---
title: "Coherent vs. Direct Detect: The Decision Your Network Will Make for the Next Decade"
type: technology_deep_dive
target_audience: technical
score: 9/10
---

There's a moment in every network upgrade cycle when you have to decide whether your capacity problem is a data center problem or a WAN problem. That decision determines whether you're buying direct-detect optics or coherent optics, and getting it wrong means buying the wrong technology at the wrong price for the wrong reasons.

The difference is not complexity — it's physics.

Direct-detect transceivers convert light directly to electrical signal at the receiver. The transmitter sends intensity-modulated light; the receiver detects whether the light is on or off (NRZ) or determines the amplitude level (PAM4). No phase information, no carrier recovery, no DSP. This is every QSFP28 100G SR4, DR4, LR4 module you've ever touched. The optics are simple. The manufacturing cost is low. The interoperability is near-universal.

Coherent transceivers transmit using the phase and amplitude of the optical carrier, encoding information in both in-phase and quadrature components, plus two polarizations. At the receiver, a local oscillator laser mixes with the received signal to recover the carrier phase. The DSP processes the electrical signal and recovers the original data. This process — called coherent detection — can be combined with powerful error correction, dispersion compensation, and nonlinear mitigation to achieve spectral efficiency that direct detect can't approach. 400G ZR modules carry 400G over 80km of uncompensated fiber using two polarizations and QPSK modulation. Direct-detect can't do that.

The reason this matters operationally: coherent optics are not better versions of direct-detect optics. They solve different problems. Using coherent optics for an intra-data-center link is like renting a semi-truck to move a box — you're paying for capability you don't need. Using direct-detect for a 400km terrestrial WAN link is physically impossible regardless of price.

The decision tree looks like this.

For links under 10km, within a data center, between adjacent buildings, or on dark fiber with no DWDM: use direct-detect. 10G SR/LR, 25G SR/LR, 100G SR4/LR4/DR4, 400G SR8/DR4/LR4/FR4. The ecosystem is mature, compatible optics are widely available and cheap, and the performance is more than sufficient. No DSP complexity, no coherent penalties, no platform-specific tuning.

For links over 80km, on DWDM infrastructure, crossing metro or long-haul fiber, or requiring line-side amplification: you need coherent. 100G DWDM pluggables (100G ZR, or older CFP/CFP2 coherent), 400G ZR (OIF implementation agreement, 80km target), 400G ZR+ (extended reach, vendor-specific, 600-2000km). These are platform-specific products. The performance depends on the DSP firmware. Interoperability exists but is tested on specific pairs, not assumed universally.

The grey zone is 10-80km. This is where the argument is genuinely complex. Direct-detect LR4 covers 10km. Direct-detect ER4 covers 40km. For some metro deployments, that's enough and the simpler optic wins on cost and operational simplicity. For higher spectral efficiency or higher reliability over that range, coherent 400G ZR makes sense. The answer depends on your fiber budget, amplification infrastructure, and whether you're running DWDM.

The operational complexity difference is real and often understated. A direct-detect link is up or it's down. The DOM data shows TX power and RX power. If both are in spec, the link works. If they're out of spec, you replace the optic. The troubleshooting flow is two steps.

A coherent link has pre-FEC BER, post-FEC BER, constellation diagrams, OSNR margin, PDL, nonlinear noise figures, phase noise metrics, and DSP convergence state. The link can be "up" — passing traffic — while accumulating correctable errors at a rate that will eventually exceed FEC capacity. The diagnosis requires understanding what normal looks like for your specific fiber plant and traffic load. You need more sophisticated monitoring. You need engineers who understand coherent signal processing, or at least understand which parameters indicate which failure modes.

That's not an argument against coherent. It's an argument for having the right tooling and people before deploying it at scale. Organizations that deploy 400G ZR without training, without enhanced monitoring, and without documenting baseline OSNR for every span will have incidents they can't diagnose.

The compatible optic question is different for coherent. For direct-detect 400G DR4, buying compatible is straightforward — the MSA standard is well-defined, the optic is commodity, and every reputable compatible vendor has thoroughly tested it. For coherent, the situation is more nuanced. OIF 400G ZR is a published standard with compliance testing. Compatible 400G ZR modules exist and from reputable vendors they work correctly on tested platform pairs. But the "tested platform pairs" part matters more than it does for direct-detect. A compatible 400G ZR module should be validated on your specific linecard firmware and your specific peer device firmware, not just on the generic platform family. This testing exists — ask for the test matrix.

The price delta matters at scale. A direct-detect 400G DR4 from a tier-1 compatible vendor: $25-40. A coherent 400G ZR: $500-1200 from OEM, $200-400 from compatible. For a 32-port switch spine, that's $800-1280 vs $6,400-12,800 in optics per box. For DCI, coherent is clearly worth it. For a data center fabric, direct-detect is clearly worth it.

The upgrade path to 800G also differs by technology. 800G direct-detect (SR8, DR8, LR8 emerging) extends the same physics with more lanes or higher baud rate. 800G coherent (800G ZR, 1.2T extended reach) similarly extends coherent capabilities. The two tracks don't converge. If you're building DCI infrastructure for 400G ZR today, you're on the coherent roadmap for 800G. If you're building data center fabric with DR4, you're on the direct-detect roadmap.

The decision is not about which technology is better. It's about which technology solves your specific problem. Metro and long-haul WAN: coherent, no compromise. Data center fabric within a campus: direct-detect, no compromise. DCI within a metro at 10-80km: evaluate based on fiber plant, amplification, and DWDM infrastructure. Everything else: the physics tells you which one to use, and the physics doesn't change based on which vendor you're talking to that quarter.

Buy the right tool. Deploy with appropriate monitoring. Train the people who will operate it. That's the whole framework.
