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Fiber Optic Sensor [Working Principle, Fiber Optic Cables Types, Features and Applications]

Fiber Optic Sensor [Working Principle, Fiber Optic Cables Types, Features and Applications]

One of the most widely used and unique sensors in the field of factory automation environments and electricity is the fiber optic sensor. Fiber optic sensors also called optical fiber sensors are fiber-based technology that is used to sense some quantities such as temperature and mechanical pressure, and sometimes to measure the displacement, vibrations, rotations, pressure, acceleration, and concentration of chemicals. 

What is a fiber optic sensor?

Fiber optic sensor are used for monitoring areas, precise spots in positioning, measuring, and sensing large distances in harsh conditions, including high vibration, extreme heat, and noisy, wet, corrosive, or explosive environments. 

They are usually cylindrical and have high accuracy as well as long operating life. Fibers optics are widely used in the field of accurate positioning or detection of small objects or remote sensing in tight spaces, harsh environments, or everywhere other sensors cannot be used because using electrical signals may cause risk of explosion or fail to operate correctly. 

Fiber optic systems don't need any electrical power in the sensing head at the remote location and they have compact and small size. 

These sensors are an ideal choice for small areas and can be positioned correctly wherever flexible fibers are required. 

Fiber optics have two distinct components, an amplifier that is made of the emitter (the light source) and receiver (detector) and some electronic components and sensor heads or cables used to transmit and receive the light. 

The sensor emits light to the target object and then by measuring the increase or decrease of the returned light, and converting it into an electrical signal can detect the target condition. 

The multiple sensors can be across the fiber using a different light wavelength conversion for each sensor or by measuring the time required to pass light across the fiber through each sensor. 

The sensor head is resistant to chemical substances, oil, and electromagnetic interference that provide reliable detection even under difficult conditions, like filling levels or leaks in aggressive fluids and applications that need highly inflammable material or high voltage electricity.

Working Principle of Fiber Optic Sensors

Fiber optic sensor consists of:

  • An optical source (light-emitting diode, laser, and laser diode)
  • Optical fiber 
  • Sensing element
  • Optical detector
  • End-processing devices (optical-spectrum analyzer, oscilloscope)

The light beam from lasers or any source of light is transmitted to in one or several fiber Bragg gratings, and then reaches a detector measuring these changes.

There are three photoelectric sensing methods for fiber optic systems including:

  • Thru beam 
  • Reflective
  • Retro-reflective 
  • Definite-reflective 

They are operated based on photoelectric sensing technology and each method is used for different applications and targets. 

In diffuse reflective sensors, the transmitter and receiver are installed next to each other in a case, but in thru beam sensors, the transmitter and receiver are separate and the light is sent by the transmitter and the receiver that is installed in front of the transmitter receives light. 

Two types of fiber-optic assemblies that are operated in these sensing modes are individual and bifurcated. 

For detection of target objects in bifurcated fiber-optic mode used for diffuse reflective and retro-reflective sensing, the receiver, and emitter cable integrated with each other. 

A bifurcated fiber-optic assembly used for both combines the emitter and the receiver cable assemblies to achieve detection. 

In the fiber optic thru beam method when the light beam that extends from the emitter to the receiver is disconnected, individual sensing systems detect the target object.

The fiber optic cable can be made of glass or plastic core covered by a layer made of cladding material.

Plastic fiber optic cables

Plastic fiber-optic cable made of a single strand allows bending radii (plural for radius) of only 1 mm which is impossible for glass fiber optic cable. 

These fibers are flexible and excellent for use in applications that need frequent bending and are also excellent for use in extremely tight areas. 

They are generally sold with a cutting machine allowing customers to modify the desired length. They can be used in drag chains because they have a tolerance to dynamic bending forces.

Glass fiber optic cables

Glass fiber cables are made of hundreds of individual fibers, each of them transfers a part of the emitted light. based on the arrangement of the fibers, they either produce a light spot or a light line. 

In addition, the glass is high quality and durable material that ensures reliable detection and long operating life.

The high-resolution analog to digital conversion in optical fiber sensors also detects even very slight changes. It is important for the detection of very small objects or distinguishing between very subtle oppositions.

Types of Fiber-Optic Sensor Systems

The fiber optic sensors are categorized into two types: 

  • Intrinsic Fiber-Optic Sensors 
  • Extrinsic Fiber-Optic Sensors

The Intrinsic fiber optic sensor measures and senses very large distances and is the best and most efficient type of sensor. In this type a fiber itself (possibly in a modified form, e.g. containing a Bragg grating) operates as the sensor and Light does not leave the fiber, and as a result, environmental changes do not affect it.

Extrinsic Fiber-Optic Sensor uses fibers just for transmitting modulated light from a non-fiber optic sensor, or electronic sensor to a fiber optic transmitter. In this sensor, fiber may be used as a carrier.

Features and Advantages of fiber optic sensor

  • High sensitivity and accuracy
  • Resistant to electromagnetic interference and radio frequency
  • Extremely Small size, lightweight, and flexible for tasks in robotics and tight spaces
  • Resistant to harsh and high-voltage environments
  • Explosion Proof
  • Fast and precise measurements
  • Very long service life due to No electronics
  • Non-contact and Remote sensing
  • Wide operating temperature range much wider than is possible for many electronic devices
  • Large sensing range
  • Easy and flexible installation and setup
  • Reliable detection in fast processes
  • Fast response time
  • High resolution and multiplexing capabilities.
  • Ability to measure various parameters

Applications

Fiber optic sensors are used in a number of different applications. 

  • In mechanical properties testing, fiber optical sensors are used to measure mechanical strain
  • They can also be used to measure acceleration, velocity, pressure, temperature, and displacement. 
  • In heritage structures, fiber optic sensors can be used to evaluate post-seismic damage, analyze cracks, monitor restoration, and monitor displacement
  • Similarly, in dams, they can detect and monitor leakages, foundation defects and measure spatial displacement. 
  • Fiber optic sensors are widely used in telecommunications, aerospace, oil and gas transmission lines, and petrochemical industries.
  • In buildings and stairs to monitor cracking and fracture, assess post-earthquake damage, monitor concrete pouring during construction, and…
  • In tunnels to identify connection line damage, assess shot Crete expansion, and…
  • In dams to monitor temperature distribution, leakage monitoring, foundation work monitoring, and…
  • Assess post-earthquake damage, monitor reconstruction, and check the cracks in old buildings
21st Mar 2022 Saeed Abd

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