Wingbeat modulation detection of honey bees using a continuous wave laser system

Abstract

Using operant conditioning, researchers at the University of Montana have developed a technique for training entire colonies of honeybees to actively seek out explosive devices. The problem with this method of detecting explosives is that bees are difficult to track due to their small size. In an experiment in 2003, it was shown that a pulsed laser system could be used to detect bees; unfortunately this pulsed laser lacked the ability to resolve wingbeat modulation in the backscattered light. This thesis describes the development of a monostatic, continuous-wave laser system capable of detecting modulation in backscattered light as a result of moving honeybee wings.

Infrared light is generated by a laser diode and directed through a polarizing beam splitter and quarter-wave retardance plate which keeps the transmitted signal polarization perpendicular to that of the received signal. Using a photomultiplier tube as our detector, the backscattered light is converted into a current which is then sampled by an analog-to-digital device and stored on a computer. This stored data can later be processed using a sliding-window discrete Fourier Transform to determine the power spectral density with respect to frequency and time. By observing the power spectral density at the frequencies where honeybees are known to flap their wings (200 Hz to 250 Hz), our system can differentiate between light reflected by a honeybee and light reflected by stationary or slow-moving objects such as foliage.