Hybrid Long-Period Fiber Grating with Multimode Fiber Core for Refractive Index Measurement

Min Shao(邵敏)$^{1,4\hyperlink{s}{**}}$, Xue-Guang Qiao(乔学光)$^{2}$, Xue-Liang Ren(任学良)$^{3}$, De-Xing Yang(杨德兴)$^{4}$

doi:10.1088/0256-307X/34/11/114205

⇦Chinese Physics Letters, 2017, Vol. 34, No. 11, Article code 114205⇨ Hybrid Long-Period Fiber Grating with Multimode Fiber Core for Refractive Index Measurement * Min Shao(邵敏)1,4**, Xue-Guang Qiao(乔学光)2, Xue-Liang Ren(任学良)3, De-Xing Yang(杨德兴)4 Affiliations 1School of Science, Xi'an Shiyou University, Xi'an 710065 2School of Physics, Northwest University, Xi'an 710069 3Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 4Department of Physics, Northwestern Polytechnical University, Xi'an 710069 Received 11 May 2017 ^*Supported by the National Natural Science Foundation of China under Grant Nos 61327012 and 61505160, the Natural Science Foundation of Shaanxi Province under Grant No 2016JQ6021, and the Shaanxi Key Laboratory of Optical Information Technology under Grant No OIT201601.
^**Corresponding author. Email: shaomin@xsyu.edu.cn Citation Text: Shao M, Qiao X G, Ren X L and Yang D X 2017 Chin. Phys. Lett. 34 114205 Abstract A refractive index (RI) sensor based on hybrid long-period fiber grating (LPFG) with multimode fiber core (MMFC) is proposed and demonstrated. The surrounding RI can be determined by monitoring the separation between the resonant wavelengths of the LPFG and MMFC since the resonant wavelengths of the LPFG and MMFC will shift in opposite directions when the surrounding RI changes. Experimental results show that the sensor possesses an enhanced sensitivity of 526.92 nm/RIU in the RI range of 1.387–1.394 RIU. The response to the temperature is also discussed. DOI:10.1088/0256-307X/34/11/114205 PACS:42.81.Wg, 42.81.Bm, 07.07.Df © 2017 Chinese Physics Society Article Text Detection of the refractive index (RI) of liquid is important in chemical, biological, food and medical fields. In many applications, such as DNA detection, there is a need for in situ nondestructive and accurate measurement of RI. Optical fiber RI sensors have been extensively exploited in the past decades because they could not only offer these facilitations, but also have some unique advantages such as high sensitivity and remote sensing, and could work under high electromagnetic interference and in harsh environments. The recent techniques on fiber RI sensors include fiber Bragg gratings (FBGs),[1] long-period fiber gratings (LPFGs),[2] titled fiber Bragg gratings (TFBGs),[3] tapered fibers,[4] and photonic crystal fibers (PCFs).[5] LPFGs were studied as a potential refractometer because they are intrinsical to the surrounding RI due to the cladding mode coupling mechanism. However, the RI sensitivity of a single LPFG is several nanometers in one RI unit, which is insufficient in applications. To improve the RI sensitivity of LPFGs, numerous works have been carried out including coating film on LPFGs,[6] hybrid LPFGs/FBGs,[7] hybrid LPFGs and thinned-core fibers (TCFs),[8] hybrid LPFGs/PCFs,[9] and LPFG pairs.[10] Among these techniques, hybrid fiber structures effectively utilize the advantages of both LPFG and cascading fiber structure, hence largely improving the RI sensitivity of LPFG, thus hybrid LPFG with other fiber structures is a promising solution for increasing the RI sensitivity of LPFGs. In this Letter, we present and demonstrate a fiber RI sensor based on a hybrid LPFG with a multimode fiber core (MMFC). The MMFC has been widely employed in strain, temperature, humidity and RI sensing application due to its sensitivity to external environment. The combination of LPFG and MMFC could improve the RI sensitivity by measuring the separation between the resonant wavelengths of the LPFG and MMFC fiber structure.

Fig. 1. Schematic diagram of the hybrid LPFG/MMFC fiber structure.

The schematic diagram of the hybrid LPFG/MMFC-based RI sensor is shown in Fig. 1. It is composed of an input and an output single mode fiber (SMF), and a section of MMFC fusion splicing with an LPFG between them. When light transmits to the MMFC section from the input SMF, cladding modes will be excited for the core diameter mismatching. These cladding modes interfere with the core mode in the MMFC section and couple to the output SMF. The MMFC strongly interact with the surrounding medium, which makes the interference more sensitive to the surrounding RI. The phase difference of the core mode and the cladding mode can be expressed as $$\begin{align} \varphi =\frac{2\pi}{\lambda}(n_{\rm eff}^{\rm co} -n_{\rm eff}^{{\rm cl},m})L,~~ \tag {1} \end{align} $$ where $\lambda$ is the resonant wavelength, $L$ is the length of the MMFC, $n_{\rm eff}^{\rm core}$ and $n_{\rm eff}^{{\rm cl},m}$ are the effective RIs of the core mode and the $m$-order cladding mode, respectively. When the phase difference is equal to $({2k+1})\pi$, the resonant wavelength of the MMFC can be expressed as $$\begin{align} \lambda _{\rm M} =\frac{2(n_{\rm eff}^{\rm co} -n_{\rm eff}^{{\rm cl},m})L}{2k+1},~~ \tag {2} \end{align} $$ where $k$ is an integer. The surrounding RI impacts on the effective RIs of the cladding modes, and causes the resonant wavelength shifting. Considering the modal dispersion, the RI sensitivity of the MMFC can be defined as[11] $$\begin{alignat}{1} \frac{d\lambda _{\rm M}}{dn_\alpha}=\,&-\frac{\lambda _{\rm M}}{n_{\rm eff}^{\rm co} -n_{\rm eff}^{{\rm cl},m}}\frac{\partial n_{\rm eff}^{{\rm cl},m}}{\partial n_\alpha}\\ &\cdot \Big[1\!-\!\frac{\lambda _{\rm M}}{n_{\rm eff}^{\rm co} \!-\!n_{\rm eff}^{{\rm cl},m}}\Big(\frac{\partial n_{\rm eff}^{\rm co}}{\partial \lambda}\!-\!\frac{\partial n_{\rm eff}^{{\rm cl},m}}{\partial \lambda}\Big)\Big]^{-1},~~ \tag {3} \end{alignat} $$ where $n_{\rm \alpha}$ is the RI of the surrounding medium. According to Eq. (3), whether or not the resonant wavelength of the MMFC has a red shift or blue shift depends on the cladding mode in the MMFC satisfying or dissatisfying $({\frac{\partial n_{\rm eff}^{\rm co}}{\partial \lambda}-\frac{\partial n_{\rm eff}^{{\rm cl},m}}{\partial \lambda}})>\frac{n_{\rm eff}^{\rm co} -n_{\rm eff}^{{\rm cl},m}}{\lambda _{\rm M}}$. To make clear the spectra response of the MMFC fiber structure, we carry out the simulation for the MMFC fiber structure using the beam propagation method, and the result is shown in Fig. 2. In the simulation, the MMFC has core diameter of 95 μm and length of 20.8 mm, the core refractive index is 1.4667, the SMF has core/cladding diameters of 9.1/125 μm, and the core/cladding refractive indices of 1.4628/1.4682. Figure 2 shows that the transmission spectrum of the MMFC fiber structure shifts to the long-wavelength band as the surrounding RI increases. It can be found that the resonant wavelength of 1558.3 nm experiences a red shift about 9 nm when the surrounding refractive index range changes from 1.33 to 1.39.

Fig. 2. Simulated transmission spectra of the MMFC fiber structure under different surrounding RIs.

At the output SMF, there is an LPFG written by a CO$_{2}$ laser. As the core and cladding modes arrive at the LPFG, the power of the core mode is coupled to certain forward-propagating cladding modes. The resonant wavelength of the LPFG is determined by the phase matching condition, which is dependent on the effective RI difference between the core and cladding modes, and can be written as $$\begin{align} \lambda _{\rm D} =(n_{\rm eff}^{\rm co} -n_{\rm eff}^{{\rm cl},m}){\it \Lambda},~~ \tag {4} \end{align} $$ where ${\it \Lambda}$ is the period of the LPFG. As the surrounding RI increases, the effective index of the core mode remains unchanged while the effective indices of the cladding modes increase.[12] Thus the resonant wavelength of the LPFG will experience a blue shift as one can see from Eq. (4). Since the resonant wavelengths of the MMFC and LPFG will shift in opposite directions when the surrounding RI changes, the separation between the resonant wavelengths of the MMFC and LPFG will be larger than the individual wavelength shifting. Therefore, the RI sensitivity of this sensor is largely improved and higher than that of solely using LPFG or MMFC. As is known, the MMFC has highly sensitive RI response to the surrounding RI, and the proposed sensor utilizes the RI response of the MMFC opposite to that of the LPFG, making the sensor more effective. Compared with other hybrid LPFG configurations, such as LPFG/FBG and LPFG/PCF schemes, the proposed sensor has the advantage of easy fabrication and could increase the RI sensitivity. The hybrid LPFG/MMFC-based sensor was fabricated by fusion splicing an LPFG with a 20.8-mm-long MMFC. The used LPFG was fabricated by a 248 nm excimer laser in a hydrogen-loaded SMF28 with a grating period of 570 μm and length of 25.6 mm. The MMFC used in experiment was fabricated by chemical etching cladding of a commercial multimode fiber (MMF) and the diameter of the MMFC was 95 μm. The used MMF (Yongtze) has a step index profile and the core/cladding diameter is 105/125 μm. The prepared MMFC is fusion spliced without offsetting between an SMF28 and the LPFG using a commercial fusion splicer (S177, FITEL Co.). To ensure the fusion quality and to reduce the insertion loss, all the fiber end faces were cleaved to be flat. It must be pointed out that the distance between the right end of MMFC and the beginning of the LPFG should be less than 5 mm during the fabrication as shown in Fig. 1. Such a small distance is expected to reduce the attenuation of the cladding modes from the MMFC. Due to the brief transmission distance, some cladding modes will reach the LPFG and will be coupled into the core mode. Hence, the sensor is not simply fusion splicing MMFC with LPFG, and it can efficiently combine the LPFG and MMFC. Unlike a structure comprising of a section of single mode-multimode-single mode (SMS) fiber structure used as temperature compensation for LPFG,[13] both LPFG and MMFC fiber structures experience the surrounding RI changing used in this study. A broadband source (1520–1600 nm) and an optical spectrum analyzer (MS9740A) were employed to detect the spectral behavior of the sensor. In our experiment, the sensor was kept straight in a glass substrate to avoid bending loss. Figure 3 shows the transmission spectra of the LPFG and hybrid LPFG/MMFC fiber structure in air. It can be seen that there is a loss dip over 20 dB at the wavelength of 1547.064 nm in the transmission spectrum of the LPFG (alone). From Eq. (4), by solving the three-layer optical fiber waveguide dispersion equation, we can calculate the dominant cladding mode in the LPFG (alone), which is the $LP_{03}$ mode.

Fig. 3. Transmission spectra of the LPFG and the hybrid LPFG/MMFC fiber structure.

Fig. 4. Transmission spectra of the sensor under different surrounding RIs.

In the transmission spectrum of the hybrid LPFG/MMFC fiber structure, there are two resonant wavelengths at 1528.950 nm and 1558.260 nm, in which the resonant wavelength of 1558.260 nm agrees with the resonant wavelength of 1558.3 nm in the simulation of the MMFC fiber structure. By solving Eq. (4), we can find that $LP_{02}$ mode in the LPFG contributes to the resonant wavelength at 1528.950 nm. This indicates that the dominant coupling mode of the LPFG has changed when assisted with MMFC. It is different from the hybrid LPFG with PCF or TCF fiber structures, in which the resonant wavelengths of LPFG keep unchanged. The reason for this is that using the MMFC has successfully excited the $LP_{02}$ mode and coupled with the core mode in the LPFG. The RI experimental investigations were carried out by dropping different concentration sucrose liquids with RI range of 1.335–1.394 while the room temperature was kept at 23$^{\circ}\!$C. After each test, the sensor is cleaned by water to remove the residual RI samples and dried in air. Until the transmission spectrum is recovered to its initial spectrum as shown in Fig. 3, the next test was given. The transmission spectral response is shown in Fig. 4. It can be seen that LPFG and MMFC fiber structures present different RI responses. As the surrounding RI increases, the resonant wavelength of the LPFG has a blue shift, while the resonant wavelength of the MMFC fiber structure has a red shift.

Fig. 5. Wavelength shift of the sensor with surrounding RI.

Figure 5 shows the measured wavelength shifts for LPFG and MMFC fiber structures versus RI. It is estimated that the LPFG has RI sensitivities of $-$50.86 and $-$105.38 nm/RIU in the RI ranges of 1.335–1.378 and 1.387–1.394, respectively, while the MMFC fiber structure has RI sensitivities of 88.15 and 421.54 nm/RIU in the RI ranges of 1.335–1.378 and 1.387–1.394, respectively. The experimental results are in accordance with the theoretical analyses. When further considering the separation between the resonant wavelengths changing, the RI sensitivities of the hybrid LPFG/MMFC fiber structure could be 139.01 and 526.92 nm/RIU in the RI ranges of 1.335-1.378 and 1.387–1.394, respectively. This sensitivity is higher than the RI sensitivities of individual LPFG or MMFC, which verifies the prediction. Moreover, the RI sensitivity of the proposed sensor is higher than that of hybrid LPFG/TCF fiber structures (97 nm/RIU)[8] and a tapered SMS fiber structure (430.94 nm/RIU).[12] When the resolution of optical spectrum analyzer is 0.02 nm, the maximum resolvable RI change is $3.79\times10^{-5}$, which is higher than that of the LPFG-taper-LPFG fiber structure ($2.3\times10^{-4}$).[14] To test the temperature dependence of the proposed sensor, the sensor was placed in a vessel full of water. Heating water from 25$^{\circ}\!$C to 65$^{\circ}\!$C, the transmission spectra of the sensor under different temperatures is recorded as shown in Fig. 6. Figure 6 shows that both the resonant wavelengths of LPFG and MMFC fiber structures shift to the long-wavelength band as the temperature increases. When the water temperature increases to 40$^{\circ}\!$C, the refractive index of water decreases about 0.005 RIU. The transmission spectrum of LPFG and MMFC fiber structures should shift 0.254 nm and $-$0.441 nm according to their RI sensitivities, respectively. This shows that the RI changing induced by water temperature has influence less than the thermo-optic effect on the temperature experiment.

Fig. 6. Spectra response of the sensor under different temperatures.

Fig. 7. Wavelength shift of the sensor with temperature.

The measured wavelength shifts for the LPFG and MMFC fiber structures versus temperature is shown in Fig. 7. From the measured results, the temperature sensitivities are 0.085 and 0.102 nm/$^{\circ}\!$C for the LPFG with MMFC fiber structures, respectively, while the temperature sensitivities of separation between the resonant wavelengths are $-$0.012 nm/$^{\circ}\!$C and 0.04 nm/$^{\circ}\!$C, and the corresponding maximum RI measurement errors caused by temperature are $8\times10^{-5}$ RIU/$^{\circ}\!$C and $2\times10^{-4}$ RIU/$^{\circ}\!$C in the temperature ranges of 25–45$^{\circ}\!$C and 45–65$^{\circ}\!$C, respectively. This implies that the sensor is reliable when the liquid temperature is below 45$^{\circ}\!$C. When applied in liquid temperature over 45$^{\circ}\!$C, the sensor could be cascaded an FBG for temperature compensation. In summary, we have proposed a hybrid LPFG/MMFC-based RI sensor. The sensor utilizes the opposite transmission spectrum shifting for LPFG and MMFC fiber structures with surrounding RI variation. By measuring the resonant wavelength separation changing, the surrounding RI is determined, and the RI sensitivity could be further enhanced. The experimental results show that the sensor has a high RI sensitivity of 526.92 nm/RIU, which is much higher than that of individual LPFG- or MMFC-based fiber sensors. In addition, the temperature response of the sensor is discussed. With its appreciable performance, the sensor is expected to be applied in chemical or biological sensing. References High Sensitive Refractometric Sensor Using Symmetric Cladding Modes of an FBG Operating at Mode TransitionA Highly Sensitive Fiber-Optic Refractive Index Sensor Based on an Edge-Written Long-Period Fiber GratingReflective tilted fiber Bragg grating refractometer based on strong cladding to core recouplingHigh sensitivity Mach–Zehnder interferometer sensors based on concatenated ultra-abrupt tapers on thinned fibersInline Microcavity-Based PCF Interferometer for Refractive Index and Temperature SensingRefractive index sensitivity of nano-film coated long-period fiber gratingsOptical fiber refractometer based on cladding-mode Bragg gratingEnhanced refractive index sensor using a combination of a long period fiber grating and a small core singlemode fiber structureLong period grating cascaded to photonic crystal fiber modal interferometer for simultaneous measurement of temperature and refractive indexSimultaneous Measurement of Refractive Index and Temperature by Using Dual Long-Period Gratings With an Etching ProcessLow-cost high-performance fiber-optic pH sensor based on thin-core fiber modal interferometerRefractive index measurement based on fiber Bragg grating connected with a multimode fiber coreTemperature compensated refractometer based on a cascaded SMS/LPFG fiber structureTapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

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