In this paper, we report recent progress in 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. The writing mechanism of this technique is reported for the first time to our knowledge. Novel sensor systems for low-cost measurement of strain and dynamic transverse load are proposed and demonstrated. In addition, phase-shifted LPFGs are fabricated and used for sensing of strain and temperature. Finally, the etching characteristics of these LPFG sensors are investigated.

In this paper, a brief review of recent advances on high-temperature distributed fiber optic sensors is presented, which includes (1) background on fiber gratings, (2) in-fiber grating fabrication methods in single crystal sapphire fiber, (3) reflection-type fiber specklegram generated in single crystal sapphire fiber, and (4) preliminary experimental results on high temperature sensing.

A number of embedded sensor protection system (ESPS) and surface-mountable sensor protection system (SSPS) for fibre optic sensor (FOS) have been developed in this paper. Fibre Bragg Grating (FBG) sensor protected by the designed protections system have been used to monitor the cure progress and structural health status of concrete cylinders. Results indicate that the sensor protection systems for the FOS perform adequately and effectively in concrete environment. The protected fibre optic sensors are suitable achieve the structural health monitoring in practical. It is also revealed that there is excellent correlation between the results obtained from the protected FOS and reference electrical resistance strain gauges.

We present a simple technique for the interrogation of fiber Bragg grating (FBG) sensors using a self-seeded Fabry-Perot laser diode (FP-LD). The FBG sensor is configured in such a way that its reflection spectrum consists of two peaks, whose separation depends on the physical parameter to be measured. When the modulating frequency of the FP-LD is set properly, stable short pulses at the two wavelengths selected from the same FP-LD mode with the FBG are generated simultaneously. The wavelength separation of the dual-wavelength pulses is converted into a time difference by transmitting the pulses through a dispersive fiber. In this way, accurate wavelength measurement is replaced by the less expensive time measurement. To multiplex similar sensors that are sufficiently far apart spectrally, it is only necessary to change the modulating frequency of the FP-LD. To balance the pulse intensities at the two wavelengths for a particular sensor, the temperature of the FP-LD can be adjusted. Using this technique, we demonstrated experimentally a number of FBG-based sensors, including displacement/bending sensors, temperature-compensated current sensors, and transverse force sensors. This technique provides an intensity-referencing-free demodulation scheme, an easy way to interrogate a series of sensors, and an effective means to overcome drifts.

LPFGs written by high-frequency CO₂ laser pulses have great potential for use in optical fiber sensing and communication. It is of importance to study the fundamental characteristics of these novel LPFGs. In this paper, a more systematic analysis is carried out to investigate the resonant wavelength-bend sensitivity of the LPFG and its bend loss of resonant peaks based on the mode field theory. It is for the first time to our knowledge, that a method is proposed for analysis of the bend orientation-dependence of the LPFG. The simulation is in agreement with experimental results. This paper provides a more systematic analysis for the bending characteristics analysis of the LPFG written by the high-frequency CO2 laser pulses exposure method.

By using the edge-filtering approach, a low-cost strain sensor system with the long-period fiber grating (LPFG) written by high frequency CO₂ laser pulses is demonstrated for the measurement of both static and dynamic strains. In this system, the wavelength-shift induced by the strain applied to the LPFG is interrogated by using a single-frequency DFB laser source. The intensity change due to the wavelength-shift is received by a photodetector. In addition, the intensity fluctuation due to instability of the fiber link is compensated by using a reference light source at 1310nm. In the static measurement, the result shows that the relationship between the electrical output of the sensor and the strain is very linear. The resolution of this sensing system is better than 10με and the error is ±10με. Dynamic strain measurement over 5kHz is also demonstrated. This sensor system offers the advantages of low-cost and fast response.

A low-cost fiber optic sensor system based on multimode fiber and an LED light source is presented. A multimode fiber Bragg grating (MMFBG) element is used as a strain sensor. In a matched grating scheme, a MMFBG similar to the sensing one was used as a reference in the receiving unit. For detection of large wavelength shift we demonstrated the feasibility of MMFBG wavelength detection using a single mode fiber fused coupler edge filter. The high cost normally associated with wavelength interrogators for single mode fiber FBG sensors was overcome by the utilization of a low cost multimode fiber pigtailed LED light source. The multimode fiber sensing system has the potential of maintaining much of the advantages of its single mode FBG sensor system counterparts. The MMFBG sensing schemes could be used for short distance, high sensitivity, high speed, strain, temperature and acoustic sensing applications

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is studied. The sensing head is comprised of one FBG and two metal strips with different lengths and different coefficients of thermal-expansion (CTE). The temperature to be measured can be converted into the strain of the FBG. One can know the temperature according to the peak wavelength shift of the FBG induced by the strain. A dynamic range of 800°C and a resolution of 1°C can be obtained by this sensor system. The experiment results well agree with the theoretical analysis.

This paper reports the development of a wavelength detection system that can be used to detect the Bragg wavelength of fiber Bragg grating sensors. The system can interrogate up to 100 sensors with wavelength detection resolution on the order of picometers. The results of medium-term test will be reported. Possible applications will be discussed.

With decades of research experience on optical sensors, Optoelectronic Technology Lab of Chongqing University (OTLCU) has studied on a variety of sensors system designed for practical use in health monitoring. In OTLCU, embedded and surface mounted fiber Fabry-Perot strain sensor has been developed for monitoring the local strain of both concrete and steel truss bridge. Optoelectronic deflect meter, with a group of optical level sensor in a series connected pipe, was developed for deflection monitoring and line shape monitoring of the bridges. Laser deflect meter, with a laser pointer and a sensors array, has been also developed for a dynamic deflection monitoring of the bridges. To monitoring the 2-Dimentional displacement of the bridge, a self-calibrating imaging system was developed. All these sensor systems have been applied in different bridges successfully. This paper briefly describes principle of these optical sensing systems, and also gives some representative results of the system in practical application of bridges.

Historically, due to the high cost of optical devices, fiber-optics sensor systems were only employed in niche areas where conventional electrical sensors are not suitable. This scenario changed dramatically in the last few years following the explosion of the Internet which caused the rapid expansion of the optical fiber telecommunication industry and substantially driven down the cost of optical components. In recent years, fiber-optic sensors and particularly fiber Bragg grating (FBG) sensors have attracted a lot of interests and are being used in numerous applications. We have conducted several field trials of FBG sensors for railway applications and structural monitoring. About 30 FBG sensors were installed on the rail tracks of Kowloon-Canton Railway Corp. for train identification and speed measurements and the results obtained show that FBG sensors exhibit very good performance and could play a major role in the realization of "Smart Railway". FBG sensors were also installed on Hong Kong's landmark TsingMa Bridge, which is the world longest suspension bridge (2.2 km) that carries both trains and regular road traffic. The trials were carried out with a high-speed (up to 20 kHz) interrogation system based on CCD and also with a interrogation unit that based on scanning optical filter (up to 70 Hz). Forty FBGs sensors were divided into 3 arrays and installed on different parts of the bridge (suspension cable, rocker bearing and truss girders). The objectives of the field trial on the TsingMa Bridge are to monitor the strain of different parts of the bridge under railway load and highway load, and to compare the FBG sensors' performance with conventional resistive strain gauges already installed on the bridge. The measured results show that excellent agreement was obtained between the 2 types of sensors.

Optical fiber sensors have received increasing attention in the fields of aeronautic and civil engineering for their superior ability of explosion proof, immunity to electromagnetic interference and high accuracy, especially fitting for measurement applications in harsh environment. In this paper, a novel FBG (fiber Bragg grating) strain sensor, which was packaged in a 1.2mm stainless steel tube by epoxy resin, was developed. Experiments were conducted on the universal material testing machine to calibrate its strain transferring characteristics. The sensor has the advantages of small size, high precision and flexible use, and demonstrates promising potentials. Ten of tube-packaged strain FBG sensors were applied in the vibration experiment of submarine pipeline model. The strain measured by FBG sensor agrees well with the electric resistance strain sensor.

Quasi-static testing of the fabric material mechanically with a novel embedded strain measurement system, which aimed at mechanical analysis of the dynamic characteristics of the parachute canopy textile with fiber optic technology, and using a tensile tester were aimed at correlating the results obtained by using the fiber optic sensors to the mechanical parameters, and in calibrating the system. To achieve better and more consistent results, FBG sensor took part in fabric samples testing in the warp, fill, and 45° directions. The mechanical tensile tests provided a traditional correlation between the loads and the elongation of the fabric structure, and were correlated to the fiber optic sensor output calibration. The comparison shows clearly that the two curves converged in the loading region before the structural failure started. The achieved results were consistent with each other, which dues primarily to the consistency in the fiber embedded technique, and will be discussed in detail in this paper.

In order to improve the initial and the response times of the Laser Dew-Point Hygrometer (LDH), the measurement simulation was developed on the basis of the loop computation of the surface temperature of a gold plate for dew depostition, the quantity of deposited dew and the intensity of scattered light from the surface of the plate at time interval of 5 sec during measurement. A more detailed relationship between the surface temperature of the plate and the cooling current, and the time constant of the integrator in the control circuit of the LDH were introduced in the simulation program as a function of atmospheric temperature. The simulation was more close to the actual measurement by the LDH. The simulation results indicated the possibility of improving both the times of teh LDH by the increase of the sensitivity of dew and that of the mass transfer coefficient of dew deposited on the plate surface. It was concluded that the initial and the response times could be improved to below 100sec and 120 sec, respectively in the dew-point range at room temperature, that are almost half of the those times of the original LDH.

We develop a simple strain sensor using polymer optical fibre Bragg grating and long period fibre grating. The sensor head is formed by a polymer optical fiber Bragg grating. A long period fibre grating is used for strain related wavelength shift demodulation. This particular combination of two quite different gratings could offer very large dynamic, up to tens of thousands micro-strains, for strain sensing. The preliminary experimental results have demonstrated that this sensing scheme provides good linearity, high resolution and large dynamic range.

In this paper, the rule of transmission characteristic of Photonic Crystal Fiber (PCF) is discussed when it is injected with different kinds of medium and stressed. Based on Finite-Different Time-Domain (FDTD) algorithm, the Maxwell equation group that changes in time-domain can be transformed into different equation on Yee Lattice. Absorbing boundary condition is added in proper position around the lattice, in order to truncate the algorithm space, and to transform the electromagnetic problem with infinite extending structure into the one with finite extending structure. Through simulating the photonic field distribution of PCF in this way, the transmission characteristics of PCF which is different from the ordinary optic fiber can be gotten. When the medium in the air holes changes, for example, they are injected with gas or liquid that has special refraction index, strong interaction will be brought about between the medium and the light that has conduction mode. In this article, the rule of transmission characteristic of PCF is analyzed, when PCF is stressed and injected with different kinds of medium.

Chlorine content in drinking water is hazardous for health. In this presentation a fiber optic evanescent field absorption sensor is demonstrated which can be implemented for the detection of the content of chlorine in drinking water. The method of demonstration is based on the absorption of evanescent waves through the development of color, which is achieved due to the reaction of diethyl phenylene diamine with chlorine present in water. A bent U-shaped probe is implemented in order to improve the sensitivity.

A dual-parameter optical sensor has been realized by UV-writing a long-period and a Bragg grating structure in D-fiber. The hybrid configuration permits the detection of the temperature from the latter and measuring the external refractive index from the former responses, respectively. The employment of the D-fiber allows as effective modification and enhancement of the device sensitivity by cladding etching. The grating sensor has been used to measure the concentrations of aqueous sugar solutions, demonstrating the potential capability to detect concentration changes as small as 0.01%.

As a chemiluminescence (CL) probe, 3,7-dihydro-6-{4-{2-(N’-(5-fluoresceinyl) thioureido)ethoxy}phenyl}-2-met -hylimi-dazo{1,2-a}pyrazin-3-one dosium salt (FCLA) can sensitively and specifically react with singlet oxygen (¹O₂ ) and superoxide(O₂""). BSA (Bovine Serum Albumin) can enlarge the CL intensity of FCLA to 860%. This report presents a novel method for determination of Aflatoxin B1 (AfB1) mediated by FCLA+BSA. The concentration of AFB1 showed an obvious positive correlation with the CL intensity mediated by FCLA+BSA. This method could measure accurately ng/ml of AfB1 concentration. At the same time, the fluorescence spectrum of FCLA+BSA and FCLA+BSA+AfB1 were measured respectively, which showed that the fluorescence intensity of FCLA+BSA+AfB1 was higher than FCLA+BSA. Comparing the peak value of FCLA, FCLA+BSA and FCLA+BSA+AfB1 had a 6nm Einstein shift (red shift). The study suggested that CL method mediated by FCLA+BSA might be applicable to the determination of AfB1 concentration.

To realize the on-line fluorescence monitoring of mineral oil pollution in water, three-dimensional spectral characteristic of oil-water intermixtures must be studied and the characteristic must be extracted. Using excitation wavelength, fluorescence wavelength and fluorescence intensity as three-dimensional system of coordinate, through sampling and surface fitting, three-dimensional fluorogram is gotten, which can provide gist for oil discrimination when presented in contour chart (finger-print map of oils). But there is little difference between characteristics of three-dimensional fluorogram because of the similarity of constituent and structure of similar oils. Therefore this paper introduces quantitative analysis method-characteristic parameter method which starts with analyzing statistical characteristic of three-dimensional fluorogram. Using RFPC fluorescence spectrometer (Shimadzu, Japan), three-dimensional fluorescence spectra of diesel oil, machine oil, gasoline oil, crude oil are measured and parameterized. The result shows that as a quantitative classified discrimination method of three-dimensional fluorescence spectra, the parameter of characteristic parameter method possesses definiteness for three-dimensional fluorescence spectra, and it is applicable, available when used in oil discrimination.

By dividing both the object and reference beam in an OCT interferometer, two independent OCT imaging channels are assembled. The depth scanning proceeds simultaneously in the two OCT channels and from the same range, however a differential optical path difference can be introduced between the two channels. In this way, two simultaneous images are generated where the depth differs in each pixel by the differential optical path difference. A dual OCT system working at 850 nm was devised and we demonstrate the capability of the method by simultaneously acquiring images from the optic nerve and fovea of a volunteer. The configuration devised insures a strict pixel to pixel correspondence between the two images irrespective of the axial eye movements while the depth difference between the corresponding pixels is exactly the differential optical path difference. The images are collected by fast en-face scanning (T-scan) which allows both B-scan and C-scan acquisition.

A Surface plasmon resonance (SPR) biosensor constructed with common path, heterodyne inteferometric system has been developed. The sensor ship consists of a BK7 substrate coated with gold film on which the receptor of the specific biomolecular or protein has been immobilized. The light source consisting of the s and p polarizations with heterodyne frequency of 60kHz is used to measure the phase difference between these two polarizations. Because the SPR sensor probes the changes of refractive index near the gold film (i.e. about one wave-length), the more the binding of molecules on the sensing surface results in the less sensitivity of the detection. In order to overcome this shortage, we set two quarter-wave plates before and after the SPR prism to make the sensitivity of measurement to be tunable. This sensor could detect the concentration of antibody of sheep IgG as low as several nanograms per milliliter. The results indicate that this system provides high sensitivity and is capable for detecting biomolecular interactions.

With high-sensitivity single-photon counter, spontaneous chemiluminescence (CL) spectrum and different temperature study on rice (Oryza sativa L.) seeds during early imbibition were carried out. The emission spectrum of whole rice seed, rice and coat had a greater proportion of red light during early imbibition. Comparing with spontaneous CL of barley (Hordeum vulgare L.) and soybean (Glycine max L. Merr) seeds, the spontaneous CL of rice seeds had a nonlinear, logarithmic-like increase of intensity in the T range 30-50°C, the Van't Hoff coefficient Q10=IT+10/IT is equal to 2, which led us to the conclusion that spontaneous CL of rice seed during early imbibition partly came from enzyme catalyzing chemistry reaction.

A 30km long distance distributed optical fiber Raman temperature sensor (DOFRTS) system has been made, it use new measuring temperature principle of optical fiber amplified anti-Stokes Raman spontaneous scattering. In the system, 1550nm erbium-doped optical fiber laser, a highness speed data acquisition card and signal processing technique are used. By using these technique, the problem of weak signal detection is resolved and signal to noise ratio is increased. All components of system are put into an intellectualized constant temperature box and work in constant temperature condition. Stability and environment adaptability are improved. By appraisal, performance of the system is listed as follows: length of single mode fiber: 31km, temperature rang:0-100°C (can be expanded), temperature measuring uncertainty:±2°C, temperature resolution:0.1°C, measurement time:432s, spatial resolution :3m.

In order to demodulate frequency division multiplexed optical fiber Fabry-Perot sensors with high accuracy and absolute cavity length, an improved cross-correlation method was presented. As the applying of a special correlating factor in it, the cross correlation function at every sensor's cavity length is a single peak and multiplexed sensors that cavity length is close to each other can be easily distinguished, and segment golden search method was employed to reduce computation and demodulating speed is fast. The multiplexing experiment of two sensors was accomplished with the help of general demodulating system constituted by broadband light source, spectrometer etc. The results showed that the system can demodulate the frequency division multiplexed signal of optical fiber Fabry-Perot sensors, and the least cavity length difference that can be distinguished among multiplexed sensors is less than 6μm and fluctuation of the cavity length is less than 0.073%.

Due to their variability Fibre Bragg Grating (FBG) sensors are introduced in a growing number of applications. Furthermore using their spectral selectivity and adjustable reflectivity these sensors are predestined for serial networking, whereby presently in most cases the addressing is realised by time division multiple access (TDMA) or wavelength division multiple access (WDMA). But these technologies have big disadvantages regarding the effective use of the prevailing resources time and wavelength, respectively. In this paper a new scheme capable of addressing a large number of FBGs in a single serial network is proposed and compared to TDMA and WDMA. Using all optical sequence inversion keyed (SIK) direct sequence (DS) code division multiple access (CDMA) it overcomes the restrictions handling the resources time and wavelength without loosing accuracy. A simulation with 20 FBGs in a spectral range of 2nm shows the abilities of this system, which can be build up with nearly the same components as TDMA systems.

In order to increase the number of channels available to a grating based strain sensor system specialised gratings were designed that would allow the sensor system to be able to distinguish between a number of gratings located in the one WDM channel independently of the amount by which they overlap each other in the wavelength domain. Distinguishing between gratings is achieved by inscribing a carrier frequency in the grating spectrum, so that each grating can be addressed in the Fourier domain via the spectral information centred around the inscribed carrier frequency. Tests performed on the gratings successfully show the ability to distinguish between three spectrally overlapping gratings. The calculated value for Young's modulus, 72+/-3GPa, was found to be in keeping with the standard value of 70.3GPa for fused silica.

Fiber Bragg grating (FBG) sensing is gaining attention in both scientific research areas and engineering applications thanks to its distinguishing advantages including wavelength multiplexing capability, miniature size, high sensitivity, immunity from electro-magnetic interference, etc. FBG sensing is based on the detection of the shifted Bragg wavelength of the light reflected by a fiber grating which is sensitive to various physical parameters such as strain and temperature. One of the challenging tasks in FBG sensing is to determine the Bragg wavelength shift, which can be done by using an optical spectrum analyzer (OSA). An OSA is suitable for laboratory tests, but not an ideal solution for field applications in term of cost and convenience. Different wavelength demodulation methods have been developed for FBG sensing purpose. One of them is employing a bulk linear edge filter to convert the wavelength shifts to intensity variations. This method offers several obvious advantages including low cost and ability for dynamic measurements. However, most of the edge-filter based FBG sensing systems are designed for single-channel measurement. In this study, we propose and develop a multi-channel FBG sensing system based on the edge-filtering technique. In order to demodulate multi-channel signal from FBG sensors, we propose to use a dense wavelength division multiplexing (DWDM) module. The light signals coming from wavelength-multiplexed FBG sensors are demultiplexed into individual channels and demodulated by the pass-band edges. In the present study, a four-channel FBG sensing system has been demonstrated.

In this paper, a novel method for multiplexing extrinsic fiber Fabry-Perot interferometric sensors (EFPIs) using the coarse wavelength-division-multiplexing technology (CWDM) is proposed and demonstrated, for the first time to our knowledge. The system and principle for sensing and multiplexing are described. A 1×4 CWDM is used to multiplex signals from four EFPIs with identical cavity length. The experimental results show that a strain accuracy of 2.5me is achieved. There is little cross-talk between adjacent sensor channels. This system improves the multiplexing capability of EFPIs by a factor of four. It is anticipated that such a sensor system could find important applications in smart materials and health monitoring for large structures.

The spatial resolution of a 30km distributed optical fiber sensor is optimized to 3m. Two methods to measure the spatial resolution of distributed optical fiber sensor were demonstrated. The lower limit of the spatial resolution of the system is determined by the width of the input laser pulse. The bandwidth of optoelectronic receiving system and the electronic system also affect the spatial resolution of the system. The bandwidth of the electronic system shall be matched with the pulse width of the laser. The spatial resolution of system can be measured directly or evaluated indirectly by the half width of the Fresnel reflection at the end of the fiber. The spatial resolution of the system reached 3m after the optimization.

This paper demonstrates the recent achievements in the field of Brillouin based distributed optical fiber temperature sensing. When a dispersion-shifted fiber was subjected to a temperature cycle between 20 and 820 °C, the Brillouin shift exhibited an undesired hysteresis with a maximum frequency discrepancy of larger than 48 MHz between heating and cooling processes. After the fiber was annealed for 9 h at 850 °C, however, the hysteresis almost disappeared for repeated temperature cycles in the ranges of 20-820 °C and of 500-800 °C with deviations of the measurements from the best-fit curve of less than ±12.5MHz. The temperature dependence of Brillouin shift in the range of 20-820 °C in the annealed fiber was well expressed by a second order function of temperature. A sensing scheme that utilizes both output signals of the fiber Mach-Zehnder interferometer used as an optical frequency discriminator has been proposed. The scheme that has the advantages of less system adjustment and fast measurement, combined with a suitably annealed fiber, offers a reliable means for the Brillouin shift-based distributed sensing over the wide temperature range.

This paper firstly introduces the principle of distributed sensing system based on Polarization sensitive Optical Time-Domain Reflectometer (P-OTDR). As known, the change of State of Polarization (SOP) in fiber is sensitive to environment, consequently this sensing method can extract very little perturbation of extra temperature or strain; another significant advantage is that the sensing information can be achieved nondestructively at one end. Hence, this technique has become a practical and helpful tool in distributed optical fiber sensing system. However backscattered light in single-mode fiber is very weak, and the optical pulse width limits the spatial resolution, so there is a compromise between SNR and sensor spatial resolution. We investigate two signal-process methods to extract the state polarization (SOP) evolution from backscatted signal. From the experiment results, we find that both the digital average and deconvolution can be used for enhancing SNR and system spatial resolution. And we also find hundred of digital average is enough, the more time we average the less improvement of SNR we get. At the same time, we view that the SOP of backscattered signal is the response of sensing system. By mean of deconvolution the actual SOP evolution is not lost, and the spatial resolution of POTDR System doesn’t degrade with a wider pulse. Compared with previous deconvolution case, the deconvolution method can effectively improve the system spatial resolution without decreasing the transmitting pulse width. Finally we give some experiment results with strain perturbation, our system can detect the point where the environment changes, which is suitable for smart structure and optic-fiber sensor application.

Laser interferometers with better than10 picometer (pm) accuracy in displacement measurement and 1-3 microns in absolute distance ranging accuracy are sought in several of NASA's planned missions, such as the Space Interferometry Mission (SIM) and Terrestrial Planet Finder (TPF). Over the past several years, we have made significant progress at JPL toward a laser heterodyne interferometer system that can provide both picometer displacement measurement and micron level absolute distance measurement. This paper presents an review on the development of high precision metrology gauges for these missions.

A Michelson based fiber optic low-coherence interferometric quasi-distributed sensing system permitting absolute length measurement in the sensors array is proposed. The main part of the sensing system is a fiber optic 3x3 star coupler. The architecture of the fiber optic sensors can be easy realized as a linear sensor array, twin sensor arrays or a loop sensor array. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application could be deformation sensing in smart structures. Experimentally, a 6 sensors array has been demonstrated.

We demonstrate a method of displacement measurement based on polarization hopping of laser with optical feedback. The measurement system is composed of a half-intracavity He-Ne laser, a quarter wave plate and an external feedback mirror. When the conditions of the second polarization flipping are satisfied, the frequency of intensity modulation can be doubled, and one polarization switching corresponds to displacement of external mirror. Meanwhile, the intensity transfer between two polarization states will come out, i.e., an increase of one polarization light intensity always accompanies a decrease of another polarization light intensity. One period of intensity transfer can be divided into four domains: e-light, e and o-light, o-light, and no light, and each domain corresponds to change of external cavity length. According to appearing sequence of the four domains, the movement direction of external mirror can be distinguished. Our method can improve the resolution of displacement measurement 8 times that of conventional optical feedback, and reaches 40nm for a laser wavelength of 632.8nm.

Most fiber optic interferometric refractometers suffer from large temperature sensitivities, especially those that employ long-period gratings. This paper presents results on a compound-cavity Michelson interferometer whose phase shift is only dependent on the refractive index of the analyte surrounding the fiber probe. This single probe Michelson interferometer uses mode coupling in a long-period grating to establish the two optical paths in a single fiber, and therefore presents a compact sensor for measuring the refractive index and other related properties (such as chemical concentration and composition) of various types of substances. It can also be used to measure the level of liquid in a container. Experimentally, we measured a phase change of 1.374 rad for a change in refractive index of 0.0132. The temperature sensitivity of one such compensated device is only -0.01 rad/°C, which is less than the temperature gradient of the refractive index of solutions of glycerine in water.

The Space Interferometer Mission (SIM), scheduled for launch in 2009, is a space-born visible light stellar interferometer capable of micro-arcsecond-level astrometry. The Micro-Arcsecond Metrology testbed (MAM) is the ground-based testbed that incorporates all the functionalities of SIM minus the telescope, for mission-enabling technology development and verification. MAM employs a laser heterodyne metrology system using the Sub-Aperture Vertex-to-Vertex (SAVV) concept. In this paper, we describe the development and modification of the SAVV metrology launchers and the metrology instrument electronics, precision alignments and pointing control, locating cyclic error sources in the MAM testbed and methods to mitigate the cyclic errors, as well as the performance under the MAM performance metrics.

A novel method for multiplexing fiber-optic Fizeau strain sensors with optical amplification is proposed and demonstrated. This method overcomes the two intrinsic disadvantages of fiber-optic Fabry-Perot (F-P) strain sensors, i.e. weak signal and difficult multiplexing. The amplified spontaneous emission (ASE) and optical amplification of an Erbium-doped fiber (EDF) pumped by a 980nm laser diode are used simultaneously to enhance the interferometric signal considerably. A Fizeau interferometer formed by two fiber ends with a quite different reflectivity is used to replace the F-P cavity in sensor head design. Such a Fizeau cavity can enlarge the cavity length by at least an order of magnitude and allows more than ten sensors to be multiplexed simultaneously by using spatial-frequency multiplexing. The operating principle of the sensor system is discussed and an experiment is carried out to verify the concept of the method proposed. It is anticipated that such a sensor system could find important applications for health monitoring of large structures.

A novel scheme of a fringe count interferometric accelerometer is proposed. In view of practicability, the sensor must be made up of only fiber. Two 3dB directional couplers and singlemode fibers are used in this scheme to compose an imbalance Mach-Zehnder interferometer. Phase modulation is achieved by modulating LD frequency to scan the interference fringes. With special technique, only one PIN is needed to count and identify the moving direction of the interference fringes. The signal can be transmitted more than several kilometers in a pair of fibers. The two arms of the interferometer work at push-pull mode, which is favorable for temperature compensation. The resolution is significantly improved by fringe sub-division. A real-time dynamic vibration signal can be measured without distortion by this system.

With the development of the technique of the digital signal processor, the using of digital signal processor in the open loop IFOG (interferometric fiber optic gyroscope) signal processing brings a great improvement of the performance of IFOG, especially in the technique of digital filtering, acquisition the values of arcsine and inverse tan function. The traditional way to get the values of inverse tan function is linear interpolation technique in digital signal circuit. This article put forward a new way to acquire the values of inverse tan function, the proportion of the second order Bessel function to the first order Bessel function and the proportion of the fourth order Bessel function to the second order Bessel function by using of the virtue of DSP, high speed of data processing. The fundamental, second and fourth harmonic signals from photoelectrical detector are sampled and demodulated by analog circuit. Then the signals from analog circuit are sampled into DSP by a 24-bit analog-to-digital converter, which is used to improve the dynamical range of the system. Then DSP computes the values mentioned above by polynomial approach. The simulation in Mathematic and Microsoft Visual C++ 6.0 proved that the method of polynomial approach to the functions has higher precision compared to the method of linear interpolation. Meanwhile the designed circuit shows it can meet the demand of real time data process. The tested results of designed circuit prove it has a great improvement in the dynamic range and precision of IFOG.

The combined two-ring-resonator schemes of fiber optic gyroscope are developed and demonstrated. The two-ring topology structure has been classified 2 kinds: two-ring in series and in parallel. The cavity lengths L1 and L2 of the two-ring-resonator are chosen as 623m and 1260m, respectively. A theoretical model of the two-ring Rayleigh backscattering fiber optic gyroscope which including the Sagnac effect is established. The experimental results are agreement with the theoretical prediction.

Fiber Ring Laser Gyroscope (FRLG) is a novel resonant gyroscope which uses erbium-doped optical fiber as the gain medium and optical fiber as the ring resonant cavity. FRLG have advantages over conventional Interferential Fiber Optical Gyroscope (IFOG) with its simplicity in both the optical configuration and the electronic signal processing. The precision of FRLG depend on the linewidth of fiber ring laser output. So how to get narrower laser linewidth is key technology for FRLG. According to recently report, FRLG's laser linewidth reach 100 kHz, can not directly applicable to rotation sensing at present. A Stand-wave in the un-pumped erbium-doped fiber can induce self-written fiber grating, which have self-adaptable narrow band pass characteristics. Base on this, we propose a novel FRLG's structure. Two filter sections were used in this structure to make sure FRLG run in narrow linewidth and single mode. Fiber circulator, un-pumped erbium-doped optical fiber and Bragg grating (FBG) as the reflect mirror make up of the filter section. In this paper, we have optimized the parameters through a computing simulation. The theoretical and experimental study on FRLG has been finished. We get experimental results from them. FRLG which use this structure can get bidirectional laser output, the linewidth of laser can reach about tens kHz level and the power of laser output can reach 1dbm.

Re-entrant fiber optic gyroscope (Re-FOG), using a shorter Sangnac sensitive ring (SSR), makes the two counter-propagandizing lights re-enter the ring before they interference with each other when they reach the photo-detector. By this means, the Re-FOG gets the same detecting accuracy as the conventional interferometric fiber optic gyroscope (IFOG) that has several times length of the sensitive ring. In this paper, the principle of the Re-FOG is first reviewed. Then the output signal is analyzed. Finally a method of signal detection is presented.

A double-pass super-fluorescent fiber source with a wavelength-selective reflector fabricated using thin film filter technology was demonstrated. The simulation on output power and bandwidth and meanwavelength were made. The optimized parameters have been obtained and the desired reflector has been fabricated. A practical prototype was made and tested, its output power is great than 8.5 mW, the bandwidth is great than 28nm. The instability of power and meanwavelength are less than 3% and 3ppm/°C within -40 °C ~+60 °C temperature range respectively.

Torque sensing on rotating shafts is important for monitoring electrical and internal combustion machines, especially for real-time computer control. Optical fiber sensors are ideally suited to this application because of many desirable properties such as immunity to electromagnetic interference, their multiplexing capability and small size. This paper discusses a dual fiber Bragg grating sensor for the independent measurement of temperature and torsion in a rotating shaft. It shows theoretically that whereas torsion affects the differential wavelength of the gratings in the proposed configuration, temperature changes in principle only the common-mode wavelength. These characteristics of the system allow one to measure both torsion and temperature with the same sensing system. For the particular system investigated experimentally, the differential wavelength change was 19.4 pm/Nm, and the corresponding temperature sensitivity was -0.67 pm/°C. In contrast to these values, the temperature sensitivity of the common-mode wavelength was 25.4 pm/°C, and its torque cross-sensitivity was -0.19 pm/Nm.

In many industrial systems, the sensor is desirable to directly respond to a vector component associated with some physical phenomenon, such as the acceleration, velocity or displacement. This paper focuses on the application of fiber-optic accelerometers in oil well logging, presents a new design utilizing the combination of bellows structure and compliant material to enhance the unidirectional response of the fiber optic accelerometer. Furthermore, we accomplished a three components (3-C) acceleration sensing system by multiplexing three unidirectional sensing elements. Theory and experimental results show that our sensor can fulfill the demand of application in oil well logging, including responsivity, operation frequency range and directivity. The demodulated signal and the acceleration of vibration measured exhibit a linear relationship, and the suitable measurement frequency range of the system is between 3Hz and 800Hz. The on-axis sensitivity, cross-sensitivity and minimum detectable level is -101dB re V/ug, -135dB re V/ug and 39.3ug/Hz1/2, respectively.

A robust-fusion optic fiber sensor technology of proximity distance and orientation integration is studied in this paper. A novel optic fiber sensing head with redundant information, which can measure distance and orientation in any pose as well as compensate fluctuation caused by changing parameters such as surface reflectivity, light intensity and characteristic shifting from photoelectric-converter device, is proposed. The implement method of sensor network compensation is introduced. An improved BP network arithmetic, which can enhance the dynamic characteristic and measurement accuracy of the sensing system, is presented. To speed up the convergence rate of BP network training, GA -BP training method is applied. An intelligent signal detecting and processing system based on DSP is designed, the strong data processing ability of DSP makes the system hardware structure simplified. The method of moderate output light power control is put forward for enlarging the measuring range. Experiment result shows that robust-fusion optic fiber proximity sensor system has the distance measuring range of 0.1~19.9mm and the orientation measuring range of 0~25°. The measuring time of each point is 92.5ms.

A miniature optical sensor for laser particle counter is described, and some calculated and test results are reported in this paper. A reflective spherical mirror coated with highly reflective optical film is applied as collecting element for scattered light, and a PIN photodiode with high performance is used as the photo-detector. A band-pass preamplifier is used to eliminate lower-frequency electromagnetic interference from external environment, as well as to filter high-frequency components from electronic noise. An air sampling system can provide a very constant flow rate. The smallest particle diameter of the optical sensor is 0.3 microns with a signal-to-noise ratio exceeding 2:1.

A novel optical fiber null coupler (OFNC) sensor based on acousto-optic interaction is developed, which can be used in the structure health monitoring of the medical materials. The OFNC sensors can be response to 10MHz supersonic wave, and their signal-to noise ratio are higher then Piezo Ceramic Transducers(PZT). A kind of Perspex with a 1mm hole is employed as the sample, where the OFNC sensor is glued on, and the reflected signal of ultrasonic wave by the hole is detected .

A fiber thermometer using the cross detection of the fluorescence lifetime and blackbody radiation was presented to measure temperature from -10°C up to 1400°C. Using a long pure YAG crystal fiber as the seed and a 0.1 at. % Cr₂O₃-doped Y₃Al₅O₁₂ sintered powder rod as the source rod, a YAG fiber thermal probe with Cr³⁺ -ions doped end was grown by laser heated pedestal growth method. A blackbody cavity was constructed by sintered a thin ceramic layer around the Cr³⁺: YAG fiber end. A phase-locked detection scheme was used for the fluorescence lifetime detection. The fluorescence characteristics of the Cr³⁺-ions doped YAG was analyzed in a temperature range from -10°C up to 500°C. From 350°C to 1400°C the blackbody radiation signal in a narrow waveband were detected. Because the fluorescence lifetime was intensity independent, it should have the long-term stability and would not change if the fiber connectors of the probes were realigned. So the fluorescence lifetime based temperature measurement could be used to recalibrate that based on the blackbody radiation detection. Preliminary experimental results showed that the system could achieve a resolution much better than 1°C over the whole temperature range from -10°C to 1400°C.

The present work reports the design and implementation of an optical-sensor based virtual instrument for visualizing and estimating the degree of corrosion of metal surfaces exposed to atmosphere. A platform carrying the specimen plates was driven by a stepper motor assembly horizontally in XY directions. An opto-electronic transmitter-receiver unit was fixed vertically very close to the platform so that the sample surfaces may be scanned by a light beam. The reflected and scattered optical signals from each point in the scanned area were acquired by the sensor module. A parameter incorporating the relative values of these signals proved to be a fairly reliable measure of the surface texture, which in turn consistently represented the degree of corrosion. These signals interfaced with a personal computer enabled 3D visualization of the magnitude of the corrosion level of the surface and 2D mapping of the surface corrosion.

In order to successfully implement the extended-life operation plan of the nuclear power plant (NPP), predictive maintenance based on on-line monitoring of deteriorated components becomes highly important. Pipe wall-thinning is usually caused by Flow-Accelerated Corrosion (FAC) under the undesirable combination of water chemistry, flow velocity and material composition. In order to increase the confidence of understanding on underlying process, a multi-disciplinary approach has been adopted in this work. Here, we apply a combination of several advanced sensors, ranging from chemical electrodes to mechanical vibration sensors to monitor the thickness change of the elbow, which can be still economical option. Electrochemical Corrosion Potential (ECP) and pH are chosen as electrochemical parameters, the change of vibration mode, displacement, and etc. are chosen as mechanical parameters to monitor the wall thinning phenomena. Electrodes are developed for the on-line monitoring of pH and ECP. Vibration is considered as a promising candidate as a mechanical parameter. Various sensors are surveyed and some are chosen based on FEM analysis result, which shows the approximate vibration range according to the thickness change. Mechanical sensors need to be sensitive enough to detect small thickness change with adequate safety margin to a pipe rupture. A few sensors are suggested to detect vibration or displacement quantitatively. Fiber optic sensors are chosen for their non-contacting property, which is appropriate for the high temperature application. Accelerometer and capacitance gage are suggested for their applicability fit to the test purpose.

Low-loss optical wave guiding along a subwavelength-diameter silica wire leaves a large amount of the guided field outside the solid core as evanescent wave and at the same time maintains the coherence of the light, making it possible to develop sensitive and miniaturized optical sensors for physical, chemical and biological applications. Here we introduce, for the first time to our knowledge, a scheme to develop optical sensors based on evanescent-wave-guiding properties of subwavelength-diameter wires. Optical wave guiding properties of these wires that are pertinent to a waveguide sensor, such as single-mode condition, evanescent field, Poynting vector and optical loss are investigated. By measuring the phase shift of the guided light, we propose a Mach-Zehnder-type sensor assembled with two silica wires. The sensitivity and size of the sensor are also estimated, which shows that, subwavelength-diameter silica wires are promising for developing optical sensors with high sensitivity and small size.

Hybrid-integrated Optical acceleration seismometer and its digital signal processing system are researched and developed. The simple system figure of the seismometer is given. The principle of the seismometer is explicated. The seismometer is composed of a seismic mass,Integrated Optical Chips and a set of Michelson interferometer light path. The Michelson Integrated Optical Chips are critical parts among the sensor elements. The simple figure of the digital signal processing system is given. As an advanced quality digital signal processing (DSP) chip equipped with necessary circuits has been used in its digital signal processing system, a high accurate detection of the acceleration signal has been achieved and the environmental interference signal has been effectively compensated. Test results indicate that the accelerometer has better frequency response well above the resonant frequency, and the output signal is in correspondence with the input signal. The accelerometer also has better frequency response under the resonant frequency. At last, the curve of Seismometer frequency response is given.

This paper reports the development of a very compact and very-high sensitivity optical fibre hydrophone system using a distributed-feedback fibre laser with a cavity length of 10cm. A theoretical system design making use of a Mach-Zehnder interferometer, homodyne demodulation scheme and digital signal processing is described. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the distributed-feedback fibre laser sensor head. Results for noise spectrum below 100kHz are presented, as well as discussions on some key issues with designing such hydrophone systems. Although not the intention for the system, initial results also indicate the suitability of the DFBFL for intensity modulated sensing.