Fiber Optic Temperature Sensors: Principle of Operation & Applications

As the name suggests these sensors employs fiber optics technology to function. A fiber optic sensor generally guides light to and from a measurement zone where the light is modulated by the measurand of interest and returned along the same or a different optical fiber to a detector at which the optical signal is interpreted. The measurement zone in this case can be intrinsic within the fiber that transports the optical signal or can be extrinsic to the optical waveguide.

The key benefits of using fiber optics for measurement include:

• The immunity to electromagnetic interference within the sensor system and within the optical feed and return leads.
• Chemical passivity within the sensor system itself and inherent immunity to corrosion.
• Small size, providing a physically, chemically and electrically non-invasive measurement system.
• Mechanical ruggedness and flexibility – optical fibers are very strong and elastic, they can withstand strains of several percentages.
• High temperature capability, silica melts over 1500 °C.
• The capacity for intrinsic distributed measurements.

The key phenomena employed in the optical techniques for temperature measurement include:

• Changes in refractive index of external media with temperature.
• Changes in fluorescence spectra and/or fluorescence rise times with temperature.
• Phases transitions in carefully chosen materials imposing mechanical modulation on optical fiber transmission properties.
• Changes within an optical path length with temperature, either within the fiber or an external interferometer element.
• Collection and detection of blackbody radiation.
• Changes in Raman or Brillouin scatter with temperature.

In this article, we will only focus on one phenomenon: changes in fluorescence spectra to illustrate the operation; therefore we will demonstrate the principle of operation of the fiber optic temperature sensor based on changes in fluorescence spectra.

Fiber Optic Fluorescence-Based Temperature Sensor

This is one of the most utilized fiber optic temperature sensors and is extensively used in a variety of applications.

To illustrate the principle of operation of this temperature sensor, consider the following diagram:

In reference to the above figure, phosphor is excited by an ultraviolet light source (which limits the length of the silica-based feed fiber to a few tens meters) and the return spectrum is divided into ‘red’ and ‘green’ components, the intensity ratios of which are a simple single-valued function of  phosphor temperature. For accurate temperature measurement, the detectors and feed fiber require calibration, and particularly for the detectors, the calibration is a function of the surrounding temperature. Nevertheless, this can be solved through curve fitting and interrogation of a thermal reference.

This instrument is capable of accuracies of about ±0.1 °C within sub-second integration times over a temperature range extending from approximately -50 °C to ±200°C.

Applications of Fiber Optic Temperature Sensors

Fiber optic temperature sensors are used for solving specific measurement problems for example where metallic probe either distorts the electromagnetic field significantly e.g. in microwave ovens or is subject to very high levels of interference, producing spurious readings. Typical applications include monitoring temperature profiles in both domestic and industrial microwave ovens, examining temperatures in power transformer oils, motor/generator windings. Therefore fiber optic temperature sensors are primarily used as measurement probes in regions of very high electromagnetic fields, in zone zero intrinsically safe areas, and so forth.

The distributed capability of fiber optic sensors is especially relevant in structural monitoring and in other specialized areas such as measuring the temperature distribution along underground power lines, tunnels or similar structures or in experimental circumstances such as the measurement of curing processes in large volumes of concrete.

Distributed temperature alarms triggering on and locating the presence of either hot or cold spots along the fiber can achieved at significantly lower costs. A typical application is a temperature alarm on liquefied natural gas storage tanks. Here, the core and cladding indices for a plastic-clad fiber cross at a temperature in the region of 50 °C, Such temperatures can only be realized when a leak occurs. Furthermore, the system has the obvious benefit of intrinsic safety and total compatibility with use within potentially explosive atmospheres.

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