Active vs Passive DAC: A 2026 Guide to DCI, OTN/WDM, and Passive Optical Components

The Interconnect Layer: Where Every Network's Economics Are Decided

The rapid development of 5G, cloud computing, and AI training clusters has pushed the demand for ultra-large bandwidth, low latency, and flexible scheduling in backbone networks to unprecedented levels. OTN/WDM technology has become the core bearing solution of modern backbone networks thanks to its high capacity, long-distance transmission, and intelligent management capabilities. Yet between the router ports and the optical transport shelf sits a less glamorous but equally decisive layer: the interconnect. Choosing between an active DAC and a passive DAC is one of the first engineering trade-offs every data center architect faces in 2026.

What Is a Direct Attach Cable (DAC)?

A Direct Attach Cable is a fixed assembly in which two copper twinax cable ends are terminated with pluggable-module-form-factor connectors permanently attached to the cable. Unlike a pluggable optical module plus patch cord, a DAC is a single, factory-terminated, factory-tested unit. The link is electrically defined by the high-speed interface requirements of the IEEE 802.3 Ethernet family, while the traffic it carries is eventually aggregated into the transport layer built on ITU-T OTN recommendations and verified against IEC optical test standards.

The DAC family splits into two branches: passive DAC and active DAC.

Passive DAC: the zero-power, zero-latency workhorse

A passive DAC contains no active electronics at all. The cable is a pure passive copper channel: no power supply, no signal-conditioning chip, no retimer. All equalization and error correction are handled by the SerDes of the host switch or NIC, complemented by forward error correction at the protocol level.

• Reach: typically up to about 3-5 m in high-speed racks, with longer spans achievable at lower per-lane rates.
• Power: effectively zero cable power, which keeps the rack thermal budget free for compute.
• Latency: only the propagation delay of copper, on the order of a few nanoseconds per meter. This matters enormously for AI/ML fabrics and latency-sensitive systems.
• Reliability and cost: fewer components means fewer failure points, and the lowest cost per link of any interconnect technology.

Active DAC: electrical conditioning inside the cable

An active DAC adds signal-conditioning electronics inside the module shells, typically equalizers plus clock-and-data recovery or a lightweight retimer. The physical copper medium can remain similar, but the electronics clean up the eye diagram, restore amplitude and timing margins, and therefore extend reach considerably.

• Reach: commonly around 10-15 m, and further at moderate line rates.
• Power: still a fraction of an optical module, but non-zero, so power and thermal budgets must be checked.
• Latency: slightly higher than a passive DAC because of the retiming stage, yet still far below optical conversion.
• Cable weight and gauge: conditioning allows thinner copper for a given span, which simplifies cable management and airflow in high-density racks.

Active DAC vs Passive DAC: Side-by-Side

A Practical Selection Checklist for 2026

• Distance first: if the link is under a few meters, a passive DAC is almost always the right answer; if it approaches or exceeds 10 m, evaluate an active DAC before jumping to AOC or optics.
• Per-lane rate: higher per-lane speeds shrink the copper budget, so higher-rate ports tend to need active DACs or optics sooner.
• Latency sensitivity: for AI/ML collectives, keep the link as passive as possible.
• Power and thermal headroom: every watt saved in the interconnect is a watt available to compute.
• Link budget verification: measure actual eye margin and pre-FEC bit error rate rather than trusting datasheet reach alone.
• Operational simplicity: fewer SKUs, fewer spares, and predictable factory termination reduce operating cost over the equipment lifetime.

Where Copper Ends: DCI and OTN/WDM Need Passive Optics

Once the distance leaves the data hall, copper is out of the question. Data center interconnect (DCI), metro, and backbone spans rely on optical transport, and the physical foundation of those systems is a family of passive optical components that add no electronics, consume no power, and simply shape, split, combine, and route light.

Fiber optic couplers: the traffic command of optical signals

A fiber optic coupler is a passive optical device that efficiently and stably transmits optical signals from one or more fibers to one or more other fibers. Its core function is optical power distribution, or splitting, and combination while keeping the signal spectrum composition unchanged. Passive design means no external power supply; bidirectional reversibility means input and output ports can be interchanged; and high-quality couplers reach insertion loss as low as 0.1 dB.

Most couplers are made by fusion tapering: two or more fibers are brought together, heated and melted, then stretched to form a double-cone coupling region. In that region the optical signal is guided through a gradient waveguide structure, and the splitting ratio is precisely set by controlling the stretching length.

Polarization-maintaining (PM) couplers

A PM coupler is manufactured with the same fused tapering process, but with extreme precision: the polarization axes of the two fibers must be aligned and kept parallel throughout. The composite waveguide formed in the coupling region then distributes or combines optical power while maximally preserving the polarization state of the input light. Outstanding polarization retention means stable transmission of two orthogonal linear polarization states, with a polarization extinction ratio better than 20 dB, and typical insertion loss below 0.1 dB, the combination that keeps polarization information intact in sensitive systems.

Large core diameter fiber couplers

When applications move from microwatt-level communication signals to kilowatt-level high-power lasers, ordinary fiber cores cannot handle the load. Large core diameter fiber couplers use fibers with significantly larger cores, in the hundreds of microns, and higher numerical apertures. The manufacturing principle is still fused tapering, but the fiber properties demand special process control to form a waveguide structure suited to large-mode transmission. The large core area allows higher optical power input and effectively prevents device damage caused by excessive power density.

High extinction ratio in-line fiber polarizers

Unstable polarization optical paths are a core bottleneck for fiber lasers, high-precision fiber sensing, and PMD monitoring. Hirundo's in-line fiber polarizer relies on an internal polarizing crystal optical structure to perform bidirectional polarization processing: it filters linearly polarized light of a single orientation out of random or natural light, and can be used in reverse as a polarization analyzer to collect real-time beam polarization data. Matched with wide-temperature-stable packaging, high-precision polarization filtering deeply suppresses orthogonal stray light, while a low insertion loss optical path design cuts attenuation and extends light source lifetime. Temperature-resistant packaging keeps polarization parameters free of drift in harsh outdoor and airborne environments, and the bidirectional reusable design lets one component perform both polarization generation and analysis, reducing material categories and overall cost. Fully automatic coupling and packaging lines with full-dimensional optical testing before delivery deliver consistent extinction ratio, insertion loss, and polarization stability for mass shipment, with customization available for polarization-maintaining pigtails, special packaging, and custom wavelengths.

WDM, isolation, and splitting for OTN/WDM

Backbone OTN/WDM systems use a layered architecture, spanning the optical channel layer, the optical multiplexing section layer, and the optical transmission section layer, in which passive devices perform the optical-layer signal processing. Hirundo provides the key building blocks:

• WDM: DWDM/CWDM multiplexers supporting transmission of more than 96 waves per fiber, with customizable channel spacing such as 0.8 nm or 20 nm, insertion loss below 0.3 dB, and wavelength isolation above 35 dB through precision optical coating.
• Optical isolator: Faraday rotating magneto-optical crystal technology delivering reverse isolation above 40 dB, protecting EDFAs and other active devices from reflections.
• Optical splitter: 1xN splitting with ratio accuracy of plus or minus 0.5 dB, wavelength independent, suited to optical power monitoring and service protection switching.

Engineering breakthroughs that enable long-haul DCI

• Anti-nonlinear design: end-face microlens radius optimization below 5 microns reduces four-wave mixing in high-power scenarios and supports 400G QPSK/16QAM modulated signal transmission.
• Wide temperature stability: operating range extended to -40 to 85 degrees Celsius for field optical relay stations.
• Intelligent integration: hybrid passive devices such as a WDM plus isolator plus splitter 3-in-1 module reduce size by about 60 percent, adapting to high-density OXC all-optical switching equipment.

Free-space products and standard CWDM

Two complementary approaches serve different points in the network. Free-space products process optical signals through lenses and optical filters in free space, offering compact packages, low insertion loss, strong transmission efficiency, and flexible multi-wavelength support. Standard CWDM builds on mature fiber-based cascading and transmission, a widely deployed and cost-effective choice for metro access layers. Choosing between them is a question of reach, capacity, and cost balance rather than one technology replacing the other.

Deployment Proof Points

In a 96-wave DWDM plus OTN electrical crossover scheme, a high-density fiber ring with insertion loss below 1.2 dB and a dimmable optical attenuator with plus or minus 0.1 dB adjustment accuracy delivered 200G service bearing across 4000 km between provinces. In data center interconnect, an all-optical mesh built with silicon-based hybrid integrated devices supports 1.6 Pbps switching capacity in a single rack while reducing power consumption by about 45 percent.

These results underline the overall advantages passive innovation brings to next-generation OTN/WDM. Ultra-high speed and large capacity come from C plus L band systems pushing single-fiber capacity beyond 40 Tb/s on a foundation of low-loss MUX/DeMUX and WSS. Flexibility and intelligence come from the maturity of WSS and tunable devices, which turn the network from a static pipeline into a flexible mesh that SDN controllers can shape on demand. High reliability comes from fast, lossless optical-layer protection switching. And lower power and cost come from device integration, automated production, and miniaturization reducing splice junctions, insertion loss, and the cost of the entire system.

Hirundo at the 15th Optoelectronics Industry Expo

On July 24th, the first day of the 15th Optoelectronics Industry Expo (2024) opened at the China National Convention Center in Beijing. The Hirundo team presented an impressive lineup of passive components including polarization-maintaining products, lasers, optical switches, and Arrayed Waveguide Gratings (AWG). At the booth, visitors showed strong interest in the PM series, couplers and PLC splitters in particular, as well as various custom-wavelength passive devices, reflecting how much of the industry's next capability step depends on exactly these building blocks.

Behind that portfolio is a closed loop of self-developed mass production and technical support. The factory is equipped with integrated precision devices for fiber component processing, coupling, and testing, with automated standardized assembly lines delivering a daily output of 300 units. Every finished product ships with a complete factory test report and fully traceable optical parameters, and engineers provide one-on-one optical path adaptation, customized component models matched to actual equipment operating conditions, and full-process support from datasheet to free sample testing and bulk supply.

Conclusion

The active DAC versus passive DAC question looks like a cable decision, but it is really a system decision about reach, latency, power, and cost. Get it right and the rack stays efficient; get the optical side right, with reliable passive components such as fiber couplers, polarization-maintaining couplers, in-line polarizers, isolators, splitters, and WDM, and the DCI and OTN/WDM layers that connect those racks will carry the bandwidth that 5G, cloud, and AI will demand next.