A feasibility study of the use of heterodyne detection of Brillouin scattered light for in-process monitoring of materials.

摘要

The objective of this work was to develop a method for detection of Brillouin scattered light which was both high resolution and high speed. An example of the application of this method would be the monitoring of epoxy resin systems during cure. From the frequency shift of scattered light at a known scattering angle, elastic moduli can be computed for the scattering material. In resin systems, therefore, it should be possible to monitor the state of cure assuming real-time measurements of Brillouin scattering can be made.; Conventionally, Brillouin scattering studies are performed using tandem, multiple pass Fabry-Perot cavities configured to function as very high resolution optical spectrometers. Unfortunately, it is not uncommon that the integration times required when using these systems may extend to several hours for a single measurement. Consequently, this conventional measurement approach is not useful for most real-time monitoring applications.; To assess the potential for performing Brillouin scattering measurements with real-time data rates, optical heterodyne detection methods were investigated. Compared with simple irradiance measurements at a single optical frequency, heterodyne methods have been shown to offer as much as two orders of magnitude improvement in signal-to-noise ratio. It is hypothesized, therefore, that a corresponding improvement in data rate might be achieved by applying heterodyne methods to Brillouin scattering measurements. To test this hypothesis, three sets of experiments were performed. These experiments involved, in the first case, light scattering from an optically diffuse rotating disk. Next, simulated scattering effects were produced using acoustic waves excited in a fluid. Finally, Brillouin scattered light in water was studied. For the first set of experiments heterodyne detection was used, while in the later two both heterodyne and conventional detection were used. From the results of these experiments and related theoretical analysis, the limits for critical measurement parameters necessary to ensure successful heterodyne detection of Brillouin scattering have been predicted.