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Abstract
We propose a high-temperature sensor based on a suspended-core microstructured optical fiber (SCMF). The sensor is constructed by fusion splicing a piece of SCMF between two sections of multimode fibers (MMFs) which act as light beam couplers. The multimode interference is formed by the air cladding modes and the silica core modes in the SCMF. Fast Fourier transform is adapted to filtering the raw transmission spectra of the MMF-SCMF-MMF structure. The wavelength shift of the dominant spatial frequency is monitored as the temperature varies from 50 °C to 800 °C. The sensitivities of 31.6 pm/°C and 51.6 pm/°C in the temperature range of 50 °C-450 °C and 450 °C-800 °C are respectively achieved. Taking advantage of the compact size, good stability and repeatability, easy fabrication, and low cost, this proposed high-temperature sensor has an applicable value.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber sensors have attracted widespread interests for temperature sensing with their well-known advantages, such as compact size, low cost, resistance to electromagnetic interference, and high reliability [1]. Especially, they are superior in applications of harsh conditions [2,3], such as high temperature and strong electromagnetic fields. Various high-temperature sensors based on optical fiber structure have been extensively investigated in fiber Bragg grating (FBG), long-period fiber gratings (LPFGs), and in-line fiber intermodal interferometers. It is well known that FBG temperature sensors are predominant in distributed sensing [3,4]. However, Type-I FBGs created by the traditional ultra-violet inscription technique will be erased at a temperature higher than 500 °C [5–8]. Type II FBGs can work at a temperature up to 1000 °C, but its fabrication requires expensive lasers with high energy density, such as excimer laser or femtosecond laser [9–11]. Although LPFG-based high-temperature sensors possess much higher sensitivity, the long length and large cross-sensitivity to bending or refractive index (RI) need to be optimized [12–14]. Alternatively, Fabry–Perot interferometer (FPI) [15–27] and intermodal interferometers structures, for instance, Michelson interferometer (MI) [28–34], and Mach-Zehnder interferometer (MZI) [34–39] have been introduced in conventional or special optical fibers for high-temperature sensing.
However, among these in-line fiber intermodal interferometers, some of them [15–19,29–31] are highly vulnerable to dust and may introduce an apparent cross-sensitivity to other variables, including bending, external refractive index, and humidity. Therefore, packages are required to protect the sensor probes in practical applications. Meanwhile, these interferometers are fabricated by femtosecond laser micromachining [15–18,34,35] or chemical etching [19–23], which not only weakens the mechanical strength but also increases the production cost and fabrication complexity. Moreover, the similar thermo-optic coefficient of the interference modes will incur a low temperature sensitivity, such as multimode-fiber-based interferometer [31–33]. As limited by the relatively low thermo-optic coefficient (TOC) and thermal expansion coefficient (TEC) of silica fiber, some FPIs [20,21,25] also suffer from very poor temperature response. Instead, a large thermo-optic coefficient difference of interference modes will heighten the temperature sensitivity. It is noteworthy that for long-term operation at high temperature, the temperature sensing elements constructed by conventional optical fibers with a doped core are impeded because the dopants in the core will diffuse into the cladding [32], which causes poor stability and reversibility. A promising alternative is to use single material optical fibers to construct sensing element, such as pure-silica optical fiber.
In this work, a high-temperature sensor based on a suspended-core microstructured optical fiber (SCMF) is proposed. The SCMF with several hundred microns is sandwiched between two sections of MMFs via simple cleaving and splicing. The MMF plays a role of light beam coupler that divides the input light into the air cladding modes and the silica core modes of the SCMF. The multimode interference is formed by the air cladding modes and the silica core modes. The large thermo-optic coefficient difference of the air cladding modes and the silica core modes in SCMF contributes to heightened temperature sensitivity. The MMF-SCMF-MMF structure undergoes twice heating and cooling cycles in the temperature range of 50 °C–800 °C. Fast Fourier transform is adapted to filtering the raw transmission spectra. The wavelength shift of the dominant spatial frequency is monitored as the temperature varies. Since the SCMF comprising of pure silica is a kind of single material optical fiber, the sensor demonstrates good stability, including wavelength stability and intensity stability, during the high temperature-maintaining process. Moreover, the sensor shows good reversibility during the twice heating/cooling processes. Standard deviation method is implemented in the recorded data of two cycles. The arithmetic average temperature sensitivities are obtained to be 31.6 pm/°C and 51.6 pm/°C in the range of 50 °C-450 °C and 450 °C-800 °C, respectively. Moreover, the proposed high-temperature sensor is inert to bending, strain, and RI, which eliminates an additional encapsulation in practical application.
2. Working principle and fabrication
Figure 1 shows the schematic diagram of this proposed SCMF-based high-temperature sensor. The light from the lead-in SMF is transmitted into the MMF. The MMF plays a role of light beam coupler, which can split the light into the air hole part and the silica core part of the SCMF. The part of the propagating modes in the suspended core can be well-confined through total internal reflection attributed to the high refractive index difference between the silica core and the inner air cladding. The modes of the air hole part and the silica core part are recoupled back to the lead-out SMF by the second MMF. Due to the optical path difference (OPD) between these propagation modes with different effective refractive index, a multimode interference pattern is formed in the transmission spectrum of the sensor.
The transmission power after propagating the SMF-MMF-SCMF-MMF-SMF structure can be described as [30]:
where Ii and Ij are the power portion carried in the ith and jth modes of the SCMF, respectively, and neff is the effective refractive index of the corresponding mode. LSCMF is the length of SCMF and λ is the input wavelength from lead-in SMF. As seen in Eq. (1), the transmission consists of multiple cosine components, indicating a multimode interference effect. Each cosine component corresponds to an interference formed by a specific pair of modes {i, j} in the SCMF. Therefore, the transmission spectrum results from the interference superposition of multiple mode pairs. The phase difference of one interference mode pair is given byAs a result of the thermo-optic effect and thermal expansion effect of the SCMF, the phase term is sensitive to temperature. The phase change induced by temperature variation can be expressed as:
It is well known that once the phase difference meets the condition of Δϕ = (2m + 1) π, (m = 0, ± 1, ± 2…), destructive interference will take place. The wavelength dip can be deduced as:
The relationship between wavelength dip and temperature can be derived by
where β is the thermal expansion coefficient of the SCMF, and Δneff is the effective refractive index difference of the ith and jth modes. Equation (5) expresses the temperature sensing principle of one interference mode pair. For multi-mode interference structures, one can use the superposition effect of multi-mode interference for sensing, while another method is to select the dominant mode interference for sensing. Equation (5) implies that the larger variations of Δneff and β with respect to temperature will enhance the wavelength change and the temperature sensitivity.The fabrication process of this high-temperature sensor is illustrated in Fig. 2. Figure 2(a) is the microscope image of the cross-section of the SCMF. The SCMF used in our sensor has three air holes. The major and minor axes of the elliptical air hole are 36.3 ± 0.1 μm and 24.8 ± 0.1 μm. The pitch size between the air holes is measured to be 2.5 ± 0.1 μm. The diameter of the core and the whole cross section are about 6.8 μm and 123 μm, respectively. Firstly, a piece of MMF with a cladding diameter of 125 μm and a core diameter of 62.5 μm) is spliced to a lead-in SMF by using a fusion splicer (FSM-45PM, Fujikura). Next, the MMF is cleaved to 1 mm and spliced to a section of SCMF. Choosing MMF is to ensure that there is sufficient light coupled into the air holes of the SCMF to form air cladding modes, 1 mm length of MMF would not excite obvious multimode interference in the MMF since high-order modes will not converge without self-imaging effect in such short region. To ensure that the air holes of SCMF are not collapsed, arc discharge deviating of 80 μm from the splice point, low arc power of −68 bit, and arc duration time of 380 ms are applied. Then, with the help of a microscope, the SCMF is cleaved to about several hundred microns. Finally, the cut end face of the SCMF is spliced to another 1 mm length MMF which has been spliced to a lead-out SMF. Figure 2(h) is the microscope image of the lateral view of the MMF- SCMF-MMF.
3. Experimental setup and measurement results
Figure 3 shows the experiment setup schematic of the proposed sensor. The incident light from a broadband source (BBS) with an effective wavelength range from 1525 to 1610 nm is launched into the SCMF, and the transmission spectrum is recorded by an optical spectrum analyzer (OSA, Anritsu CMA5000). Several samples of sensors with different SCMF lengths are fabricated in the experiment and their measured transmission spectra at room temperature are shown in Fig. 4. One can see from Fig. 4 that the transmission spectrum of the MMF-SCMF-MMF is the result of multimode interferences. Moreover, the length of SCMF is longer, the interference fringe spacing is denser, which is highly consistent with the general definition of the free spectral range of transmission spectrum [32]:
Fig. 4 The measured transmission spectra of the no MMF structure and the MMF-SCMF-MMF structures with different SCMF length.
For an SCMF-based sensor without MMF, the transmission spectrum almost has no interference thanks to a small amount of weak air cladding modes. Therefore, in this work, the MMF plays a vital role of light beam coupler that excites more air cladding modes and recouples them back to the lead-out SMF to interfere with the silica core modes.
To have a deep insight into the interference modes, fast Fourier Transform (FFT) is applied to the transmission spectra of different SCMF length to obtain the spatial frequency spectra, as shown in Fig. 5. Each sensor has one dominant spatial frequency and several weak spatial frequencies. The dominant spatial frequencies corresponding to the interference between the two most powerful modes are 0.05882 nm−1, 0.08235 nm−1, 0.1882 nm−1, 0.2235 nm−1, and 0.3059 nm−1 for the length of 334.5 μm, 457.1 μm, 1.05 mm, 1.23 mm, and 1.72 mm, respectively. That is the dominant spatial frequency shifts towards higher spatial frequencies with respect to the increment of SCMF length. Furthermore, the two dominant interference modes resulting in the dominant spatial frequency can be resolved through the relationship between effective refractive index difference and the spatial frequency ζ [33]. Consequently, the dominant spatial frequencies in Fig. 5 are in accord with the Δneff about 0.44, which means the two dominant interference modes originate from the air cladding mode and silica core mode.
The sensor with SCMF length of 457.1 μm is chosen for temperature sensing. As shown in Fig. 4, due to multimode interference, the wavelength dips of the transmission spectra are ambiguous. Therefore, fast Fourier transform (FFT) is adapted to filtering the transmission spectra. The dominant spatial frequency of the filtered spectra is converted back to the wavelength domain by a linear phase finite impulse response (FIR) filters and the wavelength shift is measured to determine temperature changes. The schematic diagram of the measurement setup is shown in Fig. 3. The SCMF is inserted into a ceramic tube, and then the ceramic tube is put into a controllable high-temperature furnace (resolution ± 1 °C). The furnace is programmed to increase the temperature from 50 °C to 800 °C with a step of 50 °C, and decrease back to 50 °C with the same step. For observing the stability, the sensor is kept at 800 °C for 3 hours before the temperature was decreased.
The spectral shifts of the dominant spatial frequency ξ = 0.08235 during the heating and cooling processes are displayed in Figs. 6 (a) and 6(b), respectively. An expected redshift is observed as the temperature increased and vice versa. The wavelength dip λ = 1567.225 nm at the temperature of 50 °C is chosen for tracking. When the temperature is increased 800 °C, the tracking wavelength dip shifts from 1567.225 nm to 1599.295 nm. A 180-min stability test is performed at 800 °C. The spectrum is recorded every 10 min and, a total of 19 data are obtained. A stable response, including the wavelength and the intensity, can be observed from Fig. 7. The deviation of wavelength and intensity mainly derive from two aspects: (i) the temperature fluctuation of the furnace; (ii) the power fluctuation and wavelength shift of the light source. A temperature control furnace with high precision can further improve stability.
Fig. 6 The spectral shift of the dominant spatial frequency ξ = 0.08235 during (a) the heating process; (b) the cooling process.
Fig. 7 (a) The spectra of 19 recorded data at the tracking wavelength dip within 3 h. (b) Stabilities in terms of wavelength and intensity within 3 h.
In order to demonstrate the repeatability of the SCMF-based temperature sensor, the heating and cooling processes are repeated twice, and the result is presented in Fig. 8(a). It is obvious that the data of two heating and cooling cycles match well, which shows good reversibility and stability for temperature sensing. As concluded in Fig. 8(b), the experimental data with an error bar are calculated using the records of two cycles and standard deviation method. A quadratic fitting y = 2.6 × 10−5x2 + 0.183x + 1567.4 is matched to all data. The temperature response curve shows a nonlinear property due to the nonlinearity of the thermo-optic effect in a large and high-temperature range [40]. The arithmetic average temperature sensitivity is obtained to be 31.6 pm/°C for the temperature range of 50 °C −450 °C and 51.6 pm/°C for the temperature range of 450 °C −800 °C.
Fig. 8 (a) The tracking wavelength dip at different temperatures during the two heating and cooling cycles. (b) The experimental data of wavelength shift with error bar as temperature rising. Black line: quadratic fitting; R2: the correlation coefficient of the quadratic fitting.
Thanks to the high refractive index difference between the silica core and the inner air cladding, propagating modes in the suspended core can be well-confined through total internal reflection. The influences from other external variables, such as RI, bending and strain, on the temperature response of the MMF-SCMF-MMF structure are investigated. Putting the MMF-SCMF-MMF structure into some mixtures of glycerol and water, the RI values of the mixtures is calibrated by an Abbe refractometer (resolution ± 0.0002). As seen in Fig. 9(a), the tracking wavelength dip has no shift with RI increasing from 1.3335 to 1.453. This is because external RI will not affect the mode interference that occurs between the air cladding mode and silica core mode. Moreover, when downward bends from 2.74 m−1 to 9.5 m−1 are applied to the MMF-SCMF-MMF structure, the wavelength dip hardly drifts, but the intensity changes slightly, which will not affect the temperature sensitivity. The strain response trial is carried out with one end of the MMF-SCMF-MMF structure glued on a fixed stage, while another glued on a translation stage. The initial distance between the two stages is 40 mm. Then the translation stage moves 100 µm with a step of 10 µm. The strain sensitivity of the MMF-SCMF-MMF structure is 0.2 pm/µɛ over the wide range of 250 µɛ −2500 µɛ. The strain -induced temperature measurement error is about 0.0063 °C/με for the temperature range of 50 °C −450 °C and 0.0038 °C/με for the temperature range of 450 °C −800 °C, which hardly affects the temperature response. Therefore, this sensor can be exempted from an additional encapsulation in practical applications.
4. Summary
In conclusion, we proposed and experimentally demonstrated a high-temperature sensor based on a suspended-core microstructured optical fiber (SCMF). The SCMF is sandwiched by two sections of MMFs that play the role of light beam coupler. The multimode interference is formed by the air cladding modes and the silica core modes of the SCMF. The MMF-SCMF-MMF structure is applied for high temperature sensing in the range of 50°C–800 °C. Fast Fourier transform is adapted to filtering the raw transmission spectra. The wavelength shift of the dominant spatial frequency is monitored as the temperature varies during the two heating and cooling cycles. This high-temperature fiber sensor shows good stability and repeatability. The linearly average temperature sensitivities of 31.6 pm/°C and 51.6 pm/°C in the range of 50°C-450°C and 450°C-800°C are respectively achieved. The proposed high-temperature sensor is inert to bending, strain, and RI, which eliminates an additional encapsulation.
Funding
National Key R&D Program of China (2018YFC1503703-3); National Natural Science Foundation of China (NSFC) (61078006, 61275066); Shanghai Aerospace Science and Technology Innovation (SAST2017-099).
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