What Is a 40GBASE-LR4 Optical Transceiver?
The 40GBASE-LR4 optical transceiver sits in a kind of middle generation of Ethernet technology. It’s not as old as 10G optics and not as new as the 100G and 400G stuff that gets all the attention now, but it quietly powers a lot of real networks. A typical 40GBASE-LR4 module uses the QSFP+ form factor and runs 40 Gbps over single-mode fiber up to about 10 km. That “LR” stands for “Long Reach,” which already tells you what it’s meant to do — connect things that are not just a few racks apart, but sometimes buildings or even campus blocks away.
Inside the module, you don’t have one single 40G lane. Instead, you have four 10G lanes running in parallel. These four lanes are carried over four different wavelengths using CWDM, usually around 1271 nm, 1291 nm, 1311 nm, and 1331 nm. Those four channels are multiplexed together for transmission and demultiplexed again on the receiving side. So under the hood it’s four 10G pipes, but to the outside world it’s one 40G link that behaves as a single logical connection.
Key Features of 40GBASE-LR4 Long-Reach QSFP+ Modules
A 40GBASE-LR4 module usually comes with a duplex LC connector and runs over standard single-mode fiber. That means you don’t need a fancy parallel-fiber cabling system or MPO connectors to get it working. For many data centers and enterprise networks that were already using SMF for 10G, this made upgrades very straightforward. You could reuse the same fiber and just replace the optics on each side.
Power consumption is another part of the story. A 40GBASE-LR4 QSFP+ module typically sits around 3.5 W or so, depending on vendor and implementation. It’s not ultra-low by modern 100G standards, but it’s reasonable enough that you can pack a lot of ports in a switch without cooking the chassis. Because the LR4 spec is mature, you’ll also find good interoperability between different brands — QSFPTEK, Cisco, Arista, and so on — which makes sourcing much easier.
Typical Use Cases for 40GBASE-LR4 in Data Centers
You usually see 40GBASE-LR4 optics in the aggregation or core layers of a network, not at the very edge. When 40G first started to roll out, it was the natural upgrade path from 10G aggregation. Many networks used 10G at the access layer and 40G at the distribution or core layer. LR4 modules were ideal for those uplinks that needed to cross between rooms, floors, or buildings without resorting to DWDM or exotic long-haul gear.
In leaf-spine architectures, 40GBASE-LR4 can be used for leaf-to-spine connections when the distances are up to 10 km. It’s enough for large campuses and metro-style layouts where different buildings or sites are tied together with single-mode fiber. It’s also handy for linking colocation cages or different areas in the same facility when the runs are just a bit too long for multimode optics.
Why 40GBASE-LR4 Still Has a Place in Modern Networks
If you look at the latest product brochures, everything seems to be about 100G and 400G, but in real deployments 40GBASE-LR4 is still alive and well. Not every environment needs to jump to 100G just to say it’s “modern.” A lot of networks are perfectly fine with 40G at the aggregation layer, especially when they’re not running huge AI clusters or extremely bursty workloads.
The cost profile is also attractive. Prices of 40GBASE-LR4 modules have dropped as newer generations have become mainstream. For many operators, that makes LR4 a sweet spot — high enough bandwidth to handle growing traffic, mature enough to be cheap and stable. Instead of ripping and replacing everything with 100G, they can keep 40G LR4 links where it makes sense and upgrade only the busiest paths.
40GBASE-LR4 vs Other 40G Options
Compared with 40GBASE-SR4, which runs on multimode fiber with MPO connectors, 40GBASE-LR4 is more fiber-efficient. SR4 needs eight fibers (four transmit and four receive), while LR4 gets away with just one pair of SMF. In environments where single-mode fiber is already in place, LR4 wins hands down in terms of cabling simplicity and long-term scalability.
Compared with PSM4-style solutions, LR4 again uses just two fibers instead of eight. That can be a big deal in older buildings, cramped conduits, or scenarios where pulling new fiber is either painful or expensive.
Conclusion
The 40GBASE-LR4 optical transceiver may not be the brightest star in networking anymore, but it plays a very steady, very important role. It delivers 40G over long distances using simple duplex SMF, keeps cabling under control, and leverages a standard that’s been tested in countless production networks. For data centers and enterprises that want reliable 40G connectivity without overcomplicating their fiber plant, 40GBASE-LR4 is still a very practical and very relevant choice.