DML vs. EML Lasers in 100G QSFP28 Transceivers

Thursday 15th February 2024

When it comes to transmitting data across varying distances, 100G QSFP28 transceivers employ different optical technologies. Shorter reaches typically utilize Vertical Cavity Surface Emitting Lasers (VCSELs), while longer reaches rely on Electro-absorption modulated lasers (EMLs) or Directly Modulated Lasers (DMLs). However, the recent scarcity of EML lasers in the market has prompted design engineers to explore alternatives for longer reach 100G QSFP28 transmitters. DML optics paired with DFB TOSA (Distributed Feedback Transmitter Optical Sub-Assembly) emerged as a cost-effective and dependable solution.

Here, we delve into the fundamentals of DML and EML based optics.

Directly Modulated Laser (DML)

DMLs commonly employ a distributed feedback structure integrating a diffraction grating within the waveguide for stable direct modulation. This design, also termed "DFB" (Distributed-Feedback laser diode), heavily relies on the spectral line-width of the laser for modulation speed and transmission distance. A narrower line-width is crucial for achieving higher modulation speeds (data rates) and longer transmission distances. Compared to a Fabry-Perot laser, the spectral line-width of a DFB is approximately one-tenth, making the DFB structure more suitable for high-speed DML applications.

In a DML, data modulation occurs by varying the Injection Current, an on/off electrical signal directly applied to the laser diode chip, resulting in a modulated optical signal. With a single-chip configuration, a DML offers a straightforward electrical circuit setup, making it suitable for compact designs and low-power applications.

However, direct modulation in a DML leads to changes in laser properties, such as refractive index alterations, causing significant chromatic dispersion. Consequently, the performance of a DML deteriorates over longer reaches (>10km) due to increased chromatic dispersions, reduced frequency response, and a relatively lower extinction ratio compared to EMLs.

Electro-absorption Modulated Laser (EML)

An EML integrates a laser diode with an electro-absorption modulator (EAM) in a single chip. While the laser diode operates under continuous wave (CW) conditions, on/off voltage signals are applied to the EAM section to generate optical output signals. Unlike DMLs, the modulation process in EMLs does not alter the laser properties themselves. EMLs offer advantages in higher-speed (>25Gbps) and longer-range (10-40km) telecom applications due to their smaller chromatic dispersion.

Compared to DMLs, EMLs exhibit smaller chromatic dispersion and maintain stable wavelengths during high-speed operations because the injection current to the laser section remains unmodulated. The frequency response of an EML depends on the capacitance within the EAM section, enabling operation speeds surpassing 40GHz. Extinction in EMLs is induced by absorption, with the coefficient altering as modulated voltage is applied to the EAM section, resulting in higher extinction ratios with increased voltage inputs (on/off electrical signals).

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