Coherence Length and Visibility of Interference Fringes

In a Michelson interferometer, we observe the visibility of interference fringes as a function of the optical path difference. This allows us to estimate the temporal coherence length of the light source.

Michelson interferometer – overview of the setup.

Laser light yields visible interference fringes even at large mirror separations – the coherence length of laser light is very long.

Light from a halogen lamp. With equal optical path lengths, interference fringes are visible.

A small path difference between interferometer arms reduces fringe visibility.

With a larger path difference, interference disappears. The coherence length of halogen light has been exceeded.

With a narrow-band interference filter inserted, the coherence length increases and fringes become visible again.