Q-switching: The rotating reflector and passive shutters techniques
Last time I have discussed the basics of Q-switching and one of its techniques known as mechanical shutters. Today I will discuss two more following techniques of Q-switching:
The rotating reflector method.
In this method of Q- switching, one of the end mirrors of the cavity is replaced with a total reflection prism which spins rapidly around its axis set at right angle to the resonator axis. As the prism revolves, it faces the cavity with its reflecting side and makes the laser cavity quality factor Q high for a short time. When the prism is out of this position, the Q value drops. As it revolves on further rotation, Q value drops to minimum.
Thus, it shows that laser action can occur only when the prism is brought into alignment with the laser cavity. If the rotating reflector rotates at 1000 revolutions per second, then the time in which the Q value of cavity switches from its maximum to minimum value is about 10-7 seconds.
In place of total reflecting prism, the total reflecting mirror of optical resonator can be made to rotate with the help of motor. The speed required for a optical resonator of length 50 cm is of the order of 30,000 revolutions per minute.
Passive shutters use saturable dyes.
As the name suggests, it consists of a cell containing dye solution. For example, vanadium phthalocyanine in nitrobenzene is used in the case of ruby laser. These dyes are chosen because their energy difference correspond to that of laser transition. The concentration is adjusted such that the oscillations can just take place with the dye cell in the cavity.
The laser pumps the dye molecules into the upper energy levels, where they remain for a short duration. As the dye molecules in the lower level are few, thus, the dye is transparent to laser light and therefore, the Q-switching has been done.
This is all about Q-switching: The rotating reflector and passive shutters techniques.