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We proposed and experimentally demonstrated a high temperature fiber sensor using a hetero-structured cladding solid-core photonic bandgap fiber (HCSC-PBGF) for the first time to our knowledge. A hetero-structured cladding solid-core photonic bandgap fiber is designed and fabricated that supports vibrant core mode and cladding mode transmission. Then, an all fiber M-Z interference sensor is constructed by splicing single mode fiber at both ends of HCSC-PBGF without any other micromachining. The transmission characteristics of HCSC-PBGF are analyzed with a full-vector beam propagation method and a full-vector finite element method, and the simulation results are consistent with experiment results. The sensitivity of this fiber sensor is as high as 0.09 nm/°C when operating from room temperature to 1000 °C, and the fringe contrast keeps stable and clear. It is obvious that this all fiber sensor will have great application prospects in fiber sensing with the advantages of a compact structure, high sensitivity, and cost-effectiveness.
© 2016 Optical Society of America
1. Introduction
Optics fiber sensor has a wide range of applications, it can precisely detect turbulences of physical parameters such as curvature, refractive index, temperature and magnetic field [1–6]. Many technologies have been created to prepare all fiber sensors, including SMF-MMF-SMF structure [5, 6], long period gratings (LPGs) or fiber Bragg gratings (FBGs) [7–10], PCF core diameter mismatch, fiber tapers [11, 12], and multicore fiber coupling [13–15]. In general, there are two ways to construct optics fiber sensor, namely fiber grating and interferometer-based systems. Fiber Bragg gratings have been widely applied in high-temperature sensor for its stability and multiplexing technology. However, its sensitivity still needs to be improved (~0.01 nm/°C) [16, 17]. Long period grating temperature sensor has higher sensitivity (~0.1 nm/°C) [18], and even can be used at the temperature as high as 1200 °C. However, it is complicated in manufacturing. In addition, it is still not practical enough and its practicability is greatly limited due to the interfering from multi-physical parameters. The Fabry-Perot interferometer temperature sensors are usually formed by creating micro-cavity in fiber using femtosecond laser, the sensitivity is not so ideal and the micro processing on fiber is costly [19–23]. MZI temperature sensor can realize light splitter and combiner through core diameter mismatch, tapers and micro-cavity, and it has great performance in sensitivity, but the structure is fragile and complicated [24]. Conclusively, fiber grating sensors have stable structure and high repeatability, but these structures are so sensitive to many physical parameters that they are not accurate enough for fiber sensor. Interferometer-based fiber sensor has better performance in sensitivity but fragile in structure, thus its repetitiveness and stability in practice are questionable.
M-Z interference in PCF is based on the interference between core mode and cladding mode, the excitation of cladding mode and its coupling with core mode can be achieved through two special splicing points. One of the splicing points is to excite the cladding mode by mismatch splicing, collapsed splicing or peanut-like splicing; the other is to realize coupling between cladding mode and core mode by tapering or collapsed splicing. However, whatever methods are used, the fragility of these structures hinders the stability and repetitiveness in fiber sensor, furthermore, the exciting of cladding mode and the preciseness in coupling process with core mode is beyond control, which leads to low extinction ratio and narrow free spectral range that need to be avoided in fiber sensor.
This work demonstrates a compact and stable all fiber M-Z interferometer based on a novel hetero-structured cladding solid-core photonic bandgap fiber (HCSC-PBGF). To realize the optical signal inputting and outputting, the structure only needs to splice standard single mode fiber at both ends of HCSC-PBGF, which is self-designed and fabricated, and the perfect M-Z interfering spectrum can be observed on the spectrometer. Experiment results show that this interference structure with stable structure and high sensitivity has excellent performance when applied to high temperature sensing application.
2. Theoretical analysis and experimental results
Unlike the conventional design ideas, we don’t expect to excite cladding mode by means of core diameter mismatch, tapers or micro-cavity structures, as this micromachining on fiber will always result in the instability and fragility of the interferometer structure. We designed and fabricated a hetero-structured cladding solid-core photonic bandgap fiber (HCSC-PBGF). Figure 1(a) shows the cross section of the HCSC-PBGF, the fiber core is pure silica, and the cladding is composed of Ge-doped rods with high refractive index. The physical and geometrical parameters of the HCSC-PBGF are as follows: the diameter of the fiber core is 10 μm; the out diameter of the fiber is 125 μm; the diameter of the Ge-doped rod and lattice period (pitch) is 1.75 μm and 3.5 μm; the refractive index difference between pure silica background and Ge-doped rod is 0.03. Based on the anti-resonant reflecting optical waveguide (ARROW) theory, the transmitted light will be scattered back into the core at the anti-resonant wavelength or leak into the cladding at the resonance wavelength [25–27]. In order to excite a strong cladding mode, we replace part of the Ge-doped rod with pure silica to form a hetero-structured cladding. The missing Ge-doped rods can change the incident angel of scattered light so that there will be a strong cladding mode mainly guides in the high-index rods.
Fig. 1 (a) The micrograph shows the cross section of the HCSC-PBGF. (b) Schematic diagram and operation of the MZI. (c)- (d) Microscopic images of the fusion splicing joints.
The input and output ends of light can be constructed by using fusion splicer (Fujikura FSM-60S) to splice single mode fiber to both ends of HCSC-PBGF. Single mode fusion program inside the fusion splicer can make perfect splicing because the HCSC-PBGF has similar physical properties to single mode fiber. A super-continuum source is linked to the input end as the signal light, and the spectrometer (YOKOGAWA AQ6370D) is connected with the output end. Schematic diagram of MZI is shown in Fig. 1(b), it can be seen that signal light enters from single mode fiber at the first fusion splicing point, and it will excite two different modes in HCSC-PBGF, then those modes are coupled into the single mode fiber at the other fusion splicing point, and induce strong mode interference between them. The fusion point of single mode fiber and HCSC-PBGF is constructed by single mode fusion program of fusion splicer which shows the perfect splicing with barely extra loss, as shown in Figs. 1(c) and 1(d), respectively. The fiber sensor with this structure is easily fabricated with stable structure and less expense in contrast to those reported in [17–20].
When the length of HCSC-PBGF is 10 cm, we can observe the strong interference spectrum shown as Fig. 2(a) in the spectrometer. Furthermore, the insertion loss is less than 1 dB and fringe contrast reaches to 20 dB. In addition, a full-vector beam propagation simulation result is shown in Fig. 2(b) when the length of the HCSC-PBGF is the same as the one we have used in the experiment, and the beam propagation direction is along z-axis. The light is coupled into the HCSC-PBGF from the single mode fiber at the first fusion splicing joint and then induce strong core mode and cladding mode. Due to the refractive index difference between the core mode and cladding mode, there is a phase difference between them. Then a dual-mode MZI is formed when the two modes are coupled into a single mode fiber and induced strong mode interferences between them. As can be seen from Fig. 2(b), the power reaches the coherent emphasis point at the wavelength of 1587.26 nm, and reaches the coherent decrease point at the wavelength of 1562.97nm, which is very consistent with the experiment results show in Fig. 2(a).
Fig. 2 (a) Transmission spectrum of the MZI with 10 cm long HCSC-PBGF. (b) Beam propagation simulation of the MZI at the wavelength of 1562.97 nm and 1587.26 nm, respectively.
According to the M-Z interference theory when the light travels along the HCSC-PBGF with a distance of L, the output intensity and the phase difference can be expressed as follows:
where and are the intensity of core mode and cladding mode; and are the effective refractive indices of the core and cladding mode, respectively; ,corresponds to the minimum wavelength value of the output light intensity. As a result, the free spectral range (FSM) of this MZI can be expressed as:where is the effective refractive indices difference between the core mode and cladding mode. In order to verify the interference between core mode and cladding mode, three samples with different lengths of HCSC-PBGF are used to measure the transmission spectrum as shown in Fig. 3. It can be seen that the output light intensity varies periodically, and the FSR becomes larger with fiber shortened. What’s more, the maximum dynamic range is larger than 200 nm which is significant to the high temperature sensing.In addition, we used full-vector finite element method to calculate the effective refractive index of the core and cladding mode, as show in Fig. 4(a). Figure 4(b) and 4(c) is the calculated core and cladding mode in HCSC-PBGF, and the value of between the core and cladding mode is 0.45 × 10−3 around the wavelength of 1550 nm. According to Eq. (3), the corresponding FSRs are calculated to be 23.7 nm, 14.9 nm and 11.9 nm while the lengths of the HCSC-PBGF are the same with experiment, which are close to the experimentally measured values in Fig. 3. Furthermore, the spectra of MZI is Fast Fourier transformed to obtain the spatial frequency spectrums in order to determine which cladding mode provides dominant contribution to the interference spectrum, as show in Fig. 4(d). It is obvious that there are several modes including the fundamental mode and cladding modes. However, the power is primarily distributed in the fundamental mode and one cladding mode, which means the other cladding modes have very small contribution to the interference pattern. It can be seen that the peak amplitudes are located at the frequency of 0.038, 0.06 and 0.08 for the MZI, which correspond to the FSR of 26.3 nm, 16.7 nm and 12.5 nm. These results are very close to the values calculated by Eq. (3) and experimentally measured values in Fig. 3.
Fig. 4 (a) The effective refractive indices of core mode and cladding mode. (b)-(c) The intensity profiles of core mode and cladding mode calculated by full-vector finite element. (d) Spatial frequency spectrum by taking the FFT for MZI with the length of HCSC-PBGF is 45 cm, 37 cm and 22.5 cm.
In order to demonstrate its feasibility in high-temperature fiber sensing, we fabricated a MZI with the HCSC-PBGF length of 21.5 cm, and make sure that the HCSC-PBGF is straight inside a tube furnace while the single mode fiber is outside the tube furnace. The temperature is changed from 29 °C to 1000 °C in steps of 50 °C and a period of approximated 30 min for each step, and the maximum temperature of the tube furnace is 1000 °C. The dip wavelength at 1609.86 nm is picked before heating, which has the largest dynamic range. Figure 5(a) shows the temperature’s response to the HCSC-PBGF-based MZI. The dip blue-shifted gently below 300 °C, and more rapidly above 300 °C with a sensitivity of 0.09 nm/°C, which is higher than those reported in [19–21]. Furthermore, the wavelength shifts linearly with temperature in low temperature region and high temperature region. Figure 5(b) is the stable spectra response with temperature rising, and the fringe contrast is still clear and stable although the temperature is as high as 1000 °C.
Fig. 5 (a) Response to high temperature for the HCSC-PBGF-based MZI. (b) Interference spectra at 29 °C, 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C.
For mode interference, the wavelength shifts as a function of temperature because of two effects: the difference of effective refractive index changes with temperature due to the influence of thermos-optic, and the length of the HCSC-PBGF changes with temperature due to thermal expansion, which can be expressed by the following equations respectively:
Then the wavelength variation with temperature can be expressed as:whereis the coherent decrease point,is the effective refractive index difference between core and cladding mode, L is the length of HCSC-PBGF and is the temperature variation. In addition, the thermal expansion coefficient of silica is 5.5 × 10−7 K−1, the thermo-optic coefficients of Ge-doped rod and pure silica is 8.6 × 10−6 °C−1 and 6.9 × 10−6 °C−1, respectively [28]. Then the effective thermo-optic coefficient calculated by the full-vector finite method is 5.3 × 10−6 °C−1, according to Eq. (6) the temperature sensitivity is calculated to be 0.084 nm/°C while neglecting the thermal-expansion coefficient which is 2 orders smaller than the effective thermo-optic coefficient.3. Conclusion
In summary, we have experimentally demonstrated a compact and stable MZI based on a hetero-structured cladding solid-core photonic bandgap fiber for high-temperature sensing. A hetero-structured cladding design makes the HCSC-PBGF favorable for core and cladding mode transmission, and it only needs to splice single mode fiber at both ends of HCSC-PBGF to construct an all fiber sensor without any micromachining. The theoretical analysis of transmission mode of HCSC-PBGF is consistent with the experiment results. The sensing characteristics of the sensor with a HCSC-PBGF length of 21.5 cm is tested at high-temperature whose sensitivity can reach up to 0.09 nm/°C even at temperature as high as 1000 °C. The proposed sensor has the advantages of compact structure, simple fabrication process and high sensitivity, which indicates the great potential in high temperature fiber sensor application.
Funding
National Natural Science Foundation of China (NSFC) (61575075, 61378070, 51672091).
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