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Abstract
We design and fabricate a novel highly integrated all-fiber single mode-multimode-single mode long period grating (SMS-LPG) refractometer. The experiment and simulation results confirm that the powerful local refractive index modulation capability of the multimode fiber (MMF) simultaneously reduces the length of the LPG and achieves excellent refractive index sensitivity. The spectral characteristics and sensing responses of different duty cycles are investigated. The optimized SMS-LPG sample achieved a refractive index sensitivity of 427.01 nm/RIU. The miniature SMS-LPG refractometer is a new sensor for measuring the external refractive index.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the technological development in biomedicine and the chemical industry, the demand for refractometers is increasing [1,2]. Fiber optic sensor has unique competitive advantages for measuring the refractive index (RI), such as small size, high measurement accuracy, and rapid response [3–6]. Extensively researched fiber refractometers are typically based on long period grating (LPG), fiber Bragg grating (FBG), and different types of interferometers [7–9]. Interferometers induced by different optical modes produce many signal interference fringe with different sensitivities in a certain wavelength range, which results in difficulty in selecting the reference wavelength. The RI response of FBG is generally low due to the limitations of its formation mechanism. These problems are solved by LPG with coupling cladding and core mode.
In previous studies of the LPG, the investigation of the sensing characteristics of the RI has focused on commercial single mode fiber (SMF). LPGs obtained from various preparation methods have produced many different sensing characteristics [10,11]. Recently, researchers have turned their attention to LPGs using special fibers. Rindorf et al. demonstrated that the use of LPG prepared by a liquid-immersed photonic crystal fiber (PCF) was beneficial for the sensitivity improvement [12]. Guan et al. obtained an RI response of 1000 nm/RIU for LPG based on hollow eccentric optical fiber [13]. Ji et al. also achieved ideal sensing characteristics using adiabatically tapered microfiber LPG [14]. The high sensitivity of LPGs with special fibers usually depends on complex preparation methods. Most of these sensors are not robustness enough and are easily damaged. In addition, the RI modulation regions are weak so that most of the LPGs are in the order of dozens of periods and several centimeters.
As a promising special fiber, it is well known that large-diameter MMF itself can stimulate and propagate a variety of modes. Due to the core diameter mismatch, the large RI modulation caused by the splicing of the SMF and MMF leads to the intense diffusion of light, which makes it possible to prepare effective sensors using a simple preparation method [15]. Therefore, many refractometers with a simple structure and excellent performance were proposed using MMF, for example, single mode-multimode-single mode (SMS) structures, multimode-single mode-multimode (MSM) structures, and multi-taper SMS structure [16–18]. In previous studies, many researchers have prepared a variety of MMF-involved gratings as RI sensor, such as the RI sensors based on fiber grating inscribed in MMF [19], and the temperature compensated refractometer based on a cascaded SMS/LPFG fiber structure [20]. Because the strong RI modulation capacity of MMF tends to cause many excitation modes, LPGs prepared using MMF are rarely studied.
In this article, we report on a novel miniature SMS-LPG refractometer. Using the stable preparation system, we investigated in detail the spectral characteristic, evolutionary process, and mode distribution of the SMS-LPGs for different duty cycles. The simulation results of the RI response matched the experimental results. An optimization of the sensing response based on the duty cycle is performed for the novel LPG refractometer. The period length of the optimized SMS-LPG is 600 µm (LMMF/SMF=200/400 µm) with a total length of 3 mm. This is shorter than the length of most LPGs [21–24]. The RI sensitivity of the optimized SMS-LPG is 427.01 nm/RIU at an RI of 1.423. The RI sensitivity increases more rapidly as the external RI approaches the RI of quartz. Moreover, The SMS-LPG also exhibits diverse temperature responses. After optimization, the LPG showed temperature insensitivity of only ∼10 pm/°C. A lower temperature response helps to reduce thermal cross-sensitivity.
2. Preparation methods and spectral characteristics
Figure 1 (a) shows the precision fiber cutting system for preparing SMS-LPG. The fiber cutting system comprises three coaxially placed precision 3D adjustment brackets, a fiber cleaver, and a set of microscopic imaging systems. The adjustment brackets at both ends are used to fix the fiber, and a fiber cleaver (Sumitomo Electric Industrial Co., Ltd. FC-6S) is fixed on the middle adjustment bracket to cut the fiber. The microscopic imaging system above the device monitors the distance between the blade of the fiber cleaver and the splice point to ensure that the actual length is consistent with the design length.
Fig. 1. (a) Schematic diagram of the SMS-LPG preparation devices. (b1)-(b4) Preparation process and microscopic images of the SMS-LPG. (c) Schematic diagram of the completed SMS-LPG.
In the first step (Fig. b1), the cleaned SMF and MMF (Corning Incorporated, SMF-28e+ and 62.5/125 µm, NA = 0.12) were placed at both ends of the fusion splicer and were electro-discharged using the MM mode (a standard fusion splicer, Fujikura, FSM- 80S). In the second step (Fig. b2), the finished fiber sample was placed in the fiber cutting system. The middle 3D adjustment bracket moved the specified distance along the fiber and then cut off the MMF. The residual MMF section was 200 µm. In the third step (Fig. b3), the SMF-MMF section obtained from the first two steps was again spliced together with the SMF, using the MM mode. In the fourth step (Fig. b4), the SMF-MMF-SMF structure was placed into the fiber cutting system and was cut off again. The residual SMF section was 400 µm. At this point, the first period of the SMS-LPG was prepared. As can be seen from the microscopic image of the preparation process, after the cutting and the fusion process, the cross-section and surface of the fiber have no obvious flaws. Due to the good repeatability of the processing method, the spectral evolution of every period can be obtained (this is discussed below) by monitoring the transmission spectrum using a super-continuum (SC) light source and a spectrum analyzer (OSA) (Agilent AQ6317B, spectral range 600-1750nm) after each connection with the SMF.
A sample of the completed SMS-LPG is shown in Fig. 1 (c). For a sufficiently long MMF, many modes (includes low-order and high-order cladding mode) which satisfies the normalized frequency is stably transmitted. However, in a shorter MMF (hundreds of micrometers or even shorter), many high-order modes cannot be excited in the MMF with a short length [25]. Therefore, the excitation of the number of modes can be suppressed. In our work, the MMF with a short length is used as RI modulation region.
In the SMS-LPG, the light is periodically leaked into the cladding. When the wavelength satisfies the phase matching condition, the co-propagating cladding mode is stimulated. But, in output SMF, the cladding mode will decay rapidly. A series of loss bands (resonance peak) will be left in the transmission spectrum. The wavelength position of the resonance peak generated in the transmission spectrum satisfies the following relationship [26]:
where $\lambda$ is the wavelength position of the resonance peak, $n_{co}^{eff}$ and $n_{k,cl}^{eff}$ are the effective refractive indices of the core mode and the k-th diffraction cladding mode, $\Lambda $ is the period length of the grating. The coupling strength ${\kappa _{mn}}(z )$ between the local guided modes is defined as:The designed SMS-LPG was numerically calculated using the Beam Propagation Method (BPM). In the simulation, the SMS-LPG has a circularly symmetric excitation mode and a standard RI distribution, which is convenient for obtaining accurate simulation results. The core/cladding diameter and RI parameters of the SMF and MMF are: 8/125 µm and 1.455/1.45; 60/125 µm and 1.456/1.451, respectively. The mesh sizes of X, Y, and transmission direction Z are 0.02, 0.02, and 0.05 µm. The length of a period is 600 µm, where the lengths of the MMF and SMF are 200 and 400 µm, respectively (duty cycle is 1/2). The number of periods is 5. The light field distribution along the fiber is shown in Fig. 2 (a). When the light passes through the SMS-LPG, the significant change in energy distribution provides favorable conditions for shortening the length of the grating, as expected by the coupling strength defined in Eq. (2).
Fig. 2. (a) The transmission light field distribution of the SMS-LPG (1531 nm). (b) Transmission spectra for three duty cycles and the cross-sectional light field distribution.
We investigated the sensing response for different duty cycles. The duty cycles used in the study are 100/500, 200/400, and 400/200 µm (LMMF/LSMF), respectively. Through wavelength scanning, we obtained the transmission spectrum of the simulation model. In the transmission spectrum, we found a significant loss peak at the resonant wavelength. After that, we injected the light of the resonant wavelength into the model while monitoring the mode field distribution of the XY section along the Z axis. Finally, the mode field distribution was obtained at the end of the model. Due to the different proportions of the work area (the portion of the MMF), the number of periods required to induce the resonance peak is different (the number of periods of the three samples is 6, 5, and 4). The scanning transmission spectra and simulated cross-sectional light field distributions of the three samples are shown in Fig. 2 (b). All three samples exhibited a resonance peak with a large dip angle (greater than 20 dB). The resonance wavelength gradually shifted to a longer wavelength as the duty cycle increased. The cross-section modes obtained from the calculation and simulation are $LP_{03}^2$ (100:500 µm), $LP_{03}^3$ (200:400 µm), and $LP_{03}^\textrm{2}$ (400:200 µm).
To determine the theoretical RI response of the designed sensor, we monitored the RI sensitivity of the three samples by changing the external RI parameters, as shown in Fig. 3. When LMMF/LSMF = 200/400 µm, the resonance wavelength exhibits the highest RI sensitivity (450 nm/RIU at an RI of 1.423). The RI responses of the other two samples were 92.86 nm/RIU and 19.92 nm/RIU, respectively. The simulation results show that the higher order diffraction mode can result in higher sensitivity. The resonance peaks induced by the similar cladding mode exhibited a similar RI response.
Further, we monitored the energy of the model surface (evanescent field). Figure 4 shows the monitoring results of the evanescent fields (x = 60 µm) and center of the model (x = 0 µm) at the resonance wavelength. In Fig. 4 (a) and (b), it is observed that the light energy is leaked periodically due to the shorter length of the MMF. However, as shown in Fig. 4 (c), the change in the light wave energy distribution is most notable because of the longest MMF in the period. The strongest refractive index modulation capability causes a resonant peak in the shortest length. Figure 4 (d) shows the evanescent field intensity of the three samples. Sample 2 has the strongest evanescent field and then achieves the highest RI sensitivity.
3. Experimental verification and discussion
Based on the aforementioned theoretical analysis and stable preparation method, we prepared three SMS-LPG samples which are the same as the simulation models, as shown in Fig. 5.
3.1 Spectral verification
The transmission spectral evolution of every period is shown in Fig. 6 (a)-(c). These samples clearly show the peak attenuation increases gradually as the number of grating period increases. An increase in the working length (MMF section) results in a reduction in the sensor overall length. However, a reduction in the number of periods also results in a slight increase in the 3 dB bandwidth. Figure 6 (d) shows the resonance wavelengths of the three samples at 1384, 1530, and 1580 nm respectively.
Fig. 6. Transmission spectral evolution of the three SMS-LPG samples (a) 100/500 µm (b) 200/400 µm (c) 400/200 µm. (d) Final transmission spectra of the three samples.
3.2 Refractive index and temperature response
Figure 7 shows the schematic of the experimental setup for measuring the sensing characteristics of the SMS-LPG. The sensor was placed on a constant-temperature furnace (the temperature was maintained at about 25°C). One end of the sensor was linked to the SC source and the other end was connected to a OSA. The index matching solutions consisted of different ratios of the glycerin solution. The OSA recorded the spectral drift behavior of the sensor in the different RI matching solutions, as shown in Fig. 8 (a-c). The results show that the experimental and simulation result are in good agreement. In the measurement process, the visibility of the resonance peak also needs to be considered. As shown in Fig. 8 (b). When the refractive index is increased to 1.432, the resonance peak almost disappears. Therefore, we determined that the measurement range of the RI is 1.333 to 1.423. The three samples have different degrees of red shift in the same RI range. The sample with a duty cycle of 1:2 has the largest red shift. The RI response function is obtained by polynomial fitting: y = 2244.54x2-5960.97x + 3958.10 and a good fit is obtained R2=0.99. The highest RI sensitivity is 427.01 nm/RIU at 1.423, as shown in Fig. 8 (d). The sensitivity of the other two samples is 79.97 nm/RIU and 22.85 nm/RIU, respectively.
Fig. 8. The spectral drift behavior of the samples at different refractive indices (a) 100/500 µm (b) 200/400 µm (c) 400/200 µm. (d) The wavelength shift characteristics of the three samples in the different refractive indices.
The temperature cross-sensitivity is also an important parameter in the field of sensors. Thus, we also studied the temperature response of the three SMS-LPGs. The sensor was placed on a constant-temperature furnace as shown in Fig. 7 and an electronic thermometer was used to ensure the accuracy of the temperature measurements. The temperature measurement range was from 36 to 145 °C. A spectrum was recorded at intervals of about 15°C. After several measurements, the transmission spectra of the three samples at different temperatures were obtained, as shown in Fig. 9 (a)-(c). As the test temperature increases in the test range, the resonance wavelengths of the three samples all shift to longer wavelengths but the temperature sensitivity is different. Figure 9 (d) shows the temperature dependence of the three samples based on linear regression analysis. The values of the three samples are 84.6 pm/°C, 32.3 pm/°C, and 10.9 pm/°C and a good fit was obtained R2=0.98. The test results show that the sample with the highest order of diffraction cladding mode has the lowest temperature response.
Fig. 9. Spectral drift behavior of the three samples at different temperatures (a) 100/500 µm (b) 200/400 µm (c) 400/200 µm. (d) The temperature response of the three samples at the resonance peaks.
4. Conclusion
We designed an SMS-LPG refractometer using a splicing and cutting method. The spectral characteristics and sensing response of the SMS-LPG were theoretically analyzed and experimentally verified. Due to the strong RI modulation capacity of the MMF, clear resonance peaks were obtained with a short total length (the shortest length is 2.4 mm) without requiring expensive equipment. Different RI responses were obtained for SMS-LPGs with different duty cycles. The sample with a duty cycle of 200/400 µm exhibits the greatest response (427.01 nm/RIU). The simulation and experimental results demonstrate that the high order diffraction cladding mode exhibits an excellent RI response. However, for the samples with the low order diffraction cladding mode, the influence of the external environment is lower and a higher temperature response is obtained. The simple preparation process, stable mechanical structure and good sensing characteristics indicate good application prospect of the SMS-LPG.
Funding
Joint Research Fund in Astronomy (U1631239, U1831115); Opening Project of Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences (CAS) (CAS-KLAOT-KF201501, CAS-KLAOT-KF201605); the Fundamental Research Funds for the Central Universities; 111 project (B13015); Aeronautical Science Foundation of China (201608P6003); National Natural Science Foundation of China (NSFC) (11603008, 11704086).
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