Optoelectronically-pulsed antennas: characterization andapplications

A brief tutorial on the picosecond photoconductive effect is given. The use of picosecond optoelectronics for the characterization of broadband antennas is described. In particular, the transient radiation properties of equiangular-spiral and exponentially tapered coplanar-strip antennas are discussed. The transient radiation behavior and the polarization and radiation patterns of these antennas are easily determined with this measurement technique, without the need for anechoic chambers. Applications of picosecond-duration transient electromagnetic radiation to filter measurements, materials measurements, and scattering studies are discussed     

Broad-band microwave measurements with transient radiation fromoptoelectronically pulsed antennas

A broadband microwave measurement technique based on picosecond transient radiation from optoelectronically pulsed antennas is described. It is performed with exponentially tapered coplanar stripline antennas which are integrated with the photoconductive devices used for ultrafast pulse generation and sampling. The signal analysis required for deriving the desired physical properties from the measured time-domain waveforms is discussed. This is a coherent technique that independently determines both the real and the imaginary parts of the dielectric constants of materials, from 10 to 130 GHz, in a single experiment. Some representative results are presented     

A novel 24-GHz bandwidth coaxial probe

An electrical probe based on specially designed coaxial tips is shown to have 24-GHz bandwidth. It can be used to test high-speed signal propagation on planar or nonplanar chip or package interconnection structures with signal/ground pads as small as 50 μm. The detailed fabrication procedure, characterization, and use of the probe are presented. A variation of the design has 500-Ω input impedance and a bandwidth of 19 GHz     

Doping-induced bandwidth enhancement in metal-semiconductor-metal photodetectors

A novel technique is demonstrated for enhancing metal-semiconductor-metal photodetector bandwidths without reducing responsivity. The authors have observed bandwidth increases of up to 10 GHz from control to enhanced photodetectors which corresponds to about 70% enhancement. This technique involves modifying the internal electric field structure, by introducing a buried n-type doped layer that is completely depleted, to reduce the relatively long hole transit time.<>     

Effect of substrate thickness on radiation properties of coplanar strip antennas

The effect of substrate thickness on the broadband (15-150 GHz) radiation characteristics of exponentially-tapered coplanar-strip antennas is explored using picosecond photoconductive pulse generation and sampling.<>     

Broad-band characterization of millimeter-wave log-periodicantennas by photoconductive sampling

Results of photoconductive sampling measurements used to characterize millimeter-wave log-periodic antennas with continuous, simultaneous frequency coverage from 10 to 300 GHz are presented. Polarization properties are investigated employing wire-grid polarizers. This study reveals new information on structure resonances and antenna polarization     

Single and double mode-locked ring dye lasers--Theory and experiment

A computer model is made of the colliding-pulse ring dye laser for mode-locked production of short pulses. Initial conditions of either noise or a small CW oscillation eventually result in the same final solution. Resolution is 10 ps in the model, and a digital filter represents dispersion. A gain-sharing long pulse mode often results, but large effective absorption cross section in the mode-locking dye and optimum separation of the two dyes (one quarter of the total resonator length) favor the desired short pulse formation. The model does not include the transient grating effect. Experiments with a ring laser using rhodamine 6G and DODCI confirmed the parameters important to the two regions. Double mode locking, in which the mode-locking dye also lases, was also studied both in the model and experimentally. It is not yet clear if the advantages of colliding pulse mode locking apply to double mode locking.     

Investigation of Absolute Photonic Band Gaps in Two-dimensional Dielectric Structures

Abstract

We examine the photonic band structure of two-dimensional (2D) arrays of dielectric holes using the coherent microwave transient spectroscopy (COMITS) technique. Such 2D hole arrays are constructed by embedding low-index rods (air) in a dielectric background of higher-index Stycast material (n = 3·60). The dispersion relation for electromagnetic wave propagation in these photonic crystals is directly determined using the phase sensitivity of COMITS. We find that both the square and triangular lattice structures exhibit photonic band gaps that are common to both polarizations for all wave-vectors along major symmetry axes. In addition, the connectivity of the high-index dielectric and the opening of a large gap for propagation with E field perpendicular to the hole cylinders are found to be important criteria for generating a large absolute photonic band gap.     

Modal dispersion and attenuation measurements of silicon nitrideand silicon oxynitride waveguides using a streak camera

By measuring short pulse propagation in silicon nitride and oxynitride waveguides fabricated on silicon, it has been possible to characterize both attenuation and group velocity dispersion accurately for the different modes that propagate. The method is self-calibrating, using the ratio between scattered light from forward and backward traveling pulses at a particular point on the waveguide. In principle, the group velocity data can be used to calculate the phase velocities for the different modes using known dispersion relations. The phase velocities were measured independently using a prism coupling technique, and the group velocities that could be derived from them matched the experimental data within a few percent. The resolution of the technique is limited mainly by amplitude fluctuations of the laser. The full-width-half-maximum resolution was at best about 12 ps but should be extendable to better than 5 ps     

Characterization of quasi-optical filters with picosecond transientradiation

The broadband microwave transmission properties of quasi-optical components are characterized with optoelectronically generated, picosecond transient radiation. The polarization sensitivity of a lithographically fabricated wire grating, from 15-140 GHz, is presented. The complex transmission functions of two different uniform arrays of cross dipoles are measured and compared with theoretical predictions