Irl W. Smith

GaAs FET device and circuit simulation in SPICE

We have developed a GaAs FET model suitable for SPICE Circuit simulations. The dc equations are accurate to about 1 percent of the maximum drain current. A simple but accurate interpolation formula for drain current as a function of gate-to-source voltage connects the square-law behavior just above pinchoff and the square-root law for larger values of the drain current. The ac equations, with charge-storage elements, describe the variation of the gate-to-source and gate-to-drain capacitances as the drain-to-source voltage approaches zero and when this voltage becomes negative. Under normal operating conditions the gate-to-source capacitance is much larger than the gate-to-drain capacitance. At zero drain-to-source voltage both capacitances are about equal. For negative drain-to-source voltages the original source acts like a drain and vice versa. Consequently the normally large gate-to-source capacitance becomes small and acts like a gate-to-drain capacitance. In order to model these effect it is necessary to realize that, contrary to conventional SPICE usage, there are no separate gate-to-source and gate-to-drain charges, but that there is only one gate Charge which is a function of gate-to-source and gate-to-drain voltages. The present treatment Of these capacitances permits simulations-in which the drain-to-source voltage reverses polarity, as occurs in pass-gate circuits.

The Multioscillator Ring Laser Gyroscope

Abstract The multioscillator (or four-frequency) ring laser gyroscope is discussed from both a theoretical and a practical point of view. Fundamentals of device operation are presented, important nonideal behaviors (error sources) are discussed and analyzed from first principles, typical multioscillator gyroscopes are described, and samples of representative data from developmental instruments in our laboratories are reviewed. A key to the development of practical multioscillator instruments has been the introduction of nonplanar ring resonators. The theoretical formalisms (geometric and wave optic) necessary for understanding the properties of nonplanar ring resonators, and nonplanar gyroscopes, are derived and discussed in detail. Much of the material presented is new.

On charge nonconservation in FET's

It is shown that source and drain charges are not state variables in an FET, especially for source-drain voltages near zero. This behavior, observed in the model proposed in [2], is a genuine manifestation of the physics of FETs and is not a sign of improper behavior in a circuit simulator. However, if necessary, one may construct a model having these charges as state variables by introducing a current generator that transfers charge from source to drain internally without producing extra currents in the leads of the device.

Channeling, exponential tails, and analytical modeling of Si implants into GaAs

The depth distribution N(x) of Si/sup 29/ ion beam implanted into GaAs for a wide range of implant doses, energies, and SiN randomizing-layer thicknesses is discussed. It is found that the variation of implant angle with position on the wafer (typically encountered in scanned-ion-beam implantation systems) results in significant channeling-induced nonuniformity of the doping profiles, even in some cases where a randomizing layer was used. Because of the existence of exponential tails in virtually all implants, an extension of the standard LSS theoretical profile is necessary for accurate characterization of the implants and prediction of the device parameters. The measured profiles were fitted to two different modified-Gaussian analytical forms: the Pearson four-parameter form, which offers excellent flexibility in fitting arbitrary distributions but is relatively devoid of physical content, and a new three-parameter form that is an extension of a Gaussian to profiles hyperbolic in log N. In almost all cases, the hyperbolic-Gaussian form fits the data as well as the Pearson form, and the hyperbolic-Gaussian form is simpler. Parameters characterizing the implants for both types of fit are presented. >

Optical Resonator Axis Stability And Instability From First Principles

It is shown that optical resonators may be broadly divided into two classes based on their mirror count and other properties, and that the two classes exhibit very different axis stability properties. When a resonator of one class is slightly misaligned, its optic axis (and hence its input axis for rotation sensing) remains close to its nominal position independently of the focussing action of the resonator optics. In contrast, when a resonator of the other class is slightly misaligned, the displacement and reorientation of its optic axis is restrained only by the focussing optics. Hence resonators of this second class are restricted in the range of mirror radii of curvatures they may employ. This has implications in noise and lockin performance for rotation sensors.

Coherent Beam Combining and Optical Space-Time Division Multiple Access

We have developed an approach to multiple-access lasercom that adopts the commercial paradigm of sharing the most expensive terminal resources among all users. Space-time division multiple access (STDMA), analogous to an optical space-time switch, hops the transmit beam and receive direction among multiple users and exchanges data while the beam dwells on a user. A key enabler of STDMA is electronic beam steering using liquid crystal optical phased arrays, which provides fast, precise, and agile beam re-pointing. We have built the first optical STDMA terminal, combining beam hopping between remote terminals with coherent combining of both transmit and receive apertures, which is an effective means for increasing antenna gain in systems for which large aperture components are impractical. Coherent beam combining provided the expected increase in antenna gain, and the terminal was found to re-point the beam among users quickly and precisely enough to suffer only minor throughput degradation. Communications test were performed using 10 Gb/s Ethernet for a single-aperture configuration. Performance is presented as a function of angle scan speed and STDMA dwell time per remote terminal. The results suggest that STDMA is a viable technology for supporting multiple-access space-based laser communication.

10 Gb/s Ethernet laser communications using optical space-time division multiple access

The use of free-space laser communications for military applications will continue to increase because of its advantages with respect to bandwidth, low probability of detection, interception and jamming, and absence of spectrum restrictions. Multiple access, the ability of a terminal to simultaneously provide network access for multiple users, will become increasingly important as the number of users increases. Decreasing cost, size, weight, and power per user is essential to increasing the number and types of lasercom users. We have developed an approach to multiple-access lasercom that accomplishes this using the commercial paradigm of sharing the most expensive terminal resources among all the users. Space-time division multiple-access (STDMA), analogous to an optical spacetime switch, hops the transmit beam and receive direction among multiple users and exchanges data while the beam dwells on a user. A key enabler of STDMA is electronic beam steering, which provides fast, precise, agile beam re-pointing. We present the results of laboratory tests of STDMA for two users. The dead-time in re-pointing the beam between users has been measured under different conditions and found to be as low as 10 ms. This is low enough to enable efficient bandwidth utilization. 10 Gb/s Ethernet traffic has been transmitted to the terminals to directly measure bandwidth efficiency, which varied from ∼85% to 98% depending on the transmission time per remote terminal.

Dynamic modeling of Optical Phased Arrays

Optical Phased Arrays (OPAs) based on liquid crystals (LCs) have useful properties for all-electronic beam steering. Typical devices contain hundreds to thousands of individually-controlled phase shifters. Their behavioral dynamics are modeled in this paper starting from the physics of the LC in a single phase shifter. Results relevant to two useful device types are presented. The first is an endless phase shifter, suitable for adjusting and maintaining coherence throughout a multiple-subaperture optical beamforming (Tx or Rx) system. The second is a beam steerer, also applicable to such systems. Throughput, phase lag, and beam-shape modifications are found and compared with experiment. A simple heuristic form describing the performance, suitable for systems-engineering use, is also presented.