Alfred P. DeFonzo

Theory of hot carrier effects on nonlinear gain in GaAs-GaAlAs lasers and amplifiers

A concise and straightforward model of nonlinear grain based on the carrier heating effect in semiconductor lasers is presented. The problem is formulated using the density matrix approach and includes a priori the effect of free-carrier absorption. Coupled field-medium equations involving photon densities, carrier densities, and carrier temperatures are derived using the results of the density matrix method. The propagation of ultrashort pulses in laser amplifiers is studied and a qualitatively new model along with results on the transient gain recovery dynamics are presented. The model accounts for the wavelength dependence of the asymmetric part of the nonlinear gain observed in direct mixing experiments observed in semiconductor lasers. >

Optoelectronic transmission and reception of ultrashort electrical pulses

We report on the recent advances in using integrated planar antenna technology to photoconductively generate and detect picosecond radiation. Detection of a single pulse of picosecond duration has been achieved using a coplanar antenna structure fabricated on a radiation‐damaged silicon‐on‐sapphire substrate.

Transient response of planar integrated optoelectronic antennas

Broadband tapered slot antennas monolithically integrated on ion damaged silicon‐on‐sapphire substrates are driven by picosecond photoconductivity to generate and detect millimeter waves. The time‐dependent electromagnetic impulse response of these transceivers is modeled by relating the antenna structure and the shape of the exciting pulse. The far‐field response is observed to consist of a traveling‐wave component and a standing‐wave component, which is also predicted by the model.

Gain nonlinearities in semiconductor lasers and amplifiers

A concise and fundamentally straightforward physical model that accounts for the nonlinear gain in both semiconductor amplifiers and lasers is presented. Calculations based on this model yield results that agree very well with observed transient gain recovery dynamics in semiconductor laser amplifiers. The value of the symmetric steady‐state gain suppression factor is found to be β=1.67×10−23 m3 in good agreement with experiment. The model accounts for a wavelength dependence of the asymmetric part of the nonlinear gain observed in direct mixing experiments observed in semiconductor lasers.

Gain suppression in semiconductor lasers: The influence of dynamic carrier temperature changes

An analytic expression for the gain suppression coefficient of semiconductor laser in terms of carrier temperature and other material parameters is presented. The expression is derived from a density matrix formulation of gain in diode lasers based on a dynamic carrier heating model. We find that in the single mode approximation the theoretical estimate of the gain suppression coefficient is of the order 10−23 m3 in agreement with experimental values. This supports recent direct experimental demonstrations that dynamic carrier heating, rather than spectral hole burning provides the dominant contribution to nonlinear gain in semiconductor lasers.

FAR-FIELD CHARACTERISTICS OF OPTICALLY PULSED MILLIMETER WAVE ANTENNAS

We describe a novel technique for measuring transient far‐field radiation patterns emitted from optically pulsed, broadband, integrated antennas using photoconductive sampling methods. These devices are capable of generating and radiating short electrical pulses which contain frequency components exceeding 80 GHz. The far‐field patterns in both the E and H planes are observed to consist of single, forward directed lobes which are shown to have a cosine‐squared dependence. In addition, there are no indications of any additional secondary sidelobes in either of the principal planes.

High power harmonically mode-locked cw-pumped Nd:YAG laser

A new cavity configured from commercially available parts including a harmonic mode locker has yielded stable 60-ps pulses and an average power of 14 W. Powers of up to 20 W are readily attained at the expense of pulse width. These are the largest peak powers yet reported for a single-lamp cw-pumped Nd:YAG laser.

FEMTOSECOND CHARACTERIZATION OF SEMICONDUCTOR-LASERS

A review of ultrafast techniques for characterizing wide modu-lation bandwidth semiconductor lasers is presented. The origin of nonlinear gain effects and their influence on the modulation bandwidth are investigated. The most recent experimental results and trends are discussed.

Characterization Of High Speed Radiative Effects In Interconnects

The dominant factors which contribute to distortion of high speed electrical signals propagating along transmission line structures are discussed. Photoconductive sampling methods are used to characterize picosecond pulse propagation and dispersion properties of coplanar radiative electrical interconnects. In addition, we show that optoelectronic sampling techniques are an effective method for determining various characteristics of these antenna elements such as the far-field radiation patterns.

Optoelectronic Generation and Sensing of Millimeter Waves

Broadband tapered slot antennas, monolithically integrated with picosecond optoelectronic switches, are used to generate, control and sense millimeter wave radiation. These devices can be easily integrated with millimeter wave circuit components due to their planar geometry. This method of transmitting information eliminates many of the limitations inherent with transmission line structures and demonstrates the potential for controlling extremely wideband electrical signals.