Low drive voltage modulators with ultra low loss serpentine electrodes for electric field sensing with optical fibers James H. Cole*, Marta M. Howerton*, Robert P. Moeller*, John Niemel* *Code 5651, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, District of Columbia 20375
Luis Cundin+ and Chris Sunderman+ +SFA Inc.9315 Largo Drive West Suite 200, Largo, Maryland 20774
Abstract: A key analog fiber-optic link parameter for RF and microwave antenna remoting is conversion gain or loss, which is a measure of the output RF power to the input RF power at the modulator. The link conversion loss can be minimized by using high-power lasers, modulators with low drive voltages, and sensitive photodetectors. It is highly desirable to achieve link gain without RF amplification. This has not been possible for broadband operation due to the required modulator drive voltages. This paper reports on the development of optical waveguide modulators that can achieve subvolt drive voltages to frequencies of approximately 20 GHz. Long interaction length, multipass LiNbO3 serpentine modulators have been fabricated employing low optical loss compact 180o turns. A record minimum RF loss achieved with a novel bridge structure electrode and used in these high performance modulators is presented. These modulators when connected to conventional antennas, operate as optical fiber electric field sensors. Keywords: lithium niobate, traveling-wave electrodes, integrated optics, fiber optic electric field sensors
1. Introduction High frequency analog fiber-optic links are needed for the transmission of radio frequency (RF) signals for military and commercial applications, including satellite communications systems, phased array antenna systems, and cell phone antenna remoting. Compared to coaxial cables, optical fibers offer immunity to electromagnetic interference and low propagation losses with nearly unlimited bandwidth. A key analog fiber-optic link parameter is conversion loss, which is a measure of the output RF power to the input RF power at the modulator. The link conversion loss can be minimized by using high-power lasers, modulators with low drive voltages, and sensitive photodetectors[1]. This paper reports on the development of optical waveguide modulators that can achieve subvolt drive voltages to frequencies of approximately 20 GHz. The intention is to use these modulators to attain zero link conversion loss without RF preamplification. A previous report of subvolt drive voltages employed digital signals with a complementary voltage drive[2]. Our goal is to obtain subvolt drive voltages so that an antenna could directly drive a modulator with no RF amplification. In order to use the dual-drive configuration of reference 2 for our application, the power from the antenna must be split into two signals, halving the power to each electrode. Furthermore, one of the signals needs to be phase shifted by 180 degrees. The required broadband phase shifters are not readily available at the frequencies of interest and their insertion loss would directly increase the link conversion loss. We employ LiNbO3 as the substrate material because it is a mature technology; it has good long-term stability, low optical loss, a strong electrooptic coefficient that leads to low drive voltages, and the capability to operate at high frequencies. The technology developed at the Naval Research Laboratory has resulted in low drive voltages for high-speed packaged LiNbO3 modulators in a single-pass configuration (5 V at 40 GHz) [3]. We have also demonstrated subvolt drive voltages in a reflection modulator to 0.5 GHz [1]. To achieve broadband performance with subvolt drive voltages to 20 GHz, a novel serpentine design that allows for very long interaction lengths has been developed. Practical implementation of the serpentine structure requires both low optical loss compact 180o waveguide turns, and traveling wave electrodes with low RF losses[4]. We also report improved electrical losses for wide gap coplanar waveguide (CPW) electrodes and a new push-pull bridge electrode structure with record low rf losses. *
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17th International Conference on Optical Fibre Sensors, Marc Voet, Reinhardt Willsch, Wolfgang Ecke, Julian Jones, Brian Culshaw, eds., Proceedings of SPIE Vol. 5855 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.623602
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2. Background Figure 1 shows a schematic diagram of our conventional single-pass Mach-Zehnder interferometric high-speed modulator. The interferometer consists of optical waveguides that are formed by Ti diffusion in LiNbO3, over-coated with a SiO2 buffer layer and Au electrodes. Long electrodes reduce the drive voltage Vʌ ҏat dc by increasing the interaction length, but they must have low RF loss to minimize the unavoidable increase in Vʌ with frequency. Very thick (20 to 45 µҏm) traveling wave electrodes are used to minimize electrode losses and to match the velocities of the microwave and optical signals, which is essential for broadband operation.
Fig.1 (a) Top view of a single-pass traveling wave modulator in LiNbO3 with interaction length L; (NOT TO SCALE) (b) Cross-section of the interaction region with conventional CPW electrodes. (NOT TO SCALE)
A schematic diagram of our new design for a traveling wave modulator that uses compact 180o turns is shown in Fig. 2a. The approach is to increase the interaction length by introducing multiple passes on the same chip that previously contained only one pass. By using a 3-pass device with three times the physical length, for example, the dc Vʌ is reduced by a factor of three.
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Fig. 2. (a) Top view of a three-pass serpentine traveling wave modulator in lithium niobate; (NOT TO SCALE) (b) Photograph of conventional CPW electrode in bend section (32 µm thick, 25 µm gap)
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Since conventional semicircular turns require too much real estate (with bend diameters exceeding 2 cm for
