Among the many ways to sense temperature, combinations of advanced optical principles used with optical fibers offer very different approaches, with application advantages but also implementation limitations.
The first part of this article discussed general issues of sensing of physical parameters, especially temperature, and total internal reflection in optical fibers. This section will look at two ways in which optical fibers and associated components can be used for temperature measurement.
Point Sensing Based on Fiber Bragg Gratings
In this FBG approach, the operation principle is a consequence of the fact that the temperature affects the Bragg wavelength – the wavelength of peak reflectivity – by a complex but precise equation. In this point sensor, a light source from a laser is coupled into the fiber and impinges on a crystal such as GaAs. This crystal acts as a temperature-sensitive cut-off filter (a Bragg grating) whereby the crystal absorbs some light wavelengths while it transmits other wavelengths. This characteristic “edge” between the reflected and transmitted parts of the optical spectrum is called the transition wavelength and is directly related to the bandgap energy and thus the absolute temperature. The equation defines this energy as related changes a temperature change ΔT and the strain ε.
In a complete system, a measurement is needed of the photon energy sufficient to excite an electron from the valence to the conduction band of the semiconductor crystal. The spectrum of a GaAs sensor (or other crystal) placed in a medium of unknown temperature is measured in the reflectance mode. The wavelength of this characteristic edge is analyzed by an optical “interrogator” (an optical spectrum analyzer) to determine the temperature (Figure 1).
The temperature resolution of a carefully made FBG-based sensor is largely a function of the accuracy of wavelength measurements and is quite high, on the order of 0.1 K or better. Note that 0.1 K resolution requires a wavelength resolution of about 0.001 nanometers.
Using FBG-based sensor technology, it is also possible to make a temperature sensor which can provide distributed readings using multiple gratings in a long optical fiber. A single interrogator is used with optical multiplexing to address all the different gratings. This multiplexing can be accomplished with a time-sequenced signal or by using gratings with each having a different Bragg wavelength. In the latter case, each grating is “addressed” by tuning the interrogation laser to the unique wavelength of that grating, in the form of wavelength division multiplexing (WDM).
There’s another way to implement FBG instead of using a discrete GaAs crystal. A short segment of optical fiber that reflects and transmits particular wavelengths of light can be used in what is called a distributed fiber Bragg grating. To construct this distributed grating, a periodic variation in the refractive index of the fiber core is created (Figure 2) (exactly how this is created within the fiber is a quite complicated story). This structure, in turn, yields wavelength-specific dielectric mirrors to act as optical filters which can block or reflect specific wavelengths. Changes in the fiber temperature then induce changes in the dimensions and thus the wavelengths which the filters pass or reflect.
The Mach-Zehnder interferometer (MZI)
This approach to sensing uses the well-known, time-tested principle of interferometry but on a small scale. In an interferometer, a single waveform (which can be optical or radio) is split into two equal parts by a beam splitter (a half-silvered mirror). One part, called the sensing beam, is sent through a sensor or some element which slows it down (resulting in a phase shift) due to some external force, while the other is the reference beam and travels unimpeded.
The two waveforms are then recombined, and any changes in the indirect path are visible as interference fringes which are due to destructive or constructive interference – hence the term “interferometer.” The position and spacing of these fringes characterize changes along the non-direct path versus the direct path and the movement of the fringes a dynamic indicator of changes as they occur. Again, as with the Wheatstone bridge, any changes which both paths undergo cancel each other and do not cause errors in the observed measurement.
Interferometry was used on a platform several meters in diameter in the classic Michaelson-Morley experiment of the 1880s, which proved the non-existence of the luminiferous ether (“luminiferous ether” was a mysterious medium with contradictory properties which was presumed to permeate all otherwise empty space through which light waves traveled). Demonstrating that this ether did not exist, despite the widely held conjecture that there must be such a medium, was one of the factors which led Einstein to his theory of special relativity and his radical assumption that the speed of light was constant regardless of the motion of its source.
Long-baseline interferometry over thousands of miles is widely used in optical and radio astronomy and was a key to the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, which demonstrated the existence of long-sought gravity waves, announced in 2016. (Interferometry applies to RF and optical waves as both represent electromagnetic energy and are governed by Maxwell’s equations.)
But interferometry is not limited to these larger dimensions; it is also being implemented on a “micro” scale for temperature sensing using various optical fibers and combinations. Depending on the design, the split beam can travel through two distinct fibers (Figure 3) or along two paths in the same multimode fiber.
In one experimental implementation of the latter approach, the MZI uses a step-index multimode fiber (MMF), and the sensing head is packaged in a capillary which is filled with glycerol-water solution (Figure 4). The MZI is built by fusion-splicing of a short piece of MMF between two other pieces of MMFs having a large lateral offset.
In operation, light from a broadband light source (BBS) is launched in the lead-in single-mode fiber (SMF) and then split into two beams by the lead-in multimode fiber (MMF). One beam travels along the cladding of the sensing MMF, and the other one propagates through the medium around the sensing MMF. The two beams are then recoupled into the lead-out SMF through the lead-out MMF, forming a Mach-Zehnder interferometer.
The transmission spectrum of the interferometer is collected by the optical spectrum analyzer (OSA). If a temperature variation is applied on the sensing head, the indices of refraction of both of the fiber’s silica and the cavity will change due to the thermo-optic effect, causing a phase shift along the path, with the optical path difference (OPD) between the two interference beams mainly due to the shift in the MMF part.
Sensing using the MZI approach may seem complicated, and it is. However, advances in optical fibers, types of optical fibers, splicing of these fibers, broadband light sources, and even optical spectrum analyzers (some of which can now be fabricated on a lithium niobate substrate) are increasing its viability.
The final part of this article looks at other temperature-sensing approaches using advanced optical principles and optical fibers, as well as the pros and cons of optical-based temperature sensing.
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- Fiber Optic Sensor Measures Tiny Magnetic Fields
- RP Photonics Consulting GmbH, “Optical Temperature Sensors”
- Opsens Solutions, “Fiber Optic Temperature Sensors”
- Micronor LLC, “TS Series Temperature Sensors”
- Micronor LLC, “Why Fiber Optic Sensing?”
- Wikipedia, “Fiber-optical thermometer”
- InTechOpen, “Optic-Fiber Temperature Sensor”
- RF Wireless World, “Fiber Optic Temperature Sensor structure, working, advantages, disadvantages”
- RP Photonics, “Optical Temperature Sensors”
- Research Gate, “Applications of fibre optic temperature measurement”
- Sensor Letters, “A New Fiber Optical Thermometer and its Application for Process Control in Strong Electric, Magnetic, and Electromagnetic Fields”
- Research Gate, “Ultra-high Sensitive Temperature Sensor Based on Multimode Fiber Mach-Zehnder Interferometer”
- Wikipedia, “Brillouin scattering”
- Washington University/St. Louis, “What is Raman scattering?”
- Georgia State University, “Raman scattering”
- BW Tech, “Theory of Raman Scattering”
- Nano Photon, “What is Raman spectrum?”
- RP Photonics, “Rayleigh Scattering”
Background and Related
- Wikipedia, “Fiber Bragg Grating”
- Laser Focus World, “Distributed fiber-optic hydrophone is based on heterodyne coherent detection”
- Laser Focus World, “Fiber-optic communications: Tailoring the fiber to the task”
- Wikipedia, “Michelson–Morley experiment”
- NASA, “Sensing Magnetic Fields: Using an Innovative Optical Waveguide Fiber Bragg Grating”
- OSA Publishing, “All-fiber-optic vector magnetic field sensor based on side-polished fiber and magnetic fluid”
- Photonics, “LIGO Continues Making Waves”
- The Optical Society, “LIGO-Virgo in OPN”