Optical fibers utilize the principles of reflections to disseminate light waves. Extrinsic sensors, where the light leaves the fiber and is reflected before going back into the fiber optic system, can be affected by dust, vibration and alignment. In the case of intrinsic sensors, the light remains within the waveguide thus avoiding these problems. Intrinsic sensors also measure the effects of environmental parameters on the optical signal as it moves down the fiber.
Intrinsic fiber optic sensors, typically some type of interferometer, can utilize the natural backscattering of light, fiber bragg gratings (FBGs) or other elements to obtain measurements. Intrinsic optical sensors can measure temperature, strain, pressure, and other parameters by adjusting the fiber so that the measurable amount modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber.
There are multiple types of interferometers that have been applied across industries and used in diverse applications. While examining each method is beyond the scope of this article, Mach-Zehnder, Michelson, Sagnac, and Fabry-Perot interferometers, the most common types, will be discussed.
Mach-Zehnder (MZ) sensors use interference from a laser pulse in two arms to determine physical parameters (Figure 1). A reference arm is isolated from the external environment so that only light in the sensing arm is affected. A fiber coupler splits the light so that it propagates down each arm and another coupler recombines the signal. When the sensing arm is affected by temperature or strain, the physical length and the refractive index of the fiber is altered. This shifts the wavelength of the optical signal in the sensing arm while leaving the signal in the reference arm unaltered. After the two signals recombine, the shift in wavelength from the sensing arm corresponds to a variation in the interference signal which can be detected by an interrogator. MZ interferometers are relatively common due to the flexibility of their configuration. This type of interferometer is commonly used in the telecommunications industry to test networks and can be used as a single point temperature sensor.
Michelson interferometers (Figure 2) are similar to the Mach-Zehnder technology in that they both use interference from a laser pulse in two arms to obtain measurements. However, Michelson interferometers use reflective modes to cause the interference. A coupler splits the incident light so a signal propagates down a reference arm and a sensing arm. A reflective surface is placed at the end of each arm and the light is recombined. Fiber Bragg gratings (FBG) or other reflective elements can be inscribed along the sensing arm to provide sensing points. Michelson sensors are typically more compact than Mach-Zehnder sensors and can be multiplexed. This enables the development of fully distributed sensors rather than just point sensors. Distributed sensors are advantageous in that they provide data along the entire length of a fiber rather than individual points. This ensures that events that occur between critical points are captured so engineers can see the entire picture of what is happening to their application rather than a handful of snapshots.
Sagnac interferometers (Figure 3) use a fiber loop in which light is split into two beams so that they propagate in opposite directions down a fiber loop. The opposing signals are recombined by the coupler but, unlike other interferometers, the polarization of the light is altered by the environment. Sagnac sensors use birefringent (the refractive index is dependent on the polarization state of the light) fiber in the sensing region. A polarization controller is used at the beginning of the sensing region to adjust the polarization. As environmental parameters affect the sensing region, the path of the signal is altered resulting in a phase shift and a different interference pattern. Sagnac interferometers are sensitive to rotation and spurred the development of gyroscopes which are now widely adopted.
Fabry-Perot interferometers (Figure 4) utilize two parallel reflective surfaces that are separated by a determined distance. The cavity between the two surfaces can either be external or internal to the fiber. Interference occurs between the reflected and transmitted beams. When strain or temperature act on the fiber, the length of the cavity is changed. This causes a wavelength shift in the reflected light and results in a different interference signal. Fabry-Perot sensors are found across industries due to their relatively simple and cost effective fabrication. They are widely used in civil applications to monitor critical structural components and in the oil and gas industry to monitor downhole pressure.
While each type of interferometer has its own unique capabilities, choosing the right method will always depend on the application. Sophisticated FOS systems enable engineers to collect and analyze material and structural data to ensure precise measurement and optimal performance every time. Contact us to learn more about fiber optic sensors and how they can benefit your application.
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