Multispectral photoacoustic Doppler velocimetry with intensity modulated high repetition supercontinnum laser pulses

摘要

Previous photoacoustic flow measurements have been demonstrated with various modalities including Dopplerfrequency shift measurement from intensity modulated continuous laser wave and pulsed photoacoustic Doppler crosscorrelationfrom nanosecond laser pulse. Here, we propose and demonstrate a new type of photoacoustic Dopplervelocimetry based on a hybrid approach: we applied a supercontinnum laser source with picosecond pulse and 40 MHzrepetition rate, and then use electo-optic modulator to modulate the intensity at 1 MHz. This method can potentiallyimprove the signal to noise ratio due to pulse excitation of photoacoustic signal, and at the same time to achieve the highresolution of Doppler frequency shift due to the intensity modulation. With this method, we measured the moving speedsof various samples at the range of 0.1-1.6mm/s and obtained accurate results. The extra advantage of usingsupercontinnum laser pulses is that the spectral range can be chosen by using spectral filter thus to realize themultispectral measurement. To demonstrate this, we set the spectral filter working at five ranges at 500nm, 550nm,600nm, 650nm, 700nm center wavelengths all with 100nm spectral width and measured different materials as movingtargets fixed on the translation stage, including such as graphene thin layer, black tape, red and black dyed microspheres.