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Core Mode-Cladding Supermode Modal Interferometer and High-Temperature Sensing Application Based on All-Solid Photonic Bandgap Fiber Volume 6, Number 1, February 2014 X. L. Tan Y. F. Geng X. J. Li Y. Q. Yu Y. L. Deng Z. Yin R. Gao

DOI: 10.1109/JPHOT.2014.2303571 1943-0655 Ó 2014 IEEE

IEEE Photonics Journal

Modal Interferometer and Temperature Sensing

Core Mode-Cladding Supermode Modal Interferometer and High-Temperature Sensing Application Based on All-Solid Photonic Bandgap Fiber X. L. Tan, Y. F. Geng, X. J. Li, Y. Q. Yu, Y. L. Deng, Z. Yin, and R. Gao College of Physics Science and Technology, Shenzhen Key Laboratory of Sensor Technology, Shenzhen University, Shenzhen 518060, China DOI: 10.1109/JPHOT.2014.2303571 1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received January 20, 2014; accepted January 24, 2014. Date of publication January 29, 2014; date of current version February 6, 2014. This work was supported by the National Science Foundation of China under Grants 61308046 and 61275125, by the Doctoral Fund of Ministry of Education of China under Grant 20114408120002, by the High-Level Talents Project of Guangdong Province, and by the Basic Research Program Funds of Shenzhen under Grant JCYJ20120613110057654. Corresponding authors: Y. F. Geng and X. J. Li (e-mail: [email protected]; [email protected]).

Abstract: A core-mode–cladding-supermode modal interferometer with all-solid photonic bandgap fiber (AS-PBF) is constructed, and a reflective Michelson-type high-temperature sensor is fabricated. Such a fiber sensor is constituted by a small segment of AS-PBF and a leading single-mode fiber. The splice region of the two fibers is weakly tapered to excite the cladding supermode. Both the interference spectra and the near-field infrared CCD images verify that the LP01 cladding supermode is effectively excited and interferes with the LP01 core mode, which agrees well with theoretical results. Benefiting from a large effective thermooptic coefficient between the two modes, temperature sensitivity up to 0.111 nm= C at 500  C is obtained in experiment. The proposed sensor is compact and easy to fabricate, which makes it very attractive for high-temperature sensing applications. Index Terms: All-solid photonic bandgap fiber, modal interference, temperature sensor.

1. Introduction Fiber-optic high-temperature sensors are important for monitoring furnace operation, volcanic events, and fire alarm systems. A considerable number of such sensors have been developed based on different technologies such as fiber Bragg gratings (FBGs) [1], long-period fiber gratings (LPGs) [2], and various interferometric temperature sensors. Using a regenerated FBG, the highest erasing temperature of the FBG is improved to 1100  C or more from 700  C, however, the temperature sensitivity is still limited to be  0:010 nm= C [1]. The LPGs based on the standard Gedoped single mode fiber (SMF) have high-temperature sensitivity ( 0.1 nm= C), but the long length of LPGs (about 30–40 mm) makes them more sensitive to bending [2]. The interferometric ones are more easy-to-fabrication and have high thermal stability over time, making them suitable for high temperature sensing. The Fabry–Perot interferometers for temperature sensing are usually formed by sandwiching different fibers between two SMFs [3], [4] or creating micro-cavity in fiber by femtosecond laser micromachine technique [5], but they always have relatively low sensitivity (below 0.020 nm= C) and need expensive equipment. The Mach–Zehnder (M–Z) high-temperature

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Modal Interferometer and Temperature Sensing

Fig. 1. Schematic diagram and operation principle of the Michelson-type core-cladding modal interferometer.

interferometers are usually constructed with fiber tapers [6], [7], core diameter mismatch [8], or micro-cavity structure [9], [10] as beam splitter or combiner, and the achieved temperature sensitivity could reach 0.109 nm= C [10]; however, the inline structures are not very suitable for the measurements in small enclosed cavities or spaces. The reflective Michelson-type hightemperature sensors with temperature sensitivity up to 0.119 nm= C are usually constructed with an abrupt fiber taper or a peanut-shape structure in SMFs; however, their thin waists of the tapers are more fragile to external force [11], [12]. In this paper, a weak abrupt fiber taper is proposed and fabricated to excite the cladding supermode in the splicing joint of the SMF and a short piece of all-solid photonic bandgap fibers (AS-PBF), and a reflective Michelson-type modal interferometer is fabricated as a high-temperature sensor. In our previous work, an M–Z high-temperature sensor based on the AS-PBF has been constructed, where the LP01 and LP11 core modes are utilized as two optical arms, and its temperature sensitivity is around 0.072 nm= C at 500  C [8]. Instead of using the high-order core mode of LP11 mode as an optical arm, here, a cladding supermode is excited and used to interfere with the LP01 core mode. Owing to the increased large effective thermo-optical coefficient of the two modes and the reflective Michelson-type structure, the temperature sensitivity is expected to be improved greatly, and the length of fiber sensor head could be reduced to be several millimeters. In experiment, the analysis of the interference spectra with different lengths and the near light field infrared CCD images verify that the excited cladding supermode mode in our scheme is LP01 supermode. The response of a 1.03 mm interferometer to high temperature proves that the temperature sensitivity could be improved to be around 0.111 nm= C at 500  C, which is much higher than that of Fabry–Perot-based interferometers and comparable to that of the above mentioned M–Z and Michelson interferometers. Moreover, the proposed sensor is compact and easy to fabricate, which makes it very attractive for high-temperature sensing applications.

2. Operation Principle The schematic diagram and operation principle of our proposed Michelson interferometer are shown in Fig. 1. It consists of a small segment of AS-PBF with one end cleaved, and the other end is spliced to a leading SMF with a weakly tapered region. The waist diameter and the taper length are denoted by d and t , respectively, and the length of AS-PBF is L. Here, such a weak fiber taper is to excite the cladding supermode of AS-PBF mainly existing in the six rods of the first ring near the fiber core. As travelling to the end flat facet of the AS-PBF tip, both the LP01 core mode and the cladding supermode as two optical arms are reflected back for Fresnel reflection; then, they are recombined again and interfere with each other when they reach the tapered region. Due to the different propagation constants, a Michelson-type interference generates. Considering the symmetry of the tapered region, it can be predicted that the excited mode also has a circular symmetry structure. The phase difference ’ of the two modes is expressed as ’¼

4
neff L 

(1)

where
neff is the effective refractive index difference between the LP01 core mode and the excited mode.

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Modal Interferometer and Temperature Sensing

Fig. 2. (a) Cross section of the AS-PBF. (b) An enlarged unit cell of the high-index rod in cladding. (c) The measured transmission spectrum of AS-PBF. (d) The calculated bandgaps (gray region) and effective refractive indices of LP01 and LP11 core modes (red and green curves, respectively), the insets are the intensity profiles of LP01 core mode and LP01 supermode at 1550 nm.

According to two-beam interference theory, the output light intensity can be expressed as follows: pffiffiffiffiffiffiffi (2) I ¼ I1 þ I2 þ 2 I1 I2 cosð’ þ ’0 Þ where I1 and I2 are the intensity of LP01 mode and the excited mode, respectively; ’0 is the initial phase. According to above equations, the fringe spacing at center wavelength 0 can be deduced as ¼

20 : 2 
neff L

(3)

So, with the knowledge of  and L in experiment, the value of
neff could be figured out, and a comparison with theoretical value can be made.

3. Device Fabrication and Experimental Setup The AS-PBF used in experiment is fabricated by the Yangtze Optical Fiber and Cable Company (YOFC Co., China) with a plasma chemical vapor deposition process. The structure of AS-PBF is similar to that of Ref. [8], but with a smaller scale. Fig. 2(a) shows the cross section of the AS-PBF. The fiber cladding consists of periodically triangular-arranged high-index Ge-doped rods, and each rod is surrounded by an index-depressed layer doped with fluorine, which could effectively reduce the transmission loss. The fiber core is formed by missing a high-index rod in the pure silica background. Fig. 2(b) shows a typical unit cell of a high-index rod in cladding. The detailed physical and geometrical parameters are as follows: the outer diameter of the fiber is 123 m, and the core diameter is around 11.8 m; the spacing pitch of the high-index rods A is 9.3 m, and the relative diameters of high-index rods and depressed layers are dGe =A ¼ 0:39 and dF =A ¼ 0:7, respectively. Compared with the pure silica background ðnsilica ¼ 1:45Þ, the average refractive index difference of the Ge-doped and fluorine-doped area are approximately
nG ¼ 3:45  102 and
nF ¼ 7:23  103 , respectively. The measured transmission spectrum is shown in Fig. 2(c). The first bandgap covers from 1140 nm to 1850 nm, and the second bandgap is from 730 nm to 1030 nm formed by periodically arranged high-index rods, and the minimum transmission loss is around 0.024 dB/m in the first bandgap. The stopbands in the transmission (e.g., 1030 nm–1140 nm) are a band of cladding supermodes mainly guided in the high-index rods, which are formed by the index-matching encounter of the core modes with the high-index rod modes [13]. According to the antiresonant reflecting optical waveguide model, the number of the supermodes in each band depends on the rod number of the fiber cladding. Fig. 2(d) shows the photonic bandgaps (gray regions) of the AS-PBF and the locations of the LP01 and LP11 cladding supermode bands calculated by plane wave expansion method, which are in well

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Fig. 3. (a) x -side and (b) y -side views of the fiber taper. (c) Experimental setup for the Michelson-type interferometer based on AS-PBF.

agreement with the measured transmission spectrum. The effective refractive indexes of the LP01 core mode and high-order LP11 core mode calculated by finite element method are illustrated as red and green solid lines, respectively. The top right inset images in Fig. 2(d) are the theoretical intensity profiles of the LP01 core mode and LP01 supermode at 1550 nm. A small segment of unjacketed AS-PBF with both ends cleaved is spliced to an SMF by a commercial splicer (Fujikura FSM-60s) in a manual mode. The duration time and discharge electric intensity are 200 ms and standard-60 bit, respectively. Once the two stabs of the fiber are pushed together and the electric arc discharge finishes, the splicing point is stretched quickly and discharged again with stepping motor drive in the splicer. Then, a fiber taper with the desired waist diameter could be constructed with this repeated stretching and discharging procedure. In experiment, the optimal waist diameter d and length t of the taper are about 100 m and 250 m, respectively, and the cladding supermode could be excited to the maximum extent, and the monitored interference spectra exhibit the largest fringe contrast. Such a weak fiber taper could simultaneously maintain a good physical strength for practical applications. Fig. 3(a) and (b) shows its side views in the x and y directions. Fig. 3(c) is the experimental setup for such a Michelson-type interferometer. An amplified spontaneous emission source (ASE) with wavelength from 1525 nm to 1610 nm is employed as an incident light source. An optical spectrum analyzer (OSA, Agilent 86146B) is used to record the spectra of interference in experiment with the resolution of 0.06 nm.

4. Interference Spectra In order to verify the modal interference between LP01 core mode and cladding supermode, several samples with different lengths of AS-PBF are fabricated. Fig. 4 shows the typical interference spectra with L ¼ 19:5 mm, 14.5 mm, 3.80 mm, and 1.03 mm. It is clearly seen that the output light intensity varies periodically as a function of wavelength, and with L decreasing, the fringe spacing increases as formula (3) describes. The fringe contrast decreases gradually with L increasing. When L is longer than 30 mm, the interference spectrum is difficult to be observed, which is due to the larger transmission loss of the cladding supermode. For the interferometer with L ¼ 1:03 mm, the fringe contrast reaches 25 dB, and the fringe spacing is broadened to be 41.3 nm. Fig. 5 shows the fringe spacing varying with the length of AS-PBF at 1550 nm, where the filled square and solid curves are the measured and fitted values, respectively. According to formula (3), the experimental effective refractive index
neff is about 0.028 at 1550 nm, which suggests that the excited mode is not the LP11 core mode certainly. In order to identify the excited mode, an infrared CCD is employed. The inset image in Fig. 5 shows the collected near field light distribution in the output port of AS-PBF, and it confirms that the interference is between LP01 mode and LP01 cladding supermode in the first layer of high-index rod. By the finite element method, the calculated value of

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Modal Interferometer and Temperature Sensing

Fig. 4. Interference spectra of Michelson-type AS-PBF interferometers with different fiber lengths (L ¼ 19:5 mm, 14.5 mm, 3.8 mm, and 1.03 mm).

Fig. 5. Fringe spacing of Michelson interferometers with different lengths of AS-PBF around 1550nm. Filled square and solid curve are the measured and fitted values, respectively. The inset is the near field light distribution in the output port of AS-PBF at 1550nm.


neff between LP01 mode and LP01 cladding supermode is 0.018 at 1550 nm, which is smaller than the experimental value. The deviation is mainly due to the measurement errors of the air-hole diameter and pitch from a microscope image of the AS-PBF cross section and the refractive index of the doped region.

5. High Temperature Sensing Applications For a modal interferometer, its temperature sensitivity of the interference fringe can be expressed as
 ¼ min ð þ Þ
T

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(4)

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Modal Interferometer and Temperature Sensing

Fig. 6. (a) Response to high temperature for the Michelson-type AS-PBF interferometer. (b) Spectra shift with temperature increasing.

where min is the fringe wavelength with the minimal optical intensity,  ¼ ð1=LÞ@L=
T is the thermal-expansion coefficient of the fiber, and  ¼ ð1=
neff Þ@
neff =
T is the effective thermo-optic coefficient. As discussed before, the thermo-optic coefficients of pure silica, Ge-doped, and F-doped areas are taken as 6:9  106 = C, 8:6  106 = C, and 6:7  106 = C, respectively. The effective thermo-optic coefficient referring to the LP01 core mode and LP01 supermode calculated by the fullvector finite element method is 7:7  105 = C, which is larger than that of Ref. [8]. Neglecting the thermal-expansion coefficient , the temperature sensitivity is calculated to be 0.119 nm= C. Due to the larger effective refractive index difference between the LP01 core mode and the LP01 cladding supermode, the constructed Michelson-type interferometer is with a more compact structure and a larger dynamic range, which makes it more suitable as a high temperature probe. Its response to high temperature is tested with the 1.03 mm interferometer above. The sensor head is put into a tube furnace with maximum temperature of 1200  C and kept straight. In order to relax the intrinsic stresses during the fiber drawing process, the sensor is annealed at 750  C for about 40 min, then cooling down to room temperature before measurement. Then, the sensor head is tested in a temperature range of 20  C to 600  C. Much higher temperature above 800  C could aggravate the germanium and fluorine thermal diffusion due to the different dopant concentration, which eventually makes the interference spectra irreproducible for the heating and cooling measurement cycles [2], [14]. In the heating process, the temperature is increased with a step of 100  C and stays approximately 30 min in each step for accuracy. The sensor head is heated up and cooled down two times, and the wavelength variation with temperature in both heating and cooling processes is recorded, as shown in Fig. 6(a), which shows a good repeatability of temperature response. It also can be seen that the wavelength dip red-shifts with temperature increasing and, moreover, red-shifts gently at relatively low temperature and sharply at higher temperature. The temperature sensitivity is 0.111 nm= C at 500  C, which agrees well with theoretical value. The sensitivity is much higher than that of Fabry–Perot-based interferometers and comparable to that of the above mentioned M–Z and Michelson interferometers. Fig. 6(b) is the spectra of sensor head at different temperatures.

6. Conclusion A core mode-cladding supermode modal interferometer with AS-PBF is constructed. It is fabricated by splicing a segment of AS-PBF to an SMF with a weak tapered region in their splicing joint. The analysis of the interference spectra with different lengths and the near light field infrared CCD image verify that the excited mode in our scheme is the LP01 cladding supermode, which is in well agreement with theoretical results. In experiment, a sensor head with the length of 1.03 mm is tested for high-temperature measurement, and a temperature sensitivity up to 0.111 nm= C at 500  C is obtained. If employing a soft glass AS-PBF with large thermo-optic coefficient, the temperature sensitivity could be increased further in low temperature with such a core-cladding modal

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interferometer [15], [16]. The proposed sensor is with more compact reflective structure, easy-tofabricate, achieved by just using cleaving and splicing processes, and also exhibits high temperature sensitivity, which makes it very attractive for high-temperature sensing.

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