Indirect optical control of microwave circuits using monolithic optically variable capacitors

Abstract

In this paper, we present an integrated circuit technology suitable for low-power bias-free optical control of microwave circuits and antennas. We have integrated miniature photovoltaic arrays with varactor diodes and thin-film resistors to form monolithic optically variable capacitors (OVC's). For the monolithic OVC described here, only 1.5 mW of optical power was required for more than 2 : 1 change in capacitance (0.9-0.4 pF). Optically controlled microwave circuits such as X-band analog phase shifters and tunable notch filters, which incorporated the monolithic OVC as the control element, were fabricated to demonstrate the potential of this technology. thereby, the microwave characteristics of the device. Since the optical signal performs both bias and control functions, there is no need for external bias circuitry and bias leads. Another advantage of indirect optical control is that, since the optical detection and microwave functions are separated, the optical and microwave components may be optimized independently of each other. Thus, it is possible to design circuits such that there is no RF penalty for using optical control and, at the same time, make the optical detection process very efficient so that minimum optical powers are required. Previously described work on indirect optical control (18)-(20) relied on commercially available components. These hybrid circuits were large in size, making them unsuitable for embedding in high-frequency circuits and antennas. Also, large-area PV arrays have slow transient response due to large junction capacitance. To address these issues, we decided to mono- lithically integrate miniature PV arrays with varactor diodes to form optically variable capacitors (OVC's), as proposed by Toyon Research Corporation in (18). The size of the integrated OVC's is substantially smaller than the corresponding hybrid implementations. The miniature PV arrays have yet another ad- vantage—ease of coupling light into the device. The array size can be matched to the core size of commercially available mul- timode fibers, thus enabling a simple butt-coupling scheme to illuminate the PV array. Large arrays need beam expanders and additional optics for efficient illumination of the array active area, making the light coupling more difficult and expensive. This paper describes the design and testing of integrated OVC's that were monolithically fabricated using gallium ar- senide (GaAs). The miniaturization of the PV arrays and their integration with the varactor diodes posed some interesting challenges that are addressed here. We have designed, fab- ricated, and tested microwave circuits that incorporate the OVC's, such as tunable notch filters and phase shifters. The performance of these circuits demonstrates the potential of the proposed monolithic OVC technology for the indirect control of microwave circuits. The circuits needed no external bias and were controlled using low optical powers ( mW). Also, there was no degradation in microwave performance due to optical control. Preliminary measurements of the transient response of the monolithic OVC were also made, and they indicate that the switching speed of the monolithic OVC is adequate for optical control applications.