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The authors deliver a complete overview of fiber-optic Fabry-Perot (FFP) sensing technology, integrating the knowledge and tools of multiple fields including optics, sensing, micromachining, instrumentation, physics, and materials science. The main chapters discuss operating principles, microstructures, fabrication methods, signal demodulation, and instrumentation. This treatment spans the full range of structures (intrinsic/extrinsic, multimode fiber vs single-mode fibers), as well as advanced micromachining technologies and major interrogating and multiplexing methods for the formation of multi-point, quasi-distributed sensing networks. Readers will also gain a summary of state-of-the-art applications in oil, gas, and electricity industries, aerospace technology, and biomedicine. Yun-Jiang Rao is Dean of the School of Communication & Information Engineering, and Director of the Key Lab of Optical Fiber Sensing & Communications at the University of Electronic Science and Technology of China. Zeng-Ling Ran and Yuan Gong are both associate professors at the Optical Fiber Technology Research Laboratory of the University of Electronic Science and Technology of China.

Using the first principles method, which is based on the density function theory (DFT), the structures and electronic properties for different concentration of H2S are adsorbed on the Fe (100) surface, and their molecular orbital and absorption energies were calculated with the generalized gradient approximation. The results show that, whether one or two molecules of H2S adsorpted Fe (100) surface, adsorption of single molecules Fe (100) surface partial density of states between pure Fe (100) and two molecules, and the total density of states is mainly composed of 3d electronic contribution.

Recent progress in using excimer and femtosecond lasers to fabricate functional fiber-optic sensing devices is reviewed extensively in this paper. The application of femtosecond lasers is to fabricate fiber gratings and in-line fiber-optic interferometers for measurements of strain, temperature and refractive index. The application of 157 nm excimer lasers is to create fiber gratings and in-line fiber interferometers for measurements of strain, acceleration, pressure, temperature and refractive index. Compared with conventional optical fiber sensors, these novel micro optical fiber sensors fabricated by laser-micromachining have outstanding advantages, such as fast and direct formation, easy mass-production with ultralow-cost, good reproducibility, excellent optical performance, low temperature cross-sensitivity, etc., which pave a way to form a new generation of miniature fiber-optic sensors for many applications.

Density functional theory calculations have been performed for sulfur atom adsorption on the Fe(111) surface at 0.5 and 1.0 ML. The geometry structures, density of the state, charge population and adsorption energy were calculated. It is found that the most favored adsorption site for sulfur atom is the hollow site, and the interaction between Fe and S comes in large part from the hybridization of Fe 3d orbital and S 3p orbital. Our results have shown that the force between iron atom and sulfur atom is very small.

Using periodic density functional theory within the generalized-gradient approximation to electron exchange and correlation, we have studied S adsorption four-fold hollow site on Fe(100) in different hydrostatic pressure. We find that the adsorption height decreases with hydrostatic pressure increasing is non-monotonic. The adsorption energy decreases with an increase with pressure is monotonic and we have obtained density of states is almost unchanged, the adsorption energy change is mainly caused by lattice deformation in the hydrostatic pressure, and the adsorption energies increase linearly with pressure.

Using the first principles method, which is based on the density function theory (DFT), the structures and electronic properties of S atoms are adsorbed on the Fe (100) surface for X(X is Cr, Ni, Mo, C, Mn ,Si,P and S) impurities in Fe, and their molecular orbital and absorption energies were calculated with the generalized gradient approximation. The results show that S adsorbed on H site for Cr, Ni, Mn, C and Mo impurities in Fe is stable but for Si, S and P is B site. The adsorption energy for Ni in impurity Fe is almost nearby for the purity Fe and the effect for Ni in S absorption on Fe (100) surface is very small. In order to prevent S absorption on Fe surface，we can reduce the percentage of Ni.

Using the first principles method, which is based on the density function theory (DFT), the structures and electronic properties of S atoms are adsorbed on the Fe (100) surface, and their molecular orbital and binding energies were calculated with the generalized gradient approximation. The results show that the S atom is adsorbed hollow site is stable. With partial density of states, we have obtained the interaction of s and p states for S and Fe. It shows that the interaction between the S adsorption on the clean Fe (100) surface does lead to FeS comes into being.

An inline fiber-optic micro-Fabry-Perot (MFP) cavity and a short fiber Bragg grating (SFBG) is overlapped to form an integrated MFP/SFBG sensor for simultaneous measurement of temperature and strain under high temperature (300 °C). The FP cavity is fabricated on a GeO 2 -doped photon sensitive fiber using the 157-nm laser micromachining technique and the SFBG is written at the same position of the MFP cavity using the standard 248-nm laser method. As the MFP cavity and SFBG have different sensitivity coefficients to temperature and strain, they can be utilized for realization of simultaneous measurement of dual parameters.

The Raman amplification can be employed to obtain the enhanced sensing performance in long-distance Brillouin optical time-domain analyzer (BOTDA) with higher temperature measurement accuracy over the whole sensing fiber. Theoretical investigation on the detected probe waveshape has been performed by using the probe-pump coupled equations combined with Raman amplification. The experimental characterization has been demonstrated to validate the theoretical prediction. The results show that, the 75 Km sensing distance can be achieved by utilizing the bi-directional Raman amplification with measurement accuracy of ±0.6°C over the whole sensing fiber. The larger Raman power and Brillouin pump power are helpful to improve the sensing signal-noise ratio (SNR). There exists an optimum range for the input probe power and the extinction ratio (ER) of input Brillouin pump pulse. The BOTDA with Raman amplification has considerable potentials to realize long- distance strain and temperature measurement for structural health monitoring and fire alarming, etc.

A Mach-Zehnder (M-Z) interferometer formed by two cascaded long-period fiber gratings (LPFGs) serving as ‘many’ multi-channel edge filters is used to detect the wavelength shift, induced by measurand of fiber Bragg grating (FBG) sensors with high optical SNR of >60 dB due to the use of a fiber ring laser, for the first time to our knowledge. Advantages of such a demodulation device are low-cost, compact, easy fabrication and implementation, good immunity of vibration and temperature, etc. In addition, in order to improve the resolution for dynamic strain measurement, a spectral analyzing algorithm called Music modern spectrum estimation is adapted. The experimental results show that a dynamic strain resolution of 0.1 μɛ/Hz 1/2 at 700 Hz is obtained, which is 10 times improvement in strain resolution when compared with conventional FFT method.

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO 2 laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m −1 at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.

In this paper, we report novel long-period fiber gratings (LPFGs) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO2 laser pulses at several kHz. Several unique characteristics of these LPFGs, such as bend and transverse-load, etc., are observed by experiments, for the first time, to our knowledge. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross-section of the LPFG induced by high-frequency CO2 laser pulses. Based on these novel LPFGs, an amplified spontaneous emission noise filter and a gain equalizer have been demonstrated for the noise figure reduction and the gain spectrum flattening of Er-doped fiber amplifiers (EDFAs), respectively. By means of the unique bending and loading features of the LPFG, two tunable gain equalizers have been demonstrated for dynamic gain flattening of EDFAs.

In this paper, we report a novel long-period fiber grating (LPFG) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO/sub 2/ laser pulses at several kilohertz. A number of unique characteristics of such a LPFG, such as bend, torsion, and transverse load, are observed by experiments, for the first time, to our knowledge. Based on these unique features, a novel bend-insensitive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurands, a novel torsion sensor that can realize absolute measurement of twist rate, and a load sensor that can achieve simultaneous measurement of transverse load and temperature using a single LPFG element are proposed and demonstrated. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross section of the LPFG induced by high-frequency CO/sub 2/ laser pulses.

The combination of Raman amplification and optical pulse coding (OPC) is proposed to reduce the frequency uncertainty in a long-distance Brillouin optical time-domain analyzer (BOTDA) under higher spatial resolution. The experimental characterization of temperature sensing with measurement accuracy ~ 1°C under ~ 2.5-m spatial resolution over a whole length of sensing fiber is demonstrated. The sensing fiber is composed of 49.6-km fiber at 10.8-GHz Brillouin frequency shift (BFS) and 25-km fiber at 11.005 GHz.

A novel tunable gain equalizer based on the long-period fiber grating (LPFG) induced by high-frequency CO/sub 2/ laser pulses is demonstrated, for the first time, to our knowledge. Both the resonant wavelength and the resonant peak amplitude of the novel LPFG have a good linearity with temperature and transverse load, respectively. The wavelength and the peak amplitude of the gain equalizer can be tuned by the means of adjusting temperature and transverse load independently, showing that this tunable equalizer is a practical approach to realizing dynamic gain equalization in erbium-doped fiber amplifier systems.

The combination of Raman amplification and optical pulse coding (OPC) is proposed to reduce the frequency uncertainty in long-distance BOTDA under higher spatial resolution. The experimental characterization of 74.6Km temperature sensing with measurement accuracy ~1oC under ~2.5m spatial resolution over the whole length of sensing fiber is demonstrated. The performance comparisons of BOTDA with purely Raman amplification and combined Raman amplification with OPC are also displayed.

A novel fiber-optic phase-sensitive optical time-domain reflectometer (Φ-OTDR) distributed sensing system is proposed and demonstrated for long-distance intrusion monitoring. By using the bi-directional Raman optical amplification technology, even intrusion signal detection along the whole sensing distance of ~62km has been demonstrated, resulting in the longest and the most sensitive Φ -OTDR distributed sensing system reported so far, to the best of our knowledge. In addition, high-power single-mode fiber laser with ultra-narrow line-width and special optical fiber cable encapsulated by fiber reinforced plastic (FRP) are used to further enhance the intrusion detection sensitivity, respectively.

A novel tunable fiber ring laser configuration with combination of hybrid Raman amplification and erbium-doped fiber amplification (EDFA) is proposed for realizing ultra-long distance (300km) fiber Bragg grating (FBG) sensor system. The experimental results show that a 300 km transmission distance is achieved with an optical SNR of 4 dB, which is the longest FBG sensing distance, to the best of our knowledge.

An in-fiber Bragg grating (FBG) dynamic strain sensing system based on a fiber ring laser configuration and the Eigenvector spectrum analysis method, is proposed and demonstrated. An ultrahigh dynamic strain resolution of 5x10 -5 ne /Hz 1/2 is achieved due to both the use of the supper white-noise suppression ability of the advanced spectrum analysis method and the high optical signal-to-noise ratio (SNR) (~65dB) of the sensing signal generated by the fiber ring laser scheme. This is the highest dynamic strain resolution achieved so far for a single FBG sensor at frequency of

We demonstrate experimentally the light coupling effects between two parallel CO(2)-laser written long-period fiber gratings. For gratings written in standard single-mode fibers, the coupling efficiency depends strongly on the fiber orientation with the strongest coupling obtained when the exposed sides of the fibers face each other, while for gratings written in boron-doped fibers, the coupling efficiency is independent of the fiber orientation. We achieve a peak coupling efficiency of approximately 86% with gratings written in boron-doped fibers by using a suitable surrounding refractive index and offset distance between the two gratings. Our results suggest the possibility of realizing efficient broadband all-fiber couplers with CO(2)-laser written long-period fiber gratings.

A novel tunable fiber ring laser configuration with multi-stage hybrid Raman/EDF amplification is first proposed and demonstrated for realizing an ultra-long-distance (~100 km) fiber Bragg grating (FBG) sensing system with a high optical SNR of >50 dB.

A novel tunable fiber ring laser configuration with a combination of bidirectional Raman amplification and dual erbium-doped fiber (EDF) amplification is proposed for realizing high optical signal-to-noise ratio (SNR), long-distance, quasi-distributed fiber Bragg grating (FBG) sensing systems with large capacities and low cost. The hybrid Raman-EDF amplification configuration arranged in the ring laser can enhance the optical SNR of FBG sensor signals significantly owing to the good combination of the high gain of the erbium-doped fiber amplifier (EDFA) and the low noise of the Raman amplification. Such a sensing system can support a large number of FBG sensors because of the use of a tunable fiber Fabry-Perot filter located within the ring laser and spatial division multiplexing for expansion of sensor channels. Experimental results show that an excellent optical SNR of approximately 60 dB has been achieved for a 50 km transmission distance with a low Raman pump power of approximately 170 mW at a wavelength of 1455 nm and a low EDFA pump power of approximately 40 mW at a wavelength of 980 nm, which is the highest optical SNR achieved so far for a 50 km long FBG sensor system, to our knowledge.

A novel tunable fiber ring laser configuration with hybrid Raman/EDFA configuration is first proposed and demonstrated for realizing high optical SNR of ~60dB, long-distance of ~50km, fiber Bragg grating (FBG) sensing systems.

In this paper, a hybrid Raman and Erbium-doped fiber amplifier (Raman/EDFA) is first proposed for enhancementof the performance of long-distance fiber Bragg grating (FBG) sensor systems. The Raman fiber amplifier is used for achieving low-noise amplification of FBG signals while the EDFA arranged remotely is used for both the compensation of the fiber loss due to long-distance transmission and the illumination of remotely-located FBGsensors as a broadband source by using the residual Raman pump power to generate ASE light and by amplifyingthe transmission light of the FBGs located before the EDFA. Such a sensor system adopts the advantages of thelow noise feature ofthe distributed Raman amplifier and the high small-signal gain ofthe EDFA. Experimentalresults show that the performance of the sensor system has been improved significantly. A signal-to-noise ratio (SNR) of l3dB has been achieved for a 50km transmission distance, which is l0dB higher than that without amplification.

We have demonstrated a novel technique for simultaneous measurement of static strain, temperature, vibration and transverse load for structural health monitoring using an integrated FBG/EFPI/LPFG sensor. Preliminary results show that this technique combines the advantages of the FBG, LPFG and low-coherence-interferometry, such as immune to EMI, high accuracy and resolution, small and compact sensor head, corrosion resistance etc. This sensor system is well-suited to health monitoring of large engineering structures, such as highways, bridges, buildings and aircrafts.

In this letter, we present a novel sensor approach allowing fast measurement of temperature. It consists of a chirped fibre Bragg grating (CFBG) and an extrinsic Fabry-Perot interferometric sensor (EFPI) in series. Both EFPI and CFBG are well integrated to provide local strain and temperature information respectively, using wavelength-division-multiplexed (WDM) technique. As the wavelength-shift of the CFBG is detected via intensity variation, the speed for temperature measurement can be very high (

An alternative structure using third-order Raman amplification based on ultralong fiber lasers (UL-FLs) is investigated numerically to further improve the quasi-lossless transmission performance. The performance comparisons on flatness of signal distribution, optical signal–noise ratio (OSNR), and noise figure (NF) among first-order, second-order, and third-order Raman pumping based on UL-FLs are discussed numerically. The results display that the gain distribution is pushed more uniformly along UL-FLs using third-order Raman pumping, which is very useful to further extend the quasi-lossless midspan distance. For transparency transmission, enhanced quasi-lossless transmission could be achieved without sacrifice to OSNR and NF. We also analyze the transmission dynamics of loss-managed fundamental solitons for different pumping configurations. The amplifier span and transmission quality are improved considerably, even for lower dispersion lengths, by employing the proposed scheme.

The influence of the non-local effect on sensing performance in a long-distance Brillouin optical time-domain analyzer (BOTDA) based on bi-directional Raman amplification has been investigated theoretically. The system error induced by the non-local effect worsens with increased powers of probe wave and Raman pump. For smaller Brillouin pump power, the non-local effect is almost independent of Brillouin pump power. For larger Brillouin pump power, the non-local effect deteriorates gradually due to Raman gain saturation and cross-gain modulation (XGM). The purely backward pumping (relative to probe light) is preferable for non-local effect suppression. The frequency-division multiplexing (FDM) and time-division-multiplexing (TDM) technologies exhibit similar performance for suppressing the non-local effect under the same conditions. The sensing range near the far-end (relative to the Brillouin pump) should be appropriately shorter to minimize the non-local effect in FDM and TDM. The physical mechanisms of the non-local effect in long-distance BOTDA with Raman amplification are also shown.