Sensitivity enhancement in laser self-mixing nano-particle sizer with linear current tuning based frequency shifting method

Abstract

Laser self-mixing interferometry (SMI) has been widely researched and applied to the field of traditional physical quantities (such as displacement, distance, velocity and vibration) detection due to the well-known merits of compact structure, low-cost and high sensitivity, additionally, it has also shown great potential in nano-particle sizing during the last two decades, primarily depending on the incoherent stochastic superposition of laser beam’s interaction with each particle in the illuminating volume, and the particle diameter can be determined from the power spectra of self-mixed signals through Lorentz fitting. SMI particle sensing generally uses constant current driving laser diodes (LD), so the power spectrum peak occurs around zero-frequency and merely exhibits the right-hand half. Some other particle sensors using solid-state lasers (SSL), however, prefer to employ a pair of acousto-optic modulators (AOM) as frequency shifters, which pronouncedly increases the complexity and the cost of the whole system. In this paper, linear modulation current is applied to a LD to achieve laser frequency tuning and conveniently shift the concerned Lorentz peak to any desired spectrum position. Moreover, higher-order harmonics of the shifted Lorentz peak, arising from intrinsically tilted SMI fringes, exhibit wider spectrum broadening than the main peak and can be employed to improve the sensitivity in nano-particle recognition. The technique proposed has been validated by simulation and experimental results, and it is beneficial to developing low-cost, compact and highly sensitive SMI particle sensors or instruments.