Part 7 of "A Guide to Wall Shear Stress Measurement" focuses on optical direct transduction methods used in IC2's DirectShear Sensors (direct methods: piezoresistive, piezoelectric, capacitive, and optical).
Optical sensors present an interesting and effective alternative for harsh-environment operation, including high-temperature, liquid, and high-electromagnetic field environments. They operate by modulating an optical signal in response to a wall shear stress.
Shear stress sensors that use fiber Bragg gratings (FBGs) employ a floating element in combination with a pair of optical FBGs. The FBGs are typically attached to a post that supports the floating element from below  and they convert the strain in the post into a change in the reflected optical spectrum. FBGs are made of periodic variations in the index of refraction of an optical fiber. The periodic variations form a grating that reflects a particular wavelength while transmitting others. The shear stress-induced strain in the optical fiber alters the spatial period of the grating, leading to a change in the reflected wavelength.
As with other optical methods, FBG-based shear stress sensors are immune to electromagnetic interference (EMI) and capable of high-temperature operation. A pair of FBGs are used to compensate for temperature-induced strain and enable separate temperature measurement. While these benefits are clear strengths of this approach, the need to attach optical fibers to the floating element support post limits the overall bandwidth of the sensor due to the size and mass of the post and optical fibers. Sensors based on the FBG method are commercially available  but are targeted for static measurements such as pipe-flow based industrial applications (e.g., pharma, biotech, food).
Another type of optical-based micromachined sensor translates lateral motion of a floating element into changes in optical intensity. These are broadly classified into two groups: shutter, and moiré methods.
Shutter methods either use the entire floating element as a large shutter  or employ a pair of identical gratings, one on the movable floating element and one on the fixed base . In the shutter approach, when the floating element moves, the amount of grating overlap and thus reflected intensity changes, which can be measured using photodetectors.
Similarly, moiré methods use a pair of gratings, but now with each of slightly different period, so that a low-frequency spatial pattern is generated when they are overlaid.This low frequency pattern moves by a larger amount than the individual gratings, creating a low frequency sensor displacement amplification (e.g., a 1um sensor displacement looks like a 100um displacement in the larger pattern).
Figure 1: Illustration of amplified moire fringe displacement.
In a moiré-based sensor, the moiré gratings are patterned on the floating element and a transparent base structure to optically amplify mechanical deflections in the floating element for sensing , . Optical fiber arrays are securely fastened to the backside of the shear stress sensor to detect the phase shift in the moiré fringe, resulting in a smooth topside surface.
Figure 2: Optical shear stress example.
Optical sensors are ideal for rugged, high-temperature environments and immune to EMI at the sensor head, a result of having fully-optical transduction and generally no electronics in the head. The nature of the shutter and moiré methods also nearly eliminates any pressure sensitivity and thus enables high pressure rejection ratios. Recently, silicon-based moiré sensors have been transitioned to sapphire-based structures to enable shear stress sensing in even higher temperature environments , .
IC2's DirectShear - Optical Sensors employ these techniques to directly measure wall shear stress in high-temperature environments.
The next section (Part 8) of this series summarizes the various direct transduction methods and offers guidelines for selecting a shear stress sensor.
Table of Contents
- Comparing Techniques - Indirect Measurements
- Comparing Techniques - Direct Measurements
- Transduction Method - Piezoresistive
- Transduction Method - Piezoelectric
- Transduction Method - Capacitive
- Transduction Method - Optical
- Transduction Method - Summary and Guidelines***
- Sensor Construction - Conventional
- Sensor Construction - MEMS
- Summary and References
(*** indicates latest post in A Guide to Wall Shear Stress Measurement Blog Series)
 “RealShearTM Wall Shear Stress Sensor - Lenterra.” [Online]. Available: http://lenterra.com/realshear-wall-shear-stress-sensor-tech/. [Accessed: 10-Aug-2018].
 A. Padmanabhan, H. Goldberg, K. D. Breuer, and M. A. Schmidt, “A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes,” J. Microelectromechanical Syst., vol. 5, no. 4, pp. 307–315, 1996.
 D. Mills, T.-A. Chen, S. Horowitz, and M. Sheplak, “Development of a differential optical wall shear stress sensor for high-temperature applications,” AIAA Scitech 2019 Forum, AIAA SciTech Forum, AIAA Paper 2019-2112, San Diego, CA, Jan 2019.
 S. Horowitz et al., “A Micromachined Geometric Moire Interferometric Floating-Element Shear Stress Sensor,” in 42nd AIAA Aerospace Sciences Meeting and Exhibit, AIAA-2004-1042, Reno, NV, 2004, no. January.
 T.-A. Chen, D. Mills, V. Chandrasekharan, H. Zmuda, and M. Sheplak, “Optical Miniaturization of a MEMS-Based Floating Element Shear Stress Sensor with Moire Amplification,”,” in 48th AIAA Aerospace Sciences Meeting, 2010, pp. 1–13.
 D. A. Mills, D. Blood, and M. Sheplak, “Characterization of a sapphire optical wall shear stress sensor for high-temperature applications,” in 54th AIAA Aerospace Sciences Meeting, 2016.
 D. Mills, D. Blood, and M. Sheplak, “Development of a sapphire optical wall shear stress sensor for high-temperature applications,” in 2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2015, pp. 1295–1298.