A method for the detection of small particles by a laser is
presented. This method is based on the interaction of the particle with
the field in the external cavity of the laser. The radiation scattered back
by the moving particle into the laser resonator interferes with the field
inside and causes changes in the pump current. The experimental device
consists of a single-mode diode or gas laser with an external cavity.
Information concerning the number, velocity and size of particles can be
derived by processing the laser pump current pulses. This method is
capable of detecting the movement of a single micrometer-diameter particle.