ECOC 2025 Tech Spotlight: Furukawa's Ultra-Compact CPO Transceiver with 1060 nm VCSELs and Multicore Fiber

Introduction

Driven by AI and cloud data centers, optical modules are evolving toward high density, low power and longer reach. Traditional 850 nm multimode VCSEL technology is mature for short-reach (<100 m) applications, but cannot support Tbps-class transmission.

At ECOC 2025, Furukawa presented a 50 Gbaud × 16-channel CPO transceiver based on a 1060 nm single-mode VCSEL array (SM-VCSEL) and multicore fiber (MCF), achieving an ultra-compact package and low-energy transmission — a sign of Japanese optical component makers pushing aggressively into next-generation CPO technology.

Details

1. Design Architecture and Packaging Features

• Optical interface: uses 19-core single-mode multicore fiber, dramatically increasing channel density.

• VCSEL array: 1060 nm single-mode bottom-emitting coupled-cavity structure, using the photon-photon resonance effect to extend bandwidth and improve mode-field-diameter matching.

• Packaging design:

  • The transceiver measures just 7.7 mm × 15.9 mm.

  • Uses a 0.3 mm pitch LGA interface for high-density electrical connections with suppressed crosstalk.

  • Adopts double-sided flip-chip bonding to shorten traces and reduce loss.

2. High-Speed Characteristics and Bandwidth Tests

• VCSEL + PD bandwidth:

  • EO response: 27.5 GHz.

  • OE response: 55 GHz.

• Transmitter/receiver system bandwidth:

  • TX (incl. driver/interposer): 34 GHz.

  • RX (incl. TIA/interposer): 54 GHz.

  • End-to-end system 3 dB bandwidth: sufficient for 50 Gbaud NRZ/PAM4 transmission.

3. Optical Link Tests

• NRZ transmission (50 Gb/s × 16 channels)

  • Over 2 km of multicore fiber.

  • Open eyes on all channels, BER below the FEC threshold.

• PAM4 transmission (106 Gb/s × 16 channels)

  • Also tested over 2 km of multicore fiber.

  • Clear electrical and optical eye diagrams, BER below the KP4-FEC threshold.

4. Energy Efficiency and Comparison

• Energy consumption: reaches 3.95 fJ/bit, one of the lowest figures reported for a CPO transceiver to date.

• Technology comparison:

  • Silicon photonics CPO (with ELS): ~6 fJ/bit, supports 2 km, but higher power.

  • 850 nm multimode VCSEL CPO: ~4 fJ/bit, limited to 100 m.

  • Furukawa 1060 nm single-mode VCSEL CPO: <4 fJ/bit, supports 2 km with multi-channel parallelism.

5. Challenges and Future Directions

• Back-reflection suppression: the design needs no isolator, relying on its structure to reduce back-reflection.

• Reliability:

  • Long-term reliability studies are still ongoing.

  • The design includes temperature control to keep the VCSEL array stable.

• Scalability: future versions could adopt higher-order modulation (PAM6/8) to reach 200 Gb/s per channel, pushing total capacity to 3.2T–6.4T.

Summary

Furukawa's work demonstrates the potential of 1060 nm single-mode VCSELs + multicore fiber for CPO:

1. Ultra-compact packaging: 7.7 × 15.9 mm, suited to high-density layouts.

2. High bandwidth: 47 GHz end to end, effectively supporting 50 Gbaud and beyond.

3. Low power: <4 fJ/bit, ahead of current silicon photonics and 850 nm VCSEL solutions.

4. Long-reach capability: breaks the traditional short-reach limit of VCSELs, achieving 2 km transmission.

This result means VCSEL technology is no longer confined to short-reach multimode applications — combined with CPO and multicore fiber, it could move into Tbps-class longer-reach transmission, offering a low-power, highly scalable solution for future AI and cloud data center networks.