A QSFP28 optical module using a Directly Modulated Laser (DML) offers compact, low-power 100G transmission but is limited in distance due to laser chirp and chromatic dispersion.

Overview of QSFP28 Modules

QSFP28 (Quad Small Form-Factor Pluggable 28) modules are designed for 100G Ethernet transmission, aggregating four 25G electrical lanes into a single optical channel. They maintain the same mechanical footprint as QSFP+ modules, enabling high-density deployment in data centers, HPC networks, and enterprise cores while supporting IEEE 802.3 standards for interoperability . QSFP28 modules can use different optical technologies, including DML and EML, depending on the required transmission distance and performance .

Directly Modulated Laser (DML) Technology

A DML laser modulates light directly by varying the current supplied to the laser diode. When the diode is powered, it emits light representing a logical "1"; when off, it represents a logical "0" . Key characteristics include:

• Compact size: The laser diode and control electronics are integrated into a single semiconductor element.
• Low power consumption: DML modules typically consume less power than EML modules at low output levels.
• Narrow spectral width: Suitable for high-frequency signal transmission over short to medium distances.

Limitations

• Laser chirp: Direct modulation causes frequency variations in the emitted light, which, combined with chromatic dispersion in the fiber, can blur the signal spectrum.
• Distance constraints: DML modules are most effective at lower supply currents and shorter distances, typically up to 10 km for 100G LR4 applications .
• Power scaling: Increasing the drive current to extend distance can exacerbate chirp, reducing signal quality.

Comparison with EML

Electro-absorption Modulated Lasers (EML) emit light at a constant intensity, with modulation applied externally via an electro-absorption modulator. Compared to DML:

• Higher power consumption: EML requires additional energy for modulation.
• Longer reach: EML can support longer distances due to reduced chirp and better spectral stability.
• Complexity: EML modules are more complex and costly but suitable for metro and long-haul applications.

Applications in Quantum Communication

While QSFP28 DML modules are primarily designed for classical 100G Ethernet, their compact size, low power, and high-speed capability make them candidates for integration into quantum communication networks where short-distance, high-bandwidth optical links are required. However, for long-distance quantum key distribution (QKD) or entanglement-based systems, EML or coherent optics modules may be preferred due to their superior spectral stability and lower dispersion-induced signal degradation .

Summary

QSFP28 DML optical modules provide a cost-effective, low-power solution for 100G short- to medium-range optical links, with advantages in compactness and simplicity. Their main limitations are distance and signal quality due to laser chirp and chromatic dispersion. For longer distances or high-precision quantum communication, alternative optical technologies like EML or coherent optics are more suitable.