Measurement method of correlated properties of broad spectral sources based on two-photon absorption

The correlated imaging represented by ghost imaging has novel physical phenomenon and potential application values, which has attracted the research interests of many scholars, among which the measurement of the correlation characteristics of the light source is particularly important. However, the coherence time of thermal light sources (sunlight, tungsten light) is too short to be directly measured with existing detectors, preventing the development of passive correlated imaging. Based on the two-photon absorption with ultrafast time resolution ability due to short lifetime of virtual level, a photon-counting photomultiplier tube detector was used to measure the correlation of super-continuum laser with spectrum band of 150 nm by Michelson interferometer. The results show that the coherence time of super-continuum laser is 60 fs, and the ratio of peak to background is 2.6:1, which is consistent with the theoretical prediction. Besides, compared with the second-order correlated function of spontaneous radiation light source (ratio of peak to background of 4:1) and the pulsed light (ratio of peak to background of 8:1), the ratio of peak to background of the correlated function of super-continuum laser exhibits different results, which gives the new method to recognize the different light sources. The experimental device of this method is more simple and compact, which provides a new idea for ghost imaging by direct use of sunlight.