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Study of strain measurement by fiber optic sensors with a sensitive fiber loop ringdown spectrometer
Optical Fiber Technology 54 (2020) 102070
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Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Study of strain measurement by fiber optic sensors with a sensitive fiber loop ringdown spectrometer
T
⁎
M. Kayaa, , O. Esenturkb a b
Eskisehir Osmangazi University, Eskisehir 26480, Turkey Middle East Technical University, Ankara 06800, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber loop ringdown Strain sensor Fiber optic sensors FLRD spectrometer
A sensitive fiber loop ringdown (FLRD) spectrometer without any additional optical component was utilized to obtain strain measurement on a single mode fiber optic sensor. The strain data were obtained by employing the theory of bending loss in single mode fibers. The best sensitivity of the sensors was obtained as 5.99 με with an 80.0 cm long sensor head when the sensor heads were stretched at the midpoint. The spectrometer system had a baseline-stability of 0.22%. Stretching the sensor head from off-midpoint positions resulted faster decays with higher optical losses. Comparison of the slopes relative to the stress positions showed that it may be utilized to obtain strain location without using any high-cost equipment. This portable, basic, and simpler FLRD spectrometer system offers high sensitivity with great baseline stability without utilization of any additional optical components and/or creating air-gap/air-cavity and encapsulation on the sensor head region. With its very attractive features of such as easy setup, low cost, and simple design, sensitive FLRD sensors may have a high potential for early detection in several applications such as structural health monitoring, biomedical sensing, mining, transportation and rail applications for continuous monitoring in real-time.
1. Introduction
sensitivity. The FBG and FPI sensors are the mostly used and commercialized strain sensors based on the phase or the wavelength shifts; however, the setup employs relatively expensive components to obtain high sensitive signals [20–22]. On the other hand, creating sensors by using the single mode fiber (SMF) itself in the FLRD spectroscopy is an alternative way to the FBG and the FPI to achieve high strain sensitivity with comparably much lower overall system costs with a simpler design. The FLRD technique offers many advantages such as enhanced detection with its multi-pass nature of a laser beam inside the fiber loop, simplicity, low cost, and portable network system setup. For the strain studies, Tarsa et al. [23] reported a ringdown spectroscopy system based on the biconical tapered single mode fiber as a sensor region. They demonstrated 74.0 nm minimum detectable displacement over 10 mm taper length, corresponding to 7.4 με strain sensitivity. However, a very short measurement region and very thin taper region makes it highly fragile, hence requires very gentle handling. Consequently, this restriction brings limitation for its application in many fields. Qiu et al. [19] demonstrated an application of a fiber ringdown strain sensor based on the multimode fiber loop ringdown spectroscopy and reported strain sensitivities of 0.35 ns/με and 0.17 ns/ με for 4.0 cm and 9.0 cm curvature radii, respectively. The baseline stability was obtained as 0.8% by considering the standard deviation of
Since the last three decades, fiber optic sensors have attracted researchers and found wide application areas due to their small size, lightweight, high sensitivity, low data loss, immunity to electromagnetic radiations, and their possibility for long distance applications or remote sensing. The Fiber Loop Ringdown (FLRD) spectroscopy is a multi-functional fiber optic measurement technique that is evolved from the Cavity Ringdown Spectroscopy (CRDS). While the space, a cavity length between two reflective mirrors is used as a waveguide for a laser beam in CRDS, an optical fiber loop is employed for the same purpose in the FLRD technique. The FLRD spectroscopy is a time-domain technique due to the fact that a portion of a laser pulse is trapped into a fiber loop for multiple interactions with the sample resulting increased sensitivity and stability of the measurement. The technique has a broad application field such as measurement of strain [1,2], refractive index [3–5], temperature [6,7], pressure [8,9], chemicals [10–12] and biological species [13,14]. One of the highly employed type of fiber optic sensors is the strain sensor. Many different types of strain sensors such as Fiber Bragg Grating (FBG) [15], Fabry-Perot Interferometer (FPI) [16], fiber bending attenuation [17], Long-Period Grating (LPG) [18] and fiber loop [2,19] were studied to obtain the best ⁎
Corresponding author. E-mail address: [email protected] (M. Kaya).
https://doi.org/10.1016/j.yofte.2019.102070 Received 21 July 2019; Received in revised form 14 October 2019; Accepted 1 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Fig. 1. Schematic illustration of the fiber loop ringdown system setup.
an important role. Internal mechanism of embedded optical fibers was explored by Wang et al. based on strain transfer analysis [26]. In the study, they provided theoretical basis of interfacial bonding mechanism of the embedded optical fiber sensors by analyzing interfacial debonding between embedded sensor and the host material and explained well by strain transfer theory. In another study [27], they have fabricated distributed and quasi-distributed optical fiber sensors and FBG sensors to monitor the three dimensional deformation of an asphalt pavement by placing sensors in different positions and layers of the pavement. They analyzed three dimensional strain and displacement of the asphalt pavement under traffic loads and environmental temperature variations. In this study, we demonstrate a simple design, easy setup, low cost, and fast response strain sensor by using a sensitive FLRD spectrometer. The sensor system composed of only basic components: a laser source, a fiber loop, a photodetector and an oscilloscope. The main aim of this study is to exhibit a bare single mode strain sensor with a simple FLRD spectrometer system, enhanced sensitivity, low baseline stability, and without a need for any modification on the sensor head such as chemical or physical treatments. Obtained detection limit is independent of any constraint of optical instruments such as laser, oscilloscope, and does not need an optical spectrum analyzer, extra components such as FBG/FPI and/or an extra effort and precise work to create special sensor region such as air-gap/air-cavity. The results suggest that such strain sensors utilizing enhanced FLRD spectrometer have high potential to be employed for the early detection in the real world applications such as land sliding, erosions, structural health monitoring, biomedical applications and crack detection with its high sensitivity.
2.12 ns and the average ringdown time of 262.90 ns. Since they used a multimode fiber, a fiber mode converter as an additional device had to be used to stabilize propagating modes inside the fiber. Hu et al. [24] proposed intensity modulation fiber optic strain sensors consisting of an elastic cantilever, a tandem rod, and a collimator for temperaturecompensation measurement. They achieved a high strain detectionrange of 1.1 × 103 με for 21.5 mm sensing length and a detection limit of 5.7 × 10−3 με by using the advantage of the small beam divergence and the incident angle sensitive collimator, which was utilized for both outgoing and incoming light. The collimator detects scattered beam from the elastic cantilever whenever a strain occurs on the sample deflecting the cantilever. They employed a cantilever and a fiber collimator instead of an optical fiber in order to increase the working distance between the collimator and the cantilever. Therefore, the sensing limit depends on the collimator coupling efficiency. In another study, Zhou et al. [1] used a Photonic Crystal Fiber (PCF) MachZehnder interferometer (MZI) in a fiber loop and obtained the minimum detectable strain as 3.6 με. A PCF of 4.8 cm is spliced to a single mode fiber to fabricate MZI and the system setup includes additional components such as an Erbium-Doped Optical Fiber Amplifier (EDFA), a variable optical attenuator, an FBG, and an Optical Spectrum analyzer (OSA). Recently, Ghimire et al. [2] presented an FLRD-based strain sensor utilizing a micro-air gap as a sensing element and aligned two clean cut ends of an SMF in a ceramic ferrule. After coupling the light into the loop, the ferrule sleeves were employed for stabilization and rigidness of the sensor head. In this study, they succeeded to measure the minimum strain as 7.5 nε with a meter-long fiber. In the other study, Ghimire et al. [25] utilized a micro air-gap strain sensor to monitor the tension from prestressed concrete beams. The sensor head was embedded into concrete beams after assembling in a ceramic ferrule and a ferrule sleeve, revealing a measured strain steps of 1.25 µε and hence a theoretical detection limit of 65.0 nε. Since modification of the sensor region such as creating a micro air-gap allows the sensors to have high sensitivity, they fabricated highly sensitive strain sensors for the measurement of strain inside prestressed concrete. Another phenomenon is encapsulation of sensor region to improve their durability and lifetime in the harsh environments. In our study, the FLRD sensors were used without any modification such as etching, tapering, coating or creation of air gap/cavity to keep the system setup as simple and robust as possible, to eliminate extra work on modification of the sensor head, to reduce the cost of the system, and to minimize the precise working effort. On the other hand, if the sensor head is delicate then it may require coating with a protective layer to improve durability and sensitivity. In this case, strain transfer analysis will play
2. Experimental 2.1. The FLRD system Fig. 1 shows the schematic illustration of the FLRD system setup with stretching point from its equilibrium position (dashed line) at the midpoint. The FLRD system setup is composed of a pulsed laser (Cobolt Tango), a fiber coupled lens, a single mode fiber loop (30.0 ± 0.2 m), a 99:1 coupler, an isolator (Lightel), a photodiode (EOT 3010, DET08CFC, Electro-Optics Technology), and an oscilloscope (Tektronix MSO 4104). The light source of the system is a diode-pumped solid state laser (DSSL) [28] at a fixed central wavelength of 1534.0 nm. The laser source produces 4.0 ns pulses of 4.0 nm pulse widths and operates at a repetition rate of 3.0 kHz. Generated laser pulses are transferred from free space into the fiber via a fiber-coupled lens. Only 1.0% of the 2
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
following equation [14] as
coupled laser pulse energy is transferred into the loop by the 99:1 coupler and the rest, 99.0% portion, is separated by the coupler to be used as an incoming light for a new sensor. Such an extra energy would be very useful for sensor systems in different configuration, i.e. parallel or series sensor network setups when needed. Once the injected laser pulse passes through the sensor region at each round trip, 1.0% of the travelling light is sent to a photodiode by an isolator for recording signal level at each round trip. The rest, 99.0% of the injected light is trapped in the fiber-loop cavity resulting in multiple passes through the sample until it decays. Another advantage of using isolator in the setup is to minimize the back reflection off of the collimator reaching to the photodiode. The length of the fiber loop and the setup of the system were designed carefully to eliminate the overlap of the laser pulses in the loop and/or on the detector. The main point to be focused is that the coupled laser pulse must decay inside the loop before the next one enters the loop. The FLRD spectrometer system monitors decay of a laser pulse in the loop due to internal (system components) and external (absorption/ scattering and/or pressure/strain) losses while the coupled laser pulse experiences multiple passes in the loop. Ringdown time (RDT, τ0) of the loop without any stress on the sensor head depends on the intrinsic fiber loss, the coupler/isolator losses, and the splice loss. Every data set is collected with 512 averages at a 50.0 Ω terminated input of the oscilloscope with 10.0 mV voltage scale. The setting parameters of the oscilloscope were optimized to minimize the effect of the oscilloscope on the system noise level. The sensor region was created between two teflon discs, in which the distance between the discs determined the sensor head length. In this study, both a lengthy 80.0 cm and a short 10.0 cm sensor regions were tested. The fiber was strongly wrapped around the discs and subsequently glued in order to prevent fiber sliding or at least minimize the loosening of the sensor head due to the pull. The RDT was measured before and after the wrapping around the discs and an ignorable decrease on RDT of the loop was observed due to the bending of fiber on the discs. The fiber movement or sliding on the discs during forced extension was also carefully monitored by marking the fiber and the discs from several points. No observable slide of the fiber was noticed during measurements while the sensor region was pulled from various points. Before each experiment, the system was thoroughly characterized and the RDTs were recorded for several minutes. Then, the sensor region was stretched for the initial measurements at the midpoint with the lab-made scaled puller perpendicular to the initial position of the sensor head. As the fiber sensor region was being pulled, an amount of bending loss was created, which cause decreases in the RDT. The fiber was stretched up to 5.5 cm with steps of 0.5 cm for 80.0 cm sensor region. The same procedure was repeated for 10.0 cm sensor head, though it could only be safely stretched up to 1.0 cm before fiber damage starts and finally breaks. In addition to the midpoint, the 80.0 cm sensor head was also stretched from three different off-center positions; 10.0 cm, 20.0 cm and 30.0 cm away from one of the fixed end. Stretching from off-center positions increased the optical loss and a faster decay was observed compared to the midpoint. The bending theory in SMF at 1550 nm is well explained in references [29,30]. Calculation of the sensitivity of sensors due to the elongation and the strain of fiber from bending points is explained in details in references [2,25,28]. Strain sensitivities (ε) were calculated by ΔL/L, in which ΔL is the extension length when the sensor head is stretched and L is the sensor head length before any extension.
I = I0 e−(Act / nL)
(1)
where A is the total fiber transmission loss of the light per round trip, c is the light speed under vacuum, n is the average refractive index, and L is the total length of the fiber loop. The time it takes for the intensity to decrease from Io to Io/e is called ringdown time (RDT), τ0, and is given by
τ0 =
nL cA
(2)
When an activity occurs on the sensor head such as strain, it results an additional optical loss, B, causing a change in RDT, τ. Additional optical loss B is given by
B=
nL ⎛ 1 1 − ⎞. c ⎝τ τ0 ⎠ ⎜
⎟
(3)
The minimum detectable optical loss can be calculated as following:
Bmin =
tr ⎛ σ ⎞ 1 σ = τ0 ⎝ τave ⎠ m τave ⎜
⎟
(4)
where tr is the round trip time of the laser pulse inside the loop, m is the round number, and σ is the standard deviation, and σ/τave corresponds to the baseline stability of the data. The minimum detectable RDT (τmin ) can be found by using the baseline stability and the sensor RDT without any strain. In this study, the round trip time for 30 m loop was obtained as 144.5 ns and the round number was found to be 5. Further details on the baseline stability calculations can be found in references [28,31–33]. 3. Results and discussion Fig. 2 shows the change in RDTs when 80.0 cm and 10.0 cm sensor heads were stretched at their midpoints from their initial flat positions up to 5.5 cm and 1.0 cm, respectively, with steps of 0.5 cm. Once a safe maximum stretching position was reached, the fiber was released back to the initial condition with the same step size. Measurements were repeated several times and the average data of all measurements are presented in Fig. 2 where each data point in the figure represents an average of 20 data. For the 80.0 cm sensor head, it was noticed that an observable change in RDT starts after 1.5 cm pull of the sensor head due to the fiber elasticity and/or any possible tolerances around the discs. Initial RDTs, τ0, of the unstretched sensor regions were measured as 772.0 ± 1.0 ns (corresponding to 0.899 ± 0.0012 dB loss) for 80.0 cm and 713.0 ± 1.0 ns (corresponding to 0.983 ± 0.0014 dB loss) for 10.0 cm sensor heads. Each loop had different RDT due to the differences in loop lengths and the total optical losses. The differences in loop lengths occur as the fabrication of new sensor regions results a change due to the cut and the splice processes. The minimum detectable loss, Bmin, for 30.0 ± 0.2 m loop used in our study was calculated by using Eq. (4) as 376 μdB with 80 cm sensor region. Fig. 3 shows linear fittings of the responses of sensors in Fig. 2 up to the maximum stretching distance of the sensor heads. For 80 cm sensor system, there were two distinct regions where linear changes in RDT were observed with the change in strain. For the sensor unit with the sensor head of 80.0 cm, the detection limit was calculated as 5.99 με with the standard deviation of 1.70 ns. The strain sensitivity was determined as 0.28 ns/με from the slope of the curve in the second linear region in Fig. 3(a). Even though the change in RDT seems very small at the beginning of stretching process up to 2.5 cm, the inset graph in Fig. 3(a) shows another linear change and a possible exponential transition between the two linear regions. For the sensor unit with the sensor head of 10.0 cm, the detection limit was calculated as 59.9 με with the standard deviation of 1.55 ns. The strain sensitivity of 10.0 cm sensor head was obtained as 0.025 ns/με from the slope in Fig. 3(b).
2.2. Working principle Once a laser pulse is coupled into the fiber loop, the pulse travels many round trips in the loop until it diminishes due to the optical losses at each round trip. In each round, a small portion of the light is captured by the photodetector. The pulse intensity (I) relative to the incident light intensity (I0) varies with the losses according to the 3
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Fig. 2. Ringdown times versus the extension of the fiber when a) 80.0 cm sensor head is stretched up to 5.5 cm, b) 10.0 cm sensor head is stretched up to 1.0 cm from their midpoint with steps of 0.5 cm and they are returned to their initial positions slowly.
measurable level. Initially, the fiber was wrapped around the discs as tightly as possible. Stretching the sensor head step by step might lead to a small change in RDT due to an unexpected further tightening around the discs, especially at the first pull. In addition, pulling of the fiber at the sensor head created a bending region. When the bending radius is greater than 15.0 mm, the optical loss is expected to be negligibly small [29,30]. Initially, the bending radius was quite high (much greater than 15 mm) and was not expected to affect the RDTs. As the sensor region was being pulled, a strong bending was introduced to the fiber that dramatically increased the optical loss of the fiber, resulting in a faster decrease in RDT. This transition was observed at the critical length/ region where the transition from the initial linear region to the second linear response region occurred (Fig. 5.a). In the study by Zendehnam et al. [29], when the bending radius was less than 7.0 mm, a faster increase in the optical loss was observed. In our measurements, we believed that the dominant optical loss mainly came from the bending of the fiber because the optical loss due to the bending is much greater than the optical loss due to the longitudinal extension. For example, the initial RDT was 730.4 ns for the sensor before the stretching started at 10.0 cm away from the fixed end. The next recorded RDT was 726.2 ns when it was stretched by 1.0 cm. The difference shows an additional loss of 1.14 × 10−3 dB, which was higher than the minimum detectable optical loss, Bmin. On the other hand, the differences in RDTs between the initial position and the one at 1.0 cm stretched case became less than the ones stretched at 20.0 cm, 30.0 cm and the midpoint. For those cases, the difference above the minimum detectable level started at 1.5 cm or longer stretching lengths. The RDT in equilibrium position when the sensor head was stretched at 20.0 cm was recorded as 728.4 ns. In this stretch point, the
This difference in the strain sensitivities between 80.0 cm and 10.0 cm sensor heads may be attributed to the curvature radius change during stretching. For details in the relation between curvature radius and strain sensitivity one may read the study by Qiu et al. [19]. The baseline stability is generally used to obtain the minimum measurable RDT from two separate data points [28,31–33]. The baseline stability for 80.0 cm sensor head shown in Fig. 4.a, σ/τave , was calculated over 100 consecutive data as 0.22%. Theoretically, minimum detectable RDT between two adjacent data, τmin, for the loop with 80.0 cm sensor head was calculated as 1.55 ns (τmin = (σ/τave).τ0 [32]). On the other hand, the baseline stability of 10.0 cm sensor head shown in Fig. 4.b was calculated over 100 data point as 0.24%. The results show that the baseline stability is consistent even though sensor head length decreased 8 times and RDTs of two sensors were different. Fig. 5 shows the data set recorded while 80.0 cm sensor head was stretched from four different positions. Here, each data point at each extension distance is an average of at least 20 data sets. In general, we have observed similar behaviors for all four cases regardless of the pull point. Up to a certain point, the decrease in RDT was very small but detectable. However, beyond a critical point the loss became very significant. The change in RDT became easily observable and somewhat linearly dependent to the length. This critical length was the shortest for the closest one to the disc, the fixing point. It increased as the stretching point changed from the disc and became the longest for the midpoint one. In addition, the maximum extension length was dependent of the stretching point as expected. The sensor head could only be extended 1.0 cm before the fiber was deformed when it was being pulled at 10.0 cm away from the disc. The initial region appears to be almost flat; however, a closer look showed that the decrease in ringdown times with extension was significant and above the minimum
Fig. 3. Linear fitting of the responses of a) 80.0 cm and b) 10.0 cm long sensor heads. 4
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Fig. 4. Baseline stabilities for the sensors of a) 80.0 cm and b) 10.0 cm sensor heads.
toward the fixed end (the disc). It proves that the optical loss was much faster at locations closer to the fixed end as the fiber was stretched. Similarly, the slopes (value of the parameter dx) increased from 0.313 to 0.623 as the stretching point moved toward the center. Total losses and unit losses were also calculated when sensor heads were pulled to the maximum extension distances and presented in Table 1. The unit loss was calculated as the total loss of the loop divided by the extension distance. The decrease in total loss until 30 cm extension position and then the increase in the midpoint extension may be attributed to an asymmetric bending of the sensor heads until 40 cm extension position, then equally bending when the extension position was at the mid-point, 40 cm, of the sensor head. Interestingly an exponential increase in the slopes of the second linear regions were observed as the stress position moved away from the fixed end (Fig. 6.a). The results also showed that the optical loss was much faster at the locations closer to the fixed end. When the midpoint was taken as a reference point, the change in slope enables a prediction of the stress point for a system that was characterized in advance. This can be utilized to predict the positions on the sensor of many type of action such as land sliding, crack in concrete, etc. without using any additional device or component. Fig. 6.b presents the change in RDT relative to the estimated losses due to the fiber extension at the midpoint from 0 to 5.5 cm. Here, the increased strain on the fiber and bending of the fiber as the fiber was stressed resulted in an increased optical loss. The results show an exponential behavior of increased sensitivity of the sensor to the strain as it is stretched. In Fig. 6.b, both the RDT and the estimated loss changes are presented respect to the extension distance.
optical loss between equilibrium position and 1.0 cm stretched position was obtained as 2.72 × 10−5 dB which was lower than Bmin. Therefore, 1.0 cm extension can be attributed to the elasticity and strengthening of the fiber. However, the optical loss between the equilibrium and 1.5 cm extended positions was 1.25 × 10−3 dB, which was higher than Bmin. After 3.0 cm extension, a fast decrease in RDT values was observed due to the high optical loss, mainly bending losses. Beyond the maximum extension distance of 4.5 cm, the fiber was broken. The RDT for the sensor head for stretching at 30.0 cm was recorded as 730.2 ns in equilibrium position. The observed behavior was similar to the 10 cm and 20 cm cases. When the sensor head was stretched 1.0 cm, RDT change between steps was very small but became greater than Bmin as the fiber was stretched. Beyond 5.0 cm extension, the fiber was broken. The RDT of the sensor head for stretching at the midpoint was recorded as 751.5 ns in equilibrium and in 1.0 cm extended positions, but as soon as the sensor head was stretched to 1.5 cm, the RDT dropped to 746.4 ns, revealing an optical loss of 1.31 × 10−3 dB. Beyond the maximum extension position of 5.5 cm, the fiber was broken. The data have been fit to two different functions; i) a combination of two linear lines, a piecewise function (Fig. 5.a) and ii) a Boltzmann function producing a sigmoidal curve (Fig. 5.b). The equations are given in the figures and the parameters are listed in Table 1. Both are very informative and can be used to deduce information about the behavior of the system. Though both fittings were very good, the Boltzmann fitting was much better especially for the 30 and 40 cm cases with R values of almost 1. A significant observation in Fig. 5.a is that the absolute value of slope (second one) increased as the extension (contact) location of the sensor head moved away from the midpoint
Fig. 5. Ringdown times of 80 cm sensor head versus extension length as the fiber was being pulled from 10 cm, 20 cm, 30 cm and 40 cm (midpoint) from one of the fixed ends. a) Piecewise linear fit and b) Boltzmann fit of the data. The equations for piecewise linear and Boltzmann functions are given within the figures. 5
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Table 1 Extension positions on 80.0 cm sensor head, fitting slopes (Fig. 5), the total loss when the sensor heads were pulled to the maximum extendable length, and the loss per unit extension. Extension Position (cm)
10 20 30 40
Linear Fit
Boltzmann
Slope 1 (ns/cm)
Slope 2 (ns/cm)
R
X0
Slope (dx)
R
−4.2 −10.2 −6.5 −14.5
−349 −253 −198 −194
0.994 0.997 0.989 0.997
2.294 3.792 4.277 4.464
0.313 0.521 0.534 0.623
0.998 0.994 0.999 0.999
Total Loss (dB)
Unit Loss (dB/cm)
0.534 0.402 0.280 0.342
0.1780 0.0893 0.0560 0.0622
The strain sensor produced in this work has detection limit in με levels and a high sensor baseline stability as 0.22%.
This relatively cheap and fairly simple FLRD sensor appeared to have comparable properties to the ones reported in the literature. The strain sensor fabricated by Ghimire and Wang [2] has a detection limit of 65.0 nε with the baseline stability of 0.4%. This high strain detection limit was obtained with integration of an air-gap into the sensor head by inserting fibers into the ceramic ferrules that were coupled by ceramic sleeve. Another strain sensor fabricated by Qiu et al. [19] has a baseline stability of 0.8%, the standard deviation of 2.12 ns, and strain sensitivities of 0.35 ns/με and 0.17 ns/με for a low (4.0 cm) and a high (9.0 cm) curvature radii. Also in this study, a fiber mode converter as an additional component is required to stabilize propagating modes inside the fiber since a multimode fiber was used. In a recent study, Liu et al. [20] fabricated a high sensitivity strain sensor based on the FPI by splicing two sections of single mode fibers with an air bubble and then tapering the joint section. The sensitivity of the strain sensor was achieved as 43.0 pm/µε with a cavity length of 61.0 µm. To obtain very high strain sensitivity, they fabricated an air-cavity based FPI. Such a study needs very precise work and high effort to establish an air cavity between the fibers. Most recently, Ghimire et al. [25] fabricated an FLRD strain sensor by integrating a micro air gap in the sensor head for monitoring prestressed concrete. They obtained highly sensitive strain steps of 1.25 µε, leading to a theoretical detection limit of 65.0 nε due to fabrication of the sensor head by creating a micrometer-sized air-gap between two fiber ends. Afterwards, the sensor head region was protected by using ceramic ferrules and ferrule sleeve. By this way, they measured the strain by the change in the air-gap size. Overall, the above mentioned fiber optic strain sensors have high sensitivities, but some of them requires special optical fibers, needs costly micro-machining equipment, and/or precise and diligent treatment. Compared to the literature, the strain sensor fabricated in this study has unique advantages such as better baseline stability, no delicate design of air gap, and no need for any additional optical devices or fiber components. As a result, better baseline stability allows measuring strain change with a quite high sensitivity. In addition, a simple follow up of the change in the RDT in time can reveal the location of the stress.
4. Conclusion The sensors were fabricated for the purpose of strain measurement by the FLRD technique with high sensitivity. The sensor heads of 80.0 cm and 10.0 cm in lengths were stretched up to 5.5 cm and 1.0 cm, respectively, and then released to the initial equilibrium positions with the steps of 0.5 cm. The minimum baseline stability, which is a critical parameter to obtain the minimum detectable change in the ringdown time of sensor, was obtained as 0.22%. To the best of our knowledge this is the lowest baseline stability obtained by an FLRD spectrometer with a simple system setup when compared with its counterparts. The sensor heads deform and eventually break when they were stretched beyond the maximum stretch points. Strain sensitivities for 80.0 cm and 10.0 cm sensor heads were obtained as 5.99 με and 65.7 με, respectively. Besides stretching the sensor head from the midpoint, 80.0 cm sensor head was stretched from three other positions; 10.0 cm, 20.0 cm, and 30.0 cm from one of the fixed end. The behaviors were similar, though the sensor showed a faster response when it was stretched closer to the one fixed end. Regardless of the stretching points, the sensors initially showed slow but linear changes in RDTs. After a critical point, the change in RDT became dramatic but still linearly proportional to the stretching length with a larger slope compare to the initial part thus allowing a prediction on the applied strain location when needed. The observed change in slopes of RDTs were exponential. This can enable detection of the stress position without a need for a complicated system. The observed strain sensitivities of these FLRD sensors in comparison to the ones reported in the literature showed that FLRD sensors without any additional optical components such as FBG, FPI and OSA or any special regions, i.e. creating air-gap, also have high potential to be employed for the early detection in several applications such as land sliding, structural health monitoring, transportation, railways, etc. to detect strain in real time.
Fig. 6. a) The linear fit slope (slope 2) versus the extension positions of the sensor heads from the fixed end and b) ringdown time versus estimated loss as the fiber was stretched from 0 to 5.5 cm at the midpoint. 6
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Declaration of Competing Interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17]
[18]
Acknowledgement
[19]
This project is supported by Middle East Technical University (METU) research funding.
[20]
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[1] W. Zhou, W.C. Wong, C.C. Chan, L.-Y. Shao, X. Dong, Highly sensitive fiber loop ringdown strain sensor using photonic crystal fiber interferometer, Appl. Opt. 50 (2011) 3087–3092. [2] M. Ghimire, C. Wang, Highly sensitive fiber loop ringdown strain sensor with low temperature sensitivity, Meas. Sci. Technol. 28 (2017) 105101. [3] J.V. Ittiarah, S. Sidhik, T.K. Gangopadhyay, Refractometry using evanescent field based fiber loop ringdown spectroscopy, Sens. Actuators A Phys. 223 (2015) 61–66. [4] C. Wang, C. Herath, High-sensitivity fiber-loop ringdown refractive index sensors using single-mode fiber, Opt. Lett. 35 (2010) 1629–1631. [5] N. Ni, C.C. Chan, L. Xia, P. Shum, Fiber cavity ring-down refractive index sensor, IEEE Photonics Technology Letters 20 (2008) 1351–1353. [6] C. Wang, Fiber ringdown temperature sensors, Opt. Eng. 44 (2005) 030503. [7] G.B. Hocker, Fiber-optic sensing of pressure and temperature, Appl. Opt. 18 (1979) 1445–1448. [8] C. Wang, S. Scherrer, Fiber ringdown pressure sensors, Optics Letters 29 (2004) 352–354. [9] H. Qiu, Y. Qiu, Z. Chen, B. Fu, X. Chen, G. Li, Multimode fiber ring-down pressure sensor, Microw. Opt. Technol. Lett. 49 (2007) 1698–1700. [10] A. Yolalmaz, F.H. Sadroud, M.F. Danışman, O. Esenturk, Intracavity gas detection with fiber loop ring down spectroscopy, Opt. Commun. 396 (2017) 141–145. [11] T.K. Gangopadhyay, A. Giorgini, A. Halder, M. Pal, M.C. Paul, S. Avino, G. Gagliardi, Detection of chemicals using a novel fiber-optic sensor element built in fiber loop ring-resonators, Sens. Actuators B Chem. 206 (2015) 327–335. [12] M. Kaya, C. Wang, Detection of trace elements in DI water and comparison of several water solutions by using EF-FLRD chemical sensors, AIP Conf. Proc. 1809 (2017) 020027. [13] C. Herath, C. Wang, M. Kaya, D. Chevalier, Fiber loop ringdown DNA and bacteria sensors, J. Biomed. Opt. 16 (2011) 050501. [14] C. Wang, M. Kaya, C. Wang, Evanescent field-fiber loop ringdown glucose sensor, J. Biomed. Opt. 17 (2012) 037004. [15] S. Seridevi, K.S. Vasu, S. Asokan, A.K. Sood, Enhanced strain and temperature
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7
sensing by reduced graphene oxide coated etched fiber Bragg gratings, Opt. Lett. 41 (2016) 2604–2607. J. Greenwood, G. Dobre, Optical pressure sensor for an aeronautical application using white light interferometry, Proc. SPIE 4075 (2000) 94–100. C. Li, Y. Zhang, H. Liu, S. Wu, C. Huang, Distributed fiber-optic bi-directional straindisplacement sensor modulated by fiber bending loss, Sens. Actuators A: Phys. 111 (2004) 236–239. Y.P. Wang, L. Xiao, D.N. Wang, W. Jin, Highly sensitive long-period fiber-grating strain sensor with low temperature sensitivity, Opt. Lett. 31 (2006) 3414–3416. H. Qiu, Y. Qiu, Z. Chen, B. Fu, G. Li, Strain measurement by fiber-loop ring-down spectroscopy and fiber mode converter, IEEE Sensors J. 8 (2008) 1180–1183. S. Liu, K. Yang, Y. Wang, J. Qu, C. Liao, J. He, Z. Li, G. Yi, B. Sun, J. Zhou, G. Wang, J. Tang, J. Zhao, High-sensitivity strain sensor based on in-fiber rectangular air bubble, Sci. Rep. 5 (2015) 7624/1-7. M. Lydon, S.E. Taylor, D. Robinson, P. Callender, C. Dohetry, S.K.T. Grattan, E.J. Obrien, Development of a bridge weigh-in-motion sensor: Performance comparison using fiber optic and electric resistance strain sensor systems, IEEE Sensors J. 14 (2014) 4284–4296. S. Liu, Y. Wang, C. Liao, G. Wang, Z. Li, Q. Wang, J. Zhou, K. Yang, X. Zhong, J. Zhao, J. Tang, High-sensitivity strain sensor based on in-fiber improved FabryPerot interferometer, Opt. Lett. 39 (2014) 2121–2124. P.B. Tarsa, D.M. Brzozowski, P. Rabinowitz, K.K. Lehmann, Cavity ringdown strain gauge, Opt. Lett. 29 (2004) 1339–1341. J. Hu, H. Huang, M. Bai, T.T. Zhan, Z. Yang, Y. Yu, B. Qu, A high sensitive fiberoptic strain sensor with tunable temperature sensitivity for temperature-compensation measurement, Sci. Rep. 7 (2017) 42430/1-8. M. Ghimire, C. Wang, K. Dixon, M. Serrato, In situ monitoring of prestressed concrete using embedded fiber loop ringdown strain sensor, Measurement 124 (2018) 224–232. H. Wang, L. Jiang, P. Xiang, Improving the durability of the optical fiber sensor based on strain transfer analysis, Opt. Fiber Technol. 42 (2018) 97–104. H. Wang, P. Xiang, L. Jiang, Optical fiber sensor based in-field structural performance monitoring of multilayered asphalt pavement, J. Lightwave Technol. 36 (2018) 3624–3632. B. Cengiz, Fiber Loop Ring Down Spectroscopy for Trace Chemical Detection, MS Thesis Middle East Technology University, Ankara, Turkey, 2013. A. Zendehnam, M. Mirzaei, A. Farashiani, L.H. Farahani, Investigation of bending loss in a single-mode optical fibre, Pramana J. Phys. 74 (2010) 591–603. L. Faustini, G. Martini, Bend loss in single-mode fibers, J. Lightwave Technol. 15 (1997) 671–679. C. Wang, C. Herath, Fabrication and characterization of fiber loop ringdown evanescent field sensors, Meas. Sci. Technol. 21 (2010) 085205/1-10. P. Sahay, M. Kaya, C. Wang, Fiber loop ringdown sensor for potential real-time monitoring of cracks in concrete structures: an exploratory study, Sensors 13 (2013) 39–57. C.E. Costa, C.R.X. Cançado, Stability check: a program for calculating the stability of behavior, Mexican J. Behav. Anal. 38 (2012) 61–71.
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Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Study of strain measurement by fiber optic sensors with a sensitive fiber loop ringdown spectrometer
T
⁎
M. Kayaa, , O. Esenturkb a b
Eskisehir Osmangazi University, Eskisehir 26480, Turkey Middle East Technical University, Ankara 06800, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber loop ringdown Strain sensor Fiber optic sensors FLRD spectrometer
A sensitive fiber loop ringdown (FLRD) spectrometer without any additional optical component was utilized to obtain strain measurement on a single mode fiber optic sensor. The strain data were obtained by employing the theory of bending loss in single mode fibers. The best sensitivity of the sensors was obtained as 5.99 με with an 80.0 cm long sensor head when the sensor heads were stretched at the midpoint. The spectrometer system had a baseline-stability of 0.22%. Stretching the sensor head from off-midpoint positions resulted faster decays with higher optical losses. Comparison of the slopes relative to the stress positions showed that it may be utilized to obtain strain location without using any high-cost equipment. This portable, basic, and simpler FLRD spectrometer system offers high sensitivity with great baseline stability without utilization of any additional optical components and/or creating air-gap/air-cavity and encapsulation on the sensor head region. With its very attractive features of such as easy setup, low cost, and simple design, sensitive FLRD sensors may have a high potential for early detection in several applications such as structural health monitoring, biomedical sensing, mining, transportation and rail applications for continuous monitoring in real-time.
1. Introduction
sensitivity. The FBG and FPI sensors are the mostly used and commercialized strain sensors based on the phase or the wavelength shifts; however, the setup employs relatively expensive components to obtain high sensitive signals [20–22]. On the other hand, creating sensors by using the single mode fiber (SMF) itself in the FLRD spectroscopy is an alternative way to the FBG and the FPI to achieve high strain sensitivity with comparably much lower overall system costs with a simpler design. The FLRD technique offers many advantages such as enhanced detection with its multi-pass nature of a laser beam inside the fiber loop, simplicity, low cost, and portable network system setup. For the strain studies, Tarsa et al. [23] reported a ringdown spectroscopy system based on the biconical tapered single mode fiber as a sensor region. They demonstrated 74.0 nm minimum detectable displacement over 10 mm taper length, corresponding to 7.4 με strain sensitivity. However, a very short measurement region and very thin taper region makes it highly fragile, hence requires very gentle handling. Consequently, this restriction brings limitation for its application in many fields. Qiu et al. [19] demonstrated an application of a fiber ringdown strain sensor based on the multimode fiber loop ringdown spectroscopy and reported strain sensitivities of 0.35 ns/με and 0.17 ns/ με for 4.0 cm and 9.0 cm curvature radii, respectively. The baseline stability was obtained as 0.8% by considering the standard deviation of
Since the last three decades, fiber optic sensors have attracted researchers and found wide application areas due to their small size, lightweight, high sensitivity, low data loss, immunity to electromagnetic radiations, and their possibility for long distance applications or remote sensing. The Fiber Loop Ringdown (FLRD) spectroscopy is a multi-functional fiber optic measurement technique that is evolved from the Cavity Ringdown Spectroscopy (CRDS). While the space, a cavity length between two reflective mirrors is used as a waveguide for a laser beam in CRDS, an optical fiber loop is employed for the same purpose in the FLRD technique. The FLRD spectroscopy is a time-domain technique due to the fact that a portion of a laser pulse is trapped into a fiber loop for multiple interactions with the sample resulting increased sensitivity and stability of the measurement. The technique has a broad application field such as measurement of strain [1,2], refractive index [3–5], temperature [6,7], pressure [8,9], chemicals [10–12] and biological species [13,14]. One of the highly employed type of fiber optic sensors is the strain sensor. Many different types of strain sensors such as Fiber Bragg Grating (FBG) [15], Fabry-Perot Interferometer (FPI) [16], fiber bending attenuation [17], Long-Period Grating (LPG) [18] and fiber loop [2,19] were studied to obtain the best ⁎
Corresponding author. E-mail address: [email protected] (M. Kaya).
https://doi.org/10.1016/j.yofte.2019.102070 Received 21 July 2019; Received in revised form 14 October 2019; Accepted 1 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Fig. 1. Schematic illustration of the fiber loop ringdown system setup.
an important role. Internal mechanism of embedded optical fibers was explored by Wang et al. based on strain transfer analysis [26]. In the study, they provided theoretical basis of interfacial bonding mechanism of the embedded optical fiber sensors by analyzing interfacial debonding between embedded sensor and the host material and explained well by strain transfer theory. In another study [27], they have fabricated distributed and quasi-distributed optical fiber sensors and FBG sensors to monitor the three dimensional deformation of an asphalt pavement by placing sensors in different positions and layers of the pavement. They analyzed three dimensional strain and displacement of the asphalt pavement under traffic loads and environmental temperature variations. In this study, we demonstrate a simple design, easy setup, low cost, and fast response strain sensor by using a sensitive FLRD spectrometer. The sensor system composed of only basic components: a laser source, a fiber loop, a photodetector and an oscilloscope. The main aim of this study is to exhibit a bare single mode strain sensor with a simple FLRD spectrometer system, enhanced sensitivity, low baseline stability, and without a need for any modification on the sensor head such as chemical or physical treatments. Obtained detection limit is independent of any constraint of optical instruments such as laser, oscilloscope, and does not need an optical spectrum analyzer, extra components such as FBG/FPI and/or an extra effort and precise work to create special sensor region such as air-gap/air-cavity. The results suggest that such strain sensors utilizing enhanced FLRD spectrometer have high potential to be employed for the early detection in the real world applications such as land sliding, erosions, structural health monitoring, biomedical applications and crack detection with its high sensitivity.
2.12 ns and the average ringdown time of 262.90 ns. Since they used a multimode fiber, a fiber mode converter as an additional device had to be used to stabilize propagating modes inside the fiber. Hu et al. [24] proposed intensity modulation fiber optic strain sensors consisting of an elastic cantilever, a tandem rod, and a collimator for temperaturecompensation measurement. They achieved a high strain detectionrange of 1.1 × 103 με for 21.5 mm sensing length and a detection limit of 5.7 × 10−3 με by using the advantage of the small beam divergence and the incident angle sensitive collimator, which was utilized for both outgoing and incoming light. The collimator detects scattered beam from the elastic cantilever whenever a strain occurs on the sample deflecting the cantilever. They employed a cantilever and a fiber collimator instead of an optical fiber in order to increase the working distance between the collimator and the cantilever. Therefore, the sensing limit depends on the collimator coupling efficiency. In another study, Zhou et al. [1] used a Photonic Crystal Fiber (PCF) MachZehnder interferometer (MZI) in a fiber loop and obtained the minimum detectable strain as 3.6 με. A PCF of 4.8 cm is spliced to a single mode fiber to fabricate MZI and the system setup includes additional components such as an Erbium-Doped Optical Fiber Amplifier (EDFA), a variable optical attenuator, an FBG, and an Optical Spectrum analyzer (OSA). Recently, Ghimire et al. [2] presented an FLRD-based strain sensor utilizing a micro-air gap as a sensing element and aligned two clean cut ends of an SMF in a ceramic ferrule. After coupling the light into the loop, the ferrule sleeves were employed for stabilization and rigidness of the sensor head. In this study, they succeeded to measure the minimum strain as 7.5 nε with a meter-long fiber. In the other study, Ghimire et al. [25] utilized a micro air-gap strain sensor to monitor the tension from prestressed concrete beams. The sensor head was embedded into concrete beams after assembling in a ceramic ferrule and a ferrule sleeve, revealing a measured strain steps of 1.25 µε and hence a theoretical detection limit of 65.0 nε. Since modification of the sensor region such as creating a micro air-gap allows the sensors to have high sensitivity, they fabricated highly sensitive strain sensors for the measurement of strain inside prestressed concrete. Another phenomenon is encapsulation of sensor region to improve their durability and lifetime in the harsh environments. In our study, the FLRD sensors were used without any modification such as etching, tapering, coating or creation of air gap/cavity to keep the system setup as simple and robust as possible, to eliminate extra work on modification of the sensor head, to reduce the cost of the system, and to minimize the precise working effort. On the other hand, if the sensor head is delicate then it may require coating with a protective layer to improve durability and sensitivity. In this case, strain transfer analysis will play
2. Experimental 2.1. The FLRD system Fig. 1 shows the schematic illustration of the FLRD system setup with stretching point from its equilibrium position (dashed line) at the midpoint. The FLRD system setup is composed of a pulsed laser (Cobolt Tango), a fiber coupled lens, a single mode fiber loop (30.0 ± 0.2 m), a 99:1 coupler, an isolator (Lightel), a photodiode (EOT 3010, DET08CFC, Electro-Optics Technology), and an oscilloscope (Tektronix MSO 4104). The light source of the system is a diode-pumped solid state laser (DSSL) [28] at a fixed central wavelength of 1534.0 nm. The laser source produces 4.0 ns pulses of 4.0 nm pulse widths and operates at a repetition rate of 3.0 kHz. Generated laser pulses are transferred from free space into the fiber via a fiber-coupled lens. Only 1.0% of the 2
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following equation [14] as
coupled laser pulse energy is transferred into the loop by the 99:1 coupler and the rest, 99.0% portion, is separated by the coupler to be used as an incoming light for a new sensor. Such an extra energy would be very useful for sensor systems in different configuration, i.e. parallel or series sensor network setups when needed. Once the injected laser pulse passes through the sensor region at each round trip, 1.0% of the travelling light is sent to a photodiode by an isolator for recording signal level at each round trip. The rest, 99.0% of the injected light is trapped in the fiber-loop cavity resulting in multiple passes through the sample until it decays. Another advantage of using isolator in the setup is to minimize the back reflection off of the collimator reaching to the photodiode. The length of the fiber loop and the setup of the system were designed carefully to eliminate the overlap of the laser pulses in the loop and/or on the detector. The main point to be focused is that the coupled laser pulse must decay inside the loop before the next one enters the loop. The FLRD spectrometer system monitors decay of a laser pulse in the loop due to internal (system components) and external (absorption/ scattering and/or pressure/strain) losses while the coupled laser pulse experiences multiple passes in the loop. Ringdown time (RDT, τ0) of the loop without any stress on the sensor head depends on the intrinsic fiber loss, the coupler/isolator losses, and the splice loss. Every data set is collected with 512 averages at a 50.0 Ω terminated input of the oscilloscope with 10.0 mV voltage scale. The setting parameters of the oscilloscope were optimized to minimize the effect of the oscilloscope on the system noise level. The sensor region was created between two teflon discs, in which the distance between the discs determined the sensor head length. In this study, both a lengthy 80.0 cm and a short 10.0 cm sensor regions were tested. The fiber was strongly wrapped around the discs and subsequently glued in order to prevent fiber sliding or at least minimize the loosening of the sensor head due to the pull. The RDT was measured before and after the wrapping around the discs and an ignorable decrease on RDT of the loop was observed due to the bending of fiber on the discs. The fiber movement or sliding on the discs during forced extension was also carefully monitored by marking the fiber and the discs from several points. No observable slide of the fiber was noticed during measurements while the sensor region was pulled from various points. Before each experiment, the system was thoroughly characterized and the RDTs were recorded for several minutes. Then, the sensor region was stretched for the initial measurements at the midpoint with the lab-made scaled puller perpendicular to the initial position of the sensor head. As the fiber sensor region was being pulled, an amount of bending loss was created, which cause decreases in the RDT. The fiber was stretched up to 5.5 cm with steps of 0.5 cm for 80.0 cm sensor region. The same procedure was repeated for 10.0 cm sensor head, though it could only be safely stretched up to 1.0 cm before fiber damage starts and finally breaks. In addition to the midpoint, the 80.0 cm sensor head was also stretched from three different off-center positions; 10.0 cm, 20.0 cm and 30.0 cm away from one of the fixed end. Stretching from off-center positions increased the optical loss and a faster decay was observed compared to the midpoint. The bending theory in SMF at 1550 nm is well explained in references [29,30]. Calculation of the sensitivity of sensors due to the elongation and the strain of fiber from bending points is explained in details in references [2,25,28]. Strain sensitivities (ε) were calculated by ΔL/L, in which ΔL is the extension length when the sensor head is stretched and L is the sensor head length before any extension.
I = I0 e−(Act / nL)
(1)
where A is the total fiber transmission loss of the light per round trip, c is the light speed under vacuum, n is the average refractive index, and L is the total length of the fiber loop. The time it takes for the intensity to decrease from Io to Io/e is called ringdown time (RDT), τ0, and is given by
τ0 =
nL cA
(2)
When an activity occurs on the sensor head such as strain, it results an additional optical loss, B, causing a change in RDT, τ. Additional optical loss B is given by
B=
nL ⎛ 1 1 − ⎞. c ⎝τ τ0 ⎠ ⎜
⎟
(3)
The minimum detectable optical loss can be calculated as following:
Bmin =
tr ⎛ σ ⎞ 1 σ = τ0 ⎝ τave ⎠ m τave ⎜
⎟
(4)
where tr is the round trip time of the laser pulse inside the loop, m is the round number, and σ is the standard deviation, and σ/τave corresponds to the baseline stability of the data. The minimum detectable RDT (τmin ) can be found by using the baseline stability and the sensor RDT without any strain. In this study, the round trip time for 30 m loop was obtained as 144.5 ns and the round number was found to be 5. Further details on the baseline stability calculations can be found in references [28,31–33]. 3. Results and discussion Fig. 2 shows the change in RDTs when 80.0 cm and 10.0 cm sensor heads were stretched at their midpoints from their initial flat positions up to 5.5 cm and 1.0 cm, respectively, with steps of 0.5 cm. Once a safe maximum stretching position was reached, the fiber was released back to the initial condition with the same step size. Measurements were repeated several times and the average data of all measurements are presented in Fig. 2 where each data point in the figure represents an average of 20 data. For the 80.0 cm sensor head, it was noticed that an observable change in RDT starts after 1.5 cm pull of the sensor head due to the fiber elasticity and/or any possible tolerances around the discs. Initial RDTs, τ0, of the unstretched sensor regions were measured as 772.0 ± 1.0 ns (corresponding to 0.899 ± 0.0012 dB loss) for 80.0 cm and 713.0 ± 1.0 ns (corresponding to 0.983 ± 0.0014 dB loss) for 10.0 cm sensor heads. Each loop had different RDT due to the differences in loop lengths and the total optical losses. The differences in loop lengths occur as the fabrication of new sensor regions results a change due to the cut and the splice processes. The minimum detectable loss, Bmin, for 30.0 ± 0.2 m loop used in our study was calculated by using Eq. (4) as 376 μdB with 80 cm sensor region. Fig. 3 shows linear fittings of the responses of sensors in Fig. 2 up to the maximum stretching distance of the sensor heads. For 80 cm sensor system, there were two distinct regions where linear changes in RDT were observed with the change in strain. For the sensor unit with the sensor head of 80.0 cm, the detection limit was calculated as 5.99 με with the standard deviation of 1.70 ns. The strain sensitivity was determined as 0.28 ns/με from the slope of the curve in the second linear region in Fig. 3(a). Even though the change in RDT seems very small at the beginning of stretching process up to 2.5 cm, the inset graph in Fig. 3(a) shows another linear change and a possible exponential transition between the two linear regions. For the sensor unit with the sensor head of 10.0 cm, the detection limit was calculated as 59.9 με with the standard deviation of 1.55 ns. The strain sensitivity of 10.0 cm sensor head was obtained as 0.025 ns/με from the slope in Fig. 3(b).
2.2. Working principle Once a laser pulse is coupled into the fiber loop, the pulse travels many round trips in the loop until it diminishes due to the optical losses at each round trip. In each round, a small portion of the light is captured by the photodetector. The pulse intensity (I) relative to the incident light intensity (I0) varies with the losses according to the 3
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Fig. 2. Ringdown times versus the extension of the fiber when a) 80.0 cm sensor head is stretched up to 5.5 cm, b) 10.0 cm sensor head is stretched up to 1.0 cm from their midpoint with steps of 0.5 cm and they are returned to their initial positions slowly.
measurable level. Initially, the fiber was wrapped around the discs as tightly as possible. Stretching the sensor head step by step might lead to a small change in RDT due to an unexpected further tightening around the discs, especially at the first pull. In addition, pulling of the fiber at the sensor head created a bending region. When the bending radius is greater than 15.0 mm, the optical loss is expected to be negligibly small [29,30]. Initially, the bending radius was quite high (much greater than 15 mm) and was not expected to affect the RDTs. As the sensor region was being pulled, a strong bending was introduced to the fiber that dramatically increased the optical loss of the fiber, resulting in a faster decrease in RDT. This transition was observed at the critical length/ region where the transition from the initial linear region to the second linear response region occurred (Fig. 5.a). In the study by Zendehnam et al. [29], when the bending radius was less than 7.0 mm, a faster increase in the optical loss was observed. In our measurements, we believed that the dominant optical loss mainly came from the bending of the fiber because the optical loss due to the bending is much greater than the optical loss due to the longitudinal extension. For example, the initial RDT was 730.4 ns for the sensor before the stretching started at 10.0 cm away from the fixed end. The next recorded RDT was 726.2 ns when it was stretched by 1.0 cm. The difference shows an additional loss of 1.14 × 10−3 dB, which was higher than the minimum detectable optical loss, Bmin. On the other hand, the differences in RDTs between the initial position and the one at 1.0 cm stretched case became less than the ones stretched at 20.0 cm, 30.0 cm and the midpoint. For those cases, the difference above the minimum detectable level started at 1.5 cm or longer stretching lengths. The RDT in equilibrium position when the sensor head was stretched at 20.0 cm was recorded as 728.4 ns. In this stretch point, the
This difference in the strain sensitivities between 80.0 cm and 10.0 cm sensor heads may be attributed to the curvature radius change during stretching. For details in the relation between curvature radius and strain sensitivity one may read the study by Qiu et al. [19]. The baseline stability is generally used to obtain the minimum measurable RDT from two separate data points [28,31–33]. The baseline stability for 80.0 cm sensor head shown in Fig. 4.a, σ/τave , was calculated over 100 consecutive data as 0.22%. Theoretically, minimum detectable RDT between two adjacent data, τmin, for the loop with 80.0 cm sensor head was calculated as 1.55 ns (τmin = (σ/τave).τ0 [32]). On the other hand, the baseline stability of 10.0 cm sensor head shown in Fig. 4.b was calculated over 100 data point as 0.24%. The results show that the baseline stability is consistent even though sensor head length decreased 8 times and RDTs of two sensors were different. Fig. 5 shows the data set recorded while 80.0 cm sensor head was stretched from four different positions. Here, each data point at each extension distance is an average of at least 20 data sets. In general, we have observed similar behaviors for all four cases regardless of the pull point. Up to a certain point, the decrease in RDT was very small but detectable. However, beyond a critical point the loss became very significant. The change in RDT became easily observable and somewhat linearly dependent to the length. This critical length was the shortest for the closest one to the disc, the fixing point. It increased as the stretching point changed from the disc and became the longest for the midpoint one. In addition, the maximum extension length was dependent of the stretching point as expected. The sensor head could only be extended 1.0 cm before the fiber was deformed when it was being pulled at 10.0 cm away from the disc. The initial region appears to be almost flat; however, a closer look showed that the decrease in ringdown times with extension was significant and above the minimum
Fig. 3. Linear fitting of the responses of a) 80.0 cm and b) 10.0 cm long sensor heads. 4
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Fig. 4. Baseline stabilities for the sensors of a) 80.0 cm and b) 10.0 cm sensor heads.
toward the fixed end (the disc). It proves that the optical loss was much faster at locations closer to the fixed end as the fiber was stretched. Similarly, the slopes (value of the parameter dx) increased from 0.313 to 0.623 as the stretching point moved toward the center. Total losses and unit losses were also calculated when sensor heads were pulled to the maximum extension distances and presented in Table 1. The unit loss was calculated as the total loss of the loop divided by the extension distance. The decrease in total loss until 30 cm extension position and then the increase in the midpoint extension may be attributed to an asymmetric bending of the sensor heads until 40 cm extension position, then equally bending when the extension position was at the mid-point, 40 cm, of the sensor head. Interestingly an exponential increase in the slopes of the second linear regions were observed as the stress position moved away from the fixed end (Fig. 6.a). The results also showed that the optical loss was much faster at the locations closer to the fixed end. When the midpoint was taken as a reference point, the change in slope enables a prediction of the stress point for a system that was characterized in advance. This can be utilized to predict the positions on the sensor of many type of action such as land sliding, crack in concrete, etc. without using any additional device or component. Fig. 6.b presents the change in RDT relative to the estimated losses due to the fiber extension at the midpoint from 0 to 5.5 cm. Here, the increased strain on the fiber and bending of the fiber as the fiber was stressed resulted in an increased optical loss. The results show an exponential behavior of increased sensitivity of the sensor to the strain as it is stretched. In Fig. 6.b, both the RDT and the estimated loss changes are presented respect to the extension distance.
optical loss between equilibrium position and 1.0 cm stretched position was obtained as 2.72 × 10−5 dB which was lower than Bmin. Therefore, 1.0 cm extension can be attributed to the elasticity and strengthening of the fiber. However, the optical loss between the equilibrium and 1.5 cm extended positions was 1.25 × 10−3 dB, which was higher than Bmin. After 3.0 cm extension, a fast decrease in RDT values was observed due to the high optical loss, mainly bending losses. Beyond the maximum extension distance of 4.5 cm, the fiber was broken. The RDT for the sensor head for stretching at 30.0 cm was recorded as 730.2 ns in equilibrium position. The observed behavior was similar to the 10 cm and 20 cm cases. When the sensor head was stretched 1.0 cm, RDT change between steps was very small but became greater than Bmin as the fiber was stretched. Beyond 5.0 cm extension, the fiber was broken. The RDT of the sensor head for stretching at the midpoint was recorded as 751.5 ns in equilibrium and in 1.0 cm extended positions, but as soon as the sensor head was stretched to 1.5 cm, the RDT dropped to 746.4 ns, revealing an optical loss of 1.31 × 10−3 dB. Beyond the maximum extension position of 5.5 cm, the fiber was broken. The data have been fit to two different functions; i) a combination of two linear lines, a piecewise function (Fig. 5.a) and ii) a Boltzmann function producing a sigmoidal curve (Fig. 5.b). The equations are given in the figures and the parameters are listed in Table 1. Both are very informative and can be used to deduce information about the behavior of the system. Though both fittings were very good, the Boltzmann fitting was much better especially for the 30 and 40 cm cases with R values of almost 1. A significant observation in Fig. 5.a is that the absolute value of slope (second one) increased as the extension (contact) location of the sensor head moved away from the midpoint
Fig. 5. Ringdown times of 80 cm sensor head versus extension length as the fiber was being pulled from 10 cm, 20 cm, 30 cm and 40 cm (midpoint) from one of the fixed ends. a) Piecewise linear fit and b) Boltzmann fit of the data. The equations for piecewise linear and Boltzmann functions are given within the figures. 5
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Table 1 Extension positions on 80.0 cm sensor head, fitting slopes (Fig. 5), the total loss when the sensor heads were pulled to the maximum extendable length, and the loss per unit extension. Extension Position (cm)
10 20 30 40
Linear Fit
Boltzmann
Slope 1 (ns/cm)
Slope 2 (ns/cm)
R
X0
Slope (dx)
R
−4.2 −10.2 −6.5 −14.5
−349 −253 −198 −194
0.994 0.997 0.989 0.997
2.294 3.792 4.277 4.464
0.313 0.521 0.534 0.623
0.998 0.994 0.999 0.999
Total Loss (dB)
Unit Loss (dB/cm)
0.534 0.402 0.280 0.342
0.1780 0.0893 0.0560 0.0622
The strain sensor produced in this work has detection limit in με levels and a high sensor baseline stability as 0.22%.
This relatively cheap and fairly simple FLRD sensor appeared to have comparable properties to the ones reported in the literature. The strain sensor fabricated by Ghimire and Wang [2] has a detection limit of 65.0 nε with the baseline stability of 0.4%. This high strain detection limit was obtained with integration of an air-gap into the sensor head by inserting fibers into the ceramic ferrules that were coupled by ceramic sleeve. Another strain sensor fabricated by Qiu et al. [19] has a baseline stability of 0.8%, the standard deviation of 2.12 ns, and strain sensitivities of 0.35 ns/με and 0.17 ns/με for a low (4.0 cm) and a high (9.0 cm) curvature radii. Also in this study, a fiber mode converter as an additional component is required to stabilize propagating modes inside the fiber since a multimode fiber was used. In a recent study, Liu et al. [20] fabricated a high sensitivity strain sensor based on the FPI by splicing two sections of single mode fibers with an air bubble and then tapering the joint section. The sensitivity of the strain sensor was achieved as 43.0 pm/µε with a cavity length of 61.0 µm. To obtain very high strain sensitivity, they fabricated an air-cavity based FPI. Such a study needs very precise work and high effort to establish an air cavity between the fibers. Most recently, Ghimire et al. [25] fabricated an FLRD strain sensor by integrating a micro air gap in the sensor head for monitoring prestressed concrete. They obtained highly sensitive strain steps of 1.25 µε, leading to a theoretical detection limit of 65.0 nε due to fabrication of the sensor head by creating a micrometer-sized air-gap between two fiber ends. Afterwards, the sensor head region was protected by using ceramic ferrules and ferrule sleeve. By this way, they measured the strain by the change in the air-gap size. Overall, the above mentioned fiber optic strain sensors have high sensitivities, but some of them requires special optical fibers, needs costly micro-machining equipment, and/or precise and diligent treatment. Compared to the literature, the strain sensor fabricated in this study has unique advantages such as better baseline stability, no delicate design of air gap, and no need for any additional optical devices or fiber components. As a result, better baseline stability allows measuring strain change with a quite high sensitivity. In addition, a simple follow up of the change in the RDT in time can reveal the location of the stress.
4. Conclusion The sensors were fabricated for the purpose of strain measurement by the FLRD technique with high sensitivity. The sensor heads of 80.0 cm and 10.0 cm in lengths were stretched up to 5.5 cm and 1.0 cm, respectively, and then released to the initial equilibrium positions with the steps of 0.5 cm. The minimum baseline stability, which is a critical parameter to obtain the minimum detectable change in the ringdown time of sensor, was obtained as 0.22%. To the best of our knowledge this is the lowest baseline stability obtained by an FLRD spectrometer with a simple system setup when compared with its counterparts. The sensor heads deform and eventually break when they were stretched beyond the maximum stretch points. Strain sensitivities for 80.0 cm and 10.0 cm sensor heads were obtained as 5.99 με and 65.7 με, respectively. Besides stretching the sensor head from the midpoint, 80.0 cm sensor head was stretched from three other positions; 10.0 cm, 20.0 cm, and 30.0 cm from one of the fixed end. The behaviors were similar, though the sensor showed a faster response when it was stretched closer to the one fixed end. Regardless of the stretching points, the sensors initially showed slow but linear changes in RDTs. After a critical point, the change in RDT became dramatic but still linearly proportional to the stretching length with a larger slope compare to the initial part thus allowing a prediction on the applied strain location when needed. The observed change in slopes of RDTs were exponential. This can enable detection of the stress position without a need for a complicated system. The observed strain sensitivities of these FLRD sensors in comparison to the ones reported in the literature showed that FLRD sensors without any additional optical components such as FBG, FPI and OSA or any special regions, i.e. creating air-gap, also have high potential to be employed for the early detection in several applications such as land sliding, structural health monitoring, transportation, railways, etc. to detect strain in real time.
Fig. 6. a) The linear fit slope (slope 2) versus the extension positions of the sensor heads from the fixed end and b) ringdown time versus estimated loss as the fiber was stretched from 0 to 5.5 cm at the midpoint. 6
Optical Fiber Technology 54 (2020) 102070
M. Kaya and O. Esenturk
Declaration of Competing Interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17]
[18]
Acknowledgement
[19]
This project is supported by Middle East Technical University (METU) research funding.
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