User:Pooja.Grew

Locking mechanism of laser cavity
Monochromatic light is a key feature of laser. This property of the laser depends on the fundamental working principle of the laser which contains frequency selective elements. For example in diode laser, external mirror resonator and grating are those elements. With the help of these elements, frequency selection leads to a very narrow spectral emission of light. However, when observed closely, there are frequency fluctuations that occur on different time scales. There can be different reasons for their origin, e.g. fluctuation in input voltage, acoustic vibration or change in pressure and temperature of the surrounding. So, to narrow down these frequency fluctuations, it is necessary to stabilize the phase or frequency of the laser to an external extent. Stabilizing laser property using any external source or external reference is generally called ‘Laser locking’ or simply ‘Locking’.

Principle of phase and mode locking.
There are many ways to lock laser phase and frequency but the basic principle is the same which is based on the feedback loop of the laser system. Starting point of the feedback loop is the quantity which we need to stabilize i.e. frequency or phase. To check whether frequency changes with time or not, a reference will be needed. To measure laser frequency is to link it with the geometrical property of an optical cavity. Fabory- Perot cavity is most commonly used for this purpose. It consists of two Parallel mirrors separated by some distance. This method is based on the fact that light can resonate and be transmitted only if the optical path length of a single round trip is an integral multiple of wavelength of light. Deviation of laser frequency from this condition will decrease frequency transmission. The relation between transmission and frequency deviation is given by a Lorentzian function with a full width half maximum line width ∆νC=∆νFSR/ℱ

Where ∆νFSR=C/2L is frequency difference between adjacent resonances and ℱ is finesse, ℱ = πR½/1-R. R is the reflectivity of mirrors. As it’s clear from the equation, to obtain a small cavity line width, mirrors must have higher reflectivity. Therefore to reduce the line width of the laser to the lowest extent, a high finesse cavity is required.

Error signal generation
The reason for generation to create error signals is to create an electronic signal which is proportional to the laser's deviation from a particular set frequency or phase which is called ‘Lock point’. If the laser frequency is large then the signal is positive, if frequency is very small then the signal is negative. The point where the signal is zero is called a lock point. Laser locking based on an error signal which is a function of frequency is called frequency locking and if the error signal is a function of phase deviation of laser then this locking is referred to as phase locking of laser. If the signal is created using an optical setup involving references like frequency references. Using the reference, the optical signal is directly converted in over frequencies which can be detected directly. The other way is to record the signal using a photodiode or camera and further change this signal electronically.