FMCW反射计检测距离影响因素分析

频率调制连续波（FMCW）反射计以其高动态范围和窄的空间分辨率优势,在光网络检测、集成光路诊断和光纤传感等领域有着广泛的应用前景。在介绍FMCW反射计基本原理的基础上,分析了光源扫描重复频率、光源功率与FMCW反射计检测距离之间的关系,并探讨了光源相干长度和相位噪声对检测距离的影响。理论分析表明,当待测光纤的长度接近光源的相干长度时,中频信号和相位噪声之间的信噪比会急剧下降,所以FMCW反射计要使用高相干性的光源和一定的相位噪声补偿方法才能应用于长距离的光纤检测。     

RF电缆的时域反射特性

本文主要介绍了RF电缆的时域反射特性。以及通过时域反射测量对电缆的特性进行分析,判定影响其反射参数的主要原因。     

Effect of the preliminary transformation of experimental data on the accuracy of results of processing X-ray reflectograms

An effect of the preliminary transformation of experimental data of relative two-wave X-ray reflectometry on the accuracy of determining parameters of multilayer structures is described. The efficiency of applying the CUDA technology to the computer processing of results is investigated. An efficient implementation of calculations in modeling the relative two-wave X-ray reflectometry using CUDA is proposed.     

Time domain reflectometry liquid level sensors

The technique of time domain reflectometry (TDR) has been used by electrical engineers for testing the characteristics of transmission lines and diagnosing faults. Although TDR instruments for testing cable are commonly used today, few instrumentation engineers are aware that the technique can measure liquid levels and fluid interfaces. In this article, I review the theory and principles of time domain reflectometry and present the application of this technology as a liquid level sensor     

Linearizing optical frequency-sweep of a laser diode for FMCWreflectometry

We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequency-modulated continuous-wave (FMCW) reflectometry. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator. The interference beat signal of the reference interferometer is phase-compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the laser diode is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearity of the proposed method, we apply this frequency-swept laser diode to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved. The backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagation loss of a single-mode glass waveguide is successfully evaluated     

Interferometer-less coherent optical range finder

A new coherent optical detection technique employing coherent frequency-domain reflectometry and a novel optical frequency sensor is demonstrated for high-precision optical path-length measurements. Using pulsed laser sources, an improvement of more than two orders of magnitude in spatial resolution over conventional optical coherent frequency domain reflectometry techniques is demonstrated. Varying degrees of spatial resolution ranging from several centimeters to a few hundred nanometers are achieved. High-precision distance measurement with long baseline is also presented     

In situ optical monitoring of OMVPE deposition of AlGaAs by laser reflectance

The feasibility of laser reflectometry for in situ monitoring and control of OMVPE AlGaAs was demonstrated. The optical constants, refractive index and extinction coefficient, of the AlGaAs alloys at the growth temperature were obtained for 633 nm wavelength. These data were used in closed loop control of thickness and composition during deposition of heterostructure layers. Laser reflectometry was also instrumental in the observation of unintentional composition gradients at the substrate-epilayer interfaces in atmospheric pressure vertical flow reactors.     

Extremely low back facet feedback by quantum-confined Stark effectabsorption in an edge-emitting light-emitting diode

The quantum-confined Stark effect is employed to form an integral reverse-biased absorber in a GaInAsP edge-emitting light-emitting diode. Optical low coherence reflectometry is used to measure the magnitude of reflections through this absorber. Front-facet-back-facet roundtrip reflection magnitudes are below -110 dB in devices having an antireflection coating on the front facet only. All other round trip reflections are below -80 dB. This device provides a wide usable dynamic range in optical low coherence reflectometry measurements     

Characteristics and reduction of coherent fading noise in Rayleighbackscattering measurement for optical fibers and components

The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in coherent-OTDR (optical time-domain reflectometry) and coherent-OFDR (optical frequency-domain reflectometry) are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically, and the relationships between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR     

Study of multilayered SiGe semiconductor structures by X-ray diffractometry, grazing-incidence X-ray reflectometry, and secondary-ion mass spectrometry

In this publication, we report the results of studying a multilayerd nonperiodic SiGe/Si structure by the methods of X-ray diffractometry, grazing-angle X-ray reflectometry, and secondary-ion mass spectrometry (SIMS). Special attention is paid to the processing of the component distribution profile using the SIMS method and to consideration of the most significant experimental distortions introduced by this method. A method for processing the measured composition distribution profile with subsequent consideration of the influence of matrix effects, variation in the etching rate, and remnants of ion sputtering is suggested. The results of such processing are compared with a structure model obtained upon combined analysis of X-ray diffractometry and grazing-angle reflectometry data. Good agreement between the results is established. It is shown that the combined use of independent techniques makes it possible to improve the methods of secondary-ion mass spectrometry and grazing-incidence reflectometry as applied to an analysis of multilayered heteroepitaxial structures (to increase the accuracy and informativity of these methods).     

Characterization of 1050-nm Pumped S-Band TDFA With Different Dopant Concentrations by Coherent Optical Frequency-Domain Reflectometry

For thulium-doped fiber amplifier (TDFA) design, knowledge of the gain distribution within the active fiber is of great value. For the first time, the distributed gain along highly thulium-doped fibers was measured with high-resolution reflectometry. The technique of coherent optical frequency-domain reflectometry is a nondestructive and noninvasive method well matched to this task due to its dynamic range, spatial resolution, and measurement range. Using thulium-doped fibers with different Tm³⁺-dopant concentrations, we show precise measurements of Rayleigh backscattering levels for obtaining the optimum gain-length ratio for S-band TDFA pumped by 1050-nm laser diodes     

Coherent FMCW reflectometry using a temperature tuned Nd:YAG ringlaser

A long-coherence-length, diode-pumped, monolithic Nd:YAG laser operating at 1.32 μm is used to perform coherent FMCW (frequency-modulated continuous wave) reflectometry measurements. Compared with semiconductor-based lasers, the low phase noise of the Nd:YAG source offers greatly increased distance range combined with increased sensitivity to optical reflections. Measurement of Rayleigh backscatter from 50 km of fiber is demonstrated with over 60 dB of two-way optical dynamic range. Two-point spatial resolution is demonstrated to be better than 10 cm. This source and coherent measurement technique may have potential for both short and long-haul reflectometry applications     

Detection and Location of Multiple Wiring Faults via Time–Frequency-Domain Reflectometry

In this paper, we propose a high-resolution time–frequency-domain reflectometry technique as a methodology of detection and estimation of faults on a wire. This method adopts the time–frequency cross-correlation characteristics of the observed signal in both the time and frequency domains simultaneously. The accuracy of the proposed method is verified with experiments using a radio-guide-type coaxial cable and comparing it with traditional time-domain as well as frequency-domain reflectometry methods. It is clearly shown here that the proposed algorithm produces excellent results compared to the conventional methods for single as well as multiple fault cables.      

Microwave measurements by Fourier analysis of network pulse response

Measurements by Fourier analysis of time-domain reflectometry data are made, and errors caused by noise are evaluated.     

Application of Two-Wavelength X-Ray Optical Scheme for Combined Measurements of X-Ray Specular Reflection and Diffuse Scattering to Study Multilayered Thin Film Structures

The complex study results of the parameters of TiN/Ti diffusion-barrier structures using the methods of relative X-ray reflectometry and diffuse scattering of X-ray radiation implemented by a two-wavelength X-ray optical measurement scheme are presented. It is shown that this scheme, by a single measurement, enables studying two different diffuse scattering regions, which increases the correctness and unambiguity of the analysis that was carried out. The considered complex of methods makes it possible to solve the ambiguity of a density/roughness type by the solution of the inverse reflectometry problem and to calculate the parameters of buried layers in the structures which were studied.     

Advances in phase-sensitive optical time-domain reflectometry

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has attracted numerous attention due to its superior per-formance in detecting the weak perturbations...     

Détection et localisation de défauts dans des réseaux filaires de topologie complexe

Reflectometry is a well known method widely used to monitor the health of lines and wired networks. This method provides information for the detection, localization and characterization of electrical defects in networks. CEA LIST works on the development and the application of this method for the diagnosis of wired networks of complex topology, targeting embedded systems. Application domains range from telecommunication to automotive and transport, energy distribution, etc. This paper presents a new theoretical model for the precise numerical simulation of reflectometry signals applied to any kind of wired network. Based on standard microwave propagation theory, this model provides simple explicit formulas for both time domain (TDR) and frequency domain (FDR) reflectometry, and is helpful to better understand and explain measurement results in relation with the network’s topology.     

Reflectometry of X-ray Whispering Gallery Waves Propagating along Liquid Meniscuses

Semiconductors - X-ray reflectometry of whispering galleries (WGs), which propagate along meniscuses of deionized water or silica hydrosols (Ludox® SM) enriched by Cs+ ions, was analyzed for...     

In situ growth monitoring of AlGaN/GaN distributed Bragg reflectors at 530 nm using a 633 nm laser

The metal-organic chemical vapor deposition(MOCVD) growth of AlGaN/GaN distributed Bragg reflectors (DBR) with a reflection peak at 530 nm was in situ monitored using 633 nm laser reflectometry.Evolutions of in situ reflected reflectivity for different kinds of AlGaN/GaN DBR were simulated by the classical transfer matrix method.Two DBR samples,which have the same parameters as the simulated structures,were grown by MOCVD.The simulated and experimental results show that it is possible to evaluate the DBR...     

Mid-range coherent optical frequency domain reflectometry with aDFB laser diode coupled to an external cavity

The optical frequency of an external double cavity coupled laser diode was controlled via the phase of the feedback light. Strong linewidth and mode hopping reductions were obtained, extending coherent reflectometry with semiconductor sources to the range of hundreds of meters