GBC Photonics 100G Optical Modules - DML and EML Lasers
Advantages and disadvantages of DML and EML laser
DML, or Directly Modulated Laser, is an element in which a diffraction grating is used to obtain internal feedback stabilizing the modulated wave. Direct modulation involves the use of variable current to power a laser diode.
Turning on the modulating current ignites the laser and this state is treated as logical “1", and turning off the modulating current extinguishes the laser and this state is treated as logical “0".
The advantages of this type of laser are small in size - the laser diode and the electrical control system are combined within a single semiconductor system - relatively small power consumption and a narrow spectrum of the emitted wave. Especially the latter characteristic causes that such lasers can be used to transmit high-frequency signals.
However, the advantage in the form of simplicity of construction of this type of laser is at the same time the source of its greatest weakness. Change of control current directly the laser diode induces changes in the frequency of the generated radiation (laser chirp), which in effect leads to rreverberation of the band of the emitted radiation spectrum. The chromatic dispersion causes this phenomenon to significantly limit the range of the DML laser.
EML, in turn, is a laser, in which the radiation source is supplied with a current of constant intensity, and to change the intensity of the radiation beam is used modulator EAM (electro-absorption modulator — electro-absorption modulator).
The laser diode in this case always shines with the same, maximum power, and modulating signal changes the intensity of the output light signal. Compared with DML laser, EML laser consumes more power and is a more complicated optoelectronic system.
Lasers of both types — DML and EML — meet the conditions defined in MSA standards (multi-source agreement — unified module construction rules to ensure their use in devices from different manufacturers) for optical modules 100G QSFP28. However, the differences between them have a great influence on the parameters of the modules built using each of them.
This is the situation we are facing in the context of the power consumed by these lasers. DML laser consumes less power than EML laser at low supply currents. Increasing the value of the current supplying (and simultaneously modulating) the diode of the DML laser causes a simultaneous increase in the frequency of the emitted wave.
In conjunction with chromatic dispersion, this causes undesirable blurring of the signal spectrum limiting its range. Therefore, the DML laser operates at lower values of the current supplying the laser light source. In the EML laser, on the other hand, the diode shines all the time at maximum power, and in addition, the EAM modulator requires additional energy for its power supply and modulation of the output of the light beam.
Application Both types of lasers with regard to the transmission speed and range of the optical modules is shown in the map below.
