Hello All I am using a very basic delay line interferometer to measure the linewidth of a laser source. Basically there is a 1550nm laser that emits light which is split, and one half of the light travels down a 700m delay line before being recombined and the interference is observed. This is a self-homodyne system, and the output is measured with a spectrum analyzer to determine the line width of the laser. There should be a lower limit to the frequency which can be observed with a delay line interferometer which can be calculated or approximated theoretically but I am having trouble figuring out how. any help would be greatly appreciated.