Submillimeter optical reflectometry

The development of optical reflectometers with a spatial resolution in the submillimeter range is reviewed. Optical time-domain reflectometers (OTDRs) and optical low-coherence reflectometers (OLCRs) for nondestructive diagnostics of waveguide devices are discussed. Techniques, system performances, and experimental results are discussed for OTDRs and for OLCRs. Fields of applications for these techniques are indicated, and some preliminary conclusions are presented. A dynamic range of ~100 dB and a spatial resolution in the range of 10-60 μm were achieved with both types of reflectometers. Nondestructive diagnostics on waveguide components and integrated optics circuits are feasible at these performance levels     

Six-port and four-port reflectometers for complex permittivitymeasurements at submillimeter wavelengths

The frequency range of six-port reflectometry is extended into the submillimeter wavelength range. The complex permittivity of low-loss microwave materials is determined using novel six-port and four-port reflectometers. These either consist of quasi-optical components or utilize oversized waveguide techniques. Permittivity measurements of materials possessing a wide range of values of ϵ' (2 to 7) and of the loss tangent (0.0003 to 0.03) were carried out at frequencies of about 380 GHz to 390 GHz. Good agreement with published permittivity data is shown. Moreover, the equivalance of the simple waveguide four-port reflectometer (which is preferred because of its easy handling and high stability against temperature fluctuations) to the quasi-optical reflectometers is shown     

Power and energy: National standards

The development of RF and microwave power standards over the last 20 years is reviewed in the light of the evolution of the commercial power meters which they are used to calibrate. Although many different types of power standard have been investigated, the microcalorimeter, the dry-load calorimeter, and in some cases the flow calorimeter continue to serve as the main primary instruments. These have been refined and extended in frequency coverage, resulting in uncertainties in effective efficiency in the range 0.1-0.2 percent up to 40 GHz and 0.5 percent or better above this frequency. Techniques for comparing power meters are also discussed. They include the use of tuned reflectometers, network analyzers, reflectometers of the six-port and multistate kind, and resistive power splitters.     

A Universal Wall-Current Detector

A universal X-band waveguide detector has been developed which offers the possibilities of a broad-band untuned detector with a stable frequency characteristic. The wall-current detector is a reflectionless two-port with an insertion loss, less than 0.05 db, no extra phase-shift, a sensitivity of about 10 mv/mw and a frequency characteristic which repeats within /spl plusmn/0.1 db over the 8.2-12.4 Gc band for any 1N26. This performance made it possible to flatten the output of a Hewlett-Packard sweep oscillator (as seen by another wall-current detector) within /spl plusmn/0.15 db over the whole X band. As a result many frequency-dependent measurements can now be done automatically with reasonable precision. Plotters and reflectometers will be simplified, resulting in a higher precision. Circular and ridge waveguide types have also been made. The latter seems very promising for an ultra broad-band detector. A sum detector and a difference detector have been made. They can be used for phase-sensitive detection, zero measurements, etc. The wall-current detector can easily be scaled down to mm waves.     

Simplified calibration equations for computer-corrected network analyser using matched-impedance normalisation

Explicit expressions are presented for a bidirectional computer-corrected network analyser using matched-impedance normalisation. It is shown that this procedure simplifies the equations and considerably reduces the number of error terms possible in measurements with network analysers. In addition errors due to source switching, if they do arise, can be rigorously accounted for. However, this requires an extended calibration procedure with a two-port network standard.     

A Network Analyzer Incorporating Two Six-Port Reflectometers

This paper outlines the theory and design of a microwave network analyzer capable of measuring the network parameters of any linear reciprocal or nonreciprocal, active or passive two port. An RF signal from one source is applied at the same time to two six-port reflectometers which measure the incident and reflected waves at the ports of the two port being measured. An experimental dual six-port network analyzer for the 2-18-GHZ range has been completed and is described briefly. Some advantages of the proposed design over existing network analyzer designs are 1) only one source is needed, 2) no phase detectors are required, 3) no flexible cables or arms are used between the reflectometers and the two port being measured, and 4) self-calibration techniques are readily applied.     

Measurements at millimeter and micrometer wavelengths

Insertion loss measurements of waveguide components have been made with an accuracy 0.2 dB at the 10-dB level and 2.8 dB at the 30-dB level. Attenuation measurements of 60-mm TE10circular waveguide have been made over the frequency range 33 to 110 GHz with total uncertainty of 0.4 dB. Intercomparison between calorimeters developed by national laboratories at 100 GHz resulted in differences less than 0.5 percent. In measuring optical fibers calorimetry was used to measure loss and power with an accuracy of 1 to 2 percent. Loss measurements by comparison with a standard have resulted in accuracies of 0.5 dB in losses of 40 dB/km in the 1.0- to 1.6-µm range. Shuttle pulse measurement of pulse spreading indicates that pulsewidths of 0.4 ns are increased to 4.0 ns by passing through 2 km of fiber at a wavelength of 0.9 µm. Interferometer techniques were developed for determination of the complex permittivity of liquids and solids over a wide temperature range in the frequency range from 10.0 GHz to 18 THz. Complex permittivities have been measured at 94 GHz by transmission through a dielectric slab. Errors reported in relative permittivity and loss tangent are 0.2 and 2.5 percent, respectively.     

A Novel Calibration Algorithm for a Special Class of Multiport Vector Network Analyzers

A new error model for a special class of multiport vector network analyzers (VNAs) is presented in this paper. This model can be applied to multiport network analyzers with noncomplete reflectometers, i.e., when the measurement of the incident waves at each port is not always available. The method used to compute the error coefficients proposed here is based on a compact and easy formulation. This method is an extension of the already existing general theory for complete reflectometer multiport network analyzers. Furthermore, the new error model generalizes the theory for three-sampler two-port VNAs. The proposed model has been tested against the complete reflectometer one and exhibits the same accuracy level.     

Advances of Reflectometry on Tore-Supra: From Edge Density Profile to Core Density Fluctuations

A new generation of broadband reflectometers based on solid state components has been installed on Tore-Supra. With reflectometers covering the range 50 to 155 GHz, the whole plasma can be scanned. Two X-mode reflectometers (V and W band) are dedicated to electron density profile measurements. Diagnostics are operated routinely with an automatic algorithm to reconstruct the density profile from shot to shot. A fast acquisition mode is available to study short time plasma evolution. For turbulence and transport studies, a third reflectometer operating between 105 and 155 GHz measures density fluctuations in the plasma centre. Sensitive to large scales, it can retrieve density perturbation due to MHD modes or broadband turbulence. Lastly, a Doppler reflectometer, based on back scattering, is being installed for measuring the poloidal rotation and fluctuations amplitude at higher wave numbers.     

Approximate formulas for the far field and gain of open-ended rectangular waveguide

Approximate formulas are derived for the far field and gain of standard, open-ended, rectangular waveguide probes operating within their recommended usable bandwidth. (Such probes are commonly used in making near-field antenna measurements.) The derivation assumes first-order azimuthal dependence for the fields, and anE-plan pattern given by the traditional Stratton-Chu integration of the transverse electric (TE_{10}) mode. TheH-plane pattern is estimated by two different methods. The first method uses a purelyE-field integration across the end of the waveguide. The second, more accurate method approximates the fringe currents at the shorter edges of the guide by isotropically radiating line sources. The amplitude of the line sources is determined by equating the total power radiated into free space to the net input power to the waveguide. Comparisons with measurements indicate that forX-band and larger waveguide probes, both methods predict on-axis gain to about 0.2 dB accuracy. The second method predicts far-field power patterns to about 2 dB accuracy in the region90degoff boresight and with rapidly increasing accuracy toward boresight.     

Thermo-optical modulation at /spl lambda/=1.5 /spl mu/m in an /spl alpha/-SiC-/spl alpha/-Si-/spl alpha/-SiC planar guided-wave structure

A planar waveguide based on an amorphous silicon-amorphous silicon carbide heterostructure is proposed for the realization of passive and active optical components at the wavelengths /spl lambda/=1.3-1.5 /spl mu/m. The waveguide has been realized by low temperature plasma enhanced chemical vapor deposition and is compatible with the standard microelectronic technologies. Thermo-optical induced modulation at /spl lambda/=1.5 /spl mu/m is demonstrated in this waveguide. Numerical simulations predict that operation frequencies of about 3 MHz are possible. The measurements have also allowed the determination of the previously unknown thermo-optical coefficient of undoped amorphous silicon at this wavelength.     

National standards and standard measurement systems for impedance and reflection coefficient

A review of the current status of national impedance and reflection standards at RF and microwave frequencies is presented. In the first part, various proposals are examined for realizing a given characteristic impedance, or a calculable value of reflection coefficient, both in coaxial lines and in hollow rectangular waveguides. The second part describes measurement techniques and systems used in national standards laboratories to obtain the highest precision and accuracy. Included are techniques using admittance bridges, reflectometers, slotted lines, fixed probes, and network analyzer systems, in particular six-port devices.     

Theoretical and Experimental Analysis of a Waveguide Mounting Structure

The induced EMF method has been extended and applied to derive the driving-point impedance of a common waveguide structure used for mounting small microwave devices. An equivalent circuit is developed and discussed in detail. Theoretical impedance curves are presented demonstrating the circuit characteristics for various configurations of the mount. The driving-point impedance of this mount has also been considered experimentally. A novel measurement technique is used based upon the use of subminiature coaxial line to gain electrical access to the terminal pair located inside the waveguide. A model of the measurement circuit, which enhances the accuracy of the results, providing excellent agreement between the theoretical and measured values, is developed. The multilateral nature of the circuit allows consideration of the mount in the waveguide as an obstacle to any incident propagating mode. Some related measurements have been made using standard techniques for the H/sub 10/ mode. It is anticipated that this formulation will permit accurate design of many components which previously required empirical methods based on limited experimental data.     

High-accuracy finite-difference equations for dielectric waveguide analysis II: dielectric corners

For part I see ibid., p. 1210, 2002. We present a discussion of the behavior of the electric and magnetic fields satisfying the two-dimensional Helmholtz equation for waveguides in the vicinity of a dielectric corner. Although certain components of the electric field have long been known to be infinite at the corner, it is shown that all components of the magnetic field are finite, and that finite-difference equations may be derived for these fields that satisfy correct boundary conditions at the corner. These finite-difference equations have been combined with those derived in the previous paper to form a full-vector waveguide solution algorithm of unprecedented accuracy. This algorithm is employed to provide highly accurate solutions for the fundamental modes of a previously studied standard rib waveguide structure.     

Single-mode transmission selective integrated-optical polarisers in LiNbO3

Novel low-loss integrated-optical polarisers and analysers are reported for single-mode operation in LiNbO3 waveguides. The operation is based on the polarisation-selective transmission of short proton-exchanged waveguide channels filling discontinuities in Ti-diffused channel waveguides. Extinction ratios exceeding 32 dB are experimentally demonstrated with 0.8 dB excess loss.     

An Automatic Network Analyzer Using a Slotted Line Reflectometer

An automatic network analyzer (ANA) based on a 5-port reflectometer is presented. The measuring circuit consists of a slotted waveguide with sliding probe. A group of three fixed probes may be used instead. A microwave source, frequency counter, and power meter are used all controlled by a desktop computer. The theory is simple and the algorithm for obtaining the complex reflection coefficient from experimental data is fast. Some results are given for measurements earned out at X-band frequencies. Successful measurement accuracy is achieved with relatively noncomplex hardware. Multioctave bandwidth operation is expected for the proposed technique.     

Subtle Differences in System Noise Measurements and Calibration of Noise Standards

Stringent system requirements and the lack of accurate standards have been the source of many controversies in low-noise receiver measurements. Some of the critical measurement problems where subtle errors commonly arise, and their significance in automatic noise temperature monitoring systems have been investigated at L-, S-, and X-band frequencies. Problems of interest include the following: 1) difficulty in determining losses associated with low-noise systems and measuring equipment, 2) differences in noise figure measurements depending upon the use of gated or nongated receivers, 3) discrepancies in excess noise ratios of secondary standard argon gas tubes, 4) non-uniformity in the output noise power of such gas tubes due to critical coupling from the gas tube proper to the waveguide (or coaxial) output flange. As a partial solution to these problems noise standards at liquid nitrogen temperature in coax and waveguide were developed at the aforementioned bands.     

A Waveguide Impedance Meter for the Automatic Display of Complex Reflection Coefficient

A waveguide impedance meter has been developed, comprising some specially designed components and some components previously designed for other applications. When this circuit is used in conjunction with the X-band rapid sweep oscillator and suitable display and control circuits, the impedance focus of a waveguide component is automatically and rapidly measured and oscilIographically displayed in the reflection coefficient plane. A waveguide component having a 11/4-inch X 5/8-inch (large X-band) waveguide input port can be continuously measured throughout the frequency range extending from 8.5 to 9.6 kmc (12 per cent X-band). The bandwidth of the system is limited by the design bandwidth of the waveguide components. The plane of the impedance measurement may be referred to a plane internal to or external to the input port of the component under test. An expanded portion of the reflection coefficient plane may be displayed on the crt when small reflections are measured. The measurements of several representative impedances by the waveguide circuit were compared with slotted line measurements of these same components. For measurements of large reflections, standard /spl infin/ db swr full scale display, the maximum observed errors of the magnitude and phase of the reflection coefficient as measured by the waveguide circuit were 10 per cent and 5 degrees respectively. These maximum errors occurred for measurements performed at the ends of the 12 per cent frequency band. The average errors of the magnitude and phase of the reflection coefficient were 2.5 per cent and 2 degrees respectively. For measurements of small reflections, with the crt display of the reflection coefficient plane expanded to 6 db swr full scale, the maximum observed deviation of the waveguide circuit measurements from slotted line measurements was 0.5 db swr, and the average deviation was 0.2 db swr. The maximum errors again occurred at the ends of the 12 per cent frequency band.     

Analysis and design of a new type NRD-guide E-plane Y-junction circulator

A new type of E-plane Y-junction circulator has been developed based on the nonradiative dielectric waveguide (NRD-guide) in Ka- band. First, the circulation principle has been analysed by the theory of polarization wave. The field components in the nonreciprocal Y-junction zone and the conclusion of 60° phase difference between clockwise and counterclockwise rotating polarization modes has been obtained. And then, the resonant frequency of the non-dc biased triangle ferrite sample has also been calculated. This kind of structure is benefit to enlarge the bandwidth of the Y-junction circulator. As an example, we had designed and fabricated a circulator in Ka-band. Its operating frequency is 35. 1GHz, the 1.5dB insertion loss and 20dB isolation bandwidth is about 1. 0GHz.     

Procedure for phase discrimination applicable to homodyne reflectometers for plasma density measurements

A phase discrimination method for homodyne reflectometers applied to plasma density fluctuations is proposed and tested in the laboratory simulating the plasma fluctuations by a vibrating metallic mirror. Applicability conditions for plasma measurements are established. The method can be used in broadband operation. Application to density profile measurements is also possible.