Introduction to Fiber Optic Sensors and their Types with Applications

In the year 1960, laser light was invented and after the invention of lasers,  researchers had shown interest to study the applications of optical fiber communication systems for sensing, data communications, and many other applications. Subsequently the fiber optic communication system has become the ultimate choice for gigabits and beyond gigabits transmission of data. This type of fiber optic communication is used to transmit data, voice, telemetry and video over a long distance communication or computer networks or LANs. This technology uses a light wave to transmit the data over a fiber by changing electronic signals into light. Some of the excellent characteristic features of this technology include light weightness, low attenuation, smaller diameter, long distance signal transmission, transmission security, and so on.

Significantly, the telecommunication technology has changed the recent advances in fiber optic technology. The last revolution appeared as designers to combine the productive results of optoelectronic devices with fiber-optic-telecommunication devices to create fiber optic sensors. Many of the components associated with these devices are often developed for the fiber-optic-sensor applications. The ability of the fiber optic sensors has increased in the place of traditional sensor.

Fiber Optic Sensors

The fiber optic sensors also called as optical fiber sensors use optical fiber or sensing element. These sensors are used to sense some quantities like temperature, pressure, vibrations, displacements, rotations or concentration of chemical species. Fibers have so many uses in the field of remote sensing because they require no electrical power at the remote location and they have tiny size.

Fiber optic sensors are supreme for insensitive conditions, including noise, high vibration, extreme heat, wet and unstable environments. These sensors can easily fit in small areas and can be positioned correctly wherever flexible fibers are needed. The wavelength shift can be calculated using a device, optical frequency-domain reflectrometry. The time-delay of the fiber optic sensors can be decided using a device such as an optical time-domain Reflectometer.

The general block diagram of fiber-optic sensor is shown above. The block diagram consists of optical source (Light Emitting Diode, LASER, and Laser diode), optical fiber, sensing element, optical detector and end-processing devices (optical-spectrum analyzer, oscilloscope). These sensors are classified into three categories based on the operating principles, sensor location and application.

Types of Fiber-Optic Sensor Systems

These sensors can be classified and explained in the following manner:

1. Based on the sensor location, the fiber optic sensors are classified into two types:

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

Intrinsic Type Fiber Optic Sensors

In this type of sensors, sensing takes place within the fiber itself. The sensors depend on the properties of the optical fiber itself to convert an environmental action into a modulation of the light beam passing through it. Here, one of the physical properties of light signal may be in the form of frequency, phase, polarization; intensity. The most useful feature of the intrinsic fiber optic sensor is, it provides distributed sensing over long range distances. The basic concept of the intrinsic fiber optic sensor is shown in the following figure.

Extrinsic Type Fiber optic Sensors

In extrinsic type fiber optic sensors, the fiber may be used as information carriers that show the way to a black box. It generates a light signal depending on the information arrived at the black box. The black box may be made of mirrors,gas or any other mechanisms that generates an optical signal. These sensors are used to measure rotation, vibration velocity, displacement, twisting, torque and acceleration. The major benefit of these sensors is their ability to reach places which are otherwise unreachable.

The best example of this sensor is the inside temperature measurement of the aircraft jet engine that uses a fiber to transmit a radiation into a radiation pyrometer, which is located outside of the engine. In the same way, these sensors can also be used to measure the internal temperature of the transformers. These sensors provide excellent protection of measurement signals against noise corruption. The following figure shows the basic concept of the extrinsic fiber optic sensor.

2. Based on operating principles, fiber optic sensors are classified into three types:

• Intensity based
• Phase based
• Polarization based

Intensity based Fiber Optic Sensor

Intensity based fiber optic sensors require more light and these sensors use a multi-mode-large core fibers.The shown figure gives an idea about how the light intensity work as a sensing parameter as well as how this arrangement makes the fiber to work as a vibration sensor. When there is a vibration, there will be a change in light inserted from one end to another end and this will make the intelligence for measuring the vibration amplitude.

Intensity based Fiber Optic Sensor

In the figure, the closer fiber optic and vibration sensor depend on the light intensity in later parts. These sensors have many limitations due to variable losses in the system that do not occur in the environment. These variable losses include loses due to splices, micro & macro bending losses, loses due to connections at joints, etc. The examples include intensity-based sensors or microbend sensor and evanescent wave sensor.

The advantages of these fiber optic sensors include low cost, ability to perform as real distributed sensors, very simple to implement, possibility of being multiplexed, etc. The disadvantages include variations in the intensity of the light and relative measurements, etc.

Polarization based Fiber Optic Sensor

Polarization based optical fibers are important for a certain class of sensors. This property can be simply modified by various external variables and thus, these types of sensors can be used for the measurement of a range of parameters. Special fibers and other components have been developed with exact polarization features. Generally, these are used in a variety of measurements, communication and signal processing  applications.

The optical setup for a polarization-based-fiber-optic sensor is shown above. It is shaped by polarizing the light from the light source through a polarizer. The polarized light is started at 45o to the selected axes of a length of  birefringent polarization protecting fiber. This section of the fiber is served as sensing fiber. Then, the phase difference between the two polarization states is changed under any external disturbances such as stress or strain. Then, according to the external disturbances, the output polarization is changed.Thus, by considering  the output polarization state at the next end of the fiber, the external disturbances can be detected.

Phase based Fiber Optic Sensor

These types of sensors are used to change emitter light on information signal wherein the signal is observed by the phase based fiber optic sensor. When a light beam is passed through the interferometer, then the light separates into two beams.Wherein one beam is exposed to the sensing environment and the other beam is isolated from the sensing environment, which is used as a reference. Once the two separated beams are recombined, then they get in the way with each other. The most commonly used interferometers are Michelson, Mach Zehnder, Sagnac, grating and polarimetric interferometers. Here, the Mach Zehnder and Michelson interferometers are shown below.

here are differences and similarities between the two interferometers. In terms of similarities, The Michelson Interferometer is frequently considered to be folded Mach Zehnder interferometer. The configuration of  the Michelson interferometer requires only one optical fiber coupler. Because the light passes twice through the sensing and reference fibers, the optical phase shift per unit length of the fiber is doubled. Thus, the Michelson canessentially have better sensitivity. Another clear advantage of the Michelson is that the sensor can be interrogated with only a single fiber between the source and source detector module. But, a good-quality reflection mirror is required for the Michelson interferometer

3. Based on application, fiber optic sensors are classified into three types such as

• Chemical Sensor
• Physical Sensor
• Bio Medical Sensor

Chemical Sensor

A chemical sensor is a device which is used to transform chemical information in the form of a measurable physical signal that is associated with the concentration of a certain chemical species.The Chemical sensor is an importantcomponent of an analyzerand may include some devices that perform the following functions: signal processing, sampling, and data processing. An analyzer may be an important part of an automated system.

The working of analyzer according to a sampling plan as a function of time acts as a monitor. These sensors include two functional units: a receptor and a transducer. In the receptor part, the chemical information is transformed into an energy that may be measured by the transducer.In the transducer part,  the chemical information is transformed into an analytical signal and it does not show sensitivity.

Physical Sensor

A physical sensor is a device that is made according to the physical effect and nature. These sensors are  used to provide the information about a physical property of the system. This type of sensors are mostly signified by sensors such as photoelectric sensors, piezoelectric sensors, metal resistance strain sensors and semiconductor piezo-resistive sensors.

Bio Medical Sensor

Biomedical sensor is an electronic device that is used to transfer various non- electrical quantities in biomedical fields into easily detectable electrical quantities. Due to this reason, these sensors are included in health care analysis. This sensing technology is the key to collecting human pathological and physiological information.

Applications of Fiber Optic Sensors

Fiber optic sensors are used in a varied range of applications such as

• Measurement of physical properties such as temperature, displacement,velocity, strain in structures of any size or any shape.
• In real time, monitoring the physical structure of health.
• Buildings and bridges, tunnels,Dams, heritage structures.
• Night vision camera, electronic security systems, Partial discharge detection and measuring wheel loads of vehicles.

Thus, an overview of fiber optic sensors and applications has been discussed. There are many advantages of using fiber optic sensors for long distance communication that include small in size, light in weight, compactness, high sensitivity, wide bandwidth, etc. All these characteristics make the best use of fiber optic as a sensor. Apart from this, for any help regarding this topic or sensor based project ideas, you can contact us by commenting in the comment section below.

Photo Credits:

• Fiber Optic Sensor by automationdirect
• Chemical Sensor by iramis.cea
• Bio Medical Sensor by imimg