EML vs. DML: Essential Laser Technologies in 100G/200G/400G/800G Optics

The key laser technologies used in 100G/200G/400G/800G transceivers are EML and DML. So what are the differences between them? This article will discuss the basics of EML and DML and highlight their key differences.

EML vs DML: What Are They?

DML refers to a directly modulated laser. This laser is also called a distributed-feedback laser diode (DFB) since it uses a distributed feedback structure. A DML uses a single chip with a simple electrical circuit design, so it can be an optimal choice for a compact circuit configuration with low power consumption.

EML refers to an electro-absorption-modulated laser. An EML diode is structurally similar to a DML one. The difference is that EML integrates a laser diode with an Electro-absorption Modulator (EAM) in a single chip. In an EML diode operating under continuous wave (CW) conditions, the optical output signals are generated through modulation of the EAM section when on/off input signals are applied.

EML vs DML: What Are the Benefits?

EMLs feature low chromatic dispersion since the process of modulation will not change laser properties

constantly a EMLs

feature low chromatic dispersion since the process of modulation will not change laser properties constantly.

EMLs can operate at higher modulation speeds and have a much lower chirp compared to DML.

EMLs are an ideal choice for high-speed and long-distance transmission because of lower dispersion in the fiber.

DMLs typically employ a distributed feedback (DFB) structure with a diffraction grating integrated in the waveguide to achieve stable operation.

DMLs feature a single chip and provide a simple circuit design, making them more compact and fit into more small-sized configurations

and lower power consumption

.

A laser's modulation speed and transmission distance depend critically on its spectral linewidth. Compared to Fabry-Perot lasers, DFB have a narrower spectral line width, enabling higher data rates and longer reaches, making them ideal for high-performance DMLs.

EML vs DML: What Are the Limits?

EMLs require a more complex electrical configuration and diode layout.

EMLs generally cost more as they use electric absorption to modulate signals.

EMLs provide a higher extinction ratio and thus better performance, but this also increases chirp, easily causing inter-symbol interference (ISI).

DMLs feature high chromatic dispersion because direct modulation directly alters the laser properties.

DML lasers utilize direct modulators to produce laser light, and they usually function at lower frequencies but with higher power.

The performance of DMLs degrades over longer reaches (>10km) due to frequency shifts and fiber dispersion.

EML vs. DML: How Do They Differ from Each Other?

The above introduction to EML and DML shows they are quite different from each other. The following part will dig deeper and highlight their differences in key aspects.

Working Principle

DML is designed with a distributed feedback structure, and its waveguide has a diffraction grating to enable stable operation, thus realizing direct modulation. EML cannot have the laser directly modulated and mainly relies on an external modulator also called an electro-absorption modulator. The difference in working principles results in their different features and functions.

Key Parameters

The following figure shows how DML and EML stack up against each other across a number of key parameters.

Parameter	DML	EML
Speed	Up to 100G	25G and higher
Reach	2-10km	10km and over
Market Maturity	Mature	More Advanced
Reliability	Good	Better
Extinction Ratio	Good	Better
Max Power Consumption	4.0W	4.5W
MSA Compliant	Yes	Yes

Use Case

Due to limitations such as greater chromatic dispersion and lower frequency response, DMLs are mainly used for relatively lower speeds (≤25Gbps) and shorter reaches (2-10km) applications in telecom and data centers.

By contrast, EMLs have an upper hand in applications with higher speeds and longer distance transmission, due to their smaller chromatic dispersion. They can operate at higher speeds with a much lower chirp, making them an ideal choice in high-performance long-haul coherent optical communication systems.

Naturally, EMLs are not an economical option in lower data rate (1G-10G) applications under 10km as they have a relatively higher cost.

Conclusion

In summary, both EML and DML have their own pros and cons. They are able to bring out the best performance if used in the right application scenario. That said, you still need to take into consideration a number of factors such as network infrastructure and business budget before rushing into a decision.