AN INEXPENSIVE TECHNIQUE TO FABRICATE HYBRID GLASS/PLASTIC OPTICAL FIBER SENSORS FOR STRUCTURAL HEALTH MONITORING

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1 4th International Conference on Earthquake Engineering Taipei, Taiwan October 1-13, 006 Paper No. 93 AN INEXPENSIVE TECHNIQUE TO FABRICATE HYBRID GLASS/PLASTIC OPTICAL FIBER SENSORS FOR STRUCTURAL HEALTH MONITORING Haiying Huang 1 and Shantilal Zanwar ABSTRACT Silica-based optical fiber sensors are widely used in structural health monitoring systems for strain and deflection measurement. One drawback of silica-based optical fiber sensors is their low strain toughness. In general, silica-based optical fiber sensors can only reliably measure strains up to %. Recently, polymer optical fiber sensors have been employed to measure large strain and deflection. Due to their high optical losses, the length of the polymer optical fiber sensors is limited to 100 meters. In this paper, we present a novel economical technique to fabricate hybrid glass/plastic optical fiber sensors. First, stress analysis of a surface-mounted optical fiber sensor is performed to understand the relationship between the mechanical properties of the fiber/host structure and the stress distributions among the fiber, the bonding material, and the host structure. Next, the concept of fabricating a polymer sensing element on the end face of an optical fiber using ultraviolet irradiation is explained. The experimental set-up and the components used in the fabrication process are described in details. Micrographic images of the fabricated polymer sensing elements are presented and the control of the fabrication parameters is discussed. Potential application of the presented technique to fabricate an intrinsic Fabry-Perot optical fiber sensor is also discussed. Keywords: Optical fiber sensor, plastic optical fiber, sensor fabrication, structural health monitoring INTRODUCTION Optical fiber sensors are the sensor of choice for strain and deflection measurement in structural health monitoring systems due to their light weight, small size, and immunity to electro-magnetic interference. These sensors can be either surface mounted on or embedded in large civil and mechanical structures to assess their reliability and functionality (Ansari 005, Friebele, et. al., 1999, Zhou and Sim, 00). Ideally, the surface mounted or embedded strain sensors should have a lower stiffness and higher strain fracture toughness than the host materials. However, most existing optic fiber sensors are fabricated from silica-based optical fibers and usually have large stiffness and can only sustain maximum strain of ~3-5%. In general, silica-based optical fiber strain sensors are only reliable up to around % strain unless special preparation procedures are followed. Due to this mismatch of mechanical properties between the optical fiber and the host material, sensor fracture and debonding before the host structure failure are commonly encountered in silica-based optical fiber structural health monitoring systems. To overcome this shortcoming of the silica-based optical fiber sensors, plastic optical fiber strain sensors are exploited for large strain measurement (Kiesel 006, 1 Assistant Professor, Mechanical and Aerospace Engineering, University of Texas Arlington, USA, huang@uta.edu Graduate Research Assistant, Mechanical Engineering Technology, Purdue University, USA

2 Kuang 00). However, the length of the plastic optical fiber is limited to 100 meters due to its high optical loss and its multimode nature. Therefore, it is not suitable for the structural health monitoring of large structures. In this paper, a novel technique to fabricate hybrid glass/plastic optical fiber sensors is presented. First, stress analysis of a surface-mounted optical fiber sensor is performed to understand the relationship between the mechanical properties of the fiber/host structure and the stress distributions among the optical fiber, the bonding material, and the host structure. Next, the concept of fabricating a polymer sensing element on the end face of an optical fiber using ultraviolet (UV) irradiation is explained. The experimental set-up and the components used in the fabrication process are described in details. Micrographic images of the fabricated polymer sensing elements are presented and the control of the fabrication parameters is discussed. Potential application of the presented technique to fabricate an intrinsic Fabry-Perot optical fiber sensor is also discussed. STRESS ANALYSIS OF A SURFACE-MOUNTED OPTICAL FIBER SENSOR When an optical fiber strain sensor is either surface mounted on or embedded in the host structure, it is required that the strain sensor should not change the stress fields of the host structure and faithfully reflect the strains experienced by the host structure. In addition, the strain sensor and the bond between the sensor and the host structure should maintain their integrity until the host structure fails. In this section, we will conduct a simple stress analysis to study how the mechanical properties of the optical fiber influence its tensile stresses and the shear stresses of the bond. As shown in Figure 1(a), an optical fiber with a Young s modulus E f and a diameter d is surface mounted on a host structure with a Young s modulus E h and a cross-section area A h. Assuming that the host structure and the optical fiber undergo the same strain ε when the host structure is subjected to an external load P, the stresses of the host structure and the optical fiber are and therefore σ h = E ε, σ = E ε Equation 1 h f f σ σ P = A + = E A + E ε. Equation h h f h h f 4 4 The strain ε can then be calculated as P P E f ε = = 1 + Equation Eh Ah Eh Ah E h Ah E f 4 E f Because is usually very small, the strain measured by the optical fiber sensor is Eh 4Ah approximately equal to the strain of the host structure when the strain sensor is not attached, i.e., P ε. Subsequently, the tensile stresses of the optical fiber sensor can be calculated as E h A h E f P σ = Equation 4 f Eh Ah Obviously, the smaller the ratio between the Young s moduli of the optical fiber and the host structure is, the smaller the tensile stresses of the optical fiber are. Therefore, it is advantageous to have an optical fiber with a smaller Young s modulus to reduce fiber breakage. In addition to fiber breakage, 1

3 we also need to consider the bonding integrity between the optical fiber sensor and the host structure. The shear stresses of the fiber/host bond are calculated from Figure 1 (b), i.e., E f P τ b Ab = σ f τ =, Equation 5 b 4 Eh Ah 4Ab where A b is the bonding area between the fiber and the bonding material. Again, the shear stresses at the bond are proportional to the ratio between the Young s moduli of the optical fiber and the host structure. As a result, reducing the Young s modulus of the optical fiber will reduce the shear stresses and therefore ensures better bonding. Unfortunately, most existing optical fiber sensors are made of silica glass and their mechanical properties can not be tailored to match that of the host structure. The advantage of a hybrid glass/polymer optical fiber sensor is that the sensing element is made of polymer and there is a large selection of polymers with different mechanical properties. In this paper, we will use a UV-curable epoxy adhesive (Norland NOA61) to fabricate the sensing element. Typical mechanical properties of the NOA61 are listed in Table 1, as well as the mechanical properties of several commonly used engineering materials (Budinski and Budinski, 004). Compared to other materials, NOA61 is much less stiff and can sustain a large strain before failure. Table 1: Comparison of the mechanical properties of NOA61 with other materials Young's Modulus (Gpa) Yield Strength (Mpa) Elongation (%) Tensile Strength (Mpa) NOA Silica 69 ~ PMMA* AL Steel *: PMMA is commonly used for plastic optical fibers PRINCIPE OF OPERATION The concept of fabricating a polymer sensing element on the end face of an optical fiber is illustrated in Figure. The polymer sensing element is fabricated by UV irradiation. UV light is first coupled into the fiber at one end and is transmitted along the optical fiber. The other end of the optical fiber is submerged into a pool of uncured photopolymer. As the UV light exits the fiber core, it irradiates the uncured photopolymer along its optical path. The photopolymer that is exposed to the UV light will be cured and can be served as a sensing element. Because the polymer sensing element is automatic aligned with the fiber core, the coupling loss between the optical fiber and the polymer sensing element is minimized. The parameters that control the shape of the fabricated sensing element include the numerical aperture (NA) of the optical fiber, the intensity pattern of the output UV light, and the refractive indices of the cured and uncured polymer. EXPERIMETAL SET-UP The experimental set-up to fabricate a polymer sensing element on the end face of an optical fiber is shown in Figure 3. First, an optical fiber is cleaved at both ends and mounted on a three-axis fiber alignment stage. The glass cover of a UV LED (Nichia NSHU550A, wavelength 375nm) is removed to expose the LED chip. The optical fiber is then manually aligned with the LED chip to achieve maximum coupling from the LED to the optical fiber. An optical power meter is employed to monitor the output power at the other end of the optical fiber and provide feedbacks for the alignment. Different techniques to couple the UV light into the optical fiber were experimented. It was found that the best coupling is achieved by butting the fiber end directly against the LED chip. With a forward

4 current of 50mA supplied to the UV LED, an output power of 180µW at the fiber end is achieved. The output power can be adjusted by varying the forward current. After one end of the optical fiber is aligned with the LED chip, the other end of the optical fiber is submerged into a pool of NOA61. NOA 61 is a clear, colorless, liquid photopolymer that will cure when exposed to the UV light. It is commonly used for bonding lenses, prisms, and mirrors as well as for terminating and splicing optical fibers. NOA 61 is cured by UV light with maximum absorption within the range of nanometers. The recommended energy required for full cure is 3 Joules/square centimeter in these wavelengths. The cure is not inhibited by oxygen; hence any areas in contact with air will cure to a non-tacky state when exposed to UV light. Curing is done in two steps. First, the epoxy adhesive is exposed to the UV radiation for about 5 seconds to set the bond (pre-curing) and allow it to be moved without disturbing the alignment. After precuring, the optical fiber is removed from the epoxy pool and is rinsed in alcohol to remove any residual uncured epoxy. The pre-cured epoxy is then post-cured under an UV LED for one minute to achieve full crosslinking and stability of the sensing element. RESULTS AND DISCUSSIONS The micrographic images of the polymer sensing elements fabricated on a single mode fiber and a multi-mode fiber are shown in Figure 4. These images clearly demonstrated that the fabricated polymer sensing elements are aligned with the fiber core perfectly. Because the cured polymer has a higher refractive index than that of the uncured polymer, the cured polymer serves as a light guide and prevents the UV light from diverging. As a result, it is possible to fabricate a long polymer wire on the end face of an optical fiber, as demonstrated in Figure 4. The shape of the polymer sensing element is controlled by several fabrication parameters, including the optical fiber length and the curing energy. Because the optical fiber is butted against the UV LED chip, both the core mode and the cladding mode are coupled into the optical fiber. For a fiber of short length, the output light at the other end of the fiber consists of both modes. Since the optical loss for the cladding mode is much higher than that of the core mode, the output power decreases as the length of the fiber increases. Eventually, the core mode is dissipated over a sufficient distance and only core mode remains. Increasing the fiber length further will not reduce the output power. As shown in Figure 5, a constant output power is obtained when the fiber is longer than 40 centimeters, indicating only the core mode is present. Inspecting the shape of the cured polymer confirm this premise. Figure 6 displays the microscopic images of the polymer sensing elements fabricated on optical fibers of different lengths. When the optical fiber is relatively short (L=10cm and 0cm), the cured polymer covers both the cladding and the core region. When the optical fiber is longer than 40 centimeters, only the polymer covering the core region is cured and the cured polymer has the same diameter as the fiber core. The tapered shape of the cured polymer is due to the intensity pattern of the output UV light. It is expected that the intensity of the output light is Gaussian distributed along the radial direction, i.e. the UV light has the highest intensity at the center of the fiber core and the lowest intensity at the perimeter of the fiber core. Therefore, the polymer at the center of the fiber core is cured faster than the polymer at the edge of the fiber core. As the curing time increases, the cured polymer increases in length while maintaining the tapered shape at the tip. POTENTIAL SENSOR APPLICATIONS The technique described above essentially fabricates a polymer core on the end face of an optical fiber and therefore can be applied to produce various optical fiber sensors. For example, an intrinsic Fabry- Perot strain sensor can be constructed by fabricating a polymer core in between two optical fibers, as shown in Figure 7. First, the two optical fibers are aligned using a V-grooved fixture. A drop of uncured epoxy is then deposited in the gaps between the two optical fiber ends. Next, UV light is

5 coupled into one fiber to cure the epoxy. Given sufficient curing time, a thin polymer wire is fabricated in between the two optical fibers. The thin polymer wire will serve as a wave-guide to keep the optical light from diverging. Therefore, the hybrid glass/polymer Fabry-Perot strain sensor can be employed to measure large strains and deformations. A micrographic image of such a sensor is shown in Figure 8. Another potential application of this technique is for the fabrication of tapered optical fiber sensors, which has become increasingly important as interferometric devices and bio-sensors (Moar 1999). CONCLUSIONS A novel technical to fabricate a polymer sensing element on the end face of an optical fiber using UV irradiation is presented. This technique is simple, inexpensive, easy to control, repeatable, and versatile. It can be applied to fabricate polymer sensing elements of various properties on the end face of an optical fiber or in between two optical fibers. Characterizing the optical and mechanical properties of the polymer sensing element will be carried out in future work. ACKNOWLEDGMENTS The authors would like to acknowledge the National Science Foundation of the United States for their financial support of this research through the Sensor Technologies for Civil and Mechanical Systems Program (award # CMS ). REFERENCES Ansari F., (005), Fiber optical health monitoring of civil structures using long gage and acoustic sensors, Smart Materials and Structures, 14, S1-S7 Budinski, K. and Budinski, M., (004), Engineering Materials-Properties and Selection, 8 th Ed., Prentice Hall Friebele E., et. al., (1999), Optical fiber sensors for spacecraft applications, Smart Materials and Structures, 8, Kiesel S., et. al., (006), Polymer optical fiber sensors for the civil infrastructure, Proc. of SPIE, 6174, Kuang K., et. al., 00, An evaluation of a novel plastic optical fiber sensor for axial strain and bend measurements, Measurement Science and Technology, 13, Moar P., et. al.,1999, Fabrication, modeling, and direct evanescent field measurement of tapered optical fiber sensors, Journal of applied physics, 85, Zhou G. and Sim L., 00, Damage detection and assessment in fiber-reinforced composite structures with embedded fiber optic sensors-review, Smart Materials and Structures, 11,

6 E f, d σ f P E h, A h σ h τ b E f, d (a) (b) Figure 1: Stress analysis of a surface-mounted optical fiber sensor Uncured polymer UV light Polymer irradiated by UV light Figure : Schematic of the fabrication technique UV LED Fiber holder Translation stage Optical fiber Figure 3: Experimental set-up

7 (a) MMF (b) SMF Figure 4: Microscopic images of the fabricated polymer sensing element Output power (uw) Optical fiber length (cm) Figure 5: output power vs. fiber length (a) (b) (c) (d) Figure 6: Shape of cured polymer vs optical fiber length. (a) L=10cm (b) L=0cm (c) L=40cm (d) L=50cm

8 UV light Optical fiber Uncured epoxy Optical fiber Cured epoxy (a) (b) Figure 7: Fabrication of an intrinsic Fabry-Perot sensor Figure 8: Micrographic image of a thin polymer wire fabricated in between two multimode fibers