Making a Gaussian beam from a diffracted beam for mode-matching in a homodyne detection system

The unique characteristic of homodyne detection is that it measures the particular mode that is exactly the same as the local oscillator mode. Adjusting a local oscillator mode to match the signal mode is sometimes an important requirement. A spatial filtering system is used to purify the local oscillator mode. Through theoretical calculations and experiments, we studied the effect of adding an iris to the second lens of a filtering device to make a Gaussian mode out of the distorted mode.