Introduction To DML And EML Modulation Methods For Optical Modules

• Background

When discussing optical transceiver parameters, modulation schemes are a key consideration, and the transmitter modulation method is specified in the datasheet of some optical modules, as shown in the figures below:

• The transmitter laser modulation mode is marked as EML in the Moduletek 25G ER optical transceiver datasheet

Figure 1 Moduletek 25G ER Optical Transceiver Datasheet (EML Marked)

• The transmitter laser modulation mode is marked as EML in the Moduletek 10G ZR optical transceiver datasheet

Figure 2 Moduletek 10G ZR Optical Transceiver Datasheet (EML Marked)

Optical transceivers primarily adopt two mainstream modulation technologies: DML and EML. This article provides a brief introduction to both.

Basic Principle of Optical Transceivers

The core function of an optical transceiver is to achieve optical-electrical conversion. Below is a simplified working principle diagram:

Figure 3 Working Principle Diagram of Optical Transceiver

The optical signal transmitted through optical fibers is not constant; instead, it is a modulated signal with varying intensity. The characteristics and application differences between DML and EML modulation technologies are explained below.

• DML Modulation

DML stands for Directly Modulated Laser.

Its basic principle is to directly control the current passing through the laser diode (LD) to generate optical signals of different intensities:

• When the modulation signal is at a high level: Modulation current flows through the LD, and the laser emits light normally.

Figure 4 DML Modulation – High Level

• When the modulation signal is at a low level: No modulation current flows through the LD, and the laser stops emitting light.

Figure 5 DML Modulation – Low Level

In direct modulation, the laser diode operates under fluctuating bias conditions, leading to various nonlinear effects that degrade the quality of the output optical signal.

• EML Modulation

EML stands for Externally Modulated Laser (corrected from "External Modulated Laser").

Its basic principle is to supply a constant current to the laser diode, ensuring the LD emits continuous, stable light. An external electro-absorption modulator (EAM) then adjusts light transmittance to generate optical signals of different intensities:

• When the modulation signal is at a high level: The electro-absorption modulator barely absorbs light from the LD, allowing most light to pass through.

Figure 6 EML Modulation – High Level

• When the modulation signal is at a low level: The electro-absorption modulator absorbs most of the light, leaving only a small amount to pass through.

Figure 7 EML Modulation – Low Level

EML technology is commonly used in high-speed (typically above 10 Gbps) and long-haul (typically over 40 km) communication systems (unified "typically" to avoid redundancy).

• Summary

The key difference between DML and EML lies in the operating state of the laser diode:

• In DML, the laser diode operates in an unstable state with fluctuating light intensity.

• In EML, the laser diode maintains a stable, constant continuous light output.

Therefore, EML is more suitable than DML for high-speed and long-distance transmission applications.

Moduletek sincerely serves you.

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Introduction To DML And EML Modulation Methods For Optical Modules