What Are Optical Interconnections?

Optical interconnections are communication links that carry data as modulated light rather than electrical signals, used to move information between chips, boards, racks, or geographically separated systems. By exploiting the low loss, high bandwidth, and immunity to electromagnetic interference of optical fiber and waveguides, they overcome the distance and speed limitations that constrain copper-based electrical interconnects at high data rates. As processor speeds and memory bandwidths have grown faster than the capacity of conventional copper traces, optical interconnections have moved progressively closer to the processor, from rack-to-rack links down to chip-to-chip and, in emerging designs, directly co-packaged with silicon dies.

The technology draws on photonic integrated circuits, semiconductor optoelectronics, and high-speed signal processing. The transition from electrical to optical signaling at progressively shorter distances has been the defining trend in data center networking over the past two decades, and it continues to accelerate under the bandwidth demands of large-scale machine learning workloads.

On-Chip and Board-Level Interconnects

At the longest ranges, between geographically separated data centers, single-mode fiber carries coherent WDM signals across hundreds of kilometers. Within a data center, multimode or single-mode fiber links connect racks at 400 Gbps and 800 Gbps per port. The architectural frontier is the board and package level, where silicon photonics allows optical waveguides, modulators, and photodetectors to be fabricated on or co-packaged with processor and memory dies. Co-packaged optics (CPO) place the optical engine directly in the switch package rather than in a pluggable module at the faceplate, eliminating the lossy electrical SerDes traces that carry signals between the switch ASIC and the transceiver. Intel's integrated optical compute interconnect chiplet, demonstrated in 2024 at the Optical Fiber Communication Conference, delivers up to 4 Tbps bidirectional bandwidth in a co-packaged format at roughly 5 picojoules per bit, compared to 15 picojoules per bit for conventional pluggable modules.

Optical Transceiver Components

The key active components in an optical interconnect are the transmitter, which converts an electrical data stream into modulated light, and the receiver, which converts incoming light back to an electrical signal. On the transmitter side, directly modulated lasers (DMLs) or externally modulated lasers driving electro-optic modulators encode bits at symbol rates from 25 to 200 GBaud per lane. Vertical-cavity surface-emitting lasers (VCSELs) dominate short-reach multimode links because of their low cost and ease of array packaging. Silicon photonic transceivers use ring modulators or Mach-Zehnder modulators on SOI waveguides paired with germanium photodetectors, enabling integration in standard CMOS foundries. Wavelength-division multiplexing multiplies the single-fiber capacity by running several wavelength channels in parallel; four-channel coarse WDM at 850 to 950 nm is standard in 400G short-reach modules. Research on silicon photonics for high-speed communications published in npj Nanophotonics documents current modulator and detector performance limits.

Performance and Power Advantages

The primary figures of merit for an optical interconnect are bandwidth density, expressed in terabits per second per millimeter of chip edge, energy consumption per bit, reach in meters or kilometers, and latency. Electrical copper traces at 112 Gbps per lane lose more than 20 dB at the Nyquist frequency across a 30-centimeter board trace, requiring power-hungry equalization. An optical link over the same distance introduces less than 1 dB of loss and requires no equalization, directly trading circuit complexity for optical component cost. As bandwidth targets approach 1 Tbps per port at the rack scale, research from Caltech's high-speed electrical and optical interconnects group and industry collaborators indicates that optical solutions below 1 picojoule per bit are achievable through further integration and improved laser efficiency.

Applications

Optical interconnections have applications in a range of fields, including:

• Intra-data-center rack and top-of-rack switching at 400 Gbps, 800 Gbps, and beyond
• High-performance computing clusters requiring low-latency memory and processor interconnects
• AI and machine learning accelerator fabrics where all-to-all bandwidth determines training throughput
• Fronthaul and midhaul links in 5G radio access networks connecting base stations to centralized units
• Spacecraft and avionics systems where electromagnetic interference immunity and weight savings are critical