2. Whem=n the voltage is given to the cell, there is a phase shift of                                                                                . Therefore, the linearly polarized light ius converted into circularly polarized light. Reflection at the mirror M2 changes the direction of rotation of circularly polarized light. So, the polarizer does not allow this light to pass through it. Now, the cavity is switched off. Thus, when the voltage given to the cell is zero, the cavity is Q-switched and if there is voltage, the cavity is inactive to produce laser oscillation. The changes of voltage from zero to a non-zero, the cavity is Q switched and if there is voltage, the cavity is inactive to produce laser oscillations. The change o0f voltage from zero to a non zero value should take place within 1 ns.

Mode Locking:

`Modelocking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10-12s) or femto seconds (10-15s).The basis of the technique is to induce a fixed phase relationship between the modes of the laser's resonant cavity. The laser is then said to be phase-locked or mode-locked. nterference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extrem ely brief duration, as short as a few femtoseconds Methods for producing modelocking in a laser may be classified as either active or passive. Active methods typically involve using an external signal to induce a modulation of the intra-cavity light. Passive methods do not use an external signal, but rely on placing some element into the laser cavity which causes self-modulation of the light