PM Fiber Array Pitch Tolerance: Hirundo’s ≤0.5 µm Precision for CPO, Silicon Photonics, and AI Supercomputing

Introduction: Why Sub-Micron Fiber Spacing Decides Who Wins in AI Computing

As large AI models scale to trillions of parameters and supercomputing centers race toward Exascale (10¹⁸ operations per second) and Zettascale (10²¹ operations per second) computing power, data transmission capacity has become the core bottleneck limiting further breakthroughs in computing performance. Conventional electrical signal transmission suffers from limited bandwidth and high energy consumption and can no longer satisfy the demand of supercomputing facilities for the instantaneous flow of massive data volumes. Against this backdrop, the coordinated rise of Co-packaged Optics (CPO) and silicon photonics has opened a new lane of optical interconnection for computing infrastructure, and the fiber array is the optical bridge that connects these two technologies. Yet behind every reliable optical bridge lies a specification that rarely appears in marketing brochures but determines whether coupling succeeds or fails: pitch tolerance.

What Pitch Tolerance Means in a PM Fiber Array

A fiber array (FA) is a standardized optical assembly in which multiple fibers are arranged, fixed, and polished at a fixed pitch on a high-precision V-groove substrate, turning numerous single-lane fibers into one multi-lane high-speed optical highway. Pitch refers to the center-to-center distance between adjacent fibers, typically 127 µm or 250 µm in standard designs, while pitch tolerance describes the maximum permissible deviation between that nominal spacing and the spacing actually achieved in mass production.

For a polarization-maintaining fiber array (PM FA), the requirement is stricter still. The array must simultaneously preserve the polarization state of the transmitted signal and hold every channel in the exact geometric position demanded by the mating photonic device. Hirundo’s PM FA family addresses this with standard fiber spacing of 127 µm and 250 µm, a pitch tolerance controlled to ≤0.5 µm, fiber types based on PM 1310 nm and PM 1550 nm (both customizable), end-face angles spanning 0° to 45°, and a polarization extinction ratio (ER) of ≥18 dB, in compliance with GR-1209/1221 and RoHS standards.

Why Sub-Micron Pitch Control Determines Coupling Loss

Silicon photonic chips deliver their optical signals through waveguide arrays whose spacing is defined at the lithography stage. When a fiber array is brought into alignment with that waveguide array, any deviation between the two pitches translates directly into lateral offset at each channel. Because the optical mode field is only a few micrometers wide, an offset of one micrometer already moves a significant fraction of the beam away from the receiving waveguide, and the misalignment penalty grows rapidly as the offset increases toward the mode field radius.

The consequences are measurable across the entire link budget:

• Insertion loss rises channel by channel, forcing higher laser drive power and increasing module energy consumption.
• Channel uniformity degrades, so the worst channel, not the average one, sets the link margin.
• Return loss suffers as imperfect end-face geometry and misalignment increase back reflection.
• Polarization extinction ratio in PM arrays can be eroded when stress-rod orientation and fiber position drift together.

This is precisely why the FA-MT used in QSFP+ PSM4 parallel modules is specified with an alignment error below 0.5 µm and a single-channel insertion loss under 1.5 dB. The same logic now governs CPO and silicon photonics packaging, where hundreds of optical paths must couple simultaneously.

How Hirundo Holds Pitch Tolerance to ≤0.5 µm

Pitch accuracy is not a polishing step added at the end of production; it is designed into the substrate from the first process. Hirundo fabricates its arrays by placing the bare fiber portion, with the coating removed, into precision V-grooves on a high-precision substrate, pressurizing the assembly with a dedicated pressing fixture, bonding it with adhesive, and finally grinding and polishing the end face to the required geometry. Because the V-groove geometry defines the fiber spacing and position directly, the precision of the groove determines the precision of the array.

This manufacturing route delivers the core advantages that Hirundo’s FA products are known for:

• High precision, multi-channel: dozens of fibers are encapsulated in a very small space without mutual interference, with V-groove substrates providing precise control of fiber spacing and position.
• Low insertion loss: precision machining and polishing bring insertion loss down to as low as 0.05 dB.
• High return loss: typical values of ≥55 dB for APC end faces effectively reduce back reflection and stabilize transmission.
• Excellent temperature stability: with optical fiber, glass substrate, and cover plate as primary materials and modern curing adhesives, operating and storage ranges of -40 °C to +85 °C are maintained.
• Full customization: end-face angles beyond flat or 8°, including 45° for vertical coupling, plus 1CH to 64CH channel counts and custom pitches.

Polarization-Maintaining Performance Depends on Geometry Too

A PM FA is built from multiple polarization-maintaining fibers fixed through a specific packaging process, delivering good polarization-maintaining performance, high arrangement accuracy, and strong stability. In optical communications it carries polarization-sensitive signals and combines with other components to realize complex functions; in fiber sensing it supports high-precision sensor construction; in laser technology it serves as a key component for laser transmission and coupling. Every one of these applications depends on the same foundation: fibers held at an exact, repeatable pitch with their polarization axes correctly oriented, so that the ER remains ≥18 dB and the polarization state survives from source to detector.

Built for CPO and Silicon Photonics

The core advantage of CPO is near-package transmission: shortening the distance between the optical module and the computing chip reduces latency and power consumption. Hirundo’s FA, with its compact structural design, can be integrated directly into the CPO package, enabling short-distance direct connection of optical signals between the silicon photonics chip and the optical module. Compared with traditional fiber connections this significantly reduces transmission loss, amplifies the low-latency advantage of CPO, and provides a stutter-free transmission guarantee for high-frequency data interaction in AI supercomputing.

For next-generation 1.6T silicon photonic and CPO modules, Hirundo supplies detachable D-FAU arrays and integrated coupling FCFA arrays. These custom ultra-thin FAs couple directly with silicon photonic waveguides and eliminate intermediate transfer paths, cutting overall module power consumption by 15% to 40%. The detachable design breaks the one-time packaging limitation of traditional FAs, so optical engine maintenance and module upgrades no longer require full disassembly, a substantial reduction in data center operation and maintenance costs.

Validated in 400G/800G and Medium-to-Long-Reach WDM Modules

In general cloud data centers running 400G and 800G SR8/DR8 parallel modules, MT-type FAs realize eight-channel parallel transmit and receive. Each channel carries 100G PAM4 signals with insertion loss controlled below 0.35 dB, return loss of ≥60 dB, channel uniformity deviation under 0.2 dB, and a bit error rate as low as 10^-12 under 24/7 full load. Compared with discrete fiber assembly, modules equipped with standardized FAs achieve higher mass production yields and cut manual assembly costs by more than 30%.

For FR/LR wavelength division multiplexing modules used in inter-room and inter-city links, dedicated WA-type FAs work together with Arrayed Waveguide Gratings (AWGs) to realize multi-wavelength multiplexing and demultiplexing, integrating multiple wavelength paths into a single component so that ultra-large bandwidth travels over fewer fibers.

Mass Production Consistency: Turning Tolerance Into Reliability

AI supercomputing centers continue to expand in scale, placing stringent requirements on large-scale deployment and long-term stability of optical links. High reliability and high consistency mean that a pitch tolerance of ≤0.5 µm must hold not only in the laboratory sample but across every unit shipped. Hirundo’s FA products are mass-produced with minimal performance deviation, meet the needs of large-scale optical link deployment in supercomputing centers, and offer flexible array specifications from 1CH to 64CH that adapt to the port count of CPO and silicon photonics chips, reserving expansion space for future computing power upgrades.

Conclusion

In the optical interconnect era, the battle for computing power is fought at micrometer and sub-micrometer scales. A pitch tolerance of ≤0.5 µm is what allows a PM fiber array to mate efficiently with a silicon photonic waveguide array, what protects polarization extinction ratio and return loss, and what makes the CPO promise of low latency and low power physically achievable. Hirundo approaches this specification as an engineering discipline rather than a marketing claim, from V-groove design and pressing fixtures to polishing angles of 0°, 8°, 42.5°, and 45°, so that every fiber in every array lands exactly where the light needs it to be.