Multi-Physical Analysis and Optimization in Integrated Lithium Niobate Modulator Using Micro-Structured Electrodes

At a Glance

Metadata	Details
Publication Date	2023-07-10
Journal	Photonics
Authors	Jianchao Su, Guoliang Yang, Dandan Guo, Ming Li, Ninghua Zhu
Institutions	Institute of Semiconductors, University of Chinese Academy of Sciences
Citations	3
Analysis	Full AI Review Included

Executive Summary

This study details the multi-physical analysis and thermal optimization of a high-speed, thin-film Lithium Niobate (LiNbO₃, LN) modulator utilizing micro-structured (T-shaped) slow-wave electrodes.

Problem Identification: At high modulation rates (100 GHz), energy loss from microwave (MW) signals generates significant heat, causing temperature rise (up to 17.72 °C above ambient) and non-uniform temperature distribution.
Performance Degradation: This thermal field alters the material properties (gold conductivity, LN refractive index), leading to reduced MW signal quality (S21 parameter degradation) and thermal phase shift noise in the optical signal.
Dominant Heat Source: Multi-physical simulation confirmed that MW electromagnetic loss is the primary heat source; optical signal heating was found to be negligible by comparison.
Optimization Strategy: A 10 µm thick polycrystalline diamond heat dissipation layer, grown via Chemical Vapor Deposition (CVD), was integrated beneath the modulator structure.
Thermal Improvement: The diamond layer significantly improved thermodynamic characteristics, reducing the maximum temperature rise by 28.84% (from 17.72 °C to 12.61 °C above ambient).
Electrical Improvement: The adverse effect of temperature on the S21 parameter (at 100 GHz) was optimized by 71.4% (reducing the temperature-induced loss from 0.07 dB to 0.02 dB).
Optical Improvement: Thermal phase shift noise was reduced by 30.2% (from 0.086 π to 0.060 π), enhancing phase stability and modulation accuracy.

Technical Specifications

Parameter	Value	Unit	Context
Modulation Frequency	100	GHz	Simulation operating point
MW Signal Power	0.03	W	Input power for thermal simulation
Ambient Temperature	20	°C	Reference room temperature
Max Temp Rise (Before Opt.)	17.72	°C	Maximum temperature increase above ambient
Max Temp Rise (After Opt.)	12.61	°C	Maximum temperature increase with diamond layer
Temperature Rise Reduction	28.84	%	Optimization effectiveness
Diamond Layer Thickness	10	µm	Polycrystalline CVD diamond heat sink
S21 Loss Reduction (100 GHz)	71.4	%	Optimization of temperature-induced S21 degradation
Thermal Phase Shift (Before Opt.)	0.086	π	Maximum phase shift noise
Thermal Phase Shift (After Opt.)	0.060	π	Phase shift noise after optimization (30.2% reduction)
LN Refractive Index (n_o)	2.21	-	Ordinary index at 1550 nm
LN Refractive Index (n_e)	2.14	-	Extraordinary index at 1550 nm
Gold Conductivity (20 °C)	41 x 10⁶	S/m	Initial electrical conductivity of electrodes
Thermal Conductivity Coefficient	20	W/(m²K)	Natural convection boundary condition
Electro-optic Coefficient (r₃₃)	28	pm/V	LN material property

Key Methodologies

The research relied on a coupled multi-physical simulation approach to analyze and optimize the device performance under high-frequency operation.

Device Modeling: A 3D model of the thin-film LN Mach-Zehnder modulator was established, featuring undoped pure LN thin film and T-shaped slow-wave gold electrodes, based on established geometric parameters.
Multi-Physics Simulation: The study utilized two primary simulation methods:
- Joint simulation using HFSS (electromagnetic analysis) and Icepak (thermal analysis).
- Integrated simulation using COMSOL Multiphysics.
Governing Equations: The simulation solved the Helmholtz equation for the optical and MW E-fields, coupled with the generalized heat transfer differential equation (Equation 3) to model temperature profile evolution.
Heat Source Calculation: Electromagnetic loss calculated from the MW signal propagation (primarily occurring between the T electrodes) was used as the heating generation rate (Q) input for the thermal model. Optical heating was calculated but deemed negligible.
Iterative Coupling: An iterative process (up to 50 iterations) was employed to achieve a steady-state result. In each iteration, the calculated temperature field updated the material properties (e.g., gold conductivity, LN refractive index), which in turn affected the electromagnetic loss calculation.
Thermal Optimization Implementation: A 10 µm thick polycrystalline diamond layer was introduced into the model as a heat sink, simulating material obtained via Chemical Vapor Deposition (CVD).
Performance Metrics: Optimization effectiveness was quantified by measuring:
- Maximum steady-state temperature rise.
- Degradation of the S21 transmission parameter versus frequency.
- Non-uniform thermal phase shift difference between the two modulator arms.

Commercial Applications

The thermal management and performance optimization techniques developed in this study are critical for advancing high-performance integrated photonics.

High-Speed Optical Communications: Enabling next-generation, high-bandwidth (100 GHz and above) electro-optic modulators required for high-capacity data centers and long-haul fiber networks.
Integrated Photonics Systems: Reducing thermal crosstalk between adjacent modulators in dense arrays, improving overall system stability and performance in complex photonic integrated circuits (PICs).
High-Power Handling: Improving the optical power-handling ability and reliability of LN devices by effectively dissipating heat generated by high-power MW and optical signals.
Defense and Sensing: Applications requiring extremely stable and accurate phase modulation, where temperature-induced phase noise must be minimized.
Advanced Material Integration: Demonstrating the effective use of high thermal conductivity materials (polycrystalline diamond via CVD) as integrated heat dissipation layers in semiconductor devices.

View Original Abstract

With the increase in the modulation rate of thin-film lithium niobate (LiNbO3, LN) modulators, the multi-physical field coupling effect between microwaves, light, and heat becomes more significant. In this study, we developed a thin-film LN modulator model using undoped pure LN thin film and T-shaped slow-wave electrodes. Furthermore, we utilized this model to simulate the microwave heating and light heating situations of the modulator. The temperature of the LN modulator was analyzed over time and with different signal frequencies. We also studied the influence of temperature rise on microwave and light signals, and we analyzed the change of S parameters and the Phase Shift of the light signal caused by temperature rise. Finally, we improved the thermodynamic characteristics of the modulator by adding a diamond heat dissipation layer. The diamond was obtained through the Chemical Vapor Deposition (CVD) technique and was a polycrystalline diamond. After adding the diamond heat dissipation layer, the temperature rise of the modulator was significantly improved, and the adverse effects of temperature rise on microwave signals were also significantly reduced.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10070795

References

2021 - Taking silicon photonics modulators to a higher performance level: State-of-the-art and a review of new technologies [Crossref]
2000 - A review of lithium niobate modulators for fiber-optic communications systems [Crossref]
2020 - Advances in on-chip photonic devices based on lithium niobate on insulator [Crossref]
2018 - Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages [Crossref]
2009 - Micro-structured integrated electro-optic LiNbO3 modulators [Crossref]
2021 - High-Speed Modulator With Integrated Termination Resistor Based on Hybrid Silicon and Lithium Niobate Platform [Crossref]
2022 - Monolithic thin film lithium niobate electro-optic modulator with over 110 GHz bandwidth [Crossref]
2004 - Photonic applications of lithium niobate crystals [Crossref]
1994 - Characterization of lithium niobate electro-optic modulators at cryogenic temperatures [Crossref]
2017 - Nanophotonic Lithium Niobate Electro-optic Modulators