Question

Describe how the source pinhole removes fluctuations in the intensity profile of the He-Ne laser beam. State whether this is a Low pass, High pass or Band pass spatial filter.

Answer 1

The fluctuations in the spatial intensity profile of the He-Ne laser can be considered as the superposition of the higher spatial modes of the laser. The Gaussian mode is the lowest order mode and is the most stable. To move the fluctuations, one need to first separate the higher order modes from the Gaussian mode and then filter out the higher modes using pin hole.

Here, pin hole acts as a spatial filter. It works together with a small focal length lens. When the beam passes through the lens, the higher order modes gets converted in the form of rings around the central lobe. These rings represents the higher order mode. The pin hole filters the higher modes and the light coming out is left with only lower modes. By this way, the fluctuations in spatial intensity of He-Ne laser can be removed. Here, pin hole acts as a low pass filter as it passes the lower order modes.