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

As large data centers migrate to ultra high-speed infrastructures, optical communication technologies are in place to keep pace with the ever-expanding number of users, devices, and applications. QSFP28 and QSFP-DD transceivers are today heavily deployed for data centers and cloud services.

Those 100G/200G/400G/800G transceivers support long distances of 2 km to 40 km. They currently use predominantly either Directly Modulated Lasers (DMLs) or Electro-absorption Modulated Lasers (EMLs). This article will discuss the basics of EML and DML and highlight their key differences.

Directly Modulated Laser (DML)

DML refers to a directly modulated laser. This laser is also called distributed-feedback laser diode (DFB) since it uses distributed feedback structure for direct modulation. 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.

DML modulates the input on/off electrical signals generated by a driver IC to place information on the optical beam. The information is then directly applied to the laser diode chip to produce a modulated optical signal.

The modulation speed and transmission distance vary slightly, depending on the spectral line width of the laser. The narrower the spectral line width is, the higher the modulation speed and the longer the transmission distance. Generally, DMLs have a relatively narrow spectral line width.

Benefits of DML

• DMLs tend to be much more stable than lasers like FP abd DBR as the grating and the reflection are not only at the two ends of the cavity of the laser but are almost continuous along the cavity.

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

• DMLs generally cost relatively less and have low power consumption since optical signals are modulated by current change in a DML.

• Compared to Fabry-Perot lasers, DMLs have a narrower spectral line width, meaning higher modulation speed and longer transmission distance.

Limits of DML

• There is high chromatic dispersion in DMLs because direct modulation changes the laser properties directly.

• DMLs have relatively low frequency response and extinction ratio as they are all limited by the relaxation frequency.

• Associated frequency shifts, coupled with dispersion in the fiber, cause the performance of a DML to degrade over longer reaches (>10km).

Electro-absorption Modulated Laser (EML)

EML refers to an electro-absorption modulated laser. An EML diode is structurally similar to a DML one. The difference is that there is an electro-absorption modulator (EAM) integrated in a single chip.

The laser diode operates under a continuous wave (CW) condition. In an EML diode, signal modulation is on the EMA section instead of the electrical section. It means that optical output signals are generated when input on/off signals are applied to the EAM section.

Benefits of EML

• 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 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.

Limits of EML

• EMLs are more power-consuming as there is an electro-absorption modulator (EAM) integrated within the chip.

• EMLs require a more complex electrical configuration and diode layout.

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

EML vs. DML

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. While EML cannot have the laser directly modulated and mainly relies on an external modulator also called an electro-absorption modulator. The difference in working principle 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.

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 its 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) application as they have a relatively higher cost.

FS Transceivers Boost 100G/200G/400G/800G Migration

FS offers a comprehensive line of 100G/200G/400G/800G transceiver modules. They take advantage of different laser technologies to help users achieve high-performance 100G/200G/400G/800G data center networks. The following two figures will all FS transceiver modules with either DML or EML.

FS 100G/200G/400G transceiver modules are equipped with either DML or EML to enable high-speed data transmission of varying reaches from 500m to as far as 80km. To boost easy migration to 800G, FS 800G transceiver modules take full advantage of EML to handle distances from 500m to 10km. They prove to be a flexible and reliable choice for various applications in your diverse 100G/200G/400G/800G data center networks.

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.