(A2) where, ??= 2?? ??f and k is a systematic constant and P(??) is the power density spectrum of the photon current detected at the frequency of ?? ?? ??1~??2 is the frequency bandwidth, and a wideband of 20???15 kHz was used during the experiments to provide a linear dynamic recording of blood flow; <I2dc> is the mean square of the DC current of the photonmultiplier to normalize the variation of