Search

In this paper, we propose a dual wavelength scheme for the detection of liquids based on optically absorbing (Co2+-doped) optical fibers. The proposed configuration makes use of a Coherent-Optical Time Domain Reflectometer (C-OTDR) operating with standard fibers at 850 nm. Besides vibration measurements over all the sensing fiber, the proposed scheme performs liquid detection in short sections of Co2+-doped fibers, heated by a modulated optical pump at 1550 nm. We demonstrate that the proposed sensor detects the liquids surrounding the doped fiber, such as water or acetone.

An experimental set-up was constructed based on an ytterbium pulsed laser with a passive holmium fiber shutter, which provides time-domain reflectometry research the length of active fiber samples after drawing and during storage. The possibility of studying non-uniformities in the fiber and quantifying the attenuation coefficient of the optical signal at the source wavelength is discussed. The results indicate the possibility of conducting studies of this type, as well as the potential development of reflectometers for the needs of laboratories of enterprises that produce active optical fibers commercially.

This study investigates variations in the velocity and sensitivity of electromagnetic waves in transmission lines configured in defective model piles for the detection of necking defects containing soil. Experiments are performed with model piles containing defects filled with different materials, such as air, sands, and clay. Five different types of transmission lines are configured in model piles. The electromagnetic waves are generated and detected using a time domain reflectometer. The velocity of electromagnetic waves is highest when the defect is filled with air, and it decreases with an increase in the water content. The velocity is lowest when the defect is filled with clay. The sensitivity of transmission lines for detecting defects decreases with an increase in soil water contents. The transmission line with a single electrical wire and epoxy-coated rebar exhibits the highest sensitivity, followed by that with three and two parallel electrical wires. Transmission lines with a single electrical wire and uncoated rebar and those with two parallel electrical wires wrapped with a sheath exhibit poor sensitivity when the defect is filled with clay. This study demonstrates that electromagnetic waves can be effective tools for detecting necking defects with wet and conductive soils in bored piles.

This paper presents a non-destructive method for detecting necking defects in bored piles using electromagnetic waves. Small-scale laboratory experiments are conducted with a defective model pile with diameter of 0.6 m and length of 1.0 m. Four artificial necking defects are introduced on three different sides of the model pile. One necking defect is introduced at the upper position on one side of the model pile, another one is introduced at the lower position on other side of the model pile, and the other two are introduced at both the upper and lower positions on the same side of the model pile. Electrical wires are installed alongside the main rebars of a reinforcement cage to configure a two-conductor transmission line. Electromagnetic waves are generated and detected using a time domain reflectometer. The experimental results indicate clear detection of the reflected electromagnetic waves at the necking defects and pile end. In addition, the calculated defect locations are similar to the actual locations. This study demonstrates that electromagnetic waves can be effective in detecting necking defects at various positions in bored piles.

25th International conference on Optical Fiber Sensors, , 24/04/2017-28/04/2017, Jeju, Corea del Sur., We review our recent work on chirped-pulse phase-sensitive optical time-domain reflectometry along optical fibers and its application in the measurement of true nanostrain variations in the kHz frequency range over km-long fibers. We also show how this technique can be used to perform distributed photothermal measurements of gas presence in suitable microstructured fibers., European Commission, Ministerio de Economía y Competitividad

We demonstrate a distributed localization of an impact source in the composite cylinder using a home-made phase-sensitive optical time domain reflectometer (Φ-OTDR). The system shows the advantages of simple configuration using semiconductor optical amplifier to improve the spectral response of the pump pulses, high spatial resolution and high signal-to-noise ratio (SNR). With a spatial resolution of 10 m, the SNR could be obtained as high as 6.5 dB. A composite cylinder is longitudinally divided in multiple sections of equal lengths by bending the sensing fibers around it at different locations, and the section of a source impact is localized using the time-delay of the signal detected by the sensing fibers.

A simple optical reflectometry with a semiconductor optical amplifier ring structure is proposed for the first time to locate the accurate fiber fault position using the chaotic correlation detection method. The experimental results show that a much higher spatial resolution of about 50 μm and a measured stability of ±150 μm are achieved.

25th International Conference on Optical Fiber Sensors, 2017, Jeju, Corea del Sur., In this communication, the effect of the probe pulse shape on the backscattered power trace of phase-sensitive (φ-) OTDR is analyzed. In particular, the power traces obtained with rectangular and Gaussian-like probe pulses are compared, both numerically and experimentally. Our analysis reveals that the use of Gaussian-like pulses can increase the operation range and sensitivity of φOTDR-based sensors in at least two-fold as compared with that obtained from traditionally-employed rectangular-like pulses., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

A single-chip Optical Time Domain Reflectometer (OTDR) system-on-chip (SoC) for fiber-optic fault detection and localization is presented. The IC can be configured into several modes including high-performance mode (external modulator and expensive off-chip laser / APD), and low-cost mode (low power/gain and inexpensive laser / APD). Experimental optical measurements demonstrate that the maximum detectable fiber length of the proposed OTDR SoC can be as long as 25km, while the spatial resolution can achieve 2.5m, 4m and 10m, for various configurations. Compared with current discrete solutions on the market, the proposed OTDR system can reduce the cost by 100x, area by 5x, and power consumption by 4x.

최근 10년간 전세계적으로 아스팔트 포장을 재활용하는 기술이 급속도로 확산되고 있으며 노후 아스팔트 포장을 폼드 아스팔트 또는 유화 아스팔트를 사용하여 현장에서 바로 100% 재활용하는 현장 상온 재생 아스팔트 포장기술이 다양하게 적용되고 있다. 특히, 아이오와 주에서는 교통량이 적은 지방도로에서 기존 포장의 수명을 연장 시켜주는 현장 상온 재생 아스팔트 공법을 많이 적용하고 있다. 일반적으로 현장 상온 재생 아스팔트 포장층은 수분의 침투나 교통하중으로부터 보호하거나 포장설계를 만족시키기 위해 가열 아스팔트 포장으로 덧씌우기를 한다. 일반적으로 현장 상온 재생 아스팔트 포장층 위에 가열 아스팔트 포장으로 덧씌우기 할 시기는 대부분에 감독자들은 일정한 양생기간 또는 최대 함수비에 근거하여 결정하고 있다. 따라서, 본 연구에서는 감독자가 최적에 덧씌우기 아스팔트 포장 시기를 결정할 수 있도록 현장 상온 재생 아스팔트 포장층의 현장 함수비를 간단하게 측정하여 덧씌우기 시기를 결정할 수 있는 수분 감소계수를 개발하는 것이다. 먼저, 현장 상온 재생 아스팔트 포장층의 함수비를 TDR 함수량계를 사용하여 측정하였고 현장 상온 재생 아스팔트 포장이 시공되는 기간 동안에 강우량, 대기온도, 습도, 바람속도 등 기상정보를 수집하였다. 마지막으로 현장 상온 재생 아스팔트 포장의 초기 함수비, 대기온도, 습도, 바람속도를 변수로 하는 수분 감소계수를 개발하였다. 실제 현장 상온 재생 아스팔트 포장에서 측정한 값을 사용하여 개발한 수분 감소계수는 감독자가 연속적으로 현장 상온 재생 아스팔트 포장층의 함수비를 측정하지 않고 최적의 덧씌우기 포장 시점을 결정할 수 있다.

A novel scheme of quasi-distributed vibration disturbances detection system based on incomplete Polarization optical time domain reflectometry was proposed. The system was enhanced by employing Fresnel-reflection caused by FC/PC connector, which can improve the signal’s SNR significantly, while the temporal depolarization effect can be almost completely suppressed. Without performing any data averaging, the intrusion event can be detected and located precisely/instantaneously with good stability. Also the frequency components of vibration events applying on sensing fiber can be obtained with large dynamic range. It shows a very good potential in intrusion detection, vibration frequency measuring, etc. Published version

An on-line supervisory 1.55 µm 10 Gb/s dense wavelength division multiplexing (DWDM) distributed fiber Raman amplifier (FRA) transmission system using either a forward- or backward-pumping scheme is investigated. A distorted optical time-domain reflectometer (OTDR) trace due to the Raman gain amplification of 1.65 µm OTDR pulsed light and unwanted Rayleigh-backscattered CW amplified spontaneous emission (ASE) lights is observed. Such distorted traces give rise to an unaccurately measured fiber-loss coefficient and a reflection profile along the fiber link. The monitoring characteristics and system power penalty for both forward- and backward-pumping schemes are examined and compared.

A digitally enhanced optical frequency domain reflectometry technique is proposed, in which the frequency-to-distance mapping is realized in digital domain. A spatial resolution of 1 m over 56 km was obtained in demonstrational experiments.

Coherent fiber-optic reflectometer with a single-mode (λ = 810 nm) semiconductor laser as a radiation source has been experimentally studied. It is demonstrated that distributed fiber-optic sensors for detecting and locating dynamic impacts can be created based on the proposed reflectometer with a fiber circuit length of up to 3 km.

The design, manufacturing, analysis, and measurement of a thermal and electrical enhanced cavity-down plastic ball grid array (PBGA) package are presented in this study. Because of the split via connection (SVC) design, the package consists of a very-thin single core of organic material and two-metal layers of copper and is manufactured with the conventional printed circuit board (PCB) process at very low cost. Furthermore, the heat spreader is made with bottom surface having saw-teeth to contact the ground planes disposed on the backside of the substrate. Improvements in electrical and thermal performance are achieved. Parasitic parameters of the package are extracted from time domain reflectometer (TDR) measurements. The thermal performances of the packages are studied by both 3D finite element analysis and wind-tunnel measurements. The results are compared with other well-known packages.

A family of cavity‐down plastic ball grid array (PBGA) packages have been designed by split via connection (SVC) and split wrap around (SWA) methods. Because of the special designs, these packages consist of a single core of organic material and two‐metal layers of copper and are manufactured with the conventional printed circuit board (PCB) process at very low cost. The electrical performances of the packages are studied by both numerical simulations and experimental measurements. Parasitic parameters are extracted from time domain reflectometer (TDR) and time domain transmission (TDT) measurements. Cross‐talk and simultaneous switch output (SSO) noise of the packages are also investigated.

A micro-displacement sensing head based on two aligned cleaved fibers inserted into an optical fiber ring and interrogated by an OTDR is presented. The sensor configuration is characterized for measuring both longitudinal and axial microdisplacement, showing a sensitivity of 0.07 dB/mm and 2.67 dB/mm for longitudinal and axial micro-displacement, respectively. A multiplexing system using two of these configurations is also studied. The authors are grateful to the European Cost Action TD-1001 “OFSeSa” and to the Spanish Government project TEC2010-20224-C02-01.

International audience; A new post-processing approach which aims at reducing the dispersion effect of the wave travelling inside a waveguide is presented. The main objective is to improve the time domain reflectometry measurements performed over long-distance cables, relative to the wavelength λ. A real-case scenario is presented so that the gain of this method can be appreciated. Introduction: Cables are present in almost all modern systems. These cables are subject to several electrical, chemical and mechanical stresses. Depending on the application area, such degradation can lead to hard faults (short or open circuits) and therefore catastrophic consequences [1]. Reflectometry methods are commonly used for assessing transmission lines [2]. This Letter overcomes reflectometry's limitations by proposing a signal processing technique based on dynamic cross-correlation for the detection and the localisation of faults over long-distance cables.

A distributed fiber strain and vibration sensor which effectively combines Brillouin optical time-domain reflectometry and polarization optical time-domain reflectometry is proposed. Two reference beams with orthogonal polarization states are, respectively, used to perform the measurement. By using the signal obtained from either reference beam, the vibration of fiber can be measured from the polarization effect. After combining the signals obtained by both reference beams, the strain can be measured from the Brillouin effect. In the experiment, 10 m spatial resolution, 0.6 kHz frequency measurement range, 2.5 Hz frequency resolution, and 0.2 MHz uncertainty of Brillouin frequency measurement are realized for a 4 km sensing distance.

A new 7-port structure for terahertz reflectometer is proposed and analyzed. Our structure for 7-port reflectometer can be designed and fabricated more easily at terahertz.. as compared with traditional 6 or 7 port reflectometers and achieved near-perfect phasor distribution. Our 300 GHz reflectometer was fabricated and validated with performances with VNA.

International audience; To study the hydraulic behavior of fouled ballast, an infiltration column 600 mm high and 300 mm in diameter was developed. Five time domain reflectometer (TDR) sensors and five tensiometers were installed at various levels, allowing the measurement of volumetric water content and matric suction, respectively. The material studied was fouled ballast that was formed in the railway track-bed by penetration of fine-grained soil into the ballast. This material is characterized by a high contrast of size between the largest and the smallest particles. During the test, three stages were followed: saturation, drainage, and evaporation. Based on the test results, the water-retention curve and the unsaturated hydraulic conductivity were determined. The quality of the results shows the capacity of this large-scale infiltration column in studying the unsaturated hydraulic properties of such fouled ballast.

Phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR), which provides a capability to achieve a very high-resolution measurement over a long range, is reviewed in this paper. Its principle and applications are also introduced and discussed.

A fiber-optic low-coherent reflectometer was developed to accurately monitor fiber length variation. A large length-coverage range was obtained by using a fiber Bragg grating array in a wavelength-division-multiplexing configuration. The polarization mismatch-induced signal fading was eliminated by applying a polarization spreading technique at the optical receiver and, therefore, no active polarization adjustment was necessary for long-term measurement.

Coherent optical frequency domain reflectometry is used to multiplex a serial array of tapered optical fibre sensors. By monitoring changes in the Rayleigh backscattered signal from tapered regions of a single mode optical fibre, the refractive index of the medium surrounding each taper can be determined. The approach is used to demonstrate a liquid flow sensor.

In this paper we present a novel method of near-field microwave microscopy using a high impedance measurement system. The proposed system combines a classical vector network analyzer to a high referenced impedance splitter via impedance matching networks. We show that the resulting high impedance reflectometer associated to a tip probe permits to compensate the limitation of the non-resonant mode by using the frequency diversity.

Recent progress on novel long-range coherent optical frequency domain reflectometry is described along with its applications to diagnosing problems with optical fiber cables. The measurement range of a conventional C-OFDR is limited to the coherence length of the laser used as a light source, since the phase noise of the laser degrades the sharpness of the beat spectrum. We have developed phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) to overcome this limit, by introducing a novel phase-noise compensation technique, and achieved a very high-resolution measurement over the fiber link length. We describe the principle of PNC-OFDR and recent related developments, and discuss its use in diagnosing issues with fiber networks.

The six-port reflectometer technique enables simple and accurate measurement of optical reflection coefficients in both magnitude and phase. The reflection coefficient is computed from four power measurements of linear combinations of the waves incident and reflected from the device under test. While the six-port technique is very successful at microwave frequencies and conceptually related to coherent optical phase diversity receivers, no optical implementations have been reported so far. In this paper, we present an experimental characterization of an optical six-port reflectometer composed of three multimode interference couplers, fabricated with a single etch step in silicon on insulator. Measurements are performed using a novel technique with a simple setup. The six-port combines the incident and reflected waves with magnitude and phase errors less than plusmn0.5 dB and plusmn6deg in the 1485-1575 nm band.

The Brillouin-optictime domain reflectometer (BOTDR) fiber strain sensing technology has been used in the optic fiber and its attachment surface supporting for the axial strain distributed monitoring, A stress test monitoring is respectively on a steel hinge cable line with the three arylene fiber-reinforced plastic (AFRP) anchor, and the distributed contingency sampling point has the minimum distance of 5cm. Test results show that distributed optic fiber sensor has a high accuracy. The relative error between optic fiber sensors and strain measurements is less than 3.8 percent. In the stage of tensing anchors, a steel cable hinge line's prestressed loss is 8.8 percent; three AFRP anchors' prestressed loss is less than 6.08 percent. After a month, the steel cable hinge line's prestressed loss is 11.1 percent; AH cable prestressed loss is less than 10.3 percent.

A soil-embedded optical fiber sensing cable is evaluated for an embedded cavity detection and sinkhole warning system in railway tunnels. Tests were performed on a decametric structure equipped with an embedded 110 m long fiber optic cable. Both Brillouin optical time-domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) sensing techniques were used for cable interrogation, yielding results that were in good qualitative agreement with finite-element calculations. Theoretical and experimental comparison enabled physical interpretation of the influence of ground properties, and the analysis of embedded cavity size and position. A 5 mm embedded cavity located 2 m away from the sensing cable was detected. The commercially available sensing cable remained intact after soil collapse. Specificities of each technique are analyzed in view of the application requirements. For tunnel monitoring, the OFDR technique was determined to be more viable than the B-OTDR due to higher spatial resolution, resulting in better detection and size determination of the embedded cavities. Conclusions of this investigation gave outlines for future field use of distributed strain-sensing methods under railways and more precisely enabled designing a warning system suited to the Ebersviller tunnel specificities.

Soil constituency plays a significant role in the dielectric permittivity-water content relationship which informs electromagnetic sensor-based readings. Sensor output generally provides a bulk determination of permittivity that must be converted to soil volumetric water content. The objective of this work was to evaluate the potential for a Time Domain Reflectometer (TDR) and open-ended dielectric probe (OEDP) to provide electromagnetic-based (EM) mobile soil water content determination. Open-ended dielectric probes were made from a variety of coaxial fixtures and tested using an Anritsu portable network analyzer (PNA). After TDR and analyzer calibrations using an open, short and load, measurements were compared for accuracy using reference dielectric liquids. Bilinear analysis was used to derive frequency dependent permittivity. Both blade and surface OEDP’s were tested for permittivity measurement capability (PNA) and waveform consistency (TDR). Additional work is needed to understand the impact of soil compaction on readings. Dynamic effects on soil structural properties need to be studied to determine the effect on sensing volume at the soil probe interface. Reliable and rapid mobile measurement of soil water content at the field-scale could greatly enhance many agronomic activities such as development of reclamation strategies, reducing fertilizer application and optimizing fresh and saline water use. This capability will require improvements in sensor design, signal analysis and novel application of the technology before real-time field-scale measurements for precision agriculture are readily available.

A six-port reflectometer circuit that is particularly suited to dielectric-type waveguides is described. Its scattering matrix is derived, allowing for flexibility in design. A phase-shifting network is incorporated in the circuit, and it is shown that a quadrature hybrid can be configured for this purpose. The reflectometer was implemented in a dielectric waveguide for W-band (75 GHz-110 GHz) operation, and measurements are presented. >

A high-spatial resolution reflectometry by synthesis of the optical coherence function is proposed for testing optical devices and circuits at the far end of optical subscriber networks. The system can measure the reflectivity distribution at a long distance without performance degradation resulting from the phase fluctuation in a long-length optical fiber. Reflectivity distribution at a distance of 5 km is measured successfully with the spatial resolution of 6 cm.

We introduce a tunable-laser-based technique for sub-millimeter resolution fiber reflectometry. We show how the technique is used to locate bends, breaks, bad splices and poor connections in short-haul single- and multimode fiber links.

Time domain reflectometry (TDR) technique is widely used in hydrology and soil science for accurate and flexible soil water content measurements. The most attractive advantages concerning the considered TDR measurement system are: good precision and accuracy, high reliability of the measuring head, a unique approach of pulsing a long coaxial probe and analysing the reflected voltage signature caused by changes in impedance, capability of multiplexing several probes, possibility of remotely acceding, controlling and electronically retrieving and transmitting data through existing telecommunication technologies. A time domain reflectometer transmits the incident signal, an ultra short rise time (200 ps), step voltage pulse, along the transmission line and records the travel time and the magnitude of all reflected signals (echo) returning from the controlled system. Changes in impedance (capacitance, inductance and resistance) causing electromagnetic discontinuities that reflect voltage can be located, particularly, for liquid level and dielectric properties monitoring purposes. The above mentioned discontinuities result from impedance changes produced by changes in the dielectric constant. The time domain reflectometry method used in this research has the purpose to monitor the behavior of different liquid substances, detecting their levels and giving information about their characteristics, such as volumetric content, dielectric constant, electrical conductivity, as well as the identification of different interfaces. Furthermore, the main objective of the present work is to develop an interpretation method for the analysis of TDR signal that, associated with a simple calibration procedure and with a suitable probe design can ensure, at the same time, quantitative and qualitative liquid monitoring

A concept demonstration of a code-correlation OTDR reflectometer is performed with improved sensitivity and < 1 cm resolution. It is shown that this approach can exceed conventional OTDR techniques in performance and eliminate dead-zones in the measurement, while being based upon commercially available optical component technology

In this talk, we will introduce a commercially available optical frequency domain reflectometry (OFDR) technique and its applications in avionics fiber-optics. We will provide multiple measurement examples within the avionics field not currently supported by conventional test tools or methods including high-resolution fault detection and distributed fiber sensing

A Time-Domain-Reflectometry (TDR) characterization technique has been investigated to enhance the detection of discontinuities in packaging and PCB structures. Three groups of test vehicles have been fabricated, on which special features were purposely introduced. From the locations of these special test features, the detection of discontinuities was improved significantly.

We have used microwave cavity spectrometer and time domain reflectometer for the measurement of permittivity and dielectric loss at different temperatures in 2,4-dimethyl substituted pyridine. The observed data of the width of resonance profile and the shift in the resonance frequency have been analyzed by using Slater perturbation equations useful for cavity spectrometer. The parameters measured from the time domain reflectometer as the function of time at different temperatures have been transformed to obtain permittivity and dielectric loss. The observed values of permittivity and dielectric loss by these two techniques have also been compared. The relaxation time has been evaluated using single relaxation time mechanism and thermodynamical parameters have been determined. This experimental study provides fruitful information about the bulk properties of 2,4-dimethyl substituted pyridine.

The time domain reflectometer (TDR) has been used for the experimental study of permittivity and dielectric loss for tetrahydrofuran and 2,5-dimethylfuran. The observed data in time domain has been transformed to frequency domain using Fourier transform method. We have also evaluated the relaxation time using distribution of relaxation time mechanism at different temperatures. The entropy, enthalpy and free energy of activation are determined for the purpose to understand the molecular collisions. This study is fruitful for the workers in the field of material science.

In the paper, the analysis of the application of a tunable semiconductor laser working in 1550 nm band in a C-OFDR construction is presented. The laser was tuned in narrow 0.24 nm range by a PZT element and in wide 70 nm range by a stepping motor. The narrow range was used for signal separation from measurement points laying along optical fiber. The wide range can be used in to separate signals from branches of a fiber optic sensor network. The reflectometer was used for signal detection from quasi-distributed sensors network. Measurements were carried out for different distances of point fiber optic sensors from the reflectometer. As the models of point sensors air cavities were used. The influence of tuning range of the laser on measurement errors is discussed.

A network analysis based high-frequency inductance technique was used to measure the package inductance of various A42 and Cu QFP packages. A continuous frequency range of 2.6 MHz to >500 MHz corresponding to the relevant speed/bandwidth of today's ICs was employed. Results indicated that while the inductance of all the Cu packages were essentially constant across the frequency band, the inductance of A42 packages varied appreciably with the frequency. The inductance of a typical A42 package was three times that of a Cu package at 2.6 MHz but decreased to only 10%-20% higher than that of a Cu package above 250 MHz. The lead resistance of A42 also increased sharply with increasing frequency and could become an important factor in determining the electrical performance. Based upon the preliminary analyses, the frequency-dependent inductance and resistance of A42 packages were attributed to the interaction between the skin effect and the non-unity permeability of ferromagnetic materials. To verify the theory and to test possible electrical enhancement for the A42, different surface-treated packages, including an Au-plated A42, a Pd/Ni-plated A42 and a Pd/Ni-plated Cu CQFP packages were characterized for their inductance and resistance. A 3-D field solver was also used to derive the permeability of the A42 materials at different frequencies. To further study the impact on high-speed digital chips where signals are usually analyzed in the time domain, a test structure made of both A42 traces and Cu traces was characterized using a time domain reflectometer for impedance, delay, and crosstalk. Both the frequency-domain and the time-domain results for A42 and Cu packages are summarized in this paper and their implications discussed. >

The ferrite absorbing panel used for suppressing TV ghost images caused by TV wave reflection from tall buildings and the RF absorber used for an anechoic chamber have been realized for many years. The convenient and accurate measuring method of the reflectivity characteristics of RF absorbers against the oblique incidence in the low frequency range (lower than 1 GHz) is investigated by using a new time domain reflectometer method. The direct wave vector from the transmitting antenna to the receiving antenna and the undesired reflection vector can be eliminated by means of a mathematical treatment or by subtraction of the receiving signal without the target from that signal with the target in advance of time gating in the desired response. By using these new methods, a convenient and efficient evaluation method for an RF absorbing panel and absorber could be realized.

Numerous techniques have been recently developed or modified for improved, in-situ soil moisture content determination. Examples of the devices used in these techniques include the time domain reflectometer, the soil moisture gauge, and the heat-pulse probe. This paper discusses the theory, description, and application of an optical reflectance technique for the in-situ measurement of soil moisture content. An encased, simple light source is proposed which produces a reflectance from a coated, porous membrane whose moisture content is a result of being in close contact with a moist section of soil. A photodiode and a photocell detector capable of sensing the 1.4 mm wavelength of light that is reflected by the moist porous membrane surface are used to quantify the moisture content. The device is approximately 125 cm/sup 3/ and can be pushed into the soil to desired depths. Reflectance measurement techniques such as this have been able to obtain soil moisture content measurements to an accuracy of better than 1%. The optical reflectance methodology presented uses a simple technique that will allow the determination of soil moisture content by using a simple equation which is based on the fraction of reflected light power incident on the detector with a dry and moist porous membrane.

A time domain spectroscopy is applied to an efficient and broad band measurement of electrical properties of biological substances. Observed reflected waves by a time domain reflectometer from beef meat were analyzed by a time domain analysis covering from dc to 10 GHz. A computational technique has been developed. >

A GaAs monolithic time-domain reflectometer (TDR) for millimeter-wave network analysis has been fabricated. The nonlinear transmission line (NLTL)-based TDR has two outputs from which the incident and reverse waves can be determined. The two channels show a 2.3-ps falltime, and hence a 150-GHz 3-dB TDR bandwidth. The reflection coefficient in the time domain obtained after a partial calibration clearly indicates a prominent reflection when the TDR is under open-circuitry load. With the use of network analysis calibration routines, corrected millimeter-wave vector measurements will be feasible with these devices. >

The author reviews the fundamentals of optical reflectometry, including the standard optical time-domain reflectometry (OTDR) measurement. Some novel reflectometry schemes are described, including a spread-spectrum approach to long-range OTDR, as well as new optical frequency-domain reflectometry (OFDR) techniques that have proven useful for high-resolution measurements. These novel reflectometry techniques are suitable for testing both fiber links and small components. The spread-spectrum technique allows improved dynamic range without sacrificing resolution. High-speed modulators and detectors now allow the extension of OFDR techniques to high frequencies. Resulting measurements show a large dynamic range and resolution better than 4 mm. >

A 400 MHz, GaAs ASIC with on-chip laser driver was designed to provide programmable precision timing and digital signal averaging for an optical time-domain reflectometer (OTDR). Semi-custom ASIC design was based on our fully-characterized GaAs cell library.

This paper presents a free-space reflectometer that has been developed to measure surface impedance of planar materials in the Ku-band (12.5 to 18 GHz). The set-up uses a microwave lens made of metal plates to create an equiphase wavefront arriving at normal incidence on the sample surface. Thus, the TEM wave assumption can be made to extract the impedance of the sample, which is not the case for other free-space methods using lenses. It also has the advantage of requiring small sample size (17 cm/spl times/17 cm). The system has been validated with plexiglass. The maximal error on the reflection coefficient was 0.02 for the amplitude over the 5.5 GHz bandwidth. The accuracy of the system makes it suitable for shielding effectiveness measurement and characterization of conductive polymers or thin metallic layers.

This paper discusses a method for extracting the frequency dependent characteristic impedance of transmission lines from Time Domain Reflectometry (TDR) measurements using an Open, Short, Load, and Shortline calibration. The frequency dependent behavior of transmission lines was successfully captured using this method. Two types of transmission lines were measured using this method namely, thick metal transmission lines in Printed Wiring Board (PWB) and thin transmission lines in MCM-L technology.