100G LR4 QSFP28 single and dual rate transceivers | DML and EML

The aim of these four articles about optical transceivers is to familiarise potential users with the technology used in the modules, the potential solutions, functional differences, areas of application and possible integration with other GBC Photonics products.

Directly Modulated Laser

A Directly Modulated Laser (DML) uses a diffraction grating to receive inner feedback which is used for stabilising a modulated wave. Direct modulation uses alternating current to power a laser diode. Turning it on activates the laser and this state is interpreted as a logical 1, while turning it off deactivates it, which is interpreted as a logical 0.

The advantages of this type of laser are compact size — both the laser diode and electrical control system co-exist within one semiconductor element — relatively low power consumption and the fact that the emitted wave has a narrow spectrum. Thanks to this last feature, this type of laser can be used to transmit high frequency signals.

Simplicity in construction is a great advantage, but at the same time it is also the greatest drawback of such a laser. Changing the current that directly controls the diode causes changes in the frequency of the generated radiation — laser chirp — which leads to the spectrum of the emitted radiation becoming blurred. This phenomenon, combined with chromatic dispersion, dramatically limits the scope of a DML laser.

Electro-absorption Modulated Laser

An Electro-absorption Modulated Laser (EML) is powered by a constant current, with the intensity of the radiation beam being controlled by a device known as an electro-absorption modulator. Here, the laser diode always emits light with the same, maximum, intensity, and the modulating signal changes the value of the output light signal. Compared to a DML laser, an EML laser consumes more power and is a more advanced optoelectronic device.

Compliance and benefits for 100G LR4 QSFP28 transceivers

Both types of lasers comply to the MSA standards for 100G LR4 QSFP28 optical transceivers, which is a multi-source agreement containing unified guidelines for constructing optical transceivers. This means that they can be used in devices from various manufacturers.

However, the differences between the two significantly affect the parameters of the transceiver modules in which they are used. A DML laser consumes less power than an EML laser at low power radiation levels. However, when the voltage of the supply current — and modulating current — that powers the diode is increased for a DML laser, it leads to greater changes in the frequency of the emitted wave. This, in combination with chromatic dispersion, causes unwanted blurring of the signal spectrum which limits its effective range.

That is why a DML laser works effectively at lower values of the supply current that powers the source of the laser light. In comparison, the diode in an EML laser constantly emits light with the maximum intensity. Moreover, an EAM modulator requires additional energy to power the laser and to modulate the output light.

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