Optical Interconnect Technologies for CPO - The role of MT Ferrule

In the implementation of Co-Packaged Optics (CPO), one of the key points of discussion is how optical interconnects should be realized in terms of their physical configuration. In particular, the choice between a detachable interface and a direct-to-board (non-detachable) design has a significant impact on assembly feasibility and scalability for volume production.

At Hakusan, we have focused on the practical realities of CPO and have advanced our technology development based on a direct-to-board design in which the performance of the MT ferrule itself becomes a critical factor. In this article, we share Hakusan’s perspective—shaped by over 35 years of experience with MT ferrule design and manufacturing - on the key challenges in optical connectivity for CPO and the technologies developed to address them.​

Challenges in Optical Interconnects for CPO

Structure of the optical interconnects in the CPO

The optical interconnect structure for Co-packaged Optics (CPO) is still evolving. In particular, there is ongoing discussion within the industry about whether the optical connections should be implemented as detachable interfaces or direct-to-board (non-detachable) design.

In a detachable interface, a receptacle (the mating interface) must be provided on the board, which increases structural complexity and introduces additional integration constraints. In contrast, direct-to-board design is often regarded as a more practical option for CPO, as it offers better compatibility with existing back-end assembly and testing processes.

Emerging technical challenges

When direct-to-board (non-detachable) design is assumed, the optical interface to the board is expected to move away from conventional connector‑housing‑based fiber connections. Instead, MT ferrules are directly interfaced with FAUs (Fiber Array Units), optical waveguides, or optical engines.

As a result, in CPO, much of the reliability and performance of the optical interconnect increasingly depends on the quality of MT Ferrule itself.
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Hakusan, with over 35 years of experience in developing and manufacturing MT ferrules, views this shift as an inevitable requirement in the CPO era and is committed to advancing the development of MT ferrules specifically designed for CPO.

When a direct-to-board (non-detachable) design is assumed, MT ferrule technologies capable of simultaneously achieving the following requirements are essential:

• High density
• Reflow resistance
• High-precise alignment without a connector housing

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Solution -Hakusan 's Optical Interconnect Technology

Leveraging more than 35 years of experience in developing and manufacturing MT ferrules, Hakusan provides four key technologies that enable the high fiber density and heat resistance required for CPO.

MTDS® -Narrow-Pitch MT Ferrule for High-Density Optical Interconnects for CPO

In CPO, advances in silicon photonics are increasing the integration density of optical circuits, creating a demand for optical interfaces that can connect optical fibers with high precision and high density in the limited space near the optical circuits.
MTDS® is Narrow-Pitch MT Ferrule developed to meet these requirements.

By adopting reduced cladding fiber (80µm) and achieving a fiber pitch of 125/127µm, it is possible to support 24-Fiber to 32-Fiber in a single row, achieving both high-density optical interconnect and space savings inside the CPO package.

Designed for compatibility with silicon photonics layouts, MTDS® provides high-density connections from optical circuits to FAUs and waveguides, contributing to improved optical interconnect efficiency within CPO packages.

▶ Product details: https://hakusan-mfg.co.jp/products/optical/mtds/
▶ Related article: https://hakusan-mfg.co.jp/project/1054/

PMT Connector - Optimized Board Edge Interconnect Interface for Optical Waveguides

The PMT connector is an optical interface designed to provide stable connections between optical waveguides and optical fibers. When combined with optical waveguide film, it enables narrow-pitch, low-loss optical interconnect within the board.

The connector adopts a structure similar to that of an MT ferrule, ensuring a high level of mating compatibility with standard MT ferrules. This allows seamless connection between the optical waveguide formed on the board and optical fibers, supporting high-speed, low-loss optical communications.

▶ Product details: https://hakusan-mfg.co.jp/products/optical/pmt/
▶ Related article: https://hakusan-mfg.co.jp/project/953/

CMF® - Heat‑Resistant, High‑Precision Ceramic Multifiber Ferrule for Reflow Processing

CMF® is a Ceramic Multifiber Ferrule developed to withstand the reflow process (approximately 260 °C) required in back-end assembly for CPO.

It suppresses deformation and fiber pistoning issues commonly seen with conventional plastic ferrules, maintaining dimensional stability even under high-temperature conditions.

hile retaining the same interface as a standard MT ferrule, CMF® supports thin and compact designs with thicknesses ranging from 0.5 mm to 1.2 mm, and accommodates ultra-high fiber counts of 36 fibers or more. This enables efficient optical connectivity even in the limited space of CPO packages and other high-density routing environments.

In addition, its high-precision design minimizes fiber core misalignment, achieving stable optical performance with insertion loss (IL) of 0.7 dB or less (standard Single-mode level).

▶ Product details: https://hakusan-mfg.co.jp/products/optical/cmf/
▶ Related article: https://hakusan-mfg.co.jp/project/935/

MTHR ® - Reflowable MT Ferrule Optimized for CPO Pigtail Applications

MTHR is Reflowable MT Ferrule developed for use in optical engines with optical fiber pigtails in CPO and OBO.

Pigtail connectors mounted together with the optical engine must pass through a reflow process at approximately 260°C, requiring exceptional heat resistance and dimensional stability.

MTHR uses a high heat-resistant material that differs from conventional PPS ferrules, and maintains stable optical characteristics even after reflow process.

Its high-precision design keeps fiber hole eccentricity within 0.7 µm, minimizing fiber core misalignment after reflow. In-house evaluation involves conducting a heating test in an environment of 260°C for 10 minutes, and measuring and confirming the ferrule dimensions, especially the eccentricity of the fiber hole, before and after the process.

Additionally, MTHR® offers a reflow-resistant boot and dedicated end-face protection cap to prevent contamination, further enhancing reliability during the mounting process.

▶ Product details: https://hakusan-mfg.co.jp/products/optical/mthr/
▶ Related article: https://hakusan-mfg.co.jp/project/924/

Summary

CPO is an essential technology for data centers in the AI era, and its implementation requires optical interconnect components that are both high-density and heat-resistant. From the perspective of those developing and supplying optical devices and optical engines, the challenge goes beyond meeting design requirements—it lies in ensuring process reliability through reflow and maintaining consistency during high-volume production.

Hakusan Inc. provides optimal solutions to these challenges, leveraging its 35 years of experience in developing and manufacturing multifiber optical connectivity components centered on MT ferrules. Our approach spans from technical evaluation at the design stage to optimizing component specifications for assembly conditions and securing stable supply during mass production, contributing to both faster product development and improved manufacturing yield.

Hakusan is not merely a manufacturer and supplier of multifiber optical connectivity components; we are a partner that supports the implementation and production processes of CPO, contributing to the realization of next-generation optical infrastructure. Contact Hakusan to discuss solutions for the demanding requirements of CPO—high density, heat resistance, and scalability for volume production.