What Is the Difference Between Active and Passive DAC, and How Do Hirundo Passive Optical Components Support OTN/WDM and Data Center Interconnect?

Data centers, telecom backbones, and AI computing clusters are scaling faster than ever, and every layer of the network — from the copper cable connecting a switch to a server, to the passive optical components that multiplex dozens of wavelengths onto a single fiber — has to keep pace. Two questions come up repeatedly during infrastructure planning: what is the difference between an active DAC and a passive DAC, and which passive optical components actually support OTN/WDM and data center interconnect (DCI) architectures? This explainer answers both questions and shows where Hirundo's passive optical portfolio fits into the picture.

What Is a DAC, and What Is the Difference Between Active and Passive DAC?

A Direct Attach Cable (DAC) is a copper-based, fixed-length assembly used to link switches, servers, and storage inside a rack or between adjacent racks. Instead of converting electrical signals into light, as an optical transceiver module does, a DAC carries the electrical signal directly between two ports. DACs are usually offered alongside optical transceiver families — the same way Hirundo's transceiver portfolio spans data rates up to 800G/1.6T and includes active and direct attach cables (AOC/DAC).

The essential difference between an active DAC and a passive DAC is whether the cable contains powered electronics.

Passive DAC

• No external power supply: the assembly is passive by design and mechanically simple — the same design philosophy that makes passive optical components such as fiber optic couplers attractive in optical networks.
• No signal-conditioning silicon: the electrical signal travels through the copper pair without active equalization or retiming.
• Lower cost and lower power draw: fewer components mean a lower bill of materials and effectively no added power budget.
• Shorter reach: without active conditioning, a passive DAC is limited to relatively short in-rack and rack-to-rack links.

Active DAC

• Built-in signal conditioning: active electronics such as equalizers, re-drivers, or retimers compensate for attenuation and jitter along the copper link.
• Requires power: the module draws power from the port, so the switch or server must supply it.
• Longer reach and thinner cable options: active conditioning extends usable distance and often enables a smaller cable diameter, which helps airflow and cable management.
• Higher complexity and cost: extra silicon increases the bill of materials and the heat load, so active DACs are selected when the link distance demands them.

Active vs Passive DAC: How to Choose

• Distance: choose a passive DAC when the link stays within a short, well-defined span; choose an active DAC when the run goes beyond what an unpowered cable can deliver reliably.
• Power and cooling: a passive DAC contributes zero active power consumption inside the cable, while an active DAC adds a small but real thermal load at every port.
• Cost: passive solutions are simpler and cheaper to deploy at scale, while active solutions cost more but can replace more expensive optical links at intermediate distances.
• Deployment speed: both are fixed-length, plug-and-play assemblies that avoid the tuning and cleaning overhead that optical connectors can require — a common reason DAC remains the default for top-of-rack connectivity.

In practice, network architects treat the active vs passive DAC question as a distance-and-power trade-off: passive where possible, active where necessary. The same logic runs through optical layer design, where passive components that need no electrical power continue to handle signal splitting, combining, filtering, and routing.

How Do Hirundo Passive Optical Components Support OTN/WDM?

OTN/WDM backbone networks use a layered architecture — the optical channel layer (OCh), the optical multiplexing section layer (OMS), and the optical transmission section layer (OTSL) — and passive devices are embedded in every one of those layers. As a manufacturer of fiber optic passives, Hirundo supplies foundational components that make these systems work.

WDM Multiplexers and Demultiplexers

Hirundo's WDM products support DWDM and CWDM technology, enabling transmission of more than 96 wavelengths per fiber with channel spacing that can be customized at 0.8 nm or 20 nm. A precision optical coating process delivers insertion loss below 0.3 dB and wavelength isolation greater than 35 dB, which is essential for dense, long-haul optical multiplexing.

Optical Isolators

Using Faraday rotating magneto-optical crystal technology, Hirundo optical isolators achieve reverse isolation of more than 40 dB, effectively preventing reflection damage to EDFA and other active devices.

Optical Splitters

Hirundo optical splitters offer 1×N splitting ratio accuracy of ±0.5 dB and support wavelength-independent splitting, making them suitable for optical power monitoring and service protection switching scenarios.

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 (splitting) or combining (combination) while keeping the signal spectrum unchanged. Key characteristics include a passive design with no external power supply, bidirectional reversibility with interchangeable input and output ports, and low-loss transmission with insertion loss as low as 0.1 dB in high-quality couplers.

The manufacturing method behind these parts is fusion tapering. Two or more fibers are brought together, heated, melted, and stretched to form a double-cone coupling region. Within that region the optical signal is energized through a gradient waveguide structure, and the splitting ratio can be precisely adjusted by controlling the stretching length. Winding and melting-stretching create the fused coupling region that defines the device's optical behavior. In polarization-maintaining (PM) couplers, the polarization axes of the two fibers must be precisely aligned and kept parallel throughout the process, so the composite waveguide structure distributes or combines optical power while maximally preserving the polarization state of the input light. Polarization extinction ratio (PER) exceeds -20 dB and typical insertion loss stays below 0.1 dB, ensuring high signal intensity and a high signal-to-noise ratio after distribution. Large core diameter fiber couplers extend the same principle to high-power transmission, using fibers with core diameters of hundreds of microns and higher numerical apertures, and a special waveguide structure in the coupling region that can handle a large mode field without being damaged by excessive power density.

In-Line Fiber Polarizers

In fiber laser, high-precision fiber sensing, and PMD monitoring applications, unstable polarization paths are a core bottleneck that limits overall equipment performance. Hirundo's in-line fiber polarizer addresses this with internal polarizing crystal optical structures that provide bidirectional polarization processing: it filters linearly polarized light of a single orientation from randomly polarized or natural light, and can also be used in reverse as a polarization analyzer that collects real-time beam polarization data. Matched with wide-temperature stable packaging, the design delivers high-precision polarization filtering that suppresses orthogonal stray light and raises system extinction ratio, a low-insertion-loss optical path that reduces power attenuation and extends source life, temperature-resistant packaging that keeps polarization parameters drift-free in harsh outdoor and airborne environments, and a bidirectional reusable design that lets one component handle both polarization generation and analysis.

Technological Breakthroughs in Passive Devices for Backbone Networks

To meet the ultra-long-distance transmission requirements of backbone networks, Hirundo products have achieved breakthroughs in several areas.

• Anti-nonlinear design: by optimizing the radius of curvature of the fiber end-face microlens (below 5 µm), the four-wave mixing effect in high-power scenarios is reduced, supporting 400G QPSK/16QAM modulated signal transmission.
• Wide temperature stability: operating temperature range is extended to -40°C to 85°C to meet the deployment needs of field optical relay stations.
• Intelligent integration: hybrid passive devices such as a WDM + isolator + beam splitter 3-in-1 module reduce size by 60% and adapt to high-density deployment of OXC all-optical switching equipment.

How Do Hirundo Components Enable Data Center Interconnect (DCI)?

DCI requires high-bandwidth, low-latency connections between facilities. A 96-wave DWDM + OTN electrical crossover scheme can deploy a high-density optical fiber ring (insertion loss below 1.2 dB) and a dimmable optical attenuator with adjustment accuracy of ±0.1 dB to carry 200G services across 4,000 km between provinces. For all-optical mesh networks, silicon-based hybrid integrated devices support 1.6 Pbps switching capacity in a single rack while reducing power consumption by 45%.

The passive innovation behind these results also enables flexibility and intelligence across the network. The maturity of WSS and tunable devices changes the network from a static pipeline into a flexible mesh, where SDN controllers can create, adjust, or remove optical channels on demand in real time. High-performance passive devices such as optical switches and WSS enable fast, lossless optical layer protection switching, providing more efficient and lower-latency recovery than electrical layer protection. Device integration reduces fiber splice junctions and independent packages, lowering insertion loss and power consumption, while automated production and miniaturization reduce the cost of a single device and therefore the construction and operations costs of the entire system.

Hirundo fiber arrays also support AI supercomputing deployment. They are designed for high reliability and high consistency, enabling mass production with minimal performance deviation, and offer flexible array specification designs from 1CH to 64CH that adapt to the port count of CPO and silicon photonics chips. End-face options include 8°, 0°, 45°, and custom angles to meet diverse customer requirements.

Free-Space Products vs. Standard CWDM: Pros and Cons

• Free-space type products process optical signals in free space through lenses and optical filters. They offer a small package size for easy installation and deployment, low insertion loss with high transmission efficiency, and strong flexibility supporting multi-wavelength transmission. The trade-offs are a large package size with low space utilization in some configurations, plus limited transmission capacity and distance compared with DWDM.
• Standard CWDM mainly performs cascading and transmission of optical signals through optical fibers without direct application of free-space technology. It is a mature technology with wide application, suitable for the metro network access layer, and relatively low cost compared with DWDM. Its drawbacks include slightly higher cost than free-space products in some cases and a need for specific technical support and maintenance.

Quality, Traceability, and Customization

Hirundo's competitiveness rests on a closed loop of self-developed mass production and technical support services. The factory is fully equipped with integrated Fujikura devices for fiber component processing, coupling, and testing, and fully automatic standardized assembly lines deliver a daily output of 300 units for stable mass delivery. Every finished product ships with a complete factory test report and fully traceable optical parameters. For customers with differentiated requirements, professional engineers provide one-on-one optical path adaptation, matching customized component models to actual equipment operating conditions, with full-process support that includes datasheet provision, free sample testing, and bulk quotation — covering the whole service chain from component selection to mass supply. Options such as polarization-maintaining pigtails, special packaging, and custom wavelengths are available for rapid sample development, meeting the demands of both mass orders and small R&D batches.

Key Takeaways

• Active DACs contain powered signal-conditioning electronics for longer copper links; passive DACs are unpowered, simple, and cost-effective for short in-rack and rack-to-rack connections.
• Choosing between them is a distance-and-power trade-off, much like the choice between active and passive approaches elsewhere in the network.
• Hirundo passive optical components — WDM MUX/DEMUX, optical isolators, optical splitters, fiber optic couplers, PM couplers, large core diameter couplers, in-line fiber polarizers, and fiber arrays — provide the building blocks for OTN/WDM backbones and data center interconnect.
• Fusion tapering, precision optical coating, anti-nonlinear design, wide temperature stability, and intelligent integration are the technical foundations behind these components.
• Automated production, full optical testing, and customization options support both mass deployment and early-stage R&D.