As network operators accelerate their migration from 40G to 100G and even 400G backbone infrastructures, one might assume that 40G optics are heading toward rapid obsolescence. However, the reality is more nuanced. 40G QSFP+ modules continue to ship in significant volumes, particularly in price‑sensitive segments, legacy network upgrades, and specialised long‑reach applications. Among these, the 40GBASE‑ER4 QSFP+ – a 1310nm LAN‑WDM transceiver capable of reaching 40km over single‑mode fibre – occupies a unique position. While 100G LR4 modules have become more affordable, the 40G ER4 remains relevant for specific use cases where cost per kilometre and existing infrastructure matter more than raw speed. This article analyses the market dynamics of 40G optics in 2025–2026, the competitive pressures from higher speeds, and the niches where 40G ER4 will continue to survive – and even thrive.
The Current State of 40G Optics in a 100G/400G Era
Why 40G Refuses to Disappear (Installed Base and Cost)
Despite the relentless push toward 100G and 400G, the installed base of 40G equipment remains enormous. Many enterprise data centres, metropolitan aggregation networks, and regional telecom backbones deployed 40G ports between 2015 and 2020, and those assets have not been fully depreciated. Replacing a functioning 40G switch line card or router interface with a 100G counterpart involves not only capital expenditure but also service disruption and re‑engineering of fibre paths. For network operators under budget constraints, keeping 40G links alive is a rational financial decision. Moreover, the per‑gigabit cost of 40G QSFP+ modules has reached a floor: a standard 40G SR4 or LR4 can be purchased from third‑party suppliers for a fraction of its original price, while 100G modules still carry a premium. This cost gap is especially pronounced in long‑reach variants like ER4, where the 100G LR4 module (10km) is significantly cheaper than a 100G ER4 (40km) or ZR4 (80km) – but a 40G ER4 remains substantially more affordable than any 100G module capable of 40km.
The ER4 Niche: Distance as a Differentiator
The defining feature of the 40G QSFP+ ER4 is its 40km reach over standard single‑mode fibre. For connections between 10km and 40km, operators have limited options. They can use 100G LR4 (only 10km) with mid‑span optical amplifiers, which adds complexity and cost, or they can deploy 100G ER4 (typically 40km) at a price point that remains two to three times higher than 40G ER4. Alternatively, they can keep using the existing 40G ER4 link and aggregate traffic onto it. In many regional metro networks, the peak bandwidth demand between two aggregation sites is still under 30Gbps, making a 40G link perfectly adequate. Upgrading to 100G would overprovision capacity by more than 200% while tripling the optical component cost. Therefore, the 40G ER4 survives not because it is technologically superior, but because it fits the actual bandwidth requirements of thousands of existing links perfectly.
The Pressure from 100G LR4 and Coherent Lite
Price Erosion of 100G Modules
The most direct threat to 40G ER4 comes from falling prices of 100G LR4 (10km) and the emergence of “coherent lite” modules such as 100G ZR (80km). As of 2025, a standard 100G LR4 QSFP28 can be found for roughly 1.5 times the price of a 40G ER4, while delivering 2.5 times the bandwidth. For links shorter than 10km, the economic argument to skip 40G altogether is already winning. However, for distances beyond 10km, the comparison changes. A 100G ER4 module (40km) still costs three to four times more than a 40G ER4, because the tighter wavelength tolerances and higher‑power EML lasers required for 40km at 100G are significantly more expensive to manufacture. This price gap is expected to narrow by 2026 but will not disappear entirely – the physical challenges of launching 25G per wavelength over 40km versus 10G per wavelength over 40km are fundamentally different.
When 400G Enters the Metro
The deployment of 400G in metro core networks began around 2023–2024, and by 2025–2026, 400G is expected to become the standard for high‑capacity backbone links (typically 80km to 120km using 400G ZR or ZR+ coherent optics). This development ironically helps the 40G ER4 rather than killing it. As network operators deploy 400G on their core rings, they often “sweep” existing 100G and 40G links into lower‑tier roles. A 40G ER4 that was once a primary link between two central offices may become a backup or overflow path once a 400G backbone is activated. Because the 40G ER4 uses simple direct‑detection optics (no coherent DSP), it consumes very little power and introduces no latency from forward error correction. These characteristics make it an excellent choice for secondary, always‑on protection paths where 100G ER4 would be overkill.
Survival Strategy: Where 40G ER4 Will Remain Relevant
Secondary Links and Backup Paths
One of the strongest survival niches for the 40G ER4 in 2025–2026 is as a dedicated backup or diverse path. Many metro rings are built with two physically separate fibre routes. The primary route may be upgraded to 100G or 400G, while the secondary route retains a lower‑cost 40G ER4 link. In a failure scenario, the secondary link only needs to carry essential traffic; it does not need to match the primary’s bandwidth. Since 40G ER4 modules are mature, widely available from multiple sources, and have well‑understood failure modes, network engineers trust them for backup roles. Additionally, because the backup link is often dark until activated, the higher loss budget of the ER4 (up to 24dB) provides a safety margin when fibre conditions are not ideal.
Greenfield vs Brownfield Networks
For new network builds (greenfield), few engineers would design a 40G link today unless the distance is exactly between 10km and 40km and the required bandwidth is below 40G with no growth forecast for five years. Therefore, greenfield deployments of 40G ER4 will be rare. However, brownfield networks – existing facilities that are being expanded or refurbished – will continue to order 40G ER4 modules well into 2026. The reason is simple: compatibility. Adding a new 40G ER4 link to an existing 40G aggregation ring requires zero changes to the switching hardware or management software. The module is plug‑and‑play, whereas adding a 100G link might require new line cards, different power supplies, and updated software licenses. For brownfield expansion, the operational simplicity of 40G ER4 is a compelling advantage.
Outlook for 2025-2026
The 40G ER4 will not disappear by 2026. Instead, its market will gradually contract at a rate of approximately 10–15% per year, driven by two forces: the falling cost of 100G ER4 (and eventually 100G ZR) and the natural retirement of older 40G switches. However, the module will continue to see steady demand from telecom carriers in emerging markets (where budget constraints are severe), from utility and transportation networks (where 40km distances are common but 100G is unnecessary), and from backup link applications in developed markets. By 2026, the average selling price of a 40G ER4 module is expected to drop below $200 for third‑party brands, making it an impulse purchase for network maintenance inventories. The 100G/400G wave does not drown the 40G ER4; it simply pushes it into deeper, quieter waters where distance and low cost matter more than headline speed. For network operators who understand this niche, the 40G ER4 will remain a reliable, cost‑effective workhorse for years to come.
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