Fiber Bragg Grating Sensor Technology

Fiber-Bragg-grating (FBG) sensing systems are able to provide faster, more accurate, and more sensitive systems. Because the temperature and strain states of FBGs directly affect their reflectivity spectrum, they can be used to service a variety of sensing applications. As the fiber-optic analogue to conventional electronic sensors, FBGs can serve as strain-gauge sensors to provide structural engineers with measurements not previously possible.

Figure 2 Principle of Fiber Bragg Grating

The physical principle behind the FBG sensor is that a change in strain, stress, or temperature will alter the center of the wavelength of the light reflected from an FBG. A fiber's index of refraction depends on the density of the dopants it contains. Developers construct FBGs by redistributing dopants to create areas that contain greater or lesser amounts, using a technique called laser writing.
The FBG wavelength filter consists of a series of perturbations in the index of refraction along the length of the doped optical fiber. This index grating reflects a narrow spectrum that is directly proportional to the period of the index modulation (Λ) and the effective index of refraction (n).
The wavelength at which the reflectivity peaks, called the Bragg wavelength (λB), is expressed by λB = 2nΛ. Because temperature and strain directly affect Λ and n, any change in temperature and strain directly affects the λB.
A change in mechanical or thermal strain on the FBG results in a wavelength/strain sensitivity of 1.2 picometers/µstrain (microstrain is a change in dimension that is one millionth of the original) and a wavelength/temperature sensitivity of 10 picometers/°C. The gauge lengths of FBGs are up to 5 millimeters, although lengths of up to 100 centimeters are being developed for civil engineering applications.
Fabrication of FBGs is done by writing an index grating directly on a doped optical fiber. Two intense ultraviolet beams angle in to form an interference pattern with the desired periodicity, which is written on one side of a bare fiber after the external coatings have been stripped away. The pattern's intense bright and dark bands cause local changes in the index of refraction by the migration of the dopants in the fiber. After the grating writes on the fiber, acrylate/polyamide recoats and covers the fiber.
Writing many sensors on a single optical fiber requires careful consideration of each FBG specification. For example, the allowable strain range for any given FBG sensor depends on the available optical bandwidth. When placing many FBG sensors on a single fiber, each sensor must have its own wavelength segment so that various signals do not overlap. As the FBGs undergo strain, they shift in wavelength within their allotted optical bandwidth range.
To measure wavelength shifts that result directly from changes in temperature or tension, FBG sensor systems must consist of an optical source that continuously interrogates the reflection spectrum and a detection module that records the shifts in the peak reflectivity versus wavelength. For applications such as maintenance checks of an airplane's structural integrity, the slower—but more accurate—systems are desirable. In other applications for which in situ monitoring is required, high update rates are more important.
Emerging applications include detecting changes in stress in buildings, bridges, railway and airplane bodies; depth measurements in streams, rivers, and reservoirs for flood control; and temperature and pressure measurements in deep oil wells, slope/rock movement for landslide early warning system and in various medical applications.

1.4 Advantages of FBG Sensor

 

Fiber Bragg grating sensors offer several significant advantages over conventional electrical sensors. The FBG sensor has high accuracy, sensitivity, and immunity to electromagnetic interference, radio-frequency interference, and radiation.

Has the ability to be made into a compact, lightweight, rugged device small enough to embed or laminate into structures or substances to create smart materials that can operate in harsh environments—such as underwater—where conventional sensors cannot work.

The ability to accommodate multiplexing and an inherent low transmission loss at 1550 nanometers. These features allow one to use many sensors on a single optical fiber at arbitrary spacing.

Installation and use are easy. Because the gratings multiplex on a single fiber, one can access many sensors with a single connection to the optical source and detector.

Potential low cost as a result of high-volume automated manufacturing process.

 The key advantages of FBG Sensors are:

1. They’re rugged passive components resulting in a high life time (> 20 years)
2. They’re small in size and can be easily embedded into desired areas.
3. They form an intrinsic part of the fiber optic cable that can transmit the measurement signal over several tens of kilometers
4. They show no interference with electromagnetic radiation, so they can function in many hostile environments where conventional sensors would fail.
5. They don't make use of electrical signals what makes them explosion safe.
6. They’ve the ability to multiplex many sensors using only one optical fibre, driving down the cost of complex control systems.
7. They’re easily installed and virtually maintenance free.
8. They’re cost effective and highly reliable.
9. They produce fast and accurate measurement.
10. They don’t require power supply at immediate site.
11. They’ve high sensitivity thus having fast response time.

 

Application of Fiber Bragg Grating Sensor

          The technology and applications of optical fibers have progressed very rapidly. Optical fiber, being a physical medium, is subjected to perturbation of one kind or the other at all times. It therefore experiences geometrical (size, shape) and optical (refractive index, mode conversion) changes to a larger or lesser extent depending upon the nature and the magnitude of the perturbation. In communication applications one tries to minimize such effects so that signal transmission and reception is reliable. On the other hand in fiber optic sensing, the response to external influence is deliberately enhanced so that the resulting change in optical radiation can be used as a measure of the external perturbation. In communication, the signal passing through a fiber is already modulated, while in sensing, the fiber acts as a modulator. It also serves as a transducer and converts measurands like temperature, stress, strain, rotation or electric and magnetic currents into a corresponding change in the optical radiation. Since light is characterized by amplitude (intensity), phase, frequency and polarization, any one or more of these parameters may undergo a change. The usefulness of the fiber optic sensor therefore depends upon the magnitude of this change and our ability to measure and quantify the same reliably and accurately.
The advantages of fiber optic sensors are freedom from EMI, wide bandwidth, compactness, geometric versatility and economy. In general, FOS is characterized by high sensitivity when compared to other types of sensors. It is also passive in nature due to the dielectric construction. Specially prepared fibers can withstand high temperature and other harsh environments. In telemetry and remote sensing applications it is possible to use a segment of the fiber as a sensor gauge while a long length of the same or another fiber can convey the sensed information to a remote station. Deployment of distributed and array sensors covering extensive structures and geographical locations is also feasible. Many signal processing devices (splitter, combiner, multiplexer, filter, delay line etc.) can also be made of fiber elements thus enabling the realization of an all-fiber measuring system. Recently photonic circuits (Integrated Optics) has been proposed as a single chip optical device or signal processing element which enables miniaturization, batch production, economy and enhanced capabilities.
            The application of FBG can be divided into four main groups comprising of the following:

1. Structural Health Monitoring
2. Environmental Monitoring
3. Surveillance & Safety Monitoring
4. New Emerging Areas

 

Erbium-Doped Fiber Amplifier (EDFA)

Optical amplifiers are devices that amplify the power of an input optical signal as shown in Fig. 2. Optical amplifiers are transparent, i.e. amplify the input optical signal regardless of the data format that is being transmitted.

 

 

 

 

 

Fig. 2. A layout of an optical amplifier.

An optical fiber amplifier as shown in Fig. 3 uses erbium doped fiber as its active medium and semiconductor laser diode as the pumping source. As the input signal traverse the optical fiber, it is coupled with the pump source signal through the coupler. The pumped light will create the required population inversion for the stimulated emission process. This results in amplifying the input signal.

 

 

 

 

 

 

Fig. 3. A typical erbium doped fiber amplifier.
The versatility of optical amplifiers has transformed classical optical communications into cutting-edge technologies by replacing the functionality of the conventional repeaters. Its ability to amplify the optical signal without converting it into electrical domain has triggered optical communication progress at the speed of light. For long-haul transmission, optical amplifiers have proven to be one of the essential components to extend the transmission distances to a few thousands kilometers.

Broadband and gain-clamped amplifiers have been studied for long-haul transmissions. These amplifiers utilize the cutting-edge technologies to achieve the ultimate performances to meet the telecommunication providers’ requirements. The tremendous growth of the Internet and data traffic spearheads the demand for data-centric bandwidth from telecommunication operators. WDM transmission systems are widely deployed around the globe utilizing the conventional Erbium doped fiber amplifiers (EDFAs) window (C-band, 1528–1563 nm). In order to cope with the explosive demand on bandwidth, new optical transmission windows are required and a merger of high speed data rates and ultra-dense WDM signals is a pre-requisite, a function fulfilled by the long-wavelength band (L-band, 1565–1605 nm). By integrating these C- and L-band amplifiers, broadband optical amplifiers with a gain bandwidth of more than 60 nm have been reported.

Even though the capacity of channels can be increased with improvement of gain bandwidth, gain control in EDFA for WDM systems still remain as the main factor for investigation. This factor is critical due to the wavelength dependent gain of an EDFA, when channels are added or dropped for network configuration. Adding channels reduces gain of existing channels, which in turn lowers the power to approach the receiver sensitivity level. On the other hand, dropping channels increases the power of the present channels with the possible consequence of surpassing thresholds for non-linear effects in optical fibers. Different solutions to these problems have been proposed such as all-optical gain control, which utilizes a lazing mechanism to clamp the population inversion, and the signal gain is clamped regardless of the input signal powers. Fast pump control in a two stage EDFA was reported, by adjusting the pump power according to the detected power of the incoming signals to the EDFA. In a similar approach, studies on pump-loss control were carried out, which controls pump power based on monitored pump power loss. Recently, a new gain-clamping EDFA was demonstrated using double-pass super-fluorescence technique.

EDFA Application

1. Metro and long haul network
2. Booster, pre-amp, in-line amplification
3. DWDM networks
4. Boost tunable laser source
5. Passive Optical Network (PON)
6. Free Space Communication
7. CATV systems
8. Fiber to the Home (FTTH)

 

Fused Biconical Tapered (FBT) Couplers

In its simplest form, an FBT fiber coupler consists of two optical fibers in which the optical cladding has been fused together. B. S. Kawasaki and colleagues produced the first fused couplers, which used multi-mode fiber and provided only simple power distribution between fibers, in 1977. The same group extended the technique to single-mode fibers in 1981. An analysis by J. Bures and others in 1983 provided insight into the inner workings of the device and opened the door to new applications, including WDM. Since then, fused couplers have grown from their simple original structure to more precise and complex structures suitable for advanced applications such as DWDM.

The basic principle of operation of all fused-fiber couplers is the same; the fusion of two or more fibers into a new wave-guide structures in which several fiber cores share a common optical cladding. Tapering this structure changes the core modes of the fibers, broadening the modal fields in the cladding. In the "down" taper direction, modes can escape the cores to become common cladding modes that overlap all the fiber cores present. In the "up" taper region, the cladding modes are converted back into fiber-core modes. Which particular cores recover power depends on the relative phase between the cladding modes. Refer figure 4 below.

 
             

  

 

The conversion between the core and cladding modes is virtually “loss-less” if the taper slopes are gradual, making FBT components the lowest excess-loss passive devices available. Furthermore, signal light never leaves the optical-fiber structure and thus never encounters an interface or discontinuity, so there is intrinsically no possibility of back reflection. These components, therefore, display very high directivity. Optical fiber couplers are used for branching or combining optical signals.

FBT couplers are fabricated by placing two standard single-mode fibers side by side, twisting them together to bring them into strong contact and thermally fusing (burning) them with flame or heater while elongating (tapering) the fused region. After the tapering process, the coupler is bonded to a quartz substrate with a special adhesive. The substrate with the coupler is inserted into a metal tube and the ends of the tube are closed with elastic epoxy glue. FBT technique of fabrication has proved to be an effective method for coupler construction because the resultant fused couplers can be potted and mounted in small and rugged packages.