Project RAINBOW: An all-integrated all-optical ultrafast dual-comb chip

Technical Summary of Project RAINBOW

Project Overview
Project RAINBOW, led by Dr. Ir. Tushar Malica at the Brussel Photonics Team (B-PHOT) of Vrije Universiteit Brussel, aims to develop a novel, all-integrated, all-optical ultrafast dual-comb photonic chip. The project seeks to bridge the gap between fundamental ultrafast laser physics and applied photonic engineering by creating a standalone device capable of emitting two phase-locked optical frequency combs with slightly different repetition rates from a single laser source. Unlike current state-of-the-art solutions that rely on bulky free-space optics or multi-chip assemblies, this project targets a compact, mass-manufacturable chip operating in the C-band (~1550 nm) suitable for telecommunications and precision spectroscopy.

Problem Statement and Rationale
While optical frequency combs have revolutionized fields such as atomic clocks and high-speed data communication (recognized by the 2005 and 2018 Nobel Prizes), their practical deployment is hindered by the size, cost, and power consumption of traditional free-space setups. Although photonic integration offers a path to miniaturization, the current landscape remains predominantly research-centric (~93% of the market), with few fully integrated, all-on-chip ultrafast sources available.
Specifically, the project addresses three critical gaps:

1. Lack of Integrated Dual-Comb Sources: No single all-optical chip has yet demonstrated the generation of two distinct, phase-locked combs from one laser source.
2. Pulse Duration Limitations: Current integrated sources (e.g., LiNbO3 hybrid platforms) achieve pulse durations in the picosecond range (~4.3 ps), whereas the goal is to reach the femtosecond regime.
3. Understanding Nonlinear Dynamics: There is a significant experimental gap regarding "chimeras"—dynamical states where coherent and incoherent oscillators coexist. While theoretically studied, experimental evidence of chimeras arising inherently in lasers without external forcing is scarce.

Methodology and Work Plan
The project is structured into three sequential Work Packages (WPs) designed to move from simulation to fabrication and finally to advanced characterization and optimization.

• WP1: Photonic Integrated Chip Design

  • Simulation: Utilizes Ansys Lumerical INTERCONNECT and a theoretical multi-mode differential rate-equation model in MATLAB to simulate laser dynamics and circuit behavior.
  • Design Strategy: The design is based on a patented multi-wavelength continuous laser emission concept, reshaped into pulses via a saturable absorber (SA). Two cavity architectures are proposed:
    • Cavity #1 (Series): Uses a chromatic dispersive waveguide to create a spatial lag between combs.
    • Cavity #2 (Parallel): Employs two independent saturable absorbers for each comb.
  • Fabrication: Designs are converted to mask layouts using Luceda IPKISS and fabricated by Smart Photonics B.V. (Netherlands) on an Indium Phosphide (InP) platform, chosen for its ability to integrate active and passive components on a single chip.

• WP2: Experimental Characterization and Phase Stabilization

  • Testing: Once fabricated, chips are characterized using high-resolution equipment (APEX AP2083, Femtochrome FR-103XL, Keysight N9021B, Teledyne oscilloscope) to measure temporal and spectral properties.
  • Mode-Locking: The project focuses on passive mode-locking (using an SA) and hybrid mode-locking (combining DC gain with RF modulation on the SA) to ensure fundamental mode-locking (single pulse per cavity) and minimal pulse-to-pulse variation.
  • Phase Locking: Task 2.2 aims to achieve phase-locking between the two combs. This involves locking individual combs to an external reference laser and measuring the beat note between the two combs to verify phase coherence, addressing the challenge of carrier-envelope offset.

• WP3: Femtosecond Pulse Generation and Chimera Exploration

  • Pulse Compression: Task 3.1 involves off-chip compression of picosecond pulses (expected 15–30 ps initially) to the femtosecond regime using diffraction gratings and mirrors to manage dispersion and nonlinear effects.
  • Platform Optimization: Task 3.2 explores hybrid and heterogeneous integration (e.g., SiN, Si, LiNbO3, AlGaAs) to overcome InP limitations and further reduce pulse duration.
  • Chimera Dynamics: Task 3.3 investigates the conditions required to observe "chimeras" (coexistence of coherent and incoherent modes). This involves manipulating material properties (e.g., using Quantum Dots for broad trailing edges) or device geometry (shortening gain medium length) to intentionally drive the system into these intermediate, non-stable dynamical states.

Key Contributions and Hypotheses
The project posits several hypotheses and aims for specific breakthroughs:

1. Inter-band Phase Locking: The hypothesis is that using a single gain medium (carrier reservoir) for two combs will facilitate inter-band phase locking, effectively creating a "super" comb with an extended frequency span.
2. All-Optical Integration: The project aims to demonstrate the first standalone chip generating dual combs without free-space optics, utilizing passive and hybrid mode-locking to avoid external AC sources.
3. Chimera Observation: A significant contribution will be the experimental observation of inherent chimeras in a laser system, a phenomenon previously limited to theory.
4. Scalability: The design targets minimal footprint, power usage, and robustness to enable mass manufacturing, addressing the current market's reliance on research-centric, bulky solutions.

Results and Claims
As a proposal and work plan, the paper does not present final experimental results but outlines the expected trajectory:

• Initial Expectations: The first-generation InP chips are expected to produce pulses in the 15–30 ps range, limited by the saturable absorber recovery time and fabrication losses.
• Long-term Goals: The ultimate goal is to achieve femtosecond pulse durations and a fully integrated, phase-locked dual-comb system.
• Significance: The project claims to advance the field by converting fundamental science into a first-time technology demonstration. It addresses the disconnect between simulation and fabrication, aims to solve the long-standing challenge of intrinsic phase-locking in multi-wavelength devices, and seeks to provide experimental evidence of complex nonlinear dynamics (chimeras) in lasers.
• Impact: By downsizing ultrafast comb technology from free-space optics to an integrated chip, the project aims to create a patented design that serves as a stepping stone toward market-friendly products in optical atomic clocks, high-speed data communication, and precision laser spectroscopy.