Wavelength-shifted chirped FBGs for temperature compensated strain measurement
Introduction
Fiber Bragg grating (FBG) sensors have been used in a variety of applications ranging from damage detection in composites [1], dynamic structural strain monitoring on bridges [2] to long-term strain monitoring in the construction industry [3], [4]. They have also been used as temperature sensors, pressure sensors etc. [5], [6], [7] with a very high measurement accuracy. The most genuine problem with the application of FBG sensors is the bulkiness and high cost of the interrogation system, optical spectrum analyzer (OSA). It limits the application areas of FBG sensors.
Several FBG interrogation designs have been proposed in the past, but they are only partially promising. A matched-filter interrogation was demonstrated for strain measurement whereby identical gratings are used as notch filters [8]. These notch filters are mounted on small stretching devices driven by piezoelectric stacks. It makes this technique complicated and limits the strain measurement range of the system to ±200 με. Mechanical strain amplification is needed to expand the range which makes the system even more complicated. An FBG demodulation system utilizing tunable Fabry-Perot wavelength filter was also developed [9]. Again, a piezoelectric transducer is used to adjust the cavity spacing in the Fabry-Perot wavelength filter. It makes this system complex and dependent on the performance of an electrical component, like a piezoelectric transducer. A passive wavelength demodulation system was demonstrated which uses a commercial infrared high-pass filter [10]. The resolution of this system is reported as 400 με, which is very poor by any standard. One other reported system employs an asymmetric grating as a wavelength-to-amplitude converter for linear sensing structures [11]. The asymmetric grating employed in this technique is difficult to fabricate. In another low-cost FBG interrogation system, a long period grating (LPG) was used as an edge filter converting strain-induced wavelength variations into optical power variations [12]. The problem with this interrogation system is that LPGs are extremely sensitive to external perturbations, like temperature, strain etc. Moreover, LPGs are known for their very high sensitivity to the refractive index variations in the surrounding medium.
An FBG demodulation method using UV-induced birefringence of the optical fiber was presented [13]. To interrogate the wavelength shift in the FBG sensor, the proposed demodulator uses the wavelength-dependent travel-length of the reflected light from a chirped fiber grating. This method requires few other expensive optical components and the range of this demodulator is limited to 3000 με. A multiplexed Bragg grating sensor configuration utilizing chirped fiber Bragg grating as interrogator was established [14]. This design is complicated and expensive as it employs Erbium doped fiber amplifier, RF generator, phase detector etc. In another complex FBG interrogation technique, a chirped fiber grating based Sagnac loop was introduced [15]. Though the strain resolution of this method is good (around ±5 με), the strain measurement range is only around ±250 με. Another interrogation technique using identical chirped FBG was proposed for strain sensing with a resolution of 5 με [16]. This interrogation method, however, can measure strain only in one direction (either positive or negative).
In summary, some of the FBG interrogation techniques presented so far can measure both the positive and negative strains, but they have at least one of the following issues, poor resolution, small measurement range or the system’s performance being dependent on electrical components such as a piezoelectric sensor/transducer. Moreover, most of these designs involve complicated experimental setup. Designs like identical chirped FBG for interrogation [16] are simple and seem robust, but they can only measure strain in one direction. None of the FBG interrogation designs presented till date seems to fulfil all the requirements desired for practical applications, including wide range of measurement, good resolution and strain measurement in both positive and negative directions.
The sensor design presented in this paper employs two pairs of wavelength shifted chirped FBGs (CFBGs). One pair of CFBGs measures strain and the other measures temperature. The spectra of both pairs are kept apart from each other to avoid cross talk. This design is capable of measuring true strain (temperature independent) in both positive and negative directions. It can also measure the temperature at the same time. The response of this system is about 750 pW/με (at an input power of 2.5 mW). A photodiode with a sensitivity of 0.3–0.4 nW would comfortably provide a strain resolution of less than 1 με. The resolution can be enhanced further by increasing the reflectivity of CFBGs and/or the power of the light source.
Section snippets
Sensor design and technical description
The schematic diagram of the sensor design is shown in Fig. 1, in which two pairs of wavelength shifted CFBGs are used. The first pair comprises CFBG1 & CFBG1′ and the second pair comprises CFBG2 & CFBG2′. The reflectivity of these CFBGs is between 80 and 85%. CFBG1 & CFBG2 work in reflection mode (sensing arms) and CFBG1′ & CFBG2′ in transmission mode (interrogation arms). In other words, CFBG1′ & CFBG2′ act as wavelength filters to CFBG1 & CFBG2, respectively. The transmission spectra of
Principle
The light reflected by CFBG1 goes to Coupler 2 through Coupler 1 and then it gets divided into two parts which pass through the two interrogation arms (CFBG1′ & CFBG2′) separately. Since the spectrum of CFBG1 is far from that of CFBG2′, the light reflected by CFBG1 goes through CFBG2′ uninterruptedly. Therefore, if the strain is applied to CFBG1, the light intensity received at Photodiode2 does not change at all. On the other hand, when the light reflected by CFBG1 goes through CFBG1′, it is
Strain measurement
As mentioned earlier, the strain is measured using the first pair of CFBGs (CFBG1 & CFBG1′). As an illustrative example, CFBG1 is glued close to the fixed end of an aluminum cantilever as shown in Fig. 4. The length, breadth, and thickness of the cantilever are 280 mm, 19 mm and 3.3 mm, respectively. The power from SLED light source in this experiment was kept about 2.5 mW.
The cantilever is loaded at its free end as shown in Fig. 4. Since CFBG1 is at the top side of the cantilever, it experiences
Conclusions
In this paper, a wavelength shifted CFBG interrogation system has been demonstrated. The strain resolution of the presented sensing module can be as good as 1 με. Altogether, it presents a cost effective, compact and high-resolution sensing module which can measure positive/negative strain and temperature simultaneously. Thus, this optical intensity based CFBG interrogation system abrogates the need for an optical spectrum analyzer. The responses from photodiodes can be easily transmitted
Muneesh Maheshwari completed his Master in Technology (M. Tech) from Indian Institute of Technology Kanpur, India. He was the recipient of Junior Research Fellowship during his M. Tech program. He was granted NTU research scholarship in 2012. He completed his Ph.D. from Nanyang Technological University in Jan 2016 in Mechanical and Aerospace Engineering. Currently, he is working as Research Fellow in the school Maritime Institute@NTU.
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Muneesh Maheshwari completed his Master in Technology (M. Tech) from Indian Institute of Technology Kanpur, India. He was the recipient of Junior Research Fellowship during his M. Tech program. He was granted NTU research scholarship in 2012. He completed his Ph.D. from Nanyang Technological University in Jan 2016 in Mechanical and Aerospace Engineering. Currently, he is working as Research Fellow in the school Maritime Institute@NTU.
Swee Chuan Tjin graduated from the University of New England, Armidale, Australia in 1987, received his Ph.D. in 1992 from the Department of Medicine at the University of Tasmania, where he worked on Fibre Optic Laser Doppler Anemometry. He joined NTU in 1991 as a Lecturer in the School of EEE. Swee Chuan’s research interests are in fibre optic sensors, biomedical engineering and biophotonics. Dr. Tjin is also recognized by his peers for his effective communication skills and was awarded the College of Engineering Teaching Excellence Award and EEE Teaching Excellence Award in 2005 and 2012 respectively.
Yaowen Yang is currently in the School of Civil and Environmental Engineering. He received his Bachelor and Master degrees in Mechanics from Shanghai Jiao Tong University, and Ph.D. degree from Nanyang Technological Universtiy. His research interests include structural health monitoring, energy harvesting using smart materials and uncertainty analysis in structural systems. He has been often invited as a technical commitee member or referee and reviewer for a number of premier conferences and journals, including SPIE symposium, IEEE congress, Smart materials and structures etc.
Anand Asundi graduated from the Indian Institute of Technology, Bombay and received his Ph.D. from the State University of New York at Stony Brook. Following a brief tenure Virginia Tech., he joined the University of Hong Kong in 1983 where he was Professor till 1996. Currently he is Professor and Director of the Advanced Materials Research Centre at the Nanyang Technological University in Singapore. His teaching area is in Solid Mechanics and his research interests are in Photomechanics and Optical Sensors. He is Editor of Optics and Lasers in Engineering.