Refractive-index-modified-dot Fabry-Perot fiber probe fabricated by femtosecond laser for high-temperature sensing

An optical fiber Fabry-Perot probe sensor for high-temperature measurement is proposed and demonstrated, which is fabricated by inducing a refractive-index-modified-dot (RIMD) in the fiber core near the end of a standard single mode fiber (SMF) using a femtosecond laser. The RIMD and the SMF end faces form a Fabry-Perot interferometer (FPI) with a high-quality interference fringe visibility (>20dB). As a high-temperature sensor, such an FPI exhibits a sensitivity of 13.9 pm/ C ° and 18.6 pm/ C ° in the range of 100500 C ° and 500-1000 C, ° respectively. The fabrication process of this device is quite straightforward, simple, time saving, and the sensor features small size, ease of fabrication, low cost, assembly-free, good mechanical strength, and high linear sensitivity. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (050.2230) Fabry-Perot; (060.2310) Fiber optics; (060.2340) Fiber optics components; (060.2370) Fiber optics sensors; (140.3390) Laser materials processing. References and links 1. V. de Oliveira, M. Muller, and H. J. Kalinowski, “Bragg gratings in standard nonhydrogenated fibers for hightemperature sensing,” Appl. Opt. 50(25), E55–E58 (2011). 2. G. Rego, O. Okhotnikov, E. Dianov, and V. Sulimov, “High-temperature stability of long-period fiber gratings produced using an electric arc,” J. Lightwave Technol. 19(10), 1574–1579 (2001). 3. A. A. Jasim, S. W. Harun, H. Arof, and H. 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Introduction
Over the past few decades, optical fiber sensors for harsh conditions in various industries, such as high-temperature in oil exploration, high-power electrical systems, and tunnel fire alarms, have attracted growing interest due to their many intrinsic advantages such as compact size, light weight, immunity to electromagnetic interference, electrically passive operation and long life.
Various fiber high-temperature sensors have been developed, including fiber Bragg gratings (FBGs), long-period fiber gratings (LPFGs), and fiber interferometers such as Mach-Zehnder interferometers (MZIs), Michelson interferometers (MIs) and Fabry-Perot interferometers (FPIs).Among them, FBG-based high-temperature sensors are widely used, but standard gratings suffer from erasure at temperature above 800 C ° [1] and require to control strict periodicity, limiting their applications.LPFG-based high-temperature sensors [2] have a large cross sensitivity to fiber bending or strain, and they are relatively long (usually several centimeters).Various schemes including fiber tapers [3], special fibers fusion splicing [4][5][6][7][8][9][10][11][12] and mismatched core diameter structures [11,[13][14][15] have been exploited to construct MZIs and MIs.However, MZIs work in transmission, which are not very compact, not suitable for working in narrow space and remote sensing.MIs are reflective probes, but their size are still not small enough.Besides, structures with fiber tapers, fusion splicing and mismatched cores usually have very poor mechanical strength.Compared with MZIs and MIs, FPIs are far more compact, simple and suitable for working in narrow space and remote sensing.
FPI high-temperature sensors are also fabricated by various techniques, including diaphragm-based [16,17], special fiber-based [18][19][20][21][22][23][24][25][26][27], multi-fusion splicing-based [28,29], chemical etching-based [30], FIB-based [31,32] and laser micromachining-based [33,34].The sensors created with the first four techniques are assembly ones, which usually need to assemble multiple separated components together with a fusion splicer.Thus, the length of the cavity is difficult or impossible to be precisely controlled.Moreover, special fiber is high cost; diaphragm or multi-fusion splicing involves complex manufacturing steps; chemical etching is dangerous.The fabrication process of FIB-based FPI is time-consuming, complicated and high cost.The reported sensors based on laser micromachining technique are mainly based on laser ablation effect, such as directly cutting a groove on SMF to form FPI [33] or drilling a microchannel through fiber core [34] with a femtosecond laser (fs-laser).However, the groove cavity FPI has poor mechanical strength as the most part of the fiber has been removed.Besides, the layer-by-layer fabrication process is time-consuming and complex.The mechanical strength of the micro-channel FPI is still not good enough because part of the cladding is removed.Moreover, an extra arc discharge procedure is required to smooth the microchannel sidewalls to obtain a high fringe contrast.
In this paper, we propose and demonstrate a new high temperature sensor based on an assembly-free robust FPI formed with a refractive-index-modified-dot (RIMD) near the end of a SMF.Previously, we have used such a FPI for refractive index sensing [35], however it had a very low fringe visibility in the air (only ~0.8dB) and thus is not suitable for high temperature measurement.Here we show that by properly adjust the fabrication condition (such as increasing the fs-laser pulse energy), high-fringe-visibility (>20dB) FPIs can also be achieved, which are suitable for wavelength demodulation based high temperature sensing.Compared with the previously published fiber sensors based on fs-laser ablation effect [33,34], the proposed RIMD based FPI sensor is much simpler, more robust, more efficient in fabrication and more cost saving.Experiment results confirmed that such a structure is very suitable for high-temperature measurement and exhibits a good linear sensitivity of 13.9 and 18.6 pm/ C °in the range of 100-500 C °and 500-1000 C °, respectively.

Experimental setup and fabrication of the FPI
The schematic structure of our proposed FPI is illustrated in Fig. 1(a).The device was fabricated by a fs-laser system (Spectra-Physics) with a center wavelength, repetition rate and pulse duration of 520nm, 200kHz, 350fs, respectively.A broadband source (BBS), a fiber circulator and an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C) were connected to real-time monitor the reflection spectrum during the fabrication.The fabrication of the FPI is quite straight-forward.First, an end well cleaved SMF (YOFC, G.652) was mounted on a 3-D translation stage (Newport).Then, the stage is adjusted to position the SMF correctly.Last, the fiber is exposed to fs-laser pulses to generate a refractive-indexmodified-dot (RIMD).An online monitoring CCD system was used to monitor the sample morphology during the direct writing process.In the fabrication, the laser was focused through a microscope objective (Olympus UMPLFL 40X) with a numerical aperture (NA) of 0.65 and the actual fs-laser energy used for fabrication was approximately 4µJ/pulse, which is much stronger than that used in [35].Figures 1(b) and 1(c) show the top and the side view of microscope images of a fabricated FPI, respectively.

Operation principle
For the proposed FPI, the normalized reflection intensity can be modeled with three-beam optical interference equation [35]: where I is the normalized reflection intensity; i R is the reflection of the three Mirrors, i is the core RI of SMF, dot n is the RI of the RIMD and n Δ is the RI change induced by fs-laser.The dense free spectral range (FSR) corresponding to the long cavity can be derived as: The temperature sensitivity of the dense fringes of the FPI probe can be obtained by: where T α is the thermo-optic coefficient and T ξ is the thermal expansion coefficient.It is obvious that by monitoring the wavelength shift of the reflection spectrum, the temperature can be measured in real time.By changing the distance between the RIMD and the SMF end face, FPI probes with different cavity length can be easily achieved.Figure 3 shows the measured reflection spectra of three FPIs samples with a cavity length of 120, 60 and 30 μm, respectively.The maximum fringe contrast of the FPIs is more than 20dB.Clearly, when the cavity length is halved, the FSR is doubled.It means that FSR is inversely proportional to the cavity length, which is in good consistency with the theoretical calculation explained by Eq. ( 2).One may also note that the secondary periods of these samples are much less clear (compared with the simulation) and appear to be some different, which indicate that the RIMDs formed in these samples are not perfect spheres and also slightly different in sizes.This can be improved if one carefully controls the beam shape and focus spots of the fs-laser.

High-temperature response and discussion
Before investigating the response of the probe to high-temperature, we first put the device in a tube furnace, being heating at 1000 C ° and maintained for 10h to eliminate residual stress in the fiber.Then, it was naturally cooled down to room temperature.After the annealing process, the sensor probe was tested stepwise by heating up and cooling down with a temperature step of 50 C °, respectively.Figure 4 shows the measured spectral response of the FPI sensor with 60μm cavity length, where the fringe dips at each temperature step during the heating process are compared, and an obvious monotonic red shift with the increase of temperature is noted.The interference dip at initial wavelength ~1558nm was chosen to monitor its wavelength shift versus temperature, as shown in Fig. 4. Figure 5 shows the measured wavelength shift the selected interference dip during the heating and cooling cycle.Two-order polynomial function:  We find that the temperature region can be also divided into two parts with good linear fitting, as shown in Fig. 6.For the lower temperature range of 100-500 C °, a linear sensitivity of ~13.9 pm/ C °is obtained (R 2 = 0.99414 and 0.99865 for the heating and cooling cycle, respectively).For the higher temperature range of 500-1000 C °, a linear sensitivity of ~18.6 pm/ C °is obtained (R 2 = 0.99713 and 0.99681 for the heating and cooling cycle, respectively).The thermal expansion coefficient of silica is 0.55 × 10 -6 -1 °C , the thermo-optic coefficient of silica is 8.3 × 10 -6 -1 °C [22].According to Eq. ( 3), the temperature sensitivity is calculated to be 13.8 pm/ C, °which is very close to experimental sensitivity (13.9 pm/ C °).
The wavelength sensitivity to high temperature of the SMF-based FPIs is typically in the range of 12-18 pm/ C ° [16, 22, 25-27, 30, 34].Obviously, our FPI is in good agreement with the theoretical calculation and the previously reported results.The resolution could be estimated to be ~0.7 C °considering that the resolution of the OSA is 0.01 nm.

Conclusion
In summary, we proposed and demonstrated a new high temperature sensor based on a highfringe-visibility (>20dB) FPI formed by inducing a strong refractive-index-modified-dot (RIMD) in a SMF core using fs-laser.Such a FPI is very suitable for high-temperature measurement and exhibits a sensitivity of 13.9 pm/ C °and 18.6 pm/ C °in the range of 100-500 C °and 500-1000 C, °respectively.The excellent features of a simple structure, ease of fabrication, ultra-compact, low-cost, assembly-free and high-linearity sensitivity could make it promising in the practical harsh conditions in various industrial fields.

Fig. 1 .
Fig. 1.(a) System schematic for inducing a refractive index modified dot in fiber core using a femtosecond laser to form FPI. (b) The top and the side (c) view of microscope images of the fabricated FPI.

1 2 , 2 , γ γ are the transmission loss factors of the cavity 1 and cavity 2
α α are the intensity attenuation factors of the Mirror 1 and Mirror 2, respectively; 1 shift in the cavity j ( j = 1, 2); j n is the effective RI of the cavity j ; j L is the length of the cavity j ; λ is the free space wavelength.

Fig. 4 .
Fig. 4. Spectral response of a FPI probe with 60 μm cavity length at different temperatures.

Fig. 5 .
Fig. 5. Interference dip wavelength at ~1558nm with temperature variation for the heating and cooling cycle.

Fig. 6 .
Fig.6.Linear fit at low temperature and high temperature.