[Paper Review] A power-efficient integrated lithium niobate electro-optic comb generator

Ke Zhang, Wenzhao Sun+5 more|arXiv (Cornell University)|Aug 20, 2022

Advanced Fiber Laser Technologies39 references4 citations

TL;DR

This paper presents a power-efficient integrated lithium niobate electro-optic frequency comb generator that achieves broadband comb generation using a 4-round-trip optical path through modulation electrodes, enabled by low-loss mode multiplexers and waveguide crossings. By routing the optical signal through the electrodes four times, the phase modulation efficiency is dramatically enhanced, reducing electrical power consumption by over an order of magnitude, resulting in a 47-line comb at 25 GHz with only 28 dBm (0.63 W) RF drive power.

ABSTRACT

Integrated electro-optic (EO) frequency combs are essential components for future applications in optical communications, light detection and ranging, optical computation, sensing and spectroscopy. To date, broadband on-chip EO combs are typically generated in high-quality-factor micro-resonators, while the more straightforward and flexible non-resonant method, usually using single or cascaded EO phase modulators, often requires high driving power to realize a reasonably strong modulation index. Here, we show that the phase modulation efficiency of an integrated lithium niobate modulator could be dramatically enhanced by passing optical signals through the modulation electrodes for a total of 4 round trips, via multiple low-loss TE0/TE1 mode multiplexers and waveguide crossings, reducing electrical power consumption by more than one order of magnitude. Using devices fabricated from a wafer-scale stepper lithography process, we demonstrate a broadband optical frequency comb featuring 47 comb lines at a 25-GHz repetition rate, using a moderate RF driving power of 28 dBm (0.63 W). Leveraging the excellent tunability in repetition rate and operation wavelength, our power-efficient EO comb generator could serve as a compact low-cost solution for future high-speed data transmission, sensing and spectroscopy, as well as classical and quantum optical computation systems.

Motivation & Objective

• To address the high electrical power requirements of non-resonant electro-optic frequency comb generators in integrated platforms.
• To improve phase modulation efficiency in integrated lithium niobate modulators without relying on high-Q microresonators.
• To enable broadband, tunable frequency combs with low power consumption for practical applications in optical communications and sensing.
• To demonstrate a scalable, wafer-scale fabrication approach for high-performance integrated electro-optic devices.

Proposed method

• Optical signals are routed through the same electro-optic modulation electrodes four times via multiple low-loss TE0/TE1 mode multiplexers and waveguide crossings.
• The cumulative phase modulation effect from four round trips significantly enhances the effective modulation index without increasing RF drive power.
• A wafer-scale stepper lithography process is used to fabricate the device, ensuring high yield and scalability.
• The device operates as a non-resonant electro-optic comb generator, relying on phase modulation in a periodically poled lithium niobate waveguide.
• The design leverages the high electro-optic coefficient of lithium niobate and minimizes propagation losses through optimized mode conversion and routing.
• The system is experimentally characterized using a 25-GHz repetition rate and a 28 dBm RF drive signal to generate a broadband frequency comb.

Experimental results

Research questions

• RQ1 Can the phase modulation efficiency of an integrated lithium niobate modulator be significantly enhanced through multiple optical round trips through the electrodes?
• RQ2 What is the maximum number of comb lines achievable with a non-resonant, low-power electro-optic comb generator using this approach?
• RQ3 How does the electrical power consumption scale with the number of round trips in a multi-pass configuration?
• RQ4 To what extent can the repetition rate and center wavelength of the comb be tuned in this integrated platform?
• RQ5 Can this approach achieve broadband comb generation with sub-watt RF power while maintaining low propagation loss?

Key findings

• The device generates a broadband optical frequency comb with 47 distinct comb lines at a 25-GHz repetition rate.
• The system achieves this performance using only 28 dBm (0.63 W) of RF drive power, representing a reduction of over one order of magnitude in electrical power consumption compared to conventional non-resonant approaches.
• The 4-round-trip optical path through the electrodes enhances phase modulation efficiency, enabling strong modulation with moderate RF power.
• The device demonstrates excellent tunability in both repetition rate and center wavelength, enabling flexible operation for diverse applications.
• The use of wafer-scale stepper lithography enables scalable, high-yield fabrication of the complex multi-pass waveguide architecture.
• Low-loss mode multiplexers and waveguide crossings maintain signal integrity across multiple passes, minimizing insertion loss and crosstalk.