Fiber Optic Temperature Sensors: Types, Working & Applications

This article explores the structure, working principles, advantages, and disadvantages of Fiber Optic Temperature Sensors.

Introduction

Temperature measurement can be achieved through various methods, including:

• Classic mercury glass thermometer
• Infrared Pyrometer
• Electronic thermometer (thermocouple, thermistor, RTD, etc.)

However, these traditional systems often suffer from limited immunity to electromagnetic interference and stray radiation, leading to inaccurate measurements. Fiber optic temperature sensors offer superior performance compared to these techniques, thanks to their numerous benefits. This makes them suitable for use in space applications and hazardous environments such as high-voltage machinery (e.g., generators, motors, transformers), nuclear power plants, and chemical power plants.

Fiber Optic Temperature Sensor Structure and Working Operation

Fiber optic temperature sensors are mainly classified into two types:

1. Interferometric
2. Non-interferometric

Non-Interferometric Fiber Optic Temperature Sensor

Figure 1 illustrates a simple non-interferometric and non-luminescent type fiber optic temperature sensor.

Figure-1: Non-Interferometric fiber optic temperature sensor

This type of sensor consists of a multi-mode optical fiber and a temperature-sensitive material. Common temperature-sensing materials include GaAs, CdTe, and Si. GaAs is often preferred due to its superior wavelength variation with temperature.

These materials exhibit changes in their optical parameters (absorption, transmission, and reflection) as temperature varies. The working principle relies on the phenomenon of energy bandgap shrinkage as the temperature of the semiconductor material increases.

In this setup, a thin semiconductor chip serves as the active element, sandwiched between a light source (e.g., LED or laser) and a photodetector. The constant intensity of the light signal is modulated by the external temperature as it travels through the optical fiber cable. The wavelength shifts towards higher values as the temperature increases, a phenomenon linked to the optical absorption edge.

Interferometric Fiber Optic Temperature Sensor

Figure 2 depicts a Mach-Zehnder Interferometric temperature sensor.

Figure-2: Interferometric fiber optic temperature sensor (Mach-Zehnder type)

This sensor offers flexible geometry and higher sensitivity, making it suitable for measuring temperature, pressure, rotation, strain, and other parameters. It operates based on phase modulation by external measurands.

The phase of the beam passing through the sensing fiber is compared to that of a reference beam. A beam splitter is used to divide the light beam into two parts: one directed into the sensing part and the other used as a reference.

Advantages of Fiber Optic Temperature Sensors

Fiber Optic Temperature Sensors offer several benefits:

• Immunity from electromagnetic interference and stray radiation.
• Suitable for environments with high levels of electrical interference or where intrinsic safety is a concern.
• Non-interferometric types offer greater accuracy (+/- 1 °C) and faster response times (~2 seconds).
• Lightweight and compact.
• Cost-effective due to low manufacturing costs.
• Support a wide temperature measurement range, typically from -10 °C to 300 °C. GaAs offers better wavelength variation with temperature, enhancing performance.

Disadvantages of Fiber Optic Temperature Sensors

Despite their advantages, Fiber Optic Temperature Sensors also have some drawbacks:

• Different types of optical temperature sensors have different temperature ranges and varying accuracies, depending on their construction and materials.
• Developing measurement systems using fiber optic sensors can be complex.
• Users often require training before they can effectively use these sensors.
• Some fiber optic temperature sensors can be expensive.

Other types of fiber optic temperature sensors include Fabry-Perot Interferometric Temperature sensors, fluorescent temperature sensors, and optical scattering temperature sensors.