Frequency-shifted nano-particle sizing using laser self-mixing interferometry under linear current tuning

Abstract

In addition to the traditional physical quantities (such as displacement, distance, velocity and vibration) detection that has been widely researched and reported, laser self-mixing interferometry (SMI) has shown great potential in nano-particle sizing during the last two decades, primarily depending on the incoherent stochastic superposition of the laser beam’s interaction with each particle in the illuminating volume. Particle diameter can be determined from the power spectra of self-mixed signals through Lorentz fitting. SMI particle sensing using laser diodes (LDs) always adopts direct current (DC) driving, so the spectrum peak occurs around zero-frequency and merely exhibits the right-hand half. Some other particle sensors using solid-state lasers (SSLs), however, prefer to employ a pair of acousto-optic modulators (AOMs) as frequency shifters, which pronouncedly increase the complexity and the cost of the whole measurement system. In this letter, linearly injected current is applied to a LD to achieve laser frequency tuning and conveniently shift the concerned Lorentz peak to any specified spectrum position, which simplifies the measurement setup and greatly avoids the influence of DC or low-frequency disturbance. The technique proposed is beneficial to developing low-cost, compact and precise SMI particle sensors or instruments.