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Ultrafast femtosecond-laser-induced fiber Bragg gratings in air-hole microstructured fibers for high-temperature pressure sensing
Optics Letters
Abstract
We present fiber Bragg grating pressure sensors in air-hole microstructured fibers for high-temperature operation above 800 degrees C. An ultrafast laser was used to inscribe Type II grating in two-hole optical fibers. The fiber Bragg grating resonance wavelength shift and peak splits were studied as a function of external hydrostatic pressure from 15 psi to 2000 psi. The grating pressure sensor shows stable and reproducible operation above 800 degrees C. We demonstrate a multiplexible pressure sensor technology for a high-temperature environment using a single fiber and a single-fiber feedthrough.
... Nevertheless, FBGs fabricated in conventional single-mode fibers (SMFs) have low pressure sensitivities of 3-4 pm/MPa [16] compared to other measurands [17,18]. Although, Bragg gratings written in grapefruit MOFs and two-hole fibers [11,19] exhibit an improved sensitivity compared to that of SMF, the pressure sensitivity remains at a value lower than -13 pm/MPa. However, through effective modification of the structure of the MOF, the pressure sensitivity can be enhanced. ...
... Besides maintaining the suspended core in the center of the structure, the ring also introduces a large holey region and a geometrical asymmetry. Owing to the significant increase in the AFF, external pressure induces a large wavelength shift in the FBG that cannot be achieved in conventional SMFs [16] or other types of MOFs [11,19]. To the best of our knowledge, the sensitivity of -43.6 pm/MPa achieved in this experiment using the proposed single-ring suspended fiber is the highest value reported in a FBG-based pressure sensor. ...
... Compared to SMF [16], the pressure sensitivity of the proposed sensor is improved by over 14 times, which is, to our knowledge, the highest value reported for a hydrostatic pressure sensor based on grating technique. Compared to FBGs in grapefruit MOFs [11] and two-hole fibers [19], the pressure sensitivity obtained in this work is enhanced by 3.4 times and is 1.6 times larger to that of a FBG in a glass-bubble housing [22]. In the glass-bubble housing technique, mechanical compliance of a hollow glass bubble which acts as an amplifier is greater than that of a solid fiber; hence, the pressure sensitivity is enhanced relative to a bare SMF. ...
We present a novel optical fiber composed of a suspended core, a supporting ring and an outer ring. To establish a large holey region, a germanium-doped core is suspended by a silica ring and the entire structure is enclosed by another silica ring. By monitoring the Bragg wavelength shift of an FBG written in such a fiber with an air filling fraction of 65%, a hydrostatic pressure sensitivity of –43.6 pm/MPa was achieved experimentally. The high-pressure sensitivity is in good agreement with the numerically calculated value of ~40 pm/MPa. Due to the significant impact of the fiber core suspended in the large holey region inside the fiber, the pressure sensitivity improved by approximately eleven times compared to a Bragg grating inscribed in a standard single-mode fiber. To the best of our knowledge, it is the highest pressure sensitivity obtained for a FBG-based sensor experimentally, when compared to other FBG-based pressure sensors reported up to date. The large air hole region and the suspended core in the center of the fiber not only make the proposed fiber sensor a good candidate for pressure measurements, especially in the oil industry where space is at a premium, but also allow the detection of substances, by exploiting interaction of light with liquids or gases.
... Femtosecond laser is a powerful tool to inscribe FBGs in various MOFs, and the fabricated FBGs can operate at a high temperature [12][13][14][15][16][17]. For example, Chen et al. fabricated an FBG in a dual side-hole fiber (DSHF) by using the femtosecond laser phase mask method and realized pressure sensing at a high temperature of 800°C [14]. ...
... Femtosecond laser is a powerful tool to inscribe FBGs in various MOFs, and the fabricated FBGs can operate at a high temperature [12][13][14][15][16][17]. For example, Chen et al. fabricated an FBG in a dual side-hole fiber (DSHF) by using the femtosecond laser phase mask method and realized pressure sensing at a high temperature of 800°C [14]. Nevertheless, the fabricated FBG exhibited a low birefringence of 1.7 × 10 −4 , leading to a small peak split, which makes a request of complicated and expensive polarization-resolved demodulation equipment. ...
We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10⁻³ was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of −21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.
... For instance, the sensitivity of the response to pressure and temperature of the fiber can be optimized, varying the lattice pitch and air-hole diameter. Furthermore, hollow-core (HC) PCFs have opened up many new opportunities for the development of linear or nonlinear sensing devices by filling the air holes in with liquids, gases, or liquids crystals, or metals [1,[5][6][7][8]. ...
... Depending on the physical mechanism that is used for sensing, various types of sensors have been reported, for example: fiber Bragg grating (FBG) [5,9], interferometric [7,[10][11][12][13] and surface plasmon resonance [4,8,14] sensors, as well as directional couplers [15,16] for the measurement of different physical sensing parameters such as refractive index, temperature, pressure, humidity, and so on. Besides the reports of these kinds of linear fiber sensors, nonlinear optical sensors based on nonlinear processes as a sensing physical mechanism have also been studied. ...
We propose a high-pressure sensing mechanism in solid-core silica photonic crystal fibers (PCF) that is based on the nonlinear optical process of degenerate four-wave mixing. A quite simple configuration for the pressure sensor is given and is theoretically investigated by obtaining signal gain spectra. We focus on the Stokes and anti-Stokes sidebands that are generated during propagation of the field along the PCF due to the interplay of dispersion and nonlinear optical effects. We have considered wave propagation in the normal dispersion regime with the inclusion of negative fourth-order dispersion. We have also considered a pumping field with wavelength of 1076 nm and peak power of 3000 W that propagates along a PCF with 40 cm of length. We have optimized sensitivity, varying lattice pitch and air-hole diameter, and we have obtained high sensitivities of the Stokes and anti-Stokes lines of 3.672 and ${-}{0.146}\;{\rm{nm/MPa}}$ − 0.146 n m / M P a , respectively. The proposed sensors have potential applicability in high-pressure environments.
... OFHTSs have attracted growing interest and different types have been proposed in recent years. Femtosecond laser inscribed fiber Bragg gratings (FBGs) were utilized for high temperature sensing [2][3][4]. However, the fabrication process is complex. ...
... The aforementioned FPIs are not easy to fabricate as tapering, micro-machining or special splicing are required. In addition, the sensitivities of OFHTSs based on FBGs, LPFGs or interferometers are required to be further improved (usually dozens of pm/°C) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. ...
This study proposes a highly sensitive and stable optical fiber probe based on Vernier effect for high temperature measurement (up to 1000 °C), utilizing photonic crystal fiber (PCF)–based Fabry-Perot interferometers (FPIs). The cascaded FPIs are fabricated by fusion splicing a section of polarization maintaining PCF to a lead-in single-mode fiber, and then a section of temperature-insensitive hollow core PCF is spliced between the PMPCF and a multi-mode fiber. The shift of the spectral envelope is monitored to measure the temperature variation. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C when operating from room temperature to 1000 °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73%. The proposed probe has great application prospects due to compactness, high sensitivity and low cost.
... Optical pulse is the key element across variety of applications covering optical communication, sensing, metrology, material processing and medical surgery [1][2][3][4]. With the burst of connected devices to applications such as Internet of Things (IoT), ultrafast femto-second pulse is indeed necessary to offer high speed data with low latency for future 5 G/6 G optical communication technologies [5,6]. ...
Dark pulse mode-locked Erbium doped fiber laser (EDFL) was demonstrated in this work by employing side polished fiber (SPF) coated with Ti3AlC2 solution using drop casting method. The modulation depth of fabricated SPF coated with Ti3AlC2 solution was characterized at 2.2%. This SPF coated structure allowed the formation of dark mode-locked pulse with improved the birefringence and nonlinearity in EDFL. The dark pulse generated was cubic-quintic nonlinear Schrödinger equation (CQNLSE) dark pulse with center wavelength at 1558.935 nm, repetition rate 1.839 MHz and pulse width 170.2 ns. Pulse stability was investigated at 65.86 dB.
... 89 Femtosecond gratings produced in similar kinds of micro-structured fiber were also able to act as simultaneous pressure/temperature sensors but at much higher temperatures. 90 Other architectures for high temperature/pressure sensing involve combinations of laser machined fiber Fabry-Perot (F-P) structures in line with femtosecond laser-induced FBGs, for example, a fs-FBG in single mode fiber was fusion spliced to a Fabry-Perot cavity that was created by F 2 excimer laser machining and HF etching of multimode fiber. 91 The concatenated device exhibited maximum strain and pressure sensitivities of 41.6 pm/με and −222 pm/MPa, respectively, up to 400 ○ C. A similar device with a simpler fabrication methodology was recently reported where a spherical F-P cavity was laser machined into the end face of a single mode fiber and then fusion spliced to a fs PbP FBG. ...
The technique of femtosecond laser-induced inscription of fiber Bragg gratings creates a structure in the optical fiber that can be used effectively as a sensor especially when deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices can be made that are suitable for sensing applications involving high temperature, pressure, ionizing radiation, and strain. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation, and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.
... Wu et al. 41,43 utilized a 193-nm excimer laser and a phase mask with a period of 1065 nm to write FBG into a grapefruit microstructured fiber (GMF) with a GMF cross section depicted in Fig. 4(a), and the measured pressure sensitivity was approximately three times that of SMFBG. Jewart et al. 44 wrote FBGs onto a dual-aperture fiber using an ultrafast laser. The core of the dual-aperture fiber is only 1 μm from the edge of one of the air holes, indicating that the air holes are large, as seen in Fig. 4(b), and that the larger air holes optimize the pressure sensitivity of the FBG. ...
... Although FBGs offer the advantage of direct wavelength division multiplexing, they have low pressure sensitivity [18] and conventional UV written FBGs cannot survive at high temperature [19]. Therefore femtosecond laser written FBGs on microstructured fiber were proposed for high temperature pressure sensing [17,20]. In contrast, the fabrication of interferometric sensors is comparatively simple and costeffective. ...
We demonstrate a multipoint optical fiber pressure sensor for high temperature operation using multimode interference. Each sensing element consists of a four-hole asymmetric multimode fiber acting as the pressure sensing fiber, spliced between two standard polarization-maintaining single mode fibers. Multiple sensing elements are serially connected for multipoint pressure sensing and demultiplexing is achieved using Fourier analysis. The spatial frequency associated with each sensing element is separated by having distinct optical path lengths. The pure silica material of the sensing fiber enables high-temperature operation. As a proof of concept, we demonstrate two-point pressure sensing at a temperature up to 900 °C with negligible crosstalk between the sensing elements and good stability. Our demonstrated sensor has potential applications in heavy industry where spatially resolved measurements of pressure at high temperature may lead to improvements in efficiency and safety.
... Few-cycle pulses have attracted much attention owing to their successful applications in such fields as sensing [1] , production of attosecond pulses [2] , generation of high harmonics [3] , extremeultraviolet pulse [4] , advancement of novel X-ray sources [5] , and frequency combs [6] . To date, systematic methods have been developed to achieve few-cycle pulses to meet specific application needs. ...
... However, conventional UV written FBG-based pressure sensors are unsuitable for temperatures beyond 300 • C due to their low thermal stability [39], [40]. A high-temperature stable femtosecond laser written FBG-based pressure sensor was fabricated using two-hole microstructured fiber and demonstrated to measure pressure at temperature up to 858 • C [41]. However, apart from having low-pressure sensitivity, the sensor was highly temperature-sensitive and lacked temperature compensation capability, which is essential for any high-temperature pressure sensor. ...
We present a high-temperature interferometric pressure sensor using a simple-design and easy-to-fabricate pure silica four-hole novel microstructured optical fiber. The asymmetric geometry of the fiber allows hydrostatic pressure to induce stress at the optical fiber core, which converts to an interferometric shift. The large core of the fiber supports the propagation of several modes. Multimode interference created between different pairs of modes is used to sense the temperature and pressure change. The use of pure silica fiber is motivated by the ability of this fiber to operate up to high temperature as dopant diffusion is avoided. The sensor is demonstrated to measure pressure at a temperature up to 800 °C. We demonstrate temperature compensation using a Fourier approach to monitor different interference pairs and their phase response to pressure and temperature change. Experimental results show that the sensor has a linear response and excellent stability with a detection limit of 8.86 kPa at 800 °C temperature. This simple, compact, and potentially low-cost sensor is promising for harsh environment applications to improve quality control, operation efficiency, and safe working conditions.
... Since the 1980's, the advancement of fiber laser has been growing rapidly with the progress of fiber technology [1,2]. Owing to its advantages of small size, high efficiency, energy saving and good stability, ultrafast fiber laser quickly becomes one of the most advanced research topics in the field of chemistry, biology, materials and information science [3][4][5][6][7][8]. The rapid development of ultrafast laser has facilitated the scientific progress in numerous important fields such as nonlinear microscopy, laser micromachining, optical metrology, biomedical imaging, etc. [9][10][11][12][13]. ...
The potential applications of Mamyshev oscillator in the generation of high-energy ultrashort pulses have aroused extensive research interest. This review focuses on the latest research progress of Mamyshev oscillator. Firstly, the working mechanism of Mamyshev oscillator is introduced. Then, the research results of Mamyshev oscillator in recent years are summarized according to three different wavebands of 1 μm, 1.5 μm and 2 μm. The Mamyshev oscillator can effectively improve the laser pulse energy and shorten the pulse duration, which has great advantages in industrial applications. Meanwhile, it contains abundant nonlinear effects and can be used as an excellent platform for nonlinear optical system, which stimulates the exploitation of nonlinear optics and promoting the development of ultrafast laser technology and nonlinear optics.
... In the past two decades, optical fiber sensors for simultaneous measurement of such two parameters have attracted considerable attention owing to their well-known advantages, especially compact size, immunity to electromagnetic interference and high stability in harsh environment compared with traditional electronic counterparts [5][6][7]. Diverse types of inline optical fiber sensors based on fiber Bragg gratings (FBGs), long period fiber gratings (LPGs), and Fabry-Perot interferometers (FPIs) have been proposed to realize temperature and gas pressure sensing [8][9][10][11][12][13]. Pressure sensors with FBGs written in single mode fibers (SMFs) and microstructured optical fibers (MOFs) generally subject to low sensitivity on the order of 0.01 nm/MPa, which makes them impractical in several-megapascals gas pressure sensing system [14,15]. ...
We have proposed and experimentally demonstrated a novel optical fiber sensor for simultaneous measurement of high temperature and gas pressure based on a Regenerated Fiber Bragg grating (RFBG) cascaded with a homemade hollow core Bragg fiber (HCBF). The HCBF functions as anti-resonant reflecting waveguide and acts as a pressure and temperature sensing unit. The sample was fabricated and tested in our lab, exhibiting a high linear pressure and temperature sensitivity of -3.747nm/MPa and 25.925 pm/ in a large-dynamic range of 2MPa and 600, respectively. The RFBG as a temperature monitor was integrated to compensate the temperature cross-correspondence of the HCBF. By utilizing a 2 2 matrix obtained with the experimental results, simultaneous measurement of temperature and gas pressure can be achieved. Besides, compact size, low cost and high sensitivity make the sensor more competitive in harsh environment application.
... At present, some singlepoint fiber pressure sensors, e.g., Fiber Bragg gratings (FBGs) [4][5][6][7] and interferometric-based ones [8,9], have already been released as commercial products due to the many features, such as minimal invasiveness, remote powering and interrogation, multiplex capability, and roughness. In particular, they can also work in high-temperature environments [10,11], also due to the technology improvement in FBG writing [12][13][14]. Nonetheless, there are still applications for which the spatial resolution attainable with single-point fiber optic sensors is not enough, such as in the geo-hydrological sector [15] and oil&gas industry [16]. Indeed, single-point fiber sensors, even when densely multiplexed into quasi-distributed fiber sensors, cannot provide the wealth of information that may come from distributed optical fiber sensors (DFOSs). ...
The measurement of pressure by using distributed optical fiber sensors has represented a challenge for many years. While single-point optical fiber pressure sensors have reached a solid level of technology maturity, showing to be very good candidates in replacing conventional electrical sensors due to their numerous advantages, distributed sensors are still a matter of an intense research activity aimed at determining the most proper and robust pressure-sensitivity enhancement mechanism. This paper reviews early and recent works on distributed pressure sensors, classifying the sensors according to the sensing mechanism. For each type of mechanism, the issues and potentials are analyzed and discussed.The measurement of pressure by using distributed optical fiber sensors has represented a challenge for many years. While single-point optical fiber pressure sensors have reached a solid level of technology maturity, showing to be very good candidates in replacing conventional electrical sensors due to their numerous advantages, distributed sensors are still a matter of an intense research activity aimed at determining the most proper and robust pressure-sensitivity enhancement mechanism. This paper reviews early and recent works on distributed pressure sensors, classifying the sensors according to the sensing mechanism. For each type of mechanism, the issues and potentials are analyzed and discussed.
... Typical resistive strain sensors are highly sensitive to temperature and require additional temperature compensation arrangements, and when it comes to extreme conditions such as temperature above 600 • C, strain gauge fails to operate. Whereas femtosecond infrared (fs-IR) FBG gratings can measure strain up to 4000 με at 800 • C [25], [54] and a special microstructure twin air-holed fiber optic sensor that measure pressure at elevated temperature up to 850 • C [55]. Similarly FP sensor can also be used for strain measurement at high-temperature, sustainable up to 1100 • C [56], [57]. ...
Real-time structural health monitoring (SHM) of engineering components exposed to high temperature and pressure is an utmost need. The hostile operating conditions such as elevated temperatures, contraction/expansion, vibrations etc. may lead to catastrophic failure due to creep, thermo-mechanical fatigue or environmental attack (oxidation and hot corrosion). Non-availability of sensors susceptible at high temperature (HT) is a reason to handicap assessment and monitoring of such degradation during operation. Sensors using micro-fabricated sensing element are the promising, non-contact technology that have significant potential in structural health monitoring of critical engineering components operated at elevated temperatures. The present paper addresses this issue by developing application specific high-temperature sensors for real time condition monitoring of the components operated at high temperature. The goal is to provide superior performance for in- situ material condition monitoring (material degradation, flaw detection, stress relaxation, and/or creep monitoring) and through-wall temperature measurement. The sensor consists of a micro-fabricated primary (drive) winding and a secondary winding adjacent to the primary for sensing the response to a material under test. Multiple coils are cascaded / stacked together to increase the SNR and sensitivity of the sensor. The effectiveness of the proposed technique is first demonstrated on synthetic dataset from an eddy current simulation model. Further work is being underway for addressing the issues at elevated temperature.
... Typical resistive strain sensors are highly sensitive to temperature and require additional temperature compensation arrangements, and when it comes to extreme conditions such as temperature above 600 • C, strain gauge fails to operate. Whereas femtosecond infrared (fs-IR) FBG gratings can measure strain up to 4000 με at 800 • C [25], [54] and a special microstructure twin air-holed fiber optic sensor that measure pressure at elevated temperature up to 850 • C [55]. Similarly FP sensor can also be used for strain measurement at high-temperature, sustainable up to 1100 • C [56], [57]. ...
With Industry 4.0 becoming increasingly pervasive, the importance and usage of sensors has increased several folds. Industry 4.0 refers to a new phase in the industrial revolution that mainly focuses on interconnectivity, automation, machine learning, and real-time data. Real-time structural health monitoring (SHM) of components in the industrial process is one of the crucial and important component of Industry 4.0. SHM of components exposed to high-temperature (
$\sim 650^{\circ }\text{C}$
) is becoming increasingly important nowadays. However, harsh and high temperature environments impose a great challenge towards their implementation. This review is an attempt to demonstrate the development, application, limitations and recent advancement of the existing sensors used for SHM. Some sensors such as eddy current (EC) sensors and fiber Bragg grating (FBG) sensors have been discussed in detail. A phenomenological study of the electromagnetic sensor for the SHM of engineering components that are exposed to high temperature has been addressed. State-of-the-art fabrication methodologies such as low temperature co-fired ceramic (LTCC) technology for such type of sensors for high temperature SHM applications have been elucidated. Future challenges and opportunities for SHM applications of high temperature sensors have been highlighted.
... Special FBGs with complex spectra, such as apodized FBGs, sampled FBGs, chirped FBGs, and phase-shifted FBGs, have also been created by femtosecond laser inscription [24][25][26][27][28][29]. Moreover, FBGs have also been inscribed successfully in various specialty optical fibers, such as pure silica core SMF [30], side hole micro-structured fiber [31], multicore fibers (MCFs) [32,33], photonic crystal fibers (PCFs) [34][35][36][37], rare-earth-doped silica fibers [38][39][40], fluoride fibers (ZBLAN) [41][42][43][44][45], and single-crystal sapphire fibers [46][47][48][49]. ...
Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments.
... Type II FBG arrays have been used in several applications: for monitoring temperature gradients of the flame tube of a low emission burner [25], to monitor an entrained flow gasifier [18] and an oxy-fuel fluidized bed combustor [26]. Type II FBGs fabricated inside an air-hole fiber were used to measure pressure in a high temperature ambient environment (800°C) [27]. ...
High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200°C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900 –1000°C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.
... FBG sensors can be configured to both measure high pressures and high temperatures simultaneously. In this design, thermally stable fs-IR laser-induced Type II gratings were inscribed in microstructured airhole fiber in order to produce a sensor that could simultaneously monitor temperature and pressure in harsh environments (Jewart et al. 2010;Chen et al. 2011). These sensors were demonstrated to perform well in pressure ranges from 0.1 to 16.5 MPa and at temperatures up to 800°C. ...
... At present, some singlepoint fiber pressure sensors, e.g., Fiber Bragg gratings (FBGs) [4][5][6][7] and interferometric-based ones [8,9], have already been released as commercial products due to the many features, such as minimal invasiveness, remote powering and interrogation, multiplex capability, and roughness. In particular, they can also work in high-temperature environments [10,11], also due to the technology improvement in FBG writing [12][13][14]. Nonetheless, there are still applications for which the spatial resolution attainable with single-point fiber optic sensors is not enough, such as in the geo-hydrological sector [15] and oil&gas industry [16]. Indeed, single-point fiber sensors, even when densely multiplexed into quasi-distributed fiber sensors, cannot provide the wealth of information that may come from distributed optical fiber sensors (DFOSs). ...
The measurement of pressure by using distributed optical fiber sensors has represented a challenge for many years. While single-point optical fiber pressure sensors have reached a solid level of technology maturity, showing to be very good candidates in replacing conventional electrical sensors due to their numerous advantages, distributed sensors are still a matter of an intense research activity aimed at determining the most proper and robust pressure-sensitivity enhancement mechanism. This paper reviews early and recent works on distributed pressure sensors, classifying the sensors according to the sensing mechanism. For each type of mechanism, the issues and potentials are analyzed and discussed.
... Among them, high temperature sensors based on fiber Bragg gratings (FBGs), long-period fiber gratings (LPFGs), and fiber interferometers attract considerable interest. Femto-second laser-inscribed FBGs are utilized for high temperature sensing up to 1000 • C [2][3][4]; however, the fabrication process is complex and the cost is high. High temperature sensors based on LPFGs have large cross talk of bend and strain, meanwhile the size is not compact (normally several centimeters) [5,6]. ...
We propose a miniaturized optical fiber Fabry–Perot probe for high temperature measurement (up to 1000°C). It is simply fabricated by fusion splicing a short section of polarization-maintaining photonic crystal fiber (PMPCF) with a single-mode fiber (SMF). The interface between the core of the SMF and air holes of the PMPCF, and the end face of the PMPCF work as the mirrors. The pure silica core of the PMPCF is employed as the sensing element. Experimental results show that the probe has a high thermal stability and the temperature sensitivity reaches up to 15.34 pm/°C, which is not affected by the length of the PMPCF. The linearity of temperature response is as high as 99.83%. The proposed sensor has promising prospects in practical applications due to simple fabrication process, low cost, compact size, and excellent repeatability.
... Ultrashort pulses with high peak power have enabled numerous applications in various fields including nonlinear microscopy [1], materials processing [2], sensing [3], femtochemistry [4], frequency comb [5,6], etc. Particularly, optical pulses of temporal durations reaching few cycles or a single cycle of the carrier frequency with spectra broader than an octave spanning have been essential for a variety of cutting-edge applications such as attosecond science [7,8], high-harmonic generation [9], and coherent X-ray generation [10]. Currently, solid-state lasers have been the forefront of few-cycle pulse generation owing to their broad gain bandwidths (BWs) [11][12][13]. ...
While the performance of mode-locked fiber lasers has been improved significantly, the limited gain bandwidth restricts them from generating ultrashort pulses approaching a few cycles or even shorter. Here we present a novel method to achieve few-cycle pulses (∼5 cycles) with an ultrabroad spectrum (∼400 nm at −20 dB) from a Mamyshev oscillator configuration by inserting a highly nonlinear photonic crystal fiber and a dispersion delay line into the cavity. A dramatic intracavity spectral broadening can be stabilized by the unique nonlinear processes of a self-similar evolution as a nonlinear attractor in the gain fiber and a “perfect” saturable absorber action of the Mamyshev oscillator. To the best of our knowledge, this is the shortest pulse width and broadest spectrum directly generated from a fiber laser.
... However, Type-I FBGs created by the traditional ultra-violet inscription technique will be erased at a temperature higher than 500 °C [5][6][7][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][10][11]. Although LPFG-based hightemperature sensors possess much higher sensitivity, the long length and large crosssensitivity to bending or refractive index (RI) need to be optimized [12][13][14]. ...
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.
... Ultrafast mode-locked lasers have enabled many broader impact applications such as nonlinear microscopy [5], materials processing [6], surgical applications [7], sensing [8] etc. For those applications, the demand for shorter and higher peak power pulses has steadily grown up. ...
While the performance of mode-locked fiber lasers has been improved significantly, the limited gain bandwidth restricts them to generate ultrashort pulses approaching a few cycles or even shorter. Here we present a novel method to achieve few cycle pulses (~5 cycles) with ultra-broad spectrum (~400 nm). To our best knowledge, this is the shortest pulse width and broadest spectrum directly generated from fiber lasers. It is noteworthy that a dramatic ultrashort pulse evolution can be stabilized in a laser oscillator by the unique nonlinear processes of a self-similar evolution as a nonlinear attractor in the gain fiber and a perfect saturable absorber action of the Mamyshev oscillator.
... Conventional FBGs fabricated using modulated ultra violet (UV) lasers are generally considered to have limited use in high-temperature applications. By using a femtosecond-pulsed ultrafast laser, it is possible to achieve stable, FBG-based pressure sensors for high-temperature operation above 800°C [5]. The FBG was inscribed in a 220 µm diameter, two-hole (air hole) fiber. ...
An extrinsic Fabry-Pérot interferometric (EFPI) sensor created at the tip of an optical fiber is presented for high-pressure measurement. The sensor, with a diameter of 125 μm, is fabricated exclusively in pure silica fiber, which makes it suitable for applications in high-temperature environments. The EFPI cavity of the sensor is formed by fusion splicing, cleaving, polishing, and femtosecond laser micromachining. Experimental studies show that the sensor exhibits high stability and good linearity over a pressure range of 10 MPa under high-temperature conditions (up to 800°C). This sensor offers great potential for use in high-temperature applications.
... It may therefore be used to write FBGs in multicore fibers [19] and for other novel devices such as waveguide couplers [20]. Furthermore, the phase correction could be adjusted to compensate for internal interfaces in specialty fibers such as photonic crystal fibers [21] and air-hole fibers [22], where the aberration cannot be mitigated by immersion oil or ferrules. ...
We present fiber Bragg gratings (FBGs) fabricated using adaptive optics aberration compensation for the first time to the best of our knowledge. The FBGs are fabricated with a femtosecond laser by the point-by-point method using an air-based objective lens, removing the requirement for immersion oil or ferrules. We demonstrate a general phase correction strategy that can be used for accurate fabrication at any point in the fiber cross-section. We also demonstrate a beam-shaping approach that nullifies the aberration when focused inside a central fiber core. Both strategies give results which are in excellent agreement with coupled-mode theory. An extremely low wavelength polarization sensitivity of 4 pm is reported.
Femtosecond laser direct writing is a powerful technique for fabricating micro-nano devices as it can modify the interior of transparent optical materials in a spatially selective manner through nonlinear multi-photon absorption. In this context, laser-induced nanogratings, i.e., a sub-wavelength assembly of nanolayers (≈20 nm in width, ≈200 nm period), are ultrashort self-organized structures created by light in the bulk of transparent materials. These have been intensively explored over the last two decades opening a novel era of micro photonic devices due to their unique physicochemical properties, like orientable form birefringence, anisotropic light scattering, highly selective chemical etching, optical chirality, and extraordinary thermal stability. This review provides a throughout overview of the advances in this field, specifically focused on the formation of nanogratings, optical properties that can be exploited in various transparent solids, and the related main applications. Also, the fundamental characteristics, formation mechanism, tuning methods of nanogratings are reviewed.
We propose and experimentally demonstrate an ultrasensitive temperature and strain sensor based on a hybrid fiber interferometer (HFI) with the Vernier effect. The HFI consists of a fiber coupler, a section of hollow core photonic crystal fiber (HCPCF), and a certain length of high birefringence polarization maintaining fiber (Hi-Bi PMF) to form a composite structure with a FPI built into the FSI. The HCPCF-based FPI is insensitive to temperature and strain and is an ideal reference interferometer. By adjusting the length of the HCPCF and Hi-Bi PMF, the Vernier effect can be created, which significantly increases the sensitivity of the sensor. Experimental results show that the proposed sensor has high temperature and strain sensitivities up to 50.671 nm/°C and -81.69 pm/με, respectively, with a high resolution of ± 0.0018 °C and ± 0.86 με. The proposed sensor is low-cost, compact, and easily fabricated, making it ideal for applications that require high-precision temperature and strain measurement.
This study presents a novel sensor that combines air pressure and temperature measurement using dual-interference microfiber resonators. The sensor leverages intermode interference and the resonant ring resonance effects within the microfiber loop structure, enabling simultaneous demodulation of temperature and air pressure. We have developed a mathematical model that is closely monitored alongside the reflectance spectral structure. The sensor exhibits two distinct linear sensitivities for pressure and two different linear sensitivities for temperature, as revealed by our testing results. Specifically, microfiber mode interference demonstrates a temperature and pressure sensitivity of 160 pm/°C and 167/kPa, respectively, while the ring resonance effect yields a temperature and pressure sensitivity of 132 pm/°C and 62 pm/kPa, respectively. The maximum temperature resolution stands at 0.025°C, and the pressure resolution is 0.023 kPa. These findings are supported by the concept of a linear connection, which serves as the foundation of the transmission matrix and enables concurrent discrimination of air pressure and temperature. This sensor offers advantages due to its straightforward structure, high extinction ratio, superior electrical safety, and effective protection against electromagnetic interference.
Pressure sensors based on air-hole fiber have limitations in design and materials. We fabricate and test pressure sensors with complex 3D structures in commercial fibers, with potential for many types of high-performance fibers.
We demonstrate a fiber-optic hydrophone using a multimode microstructured pressure-sensitive optical fiber, which has a sensitivity comparable to a commercial electric hydrophone and 14 to 17 dB improvement over conventional single mode and multimode optical fiber. We then configure the pressure-sensitive fiber with a multi-fiber fan-out output where each output channel encodes different coupling from the pressure-sensitive fiber’s modal interference pairs. Thus, each channel has a different response to both the acoustic wave and environmental noise. We use a neural network model as the signal processing module to combine the acoustic signals from the multiple fiber channels to reconstruct the original signal. By combining the channels, the neural network model can effectively reduce crosstalk from environmental noise. We then demonstrate the ability to reconstruct complex audio signals using several classical music symphonies.
A sensor combining a polydimethylsiloxane-coated Mach-Zehnder interferometer with a tilted fiber Bragg grating is proposed for dual-parameter sensing of liquid temperature and refractive index. Mach Zehnder interference is used for high sensitivity temperature sensing while the core mode of tilted fiber Bragg grating is used to distinguish the temperature range and avoid spectral overlap. Meanwhile, the cut-off mode of tilted fiber Bragg grating is used for refractive index sensing, which can effectively eliminate crosstalk between temperature and refractive index. The average temperature sensitivity of the sensor can reach 4.88, 5.15, 4.53 and 4.38 nm/°C over the range of 20, 30, 40 and 50°C, respectively. The refractive index is in the range of 1.3330-1.3614, and the sensing sensitivity can reach 521.92 nm/RIU. Compared with other optical fiber sensors, the sensor proposed in this paper has the advantages of simple fabrication, compact structure, stable mechanical structure and high sensitivity, which is expected to be applied in the sensing field where refractive index and temperature need to be detected simultaneously.
We propose and experimentally demonstrate a highly sensitive gas pressure sensor in parallel configuration based on harmonic Vernier effect. The sensor consists of two segments of 75 μm hollow-core capillaries as the sensing and reference Fabry Perot cavity. An open passage is formed by a 5 μm capillary in the sensing cavity while the reference cavity is sealed by a single-mode fiber. A pressure sensitivity of 279.52 pm/kPa is achieved. Our scheme uses “air” instead of “silica” reference cavity, which allows higher sensitivity for a given accuracy of cavity length control. In addition, we also developed a “superposition envelope” method, which improves the contrast by at least 3 dB for the Vernier envelope. This design for high-sensitivity optic fiber gas sensor can be a good candidate for ultrasensitive gas pressure applications.
This paper presents a comprehensive review of optical fiber sensors (OFSs), including FBG, distributed optical fiber sensor and Fabry-Perot interferometer, and their applications within harsh environments, which include extremely high temperatures (from 275 °C to 1750 °C) and low temperatures (from −271.15 °C to −40 °C, namely cryogenic conditions: from 2 K to 233.15 K), and high levels of ionizing radiation (with a maximum gamma dose up to 2 GGy, and a maximum neutron fluence of approximately
$5 \times 10^{19}$
n/cm
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>
). After a brief introduction of the principles of OFSs and mechanisms of interrogation, this paper focuses on the existing works for the above three operating environments. Attention have been paid to material selection for fabricating fibers, effects of doping with rare earth elements, femtosecond laser engraving, pre-processing and post-processing (i.e., annealing) that are employed to overcome issues faced byOFSs in extreme temperatures and radiation environments. Application examples and practical test cases are also presented. Through these examples, the limitations in the current state-of-the-art are acknowledged and the key problems are identified. Potential solutions to some of these problems are also elucidated. A feature of this paper is the amalgamation of many research methodologies and outcomes in three seemingly distinct environmental conditions in one place so that different solution techniques can be integrated to advance OFS technologies, especially for extreme environment applications.
We investigate a novel and compact fiber-tip pressure sensor based on Fabry-Perot interferometer (FPI). The proposed device is fabricated by welding a Section of hollow silica tube (HST) on the end of single mode fiber (SMF) and then coating with ultraviolet (UV) curable polymer film at the tip of HST. A simple suspension curing method is introduced to control the formation of the UV curable polymer layer. Therefore, different thicknesses of the polymer film can be easily controlled. The pressure sensing mechanism is based on the cavity length change induced by ambient pressure. The sensor characteristics are studied theoretically and experimentally. The proposed fiber-tip pressure sensor with polymer film thickness of
$5.7\,\, \mu \text{m}$
exhibits a pressure sensitivity of 396 pm/kPa in the range of 0–30 kPa based on wavelength shift of the interference dip. Moreover, temperature crosstalk is measured, and the thermal-induced deformation of the polymer film is simulated using the Finite Element Analysis method. The proposed fiber-tip Fabry-Perot cavity pressure sensor has advantages of high sensitivity, fast response, excellent repeatability, compact size, cost effectiveness and easy fabrication, which makes it suitable for high sensitivity pressure detection in a limited space.
A highly sensitive fiber-optic Fabry-Perot sensor based on femtosecond (fs) laser micromachining is proposed and demonstrated for gas pressure measurement. The sensor is fabricated by inserting two single mode fibers (SMFs) into a capillary. A fs laser is employed to drill a micro-channel on the wall of the capillary to allow gas diffuse in. Due to the linear relationship between the pressure and the refractive index (RI) of the gas, the gas pressure can be detected by investigating the optical cavity length. The signal is demodulated by using the white-light interferometry (WLI) method based on digital cross-correlation technology. The sensitivity of the proposed pressure sensors is investigated theoretically and experimentally. The sensitivity of the sensor with a cavity length of 2926 μm is 7702 nm/MPa. In addition, a good repeatability and a high resolution of 4.9 Pa of the proposed sensor have been demonstrated. Due to the miniature size, wide dynamic range, high stability, high sensitivity and high resolution, the membrane-free fiber-optic Fabry-Perot sensor with Pa-level resolution has a wide application for gas pressure detection.
Over the past decade, a brand‐new pressure‐ and tactile‐sensing modality, known as iontronic sensing has emerged, utilizing the supercapacitive nature of the electrical double layer (EDL) that occurs at the electrolytic–electronic interface, leading to ultrahigh device sensitivity, high noise immunity, high resolution, high spatial definition, optical transparency, and responses to both static and dynamic stimuli, in addition to thin and flexible device architectures. Together, it offers unique combination of enabling features to tackle the grand challenges in pressure‐ and tactile‐sensing applications, in particular, with recent interest and rapid progress in the development of robotic intelligence, electronic skin, wearable health as well as the internet‐of‐things, from both academic and industrial communities. A historical perspective of the iontronic sensing discovery, an overview of the fundamental working mechanism along with its device architectures, a survey of the unique material aspects and structural designs dedicated, and finally, a discussion of the newly enabled applications, technical challenges, and future outlooks are provided for this promising sensing modality with implementations. The state‐of‐the‐art developments of the iontronic sensing technology in its first decade are summarized, potentially providing a technical roadmap for the next wave of innovations and breakthroughs in this field.
Birefringent π-phase-shifted Bragg gratings for multi-parameter sensing at temperatures ~1000 °C are written inside a standard single mode silica optical fiber (SMF-28) with infrared femtosecond pulses and a special phase mask one half of which is shifted with respect to the other by 5/4 of the mask period. The birefringence is caused by the presence of light-induced sub-wavelength periodic planar nanostructures in the fiber core, whose orientation is controlled by the laser polarization, and is maximized when the laser pulse polarization is aligned perpendicular to the fiber core. The birefringence can reach ~4.2 × 10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>
at room temperature at the 1.5 × 10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>
level after 100 h annealing at 1000 °C. Erasure and rewriting of the planar nanostructures inside fiber Bragg gratings by changing the laser pulse polarization is demonstrated.
We propose a sensitivity magnified polarization maintaining photonic crystal fiber (PMPCF) Fabry-Perot probe cascaded with a Sagnac loop (SL) for high temperature measurement (up to 1000 °C). The Fabry-Perot interferometer (FPI) fabricated by fusion splicing a PMPCF tip with a single mode fiber works as a sensing interferometer. The cascaded SL consisting of a temperature insensitive elliptical core polarization maintaining fiber (ECPMF) is applied as a reference interferometer. The free spectral range of the SL is easily tailored by adjusting the ECPMF length, making it similar to that of the sensing FPI, to generate Vernier effect. Experimental results show that the temperature sensitivity of the single FPI is 13.71 pm/°C, and the sensitivities of cascaded FPI-SLs are 143.52 pm/°C, 418.7 pm/°C and 1.15 nm/°C. The measured magnification factors are about 10, 35 and 91, which are consistent with theoretical results. The linearity of temperature response is 99.94 % from room temperature to 1000 °C. The proposed ultra-sensitive sensor has promising prospects for high temperature measurement.
Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology. Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field. Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications. Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors. Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber. Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed. A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper. © 2020 Society of Photo-Optical Instrumentation Engineers (SPIE).
A tip packaged high-temperature miniature sensor constructed by a suspended-core optical fiber (SCF) is presented and experimentally demonstrated. An SCF with a short length spliced to the single-mode fiber (SMF) is utilized to build the Fabry-Perot interferometer (FPI). A section of SMF acting as a tip package is spliced to the end of the SCF, which gives the sensor immunity of the external refractive index (RI). The test processes of heating up and cooling down are repeated three times to evaluate the repeatability and hysteresis. The arithmetic means temperature sensitivity increases from 9.42 pm/°C to 12.51 pm/°C over the temperature range of 50-800 °C. Moreover, the maximum stability error is 5 °C within the high-temperature 800 °C duration period. This tip packaged sensor with simple fabrication, good repeatability, and good stability within a large dynamic range is advantageous to practical temperature measurement and massive production.
This study is focused on the fiber Bragg gratings central resonant wavelength thermal stability, fabricated by the point-by-point inscription method, using an infrared femtosecond laser. A new methodology is proposed to analyze the refractive index decay using the central wavelength shift of fiber Bragg gratings, executing stepped and continuous annealing regimes up-to a maximum temperature of 800 °C. The obtained thermal refractive index decay follows a typical power-law function, and from the theoretical model fit to the experimental data was possible to simulate and predict the refractive index decay over a large time period, up-to 1000 years. With the proposed approach it is possible to adjust the fiber Bragg gratings pre-treatment conditions to ensure effective operation during a certain time period, at a specific working temperature, without any degradation. An experimental method to mitigate this decay is proposed, and the results demonstrate the high potential of using infrared femtosecond inscribed fiber Bragg gratings for high temperature monitoring.
A micro electromechanical systems (MEMS) resonant sensing chip with microcantilever has been developed to measure gas pressure by immersing it in gaseous environment. The microcantilever was designed to sense surrounding gas molecules loading, owing to the gas density sensitive to the pressure, then the resonant frequency shifts of sensing chip were induced under different pressures. Especially, the sensing chip featuring no diaphragm realized embedded package and installation for the immersive measurement. The resonance response of the sensing chip for target pressure was theoretically analyzed and simulated, and a packaged pressure sensor with the proposed sensing chip was tested under flexural and torsional modes of the microcantilever. The experimental results proved that the proposed sensor had preferable measuring performance under the torsional mode with the RSS (root sum square) accuracy of 0.21%FS in the working range of 10–560 kPa. The temperature compensation was presented to alleviate the temperature disturbance for the sensor, and the maximum deviation of the frequency was 59 ppm over the full pressure and the temperature range of 26–55 ℃. The proposed sensing chip is potentially a better choice for pressure sensors with measurement demand for immersive gas pressure.
We propose and demonstrate an ultrasensitive temperature sensor based on a fiber-optic Fabry–Pérot interferometer (FPI) with the Vernier effect. The sensor is prepared by splicing a section of silica tube and single-mode fiber (SMF) to a section of SMF in sequence, which formed two-cascaded FPIs. Their superimposed spectrum can produce the Vernier effect and form the interference spectrum envelope due to a similar free spectrum range (FSR). The shift of the interference spectrum envelope is much larger than that of a single FPI, when the temperature changes. Experimental results show that the designed sensor can provide a high temperature sensitivity of 183.99 pm/°C, which is almost 220 times higher than that of a single air cavity (0.86 pm/°C) and about 20 times higher than that of a single silica cavity (9.14 pm/°C). The sensor designed has compact structure (< 1 mm) and high sensitivity, providing a prospect for successful applications.
We propose and experimentally demonstrate a highly sensitive gas pressure sensor based on a near-balanced Mach-Zehnder interferometer (MZI) and constructed by hollow-core photonic bandgap fiber (HC-PBF) in this paper. The MZI is simply constructed by fusion splicing two HC-PBFs, which are of slightly different lengths, between two 3-dB couplers. The two output ends of each coupler are approximately equal in length, to ensure that the optical path variations of the MZI only result from the differences in the lengths between the two HC-PBFs. To apply the MZI for gas pressure sensing, a femtosecond laser is employed to drill a micro-channel in one of the two HC-PBF arms. The experiment result shows that the proposed MZI based gas pressure sensor achieves an ultrahigh sensitivity, up to 2.39 nm/kPa, which is two orders of magnitude higher than that of the previously reported MZI-based gas pressure sensors. Additionally, the effects resulting from the absolute length and relative length of the two HC-PBFs on gas pressure sensing performance are also investigated experimentally and theoretically, respectively. The ultra-high sensitivity and ease of fabrication make this device suitable for gas pressure sensing in the field of industrial and environmental safety monitoring.
Nitrogen-doped silica-core fibre is discussed in the context of a technological basis for the fabrication of Bragg gratings for sensors with enhanced temperature resistance up to 900 °C. To estimate the applicability of this fibre type for the manufacture of high-temperature Bragg grating sensors, the following features are analysed: technology for fabricating fibre preforms, fibre characteristics and particularities of Bragg gratings written therein. Experimental data on the degradation of the gratings' characteristics resulting from a long-term (up to 4 months) annealing at elevated temperatures are presented and discussed. The practical application of this type of sensor in thermometry is given as an example.
Using an ultrafast Ti:sapphire 800 nm laser and a phase mask, fibre Bragg gratings (FBGs) with high thermal stability were fabricated in Ge-doped SMF-28 fibre for sensor applications and were subjected to long annealing tests at 1000 °C. FBGs that maintained more than 99.95% reflectivity after several hundred hours at this temperature are demonstrated. The gratings perform well in cycling experiments up to 1000 °C, and hysteresis in the wavelength response was not detected. At a temperature of 1050 °C, a permanent drift of the central wavelength is observed which is associated with a reduction of the grating strength. The capability of this new type of FBG to be used for high temperature sensors is discussed.
The dependency of the pressure-induced birefringence of a side-hole fiber on its geometry has been numerically investigated using the finite element method. We demonstrate that the pressure sensitivity of such a fiber shows a linear dependence on /spl phi//sup 2/, where /spl phi/ is the angle between the side hole center and core center axis and the core center to side-hole tangent. Experimental data obtained with two different side-hole fiber sensors are shown to agree extremely well (to within 10%) with theoretical predictions.
This paper presents sensitivity enhancement of fiber Bragg grating sensors written in two hole fibers to external hydrostatic pressure. Finite element analysis was used to optimize the size, diameter, and configuration of air holes. The fiber core was then fabricated in the region with the maximum birefringence induced by external pressure. Resonant peak splitting of fiber Bragg gratings were used to gauge the external hydrostatic pressures. By using 220-mum diameter two hole fibers with 90-mum air holes, the optimized fiber structure with a fiber core fabricated on the edge of the air hole registered 0.102 pm/psi hydrostatic pressure response, yielding 6.5 times enhancement than previously reported in two hole fibers. The sensitivity enhancement of fiber sensors is further demonstrated by controlling the size of air holes.
A novel fiber optic pressure sensor system with self-compensation capability for harsh environment applications is reported. The system compensates for the fluctuation of source power and the variation of fiber losses by self-referencing the two channel outputs of a fiber optic extrinsic Fabry-Pérot interfrometric (EFPI) sensor probe. A novel sensor fabrication system based on the controlled thermal bonding method is also described. For the first time, high-performance fiber optic EFPI sensor probes can be fabricated in a controlled fashion with excellent mechanical strength and temperature stability to survive and operate in the high-pressure and high-temperature coexisting harsh environment. Using a single-mode fiber sensor probe and the prototype signal-processing unit, we demonstrate pressure measurement up to 8400 psi and achieved resolution of 0.005% (2sigma=0.4 psi) at atmospheric pressure, repeatability of +/-0.15% (+/-13 psi), and 25-h stability of 0.09% (7 psi). The system also shows excellent remote operation capability when tested by separating the sensor probe from its signal-processing unit at a distance of 6.4 km.
A thermally insensitive pressure measurement up to 300 degree C was demonstrated utiliz ing fiber Bragg gratings written onto a side -hole single mode fiber. The resulting temperature sensitivity is about 300 times lower than normal FBGs.
The changes of birefringence in Type I-infrared (Type I-IR) and Type II-IR fiber Bragg gratings induced by an ultrafast-IR laser in SMF-28 fibers are examined after and/or during grating inscription. The gratings are then annealed at increased temperatures up to 800 degC, and their polarization properties are monitored. It is shown that the birefringence in Type I-IR gratings inscribed in hydrogen (H<sub>2</sub>)-loaded fibers is small (~10<sup>-6</sup>) and can be decayed at room temperature, while the birefringence in Type I-IR gratings inscribed in non-H<sub>2</sub>-loaded fibers is relatively higher (~10<sup>-5</sup>) and shows strong dependence on the polarization of the IR laser beam. It has the same annealing resistance as the induced index. For Type II-IR gratings, the birefringence is an order of magnitude higher than in Type I-IR gratings (~10<sup>-4</sup>) and shows strong temperature variation during annealing
This paper presents a miniature fiber-optic high-temperature pressure sensor fabricated on the tip of a singlemode (SM) fiber by means of fusion splicing, cleaving, and wet chemical etching. A new approach was developed to simplify the fabrication and greatly improve the sensitivity. The sensor is made entirely of fused silica, whose high-temperature sensing capability is explored in detail for the first time. Two sensors were tested up to 611°C and 710°C, respectively, showing excellent repeatability better than 0.62% and 1.4%. The maximum operating temperature is limited by the mechanical creep of the fused silica diaphragm.
- D Grobnic
- C W Smelser
- S J Milailov
- R B Walker
D. Grobnic, C. W. Smelser, S. J. Milailov, and R. B.
Walker, Meas. Sci. Technol. 17, 1009 (2006).
- Y Zhu
- K L Cooper
- G R Pickrell
- A Wang
Y. Zhu, K. L. Cooper, G. R. Pickrell, and A. Wang, J.
Lightwave Technol. 24, 861 (2006).
- P Lu
- D Grobnic
- S J Mihailov
P. Lu, D. Grobnic, and S. J. Mihailov, J. Lightwave
Technol. 25, 779 (2007).
(Color online) FBG inscribed by a KrF 248-nm excimer laser in the same type of two-hole fiber and its response to external pressure at the room temperature
- Fig
Fig. 5. (Color online) FBG inscribed by a KrF 248-nm
excimer laser in the same type of two-hole fiber and its
response to external pressure at the room temperature.
- R Clowes
- S Syngellakis
- M N Zervas
R. Clowes, S. Syngellakis, and M. N. Zervas, IEEE Photon. Technol. Lett. 10, 857 (1998).