[Paper Review] Hybrid integrated near UV lasers using the deep-UV Al2O3 platform

Cornelis A. A. Franken, W. A. P. M. Hendriks+10 more|arXiv (Cornell University)|Feb 22, 2023

Photonic and Optical Devices19 citations

TL;DR

Demonstrates the first hybrid integrated near-UV laser using an Al2O3 deep-UV platform, achieving 0.74 mW fiber-coupled output (≈3.5 mW on chip) and >4.4 nm tuning from 408.1 to 403.7 nm.

ABSTRACT

Hybrid integrated diode lasers have so far been realized using silicon, polymer, and silicon nitride (Si3N4) waveguide platforms for extending on-chip tunable light engines from the infrared throughout the visible range. Here we demonstrate the first hybrid integrated laser using the aluminum oxide (Al2O3) deep-UV capable waveguide platform. By permanently coupling low-loss Al2O3 frequency-tunable Vernier feedback circuits with GaN double-pass amplifiers in a hermetically sealed housing, we demonstrate the first extended cavity diode laser (ECDL) in the near UV. The laser shows a maximum fiber-coupled output power of 0.74 mW, corresponding to about 3.5 mW on chip, and tunes more than 4.4 nm in wavelength from 408.1 nm to 403.7 nm. Integrating stable, single-mode and tunable lasers into a deep-UV platform opens a new path for chip-integrated photonic applications.

Motivation & Objective

• Demonstrate a chip-scale hybrid integrated diode laser in the deep-UV using an Al2O3 platform.
• Couple a GaN/InGaN double-pass amplifier to Al2O3 feedback circuits for tunable, single-wavelength operation.
• Achieve hermetically sealed packaging and UV-compatible bonding for long-term stability.
• Characterize waveguide losses, coupling, and Vernier filter performance to enable single-mode, tunable UV output.

Proposed method

• Fabricate Al2O3 cores embedded in SiO2 cladding to provide tightly guided single-mode UV propagation.
• Design and fabricate sequential Al2O3 microring resonators in a Vernier configuration for frequency-selective feedback.
• Integrate a 405 nm InGaN/GaN double-pass amplifier with the Al2O3 feedback chip through UV-compatible bonding.
• Implement thermo-optic microheaters for tuning phase and coupling in the Vernier filter and output coupler.
• Pack the amplifier and feedback chip in a hermetically sealed butterfly package with a gas environment (nitrogen/argon) for stability.
• Characterize propagation loss, coupling, and Vernier filter transmission to set optimal coupling (κ^2) and Q-factors.

Experimental results

Research questions

• RQ1 Can the deep-UV Al2O3 platform support stable, tunable hybrid integrated lasers when coupled with a UV GaN/inGaN amplifier?
• RQ2 What are the achievable output power, tuning range, spectral purity (SMSR), and coherence/linewidth in this platform?
• RQ3 How do propagation losses, coupling, and Vernier filter design affect laser performance and stability in the near-UV?
• RQ4 What packaging and hermetic sealing requirements are necessary for long-term UV operation?

Key findings

• Maximum fiber-coupled output power of 0.74 ± 0.04 mW (≈3.5 ± 0.5 mW on-chip).
• Tuning of the laser exceeding 4.4 nm from 408.1 to 403.7 nm.
• Single-wavelength operation with SMSR of 42–43 dB under different κ^2 settings.
• Laser operates without mode hops for at least 84 minutes, with drift below 1.6 GHz and final stabilization within tens of MHz.
• Coherence time inferred as ≈-based on a delayed self-heterodyne measurement indicating linewidth around 25 MHz or lower.
• Propagation losses in Al2O3 waveguides measured at 2.8 ± 0.3 dB/cm; chip-to-fiber coupling loss ~10.2 ± 0.8 dB/facet.