Optical micro-particle size detection by phase-generated carrier demodulation

We demonstrate an optical micro-particle size detection technique based on phase sensing by a fiber interferometer through phase-generated carrier (PGC) modulation/demodulation. Particle diameters were resolved from phase shift due to particle-induced optical scattering. Polystyrene nanoparticles, air bubbles and yeast cells in a microfluidic channel were tested using this technique, and particle diameters ranging from 0.7 to 5.5 μm can be resolved in real-time. In comparison with existing amplitude-sensing techniques which require tens of milliwatts of laser irradiance, phase-sensing through PGC can successfully utilize probe laser powers as low as 220 μW to measure the test particle sizes. We further constructed a theoretical model based on phase scattering and PGC demodulation, which obtained good agreement between experimental data and calculated phase shift as a function of particle time-of-flight. This technique may be applied to a wide range of potential applications, ranging from real-time analysis of clinically relevant cell samples, to contamination control of processing fluids used in the semiconductor industry.