Subvolt Broadband Lithium Niobate Modulators

M.M. Howerton, R.P. Moeller, and J.H. Cole
Optical Sciences Division

J. Niemel
SFA, Inc.

Introduction: Analog fiber-optic links have been extensively investigated for the transmission of radio frequency (RF) signals for military applications, including satellite communications systems and phased array antenna systems. This field is known as microwave photonics. Compared to coaxial cables, optical fibers offer immunity to electromagnetic interference and low propagation losses with nearly unlimited bandwidth. A basic fiber-optic link is composed of an optical source, an optical modulator that uses an electrical signal to modulate an optical signal, fiber for transmitting the modulated optical signal, and a photodetector. One key link parameter is conversion loss, which is a measure of the output RF power (optical power converted to electrical by the photodetector) to the input RF power at the modulator. It is important to minimize conversion loss. This can be done by using high-power lasers, modulators with low drive voltages, and sensitive photodetectors.¹

The objective of this program is to develop optical waveguide modulators that achieve subvolt drive voltages to 20 GHz, with the intention of using the modulators in RF links to attain zero conversion loss. We use lithium niobate as the substrate material because it is a mature technology with good long-term stability, and it has a strong electrooptic coefficient that leads to low drive voltages. Other advantages are its low optical loss and its capability to operate at high frequencies. Lithium niobate modulators are available commercially and are used in optical communications systems. The technology we have developed at the Naval Research Laboratory has been at the leading edge of the field, and has resulted in record low drive voltages for high-speed packaged, lithium niobate modulators in a single-pass configuration (5 V at 40 GHz).² We have also demonstrated subvolt drive voltages in a reflection modulator to 0.5 GHz.¹ To achieve subvolt drive voltages to 20 GHz in the current program, we have developed a novel serpentine design that allows for very long interaction lengths.

High-Speed Modulator Operation: Figure 1 shows our conventional single-pass modulator, which is a Mach-Zehnder interferometer. The interferometer consists of optical waveguides that are photolithographically formed by titanium diffusion in lithium niobate, and overcoated with a silicon dioxide buffer layer and gold electrodes. Long interaction lengths help reduce the drive voltage V_π at dc, but at the same time demand low electrode losses to minimize the unavoidable increase in V_π with frequency. Very thick (20 to 30 μm) 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. We have recently developed the capability to form ion milled ridges in lithium niobate to impedance-match the modulator structure to the external microwave source, so the complete fabrication of the modulators is done in-house. Sophisticated computer modeling is used to select layer thicknesses and gap widths that result in optimal velocity and impedance matching while maintaining low electrode losses.

Fig1 Image
FIGURE 1
(a) Top view of a single-pass traveling wave modulator in lithium niobate with interaction length L; (b) Cross-section of interaction region.

Figure 2 shows our new design for a traveling wave modulator that uses 180-deg turns to increase the interaction length. By using three passes, for example, the dc V_π is reduced by a factor of three. Since conventional semicircular turns require too much real estate (with bend diameters exceeding 2 cm for <1 dB loss), we are now developing a serpentine traveling wave modulator with low-loss compact turns in lithium niobate waveguides. The idea is to pack multiple passes on the same chip (6 cm long X 6 mm wide) that previously contained only one pass. We have successfully accomplished our goal of low-losscompact turns by using a novel approach that consists of a reflective s-bend placed at the edge of the substrate. This extremely promising technique has resulted in losses as low as 0.6 dB, while simultaneously reducing lateral space requirements by over two orders of magnitude when compared to semicircular turns. A key fabrication issue is ensuring that the mirrored edge is within a few microns of the sbend apex. Reflective s-bends are also an attractive technique for integrating different devices on the same chip.

FIGURE 2
Schematic of the traveling wave serpentine modulator. The underlying waveguide layer is emphasized for clarity. Compact waveguide turns are formed by reflective s-bends at the lithium niobate edges, as shown in inset.

Figure 3 is a model of drive voltage as a function of frequency from dc to 20 GHz for modulators with different interaction lengths at the 1.55 μm wavelength. The "previous" curve with 0.04 (GHz^1/2 -cm)^-1 electrode loss corresponds to our recently published single-pass modulator. Current electrode loss has been improved to 0.025 (GHz^1/2-cm)^-1 by increasing electrode thickness and gap widths. Note that while drive voltages of the single-pass device are around 3 to 4 V at 20 GHz, we can now push V_π near and below 1 V with the three-pass and ten-pass modulators as long as the electrode losses are kept small.

FIGURE 3
Drive voltage as a function of frequency for single-pass, three-pass, and ten-pass modulators, with electrode loss as a parameter.

Summary: We are developing broadband lithium niobate optical modulators designed to obtain subvolt drive voltages. Low drive voltage, high-speed modulation is achieved with traveling wave electrodes having a serpentine design, along with novel reflective sbends that increase the interaction length.

[Sponsored by DARPA]

References
¹ W.K. Burns, M.M. Howerton, and R.P. Moeller, "Broad-band Unamplified Optical Link with RF Gain Using a LiNbO₃ Modulator," IEEE Photon. Technol. Lett. 11, 1656-1658 (1999).
² M.M. Howerton, R.P. Moeller, A.S. Greenblatt, and R. Krähenbühl, "Fully Packaged Broad-band LiNbO₃ Modulator with Low Drive Voltage," IEEE Photon. Technol. Lett. 12, 792- 794 (2000).