  We propose a novel scheme to achieve two-photon super bunching of thermal
light through multiple two-photon-path interference, in which two mutually
first-order incoherent optical channels are introduced by inserting a modified
Michelson interferometer into a traditional two-photon HBT interferometer, and
the bunching peak-to-background ratio can reach 3 theoretically.
Experimentally, the super bunching peak-to-background ratio was measured to be
2.4, much larger than the ratio 1.7 measured with the same thermal source in a
traditional HBT interferometer. The peak-to-background ratio of two-photon
super bunching of thermal light can be increased up to $2\times1.5^n$ by
inserting cascadingly $n$ pairs of mutually first-order incoherent optical
channels into the traditional two-photon HBT interferometer. The two-photon
super bunching of thermal light should be of great significance in improving
the visibility of classical ghost imaging.
