100G QSFP28 transceivers use different optical technologies to transmit data to different distances. Shorter reaches typically use Vertical Cavity Surface Emitting Lasers(VCSELs) and longer reaches use Electro-absorption modulated lasers (EMLs) or Directly Modulated Lasers (DMLs). The recent shortage of EML lasers in the market forced design engineers to come up with an alternate solution for the longer reach 100G QSFP28 transmitter. DML optics with DFB TOSA proved to be a cost effective and reliable alternative.
In this post, we take a look at the basics of DML and EML based optics.
Directly Modulated Laser (DML)
DMLs generally use distributed feedback structure with a diffraction grating in the waveguide for stable operation for direct modulation, and so this laser is also called a “DFB” (Distributed-Feedback laser diode). The modulation speed and transmission distance strongly depends on the spectral line-width of the laser. A narrower line-width is required for higher modulation speed (data rate) and longer distance. Compared to a Fabry-Perot laser, the spectral line-width of a DFB is about 1/10th, and so a DFB structure is more suitable as a high data rate DML.
In a DML, data is placed on the optical beam by modulating the Injection Current, which is an input on/off electrical signal and is directly applied to the laser diode chip to output a modulated optical signal. A DML is a single chip and provides a simple electrical circuit configuration for operation, so it may fit to a compact design and low power application.
Direct modulation changes the laser properties like its refractive index leading to a large chromatic dispersion. Performance of a DML degrades over longer reaches (>10km) due to larger chromatic dispersions, lower frequency response, and a relatively low extinction ratio when compared to EMLs.
Electro-absorption Modulated Laser (EML)
An EML is a laser diode integrated with an electro-absorption modulator (EAM) in a single chip. The laser diode section, normally with the same device structure as DML operates under a continuous wave (CW) condition, and input voltage on/off signals are applied to the EAM section to generate optical output signals. The laser properties themselves are not changed by the process of modulation, as they are in a DML. EMLs are advantageous in applications with higher speeds and longer distance transmission, compared to a DML, because of its smaller chromatic dispersion. An EML is mainly used for higher speeds (>25Gbps), and longer reaches (10-40km) in telecom applications.
Compared to a DML, an EML has smaller chromatic dispersion with a stable wavelength under high-speed operation, because the injection current (input signal) to the laser section is not modulated and therefore does not change. The frequency response of an EML depends on a capacitance in the EAM section which enables high operating speed, even greater than 40GHz. Extinction in an EML is caused by absorption as the coefficient changes with the modulated voltage applied to the EAM section, and the extinction ratio becomes higher with a large voltage input (on/off electrical signal).