[Paper Review] Breaking voltage-bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes

Prashanta Kharel, Christian Reimer+3 more|arXiv (Cornell University)|Nov 26, 2020

Photonic and Optical Devices2 references34 citations

TL;DR

The paper demonstrates segmented micro-structured traveling-wave electrodes on thin-film lithium niobate modulators to drastically reduce RF loss and achieve sub-volt drive at high bandwidth, including a 1.3 V DC Vπ at 1 GHz and 1.6 V at 50 GHz with 1.8 dB EO roll-off at 50 GHz.

ABSTRACT

Electro-optic modulators with low voltage and large bandwidth are crucial for both analog and digital communications. Recently, thin-film lithium niobate modulators have enable dramatic performance improvements by reducing the required modulation voltage while maintaining high bandwidths. However, the reduced electrode gaps in such modulators leads to significantly higher microwave losses, which limit electro-optic performance at high frequencies. Here we overcome this limitation and achieve a record combination of low RF half-wave voltage of 1.3 V while maintaining electro-optic response with 1.8-dB roll-off at 50 GHz. This demonstration represents a significant improvement in voltage-bandwidth limit, one that is comparable to that achieved when switching from legacy bulk to thin-film lithium niobate modulators. Leveraging the low-loss electrode geometry, we show that sub-volt modulators with $>$ 100 GHz bandwidth can be enabled.

Motivation & Objective

• Motivate and enable low-voltage, high-bandwidth electro-optic modulation for analog and digital communications.
• Overcome microwave loss limits in thin-film LN modulators while preserving modulation efficiency and speed.
• Show that segmentation and low-permittivity substrates can push EO bandwidth toward >100 GHz with sub-volt drive.
• Compare segmented electrode performance to regular designs and quantify gains in loss and bandwidth.

Proposed method

• Use traveling-wave, thin-film LN modulators on quartz substrate to enable low RF loss and velocity matching.
• Introduce micro-structured (segmented) electrodes that divert current away from narrow gaps to reduce ohmic loss.
• Simulate electrode designs with HFSS to compare Vπ·L, impedance, RF loss, and estimated EO bandwidth.
• Fabricate LN-on-quartz devices with 600 nm LN and 800 nm Au electrodes, 1 μm SiO2 cladding, and 20–20 mm electrode lengths.
• Characterize RF loss with 50 Ω VNA de-embedded measurements and EO performance with 1560 nm light.
• Measure Vπ at DC/1 GHz and EO roll-off up to 50 GHz, and compare to simulations and legacy designs.

Experimental results

Research questions

• RQ1 Can segmented micro-structured electrodes on thin-film LN reduce RF loss without sacrificing modulation efficiency?
• RQ2 What is the impact of substrate permittivity and current crowding on velocity matching and EO bandwidth?
• RQ3 How far can voltage-bandwidth performance be pushed beyond existing thin-film LN modulators using micro-structured electrodes?
• RQ4 What are the practical limits (loss, velocity mismatch) for achieving >100 GHz EO bandwidth with sub-volt drive?

Key findings

• Segmented (micro-structured) electrodes on LN-on-quartz dramatically reduce RF loss from 7 dB/cm to 2 dB/cm at 50 GHz for a 10 mm device.
• Measured RF phase index is 2.23, aligning with simulations and enabling good velocity matching with LN optical group index (~2.2).
• DC Vπ is 1.35 V for a 20 mm modulator; RF Vπ at 50 GHz is 1.6 V for the same device, yielding 1.8 dB EO roll-off at 50 GHz.
• A 10 mm segmented device shows only 0.8 dB roll-off at 50 GHz relative to DC/1 GHz Vπ, with S11 < -15 dB across measurements.
• Simulations and measurements indicate that segmented design can achieve ~228 GHz estimated 3-dB EO bandwidth for 20 mm length, vastly surpassing regular designs.
• Extrapolations suggest sub-volt modulators with >100 GHz bandwidth are feasible on low-permittivity substrates; potential >180 GHz conductor-loss-limited bandwidth for 20 mm length.