Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages

Abstract

Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation.

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Fig. 1: Nanophotonic LN modulators compatible with CMOS drive voltages.
Fig. 2: Directly CMOS-driven data transmission at 70 Gbit s–1.
Fig. 3: Ultra-high-speed data transmission at 100 Gbaud and 210 Gbit s–1.
Fig. 4: Towards ultimate modulator performance.

Data availability

The data sets generated and/or analysed during the current study are available from the corresponding authors on reasonable request.

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Acknowledgements

We thank J. Khan for discussions on the LN platform, H. Majedi for help with the equipment, and C. Reimer, S. Bogdanović, L. Shao and B. Desiatov for feedback on the manuscript. This work is supported in part by the National Science Foundation (NSF) (ECCS1609549, ECCS-1740296 E2CDA and DMR-1231319) and by Harvard University Office of Technology Development (Physical Sciences and Engineering Accelerator Award). Device fabrication is performed at the Harvard University Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which is supported by the NSF under ECCS award no. 1541959.

Reviewer information

Nature thanks M. Hochberg and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

C.W., M.Z., X.C., P.W. and M.L. conceived the experiment. C.W., M.Z. and A.S. fabricated the devices. M.Z. and M.B. performed numerical simulations. C.W., M.Z., X.C. and S.C. carried out the device characterization. C.W. wrote the manuscript with contribution from all authors. P.W. and M.L. supervised the project.

Corresponding author

Correspondence to Marko Lončar.

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Competing interests

C.W., M.Z. and M.L. are involved in developing lithium niobate technologies at HyperLight Corporation.

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Extended data figures and tables

Extended Data Fig. 1 Half-wave voltages of devices with different active lengths.

ac, Normalized optical transmission of the 20-mm (a), 10-mm (b) and 5-mm (c) device as a function of the applied voltage, showing half-wave voltages of 1.4 V, 2.3 V and 4.4 V, respectively. The inset of a shows the measured normalized transmission (NT) on a logarithmic scale, revealing an extinction ratio of 30 dB.

Extended Data Fig. 2 High-speed measurement set-ups.

a, Set-up for measuring the modulator electro-optic responses from 35 GHz to 100 GHz. b, High-speed data modulation set-up. For direct CMOS driving, the RF amplifier is bypassed. EDFA, erbium-doped fibre amplifier; FPC, fibre-polarization controller; MZM, Mach–Zehnder modulator (commercial); OSA, optical spectrum analyser; VOA, variable optical attenuator.

Extended Data Fig. 3 Electrical eye diagram at 100 Gbaud.

The measured electrical BER is 3.6 × 10−5, limited by the signal distortion from the electronic circuit.

Extended Data Fig. 4 OSNR measurements.

BER versus OSNR for the three modulation schemes at 70 Gbaud.

Extended Data Fig. 5 Comparison of integrated and conventional LN modulators.

a, b, Schematics of the cross-sections of thin-film (a) and conventional (b) LN modulators. Our thin-film modulator (a) has an oxide layer underneath the device layer, so that velocity matching can be achieved while maximum electro-optic efficiency is maintained. A conventional modulator (b) also uses a buffer oxide layer for velocity matching, but on top of LN which further compromises the electro-optic overlap. c, d, Numerically simulated microwave (c) and optical (d) field distributions (both shown in Ez components) in the cross-section of the thin-film modulator. For microwave simulations, the electric-field values are obtained when a voltage of 1 V is applied across the two electrodes. e, Group refractive indices for both optical and microwave signals as a function of the buried oxide thickness. Velocity matching can be achieved with an oxide thickness of about 4,700 nm.

Extended Data Table 1 Comparison of modulator voltage, bandwidth and loss performance

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Cite this article

Wang, C., Zhang, M., Chen, X. et al. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature 562, 101–104 (2018). https://doi.org/10.1038/s41586-018-0551-y

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Keywords

  • Microwave Photonic Systems
  • Complementary Metal Oxide Semiconductor (CMOS)
  • Velocity Matching
  • Optical Signal-to-noise Ratio (OSNR)
  • Electrical Energy Dissipation

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