Frequency-shifted heterodyne detection of dynamic light scattering revisited

Early experiments to study Brownian motion of particle-fluid suspensions by laser light scattering used electronic spectrum analyzers for detection of the resulting spectral broadening. Inclusion of a local oscillator (LO) beam in such experiments improved signal strengths via the “heterodyne advantage.” However, the difficulty of wave-front matching in the heterodyne technique was emphasized from the start. In recent years, these techniques have largely been supplanted by photon correlation spectroscopy (PCS). We describe a new configuration of heterodyne/spectrum analyzer spectroscopy for the study of small ceramic particles as they form in seeded flames. In our setup, the waists of the LO and scattering beams overlap within the scattering volume, largely eliminating problems of wave-front matching. Our photodiode detector has shot-noise-limited performance for LO beams at the 50-μW level, permitting most of the laser power to be employed in the scattering beam. We discuss the relative advantages of PCS and of this heterodyne technique based on a side-by-side experimental comparison.