[Paper Review] High-efficiency electro-optic modulator on thin-film lithium niobate with high-permittivity cladding

Nuo Chen, Kangping Lou+3 more|arXiv (Cornell University)|Apr 14, 2023

Photonic and Optical Devices4 citations

TL;DR

This paper proposes a high-efficiency electro-optic modulator on thin-film lithium niobate using high-permittivity cladding to enhance electric field strength and overlap with optical modes, achieving a record-low half-wave voltage-length product of 1.41 V·cm, excess loss of 0.5 dB, and a 3 dB bandwidth exceeding 40 GHz, enabling compact, high-performance photonic integrated circuits.

ABSTRACT

Thin-film lithium niobate is a promising platform owing to its large electro-optic coefficients and low propagation loss. However, the large footprints of devices limit their application in large-scale integrated optical systems. A crucial challenge is how to maintain the performance advantage given the design space restrictions in this situation. This article proposes and demonstrates a high-efficiency lithium niobate electro-optic (EO) modulator with high-permittivity cladding to improve the electric field strength in waveguides and its overlap with optical fields while maintaining low optical loss and broad bandwidth. The proposed modulator exhibits considerable improvement, featuring a low half-wave voltage-length product of 1.41 Vcm, a low excess loss of 0.5 dB, and a broad 3 dB EO bandwidth of more than 40 GHz. This modulation efficiency is the highest reported for a broadband lithium niobate modulator so far. The design scheme of using high-permittivity cladding may provide a promising solution for improving the integration of photonic devices on the thin-film lithium niobate platform and these devices may serve as fundamental components in large-scale photonic integrated circuits in the future.

Motivation & Objective

• To address the challenge of large device footprints in thin-film lithium niobate (TFLN) electro-optic modulators while preserving high performance.
• To enhance the electric field overlap with optical modes in TFLN waveguides without increasing propagation loss.
• To enable high-bandwidth, low-power, and compact electro-optic modulation for large-scale photonic integrated circuits.
• To explore the feasibility of high-permittivity cladding as a novel design strategy for improving modulator efficiency.

Proposed method

• The modulator employs a thin-film lithium niobate waveguide with high-permittivity dielectric cladding layers deposited on top and bottom.
• The high-permittivity cladding increases the electric field confinement in the waveguide core, enhancing the electro-optic effect.
• The design maintains low optical propagation loss through optimized waveguide geometry and material quality.
• The device is fabricated using standard semiconductor processing techniques, including electron-beam lithography and reactive ion etching.
• The electro-optic response is characterized via S-parameter measurements and direct modulation testing up to 40 GHz.
• The half-wave voltage-length product (VπL) is extracted from the measured extinction ratio and bandwidth.

Experimental results

Research questions

• RQ1 Can high-permittivity cladding significantly enhance the electric field overlap with the optical mode in thin-film lithium niobate waveguides?
• RQ2 Does the use of high-permittivity cladding enable a reduction in VπL without degrading optical loss or bandwidth?
• RQ3 What is the maximum achievable electro-optic bandwidth in a compact TFLN modulator using this cladding approach?
• RQ4 How does the modulator performance compare to state-of-the-art TFLN devices in terms of efficiency and loss?

Key findings

• The modulator achieves a record-low half-wave voltage-length product of 1.41 V·cm, indicating the highest reported modulation efficiency for a broadband lithium niobate modulator.
• The device exhibits an excess loss of only 0.5 dB, demonstrating excellent optical propagation performance.
• The 3 dB electro-optic bandwidth exceeds 40 GHz, confirming broad operational bandwidth suitable for high-speed applications.
• The high-permittivity cladding effectively enhances the electric field intensity in the waveguide core, increasing the electro-optic interaction efficiency.
• The design maintains low optical loss while significantly improving the VπL metric, enabling compact integration.