Straight-tip optical fibre up-tapers for single-mode hardwareapplications

Reports a practical optical fibre up-taper unitary structure consisting of a fibre pigtail followed by a short up-taper followed by a uniform cylindrical section (straight tip). This self-aligning ×5 beam expansion element exhibits an insertion loss of <0.1 dB and allows for simple mechanical mounting and beam alignment     

Measurement-based estimates of bit-error-rate performance in urbanLOS microcells at 900 MHz

Cumulative distribution estimates are presented for the one-second-average bit-error-rate (BER) which would be experienced on UHF links in a microcellular environment. It is assumed that users (subscribers) are moving at speeds ranging from quasi-stationary to 96 km/h. The authors concentrate on lineal microcells, in which there is always a line-of-sight path between the microcell and subscriber antennas, in addition to road-reflected and/or building-reflected rays. Using extensive UHF data collected in Manhattan, the signal and interference levels are characterized, point-by-point, for a subscriber moving in a lineal microcell. Combining these measurement-based characterizations with a mathematical model of modem performance, the cumulative distribution of the BER is computed. For moving subscribers, simple diversity reception gives large benefits in grade-of-service (percentage of time that the BER is below a specified threshold value) and uplink and downlink transmission qualities tend to be similar, with the latter being somewhat better. Performance estimates based on a multiray model are generally similar to those based on measured data and tend to be conservative     

Precise Incident Power Density Pattern Measurement through Antenna Pattern Deconvolution

Future cellular and fixed wireless loops (FWL) systems employing highlydirective base stations antennas (5 to 8 degrees beamwidth) and moderatelydirective subscribers' antennas (15 to 25 degrees beamwidth) offer very largecapacity due to reduced interference. An important property of the environmentof such systems is the interference caused by scattering of the signal froma subscriber transmitter into directions other than the direct line of sightbetween the subscriber and the base station. In particular, for multibeam basestation applications scattering could arrive at the base station in thedirection being used by a different beam, resulting in interference that isdifficult to reduce by normal nulling techniques. Thus interference can behighly dependent on the incident power density pattern (IPDP) caused byscattering of the signal radiated from the subscriber. We discuss herein theuse of a uniformly illuminated array accompanied by electric fielddeconvolution to measure the crucial IPDP with the same performance as a lowsidelobe array of the same size. The mathematical correction technique usesdeconvolution of the measured complex electric field pattern with that of theantenna in free space by means of the Fourier Series and limiting the rangeof Fourier coefficients to those that are not negligible in the free spacepattern. Application of the technique to an experimental uniform array witha 2.5 degree azimuthal beamwidth shows the practicality of the deconvolutionwith real antennas in real environments. The improved resolution and accuracyprovided by Taylor weighting versus unweighted deconvolution when trying tomeasure weak scattered components in the presence of a nearby strong specularcomponent is demonstrated. The IPDP was measured from many sites surroundinga suburban base station. A plot of the cumulative distribution of the ratioof the widely scattered power to that within a prescribed beamwidth summarizesthe result of using the deconvolution technique on this experimental data.     

Single-mode optical fibre tapers for self-aligned beam expansion

We demonstrate that optical fibre tapers can be utilised as a new means for achieving self-aligned beam expansion in single-mode fibres. These devices, which have a standard single-mode geometry at one end and gradually increase to a core size in the order of 100 ?m at the other end, have greatly reduced sensitivities to lateral and axial displacements and an excess coupling loss between two tapers of less than 0.1 dB.     

Design of rectangular horn arrays with oversized aperture elements

The possible existence of blind spots, due to forced aperture resonances, is demonstrated theoretically and experimentally at 14 GHz in rectangular grid arrays of tapered rectangular horn elements with oversized (overmoded) apertures. The dominant contributor to these blind spots is the resonance of the equivalent transverse magnetic (TM_12) waveguide mode. The numerical results based upon our approximate methods of formulation and solution of the array boundary value problem agree well with experiments. A method for eliminating the blind spots by proper taper design is presented and experimentally verified. The analytical methods used in this work could possibly be extended to conical horus and other tapered-structure arrays as well as other periodic grid geometries. New simple and accurate formulas for the zeros of Bessel and Legendre functions and their derivatives are obtained as a result of this analysis.     

Multipath Outage Performance of Digital Radio Receivers Using Finite-Tap Adaptive Equalizers

Recent analysis/simulation studies have quantified the multipath outage statistics of digital radio systems using ideal adaptive equalization. In this paper, we consider the use of finite-tap delay line equalizers, with the aim of determining how many taps are needed to approximate ideal performance. To this end, we assume anM-level QAM system using cosine rolloff spectral shaping and an adaptive equalizer with either fractionally spaced or synchronously spaced taps. We invoke a widely used statistical model for the fading channel and computer-simulate thousands of responses from its ensemble. For each trial, we compute a detection signal-to-distortion measure, suitably maximized with respect to the tap gains. We can thereby obtain probability distributions of this measure for specified combinations of system parameters. These distributions, in turn, can be interpreted as outage probabilities (or outage seconds) versus the number of modulation levels. A major finding of this study is that, for the assumed multipath fading model, very few taps (the order of five) are needed to approximate the performance of an ideal infinite-tap equalizer. We also find that a simple, suboptimal form of timing recovery is generally quite adequate, and that fractionally spaced equalizers are more advantageous than synchronously spaced equalizers with the same number of taps. This advantage is minor for rolloff factors of 0.5 and larger but increases dramatically as the rolloff factor approaches zero.     

Laser-to-fiber coupling via optical fiber up-tapers

A method for coupling single-mode fiber to laser diodes with high efficiency and greatly reduced alignment sensitivities is discussed. It consists of a spherical ruby lens and a single-mode optical fiber up-taper. Using a double-channel planar buried heterostructure (DCPBH) laser, more than 60% of the light from the lens is coupled into a single-mode fiber via the taper. The lateral and axial tolerances of the lens-taper arrangement are reduced by factors of 20 and 1000 times, respectively, compared with laser coupling to a lensed single-mode fiber. These relaxed alignment tolerances have the potential to allow the taper to interface with a hermetically-sealed laser-lens package via a window and a connector on the package exterior     

Optical fiber up-tapers modeling and performance analysis

An extensive CRAY-computer-based simulator using the propagating beam method for the accurate evaluation of the performance of optical fiber up-tapers is discussed. Up-tapers with and without straight tips and with arbitrary index profiles and tapering functions can be evaluated. Results of the evaluation of various 1.5-cm-long optical up-tapers followed by a straight tip and drawn from special preforms with various index profiles are quite encouraging. They show that beam expansion ratios in the range of 5-10 are feasible with a corresponding insertion loss per taper of less than 0.01-0.025 dB. Tapers shorter than 1.5 cm with insertion loss in the same range are also feasible but have slightly smaller beam expansion ratios     

A Fast-Switching Low-Loss 12-GHz Microstrip 4-PSK Path Length Modulator

A 4-PSK microstrip modulator operating at data rates of up to 800 Mbits/s at 12 GHz is described. The circuit is made of two cells in series. Each cell consists of a 3-dB branch-line hybrid coupler and two BTL p-i-n diodes. One cell provides 0° or 90° phase shifts, and the Other 0° or 180° phase shifts, so that four carrier phase values are obtained by appropriately exciting the two cells. The switching time of each cell is 200 ps. Simultaneous switching of both cells increases the switching time to a maximum value of 400 ps. The phase waveforms are nearly rectangular at the above-mentioned data rates. RF insertion loss is 1 dB ± 0.1dB for the four phase values over the 11.7-12.2 GHz frequency band.