In the past 60 years, optical fiber sensing (FOS) has been used by many industries to enhance and test the integrity, efficiency, safety and durability of buildings, vehicles, medical equipment, etc. In the past five years, the development of optical fiber sensing technology has made it possible to achieve the level of data and detection density before air, energy and even medical applications. This will help engineers solve their current problems and improve their designs through innovation. Today, optical fiber technology has a lot of practical significance, and the possibility of future application is very wide. In this paper, we will discuss the latest development of internal optical fiber sensing technology, including 3D shape detection and optical frequency domain reflectometer. In addition, it will discuss how engineers today make good use of these technologies and look forward to the future development.

The first optical fiber sensor was patented in the 1960s, based on free space optics. About 10 years later, researchers developed the first intrinsic optical fiber sensor. This new technology can provide more obvious engineering benefits than free space sensor, and can be used to obtain reliable mechanical measurement results. The use of optical fiber allows the signal to be transmitted in the deployable medium, while the light in free space needs to rely on the sight distance propagation and cannot be deployed in the operation building or vehicle. Fiber optic gyroscope, which was formally commercialized in 1980s, is one of the earliest applications of fiber optic sensors. Now it has become a key component in the stability system and navigation system. In the early 1990s, the civil industry began to realize various types of optical fiber sensors in various applications, which were used to measure temperature, strain, pressure and other parameters. Engineers have also begun to test sensors based on Fiber Bragg grating (FBG). With its multiplexing and quasi distributed functions, FBG sensor has unique advantages compared with the existing fiber sensing technology. By 2000, there have been many common applications in the civil industry, including monitoring the deformation of key components in historical buildings, monitoring the strain of key points in bridges and observing the behavior of concrete when it solidifies. Most of these applications use a variety of interferometer sensors, most of which can not be reused. Fiber Bragg grating sensor has largely replaced these technologies in civil, oil and gas and aviation applications. For example, FBG sensors are often used in the field of oil and gas to monitor the pressure and other parameters on key drilling tools. Similarly, the aviation industry has been using FBG sensors for building health monitoring, load testing and fatigue testing. At the beginning of the 21st century, another optical fiber sensing technology, distributed sensing, appeared and showed great potential in the oil and gas industry. These technologies are mainly used to measure the temperature along the whole optical fiber and help improve various drilling technologies, including leak detection, injection process monitoring and flow chart creation. Although they provide distributed measurement, these technologies have a very slow refresh rate (at least a few seconds between acquisitions) and a spatial resolution of meters.

The latest development of optical fiber sensing

Intrinsic and extrinsic sensors are two categories of fiber optic sensors. External sensors use fiber optics to direct light into the detection area, where light leaves the waveguide and is modulated in another medium. For intrinsic sensors, light stays inside the waveguide, so it measures the effect of light signals as the light travels along the fiber.

Optical fiber itself is the intrinsic fiber sensor technology of the sensor. There are two different technologies for intrinsic sensors: scattering or fiber gratings. Scattering technology can provide fully distributed data points along the fiber, while fiber grating technology can achieve both a small number of detection points and quasi-distributed data points. By placing fiber gratings across the fiber, engineers can analyze changes in reflected light and provide accurate measurements by demodulating this information. Scattering technology does not use fiber gratings at all, but uses random imperfect signals that occur naturally in the fiber to obtain readings. Because fiber gratings are usually manufactured as sophisticated sensors, they have a much higher signal-to-noise ratio than scattering technology.

Strain gauges, thermocouples, and level sensors only care about some key points, while distributed fiber optic sensors can provide more information between key points, so they can help engineers achieve accurate measurements of the entire strain domain, temperature distribution, and other parameters . Scattering and fiber gratings use different demodulation techniques. Scattering technology obtains useful data by demodulating naturally occurring Raman, Brillouin, or Rayleigh backscatter signals. The most commonly used demodulation technology for fiber grating technology is wavelength division multiplexing (WDM). However, in some cases, the optical frequency domain reflectometer (OFDR) has a greater advantage than wavelength division multiplexing.

Wavelength division multiplexing can cover long distances and obtain data quickly, and this technology supports multiple gratings on one fiber; however, each additional grating will significantly reduce the data refresh rate. Typical parameters of WDM measurement include strain and temperature, although in some cases it can also be connected to a single accelerometer or pressure sensor. In addition, WDM only allows users to monitor key points, not the entire information domain. For this reason, applications that require very high acquisition speeds and require only a few data points, such as monitoring parts in automotive crash tests, are well suited to use WDM technology.

Raman, Brillouin or Rayleigh scattering technologies can cover distances of several kilometers and provide complete distributed information. Unlike wavelength division multiplexing, scattering technologies are completely distributed, which means that they can obtain data on the entire fiber, not just a few key points. Although strain data can be obtained by Rayleigh scattering, many systems on the market can only measure temperature or acoustic signals. These systems are called distributed temperature sensing systems (DTS) or distributed acoustic detection systems (DAS). Scattering technology is ideal for applications that must cover several kilometers but do not require high accuracy and high refresh rates. For example, monitoring pipelines to prevent disrupted applications requires only spatial resolution on the order of meters and does not require high data acquisition speeds.

Optical frequency domain reflectometer (OFDR) is another different demodulation technology often used with fiber grating sensors. Gratings are placed at both ends to achieve a completely distributed sensing fiber. OFDR has much higher spatial resolution than scattering technology, and the number of gratings is much more than that of wavelength division multiplexing. A unique advantage of OFDR is that it can maintain a high data refresh rate even if the number of sensors is increased. High spatial resolution, fast refresh rate, a large number of sensors, and fully distributed characteristics make OFDR one of the most complex sensing technologies on the market today. Unlike scattering technology and wavelength division multiplexing technology, some applications of OFDR can integrate multiple technologies into a single powerful platform. In addition to detecting strain and temperature, OFDR technology can also judge 2D deformation, 3D shape, liquid level, pressure, workload and magnetic field. Due to the universality of the platform, engineers can solve multiple problems with one system, thereby making the industry more efficient and effective.

Practical use cases of optical frequency domain reflectometers

Aerospace

Stress and strain are the main parameters for judging aircraft life and operational safety. Airlines and space agencies have been working to find safer equipment and processes. However, existing technologies make it difficult and costly to monitor and maintain the safety of aircraft and spacecraft structures. In addition, the prior art cannot clearly indicate when an aircraft or spacecraft has reached the end of its life.

Because thousands of sensors can be included in thin fiber optic fibers, fiber optic sensor solutions can provide detailed aircraft health information. For example, by using fiber optic sensors in the aerospace industry, engineers can:

● Minimize the aircraft's failure time and adjust the maintenance plan accurately

Improve fuel consumption through intrinsically safe fuel quantity measurement

● Monitor the shape of wings and other deformable parts

● Determine when the aircraft has reached the end of its life

● Understand the response of complex airframes to flight conditions

Provide feedback data to the control system during the flight

Using fiber-optic sensor technology, engineers can continuously monitor strain, temperature, stress, load, out-of-machine deformation, and 3D shapes to test, monitor, and analyze the integrity of material structures and capture position feedback data from aircraft components. Engineers can use this data to improve aircraft safety, extend service life, reduce maintenance time, and increase flight efficiency—all these results will ultimately be reflected in reduced costs.

Medical

The small diameter and chemical inertness of optical shape sensors make fiber optic sensor technology ideal for medical applications. These characteristics allow fiber optic sensors to be combined with existing minimally invasive technologies. Using fiber-optic sensor technology provides surgeons with position information about the entire length of the instrument, eliminating the need for x-rays or ultrasound. 3D data can be plotted in real time and displayed on a monitor to show the position of the instrument. This image can also be compared to known position coordinates in the body, helping the physician to combine the reference video sent by the endoscope tip with how and where the rest of the instrument is located. This improved position awareness facilitates real-time instrument guidance, minimizing the injection of foreign materials into the patient's body, and keeping it away from radiation.

Benefits of using fiber optic sensors in the medical industry include:

● Improve imaging technology in MRI system

● Assist blood vessel operation and detection to identify the severity of blood vessel blockage

● Judge target shape during minimally invasive surgery and detection

● Achieve higher resolution instrument tracking while minimizing the complexity associated with traditional imaging methods

● Minimize the injection of foreign materials into the body

energy

Fiber optic sensors are also ideal for submarine riser monitoring applications because it collects real-time tension, torque and shape information in advance. Submarine risers are designed to withstand some of the most complex loads and harsh environments that engineers have never seen. The dynamic characteristics of the riser, its components and its environment make it susceptible to structural stresses, fatigue stresses, material wear, deterioration of mechanical properties, impact and environmentally induced loads. Because of these and other factors, the ability of sensors and instruments to measure the structural response of a riser to a load becomes very important.

By using fiber optic sensors in a variety of energy applications:

● Maximize the integrity of risers and rigs

● Provide control system feedback to wind turbine blades based on deformation and rotation information

● Monitor the structural integrity of wind turbine blades

● Structural structural health and calibration information for nuclear power plant components

The future of fiber optic sensors

The price and size of fiber optic sensors are the two major obstacles currently facing the popularization of fiber optic sensing technology. Once these issues are resolved, we are expected to see more use cases in new industries.

Take the fashion industry, for example. In the future, people can insert the sensor into a certain stitch in the clothing, and provide all data and information about the individual's body shape, height, and weight distribution. This data is then used to design specialized clothing for the wearer. This will completely impact the fashion industry and fundamentally change the method of fashion design and production. Imagine shopping online and the clothes have been tailored to fit your body perfectly when they arrive in your hands. It's so cool.

Let us look at the automotive industry again. By inserting fiber-optic sensors throughout the car's structural components, we can receive real-time feedback on how the car responds to changes in the surrounding environment, or monitor when a car component needs to be replaced. These tasks can be done in real time and alert drivers and occupants before an emergency may occur.

In the construction field, optical fibers can be placed in buildings or roads to monitor and judge the degree of environmental impact of building materials during long-term use, and detect them before problems occur.

Summary of this article

The advantages of intrinsic fiber-optic sensing technology in terms of spatial resolution, refresh rate, and detection length have helped improve the ability of many industries to solve problems. The data and information collected by fiber-optic sensors not only help engineers solve current problems, but also help innovate in the future. With the continuous development of this technology, the design and application of fields such as aerospace, energy and medical will also become more advanced. As engineers continue to tap the potential of technology through innovation, sensory detection systems can also solve budding problems. Fiber optic sensors are flexible enough to be implemented as a platform, and then integrated into the design as a component of a critical system to implement the necessary real-time monitoring functions, or as stand-alone test suites.

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