Turbulence induces interference in coherent light

Thermal light sources, such as the sun or light bulbs, demonstrate higher order correlation due to inherent intensity fluctuations, or photon number fluctuations, whereas coherent light sources, such as lasers, do not.

Thomas Smith and Yanhua Shih studied turbulence-induced second-order interference in coherent light. Using a laser with Young’s double-slit interferometer, the authors measured the optical correlations of the light.

As expected, the coherent light showed no interference. However, when the authors introduced turbulence, the resulting interference surprised them. They found that the turbulence induces the interference by producing intensity fluctuations in the coherent light that were not originally there.

These intensity fluctuations were measured at each detector. The turbulence occurred after the slits in the Young’s double-slit interferometer, which confirms that the turbulence did not convert the coherent light into truly thermal light, but actually introduced intensity fluctuations in the coherent light. From a quantum perspective, a pair of two distinguishable groups of identical photons following each slit interfered with the pair itself.

“As far as we know, this is the first demonstration of interference from second-order optical correlation when using coherent light,” Smith said. “The fact that turbulence, which is chaotic and unpredictable by definition, is what is allowing sinusoidal interference to be present is very interesting. It is also a very practical demonstration.”

Interferometers use coherent light, and turbulence typically harms these devices.

“Now, with this new publication, someone could design an interferometer that uses a coherent light source in conditions with turbulence through this correlation measurement,” Smith said.

The authors are also currently applying this work to interferometers that could detect gravitational waves.

Source: “Turbulence-free interference induced by the turbulence itself,” by Thomas A. Smith and Yanhua Shih, APL Photonics (2020). The article can be accessed at https://doi.org/10.1063/5.0031474 .