This paper presents a method of using femtosecond laser inscribed nanograting as low-loss– and high-temperature–stable in-fiber reflectors. By introducing a pair of nanograting inside the core of a single-mode optical fiber, an intrinsic Fabry-Perot interferometer can be created for high-temperature sensing applications. The morphology of the nanograting inscribed in fiber cores was engineered by tuning the fabrication conditions to achieve a high fringe visibility of 0.49 and low insertion loss of 0.002 dB per sensor. Using a white light interferometry demodulation algorithm, we demonstrate the temperature sensitivity, cross-talk, and spatial multiplexability of sensor arrays. Both the sensor performance and stability were studied from room temperature to 1000°C with cyclic heating and cooling. Our results demonstrate a femtosecond direct laser writing technique capable of producing highly multiplexable in-fiber intrinsic Fabry-Perot interferometer sensor devices with high fringe contrast, high sensitivity, and low-loss for application in harsh environmental conditions.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optical fiber sensors play an important role in a wide variety of structural health monitoring applications for energy systems due to their superior robustness in harsh environments. Through the exploitation of the various light scattering processes in optical fibers, distributed measurements of physical parameters can be achieved with high spatial resolution. However, applications of scattering-based distributed fiber sensors are limited by expensive interrogation instruments [1–4]. On the other hand, point fiber sensors, such as Fiber Bragg Grating (FBG) or Fabry-Perot Interferometers (FPI) devices can be demodulated with simpler interrogation systems, and at a lower cost. Multiplexing of these sensors has been a promising alternative to performing distributed sensing. Fabry-Perot Interferometer (FPI) has been known for its easy fabrication of a pair of reflectors. There are two main types of FPIs. An Extrinsic Fabry-Perot Interferometer (EFPI) which is fabricated from inserting an external fiber part  or an air gap  between two cleaved optical fibers sections. EFPIs usually have low mechanical stability, and their multiplexing capability suffers from high insertion loss and degradation which occurs with increased cavity length . On the contrary, an Intrinsic Fabry-Perot Interferometer (IFPI) can be fabricated by the integration of the reflector inside an optical fiber, such as a pair of FBGs  or micro-reflectors . Compared to EFPIs, IFPIs are easier to package and handle, and have lower insertion loss and are well suited for multiplexed measurements. Dense multiplexing of IFPI sensors can be achieved by weak FBG arrays inscribed by UV laser . However, IFPI sensors formed by FBG reflectors only produce interference fringe within FBG resonance wavelengths. This could potentially reduce the accuracy of demodulation algorithms to determine the cavity length of devices. Additionally, FBGs inscribed by UV lasers do not have good stability at high temperatures. FBG fabrication also requires a hydrogen loading process for standard optical fibers or photosensitive fibers .
On the other hand, femtosecond laser processing has been widely applied in the fabrication of high-quality sensors for use in extreme environments [1–3]. Femtosecond lasers have been widely used in both micro- and nano- scale manufacturing [11,12]. Several manufacturing schemes have been reported to fabricate FPIs in optical fibers using femtosecond laser processing to perform point or quasi-distributed pressure , refractive index [14,15], temperature [9,16–23], strain [22,23], gas concentration , and micro-fluidic  measurements. Femtosecond lasers have been used to perform micromachining on a cleaved optical fiber end-facet to create a cavity, followed by fusion splicing to form EFPI diaphragm sensors for pressure  and refractive index  measurements. By depositing a Pd film over the femtosecond laser micromachined EFPI cavity, hydrogen sensing can also be performed . The femtosecond laser ablation technique has also been used to produce a micro-notch to form an IFPI inside a single-mode optical fiber and a photonic crystal fiber for both refractive index  and temperature measurements [16,26]. Finally, utilizing femtosecond laser-assisted etching, multiplexed IFPIs can be formed in silica fibers for refractive index sensing . However, FPI devices formed by laser micromachining techniques can incur high insertion losses that limit multiplexing capability and performance. Moreover, physical features produced by the laser micromachining processes have significant impact on mechanic integrities of fibers. Meanwhile, femtosecond laser-induced multiphoton absorption has been widely used to fabricate photonic structure on transparent bulk platforms and in-fiber optical device . The laser-induced refractive index modification, also known as Type-I modification, has been applied to form a reflector inside the core of the optical fiber, which interacts with the cleaved end of a silica fiber to form an IFPI for temperature sensing . Laser introduced refractive index change points could also be used to form cascaded micro-mirror reflectors for single IFPI to increase the fringe contrast, improve the sensitivity [18–20], and for use in multiplexed IFPIs for temperature or strain sensors [21–23].
Compared to Type-I refractive index modifications used in [9,17–23], femtosecond lasers-inscribed Type-II modifications has shown superior performance up to a temperature of 1100°C for high-temperature stable FBG sensing applications [27,28]. Anisotropic self-organized nanostructures, known as nanograting, formed at the fiber cross-sections in the Type-II modification regime [11,12], which has been widely explored in femtosecond laser selective etching to fabricate microfluidic chips  and FBG for use as sensors in extreme environments for their exceptional high-temperature stability [30,31]. More recently, femtosecond laser-induced nanograting was inscribed in unmodified fibers to improve the signal-to-noise ratio and high-temperature stability of distributed fiber sensors for use in environments with extreme temperatures of up to 800°C under active hydrogen gas  and in environments with extreme radiation .
Comparing with devices formed by laser-induced Type-I modification, fiber devices based on Type-II modification exhibit better temperature stability but incurs higher loss [27,28,30]. This paper presents a systematic study of femtosecond laser-induced Type-II nanograting as in-fiber reflectors for use in the formation of multiplexable high-temperature stable IFPIs. In this work, a roll-to-roll femtosecond laser direct writing setup was used to produce an IFP array in a standard single-mode silica-core fiber. By tuning the properties of laser-induced nanograting, we present an optimized IFPI with an insertion loss of 0.002 dB with high fringe visibility. We studied the high temperature sensing performance of a single IFPI under repeated cycles up to 1000°C, using a low-cost and real-time white light demodulation system. The method presented in this paper shows that nanograting induced by femtosecond laser direct writing can yield in-fiber reflection that is sufficient to form multiplexable IFPI sensors at extreme temperatures up to 1000°C.
2. Fabrication and characterization
A standard telecom single-mode optical fiber (Corning SMF-28e+) was selected as the fiber platform for sensor fabrication. The single-mode fiber reel was mounted on a customized roll-to-roll fabrication setup for translation. The roll-to-roll setup is composed of a nano-precision motion stage, a rotary stage, an imaging system, 3-D printed parts, and mechanical pulleys, that enable precise inscription of sensors in a fiber core along infinite distance with a simple alignment procedure. The fabrication process was done with the protective jacket to preserve the mechanical strength of the optical fiber. A Coherent RegA 9000 Ti:sapphire laser and amplifier system operating at 800-nm, 270-fs, and 250-kHz repetition rate was used for fabrication. A linearly polarized laser beam was tightly focused inside the core of the fiber sample through a 100× oil-immersion objective (Olympus 1-U2B235, NA 1.25) with index-matching oil, to compensate for the aberration at the spherical fiber shape (Fig. 1(a)). The input laser beam diameter is ∼ 2 mm, which fills the rear aperture of the objective. Each FPI reflector was created by femtosecond laser pulses with 160-nJ on-target pulse energy. Figures 1(b) and (c) show the cross-sectional view and side-view of an inscribed reflector, respectively. In this work, the length of a laser-induced reflector in fiber core is 3 µm. This laser modified cross-sectional area is approximately 2 µm by 2 µm, which is much smaller than the diameter of the fiber core (8 µm). The reflectors appear black due to the random scattering of light caused by the physical modification at the laser focus spot inside the fiber core. A 630-nm red laser light was injected to observe the scattering feature of the reflectors (Fig. 1(d)). No physical modification could be identified except inside the fiber core, that is a distinct feature of the highly localized femtosecond laser-material interaction.
The fabrication was characterized in real-time using a commercial Rayleigh scattering-based Optical Backscattering Reflectometer (Luna OBR4600) to verify the location of the inscribed points. Figure 2(a) demonstrates the Rayleigh backscattering profile of an IFPI composed of a pair of laser-induced reflectors with a 1.3-mm cavity length. The femtosecond laser could induce a reflector with a 50-dB increase of the reflection signal above the intrinsic Rayleigh backscattering signal in a standard telecom fiber. After verification, a broadband method was used for IFPI characterization. Broadband light with a 60-nm bandwidth that ranged from 1510 nm to 1570 nm from a Super-luminescent Light Emitting Diode (SLED, EXS210059-01) was coupled to the IFPI fiber sample through a circulator. The interference fringes were detected by a 512-pixel CCD-based spectrometer (Bayspec, FBGA), with a spectral range from 1510 nm to 1590 nm (Fig. 2(b)). Figure 2(c) shows the spectrum of a single IFPI. Assuming an identical reflection strength from the two reflectors, the interference spectrum of an IFPI can be expressed as a function of the wavevector k:
The Scanning Electron Microscope (SEM) was used to characterize the cross-sectional morphologies of the reflectors. Self-organized nanostructures could be identified at the femtosecond laser inscribed reflectors (Fig. 3(a)). These structures, known as nanograting, are generated from the Type-II femtosecond laser-material interaction [38,39], which are known for their exceptional stability under high-temperature. This feature has been widely utilized to fabricate Type-II-IR FBGs [30,31]. Rayleigh scattering takes place in features much smaller than the size of the wavelength, from the random fluctuations or inhomogeneities inside the optical fiber core. The laser-induced nanograting causes nano-scale physical damages in the fiber core area, that can serve as scattering centers for the increased Rayleigh backscattering signal in Fig. 2(a). Previous studies showed that using nanograting, the Rayleigh scattering property in femtosecond laser induced waveguides can be artificially increased [32,40–42]. The morphologies of nanograting have also been systematically controlled to fabricate micro-fluidic channels  and birefringent devices . Single IFPI cavities with 1-mm cavity length were inscribed on the standard telecom optical fiber with different irradiation pulse energies. The dependence of fringe visibility on the pulse energy is presented in Fig. 3(b). We observed an optimized visibility at an inscription pulse energy of 160-nJ at 0.49 which is portrayed as an inverse U-shape relationship. The dependence of the insertion loss on the fabrication condition was also characterized by measuring the optical transmission power of the fiber device at 1550-nm (Fig. 3(c)). At the optimized parameter of 160-nJ pulse energy, an insertion loss as low as 0.0012-dB per nanograting reflector was achieved, enabling the dense multiplexing capability of IFPIs using Rayleigh scattering centers. For further characterization, we cleaved the optical fiber at the inscribed Rayleigh scattering center reflectors at different pulse energies and observed their surface morphologies using the SEM. Figures 3(d)-(g) shows the surface morphologies of nanograting fabricated with different pulse energies at: 100-nJ, 120-nJ, 160-nJ, and 200-nJ.
Periods of nanograting observed in Figs. 4(d)–(g) are estimated to be 342, 258, 233, and 196 nm, respectively. From the previous literature, the nanograting period is dependent on the laser wavelength, the number of irradiation pulses, the laser writing speed, the pulse energy, and the laser repetition rate. It has also been found that the period of nanograting decreases with the increase of the pulse energy [43,44], which is consistent with our observations. As the size of the nanograting is smaller than the size of the optical fiber core, increasing the pulse energy increases the scale of the nanograting. Thus, the insertion loss increases monotonically with laser pulse energy, which is consistent with our observation of insertion loss in Fig. 3(c). The reason for the peak at an intermediate pulse energy is likely due to the tradeoff between the reflecting power and insertion loss. We propose the following mechanisms to explain this. The optical loss and enhanced Rayleigh backscattering of in-fiber reflectors are likely caused by laser-induced stress build-up and release during the formation of nanograting and nanoscale porous structures [45,46]. It was found that the optimized visibility occurs when the reflected optical power from both reflectors are identical . When the pulse energy used to fabricate the in-fiber reflectors is lower than the optimized energy, the reflected light from both reflectors are weak and are subject to the impact from continuous background Rayleigh backscattering of the pristine fiber. When the pulse energy continues to increase after the optimized energy is reached, both the insertion loss and the reflected light of the first reflector increase, leading to a decrease in the transmitted light to reach the second reflector. The fringe visibility declines as the optical power difference between reflected light from the first reflector and the second reflector becomes larger.
3. Experimental results
To test the high-temperature stability of femtosecond laser fabricated IFPI devices, a single IFPI sensor with 1346.9-um cavity length fabricated with a pulse energy of 160-nJ was placed inside a box furnace for temperature cycling. Formation of femtosecond laser-induced nanograting depends on the fabrication conditions, including pulse energy, pulse duration, and laser repetition rate. Results in our works were obtained using a femtosecond laser with a fixed pulse duration of 270 fs and repetition rate of 250 kHz. SEM studies presented in Figs. 3(d)–(g) show that a pulse energy larger than 100 nJ is above the threshold of nanograting formation. This is consistent with laser processing conditions reported previously for nanograting formation in fused silica [43,44], which is the main composition of an optical fiber core. The backscattering profile, wavelength spectrum, and FFT spatial domain are presented in Figs. 2(a), (c), and (d). The fiber sensor was first annealed at 1000°C for 5 hours, then went through repeated heating and cooling cycles. Each heating cycle started from room temperature, then raised to 1000°C in 100°C increments at a ramp rate of 5°C/min, and held at a steady temperature at each step of 100°C for 1 hour. The temperature of the furnace was continuously tracked using a type-K thermocouple with a USB temperature logger (Lascar EL-USB-LCD) every 20 seconds. Afterwards, the sample was cooled down in a furnace at a natural cooling rate. The cavity length was calculated with respect to the measurement from the electronic thermocouple to obtain the characterization curve in Fig. 4(a). The sensitivity was 11.1 nm/ at a temperature below 300°C and was found to be 14.4 nm/°C at a temperature above 300°C – these measurements are similar to the behavior that was previously reported for high temperature measurements for femtosecond laser-inscribed FBGs [19,32,48,49]. Figure 4(b) presents the measured FPI temperature against the thermocouple measurements, using the obtained characterization curve, and shows the precise sensor response of the IFPIs during repeated heating and cooling cycles.
To characterize the cross-talk between multiplexed IFPI sensors interrogated with a single instrument, we fabricated two multiplexed IFPIs. A single-mode fiber was translated along the axial direction using a customized roll-to-roll writing setup to inscribe IFPI sensors at points of interest. The optimized pulse energy of 160 nJ was used to achieve high fringe visibilities. Two IFPIs were inscribed with cavity lengths of 934.9 µm (Sensor 1) and 1050.2 µm (Sensor 2) respectively. Figure 5(a) shows the wavelength spectrum and demodulated results for the two-sensor array. Sensor 1 was placed under constant room temperature, while Sensor 2 was placed inside a tube furnace. The temperature of the furnace was increased from room temperature to 500°C. The cavity length change was demodulated in real time. Figure 5(b) shows the relative cavity length change during two phases of the measurement process. The top subplot shows the measurement results when both IFPI sensors were kept at room temperature, while the bottom subplot shows when Sensor 2 was placed at 500°C and Sensor 1 was kept at room temperature. In the bottom subplot, the cavity length of Sensor 1 remained unchanged while Sensor 2 experienced an elongation of ∼ 4720.2 nm as the temperature rise from the ambient to 500°C . The static test results shown in Fig. 5(b) reveal minimal cross-talk between the two sensor measurements. Additionally, fluctuations could also be identified from the results. Sensor 1 fluctuated at room temperature with a standard deviation of 0.0604 nm, corresponding to a temperature variation of ∼ 0.007°C (Fig. 5(b) top). The cavity length of Sensor 2 fluctuated at 500°C with a standard deviation of 0.5722 nm, which corresponds to a temperature fluctuation ∼ 0.06°C (Fig. 5(b) bottom). This could be can be attributed to the fluctuation of the furnace temperatures.
To demonstrate multiplexing capabilities, a string of six IFPI sensors were inscribed on a single-mode fiber sample using the roll-to-roll setup and 160-nJ pulse energy. Our cross-talk experiment in Fig. 5 demonstrated that the sensor cavity length difference of 115 µm is sufficient to eliminate identifiable cross-talk between multiplexed sensors. As such, we inscribed multiplexed IFPI array with the cavity lengths to be 398, 599, 888, 1190, 1527, and 1814-µm, respectively. A 1-cm spacing between two adjacent sensors was chosen in order to locate the IFPIs at different positions inside the box furnace. The location and reflection strength of the nanograting reflectors were characterized by the OBR-measured backscattering profile in Fig. 6(a). The interference fringe and FFT transformation of the multiplexed sensors are shown in Figs. 6(b) and (c). The six IFPI peaks are highly distinguishable in the FFT domain. Even though the laser-modified cross-sectional area using 160-nJ pulse energy is much smaller than that of the fiber core, as shown in Fig. 3(f), the optical alignment to produce the nanograting in the center of the fiber core is still challenging to achieve. This can be seen in the Rayleigh backscattering profile in Fig. 6(a) that each of the twelve nanograting reflectors has slightly different reflectance.
Similar to the high-temperature performance test for the single IFPIs, the multiplexed IFPIs were first annealed at 1000°C, then went through repeated heating and cooling cycles. Again, each heating cycle started from room temperature, was heated up to 1000°C at a speed of 5°C/min, and each step was held at 100°C for 1 hour. The furnace temperature was tracked using an electronic temperature data logger while cavity lengths of the IFPIs were demodulated and recorded every 1 second. The calibration curves shown in Figs. 7(a)–(f) exhibit a linear relationship between the cavity length and temperatures for each of the six multiplexed IFPIs. The temperature sensitivity of the sensors obtained from the linear fitting are: 3.71, 5.66, 8.39, 11.15, 14.28, and 17.15-nm/°C for cavity lengths of 398, 599, 888, 1190, 1527, and 1814-µm, respectively, as recorded in Table 1. We substituted lOPD with the real cavity lengths into Eq. (2) to calculate the theorical sensitivity. The experimental data was found to be consistent with the theorical thermal expansion data.
Using these calibrations curves, the temperature measured by these IFPIs are compared against the measurements obtained by the thermocouple in a box furnace for the third heating cycle (Fig. 7(g)). Figure 7(h) presents a zoomed-in area in Fig. 7(g) when the temperature of the box furnace was held at 817°C. The thermocouple sensor measurement shows a periodic temperature fluctuation, caused by the PID control for the furnace’s temperature modulation. The measurement from the six multiplexed IFPI sensors traces that of the thermocouple. While the temperature variation inside the box furnace is highly inhomogeneous, the IFPI sensors measured a different temperature depending on their position inside the furnace, however, the overall trend remained consistent with the thermocouple.
In conclusion, this paper demonstrates a femtosecond laser direct writing method to inscribe multiplexable IFPI sensors using pairs of femtosecond laser induced nanograting in a single-mode fiber core. Through engineering of the nanograting morphology, highly multiplexable IFPIs could be fabricated with high fringe contrasts of 0.49 and a low loss 0.002 dB/sensor. By employing a white light interferometry demodulation algorithm, temperature sensitivity, cross-talk, and spatial multiplexability of sensor arrays are discussed. Furthermore, we validated high temperature sensing performance and sensor stability by repeated temperature cycling from room temperature ranges to 1000°C. The sensitivities of the multiplexed Fabry-Perot Interferometer sensors ranged from 3.71 to 17.15 nm/°C. Overall, we present a direct laser writing technique to produce highly multiplexable in-fiber Intrinsic Fabry-Perot Interferometer sensor devices with high fringe contrast, high sensitivity, and low-loss for use in extreme environmental conditions.
U.S. Department of Energy (DE-AC07-05ID14517, DE-FE00028992, DE-NE0008686).
The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
The authors declare no conflicts of interest.
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