1. Introduction

Ultrahigh temperature sensing is of significance because of the wide applications in turbine engines, power plants, petrochemical, gas industry, etc. Compared with the electrical counterparts, optical fiber based sensors have a number of advantages including immunity to electromagnetic interference, high sensitivity, compact structure, light weight, capability of remote and distributed sensing, etc [1]. To enable optical fiber to withstand high temperature with high sensitivity, a great deal of work has been done into the causes involving the fiber materials [2–4], the fiber structures [5,6], and the fabrication techniques [7,8].

Generally, the special structures are created in the telecom standard Ge-doped silica fibers (SMF28) for enhancing temperature sensitivity including fiber Bragg gratings (FBGs) [9], long period gratings (LPGs) [10], Mach-Zehnder interferometers (MZIs) [11], and Fabry-Perot interferometers (FPIs) [12]. However, the performance degradation of the created structures limits the working temperature of the fiber sensors. For example, the FBG sensor could be erased around 600°C due to its weak bonds of germanium and oxygen [13]. An alternative solution to increase the temperature stability of FBG is applying a special thermal treatment on seed grating, which is called regenerated FBG [14,15]. The regenerated FBG can measure temperature up to 1295°C [16], but it requires strong seed gratings and typically provides only low reflectivity. By contrast, previous reports have shown that LPGs written in SMF28 by CO₂ laser can have high stability subjected to temperatures up to 1200°C for 1 hour, however, the possibility of LPGs withstanding higher temperature has not been reported [10]. Different from gratings with period structure, MZIs and FPIs are created in SMF with less modified points of fiber in terms of material and geometry. But both created structures in SMF28 show the application with highest temperature limited to 1100°C [11,12].

For better survivability and stability in high temperature, single crystalline sapphire fibers have been introduced due to the high melting point of sapphire >2000°C [17]. The single crystalline sapphire fibers being sensors can measure temperature up to 1745°C [18–20]. However, the used sapphire fibers are large core and unclad structure. Such large core could result in intermodal dispersion, which makes it difficult to generate good fringes for temperature sensing. Moreover, any containment on such unclad fiber can attenuate the signal so that the performance of the sensors will degrade in harsh environment. Recently, sapphire-derived fiber (SDF) with an aluminosilicate core has attracted much attention due to small Brillion gain coefficient, controllable core diameter, confined modes by cladding protection as well as wide temperature measurement range [21,22]. Elsmann et al have successfully written FBG with high reflectivity on SDF for high temperature sensing, however, the grating-induced peaks drop by 20% within 110 minutes at 1000°C [23]. Grobnic et al have shown two types of FBGs written on both high-content and low-content alumina fibers, but both FBGs on SDF work below 1000°C [24].

In this work, we have demonstrated crystallization-induced refractive index (RI) modulation on SDF showing superheat resistance and explored it for ultrahigh temperature sensing. The SDF is a special fiber with high concentration of alumina to silica in the fiber core region. Reheating and cooling the SDF by the arc discharge method can generate mullite particles in the core region of SDF. Such crystallization can achieve a maximum RI modulation of ~0.015. Utilizing the crystallized SDF as mirrors, a FPI is created in SDF for temperature sensing. The crystallized SDF based FPI shows a good linear response to temperature with sensitivity of ~13.2 pm/°C. Benefiting from superheat resistance of crystallized SDF, the developed SDF-FPI is capable to withstand high temperature up to 1600°C, which is the highest working temperature for amorphous fiber. Moreover, the SDF-FPI sensor exhibits 6-hours stability at temperature of 1200°C. The crystallization-induced RI modulation with superheat resistance opens possibility to achieve functional fiber devices that can work under ultrahigh temperature. The obtained crystallized SDF-FPIs with compactness, wide temperature working range, high sensitivity, and robustness show great potential application in space-limited harsh environments such as turbine engines, power plants, petrochemical and gas industry, etc.

2. Crystallized sapphire-derived fiber for Fabry-Perot interferometer

The SDF is fabricated based on the rod-in-tube method by inserting a sapphire rod into a quartz tube [25,26]. It has alumina-silica (Al₂O₃-SiO₂) composite core with high concentration of alumina of up to 32 mol%. The diameter of the core and the cladding are measured to be 16 μm and 125 μm, respectively. In order to create mirrors, we reheat and cool the core region of SDF by arc discharge method through fiber fusion splicer (FITEL-s178). Due to high intensity of the arc discharge, opaque region in SDF appears near the center of the arc discharge point as shown in the inset of Fig. 1(a).

 

Fig. 1 Procedure of FIB machining of a crystallized SDF: (a) Pt spraying on cross-section of crystallized region; (b) sliced sample in the core; (c) ion beam thinning; (d) aluminum and silicon element distribution along the marked line in (c); (e) the electron diffraction pattern of crystallized particle; (f) 3D refractive index profile along the SDF near the crystallized region.

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To analyze the material of the opaque region, the SDF is examined by transmission electron microscope (TEM; JEM-2010F) combining with energy dispersive spectrometer (EDS; OXFORD). The focused ion beam (FIB) microdissection technology is used for the sample pre-process of TEM. The FIB machining proceeds as follows: firstly, spray Pt on the cross-section of crystallized region as shown in Fig. 1(a); secondly, use FIB to microdissect the targeted region with a typical dimension 10 × 2 × 3 µm (L × W × H) as shown in Fig. 1(b); thirdly, pick up the pre-molding sample and reduce the thickness of the pre-molding sample below 100 nm using the ion beam thinning method as shown in Fig. 1(c). Figure 1(d) shows the composition of particle containing Al and Si elements along the marked line analyzed by the EDS. Al is gathered in the particles’ position, while Si is complementary to Al and less contained in the particles. Figure 1(e) is the electron diffraction pattern taken on one particle, indicating that the material is monocrystal crystalline as viewed along $\text{[1}\overline{\text{2}}\text{0]}$ zoon axis. Such particles are confirmed to be mullite monocrystal that is generated during the reheating and cooling process [25]. We investigate the RI of the crystallized region of SDF based on the digital holographic tomography technology [27]. Due to the symmetric Gauss distribution of the arc discharge intensity along the fiber, x-axis starts from the center of the fiber. Figure 1(f) refers to the three-dimensional (3D) RI profile along the SDF near the crystallized region. RI increases gradually and then decreases to a constant value with the distance ranging from ~136 µm to ~203 µm. Such a change in RI verifies that crystallization plays an important role in RI modulation. By optimizing the discharge condition with intensity of 16.6 mA and duration of 3 ms, we can achieve the RI modulation up to ~0.015, which is much higher than the previous reported material-modification methods with RI modulation <10⁻³ [28–31].

In order to transmit signal in traditional fiber link, we splice a piece of bare SDF with the standard SMFs. To find the loss at the interface between the bare SDF and the standard SMF, firstly, we built a SMF-SDF-SMF structure without crystallization region in SDF. To minimize the attenuation from SDF itself, the access length of SDF is only ~655 μm. An attenuation of ~0.45 dB in transmission is found for SMF-SDF-SMF structure at 1550 nm, which is mainly attributed to the two spliced points. Secondly, the crystallization in SDF is introduced by optimizing the discharge condition and locates near the spliced points. An excess loss is evaluated to be ~5 dB in transmission over repeated experiments [26]. On the other band, comparing with the interface without the crystallization region in SDF, the enhancement of reflectivity is measured to ~6 dB due to the RI modulation of 0.015, which is consistent with the calculation based on Fresnel reflection theory.

Such crystallized regions are explored as mirrors, and FPIs in SDF can be constructed. Figures 2(a)-(c) show the microscopic images of three SDF-FPI samples with 1-mm-long SDF to experience the arc-discharge heating. Due to the crystallization, the center of the core of SDFs becomes dark across ~65 µm along the fiber, which is consistent with the RI change region shown in Fig. 1(f). In the experiment, the cavities of the three SDF-FPI samples start and end in the middle of each crystallized region where the largest RI change take places for simplicity. The corresponding cavity lengths are estimated to be 285, 416 and 476 µm, respectively. Their reflection spectra are measured in Figs. 2(d)-(f), showing the free spectral range (FSR) of ~2.5, ~1.8, and ~1.5 nm, correspondingly.

 

Fig. 2 The microscopic images of three SDF-FPI samples with FPI cavities of (a) 285 µm; (b) 416 µm; and (c) 476 µm; (d)-(f) the corresponding reflection spectra showing different free spectral range (FSR) and extinction ratio (ER).

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Based on the theory of FPI, the FSR is determined by:

3. Crystallized SDF-FPI for ultrahigh temperature sensing

When a FPI is subjected to temperature variation, the effective RI and cavity length will change due to thermo-optic effect and thermal expansion, causing wavelength shift of interference spectrum. We further investigate experimentally the high temperature performance of the crystallized SDF-FPIs.

Figure 3 shows the schematic diagram of the experimental setup. The reflection spectra of the FPIs are recorded by an optical sensing analyzer (Si725). The integrated laser source can scan the wavelength over the range of 1510-1590 nm with a wavelength resolution of 5 pm and an output power of −20 dBm. The two fiber pigtails are connected to the optical sensing analyzer and immersed into RI matching oil, respectively. In the experiment, to characterize the high-temperature sensing performance, the developed fiber sensors are placed into a tube furnace with the maximum temperature up to 1200°C. The temperature rising rate is 2.5°C/min. A series of reflection spectra are measured per 100°C, and 10 minutes is kept at each measurement point. For higher temperature investigation, the tube furnace is substituted by a modified chemical vapor deposition (MCVD) tube that is a graphite furnace with temperature of >2000°C.

 

Fig. 3 Experimental setup for high temperature sensing: sapphire-derived fiber based Fabry-Perot interferometer (SDF-FPI); crystallized region (CR); single mode fiber (SMF); refractive index (RI).

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Figures 4(a) and 4(b) show the reflection spectra of the crystallized SDF-FPI with the cavity length of ~476 µm at various temperatures. As temperature rises, the interference spectrum shifts toward longer wavelengths and the spectral profile of SDF-FPI sensor remains almost unchanged below 1000°C. The marked resonant dip shifts from 1545.2 nm to 1555.7 nm when the temperature rises from 13°C to 1000°C. Figure 4(b) plots the spectra of the crystallized SDF-FPI between 1000°C and 1600°C. Here the spectrum at 1000°C in Fig. 4(a) is the same as the one given in Fig. 4(b). The offset spectra are taken in Fig. 4(b) to trace the detailed evolution of interference spectrum, showing little degradation in ER. The observation proves clearly that the mirrors served by the crystallized SDF can withstand temperature up to 1600°C. However, the performance of SDF-FPI tends to degrade gradually at temperature >1600°C.

 

Fig. 4 Reflection spectra of crystallized SDF-FPI at (a) 13°C, 1000°C, and (b) 1000°C-1600°C; SMF-FPI at (c) 13°C, 1000°C, and (d) 1200°C.

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By comparison, we create a FPI with cavity length of ~369 µm in a standard SMF28 by using a femtosecond laser, and place it in the same test environment. The used femtosecond laser is Ti:sapphire laser with the wavelength of 800 nm, the duration time of ~120 fs, the repetition rate of 1 kHz, and the power level of ~15 mW [32]. The spectral response of the SMF-FPI exposed to various temperatures is shown in Figs. 4(c) and 4(d). As temperature increases, the spectrum of the SMF-FPI shifts towards longer wavelength. The marked resonant dip shifts from 1541.1 nm to 1555.8 nm when the temperature increases from 13°C to 1000°C. From Fig. 4(d), we observe that the ER of the interference spectrum of SMF-FPI drops significantly at 1200°C, and all interference dips vanish rapidly within 10 min. The vanished interference spectrum cannot be recovered even though the temperature decreases to room temperature, indicating that the reflection in the SMF-FPI is destroyed. This observation is consistent with previous reports of SMF-FPIs withstanding the temperature <1100°C [12].

Different from the interference from a pure FPI exhibiting a flat upper and lower envelope [8,12], the upper and lower envelopes of interferences for both SDF-FPIs and SMF-FPIs show fluctuation. Previous study has figured out when the mirror in FPI has a short length rather than a single facet, an extremely short cavity within the mirrors exists and thus induces a fluctuation to the upper and lower envelope of interference [33]. Moreover, the ER of the interference increases when the RI of such mirror increases. Therefore, the strong RI modulation of mirrors and its high temperature stability is the key to maintain a large ER under high temperature. In our experiment, the mirrors of both SDF-FPIs and SMF-FPIs occupy a certain distance, and thus the cavities within the mirrors could affect the envelope and ER of interference. Benefiting from the superheat resistant of mullite, the envelope profile as well as the ER of SDF-FPIs keeps little changed even under high temperature.

In order to trace the spectral change of both SDF-FPI and SMF-FPI when temperature rises, the wavelength shifts as well as ER of resonant dips are analyzed as shown in Figs. 5(a) and 5(b), respectively. The wavelength shift ($\delta \lambda$) of dip (${\lambda }_m$) at various temperature vibration ($\Delta T$) can be expressed as [34]:

 

Fig. 5 Comparison of crystallized SDF-FPI and SMF-FPI in terms of (a) wavelength shift (dots symbols refer to experimental data, dash lines refer to data fitting); and (b) extinction ratio under various temperatures.

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Note that any destroy in the mirrors of FPIs could lead to a failure of large ER in the reflection. Although the ERs of FPIs are different for different dips due to the length of mirrors [33], in turn, a well maintained and large ER at high temperature indicates the stable mirrors in FPIs. Figure 5(b) compares the ER of the SDF-FPI and that of the SMF-FPI. It shows that the ER of SMF-FPI drops significantly >800°C. By contrast, the ER of SDF-FPI keeps constant even though the temperature is up to 1600°C. The SDF-based sensors show good property both in reflective intensity and contrast, which is attributed to the stable material property of the mirrors made from crystal boundary of mullite in silica-mullite composite. The upper temperature limitation of SDF-FPI at 1600°C can be explained by the silica-mullite eutectic temperature of ~1600°C [35]. When the SDF-based sensor is exposed to temperature >1600°C, the dissolution of mullite in crystallization occurs, grain boundary deforms and alumina starts to reprecipitate. Therefore, a decrease in reflectivity results in a change of spectrum profile as well as ER.

We further study the repeatability and stability of SDF-FPIs being high temperature sensors. Although mullite has good chemical stability and superheat resistance and thus the crystallized SDF-FPI can withstand high temperature up to 1600°C, the fiber has become brittle. Therefore, it is believed that the defects in silica cladding and SDFs could limit the long-term high-temperature measurement. As the silica cladding is generally able to survive below 1200°C [10–12], we have conducted the repeatedly work with two temperature cycles between 13°C and 1200°C lasting 48 hours totally. Figure 6(a) plots the wavelength shift of SDF-FPIs versus temperature, showing well-matched temperature sensitivity in the process of temperature cycle tests. For high temperature measurement between 1000°C and 1200°C, the spectral evolution of the crystallized SDF-FPI is given in Fig. 6(b). It is observed clearly that the ER is well maintained at such high temperature for most of resonant dips. Moreover, we have tested long-term performance of the SDF-FPI at 1200°C with 6-hours heating as plotted in the inset of Fig. 6(a). It shows ERs without strong degradation, proving that the crystallized SDF-FPI being ultrahigh temperature sensor can work at 1200°C for 6 hours.

 

Fig. 6 (a) Reheating and cooling the crystallized SDF-FPI for two temperature circles lasting 48 hours totally; (b) the measured spectra of the crystallized SDF-FPI at temperatures of 1000°C, 1100°C and 1200°C during the two temperature circles.

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4. Conclusion

A mechanism for superheat resistance fiber mirror has been proposed. By using arc discharge method, the sapphire-derived fiber can be crystallized in the core region. Such crystallized region consists of mullite particles, which is considered as stable material. The crystallization modulates refractive index of SDF around ~0.015, and the crystallized region serves as mirrors of Fabry-Perot interferometer for ultrahigh temperature sensing. The developed crystallized SDF-FPI shows a linear response to temperature with sensitivity of ~13.2 pm/°C. Since mullite is superheat resistant, the crystallized SDF being mirrors are capable to withstand high temperature up to 1600°C, which is the highest working temperature for amorphous fiber. Moreover, the SDF-FPI sensor exhibits 6-hours stability at 1200°C. Compared with the crystal growth method to produce fibers, the SDF has two main advantages: one is the core-cladding structure; the other is capable fabrication of long fiber. Such core-cladding structure can protect the light confined in the core from any containment on the fiber, and thus the developed sensors are able to endure long time in harsh environment. The obtained crystallized SDF-FPI with compactness, wide temperature working range, high sensitivity, and robustness shows great potential for turbine engines, power plants, petrochemical and gas industry, etc.