DML or EML?
Two types of DML and EML lasers are used in 100G optical modules. Which one to choose?
Optical modules 100G QSFP28wand use various optical technologies to transmit data. In the case Shorter ranges Lasers are usually used VCSEL (Vertical-Cavity Surface-Emitting Lasers - surface emission lasers with vertical cavity). For longer ranges In turn, electroabsorption modulated lasers are used (EML) or Directly Modulated Lasers (DML).
The temporary problem with the availability of EML lasers in the market has become the reason for the search for an alternative solution for the 100G QSFP28 modules with a greater range. DML optics with DFB TOSA (Transmitter Optical Sub Assembly) turned out Cost-effective and reliable alternative.
DML - directly modulated laser
DML, or Directly Modulated Laser, uses diffuse feedback structure with diffraction grating in waveguide for direct modulation, which is why it is also called “DFB” laser (diffused feedback laser diode). In DFB lasers diffused Bragg headlight is designed to lock in the desired wavelength. With DML, the laser power is modulated directly via an internal driver chip. They are usually quick electronic silicon-germanium controllers.
The modulation rate and transmission distance strongly depend on the spectral width of the laser line. A narrower line width is required for higher modulation speed (data rate) and greater distance.
In DML, information is placed on the optical beam by modulating the supply current, which is input electrical signal on/off generated by the driver integrated circuit and is directly transmitted to the laser diode system to input the modulated optical signal. DML is single integrated circuit with a simple electrical circuit, making it ideal for circuits requiring small area and low power consumption, so it can be used in a small QSFP28 package.
Direct modulation of the properties of the laser changes the refractive index, which leads to a large chromatic dispersion. DML performance therefore deteriorates over longer ranges more than 10 km due to larger chromatic dispersions and lower frequency response.
The relatively low extinction coefficient compared to EML (output optical signal “1" versus “0") of DML caused by switching the supply current on/off becomes higher as the supply current (input/off electrical signal) increases, however, it decreases if the current becomes too large.
The chromatic dispersion in DML is caused by the drift of the laser wavelength due to changes in refractive index in its active area, due to the changing amounts of current introduced. DML lasers are typically used in optical modules: 100G QSFP28 PSM4/25G SFP28 LR, 100G QSFP28 CWDM4/25G SFP28 BiDi with 10km range and 100G QSFP28 LR4.
EML - electro-absorption modulated laser
EML, or electroabsorption modulated laser, consists of diode lasers with Distributed feedback (DFB) integrated with an electroabsorption modulator (EAM) in a single integrated circuit. In the case of DFB, a diffuse Bragg reflector is used to accurately block the desired wavelength. The DFB operates in continuous wave (CW) mode and input voltage on/off signals are transmitted to the EAM section to generate optical output signals.
Therefore, the laser properties themselves are not changed by the modulation process, since they are in the DML. The output power of the DFB is adjusted to obtain the appropriate output power, then the EAM is responsible for modulating light for NRZ or PAM4 output according to your needs.
Compared to DML, EML has lower chromatic dispersion with stable wavelength when operating at high speed, since the input signal current to the laser section is not modulated, and therefore does not change. The frequency response of the EML depends on the capacity in the EAM section, which allows a high operating speed, greater than 40 GHz. The extinction in the EML is due to absorption, as the coefficient changes with the modulated voltage transmitted to the EAM section, and the extinction coefficient becomes higher at a large input voltage (electrical signal on/off).
Due to their lower chromatic dispersion, EML lasers are mainly used in Higher bit rates (>25 Gbps) and longer ranges (>10-40 km).
EML lasers are typically used in 100G QSFP28 ER4 and ZR, 100G PAM4 CWDM and CFP/CFP2/CFP4 optical modules.
Comparison of DML and EML
In general, DML are used in applications with lower data rates and shorter distances (up to 10 km), while EML supports greater distances at higher data rates.
Optical modules with DML laser are cheaper They are used in approximately 95% of all 100 Gbps applications. Optical modules with EML laser, due to the use of an external modulator, allow to achieve longer distances and “cleaner” signal, are more expensive and are usually chosen for long optical links.
Power consumption of optical modules with EML laser
The power consumption of cartridges with EML laser is greater than that of cartridges with DML laser and sometimes it happens that it exceeds the value of the maximum power consumption of the cartridge specified MSA standard class 6 - 4.5 W. Due to the emergence of higher bitrates 100/200/400 Gbps... technology development and changes in hardware requirements, in September 2019 the MSA standard was expanded and two additional classes appeared: 7 to 5 W and 8 to 10 W.
However, this is a relatively recent change, which is why most of the available devices do not yet support this standard. When choosing inserts with an EML laser, necessarily check what power consumption they have and whether your network device can handle it. The latest at the time of this writing, Class 8 of the MSA standard is as much as 10W, which is twice as much heat to dissipate compared to the previous standard and some devices may have a big problem dissipating so much heat.
The use of an insert with too much power consumption is, at best, not to pick up the link, and in the worst case - even damage to an expensive network device.
At Salumanus, we believe that good networks are economic networks, thanks to which our partners can function efficiently and achieve a high rate of return on investment.
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