Direct vs. Coherent Detection: The Ultimate Guide for DCI Networks

Data center traffic is exploding worldwide, driving the need for high-capacity interconnection between facilities. 100 Gb/s links (and higher) are now common between regional data centers, often stretching tens of kilometers. In this “metro” DCI context, network designers must choose between direct detection and coherent detection of optical signals. Each approach has different performance, cost, and complexity characteristics. This article reviews both techniques and compares their suitability for 100G metro DCI, helping readers make an informed choice.

Technology Overview：Direct vs Coherent Detection

Block diagram of a coherent-detection transceiver (A) and a direct-detection transceiver (B). ADC, analog-to-digital converter; DAC, digital-to-analog converter.

Direct detection, also known as intensity-modulation direct-detect (IM/DD), encodes data by modulating the light intensity. At the receiver, a photodiode directly converts the optical power into an electrical signal, recovering only the amplitude of the optical carrier without capturing phase or polarization information.

Coherent detection, in contrast, mixes the incoming optical signal with a local oscillator at the receiver. This allows the recovery of both amplitude and phase, and often polarization, enabling the extraction of any phase or frequency information carried by the transmitted signal. Digital signal processing (DSP) is then used to process the electrical outputs and decode the transmitted data. Coherent detection is widely used for long-haul and high-capacity optical links, supporting advanced modulation formats and enabling higher spectral efficiency.

Coherent and direct detection transceivers differ not only in signal recovery capability but also in hardware architecture. The image above shows a simplified block diagram of the two transceiver types.

Key Performance Comparison：Direct vs Coherent Detection

In metro-scale DCI environments, where link distances typically range from tens to a few hundred kilometers, both detection types show distinct operational characteristics.

The table below summarizes the performance differences between direct and coherent detection.

Feature	Direct Detection (IM/DD)	Coherent Detection
Signal Information	Amplitude	Amplitude + Phase + Polarization
Modulation Format	NRZ, PAM4	DP-QPSK, QAM
DSP & Dispersion Tolerance	Limited; may need dispersion-managed fiber	High; DSP compensates chromatic dispersion and polarization effects
Power Consumption	Low (4–6 W per 100G)	Higher (20–50 W per 100G)
Cost & Size	Smaller, simpler, cost-effective	Larger, more expensive, complex
Reach	Up to ~80 km	Hundreds to thousands of km

In short, IM/DD suits short-reach metro DCI, while coherent optics extend reach and capacity for more demanding regional or inter-city links.

Application Scenarios：Direct vs Coherent Detection

In practical DCI deployments, both direct detection (IM/DD) and coherent detection have distinct roles based on reach, capacity, and network requirements.

In real-world Data Center Interconnect (DCI) deployments, both direct detection (IM/DD) and coherent detection play essential but different roles depending on network reach, capacity, and architecture requirements.

Direct Detection (IM/DD) – Short to Medium Reach DCI

Metro and Regional Interconnects:

Pluggable IM/DD modules (e.g., PAM4-based) enable cost- and power-efficient connections between data centers across short to medium distances, typically up to 80 km, without the need for complex line systems.

Campus and Enterprise Links:

Ideal for short-range, point-to-point connections in campus or enterprise networks, eliminating the need for optical amplification or dispersion compensation.

Edge Site Connectivity:

Compact, low-power modules support flexible and scalable connectivity for edge data centers and distributed facilities.

Evolving Capabilities:

Enhancements such as carrier-assisted or advanced direct detection schemes can improve spectral efficiency and dispersion tolerance, further extending the applicability of IM/DD in modern metro and regional DCI scenarios.

Coherent Detection – Long Reach and High-Capacity DCI

Extended Metro and Regional DCI:

Standardized

pluggable coherent modules

(e.g.,

400ZR

, 400ZR+) are increasingly deployed in metro and regional networks, offering high spectral efficiency and seamless integration with switches or routers.

Long-Haul and High-Capacity Links:

Coherent detection enables multi-terabit per fiber transmission over hundreds to thousands of kilometers, supporting dense WDM systems and advanced network topologies.

Advanced Modulation and DSP:

Digital signal processing compensates for chromatic and polarization-mode dispersion, enabling the use of complex modulation formats and reducing operational complexity for long-reach DCI deployments.

In summary, direct detection is best suited for short-reach, cost-sensitive, and power-efficient DCI links, while coherent detection is preferred for longer-reach, high-capacity, and scalable network interconnects.

Selection Guide：Direct vs Coherent Detection

When selecting between direct detection (IM/DD) and coherent detection, operators should evaluate not only distance, cost, and power, but also spectral efficiency, network scalability, and operational simplicity:

Distance:

IM/DD is most cost-effective for short to medium spans (up to ~80 km). For longer distances or higher-capacity demands, coherent optics are typically preferred.

Spectral Efficiency and Capacity:

Coherent detection enables advanced modulation and dual-polarization formats for high per-fiber throughput and efficient DWDM use. Evolving IM/DD technologies can boost spectral efficiency for metro and regional links.

Cost and Power:

IM/DD modules offer lower cost and power consumption. Coherent pluggables, while standardized and compact, still involve higher power and cost.

Operational Complexity:

Coherent DSP simplifies planning and maintenance by compensating for impairments automatically. IM/DD long-reach links may need dispersion management or amplification, increasing operational overhead.

Future Scalability:

IM/DD is advancing with higher-baud PAM formats to extend reach and capacity. Coherent optics are scaling toward 400G/800G and beyond, supporting future high-capacity DCI needs.

Modern networks often adopt a hybrid strategy: short, cost-sensitive links use direct detection, while medium-reach high-efficiency or long-haul high-capacity routes utilize pluggable coherent modules, achieving reliable, flexible, and scalable DCI networks.

Conclusion

Both Direct Detection and Coherent Detection play essential roles in modern DCI networks. Direct detection provides a simple, low-power, and cost-effective solution for short-to-medium reach applications, while coherent detection delivers long reach, high capacity, and superior spectral efficiency for more demanding network scenarios. By understanding the trade-offs and evaluating network requirements, operators can select the most suitable solution—or a hybrid approach—to build reliable, scalable, and cost-efficient DCI architectures.

FS offers a comprehensive portfolio of 100G PAM4 direct-detection modules, 100G coherent transceivers and 400G coherent transceivers supporting both short-reach metro links and longer regional DCI deployments. All FS products undergo rigorous real-world testing to ensure perfect compatibility with a wide range of switches and routers. This guarantees seamless deployment, simplifies network planning, and enhances scalability, helping operators build high-performance, reliable, and future-ready DCI networks.