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Abstract
A high-temperature resistant fiber Bragg grating (FBG) with pronounced cladding modes is fabricated using a simple, fast, and flexible point-by-point femtosecond laser inscription. Cladding modes of different orders exhibit various sensitivities to changes in refractive index of the surrounding medium and temperature, while the Bragg wavelength is only dependent on the ambient temperature. By combining the resonance of cladding modes and the Bragg resonance, measurement of variations of temperature and surrounding refractive index can be achieved. Survivability of the cladding modes and the core mode in the fabricated FBG at elevated temperatures up to 1000°C is demonstrated.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber Bragg gratings (FBGs) are compact fiber in-line components that have been widely used in a variety of optical applications, including communications, lasers, and sensing [1,2]. Traditional fabrication of FBGs relies on exposure of a hydrogen-loaded optical fiber to UV light with spatially varying patterns (e.g., typically formed by interference or using a phase mask). The FBGs afford great spectral characteristics due to the UV light-induced periodic modulation of the index of refraction along the fiber core. The strong coupling between the co-propagating core mode and counter-propagating core mode results in a high reflection of the light at a certain wavelength, i.e., Bragg wavelength [1]. The Bragg wavelength of an FBG shows large dependence on the period of the modulation of refractive index (i.e., grating period) and the effective refractive index of the fiber core. The strong dependence makes FBGs an excellent candidate for sensing variations of temperature and strain [2]. This is because changes in temperature modify the effective refractive index of the fiber core due to the thermal-optic effect; variations of strain unavoidably change the grating period due to the elongation of the optical fiber. On the other hand, FBGs are intrinsically insensitive to variations of refractive index of the surroundings because the light is confined in the fiber core. A tilted FBG (TFBG) was proposed to create a refractometer [3]. TFBGs have grating fringes that are tilted with respect to the perpendicular to the optical fiber axial direction. The tilted grating fringes result in the coupling between the core mode and numerous cladding modes. The excited cladding modes are sensitive to the surrounding media due to the interaction of the evanescent field, thus monitoring of surrounding refractive index is made possible using a TFBG [4,5].
However, when it comes to high-temperature applications, the traditional hydrogen-loaded FBGs fabricated based on UV photosensitivity mechanism showed poor stability due to the erasion of the grating pattern in elevated temperatures [6]. FBGs fabricated on regular optical fibers without hydrogen loading using ultrahigh peak power radiation generated by femtosecond (fs) laser systems were reported to address this issue [7]. The refractive index of an optical fiber core can be permanently modified by exposure to tightly focused fs-laser pulses, which ensures the high-temperature stability of the femtosecond-laser written FBGs [8]. In addition, the properties of the inscribed pattern (e.g., period, strength of modulation of refractive index) can be custom-tailored with great freedom in a fs-laser micromachining system. Different techniques have been proposed to fabricate FBGs using fs-laser micromachining for high-temperature applications, including point-by-point (PbP) inscription [9,10], line-by-line (LbL) inscription [11], and phase mask technique [12]. TFBGs with strong cladding modes were also successfully demonstrated using fs-laser LbL inscription [13]. However, fabrication of TFBG using fs-laser LbL inscription method is time-consuming. Alternatively, PbP method is faster (e.g., only a few minutes for fabricating an FBG) and more flexible. Using an off-axis inscription (i.e., off-axis FBG) or high pulse energies (highly localized FBG), PbP fabrication was demonstrated as an effective way to excite cladding modes that can be used for sensing surrounding refractive index [14–16].
In this paper, a high-temperature resistant FBG with stable core and cladding modes up to 1000°C fabricated using PbP fs-laser inscription is presented. The capability of the fabricated FBG for multi-parameter sensing (i.e., measurements of refractive index and temperature) is experimentally investigated. The survivability and no-degradation of the cladding modes at high temperatures up to 1000°C are demonstrated. The work presented here introduces a rapid-production method of fabricating cladding-modes FBGs that can be used for multi-parameter sensing in elevated temperatures up to 1000°C.
2. Sensor fabrication
A fs-laser system (Spirit One, Spectra-Physics) producing laser pulses with duration <400 fs at a repetition rate of 200 kHz was employed in the fabrication. The laser amplifier is integrated with a high efficiency second harmonic generation (SHG) module, which enables the output central wavelength of the laser amplifier to switch between 1040 nm and 520 nm. In the experiment, the laser system was configured to work at 520 nm which offers higher fabrication accuracy. The home-built fs-laser micromachining system includes the fs-laser amplifier and a commercial versatile workstation (femtoFBG, Newport Corporation). The translation stage assembly in the workstation offers a displacement resolution of 0.05 µm along the horizontal directions and a displacement resolution of <1 nm along the vertical direction. The average laser power delivered to the section of optical fiber secured to the workstation for irradiation can be varied using a half-wave plate and a Glan-laser polarizer. The alignment and fabrication process is observed in real-time with a CMOS camera. The fs-laser micromachining system is fully controlled by a computer through a graphical user interface (GUI). By inputting appropriate parameters in the GUI, different types of FBGs (e.g., Gaussian apodized, chirped, sampled, phase-shifted, etc.) can be fabricated using PbP and LbL techniques by the micromachining system.
Specifically, for fabricating the cladding-modes FBG, a PbP method was used. An unstripped single-mode fiber (Corning SMF-28) with a section length of 5 cm was immersed in a refractive index matching gel and immobilized using two glass slides, i.e., a substrate slide and a cover slide. The index matching gel was utilized to eliminate significantly the detrimental effects caused by the cylindrical geometry of the optical fiber. The glass slides with the optical fiber were then secured to the high-precision three-dimensional (3-D) translation stage assembly in the workstation. Through precisely adjusting the 3-D movement of the stage, the center of the core of the optical fiber was positioned at the focal point of the fs laser for irradiation. In the fabrication of the FBG, the translation stage with the secured optical fiber moved along the axial direction of the fiber with a constant velocity (vtran), and the repetition rate of the fs-laser was set to a constant, i.e., fr. The PbP method used a single pulse to create a grating plane in the optical fiber core. Therefore, according to the phase-matching condition, the Bragg wavelength λBragg of the fabricated FBG is determined by
where neff, core is the effective refractive index of the optical fiber core; m is the resonance order of the FBG; and, Λ is the grating pitch. By tuning the ratio between the moving velocity of the translation stage and the repetition rate of the laser, the Bragg wavelength of the fabricated FBGs can be adjusted. The total length of the fabricated FBG was controlled by the displacement of the translation stage. In the fabrication, the fiber was translated along its axial direction at a constant velocity of 102.8 µm/s for a total distance of 7 mm; the repetition rate of the laser was reduced to ∼63.1 Hz by means of the laser internal pulse picker; the energy of each fs-laser pulse delivered to the fiber core was ∼500 nJ. The whole fabrication process took ∼70 seconds. The third-order Bragg wavelength of the fabricated FBG was expected to be 1594.62 nm according to Eq. (1). Please note that a common property of FBGs fabricated by fs laser direct-writing is birefringence, meaning that the geometry of the laser focal point may create local polarization dependence of refractive index so that different grating strengths can be observed for orthogonal input polarizations [17]. The birefringence property is not investigated in this work.An optical spectrum analyzer (OSA, ANDO AQ6317B) and a broadband light source (Thorlabs ASE-FL7002-C4, 1530-1610 nm) were employed to characterize the transmission of the fabricated FBG. Figure 1 shows the transmission spectra of the fabricated FBG. The transmission spectra of the FBG before and after the polymer coating was stripped are shown in Figs. 1(a) and 1(b), respectively. The inset in Fig. 1(a) shows a microscope image of a short section of the fs-laser inscribed FBG. Micro-voids at the center (the dark region) surrounded by a shell (the bright region) could be observed in the fs-modified spot. The grating period and the Bragg wavelength (i.e., the sharp notch wavelength) were found to be ∼1.6 µm and 1595.15 nm (corresponding to a third-order Bragg resonance), which matched well with the designed values. The total length of the FBG was 7 mm. Noticeable transmission loss could be observed in the left region of the Bragg wavelength, which originates from Mie scattering. Meanwhile, weak cladding modes could be observed before the polymer coating was stripped, as shown in Fig. 1(a). After the polymer coating of the FBG was stripped, the resonance depth at the Bragg wavelength remained constant indicating that the core mode was well-confined inside the optical fiber core; resonances of cladding modes were significantly increased, as shown in Fig. 1(b). Each of the resonances on the left region of the Bragg resonance was attributed to the coupling of the core mode to one or more cladding modes defined by the radial and azimuthal mode numbers. The resonance wavelength of i-th cladding mode can be expressed as
where neff, clad, i is the effective refractive index of the i-th cladding mode, the higher the mode order, the smaller the effective refractive index. As the surrounding medium of the cladding changed from the original polymer coating (with refractive index of ∼1.5) to air (with refractive index of 1), an enhanced boundary condition resulted, leading to pronounced resonances of cladding modes. A detailed three-layer model for the cladding modes can be found in [15].
Fig. 1. Measured transmission spectrum of the fabricated FBG (a) before and (b) after the polymer coating was stripped.
3. Experimental results and discussion
3.1 Refractive index sensing
The sensitivity of the cladding modes of the FBG to variations of surrounding refractive index was first characterized; the investigation results are presented in Fig. 2. In the experiment, the FBG was placed in a container with liquids with different indices of refraction. The measured transmission spectra of the FBG at around 1532 nm, 1553 nm, 1573 nm, and 1595 nm are plotted in Figs. 2(a), 2(b), 2(c), and 2(d), respectively. The resonance wavelength at ∼1532 nm, corresponding to the highest order of cladding mode in the observation bandwidth (1530-1610 nm), showed the largest shift as the refractive index increased; as the refractive index of the surrounding increased to 1.3567, the resonance disappeared due to degraded boundary condition. For the resonances at ∼1553.2 nm and 1573.6 nm, the resonance depth remained constant, meaning that the indices of refraction of these two cladding modes are larger than 1.3800, and these two cladding modes are well-guided low-order modes with most of their power concentrated away from the cladding-surrounding boundary. As can be seen from Fig. 2(d), the Bragg wavelength of the FBG showed approximately 2 pm shift as the refractive index of the surrounding increased. The little shift might be due to variations of temperature of the liquids used in the experiment (e.g., 0. 2°C change in temperature). Figure 2(e) shows the measured shift in resonance wavelengths as functions of refractive index of the surrounding liquid. Linear curve fits were applied to the measured datasets; the fitted slopes, i.e., the measurement sensitivity, are indicated in the figure. According to Eq. (2), the sensitivity of i-th cladding mode to changes in surrounding refractive index can be expressed as
As can be seen in Fig. 2(e), the higher the resonance order of the cladding mode (i.e., the smaller the resonance wavelength), the larger the refractive index measurement sensitivity.
Fig. 2. Experimental investigations of the fabricated FBG for measurements of refractive index of its surrounding liquid. The measured transmission spectra of the FBG at (a) 1532 nm, (b)1553 nm, (c) 1573 nm, and (d) 1595 nm. (e) Measured shift in resonance wavelengths as functions of the refractive index of the surrounding liquid. The fitted slopes are indicated in the figure.
3.2 Temperature sensing
The responses of the FBG to temperature variations in a range of 25-65°C were studied and the results are shown in Fig. 3. The measured transmission spectra of the FBG at around 1532 nm, 1553 nm, 1573 nm, and 1595 nm are plotted in Figs. 3(a), 3(b), 3(c), and 3(d), respectively. For all the resonance wavelengths shown, including three cladding modes and the core mode (i.e., Bragg wavelength), the spectrum shifted to the long wavelength region as temperature increased. Figure 3(e) shows the measured shift in resonance wavelengths for the three different orders of cladding modes and the Bragg resonance as functions of temperature. By means of linear curve fit, the temperature sensitivity for the resonance dips shown in Figs. 3(a), 3(b), 3(c), and 3(d) were determined to be 9.393, 9.789, 9.811, and 10.07 pm/°C, respectively. According to Eq. (2), the temperature sensitivity of i-th cladding mode is approximated as
Fig. 3. Experimental investigation of the fabricated FBG for measurements of temperature ranging from 25 to 65 °C. The measured transmission spectra of the FBG at (a) 1532 nm, (b)1553 nm, (c) 1573 nm, and (d) 1595 nm. (e) Measured shift in resonance wavelengths as functions of applied temperature. The fitted slopes are indicated in the figure.
Thus, the temperature sensitivity of i-th cladding mode depends on the cladding mode as well as the core mode. As can be seen in Fig. 3(e), the lower the resonance order of the cladding mode, the slightly larger the temperature sensitivity, approaching the sensitivity of the Bragg resonance.
The results shown in Figs. 2 and 3 indicated that the fabricated FBG can be used for multi-parameter sensing, i.e., measurements of temperature and refractive index of the surrounding medium. Specifically, the cladding modes of the FBG are sensitive to variations of temperature and refractive index of ambient liquid; while the Bragg wavelength of the FBG only responds to changes in temperature. Therefore, a matrix coefficient of the FBG for simultaneous measurements of temperature and refractive index variations (ΔT and Δn) can be built based on the calibrations
The high-temperature survivability of the fabricated FBG was subsequently investigated, and the results are shown in Fig. 4. Figure 4(a) includes the measured transmission spectra of the FBG at room temperature before heating and after heating up to 1000°C. The transmission spectrum of the FBG at 1000°C temperature setting is also included. After the FBG was heated to 1000°C, the Bragg wavelength and the cladding-modes resonance wavelengths shifted to the high wavelength region, as expected [18]. After the FBG was cooled down, the Bragg wavelength and the cladding-modes resonance wavelengths recovered. Figures 4(b) and 4(c) show the evolution of the dip wavelength at 1532 nm (the cladding mode) and 1595 nm (the Bragg wavelength) as the temperature increased from 22°C to 1000°C. As the temperature increased to 1000°C, the resonance depth of the Bragg wavelength increased. The resonance of the cladding mode also survived at 1000°C.
Fig. 4. Demonstration of the high-temperature survivability of the fabricated FBG. (a) Measured transmission spectra of the FBG at room temperature before heating and after heating, and at a temperature setting of 1000°C. The evolution of the resonance wavelength of the FBG at (b) 1532 nm (c) 1595 nm as temperature increased from room temperature to 1000°C.
4. Conclusion
In conclusion, a high-temperature resistant multi-parameter optical fiber sensor based on an FBG was reported. The FBG was fabricated using fast and flexible PbP fs-laser inscription. Cladding modes were obtained in a regular uniform FBG in a standard single-mode fiber. Combing the cladding-modes resonances and the Bragg resonance, simultaneous measurements of surrounding refractive index and temperature could be achieved. The survivability of the FBG, including the cladding modes and the core mode (i.e., corresponding to the Bragg wavelength) at elevated temperatures, up to 1000°C, was experimentally demonstrated. In addition to refractive index, the fabricated FBG with cladding modes can also be used for measurements of strain/temperature and bending/temperature at high-temperature harsh environments. The work demonstrated herein suggests a fast and flexible way to fabricate FBGs with cladding modes that can be used for multi-parameter sensing in elevated temperatures up to 1000°C.
Funding
Leonard Wood Institute (LWI-2018-006); Army Research Laboratory (W911NF-14-2-0034); U.S. Department of Energy (DE-EE0009119).
Acknowledgments
Research was sponsored by the Leonard Wood Institute in cooperation with the U.S. Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-14-2-0034. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Leonard Wood Institute, the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.
Disclosures
The authors declare no conflicts of interest.
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