Nearly every fiber optic sensing (FOS) system uses some type of interferometer to obtain measurements on parameters like stress, strain and temperature. The majority of FOS systems in the market employ the use of fiber Bragg grating sensors to reflect light back to the interrogator, with each manufacturer utilizing a unique configuration and demodulation technique (the method used to obtain and interpret the optical signal provided by the sensors). The selected technique has a significant impact on the performance that is possible with a FOS system. The refresh rate, sensing length, number of sensors, spatial resolution and the interplay of each is largely determined by the demodulation technique. Certainly, there are many other variables that impact the performance of a FOS system, but understanding the capabilities and limitations of demodulation techniques is essential when determining which FOS system to purchase.
Fiber Bragg Gratings
Fiber Bragg gratings (FBG) operate as microscopic mirrors that reflect a very narrow wavelength range called the Bragg wavelength. Each grating is comprised of periodic modulations in the core of the fiber with spacing between each modulation. This changes the refractive index of the fiber so that a single wavelength is reflected, while the rest of the light is transmitted and continues down the fiber. The spacing between modulations changes when a grating is subjected to strain or a change in temperature. This alters the refractive index of the grating and causes the Bragg wavelength to shift. The FOS system then uses a demodulation technique to observe the change in wavelength and translates it into a measurement. A number of demodulation techniques are discussed below.
Wavelength Division Multiplexing
Wavelength division multiplexing (WDM) is the most common demodulation technique used on the market today. Each grating must be written at a different Bragg wavelength and the method allows 60 sensors on average to be multiplexed on a single channel. However, only one channel can be interrogated at a time.
The amount of sensors per channel is limited because the reflected wavelengths cannot be written close to each other. If the Bragg wavelengths are too similar, a grating under strain could shift to reflect the same wavelength as another grating making the data useless. Long lead lengths can be used with WDM systems. However, only points of information can be obtained. If an event occurs between sensors, the user will miss critical data.
While WDM systems can have high refresh rates, each additional sensor added to a channel will significantly reduce its capabilities. Another limiting factor for WDM systems is that the user must specify exactly where the gratings need to be written on the sensing fiber. This means that each fiber must be customized for each project, a cumbersome process. The process of identifying where the gratings need to be inscribed, as well as lead times for custom fibers can be lengthy.
Applications well suited for using WDM systems are ones that only require a handful of sensing points at very high speeds. For example, WDM systems can be used to measure strain on crash test dummies or monitor detonation velocity.
Optical Frequency Domain Reflectometry
Optical frequency domain reflectometry (OFDR) can be applied to FBG-based sensors or to scattering technologies. Unlike WDM, when using OFDR, each grating is written at the same wavelength. When a grating is under strain, the shift in the Bragg wavelength is used to identify the degree of strain and location along the length of the fiber. Writing each grating at the same wavelength enables the gratings to be spatially continuous along the entire length of the fiber. As a result, OFDR based fiber optic sensing systems can provide fully distributed measurements instead of a handful of points.
OFDR technology has significantly higher spatial resolution and exponentially more gratings than WDM. One advantage that is unique to Sensuron’s implementation of OFDR is that it is able to maintain high refresh rates even as the number of sensors increases. The combination of high spatial resolution, quick refresh rate, additional number of sensors, and full distribution set OFDR apart as the most sophisticated technology on the market today.
Unlike other fiber optic sensing technologies on the market, Sensuron’s OFDR systems consolidates multiple technologies into a single, robust platform. In addition to sensing strain and temperature, Sensuron’s OFDR technology can determine 2D deflection, 3D shape, liquid level, pressure, operational load, and magnetic fields.
Another advantage of using an OFDR system is that the same fiber can be used for every project. For a new project, engineers can simply cut a new length of fiber, rather than waiting weeks or months for the custom fiber to arrive. The versatility of Sensuron’s OFDR platform empowers customers to solve multiple problems with a single system.
OFDR based systems can be used in diverse applications. For example, Sensuron’s systems have been used to monitor the strain and shape of a wing in real-time while in flight, the shape of medical tools, the liquid level of cryogenic fuel tanks, and much more. These applications would not be achievable with the single points of information provided by WDM systems.
While WDM is useful in certain applications, OFDR offers a solution for the future of monitoring technologies. Providing engineers with fully distributed data in real-time allows them to solve the problems they face today and to innovate beyond the problems of tomorrow.
Sensuron is a leading global provider of fiber optic sensing systems that use light to test, measure, control, inspect, assist with operation and ensure safety of innovations across aerospace, medical and energy industries.