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Nov 1, 2015 - refractive index sensor by bent-fiber interference ... The bent-fiber intermodal interferometer has a simple structure, which consists of a bare ...
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Research Article

Vol. 54, No. 31 / November 1 2015 / Applied Optics

Optimization of long-period grating-based refractive index sensor by bent-fiber interference XINPU ZHANG, LINGXIAO XIE, YANG ZHANG,

AND

WEI PENG*

School of Physics and Optoelectronic Technology, Dalian University of Technology, 2 Linggong Rd, Ganjingzi District, Dalian 116024, China *Corresponding author: [email protected] Received 17 August 2015; revised 19 September 2015; accepted 1 October 2015; posted 1 October 2015 (Doc. ID 248002); published 26 October 2015

In this paper, we propose and demonstrate a novel approach to enhance the refractive index (RI) sensitivity and eliminate the temperature cross-sensitivity of a long-period grating (LPG) -based refractive index sensor by bentfiber interference. The approach is based on a hybrid structure composed of an LPG and a bent-fiber intermodal interferometer. The bent-fiber intermodal interferometer has a simple structure, which consists of a bare fiber semi-circular bending region with a 5 mm bending radius. As the RI increases, the resonance wavelength of the LPG moves toward a shorter wavelength, while the resonance wavelength of the bent-fiber intermodal interferometer shifts to a longer wavelength. The separation of two resonance dips increases with the RI; using two resonance dips allows us to measure an RI with a higher sensitivity than if we had only used one resonance dip. However, as the temperature increases, the separation of the two resonance dips is constant. This approach can effectively enhance the RI sensitivity and eliminate temperature cross-sensitivity. © 2015 Optical Society of America OCIS codes: (060.2430) Fibers, single-mode; (120.3180) Interferometry; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/AO.54.009152

1. INTRODUCTION Fiber-optic refractive index (RI) sensors have been extensively investigated recently [1,2] due to their many distinctive advantages over traditional RI sensors, such as high sensitivity, miniature size, immunity to electromagnetic interference, and multiplexing capability [3]. Many types of fiber-optic RI sensors have been demonstrated via fiber gratings (Bragg grating or long-period grating [LPG]) [4–7], single-thmode– multimode–single-mode structures [8], microfiber interferometers [9,10], in-line fiber interferometers [11–13], and Fabry–Perot interferometers [14]. Due to their specific transmission characteristics, LPGs have been widely applied in telecommunications and optical sensing fields [15,16]. In general, LPGs possess higher sensitivity to the variation of the RI because the evanescent field extends out of the fiber cladding as a result of core-cladding mode coupling. LPGs are therefore considered to be an ideal type of optical sensing architecture, particularly by [17]. However, the intrinsic shortcomings associated with the temperature cross-sensitivity will decrease the measurement accuracy of LPGs. The separation of various physical effects is, therefore, an important issue in the development of practical LPG sensors. For an RI measurement, in particular, the temperature effect must be eliminated. The temperature sensitivity of an LPG can be reduced with a specially tailored fiber [18,19], a carefully chosen polymer 1559-128X/15/319152-05$15/0$15.00 © 2015 Optical Society of America

coating [20], or the bending effect [21]. Although some of these devices provide high RI sensitivities, most of them require a special fiber or polymer, which can increase the cost and complexity of fabrication and operation. Recently, there have been some studies employing bent fibers as optical sensing elements [22,23], especially the semi-circular fiber-based sensor, which as been proposed for displacement sensing applications [23]. However, it is difficult to ensure its precision, and also its operating principle is complicated. There are two orthogonally polarized beams excited by a bending fiber rather than higher-order cladding modes, and they will be recombined and cause interference at the output end of the fiber. In this paper, we propose and demonstrate a novel approach to optimize LPGs based on bent-fiber interference; our approach includes RI sensitivity enhancement and temperature cross-sensitivity elimination. The approach we proposed is based on a hybrid structure composed of an LPG and a bent-fiber intermodal interferometer. This bent-fiber intermodal interferometer has a simple structure, which consists of a bare single-mode fiber (SMF) semi-circular bending region with a 5 mm bending radius. With the increasing RI, the resonance wavelength of the LPG moves toward shorter wavelengths, while the resonance wavelength of the bent-fiber intermodal interferometer shows a red wavelength shift. The separation of the two resonance dips increases with the increase

Research Article

Vol. 54, No. 31 / November 1 2015 / Applied Optics

of the RI. Two resonance dips can be used to measure an RI with a higher sensitivity than using only one dip. However, as the temperature increases, the separation of the two resonance dips is constant. This approach can effectively enhance the RI sensitivity and eliminate temperature cross-sensitivity. 2. SENSOR DESIGN AND OPERATING PRINCIPLE The hybrid structure is schematically shown in Fig. 1. It consists of an LPG and one section of semi-circular, bare, standard SMF with a specified bending diameter. We inscribed an LPG in an SMF obtained by the use of a CO2 laser. An LPG can couple light at resonant wavelengths between the guided fundamental mode and some co-propagating cladding modes. The coupling matches the phase-matching condition: βCo − βm;Cl  2π∕Λ;

(1)

where βCo and βm;Cl are the propagation constants of the fundamental core mode and the mth cladding mode, respectively. Λ is the grating period. Therefore, the resonant wavelength is given by: λRes  nCo − nm;Cl Λ;

(2)

where λRes is the initial resonant wavelength, and nCo and nm;Cl are the effective indices of the fundamental core mode and the mth cladding mode, respectively. The characteristics of the LPGs are affected by external perturbations, such as temperature and RI. An interesting characteristic of LPGs is their sensitivity to the RI of the environment. When the external RI increases, only nm;Cl depends on the RI of the external medium. It also increases, while the difference between the effective indexes of the two modes Δn decrease, and a blue shift in the resonant wavelengths can be observed. In addition, the variation in the grating period Λ and the modal effective indices due to temperature causes the resonant wavelength to red shift. Therefore, the shift in resonant wavelength in a grating transmitted spectrum can be used to monitor the change in the RI and temperature. After the LPG inscription, a bent-fiber intermodal interferometer structure consisting of one section of semi-circular, bare, standard SMF with a chosen bending diameter was introduced. The protective coating of the bent section is stripped off to reduce the transmission loss of the cladding modes and expose the light field of the cladding modes to the environment. Due to the fiber bending, the fundamental core mode couples to several cladding modes, which can also couple back to the core modes as they propagate through the length of the bending. Therefore, the different optical paths of the residual core mode and cladding modes form an intermodal interferometer.

Fig. 1. Schematic diagram of the hybrid structure including an LPG and a bent-fiber intermodal interferometer.

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The detailed theoretical and experimental analyses are discussed in [24], such as bend-induced mode leakage, higher-order mode excitation, and mode coupling. It is known that the effective RIs of the cladding modes depend on the surrounding RIs (SRIs). The change of the SRI leads to the spectral shift of the transmission spectrum. For simplicity, the cladding modes can be grouped together and the interference transmission can be modeled as the two-mode interference process, for which the transmitted light intensity can be written as pffiffiffiffiffiffiffiffiffi I  I 1  I 2  2 I 1 I 2 cosΔφk ; (3) where I 1 and I 2 are the light intensities of the core and cladding modes, and Δφk is the phase difference between them. Depending on the phase difference between the core mode and cladding modes, the intermodal interference can occur, and the interference pattern will show up in the transmission spectrum. Since the resonance wavelength depends on the external RI and temperature, the bent-fiber intermodal interferometer can be used for sensing applications. As mentioned above, with the increasing RI, the resonance wavelength of the LPG moves toward the shorter wavelength, while that of the bent-fiber intermodal interferometer shifts to the longer wavelength. The separation of the two resonance dips increases with the increase of the RI, which can be used to measure RIs with higher sensitivity than using only one resonance dip. However, as the temperature increases, the separation of the two resonance dips is constant. This approach can be used to optimize an LPG for RI sensitivity enhancement and temperature cross-sensitivity elimination. 3. EXPERIMENTS AND DISCUSSION We used a commercial, standard SMF to fabricate the designed hybrid structure. A 40 mm-long LPG with a 400 μm period was inscribed in the SMF and irradiated by a CO2 laser. The resonance wavelength of the LPG is located at 1555.4 nm, appearing on the transmission spectrum in the wavelength range 1520–1590 nm. Then, a bent-fiber intermodal interferometer structure consisting of one section of semi-circular, bare, standard SMF with a specific bending radius is introduced. A polymethyl methacrylate plate is used as the platform to which the bare semi-circular fiber is bound. First, we fix one end of the bare fiber by using adhesive bonding, then bend the free end with a selected bending radius and fix it. Once the semi-circular fiber is obtained, its profile will not change. To determine the appropriate bending radius of the bent-fiber intermodal interferometer configuration, we investigated the spectra of individual interferometers with different bending radii [24]. Finally, in order to avoid the crosstalk between the dips (the resonance wavelengths of the LPG and bent-fiber intermodal interferometer), the 5 mm bending radius was selected to form the hybrid structure with the LPG. In the experimental setup, a homemade optical sensing interrogator with a spectral resolution of 4 pm was used to measure the transmission spectrum of the structure. In the homemade interrogator, a swept laser provides an output wavelength range from 1510 to 1590 nm. The peak power output of the light source is about 5 mw. The transmitted light signal is detected by the photoelectric converter in the optical sensing

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Fig. 2. Spectra of the individual and the hybrid structures.

interrogator (OSI), demodulated by a data acquisition unit and software, and then the spectra signal is displayed on a computer. Figure 2 shows a typical transmission spectrum of the individual and the hybrid structures immersed in deionized (DI) water. In Fig. 2, it is shown that there are two obvious dips at 1555.4 and 1576.9 nm appearing on the transmission spectrum generated from the LPG and the bent-fiber intermodal interferometer with a 5 mm bending radius, respectively. The RI sensing of the hybrid structure was experimentally investigated. The responses to different SRIs of the hybrid structure are shown in Fig. 3, when the hybrid structure was immersed into the solutions with the RI values increasing from 1.3269 to 1.3721. After the transmission spectrum was recorded, the structure was cleaned with ethanol and DI water and dried to recover its original spectrum. During the RI measurement, the interference fringes show good visibility that is sufficient for interference signal demodulation. Figure 3(a) shows the transmission spectra at different RIs, indicating that, as the SRI increases, the dip wavelength of the bent-fiber intermodal interferometer structure exhibits a red shift and the resonance wavelength of the LPG moves toward shorter wavelengths. In the bent-fiber intermodal interferometer, when the order of the stimulated cladding mode is relatively low, the change in the difference of the core mode and cladding mode effective indices is negative when the SRI is increasing, and the dip wavelength will have a blue shift, while in the case of the higher-order cladding mode, the change is positive with

Fig. 3. Responses to RIs. (a) Transmission spectra with decreasing RI. (b) Wavelength shift and separation as a function of the RI. The wavelength separation changes with RI evolution.

Research Article the increasing SRI, resulting in the dip wavelength shifts to longer wavelengths [25]. Figure 3(b) presents the wavelength of the spectral dips as a function of the SRI. From the linear fit, the RI sensitivities are −31.79 and 152.18 nm/RIU for the LPG dip and the bent-fiber dip, respectively. Due to the different variation trends of the LPG dip and the bent-fiber dip, and because the separation of the two resonance dips increases with the increase of the RI, the calculated RI sensitivity based on the dip separation is 183.44 nm/RIU, which can be used to measure an RI with a higher sensitivity than using only one resonance dip. In practical applications, temperature cross-sensitivity is an important issue for many RI sensors, so we investigated the temperature response of the hybrid structure. The structure was mounted on a glass plate, and our experiment is performed by increasing the temperature gradually from 18°C to 30°C, with a temperature interval of 3°C. Figure 4(a) presents the transmission spectra of the hybrid structure versus the temperature changes. As shown in Fig. 4(a), the dip wavelengths moved toward longer wavelengths. The temperature response is shown in Fig. 4(b). The resonance wavelengths moved toward longer wavelengths as the temperature increased, giving an average temperature sensitivity of 89.4 and 92.8 pm/°C for the LPG dip and the bent-fiber dip, respectively. The temperature sensitivity of the bent-fiber interferometer is similar to that of an LPG. The separation of the two resonance dips increased by only 36 pm when the liquid temperature increased from 18°C to 30°C. The temperature sensitivity evaluated by the dip separation is 3.2 pm/°C. The temperature dependence of the device was small and contributed less than 0.4% to the total RI variation over the 12°C temperature range. Therefore, the temperature and RI cross-sensitivity can be solved by this hybrid structure. To further verify the temperature cross-sensitivity, we investigated the temperature response of the structure when it is immersed into deionized water. Our experiment is performed by increasing the liquid temperature gradually from 20°C to 32°C,

Fig. 4. Responses to temperature in air. (a) Transmission spectra with increasing temperature. (b) Relationship between temperature and wavelength shift. The wavelength separation changes under multiple temperature cycles.

Research Article

Fig. 5. Transmission spectra with increasing temperature immersed in deionized water. Inset: Relationship between temperature and wavelength shift.

with a temperature interval of 3°C. Figure 5 presents the transmission spectra versus the temperature changes. As shown in Fig. 5, the dip wavelengths moved toward longer wavelengths. The temperature response is shown in Fig. 5. The resonance wavelengths moved toward longer wavelengths as the temperature increased, giving an average temperature sensitivity of 93 pm/°C and 95.2 pm/°C for the LPG dip and the bent-fiber dip, respectively. Additionally, as shown as the inset of Fig. 5, the wavelength separation is constant when the temperatures change. The temperature sensitivities measured in deionized water are similar to that in air. This conclusion demonstrated that this sensor is more appropriate for RI measurement and it can realize the temperature-insensitive RI sensing. Finally, to further investigate temperature sensitivity, a higher temperature range is measured; we performed our experiment by increasing the temperature gradually from 42°C to 53°C, with a temperature interval of 1°C. Figure 6(a) presents the transmission spectra versus temperature changes. As shown in Fig. 6(a), the dip wavelengths moved toward longer wavelengths. However, the bent-fiber dip shifts faster than the LPG dip. The temperature response is shown in Fig. 6(b). The dip wavelengths moved toward longer wavelengths as the temperature increased, giving an average

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temperature sensitivity of 89 pm/°C and 98 pm/°C for the LPG dip and the bent-fiber dip, respectively. Thus, the temperature sensitivity evaluated by the dip separation is 9 pm/°C, which is higher than the temperature range between 18°C and 30°C, but the experimental results can illuminate that temperature sensitivity calculated by the separation remained far below the LPG temperature sensitivity. This approach also can effectively improve the temperature and RI cross-sensitivity of the LPG. The different temperature responses under different temperature ranges are induced by the thermal properties of the glue. This conclusion demonstrated that this structure can effectively improve the temperature and RI cross-sensitivity of an LPG over a wide temperature range, which can realize the temperature-insensitive RI sensing. 4. CONCLUSIONS In summary, we proposed and demonstrated a novel approach to enhance the RI sensitivity and eliminate the temperature cross-sensitivity of an LPG-based refractive index sensor by bent-fiber interference. We have fabricated a hybrid structure with an LPG and a bent-fiber intermodal interferometer with a 5 mm bending radius and experimentally demonstrated the feasibility for RI sensitivity enhancement and temperature cross-sensitivity elimination. The RI sensing is achieved by measuring the wavelength shift of the resonance dips in its transmission spectrum. In a 1.3269–1.3721 RI range, the corresponding RI sensitivity calculated based on the dip separation is 183.44 nm/RIU, which can be used to measure an RI with a higher sensitivity than using only resonance dip. We investigated the temperature response of the structure. When the hybrid structure is immersed in air, the dip wavelengths move toward the longer wavelength direction as temperature increases, giving an average temperature sensitivity of 89.4 pm/°C and 92.8 pm/°C for the LPG dip and the bentfiber dip, respectively. The temperature sensitivity evaluated by the dip separation is only 3.2 pm/°C. The temperature dependence of the device was small and contributed less than 0.4% to the total RI variation over the 12°C temperature range. In addition, the temperature sensitivities measured in deionized water are similar to those in air. Therefore, the temperature and RI cross-sensitivity can be solved by this hybrid structure. Our future works will be focused on RI sensitivity enhancement by optimizing the sensor structure, parameters, and materials. Funding. Ministry of Education of the People’s Republic of China (MOE) (SRFDP-20120041110040); National Natural Science Foundation of China (NSFC) (61137005, 61520106013). REFERENCES

Fig. 6. Responses to temperature in air when the structure is exposed to higher temperatures. (a) Transmission spectra with increasing temperature. (b) Relationship between temperature and wavelength shift under multiple temperature cycles.

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