1997 In-fibre Bragg grating sensors

Meas. Sci. Technol. 8 (1997) 355–375. Printed in the UK PII: S0957-0233(97)72999-0 REVIEW ARTICLE In-fibre Bragg grating sensors Yun-Jiang Rao Applied Optics Group, Physics Department, University of Kent at Canterbury, Kent CT2 7NR, UK Received 12 August 1996, in final form 22 October 1996, accepted for publication 21 November 1996 Abstract. In-fibre Bragg grating (FBG) sensors are one of the most exciting developments in the field of optical fibre sensors in recent years. Compared with conventional fibre-optic sensors, FBG sensors have a number of distinguishing advantages. Significant progress has been made in applications to strain and temperature measurements. FBG sensors prove to be one of the most promising candidates for fibre-optic smart structures. This article presents a comprehensive and systematic overview of FBG sensor technology regarding many aspects including sensing principles, properties, fabrication, interrogation and multiplexing of FBG sensors. It is anticipated that FBG sensor systems will be commercialized and widely applied in practice in the near future due to the maturity of economical production of FBGs and the availability of cost effective interrogation and multiplexing techniques. Yun-Jiang Rao was born in Yunnan, China. He received his MEng and PhD degrees in Optoelectronics at the Uni- versity of Chongqing, China, in 1986 and 1990. In 1988 he was employed as a lecturer at the University of Chongqing, where he led a research team to develop the first fully automatic single-mode fibre arc-fusion splicing ma- chine in China. From 1991 to 1992 he worked on all- fibre optically-addressed sil- icon microresonator sensors as a Postdoctoral Research Fellow at the University of Strathclyde in Scotland. In 1992 he joined the Department of Physics at the University of Kent at Canterbury as a Research Fellow, where he has been working on fibre-optic low coherence interferometry, development of fibre-optic pressure and temperature sensors, advanced fibre-optic sensor multiplexing techniques and in-fibre Bragg grating sensors. He has developed a universal fibre-optic point sensor system for quasi-static absolute measurements of multiparameters based on low-coherence interrogation. His current research interests are intrinsic and extrinsic single- mode fibre-based sensors, multiplexing techniques, fibre- optic interferometry and in-fibre grating sensors. He has authored or co-authored over 30 journal and 40 conference papers. 1. Introduction The formation of photogenerated gratings in germanosil- icate optical fibre by sustained exposure of the core to the interference pattern produced by oppositely propagat- ing modes of argon-ion laser radiation was first reported in 1978 (Hill et al 1978). However, pioneering work at the United Technology Research Centre, which is regarded as a milestone for in-fibre Bragg grating (FBG) sensors, was published eleven years later (Meltz et al 1989). This side-writing technique creates a Bragg grating directly in the fibre core using a holographic interferometer illumi- nated with a coherent ultraviolet source. Versatility in the fabrication of FBGs has been gained from the fact that the Bragg wavelength is independent of the writing laser used. Subsequent to this initial work interest in FBGs has increased considerably in recent years. There are two prob- able main reasons for this: (1) the FBG has become a key passive device for applications in optical fibre telecommu- nications (Mizrahi 1993), including wavelength-division- multiplexing, fibre laser and amplifier pump reflectors (Ball and Morey 1992, Farries et al 1992), gain flattening devices (Kashyap et al 1993), and dispersion compensation ele- ments (Ouellette 1991, Williams et al 1994); (2) it has been demonstrated that FBGs have great potential for a wide range of sensing applications where quasi-distributed mea- surements for important physical quantities, such as strain, temperature, pressure, ultrasound, acceleration, high mag- netic field and force, are required (Morey et al 1989, 1992, Xu et al 1993b, Foote et al 1996, Kersey and Marrone 1994, Rao et al 1996f, Webb et al 1996, Theriault et al 1996, Bjerkan et al 1996). Compared with other implementations of fibre-optic sensors, FBG sensors have a number of distinguishing advantages. (1) They can give an absolute measurement that is insensitive to fluctuations in the irradiance of the illuminating source, as the information is usually 0957-0233/97/040355+21$19.50 c© 1997 IOP Publishing Ltd 355 Yun-Jiang Rao obtained by detecting the wavelength shift induced by the measurand. (2) They can be directly written into the fibre without changing the fibre diameter, making them compatible with a wide range of situations where small diameter probes are essential, such as in advanced composite materials for strain mapping, or the human body for temperature profiling. (3) They can be mass produced at low cost, making them potentially competitive with conventional electrical sensors (Askins et al 1994). (4) They can be multiplexed using similar techniques that have been applied for use with fibre-optic sensors, including wavelength-division- multiplexing (WDM), spatial-division-multiplexing (SDM), time-division-multiplexing (TDM), and their combinations (Kersey 1993, Rao and Jackson 1996a), making quasi- distributed sensing feasible in practice. One of the most important applications of FBG sensors that has been demonstrated to date is for the so-called ‘fibre-optic smart structures’, where FBGs are embedded into the structure to monitor its strain distribution (Udd 1995). Fibre-optic smart structure technology could in the future lead to structures that are self-monitoring and even self-scheduling of their maintenance and repair by the marriage of fibre-optic sensor technology and artificial intelligence with material science and structural engineering. In order to see the significance of FBG sensors, a comparison between FBG sensors and other fibre-optic strain sensors is given in table 1. It can be seen that FBG sensors have more advantages than other fibre-optic strain sensors, making them ideal for fibre-optic smart structures. However, temperature compensation of the strain error caused by thermal fluctuation is essential for practical applications and this issue will be discussed later. The aim of this article is to give a comprehensive and systematic overview of FBG sensor technology in six sections. Following the introduction, the sensing principles and properties of FBG sensors are provided in section 2. In section 3, various fabrication techniques for FBGs are outlined which make the low-cost mass production of FBGs feasible. Section 4 deals with a range of interrogation techniques for FBG sensors, which can achieve high- resolution wavelength-shift detection. A wide variety of multiplexing techniques that have been applied to FBG sensors are discussed in section 5. This article concludes in section 6 with a summary and discussion of likely future developments in FBG sensor technology. 2. Principles and properties of FBG sensors 2.1. Optical theory of in-fibre Bragg gratings An FBG is written into a segment of Ge-doped single- mode fibre in which a periodic modulation of the core refractive index is formed by exposure to a spatial pattern of ultraviolet light in the region of 244–248 nm, as shown in figure 1. This fabrication process is based on the photorefractive effect in the germania oxygen-vacancy defect band, which was observed in Ge-doped optical fibres by Hill et al (1978). The lengths of FBGs are normally within the region of 1–20 mm and grating reflectivities Table 1. Comparison of fibre-optic strain sensors. FBG FP TM PM Linear response yes yes.1/ yes.1/ yes.1/ Absolute measurement yes yes.2/ yes.2/ yes.2/ Range to resolution high high low low Sensor gauge length short short long long Mechanical strength high low.3/ high high Multiplexing yes yes.4/ yes.4/ yes.4/ Mass production yes yes.5/ yes.5/ yes.5/ Potential cost low low.6/ low.6/ low.6/ FP: Fabry–Pe´rot interferometric sensors (Kersey et al 1983, Lee et al 1992, Kist et al 1984, Claus et al 1993). TW: two-mode fibre-optic sensors (Blake et al 1987, Lu and Blaha 1992). PM: polarimetric fibre-optic sensors (Varnham et al 1983, Bock and Wolinski 1991). .1/ Requires quadrature signal demodulation and it is difficult to achieve high stability in practice. .2/ Requires suitable signal processing, such as low-coherence interferometry (Rao and Jackson 1996c). .3/ Except for in-line fibre etalon strain sensors (Sirkis et al 1993). .4/ Difficult if the number of sensors is large, unless spatial-division-multiplexing is used. .5/ Requires some handling work either for cavity construction or for lead-in/lead-out fibre splicing, except for intrinsic Fabry–Pe´rot sensors made from two FBGs reported recently (Morey et al 1991). .6/ Good skills for fibre splicing or sensor assembly are needed. Figure 1. Schematic diagram of in-fibre Bragg grating. can approach �100%. When the FBG is illuminated by a broadband light source, a set of beams reflected from a set of partially reflecting planes formed by the periodic core index modulation interfere with each other. The interference is destructive unless each beam is in phase with all the others. According to Bragg’s law which gives this condition, only one wavelength, i.e. the Bragg wavelength �B , is selected, which is given by �B D 2n3 (1) where n is the effective core index of refraction and 3 the period of the index modulation. The reflectivity at the Bragg wavelength can be estimated using the equation (Lam and Garside 1981) R D tanh2 (2) 356 In-fibre grating sensors where  D �n.L=�B/.1n=n/�.V /: (3) The factor �.V / ’ 1−1=V 2, V � 2:4 is the fraction of the integrated fundamental mode intensity contained in the core (V is the normalized frequency of the fibre). It is seen that R is directly proportional to the grating length L and the index perturbation (1n=n) which is normally determined by the exposure power and time of the UV radiation for a specified fibre. The full width half maximum (FWHM) bandwidth, 1�, of a grating is approximately given by (Russell et al 1993) 1� D �Bs s� 1n 2n �2 C � 1 N �2 (4) where s � 1 for strong gratings (near 100% reflection) and s � 0:5 for weak gratings, and N is the number of grating planes. Because the change in the index is small, the main contribution to the linewidth change is attributed to the change in the modulation depth of the index perturbation. Unlike conventional fibre-optic interferometric sensors illuminated by highly coherent lasers, FBG sensors generally require a broadband light source and a high- resolution wavelength-shift detection system, which will be discussed later. From the light source point of view, a wide wavelength bandwidth and high optical power is normally required to achieve a large range to resolution for the wavelength shift induced by the measurand. Most of the sources that have been used for FBG sensors are similar to those used for fibre-optic gyroscopes and low-coherence interferometric sensors (Lefevre 1993, Jackson 1994). They include edge-emitting light emitting diodes (ELEDs), superluminescent diodes (SLDs), superflourescent fibre sources (SFSs) and tuneable fibre lasers (TFLs). Table 2 gives a comparison of these sources. 2.2. Sensing principles FBG sensors have been reported for measurements of strain, temperature, pressure, dynamic magnetic field, etc. The FBG central wavelength will vary with the change of these parameters experienced by the fibre and the corresponding wavelength shifts are as follows. 2:2:1. Strain. The wavelength shift, 1�BS , for an applied longitudinal strain 1" is given by 1�BS D �B.1− ��/1" (5) where �� is the photoelastic coefficient of the fibre, given by �� D n 2 2 [�12 − �.�11 − �12/] (6) where �11 and �12 are the components of the fibre-optic strain tensor and � is Poisson’s ratio. For the silica fibre, the wavelength–strain sensitivities of 800 nm and 1.55 �m FBGs have been measured as � 0:64 pm �"−1 and � 1:15 pm �"−1 respectively (Morey et al 1989, Rao et al 1995b). For the measurement of acceleration, ultrasound Figure 2. Linearly chirped FBG sensor. and force, equations (5) and (6) are still applicable as these measurands are converted from strain. Linearly chirped FBGs have been demonstrated as dispersion compensation elements for high-speed optical fibre communication systems (Ouellette 1991, Williams et al 1994). These devices act as a wavelength bandpass filter in which the pitch of the grating is varied along the position of the grating length and the chirped FBG reflects a large number of wavelengths, i.e. optical frequencies, from different positions of the grating, as shown in figure 2. The principle of using this chirped device for strain sensing is based on the effective change in reflection point, �b, which is given by (Kersey and Davis 1994) �b D − � 1�C B�1" (7) where � is a constant determined by the photoelastic properties of the fibre and B is the grating length. � is a fixed source wavelength and 1�C D �B1 − �B2 with �B1 < �B2 (see figure 2). For comparison, the optical length change for a length of fibre B is given by �l D B�1": (8) The ratio of equations (7) and (8) is thus expressed in the form �b �l D − � 1�C : (9) It can be seen that because � � ��C; �b � �l, hence the chirped FBG gives a very large strain transduction amplification factor. From the high-sensitivity point of view, linearly chirped FBGs have potential advantages for dynamic strain measurement. Another potential use of chirped FBGs is for distributed strain measurement by sequentially interrogating short sections along the grating with low-coherence interferometry (Volanthen et al 1996). 2:2:2. Temperature. For a temperature change of 1T , the corresponding wavelength shift 1�BT is given by 1�BT D �B.1C �/1T (10) where � is the fibre thermo-optic coefficient. For the silica fibre, the wavelength–temperature sensitivities of 800 nm 357 Yun-Jiang Rao Table 2. Comparison of light sources for FBG sensors. ELEDs SLDs SFSs TFLs Optical power� 1–10 �W 0.1–2 mW 1–10 mW 0.1–10 mW FWHM bandwidth 40–100 nm 15–30 nm 20–40 nm 1–10 nm�� Device cost low medium high high � Optical power coupled into single-mode fibre. �� Tuneable wavelength range. Table 3. Strain and temperature sensitivities of FBG sensors with different wavelengths. Wavelength Strain sensitivity Temperature sensitivity (�m) (pm �"−1) (pm �C−1) 0.83 �0.64 �6.8 1.3 �1 �10 1.55 �1.2 �13 and 1.55 �m FBGs have been measured with values of �6:8 pm �C−1 and �13 pm �C−1 respectively (Morey et al 1989, Rao et al 1995b). Table 3 gives a summary of the wavelength–strain and wavelength–temperature sensitivities for FBGs with different wavelengths (Morey et al 1989, Xu et al 1994b and Rao et al 1995b). 2:2:3. Pressure. For a pressure change of 1P , the corresponding wavelength shift 1�BP is given by 1�BP �B D 1.n3/ n3 D � 1 3 @3 @P C 1 n @n @P � 1P: (11) In the weakly guided single-mode region, the contribution to the fractional change in optical propagation delay arising from a small fractional change in fibre diameter due to the applied pressure is negligible when compared with the change in refractive index and physical length. The changes in physical length and refractive index are given by (Hocker 1979, Morey et al 1992) 1L L D − .1− 2�/P E (12a) 1n n D n 2P 2E .1− 2�/.2�12 C �11/ (12b) where E is Young’s modulus of the fibre. As a result of 1L=L D 13=3 the normalized pitch-pressure and the index-pressure coefficients are given by 1 3 @3 @P D − .1− 2�/ E (13a) 1 n @n @P D n 2 2E .1− 2�/.2�12 C �11/: (13b) By substituting equations (13a) and (13b) into (11), we obtain the wavelength–pressure sensitivity, given by 1�BP D �B � − .1− 2�/ E C n 2 2E .1− 2�/.2�12 C �11/ � 1P (14) For a Ge-doped FBG at 1.55 �m 1�BP =1P was measured as −3 � 10−3 nm MPa−1 over a pressure range of 70 MPa (Xu et al 1993b). The pressure sensitivity has been enhanced by a factor of four by mounting the FBG in a hollow glass bubble to achieve mechanical amplification (Xu et al 1996). 2:2:4. Dynamic magnetic field. FBGs can also be used for dynamic magnetic field detection with the Faraday effect to induce a slight change in the index of the fibre experienced by left- and right-circularly polarized light in an FBG (Kersey and Marrone 1994). In the presence of a longitudinal magnetic field applied to the FBG, the index is changed for the two circular polarizations and in consequence two Bragg wavelengths are obtained, as shown in figure 3, i.e. �BC D 2nC3 (15a) �B− D 2n−3 (15b) where the subscriptsC and− represent the index and Bragg wavelength for right- and left-circularly polarized light at the FBG. For the silica fibre, the sensitivity of this effect is weak as it is determined by the inherent Verdet constant of the silica which is only �8�10−1 rad T−1 m−1 at �1:3 �m wavelength. The change in index is given by nC − n− D VdH�2� (16) where Vd and � are the Verdet constant and the working wavelength and H is the applied magnetic field. Consequently, the resulting wavelength shift is very small and has been detected with the interferometric interrogation scheme which can achieve a very high sensitivity (Kersey and Marrone 1994). Based on this work, dynamic measurement of large values of current and voltage would be possible. 2:2:5. Simultaneous strain and temperature measure- ment. For quasi-static and static strain measurements, large temperature variations are common in practical ap- plications. Hence a temperature compensation for the ef- fect of ambient temperature fluctuation is required (Farahi et al 1990). A range of techniques have been proposed to achieve this goal by measuring strain and temperature simultaneously. 358 In-fibre grating sensors Figure 3. Splitting of the Bragg resonance due to the circular birefringence induced by a magnetic field. 2.2.5.1. Reference FBG method. On a single measurement of the Bragg wavelength shift it is impossible to separate the effects of changes in strain and temperature. Therefore a reference is required for temperature measurement. The most straightforward way is to use a separated, strain-free FBG as the temperature sensor to measure the temperature of the strain sensor directly (Morey et al 1992, Xu et al 1994a). This reference FBG is located in the same thermal environment as the strain sensor. The strain error caused by the temperature variation can be compensated to first order by subtracting the wavelength shift induced by temperature variation from the total wavelength shift obtained with the strain sensor. It has been found that a chirped FBG in a tapered optical fibre can be temperature independent (Xu et al 1995). The taper profile is designed such that the FBG is linearly chirped when tension is applied, creating a strain gradient along the FBG. By measuring the effective bandwidth variation rather than the Bragg wavelength change, the reflected intensity signal is insensitive to temperature. This special type of FBG sensor is attractive as no temperature compensation is required, but one problem is that at the tapered region of the fibre mechanical strength becomes much weaker and hence the fibre is easy to break. Furthermore, investigation might be needed to assess the measurement accuracy as any intensity fluctuation along the lead fibre would cause error. 2.2.5.2. Dual-wavelength superimposed FBGs method. This method is based on the use of two sets of wavelength- shift data obtained from two superimposed FBGs written at the same location in the fibre (Xu et al 1994b). Assuming that the wavelength shifts in strain and temperature are linear, the Bragg wavelength-shift, 1�B , in response to a strain change, 1", and a temperature change, 1T , is given by