This PDF file contains the front matter associated with SPIE Proceedings Volume 7677, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Long distance sensing based on Brillouin scattering with centimeter spatial resolution, and yet high strain or temperature resolution requires the optimization of the optical and electronic system. In optical domain the limiting factors include gain saturation of the Stokes signal and pump depletion induced the Brillouin spectrum distortion, and thus a low gain is desired that requires low pump power, which sets a limit in the signal to noise ratio (SNR). The detection system must have high gain and narrow bandwidth to reduce electronic noise. The coded pulse offers the best solution as a low power solution of long distance sensing based on BOTDA to improve the signal to noise ratio (SNR), comparing two most common used formats: non-return-to-zero (NRZ) and return-to-zero (RZ), RZ coded pulses offer minimum distortion in the spatial resolution and the Brillouin spectrum, because the signal in RZ format returns to zero in very bit, while in NRZ coded pulse the signal returns to zero after continuous "1"s, which brings the higher gain and lower bandwidth comparing that in RZ coded pulse for BOTDA system. Hence NRZ coded pulse BOTDA would introduce spatial broadening and lower the spatial resolution. With minimum distortion of RZ signal we can use differential Brillouin gain to realize DPP-BOTDA technique for sub-meter spatial resolution. The minimum coded pulse width must be larger than the acoustic wave relaxation time to avoid the distorted Brillouin gain spectrum. Using LEAF fiber we achieved 50km sensing length and 50cm spatial resolution with the strain resolution of 8με which is equivalent to 0.7MHz Brillouin frequency shift, this is the 1st sub-meter spatial resolution for 50km sensing length combined with high strain resolution.

This paper describes a very simple generic approach to detecting leaks and spills using fibre optic sensing technologies. The system has been configured for distributed sensing and the sensitive fibre cable can detect one, or several wet section(s) one metre in length over distances extending to 10km with location accuracies of the order of 1 metre. Furthermore, by modifying the interface chemistry, the system will respond to either aqueous solutions or to hydrocarbon fluid with no cross talk from one to the other. The system also responds in time scales of seconds, and is reversible over - to date - indefinite number of cycles. The system can also be configured to respond to water or (some) hydrocarbon vapours, predominantly at high vapour pressures. Finally, much simplified shorter range versions of the sensors are currently being investigated with a view to detecting the occurrence of a leak or spill at any point in the path length of metres or tens thereof.

Redondo Optics in collaboration with the Cortland Cable Company, TMT Laboratories, and Applied Fiber under a US Navy SBIR project is developing an embedded distributed fiber optic sensor (EDIFOS^TM) system for the real-time, structural health monitoring, damage assessment, and lifetime prediction of next generation synthetic material arresting gear cables. The EDIFOS^TM system represents a new, highly robust and reliable, technology that can be use for the structural damage assessment of critical cable infrastructures. The Navy is currently investigating the use of new, all-synthetic- material arresting cables. The arresting cable is one of the most stressed components in the entire arresting gear landing system. Synthetic rope materials offer higher performance in terms of the strength-to-weight characteristics, which improves the arresting gear engine's performance resulting in reduced wind-over-deck requirements, higher aircraft bring-back-weight capability, simplified operation, maintenance, supportability, and reduced life cycle costs. While employing synthetic cables offers many advantages for the Navy's future needs, the unknown failure modes of these cables remains a high technical risk. For these reasons, Redondo Optics is investigating the use of embedded fiber optic sensors within the synthetic arresting cables to provide real-time structural assessment of the cable state, and to inform the operator when a particular cable has suffered impact damage, is near failure, or is approaching the limit of its service lifetime. To date, ROI and its collaborators have developed a technique for embedding multiple sensor fibers within the strands of high performance synthetic material cables and use the embedded fiber sensors to monitor the structural integrity of the cable structures during tensile and compressive loads exceeding over 175,000-lbsf without any damage to the cable structure or the embedded fiber sensors.

Terrorists and criminals more and more attack and destroy important infrastructures like routes, railways, bridges, tunnels, dikes and dams, important buildings. Therefore, reliable on-line and long-term monitoring systems are required to protect such critical infrastructures. Fiber optic sensors are well-suited for that. They can be installed over many kilometers and are able to measure continuously distributed strain, pressure, temperature and further mechanical and physical quantities. The very tiny optical fibers can be integrated into structures and materials and can provide information about any significant changes or damages of the structures. These so-called smart materials and smart structures are able to monitor itself or its environment. Particularly smart technical textiles with embedded fiber optic sensors have become very attractive because of their high importance for the structural health monitoring of geotechnical and masonry infrastructures. Such textiles are usually used for reinforcement of the structures; the embedded fiber optic sensors provide information about the condition of the structures and detect the presence of any damages and destructions in real time. Thus, critical infrastructures can be preventively protected. The paper will introduce this innovative field and will present the results achieved within several German and European projects.

Discriminating between intrusion and nuisance events without compromising sensitivity is a key performance parameter for any outdoor perimeter intrusion detection system. This is especially the case for intrusion and nuisance events which may have a similar impact on a perimeter fence. In this paper, a robust event classification system using features based on level crossings is presented for the detection and recognition of intrusion and non-intrusion events in an outdoor fence-mounted intrusion detection system for a range of operating environments and fence styles. The proposed classification system is applied to a distributed fiber-optic Mach Zehnder (MZ) mounted on a perimeter fence. It consists of a pre-processing stage employing high resolution time-frequency distribution, a novel event detection and feature extraction scheme based on level crossings, and a classification algorithm using a supervised neural network. Experimental results are presented showing accurate classification of different intrusion and non-intrusion events such as fence-climbing, fence-cutting, stone-throwing and stick-dragging. These results demonstrate the robustness of the proposed algorithm for various types of fence fabric and operating environments.

Field testing of fiber-based security sensors for use in Shallow-Water Perimeter Intrusion Detection System (SWPIDS) is presented in this paper. These devices were evaluated for continued performance and environmental resilience while deployed in a shallow-water marine environment for several months. These devices were previously untested in such conditions, and were evaluated for consideration in a high-security, low-nuisance alarm, environmentally-friendly underwater security system. The tested sensors include a breach-sensitive stainless steel grate and magnetic tamper switch, both from Woven Electronics, Inc., and a disturbance-sensitive fiber cable from Fiber SenSys, LLC. Preliminary results are presented, including nuisance alarm rates, functionality over the trial period, and resistance to marine effects.

Dual Mach-Zehnder interferometric fiber sensor is one of the appropriate solutions to the application of submarine cable security. While the seabed environment makes the vibration output a narrow bandwidth lowpass interference signal and these property makes the correlation based event positioning algorithm hard to design. A preprocessing method intends to increase signal power spectral range before correlation is proposed. A bandpass filter is imposed to enhance the effect of high frequency components, despite the filtered signal is different from the original one. A post-integration SNR threshold is calculated to determine the proper center frequency and bandwidth of the filter. Experiment results show a good accordance with theoretical prediction.

Fiber grating sensors can be used to support a wide variety of high speed measurement applications. This includes measurements of vibrations on bridges, traffic monitoring on freeways, ultrasonic detection to support non-destructive tests on metal plates, and providing details of detonation events. This paper provides a brief overview of some of the techniques that have been used to support high speed measurements using fiber grating sensors over frequency ranges from 10s of kHz, to MHZ and finally toward frequencies approaching the GHz regime.

The increasing use of thermoplastic carbon fiber-reinforced plastic (CFRP) materials in the aerospace industry for primary aircraft structures, such as wing leading-edge surfaces and fuselage sections, has led to rapid growth in the field of structural health monitoring (SHM). Impact, vibration, and load can all cause failure, such as delamination and matrix cracking, in composite materials. Moreover, the internal material damage can occur without being visible to the human eye, making inspection of and clear insight into structural integrity difficult using currently available evaluation methods. Here, we describe the detection of impact and its localization in materials and structures by high-speed interrogation of multiple-fiber Bragg grating (FBG) sensors mounted on a composite aircraft component.

We demonstrated a novel twin-core fiber grating sensor system for accurate and simultaneous measurements of temperature and strain. Specifically, we designed two cores with custom index and dopant profiles such that the gratings in these two cores have different temperature coefficients, but almost same strain coefficients. Our data also showed that both gratings had almost the same wavelength shift at the same deuterium concentration level in the fiber. This, together with the strain characteristics of the gratings, enables us to isolate the temperature measurement when strain, temperature and gaseous diffusion all apply to the sensor simultaneously. It makes our grating sensor extremely useful for hydrothermal down-hole temperature mapping applications.

Cryogenic fuels are often considered as major energy alternatives to coal and petroleum based fuels. Safe and reliable sensor networks are required for on-demand, real-time fuel management in cryogenic environments. In this paper, a new sensor design is described that enhances the low-temperature performance of fiber sensors. FBGs inscribed in high attenuation fiber (HAF) are used to absorb in-fiber power light to raise the local sensor temperature in the cryogenic environment. When in-fiber power light is turned off, FBG sensors can serve as passive sensors to gauge temperature and stress in the cryogenic system. When the in-fiber power light is turned on, the heated sensors can be used to rapidly gauge fuel level and fuel leaks. In one example, a hydrogen gas sensor is demonstrated with a palladium-coated fiber Bragg grating (FBG). The low-temperature performance of the sensor was improved by heating the gratings as much as 200 K above the ambient temperature, and hydrogen concentration well below the 4% explosion limit was measured at 123K. In a second example, an array of four aluminum coated fiber Bragg gratings was used to measure liquid level in a cryogenic environment.

In this paper, we present fiber Bragg grating pressure sensors written in twin-hole microstructured fibers for high temperature operation above 800 °C. A Ti: Sapphire ultrafast laser is used to inscribe type II grating in two-hole optical fibers. The fiber Bragg grating resonance wavelength shift and peak splits are studied as a function of external hydrostatic pressure from 15 psi to 2000 psi. The ultrafast laser inscribed grating pressure sensor shows stable and reproducible operation above 800 °C. This paper demonstrates highly multiplexible pressure sensor technology for high temperature environment.

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers without hydrogen loading and with the use of UV nanosecond laser pulses. We observe a complex regenerative process which indicates a secondary grating growth in an optical fiber by thermal activation. This process leads to an increased temperature stability of the gratings up to 600 °C which differs from the commonly known Type I gratings. With the use of an interferometric writing technique it is possible to generate arrays of regenerated fiber Bragg gratings (RFBGs) for sensor networks. The writing conditions of such new type of gratings are investigated and the temperature behavior of these RFBGs is analyzed. This type of gratings is suitable for high temperature sensor networks by combining the attributes of good spectral shape and high reflectivity with high temperature stability showing no drift or hysteresis.

A novel liquid-level sensor based on fiber Bragg grating and carbon fiber composite diaphragm is proposed and demonstrated. The sensing principle and finite element analysis result are described. From the experimental result, this sensor shows high sensitivity and good repeatability. This sensor can find applications in the area of the liquid level sensing.

In this paper, we propose to use an inverse-based method that is implemented by a Genetic algorithm (GA) for obtaining the strain and temperature profiles in a 10-mm fiber Bragg grating (FBG) sensor and a series set of ten 10-mm sensors. The changes of strain and temperatures are analyzed by utilizing the sensitivity of the refractive index and grating period of the fiber Bragg grating sensor. This can be accomplished by reconstructing the FBG structural shape by using a Genetic algorithm that is compared with the measured output data. Our ultimate objective for utilizing these results are intended for real-time sensing of strain and temperature of these sensors which are ideally suited for smart structures health monitoring and diagnostics applications.

There are great amount of information in the safety monitoring of huge structures and equipments and many kinds of sensors with promising long-term stability will be needed. Fiber Bragg grating (FBG) sensing technology is an effective means to realize the safety monitoring of huge structures and equipments. In this paper, the typical problems in the engineering application of safety monitoring of huge structures and equipments using FBG sensing technology have been analyzed. The key technologies of its engineering application have been studied, including FBG detecting technology in the harsh environment with high temperature and huge strain, the multipoint monitoring technology with huge capacity, and the demodulating technology. On that basis, several kinds of FBG sensing methods and products have been developed. The application examples of these products in a number of huge structures and equipments such as bridges, tunnels, huge rotating machines, and oil tanks have been presented, which will be helpful to the settlement of the related technical problems.

A temperature-compensated strain-sensing sensor based on fiber Bragg gratings (FBGs) that is suitable for strain mapping of rock in coal exploring application is demonstrated. FBGs were bonded to carbon fiber laminated composite (CFLC) and they were arranged in a FBG rosette configuration used to determine the direction and magnitude of the principal strain, information that is required for strain mapping. An extra reference grating was utilized for temperature compensation measurement.

Fiber optic sensors offer several advantages over their electrical counterparts, especially for applications in hostile, spark-sensitive environments, because no electrical power is required at the sensors. In addition, the installation of fiber sensors external to fluid carrying conduits facilitates access for troubleshooting and replacement, unlike in-line diaphragm-based sensors. Furthermore, glass fiber pressure sensors have a much higher operating temperature range, which makes them more practical for flammability-prone environments. Multiple fiber Bragg grating (FBG) sensors can be multiplexed along a single fiber optic cable, as opposed to traditional resistive strain gauges, which require individual shielded metal cabling. Applications for such fiber-optic pressure detection systems include the pressure monitoring of flow in fuel lines and their pressure valves. This paper characterizes the application of FBG sensors, with remote access capability, for the nonintrusive pressure monitoring of different types of metallic pipes. We show that pressure changes smaller than one psi can be detected with a tunable diode laser-based detection system. Standard metal pipes of steel, inconel, copper-nickel alloy and titanium are characterized, and the resilience of FBG sensors to an overpressure of up to 1500 psi is demonstrated.

Sensing systems for defense and security operate in evermore demanding environments, increasingly leaving the comfort zone of fiber laser technology. Efficient and rugged laser sources are required that maintain a high level performance under large temperature excursions and sizable vibrations. This paper first presents a sample of defense and security sensing applications requiring laser sources with a narrow emission spectrum. Laser specifications of interest for defense and security sensing applications are reviewed. The effect of the laser frequency noise in interferometric sensing systems is discussed and techniques implemented to reduce phase noise while maintaining the relative intensity noise performance of these sources are reviewed. Developments towards the size reduction of acoustically isolated narrow-linewidth semiconductor lasers are presented. The performance of a narrow-linewidth semiconductor laser subjected to vibrations is characterized. Simulation results of interferometric sensing systems are also presented, taking into account both the intensity and phase noise of the laser.

OPLL based on 1550 nm narrow linewidth Planar External Cavity Lasers (PLANEX) with PM output offer several advantages over OPLL based on the conventional DFB or fiber lasers such as bandwidth requirements, high coherent efficiency, absence of the phase reversal and long term locking stability over ambient temperature changes. Such requirements are critical in the field of microwave photonics, LIDAR, coherent optical communications and optical metrology. We report a development of OPLL, optimized for distributed BOTDA/BOTDR Brillouin sensing applications. Conventional approach for distributed fiber optic Brillouin scattering (BOTDR) use a heterodyne architecture for detection of Brillouin scattering signals. With such approach bandwidth (BW) of the optical detector play one of the most critical roles in accuracy of the BOTDR detection. Such coherent detection require a 12 GHz bandwidth of microwave detector which bring excessive noise and high cost of the implementation. OPLL with LO having frequency offset of the order of Brillouin frequency, i.e. 9-12 GHz allow to use low RF BW detection of BOTDR signal. Such detection allow much higher sensitivity, lower noise contribution and offer considerable cost saving for BOTDR distributed sensing and monitoring Beat frequency stability of OPLL was on the order of few kHz and linewidth of the locked lasers was less then 20 kHz. Coherent efficiency of OPLL was better the 85%. Inherent wavelength stability of ECL (order of magnitude better then any of DFB lasers) allows continuous operation of OPLL without losing locking accuracy. OPLL stability was demonstrated over 48 hours of continuous operation.

We review the technologies of fiber fusion and fused device fabrication for fiber sensing applications. Fiber interconnection and fused fiber components are integral parts in optical fiber sensing modules and systems. Dissimilar fibers need to be fused together with low loss and high strength to ensure long-term performance stability. Fused fiber components, such as fused couplers and polarization-maintaining (PM) components, are important for both coherent and incoherent detection. In addition, the latest advances in nano/micro-fiber device and multi-port signal combiners pose new challenges to the fiber fusion and fused component technologies. We describe fundamental optics, discuss fabrication techniques for these devices, and present some application examples.

An analysis and performance evaluation of several macrobending high-bend loss fiber based filters utilizing the well-known Whispering Gallery mode (WGM) effect is investigated and presented. Experimental results indicate that the WGM spectra of the macrobend fibre can be utilized for different types of optical filters by adjusting the bending diameter and the length of the fiber loop.

The last couple of decades had witnessed a rise in the research of optoelectronic and fiber optical communication fields, which resulted in applications focused initially in military and aerospace equipments, and later in health monitoring for medicine, heritage culture and various engineering fields. The monitoring of existing or /and new engineering, biomedical structures has become a regular feature throughout the world. Monitoring is fast emerging as a pioneering field with high precision and quality equipments. This field is very vast, consisting of both traditional as well as smart materials based methods. The fiber optics belong to the finest class of smart materials, there are many types and classifications based on the necessity, manufacturer and the end user. In this paper, a complete over view of fiber sensing systems and their usefulness is briefly presented.

An in-situ Dynamic Measuring System is developed and tested to measure the vibratory and translational displacement of the airfoil modes of a combustion turbine compressor vane via an optical non-contact method. The Dynamic Measurement System measures the dynamic contact displacement in the radial direction of the combustion turbine, which can be implemented as measurement of radial displacement on the axial contact surface. The dynamic system is a non-contact device that uses a fiber optic bundle for the sensor element probe. The probe's fibers are connected to a module which in turn plugs into the display controller. The probe is placed in proximity of the target area. Motion of the target is determined by the amount of light received by the probe. The fiber bundle system is capable of simultaneous measurement of vibratory (AC) motion and free body or translational (DC) motion. The AC modes are resolved spectrally by an FFT of the raw fiber optic bundle signal. The fiber optic bundle correctly identified the modes and their associated magnitude on the airfoil and measured the displacement of the target area.

An optical fiber Single/Multi-/Single-mode Intrinsic Fabry-Pérot Interferometer (SMS-IFPI) pressure sensor has been demonstrated using a silica tube-based pressure transducer hermetically sealed by thermal fusion bonding. The sensor, made entirely of fused silica, contains an IFPI strain sensor enclosed by a CO₂ laser-bonded outer tube. A sensor prototype is constructed and demonstrated for single point pressure sensing at high temperature (600°C), with temperature compensation achieved through co-location of an SMS-IFPI temperature sensor. The inline geometry and low transmission loss of the SMS-IFPI sensor makes it suitable for frequency division multiplexing (FDM) in a single fiber branch. In future work, we envision multiplexing of up to eight such IFPI pressure sensors along a single fiber branch for quasi-distributed pressure measurement.

We have fabricated a variety of chiral fiber sensors by twisting one or more standard or custom optical fibers with noncircular or nonconcentric core as they pass though a miniature oven. The resulting structures are as stable as the glass material and can be produced with helical pitch ranging from microns to hundreds of microns. The polarization selectivity of the chiral gratings is determined by the geometry of the fiber cross section. Single helix structures are polarization insensitive, while double helix gratings interact only with a single optical polarization component. Both single and double helix gratings may function as a fiber long period grating, coupling core and cladding modes or as a diffraction grating scattering light from the fiber core out of the fiber. The resulting dips in the transmission spectrum are sensitive to fiber elongation, twist and temperature, and (in the case of the long period gratings) to the refractive index of the surrounding medium. The suitability of chiral gratings for sensing temperature, elongation, twist and liquid levels will be discussed. Gratings made of radiation sensitive glass can be used to measure the cumulative radiation dose, while gratings made of radiation-hardened glass are suitable for stable sensing of the environment in nuclear power plants. Excellent temperature stability up to 900°C is found in pure silica chiral diffraction grating sensors.

A novel Fabry-Perot interferometer pressure/acoustic sensor has been designed, fabricated, and tested. The sensor consists of an angle-polished fiber, a V-shaped groove on a silicon substrate, and a silicon nitride diaphragm on the side wall of the groove. The design uses MEMS technology to ensure precise cavity length control and diaphragm design flexibility. Two shock wave tests were performed on the sensors: one where a balloon was popped near the sensors, and another that used a shock tube to simulate a blast event. Multi-sensor assemblies, where all the sensors were calibrated to have similar center wavelengths, were also put together. The assemblies were tested simultaneously using a single laser source. The results of all these tests showed that the performance of the Fabry-Perot sensors closely matched that of the reference sensors used.

A novel ultrasound generator consisting of a single mode optical fiber with a layer of gold nanoparticles on its tip has been designed. The generator utilizes the optical and photo-acoustic properties of gold nanoparticles. When heated by laser pulses, a thin absorption layer made up of these nanoparticles at the cleaved surface of a single mode fiber generates a mechanical shock wave caused by thermal expansion. Mie's theory was applied in a MATLAB simulation to determine the relationship between the absorption efficiency and the optical resonance wavelengths of a layer of gold nanospheres. Results showed that the absorption efficiency and related resonance wavelengths of gold nanospheres varied based on the size of the gold nanosphere particles. In order to obtain the bandwidths associated with ultrasound, another MATLAB simulation was run to study the relationship between the power of the laser being used, the size of the gold nanosphere, and the energy decay time. The results of this and the previous simulation showed that the energy decay time is picoseconds in length.

A novel ultrasound generator-receiver built on a single-mode optical fiber using a layer of gold nanoparticles has been designed. The generator takes advantage of the optical and photo-acoustic properties of gold nanoparticles. Thermal and pressure waves are generated in the nano-particle layer when it is exposed to high intensity, short duration laser radiation. The laser radiation is applied in an intensity range that creates an instantaneous surface heating of the layer material that, in turn, drives a pressure wave into the layer. The pressure wave interacts with the layer-substrate interface to create stress distributions of varying strengths and qualities, depending on the intensity and duration of the initial laser pulse. The radiation due to laser-induced heating on the nano-particles was investigated using FEA analyses. The maximum principal stress distribution was investigated by the FEA. Results indicate that the ultrasound generation elements have almost zero effect on the diaphragm.

The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmission cell allows synergistic integration of optics and microfluidics to form an unconventional optofluidic platform of long interaction path limited only by the fiber length. We report the strategy and methods in realizing full-length surface-enhanced Raman scattering (SERS) PCF optofluidics by immobilization of negatively charged Ag nanoparticles (NP) through polyelectrolyte-mediated approach or direct deposition of positively charged Ag NP on the PCF air channels. Through forward propagating Raman measurements, we demonstrate the full-length SERS-active PCF optofluidics with accumulative Raman signal gain along the entire fiber length. We show SERS measurements of 1x10^-7 M (~48 ppb) Rhodamine 6G and 1x10^-8 M (~0.8 ppb) sodium thiocyanate in a minute volume of ~10^-7-10^-8 liter aqueous solution using PCF with immobilized Ag NP over ~20 cm in length. The combination of high detection sensitivity and small sampling volume renders the SERS-active PCF optofluidic platform excellent potential for a multitude of applications ranging from label-free chemical and biological sensing to process monitoring in geometrically confined systems.

An evanescent field sensing platform is being pursued through excitation of cladding modes using long-period (LPGs) inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO₂ laser irradiation. Core-cladding mode coupling and recoupling has resulted in significant improvement in the evanescent field overlap throughout the cladding air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization of the PCF-LPG configuration can lead to a field power overlap as high as 23% with a confinement loss of less than 1 dB/m in the cladding mode.

We have designed and built a hollow-core fiber frequency reference cell, filled it with CO₂, and used it to demonstrate frequency stabilization of a 2.05 μm Tm:Ho:YLF laser using frequency modulation (FM) spectroscopy technique. The frequency reference cell is housed in a compact and robust hermetic package that contains a several meter long hollow-core photonic crystal fiber optically coupled to index-guiding fibers with a fusion splice on one end and a mechanical splice on the other end. The package has connectorized fiber pigtails and a valve used to evacuate, refill it, or adjust the gas pressure. We have demonstrated laser frequency standard deviation decreasing from >450MHz (free-running) to <2.4MHz (stabilized). The 2.05 μm laser wavelength is of particular interest for spectroscopic instruments due to the presence of many CO₂ and H₂0 absorption lines in its vicinity. To our knowledge, this is the first reported demonstration of laser frequency stabilization at this wavelength using a hollow-core fiber reference cell. This approach enables all-fiber implementation of the optical portion of laser frequency stabilization system, thus making it dramatically more lightweight, compact, and robust than the traditional free-space version that utilizes glass or metal gas cells. It can also provide much longer interaction length of light with gas and does not require any alignment. The demonstrated frequency reference cell is particularly attractive for use in aircraft and space coherent lidar instruments for measuring atmospheric CO₂ profile.

Recently, many accidents were reported that some passengers were unfortunately restrained and killed at the gaps between the Platform Screen Doors (PSDs) and the doors of the subway trains. In this paper, one proposal of real time monitoring system based on optical time domain reflectrometer (OTDR) to detect the intrusions at these gaps is presented. In this method the locations and weight of intrusions can be obtained by detecting the abrupt power loss of backscattering light caused by the weight of intrusions upon the microbending sensor heads. This method can be easily multiplexed and extended into a multi-function sensing system, such as monitoring the temperature, smoking and the strains in the tracks, or applied in other fields, such as escalators, board gates for ferries or planes.

Safety flight of aircrafts requires that the aircraft center of gravity (CG) must fall within specified limits established by the manufacturer. However, the aircraft CG depends not only on the structure of planes, but also on the passengers and their luggage. The current method of estimating the weight of passengers and luggage by the average weight may result in a violation of this requirement. To reduce the discrepancy between the actual weight and estimated weight, we propose a method of improving the accuracy of calculating the CG of the plane by weighing the passengers and their personal luggage. This method is realized by a Weigh-In-Motion (WIM) system installed at boarding gates based on optical fiber Bragg grating (FBG) technology. One prototype of WIM is fabricated and tested at lab. The resolution of this system is 2 kg and can be further improved by advanced manufacture technology. With the accurate weight of passengers and luggage coming from the WIM system and the locations of passengers and luggage obtained from boarding cards, the aircraft CG can be calculated correctly. This method can be applied into other fields, such as escalators, boarding gates for ferries.

A novel detection technique to estimate the amount of chirp in fiber Bragg gratings (FBGs) is proposed. This method is based on the fact that reflectivity at central wavelength of FBG reflection changes with strain/temperature gradient (linear chirp) applied to the same. Transfer matrix approach was used to vary different grating parameters (length, strength and apodization) to optimize variation of reflectivity with linear chirp. Analysis is done for different sets of 'FBG length-refractive index strength' combinations for which reflectivity vary linearly with linear chirp over a decent measurement range. This article acts as a guideline to choose appropriate grating parameters in designing sensing apparatus based on change in reflectivity at central wavelength of FBG reflection.

A novel high sensitive fiber Bragg grating (FBG) strain sensing technique using lasers locked to relative frequency reference is proposed and analyzed theoretically. Static strain on FBG independent of temperature can be measured by locking frequency of diode laser to the mid reflection frequency of matched reference FBG, which responds to temperature similar to that of the sensor FBG, but is immune to strain applied to the same. Difference between light intensities reflected from the sensor and reference FBGs (proportional to the difference between respective pass band gains at the diode laser frequency) is not only proportional to the relative strain between the sensor and reference FBGs but also independent of servo residual frequency errors. Usage of relative frequency reference avoids all complexities involved in the usage of absolute frequency reference, hence, making the system simple and economical. Theoretical limit for dynamic and static strain sensitivities considering all major noise contributions are of the order of 25 pε / √Hz and 1.2 nε / √Hz respectively.