Low-Power Indirect Optical Reactance Control using

Electronic control of microwave antennas and circuits is usually based on the use of reverse biased semiconductor junctions. Optical control of such circuits has been by directly illuminating the active (depletion) regions of the semiconductor junctions. In this paper we report indirect optical control of microwave antennas and circuits using a photovoltaic array that generates a light dependent reverse bias voltage across a conventional varactor diode. This technique requires very little optical power, and allows for the independent optimizing of both varactor and photodetector performance. We report a monolithic implementation of the Optically Variable Capacitor (OVCTM) circuit in GaAs using a novel lateral oxidation technique for device isolation. Introduction Tunable antennas have tremendous potential for use in shared aperture or frequency agile antenna arrays. One method that has been demonstrated [1] to be useful for implementing tunable antennas is the use of embedded reactive loads such as varactor diodes within the radiating structure. Thus by changing the value of the reactive loads the radiation characteristics, return loss, frequency of operation, etc. may be altered. Optical control of varactor diode reactance for reconfigurable antennas offers all of the advantages normally associated with optical links—low-loss, lightweight, immunity to noise, isolation from RF circuit—but has the additional advantage of extremely low optical power requirements as compared with other reconfigurable antenna technologies. Varactor diodes may be controlled optically using two techniques: Direct Control, or Indirect Control. In direct control, the active region of the device is illuminated with the optical control signal. The same device thus performs both optical and microwave functions, which often involve contradictory design requirements. The maximum achievable capacitance tuning range is also limited for a given range of illumination intensities. Illumination of the varactor diode also leads to a reduction in the Q-factor of the diode. An alternative scheme is indirect control, where the optical control signal is first converted to a suitable electrical form by a dedicated detector. The electrical control signal governs the bias point of the varactor diode which is part of the microwave network. In the approach used here, the optical control signal is converted to a light dependent voltage by a miniature photovoltaic (PV) array. The photovoltaic array comprises of Schottky or PNjunction diodes connected in series such that the open circuit voltages add up. The voltage developed across the PV-array reverse biases the varactor diode and hence controls the junction capacitance, as shown in figure 1a. Since the reverse biased varactor draws a small current, the voltage generated by the PV-array is essentially the open circuit voltage. Using this technique it is possible to obtain a larger swing in the voltage (and thereby in varactor capacitance) by simply using more diodes in series in the PV-array. The design is simplified because there are no microwave performance requirements on the PV-array and no optical functions to be performed by the varactor diode. Two circuits for indirect optical control are shown schematically in figure 1. Figure 1a illustrates the basic concept, as described above. Figure 1b is a modification by Toyon personnel whereby the optically-induced bias voltage does not appear at the output terminals of the circuit, which would be connected to the RF circuit. This would be especially useful if multiple programmable reactances are required. This Optically Variable Capacitor (OVCTM) circuit is made possible by the low drive currents required by the varactors (a negligible leakage current), which enables an RF choke resistor to be placed in the bias loop to present a high impedance to the RF circuit. In both cases a very large shunt resistor is required in parallel with the PV array to improve the induced voltage swing under low bias conditions. photodiode cascade v a ra ct o r light to external circuit v a r a ct o r ph ot od io de c as c a de light v a r a ct o r to external circuit Figure 1 (a) Basic concept of indirect optical control of varactors. (b) Toyon OVCTM circuit which isolates the optical bias from the RF circuit. In both cases a large shunt resistor is required in parallel with the photovoltaic cascade in order to realize maximum voltage swing Photovoltaic Arrays The first step in the fabrication of monolithic OVC's was the fabrication and testing of miniature photovoltaic arrays. The photovoltaic array had to be small so that the entire OVC could be embedded within a high frequency antenna. A prototype PV-array was implemented by connecting Schottky diodes in series by airbridges. The Schottky diodes were made on ntype GaAs with epilayers grown by MBE. The diodes were isolated by etching mesas down to the semi-insulating substrate around each device. The turn on voltage for the array is approximately 7 volts corresponding to about 0.7 volts per Schottky diode as expected. Early prototypes did not generate any open circuit voltage, and a close examination of the I-V curves for various levels of illumination showed photoconductor like behavior instead of photovoltaic behavior. This was identified to be a result of leakage through the substrate under illumination, and demonstrated the inadequacy of mesa isolation. Schottky Contact n GaAs n + GaAs Al2O3 Insulating Layer Semi-Insulating GaAs Ohmic Contact Schottky Contact n GaAs n + GaAs Al2O3 Insulating Layer Air Bridge From Previous Device