How optical sensing solves the toughest sensing challenges

Kellis Garret, NI -November 01, 2012

Measurement systems that are able to accommodate evolving application requirements are preferred as growing cost pressures demand long system life and functional flexibility. The most efficient way to develop such a system is to use a software-defined, modular architecture like PXI.  Various PXI modules can be mixed and matched for a highly customized system. The majority of these modules perform electrical measurements, but many applications have environmental or physical constraints that make the use of electrical sensors extremely challenging. Fortunately, the inherent characteristics of fiber optic sensors address or eliminate many of these concerns. Learn the basics of fiber optic sensing, how this new technology solves many of the issues faced with electrical sensors, and how its incorporation into the PXI platform has fostered successful measurement applications.

Optical Sensing Basics
Conventional electrical sensors use transducers to convert physical phenomena into electrical signals, which can then be conditioned, digitized, and scaled to the expected values by a data acquisition system. Despite their ubiquity, these sensors have inherent limitations and the use of electrical sensors is impractical, if not entirely impossible, in certain types of applications. Fiber optic sensors offer an excellent solution to these challenges.

Fundamentally, fiber optic sensors work like their electrical counterparts, but use light instead of electricity and glass fiber instead of copper wire. Where an electrical sensor might modulate electrical properties like current, resistance, or voltage, a fiber optic sensor modulates one or more properties of light including intensity, phase, polarization, or wavelength.

Optical sensing technology hinges on the optical fiber — a strand of glass thinner than human hair that transmits light within its core. This fiber is composed of three main components: the core, the cladding, and the buffer coating. The cladding reflects stray light back into the core, ensuring transmission through the core with minimal loss of light. This is achieved by ensuring the core material has a higher refractive index than the cladding, causing a complete internal reflection of light. The outer buffer coating protects the fiber from external conditions and physical damage. It can incorporate many layers, depending on the desired level of ruggedness.

Figure 1: Cross section of typical optical fiber

Although many types of fiber optic sensors exist, one of the most commonly used is fiber Bragg grating (FBG). Bragg gratings are variations of the refractive index which are ‘written’ within the core of an optical fiber at a periodic interval called the grating period. When an input light signal is shone onto an FBG, the spacing between the gratings causes reflections from each grating to interfere constructively and reflect a specific wavelength of light, called the Bragg wavelength (see figure 2).

Figure 2: Operation of FBG optical sensor
Changes in strain and temperature affect both the effective refractive index (ne) and the grating period (Λ) of an FBG, which results in a shift in the reflected Bragg wavelength (B) according to Equation 1 below. Thus, the wavelength shift can be measured to determine the corresponding change in strain and/or temperature.  Because both strain and temperature affect the Bragg wavelength (and thus the measurement), temperature compensation is an important consideration for large temperature or strain range tests.

FBGs can be manufactured with various grating periods and therefore various Bragg wavelengths, enabling different FBG sensors on the same fiber to reflect unique wavelengths of light. This makes each wavelength distinguishable from each other across the optical spectrum. The process of differentiating between FBGs on the same fiber based on their individual Bragg wavelengths is called wavelength division multiplexing. Dozens of sensors on the same fiber can take independent measurements, as long as the wavelength shift associated with each measurement does not result in the Bragg wavelength of one FBG sensor crossing over that of another.

Problem Solving with Optical Sensing
Anyone who has ever struggled with noise filtering, shielding, cabling concerns, or a damaged sensor can tell you there are some applications that electrical sensors are hard-pressed to accommodate. Four of the toughest challenges encountered when using electrical sensors are maintaining reliability in electrically hostile conditions, resisting degradation in harsh environments, economically instrumenting large areas with multiple sensors, and fitting traditional sensors into constrained spaces. Each of these tough issues can be addressed by use of fiber optic sensors instead of electrical sensors.

High Electromagnetic Interference & High-Voltage Environments
Electromagnetic interference (EMI) is one of the most common sources of measurement errors and malfunctions with electrical sensor systems. Electrical sensor signal measurements in high EMI scenarios, such as near high-power generators, motors, or other AC sources, are particularly vulnerable to distortion. These environments often include components at high voltages that can damage or even destroy traditional sensor systems. Filtering and isolated instrumentation can mitigate the risks of high EMI and high voltages to an extent, but they have limited noise rejection and isolation levels.

Again, fiber optic sensors are made of glass, and are completely nonconductive and electrically passive. This makes them immune to even the highest levels of EMI and completely unaffected by high voltages or currents in the environment. For example, you can attach fiber optic temperature sensors directly to very high-power components, such as motor windings, transformers, and power lines, for high-precision thermal characterization during operation.

>>Harsh and hazardous conditions
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