Fiber Optic Sensors: Types, Working Principle & Applications

This article explores the different types of Fiber Optic Sensors, their working principles, and various applications. We’ll delve into Intrinsic, Extrinsic, and Hybrid fiber optic sensors, explaining how they function.

Introduction to Sensors

A sensor is a device that measures a physical quantity and converts it into a signal. This signal can then be measured by an instrument or interpreted by a user. In essence, a sensor reacts to a physical, chemical, or biological condition.

For example, a thermocouple is a sensor that detects temperature and converts it into an electrical current or voltage. This information is then displayed in analog or digital form.

What is a Fiber Optic Sensor?

• A fiber optic sensor measures a physical quantity by modulating the intensity, spectrum, phase, or polarization of light traveling through the optical fiber system.
• It’s a device that converts light rays into electronic signals.
• Think of it like a photoresistor, which changes its resistance based on the amount of light hitting it. The instrument then reads this change.

Figure 1: Basic elements of an optical fiber sensing system.

Fiber optic sensors are prevalent in various applications, from computers and printers to motion detectors. For instance, when a printer or copier door is open, light falls on the sensor, stopping the machine for safety. When the door is closed, the sensor is usually in complete darkness.

Fiber Optic Sensor Working Principle

Here’s how fiber optic sensors work:

• The system includes a light source, optical fiber, sensing element (or transducer), and a detector.
• The transducer modulates a parameter of the optical fiber system, such as intensity, wavelength, polarization, or phase.
• This modulation alters the characteristics of the optical signal that reaches the detector.

Applications of Fiber Optic Sensors

Fiber optic sensors are used in a wide range of fields, including:

• Mechanical Measurements: Rotation, acceleration, electric/magnetic fields, temperature, acoustic sensing, pressure, vibration, linear & angular position, strain, humidity, viscosity, etc.
• Chemical and Biological Sensing
• Electrical and Magnetic Measurements
• Structural Health Monitoring: Real-time monitoring of the physical condition of structures.
• Buildings and Bridges: Inspection and monitoring of deformation (creep and shrinkage), displacement, damage evaluation, neutral axis evolution, concrete-steel interaction, etc.
• Dams: Joint expansion monitoring, foundation monitoring, spatial displacement measurement, leakage monitoring, distributed temperature monitoring, etc.
• Tunnels: Monitoring optical extensometers, convergence monitoring, etc.
• Heritage Structures: Displacement monitoring, post-seismic damage evaluation, crack opening analysis, etc.
• Leak Detection

Fiber Optic Sensor Types

Figure 2: Types of Fiber Optic Sensors

Fiber Optic Sensors can be categorized based on their construction and operating principles:

1. Intrinsic Sensors

• These sensors are embedded within or are part of the fiber optic system, resulting in modifications to the optical fiber itself.
• The fiber itself acts as the sensing element, directly affected by the measurand (the quantity being measured).
• The measurand affects the light transmitted within the fiber.
• The light beam remains within the optical fiber throughout the process.

2. Extrinsic Sensors

• In this type, transducers are external to the fiber optic system. The fiber simply registers and transmits the sensed quantity.
• The fiber primarily transports light to and from the sensing element.
• The modulation occurs outside of the optical fiber.

3. Hybrid Sensors

• These sensors combine elements of both intrinsic and extrinsic sensors.
• The fiber carries light both into and out of the device.

Other Types of Fiber Optic Sensors

• Point sensor, Distributed sensor
• Extrinsic sensor, Intrinsic sensor
• Through Beam Sensor, Diffuse reflective Sensor, Retro-reflective sensor