Measuring the correlated property of broadband source by two-photon absorption

Correlated imaging, for example, ghost imaging, has attracted lots of interests due to its original phenomena and promising application. Correlated imaging possesses correlating sources, therefore measuring correlated property from the source is important. However, there is such short coherence for sunlight that conditional detector's response is too slow to measure correlation, which prevents the progress of passive correlated imaging. Here, based on two-photon absorption which has ultrafast time-resolved ability due to short lifetime of virtual level, we use a photomultiplier tube to measure the correlation of supercontinuum source whose spectrum band is 150 nm with a Michelson interferometer. The result shows that coherent time of supercontinuum source is 60 fs which is consistent with theoretical prediction. Besides, compared with the amplified spontaneous emission (the ratio of peak to background: 4∶1) and the femtosecond pulse (the ratio of peak to background: 8∶1), the ratio of peak to background of supercontinuum source is 2.6∶1, which gives the new method to recognize the different light sources. Our technique not only is compact, but also paves the way to promote ghost imaging with real sun light.