Interferometric distributed optical-fiber sensor

An interferometric technique is described for detecting and locating perturbations along an optical fiber. This distributed sensor, based on a modified fiber-ring interferometer, has a position-dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift that is detected and converted to spatial information. The sensor consists of two parts, namely, a reflecting-fiber-ring interferometer and a differentiating-ring interferometer. The reflecting ring consists of a fiber ring with one port of the coupler connected to a reflector. Consequently the output port of the reflecting-ring interferometer is the same as the input port. Because it is an inherent zero-path-imbalanced system, a short-coherence-length source such as a light-emitting diode can be used. Any time-varying perturbation on the fiber in the ring results in a detector signal proportional to the product of the rate-of-phase change caused by the perturbation and the distance of the perturbation relative to the center of the fiber ring. The second part of the system, a differentiating-ring interferometer, consists of the same fiber-ring interferometer modified only slightly. The output of this part of the sensor is proportional only to the rate of phase change as a result of the unknown perturbation and contains no distance information. By dividing the output of the reflecting-ring interferometer by the output of the differentiating-ring interferometer, we determine disturbance location. Results obtained with a 155-m distributed fiber sensor are discussed.     

Electric time-domain reflectometry distributed flow sensor

Liquid composite molding (LCM) has become an important processing technique to manufacture high-performance composite parts. The sensing of the process parameters, such as resin fill of the porous material, are key to improve repeatability, maximize quality and minimize cost. This paper describes a distributed flow sensor, which considerably decreases tooling integration costs and improves spatial resolution by allowing sensing of hundreds of sensing elements with a single input/output port. The transmission line sensor is virtually divided into a large number of small discrete transmission lines treated as a long array of sensing elements. Piecewise sensing is achieved by electric time-domain reflectometry and inversion of a non-uniform transmission line model. The paper describes the distributed sensing approach, experimentally validates distributed sensing in a LCM setup, and analyzes critical sensor parameters.     

Optical time-domain reflectometry of bent plastic optical fibers

Optical time-domain reflectometry (OTDR) signals of step-index plastic optical fibers (POF's) and graded-index POF's were measured with a laser diode and an avalanche photodiode. When bent step-index and graded-index POF's were used, the OTDR signal behavior differed. The OTDR signal of the bent graded-index POF's had a step that corresponds to a curvature loss, but the step-index POF's had a spike signal at a bend, which indicated the occurrence of backscattering. The peak intensity was proportional to the square of the curvature. The refractive-index variation of the bent step-index POF's was measured, and the dependence of the peak intensity on the curvature was shown to agree with that predicted by the scattering from the refractive-index perturbation.     

Characterization of time-resolved fluorescence response measurements for distributed optical-fiber sensing

A distributed optical-fiber sensing system based on pulsed excitation and time-gated photon counting has been used to locate a fluorescent region along the fiber. The complex Alq3 and the infrared dye IR-125 were examined with 405 and 780 nm excitation, respectively. A model to characterize the response of the distributed fluorescence sensor to a Gaussian input pulse was developed and tested. Analysis of the Alq3 fluorescent response confirmed the validity of the model and enabled the fluorescence lifetime to be determined. The intrinsic lifetime obtained (18.2±0.9 ns) is in good agreement with published data. The decay rate was found to be proportional to concentration, which is indicative of collisional deactivation. The model allows the spatial resolution of a distributed sensing system to be improved for fluorophores with lifetimes that are longer than the resolution of the sensing system.     

一种基于相干光时域反射的分布式光纤振动传感系统研制

针对基于相干光时域反射的分布式光纤振动传感系统中数据量庞大,且需要信号发生器外部驱动,集成度低的缺点,利用现场可编程逻辑门阵列实现传感数据的高速采集与声光调制器的同步驱动,采用USB 3.0实现采集模块与上位机的实时数据传输。搭建一种基于相干光时域反射的分布式光纤振动传感系统,利用本地光与后向瑞利散射光的拍频效应,实现对微弱后向瑞利散射光信号的探测,并提高系统的传感距离。采用正交相位解调方法获取振动信号的位置信息。实验结果表明:该系统可在22 km传感光纤上对振动信号进行有效定位,定位误差在20 m以内,且系统对正弦波和方波等不同形态的振动信号定位效果一致。     

Frequency-derived distributed optical-fiber sensing technique: theory and characterization

Frequency-derived distributed optical-fiber sensing is a method for remote measurement of the spatial distribution of linear birefringence in an optical fiber, allowing a corresponding measurement of those external measurands that influence this birefringence. The method employs a pump-probe scheme, which, by use of the optical Kerr effect, generates an optical modulation of the probe beam, with a modulation frequency whose temporal variation maps the spatial distribution of birefringence. We provide a complete theoretical analysis of this method by using Jones calculus and graphic representation on the Poincaré sphere. The relevant characterization of the technique and some experimental results are also presented; these show good agreement with the theory.     

Two-dimensional distributed strain sensing with an Archimedean spiral arrangement in optical frequency domain reflectometry

We demonstrate a distributed two-dimensional(2D)strain-sensing system in optical frequency domain reflectometry(OFDR)with an Archimedean spiral arrangement of the sensing fiber.The Archimedean spiral describes a simple relationship between the radial radius and polar angle,such that each circle(the polar angle from0 to 2π)can sense the 2D strain in all directions.The strain between two adjacent circles can also be easily obtained because an Archimedean spiral facilitates sensing of every angle covering the full 2D range.Based on the mathematical relation of Archimedean spirals,we deduce the relationship between the one-dimensional position of the sensing fiber and 2D distribution in polar coordinates.The results of the experiment show that an Archimedean spiral arrangement system can achieve 2D strain sensing with different strain load angles.     

Electric time-domain reflectometry sensor for online flow sensing in liquid composite molding processing

This paper describes the development of a new sensor type for resin flow detection in liquid composite molding (LCM) processing. The sensor can be applied in any LCM process where lineal and high-resolution flow front detection is required, i.e. during prototyping or as a feedback sensor during flow front control. The operating principle of the embedded sensor is based on electrical time domain reflectometry (E-TDR). The system analyzes changes in the transmission line response during wet-out of the preform. The flat band transmission line, which is interrogated by E-TDR changes its dielectric properties when the resin is on or near the sensor resulting in reflection of the electrical signal. The signal is evaluated in the time-domain by mapping the areas of resin on the sensor. The study illustrates an analytical model to predict the TDR response for different resin systems, validates the accuracy and resolution of the system in a lab-scale set-up and implements the sensor in several different injection scenarios.     

Intrinsic optical-fiber sensor for nerve agent sensing

A novel chemical-sensing technique to detect the nerve agent sarin stimulant dimethylmethylphosphonate (DMMP) is presented. This technique uses a combination of doped polypyrrole as an active chemical material coated on an optical fiber to form an intrinsic fiber-optic sensor. Sensitivity of up to 26 ppm of DMMP with response time of a few seconds is demonstrated. Influence of three different dopants, i.e., 1,5 naphthalene disulphonic acid, anthraquinone 2 sulphonic acid, and hydrochloric acid is investigated for sensor response and sensitivity. Two polymer processing techniques, i.e., in situ deposition and monomer vapor phase deposition is investigated for optimal polypyrrole morphology for DMMP sensitivity. The influence of substrate nature, i.e., hydrophilic and hydrophobic, on sensor sensitivity is studied. Organophosphate specific binding sites have been created in polypyrrole structure using Cu/sup 2+/ ions to enhance DMMP response. The selectivity issue is addressed by testing the sensor in the presence of other gases like ammonia, water vapor, and acetone which influence the electronic properties of polypyrrole.     

Optical time-domain reflectometry in optical fiber with reflection delay time matched to the period of the optical frequency modulation

A method of optical time-domain reflectrometry in optical fiber is described that uses a single distributed feedback diode laser and a reference reflector. When the period of the frequency modulation of the laser matches the time difference between the reference reflection and the reflection of interest an increase in the noise at the detector occurs. The locations of reflections within the fiber are then mapped to frequencies at which the noise at the detector increases. A sinusoidal frequency modulation is analyzed, and an experiment is described in which the system is used to measure the location and the wavelength of two Bragg gratings located 10 cm apart in an optical fiber. Wavelength measurement is accomplished by temperature tuning the diode laser.     

Analytical description of backscattered states of polarization in polarization optical time-domain reflectometry measurements on uniformly twisted linearly birefringent optical fiber

We present a detailed derivation of the locus of Rayleigh backscattered states of polarization for polarization optical time domain reflectometry in uniformly twisted optical fiber with intrinsic linear birefringence. The locus is algebraically a quartic whose topology is determined by the relative orientation on the Poincaré sphere of the input SOP and the effective birefringence vector. We present an analysis that indicates how experimental data may be interpreted, through geometric parameters of the locus, for evaluating the fiber parameters. The analysis also indicates the minimum number of experimental data points required for meaningful values for the fiber parameters to be obtained.     

Interferometric probe for near-field optical microscopy systems

We have studied the possibility of creating a new type of the interferometric near-field pinhole probe for near-field optical microscopy systems based on a Fabry-Perot fiber microresonator with a nanodimensional pinhole in one of its output mirrors. The dependence of the resonance wavelength shift in the Fabry-Perot interferometer on the distance from the output diaphragm to the object has been determined using the finite-difference method in the time domain. It is shown that the proposed technique ensures a spatial resolution of no worse than λ/15.     

Interferometric optical sensor for measuring glucose concentration

With a specially designed probe, the phase difference between s andp polarization of light reflected under surface-plasmon resonance is measured by use of a common-path heterodyne interferometer. For specific ratios of phase difference to glucose concentration, the glucose concentration can be estimated as a function of the measured phase data. A prototype was set up to demonstrate the feasibility of this sensor, which was experimentally tested in the range 40-500 mg/dl with a small quantity of solution and had a measurement resolution of 1.41 mg/dl at 25 degrees C.     

长距离分布式布里渊光纤传感关键技术进展分析

布里渊光纤传感通过光纤中受激布里渊散射效应实现温度和应变测量,具有空间分辨率高、传感距离长和测量精度高等特点,因此分布式布里渊光纤传感成为近年的研究热点。本文通过对长距离分布式布里渊光纤传感研究进展的调研和分析,概括了长距离布里渊传感面临的主要限制因素和解决的关键技术,重点介绍了基于时分复用、频分复用、脉冲编码、宽带频率调制和图像处理算法的长距离布里渊光纤传感技术。随着长距离布里渊光纤传感器的实际工程化,对于快速测量的需求愈发显著,这将是未来长距离布里渊光纤传感的主要研究方向。

     

Interferometric optical vortex array generator

Two new interferometric configurations for optical vortex array generation are presented. These interferometers are different from the conventional interferometers in that they are capable of producing a large number of isolated zeros of intensity, and all of them contain optical vortices. Simulation and theory for optical vortex array generation using three-plane-wave interference is presented. The vortex dipole array produced this way is noninteracting, as there are no attraction or repulsion forces between them, leading to annihilation or creation of vortex pairs.     

Three polarization reflectometry methods for determination of optical anisotropy

Three novel methods for the determination of optical anisotropy are proposed and tested. The first, the special points method, may be applied to any uniaxially anisotropic medium and is based on the measurement of s- and p-polarized light reflectances under near-normal or grazing angles (or both) and of the Brewster angle. The second method is based on the use of the Azzam universal relationship between the Fresnel s- and p-reflection coefficients. For a flat surface and an isotropic medium, the Azzam combination of coefficients becomes zero and thus is independent of the incidence angle, whereas for a uniaxial or biaxial anisotropic sample it acquires a certain angular dependence, which may be used to determine the anisotropy of the sample. Finally, for those cases in which the anisotropy of the material of a film deposited on an isotropic substrate is itself of interest, a third method, the interference method, is suggested. This technique makes use of the different dependences of s- and p-polarized beam optical path-length changes on the variation of the angle of incidence.     

Fabrication and characterization of optical-fiber nanoprobes for scanning near-field optical microscopy

The current scanning near-field optical microscopy has been developed with optical-fiber probes obtained by use of either laser-heated pulling or chemical etching. For high-resolution near-field imaging, the detected signal is rapidly attenuated as the aperture size of the probe decreases. It is thus important to fabricate probes optimized for both spot size and optical transmission. We present a two-step fabrication that allowed us to achieve an improved performance of the optical-fiber probes. Initially, a CO(2) laser-heated pulling was used to produce a parabolic transitional taper ending with a top thin filament. Then, a rapid chemical etching with 50% buffered hydrofluoric acid was used to remove the thin filament and to result in a final conical tip on the top of the parabolic transitional taper. Systematically, we obtained optical-fiber nanoprobes with the apex size as small as 10 nm and the final cone angle varying from 15 degrees to 80 degrees . It was found that the optical transmission efficiency increases rapidly as the taper angle increases from 15 degrees to 50 degrees , but a further increase in the taper angle gives rise to important broadening of the spot size. Finally, the fabricated nanoprobes were used in photon-scanning tunneling microscopy, which allowed observation of etched double lines and grating structures with periods as small as 200 nm.     

Multiscan time-domain optical coherence tomography for retina imaging

A versatile time-domain optical coherence tomography system is presented that can generate cross-sectional images by using either transverse priority or depth priority scanning. This is made possible by using a transmissive scanning delay line compatible with balance detection operating at a speed similar to that of the transverse scanner used to scan the beam across the target. In vivo images from the retina are generated and shown using the same system switched to either transverse or depth priority scanning regime, by using the scanning delay line either in slow or fast scanning modes, respectively. A comparative analysis of different scanning regimes depending on image size to fit different areas to be imaged is presented. Safety thresholds due to the different continuous irradiation time per transverse pixel in different scanning regimes are also considered. We present the maximum exposure level for a variety of scanning procedures, employing either A scanning (depth priority) or T scanning (transverse priority) when generating cross-sectional images, en face images, or collecting 3D volumes.     

Phase-sensitive optical low-coherence reflectometry for the detection of analyte concentrations

Optical techniques may potentially be used for noninvasive glucose sensing. We investigated the application of phase-sensitive optical low-coherence reflectometry (PS-OLCR) to the measurement of analyte concentrations. The dependence of the PS-OLCR signal on the concentration of various analytes, including aqueous solutions of glucose, calcium chloride, magnesium chloride, sodium chloride, potassium chloride, potassium bicarbonate, urea, bovine serum albumin, and bovine globulin, were determined in clear and turbid media. Obtained results demonstrated (1) a high degree of sensitivity and accuracy of the phase measurements of analyte concentrations with PS-OLCR; (2) a concentration-dependent change in the phase-shift for glucose that is significantly greater than that of other analytes sampled over the same physiological range; and (3) a high submillimolar sensitivity of PS-OLCR for the measurement of glucose concentration. Further exploration of the application of PS-OLCR to the noninvasive, sensitive, and specific monitoring of glucose concentration seems warranted.