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EML vs. DML: What's the difference?

While discussing optical transceivers (especially 100G), we’re often asked about the two different types of laser technology: DML and EML. What is the difference between these two technologies? How important is this distinction on a day-to-day level? This article will answer all of these questions.

What do they mean?

DML refers to a directly modulated laser diode. This laser can also be called a distributed feedback (DFB) due to the structure used to build it. A DML features a single chip with a simple electrical circuit, making it ideal for circuit designs that require a small footprint and low power consumption. It places information on the optical beam by modulating an on/off electrical input generated by a driver IC. This is in turn directly applied to the laser dioide, which generates the modulated optical signal output.

On the other hand, EML refers to an electro-absorption modulated laser. This design integrates a laser diode (structurally similar to a DML diode) with an electro-absorption modulator (EAM) in a single chip. The laser diode operates under a continuous wave (CW) condition, and input on/off signals are applied to the EAM section to generate optical output. This design features lower chromatic dispersion, so it’s better suited to higher data rate transmission over longer distances.

When are they used?

Generally speaking, DML are used for lower data rate applications and shorter distances, while EML handle longer distances at higher data rates as shown below:

So… which one do I choose?

As the graph shows, you won’t have to make a choice for most applications. DML’s technical limitations restrict its use to 10km and under, whereas EML’s relatively higher cost makes its use economically infeasible for 1G and 10G data rates, and distances under 10km. However, the demand for EML lasers for 400G and 100G single lambda applications may decrease the cost differential going forward.

The only meaningful overlap occurs between 25G and 100G at 10km. For these applications, here’s how the two stack up across a number of key parameters:

Despite slight differences in cost and performance, this is not a significant disparity for most applications on a day-to-day operational level. In fact, both laser technologies are fully interoperable with one another.

If you would like to discuss these transceivers with our experts, contact us today.

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