Architectures are discussed in which power amplifiers are able to vary their performance characteristics in response to changing output power levels, frequencies, load impedance levels and linearity constraints. Necessary elements for implementation of such systems are reviewed, including sensors to detect changes in the external environment, actuators to adapt the amplifier characteristics, and algorithms to implement the associated control functions. An example is described of a power amplifier for cell phone applications that measures the load impedance provided by the antenna and varies its output impedance match accordingly, in order to preserve output power, efficiency and linearity as the external antenna is handled. The system automatically provides output tuning for a 1W linear amplifier, and can accommodate output standing wave ratios up to 8:1. It improves output power and efficiency by x2 in representative mismatch scenarios. The insertion loss of the system is comfortably low at 0.5dB.
Recent advances in electronic miniaturization have facilitated the design and development of a deployable, stand-alone sensor for battlefield RF (radio frequency) MASINT (measurement and signature intelligence). Recent results of a Phase I effort to assess battlefield RF signatures and compare sensor sensitivity, size, and number for optimal coverage will be presented. An RF sensor suite is being designed that will be networked for robust and redundant data gathering and wireless, stand-alone operation. The RF sensor will essentially function as a swept-frequency spectrum analyzer, measuring frequency content, amplitude and modulation characteristics over predetermined (or user programmable) bandwidths of interest. Recommendations and preliminary schematics for a compact, ruggedized, low-power RF sensor will be presented together with a design for a low-power centralized wireless network for data transfer and processing. The RF sensor technology developed in this effort will have a predominantly military application but should also find use in security and surveillance applications.
Analog-to-digital converters operating at unprecedented speeds and resolutions are presently under development using a combination of photonics and electronics techniques. These systems impose stringent performance constraints on the photoreceivers used for photonic - electronic conversion, particularly in regard to linearity and noise. Photodetectors must accommodate optical pulses with very high input powers iwthout saturation, and the pulse input energy must be accurately determined. This paper presents design considerations and simulations of photodetectors and associated preamplifiers to meet these goals.
Semiconductor electroabsorption modulator (EAM) is a promising alternative to lithium niobate modulator for digital and analog fiber optic links due to its inherent small size, high modulation efficiency, and potential of monolithic integration with other electronic and optoelectronic components. For high-speed application, the bandwidth of the lumped element EAM is known to be RC-time limited. To achieve an ultra large bandwidth in lumped element EAM, the modulation efficiency has to be greatly sacrificed. This is especially critical in analog operation where RF link loss and noise figure must be minimized. To overcome the RC bandwidth limit and to avoid significantly compromising the modulation efficiency, the traveling wave electroabsorption modulator has been proposed and experimentally investigated by several authors.
External modulation of cw laser radiation by multiple quantum well electroabsorption modulators will potentially play an important role in rf photonic links, especially at high microwave frequencies and millimeter waves. InAsP/GaInP MQW on InP and GaInAs/InAlAs MQW on GaAs modulators have been grown by MBE and fabricated into p-i-n modulators. Performance with -26 dB link efficiency without amplification, 5 dB insertion loss, 15 mW of optical power and 17 GHz bandwidth has been experimentally demonstrated. Extension to 100 GHz bandwidth with -39 dB link efficiency (without amplification) can be expected. Traveling wave modulators and on-chip impedance matching of p-i-n modulators have been designed, fabricated and evaluated. Traveling wave modulators with flat frequency response over 40 GHz have been experimentally demonstrated.
Formed in January 1995, WEST is a DARPA-supported consortium investigating technologies for implementing add-drop and cross-connect switches operating at 10 Gbit/s. Using wavelength division multiplexing (WDM), each fiber supports 40 Gbit/s (4 by 10 Gbit/s) aggregate bandwidth for SONET/SDH operation. Consortium members include Rockwell Corporation, Ortel Corporation, UCSB, UCSD, UCLA, and Caltech/JPL.
To implmenet millimeter wave photonic links using high speed optical modulators, bandwidth and modulation efficiency are important considerations. In this paper we discuss design and fabrication of novel traveling-wave multiple quantum well (MQW) electro-absorption modulator structures which can be used for wide-band applications, covering dc to 40 GHz or higher frequencies, that promise to provide better bandwidth and efficiency than conventional lumped modulators. From the microwave point of view, traveling wave modulators are constant impedance transmission lines, and are not limited by the RC roll off associated with modulator capacitance. Their sensitivity can be increased by increasing device length without significantly sacrificing the bandwidth. The principal bandwidth limitation comes from microwave loss. In this work, ridge co-planar waveguide structures were designed and fabricated to achieve good impedance matching, low microwave loss, low dispersion and reasonable phase velocity matching between lightwave and microwave. Two port measurements for these waveguides were performed up to 40 GHz with a network analyzer. The results show effective microwave index in the range of 4.5 to 3.6 (which is close to the effective index of the guided light wave), characteristic impedance around 30 Omega, microwave attenuation less than 6 dB/mm at 40 GHz and low dispersion. These characteristics are all promising for wide band high efficiency traveling wave modulators.
To implement millimeter wave photonic links using high speed optical modulators, RF input efficiency to the modulator is an important consideration. In this paper we discuss design and fabrication of ultra high speed multiple quantum well electro-absorption modulators for narrow band applications up to 40 GHz. In order to obtain higher RF efficiency at working frequencies of 20 GHz, 25 GHz and 40 GHz, modulators with monolithically integrated matching circuits were designed and fabricated utilizing co-planar waveguide MMIC technologies. Measurement results show excellent matching at specific frequencies with S11 of -16 dB for the 20 GHz devices, -20 dB for the 25 GHz devices and -36 dB for the 40 GHz devices. At least 6 dB of improvement on optical modulation efficiency can be expected over modulators without impedance matching.
To implement high-bandwidth optical analog or digital communication links based on optical modulators, ultrahigh performance modulator driver amplifiers are required. Design considerations for such amplifiers are discussed here. Designs are significantly different than for laser driver amplifiers. For modulator drivers, an emitter follower output stage is appropriate. Active pull-down circuitry is beneficial to reduce power dissipation in a digital driver. Inductive tuning is beneficial to extend frequency response. Flip-chip mounting is beneficial to reduce bonding parasitic capacitance. Examples are given for driver designs for 10Gbit/s digital applications, and for 20GHz analog links, employing GaAs/AlGaAs HBT technology.
Digital GaAs FET technology, with its high level of integration, can be employed in high-speed optical systems for optical detector amplifiers as well as for baseband processing. However, the WSi gate of the digital GaAs FETs results in high gate resistance, which increases the noise figure as compared to the Ti/Pt/Au `T' shaped gates used in microwave FETs. In high-speed optical receiver applications, the noise in the FET based amplifier dominates the entire system noise. In this work, we use the 1st and 2nd layer metal to reduce the gate resistance in order to improve the minimum noise figure through layout variations compatible with production digital technology.
The microwave power performance of AlGaAs/GaAs self-aligned HBTs from 10 to 35 GHz is described. A record value of 68% power added efficiency was obtained at 10 GHz. At 18 GHz, 16.3 dB associated gain was achieved with 1.83 W/mm power density and 40% efficiency. At 35 GHz, a 15 dB small signal gain was observed. The tested HBTs have 2 micron feature size. Further improvement is expected with optimization of the HBT structure.
AlGaAs/GaAs Pnp HBTs have the potential for high frequency performance approaching that of Npn HBTs. To achieve this performance, it is necessary to dope the base as heavily n-type as possible. This heavy base doping results in large degeneracy in the base, which reduces the heterobarrier to reverse injection of electrons from the base into the emitter. High A1 content in the emitter is desirable to maintain good injection efficiency. Incorporating a gradient in the base doping can introduce fields to sweep injected holes across the neutral base region, which reduces base transport time. DC and RF characteristics of Pnp HBTs with 40% and 75% Al in the emitter will be presented. ft of 17 GHz and fmax of 39 GHz has been achieved in 2 ?m x 11 ?m HBTs fabricated using a self-aligned ohmic contact process. Further improvement in performance should be possible.