Fibre Bragg Grating Sensors An ntroduction to Bragg gratings and interrogation techniques Dr Crispin Doyle Senior Applications Engineer, Smart Fibres Ltd. 2003 1) The Fibre Bragg Grating (FBG) There are many different types of Optical Fibre Sensor (OFS), working on many different principles: intensity modulation (e.g., microbending), interferometry, polarization effects, refractive index changes, reflectometry and so on. One relatively mature type which appears to be particularly attractive in many applications is the Fibre Bragg Grating (FBG). Bragg gratings are made by illuminating the core of a suitable optical fibre with a spatially-varying pattern of intense UV laser light. Short-wavelength (<300 nm) UV photons have sufficient energy to break the highly stable silicon-oxygen bonds, damaging the structure of the fibre and increasing its refractive index slightly. A periodic spatial variation in the intensity of UV light, caused by the interference of two coherent beams or a mask placed over the fibre, gives rise to a corresponding periodic variation in the refractive index of the fibre. This modified fibre serves as a wavelength selective mirror: light travelling down the fibre is partially reflected at each of the tiny index variations, but these reflections interfere destructively at most wavelengths and the light continues to propagate down the fibre uninterrupted. However, at one particular narrow range of wavelengths, constructive interference occurs and light is returned down the fibre. Maximum reflectivity occurs at the so-called Bragg wavelength λ Β, given by: λ Β =2n eff Λ (1) where n eff is the effective refractive index of the mode propagating in the fibre and Λ is the FBG period. Λ Figure 1. FBG, indicating core refractive index variations Equation (1) implies that the reflected wavelength λ Β is affected by any variation in the physical or mechanical properties of the grating region. For example, strain on the fibre alters Λ and n eff, via the stress-optic effect. Similarly, changes in temperature lead to changes in n eff via the thermo-optic effect and in an unconstrained fibre, Λ is influenced by thermal expansion or contraction. This situation is expressed in Equation 2, where the first term on the RHS gives the effect of strain on λ Β and the second describes the effect of temperature.
λ Β = λ Β (1-ρ α ) ε + λ Β (α+ξ) T (2) where λ Β is the change in Bragg wavelength, ρ α, α and ξ are respectively the photoelastic, thermal expansion and thermo-optic coefficients of the fibre, ε is the change of strain and T is the temperature change. For a typical grating written in a silica fibre and with λ B 1550 nm, sensitivities to strain and temperature are approximately 1.2 pm/µε and 10 pm/ C respectively. The FBG has certain useful characteristics. 1) The sensor is a modified fibre. t has the same size as the original fibre and can have virtually the same high strength. This is in marked contrast to many other types of optical fibre sensor which are either bigger, weaker or both. 2) Because information about the measurands is encoded in the wavelength of the reflected light, FBG sensors are immune to drifts and have no down-lead sensitivity. The responses to strain and temperature are linear and additive and the FBG itself requires no on-site calibration. 3) Multiple gratings can be combined in a single fibre by taking advantage of multiplexing techniques inspired by the telecommunications industry. This gives FBG sensor systems the important property of being able to simultaneously read large numbers of sensors on a very few fibres, leading to reduced cabling requirements and easier installation. 4) Temperature and strain can be measured with the same sensor. n practise, this property can be a two-edged sword. Accurate measurements of strain in the presence of significant temperature excursions require gratings to be deployed in pairs, one sensitive to temperature and strain, bonded securely to the structure of interest and one close to it but isolated from strain, responding to temperature only. n practise, this doubling-up of sensors is not too problematical because there are almost invariably spare FBG which a given unit can address, specifically for the purpose of temperature compensation. 2) FBG nterrogation To use an FBG as a sensor, it is illuminated by a light source with a broad spectrum and the reflected wavelength is measured and related to the local measurands of interest. Shifts in the Bragg wavelength can be monitored by any of the following techniques: An interferometer may be used to convert wavelength shifts into phase shifts, which can be detected by measuring variations in the light intensity as the path difference in the interferometer is varied. This technique potentially allows for very high sensitivity, but the equipment to do it is expensive and prone to environmental interference. No commercial equipment employs interferometry. A sloped optical filter, which may be another Bragg grating, can be used to convert wavelength shifts directly into intensity changes. f the filter is designed to have a known pass/reject ratio which varies with wavelength, then the wavelength of a narrowband reflection from a single grating can be determined simply by measuring and comparing the passed and rejected intensities. For the filter with a transmission spectrum shown on the left in the figure below, as the Bragg wavelength
increases from to, the transmitted intensity t decreases and the reflected or rejected intensity r increases correspondingly. This is the simplest and cheapest way of demodulating FBG, but it has the important disadvantage that it can address only one grating at a time. t reject r transmit λ Figure 2. Demodulating FBG with a passive filter. Wavelength shifts (left) are converted into intensity changes (right). A widely-employed approach is to illuminate the FBG with a narrowband tunable light source. This is the basis of Smart Fibres current products. This method will be dealt with in more detail in the following section, Wavelength Division Multiplexing Wavelength-Division Multiplexing (WDM) The principle behind WDM is simple. Many gratings can be combined on a single fibre and addressed simultaneously provided each has a different Bragg wavelength. This is achieved in practise either by using a broadband light source and a spectrometer for detection or a tunable, swept-wavelength light source and simple photodiode detectors. Smart Fibres employs the latter method in its interrogation units and the diagrams below will help to illustrate how they work. a b d 1 2 3 e c g 4 f λ 3 λ 4 h t 1 t 2 t 3 t 4 time λ 3 λ 4
Figure 3. Schematic and operating principle of WDM equipment. Key: a) light source, b) scanning filter, c) scan generator, d) coupler network for channels 1-4, e) FBG arrays, f) photodetectors, g) processor and h), timevarying output of the detector on channel 4, showing times t i converted into Bragg wavelengths λ i. The scan generator tunes the light source, sweeping it back and forth across its range such that at any given instant the wavelength of light being transmitted down the fibres is known. When this wavelength coincides with the Bragg wavelength of an FBG, light is reflected back down the fibre to a photodetector. The scan generator also supplies a timing signal to the processor, allowing it to convert the intensity vs. time information into a spectrum. Further processing is performed to identify peaks in this spectrum, find their peak positions and convert these to strain or temperature. Typical characteristics of a WDM system are: High sensitivity and accuracy. State-of-the-art WDM systems routinely achieve 1 pm wavelength resolution and long-term stability of better than 3.5 pm. Therefore, strain sensitivity and accuracy are 0.8 µε and 4 µε respectively. Moderate speed. Scan rates are usually around a hundred Hz. Some sources can scan up to 10 khz but data processing becomes difficult for large numbers of sensors at higher speeds. Flexibility. As long as they have different Bragg wavelengths, any reasonable number of sensors can be placed anywhere on a given fibre. Sensors could be 10 mm or 1 km apart, they are read in the same way. Because the FBG reflections and the light sources both have finite widths (typically < 0.5 nm and 50 nm respectively) a practical limit on the number of sensors that can be accommodated is about 100 per channel. Relatively large size. The swept light is a fairly complex, delicate component. Together with the need for significant on-board processing power, this dictates a unit of comparable size to a desktop PC. Smaller size can of course be achieved, but at a cost. Thus, WDM units have historically been more suited to laboratory use or certain environments, such as may be found in civil engineering or marine applications, where their size has not been a drawback. They are less suited for use in aircraft or wheeled vehicles. Note that devices based on grating spectrometers with array detectors can be small and robust. Time-Division Multiplexing (TDM) A TDM system employs a pulsed broadband light source and identifies different gratings by the time taken for their return signals to reach a detector. The pulses from closer gratings are received before those from more distant FBG. The figure below shows an array of FBG at different distances l from the interrogation unit. The time t i required for a pulse return from an FBG at l i is given by: t = l c i (3) n i 2 where c is the speed of light in vacuo and n is the fibre refractive index. Having established the position of a grating in an array, a system of passive sloped filters, as described previously, is used to determine the wavelength of each pulse as it arrives.
l 3 l 2 a b l 1 d e c λ 3 output pulse reflected pulses t 0 t 1 t 2 t 3 time Figure 4. TDM system. Top: Pulses from light source (a) pass through coupler (b), which is also connected to detector (c), to fibre (d) containing FBG (e). Bottom: pulses emanating from source at time t 0 are reflected from FBG at l 1, l 2 and l 3, and return at t 1, t 2 and t 3. Characteristics of a TDM system are: Low cost. There are no sophisticated tunable lasers or filters so the unit can be cheaper than a WDM system Light and rugged. All processing is done with solid state electronics. There are no moving parts so a TDM-based system has the potential to be very small and robust, hence suitable for use in more hostile environments and applications. High sampling rate. Sampling rate is determined only by processing speed and not restricted by the scan rate of the light source. Sample rates of a few khz are readily attainable. Sensor spacing. The main restriction in a TDM system is that the sensors must be sufficiently far apart for pulses returning from adjacent sensors to be separated temporally. 3) Summary FBG can be used as sensitive, stable, strain and temperature sensors or built into transducers for other measurands such as pressure, acceleration etc. Key advantages of FBG over other types of fibre-optic sensor are: minimal size, high strength, linearity, robust signal and ease of multiplexing. Cross-sensitivity to strain and temperature can be a drawback. WDM interrogation methods offer high resolution, accuracy and stability, flexibility in addressing moderate numbers of sensors and relatively low sampling rates. They are well-developed but relatively high cost. Commercial TDM-based systems are less common but they offer faster sampling in a lower cost, more rugged unit than WDM. Sensor spacing is less flexible than is allowed by WDM.