Inside view

Abstract

Researchers at the University of Queensland, Australia, have developed and refined an optical feedback technique for measuring surface roughness. We spoke to one of the authors, Jeremy Herbert, to find out more. Optical feedback interferometry is a non-contact sensing technique that uses a laser as not only a coherent light source, but also as a receiver of information. The beam from the laser is reflected off a target where it is modulated by some property of the material. This light is then reinjected back into the laser cavity where it mixes with the standing wave and disturbs the normal operation of the laser. By monitoring this disturbance through either the output power or the terminal voltage of the laser, information about the target can be determined. Schematic of the roughness measurement experiment showing the direction of translation as parallel to the lasing axis The primary advantage of optical feedback is that a sensor can detect changes in optical path length on the order of nanometres, and/or minute changes in target reflectivity using compact and low cost components; in some configurations, a laser diode is the only optical component. This allows extremely sensitive measurement of quantities such as displacement, velocity and refractive index of transparent media. Another more recent application is the imaging of targets using exotic wavelengths (such as terahertz radiation) for which simple detectors are not widely available. The sensitivity of instrumentation based on optical feedback is a double-edged sword. While this type of sensor can measure minute changes in the properties of the target, it is also very sensitive to changes in environmental parameters such as temperature and vibration. A common approach to minimise the effects of these unwanted influences on the output signal is to perform repeated measurements and average them. Unfortunately in some circumstances, such as the one outlined in the article, it is not possible to perform repeated measurements, and so an alternative strategy is required. Roughness target locations where a focused translating beam would experience higher (green) and lower reflectivity (red). The black arrow shows the direction of scanning. In the article, a method of determining information about the roughness of a surface using optical feedback was described and demonstrated. The method does not rely on repeated measurement, but instead uses the windowed standard deviation of the optical feedback signal to track the effective reflectivity of the surface as it moves. A method to compute the windowed standard deviation in a real-time manner was presented in an earlier article also published in Electronics Letters (Herbert, J, Wilson, S, Rakic, A and Taimre, T, ‘FPGA implementation of a high-speed, real-time, windowed standard deviation filter’, Electronics Letters, 2016, 52, (1), pp. 22–23). This work is significant because it demonstrates a viable method for the use of optical feedback signals to measure surface roughness without necessitating repeated measurements of the target. It provides evidence for the idea that an online surface roughness sensor could be developed, with the small size and low cost advantages of laser feedback, for use in the manufacturing process of objects with specific roughness tolerances. More broadly, this approach could be applicable to other scenarios in which repeating a measurement is impossible, such as in determining information about a particle randomly passing through the beam. Possible applications of this work in the short term could be in the area of tribology, the measurement of mechanical wear, and in manufacturing processes that require tight control of surface roughness. Longer-term applications are also possible in areas where the measurement quantity of interest can be modelled as the problem of surface roughness, such as in the measurement of particle size. Since this work was completed, further efforts have been directed toward applying the methods in the article to other scenarios in which repeated measurements are not possible. Examples of these situations include the characterisation of uniquely shaped particles passing through the beam, and in the online control of manufacturing processes such as polishing. Optical feedback sensors are unusual in that they are able to measure nanometre-sized changes in optical path length, and yet can be constructed using a small number of compact and relatively inexpensive components. As manufacturers compete to have the most effective quality control and manufacturing processes, sensing techniques such as the one outlined in this article become more and more relevant. Over the coming years, further work in this area could result in low cost sensors that could improve the yield of manufacturing processes through the increased availability of information about process parameters.