Modern aerospace, civil infrastructure, and composite manufacturing increasingly require low-profile, non-electrical strain measurement across large surface areas. Optical strain gauges fulfill this need by using hair-thin fiber optic lines (~125 µm diameter) to measure mechanical strain via changes in light transmission rather than electrical current.
By detecting micro-wavelength shifts in reflected light, a single optical fiber can monitor thousands of sensing locations over several meters without requiring electrical excitation, making it ideal for high-voltage, explosive, or high-EMI environments where traditional strain gauges struggle.
Key Takeaways
- Optical Strain Gauge Physics: Optical strain gauges use laser-etched Fiber Bragg Gratings (FBGs) inside silica glass fibers to measure micro-strain via reflected light wavelength shifts (\(\Delta \lambda_B\)).
- EMI Immunity & Explosive Safety: Because they transmit light rather than electrical current, optical sensors are completely immune to electromagnetic interference (EMI/RFI) and inherently safe in explosive or high-voltage environments.
- Structural Embedding: Hair-thin fiber optics (125\(\mu\text{m}\) diameter) can be embedded directly inside carbon fiber composites and 3D woven materials without compromising structural integrity.
- Commercial Trade-offs: While optical gauges excel in multi-point distributed sensing over kilometers, traditional resistive strain gauge load cells remain far more cost-effective for localized, ultra-precise force measurement.
Fiber Optic Technology
Fiber optic cables, or fiber optic lines, are very thin strands of glass capable of transmitting high-speed data as light signals through ultra-thin glass strands. They provide high bandwidth, minimal signal attenuation, and total immunity to electromagnetic noise over long distances.
A fiber optic cable has three main components (See Figure 1). The core is a bundle of hair-thin glass strands that transmit a signal via light waves. The cladding is a reflective glass that bounces light waves back into the core. This is critical for signals to travel long distances even though the cable is not perfectly straight. Lastly, the buffer is a protective coating of plastic or rubber that prevents environmental damage.
Strain
Most objects tend to change shape when they are subjected to an applied force. This is called deformation. Picture a fishing pole bending under the weight of a fish, or a rubber ball flattening when a person steps on it. The harder the object, the more subtle the deformation. Nonetheless, this deformation is detectable by exploiting a material’s physical property.
Strain gauges exploit the physical property of strain (hence the name) to quantify loads and forces. Strain is defined as the ratio of the change in length of a material in the direction of an applied force, to the original length in that direction. Electronic strain gauges undergo a change in resistance when loaded vs. unloaded. By applying a constant excitation voltage to the cell, a measuring device generates a change in output signal vs. the baseline signal when the object is unloaded.
Likewise, optical strain gauges produce an altered light signal proportional to changes in strain that is measurable. The exact relationship of strain to load is a function of the known material properties of the load-bearing object.
Design of Optical Strain Gauges
For the fiber in an optical strain gauge to effectively detect changes in strain, its design must differ from conventional fiber optic lines for telecommunications. That is, the core of the cable undergoes inscription (a process of laser etching) during production. These inscriptions are arranged in unique patterns called Fiber Bragg Gratings, or FBGs. They create material interference in the fiber, reflecting light differently than the rest of the core.
Think of each core strand as an extremely long glass cylinder. The FBGs behave like windows spaced out through the cylindrical fiber. When light from a laser hits each window, certain wavelengths pass through, while others reflect. When the FBGs change shape due to strain, there will be a measurable difference in the light waves transmitted. The interval between each window or inscription is critical for recording strain. Figure 2 shows a simplified depiction of a Fiber Bragg Grating.
When the fiber is in tension, or stretched, it experiences positive strain. This means the interval between each inscription lengthens. Similarly, when the fiber undergoes compression, these gaps shorten, and the strain is negative.
Placing two FBGs in opposite directions allows one to read compression with its counterpart reading tension. This has benefits including partial compensation for temperature effects, because both gauges will be affected by the same thermal expansion.
How Fiber Bragg Gratings (FBG) Quantify Strain
The core mechanism of an optical strain gauge relies on Fiber Bragg Gratings (FBGs)—periodic, laser-etched variations in the refractive index of the fiber’s glass core. When broad-spectrum laser light travels down the fiber, each grating acts as a selective filter, reflecting a specific narrow wavelength while transmitting all others.
The fundamental Bragg wavelength (\(\lambda_B\)) is expressed by:
$$\lambda_B = 2 n_{eff} \Lambda$$
Where:
- \(n_{eff}\) is the effective refractive index of the optical fiber core.
- \(\Lambda\) is the physical grating period (spacing between etched lines).
When the host structure undergoes mechanical load, the bonded or embedded optical fiber stretches or compresses, physically altering the grating period (\(\Lambda\)). This mechanical strain (\(\epsilon\)) induces a proportional shift in the reflected wavelength (\(\Delta \lambda_B\)):
$$\frac{\Delta \lambda_B}{\lambda_B} = C_\epsilon \epsilon + C_T \Delta T$$
Where \(C_\epsilon\) is the photo-elastic strain coefficient, \(C_T\) is the thermal expansion coefficient, and \(\Delta T\) is the temperature change. Because temperature also shifts the refractive index, dual-grating configurations or dedicated FBG thermal sensors are used to isolate mechanical strain from thermal drift.
Advantages and Disadvantages of Optical Strain Gauges
Strain gauges that use fiber optic technology offer several advantages over their electrical counterparts. For one, they do not need electricity or an excitation voltage to operate. This makes them suitable for environments that will experience high levels of electromagnetic interference.
Also, because they have a slim profile, they can either be embedded in structures during production, or discretely attached to the structure’s surface. When fiberglass composites or carbon fiber with resin make up the optical strain gauge, composite fabricators can easily embed the inscribed glass fiber directly into resin laminates during part construction. This allows for internal strain measurements unattainable with conventional strain gauges. A practical application of this is in the production of aircraft structures. In these applications, an array of such embedded optical fibers enable the real-time measurement of forces on these aircraft components.
On the other hand, the materials in optical/fiber-optic strain gauges have a relatively high coefficient of thermal expansion. Because of this, they often need to be combined with temperature sensors to compensate for temperature in their measurements.
Foil Strain Gauges vs. Optical FBG Strain Gauges
Choosing between traditional resistive foil strain gauges and optical Fiber Bragg Grating (FBG) sensors comes down to your operating environment, space constraints, and DAQ budget. The comparison table below highlights the key trade-offs between electrical and optical strain measurement technologies.
| Performance Metric | Strain Gauge Sensing Technology | |
|---|---|---|
| Resistive Foil Strain Gauges (Standard) | Optical FBG Strain Gauges | |
| Primary Signal Output | Electrical resistance change (\(\Delta R / R\)) | Optical wavelength shift (\(\Delta \lambda_B\)) |
| EMI / RFI Sensitivity | Requires shielding & filtered amplifiers | 100% immune to electromagnetic fields |
| Multiplexing Capacity | 1 channel per gauge bridge (wired individually) | Up to 80+ FBG sensors on a single fiber string |
| Structural Integration | Surface-mounted to metal flexures / structures | Embeddable inside composite resins & laminates |
| Interrogator / Electronics Cost | Low-cost Wheatstone bridge DAQ & readouts | High-cost optical interrogator / spectrometer units |
| Best Application Target | Discrete force, torque, and scale weight measurement | Long-range structural health monitoring (bridges, aircraft) |
Optical Strain Gauge Applications
Recall conventional strain gauges use an electrical element that measures a change in its resistance proportional to the mechanical strain induced by an applied force. While optical strain gauges employ a different technology than this, they can be used for similar applications.
One common application of optical strain sensors is the monitoring of long-term civil structures like bridges, buildings, tunnels, and pipelines. Long fiber-optic lines allow the distributed sensing of strain localization. As discussed in the article, Advantages and Applications of Wireless Load Cells, continuous monitoring of loads on the structure can help pinpoint damage, improve the life of the structure, and automate maintenance and inspection time.
Optical strain gauges can detect small cracks and material defects in other structures as well. Because of this they are important in static and fatigue testing of components such as aircraft parts and wind turbine blades.
Optical strain gauges or fiber-optic sensors measure more than just strain. They also serve as pressure sensors, temperature gauges, and vibration transducers.
Conclusion
The optical strain gauge is a versatile alternative to conventional electronic strain gauges. While electronic strain gauges are a mature technology, optical strain gauges continue to develop capabilities in defect detection on large buildings and structures, FEA model validation, and structural health monitoring of composite materials. They continue to be a developing technology in instrumentation. And their uses and applications are constantly expanding.
References
- Handbook of Optical Sensors José Luís Santos, Faramarz Farahi
- Measurement and Instrumentation Principles, 3rd Edition Alan S. Morris.
- Optical frequency domain reflectometry: principles and applications in fiber optic sensing, Stephen T. Kreger, Nur Aida Abdul Rahim, Naman Garg, Sandra M. Klute, Daniel R. Metrey, Noah Beaty, James W. Jeans, and Robert Gamber
- Three-Axis Distributed Fiber Optic Strain Measurement in 3D Woven Composite Structures, Matt Castelluccia, Sandra Klutea, Evan M. Lallya, Mark E. Froggatta, David Lowryba
