Quantum Key Distribution G652 Vs G655 Fibers Tested in Metro Networks

Imagine a world where your bank accounts, medical records, and even state secrets are protected by mathematical puzzles that could be solved in an instant. The advent of quantum computing has created this precarious reality, threatening to render current encryption methods obsolete. Quantum Key Distribution (QKD) emerges as a revolutionary solution—a technology leveraging the principles of quantum mechanics to create unconditionally secure cryptographic keys.

Current encryption systems rely on computational complexity that quantum computers may soon overcome. QKD offers a fundamentally different approach—its security is guaranteed by the laws of physics rather than mathematical difficulty. As the first quantum communication protocol to achieve industrialization and commercialization, QKD has sparked national and international initiatives to integrate it into existing telecommunications infrastructure.

Telecommunication networks predominantly use two types of single-mode fibers: G.652 and G.655. Both comply with ITU-T standards and support transmission at 1310 nm and 1550 nm wavelengths, but their dispersion characteristics differ significantly:

• G.652 fiber is optimized for 1310 nm operation with zero dispersion at this wavelength
• G.655 fiber excels in the 1550 nm band (C-band: 1530-1660 nm) with lower dispersion values

G.655 fiber's higher refractive index provides greater numerical aperture and wider acceptance angle, making it ideal for challenging environments like long-haul or submarine communications. Its compatibility with erbium-doped fiber amplifiers (EDFA) also makes it preferable for wavelength-division multiplexing (WDM) systems.

Despite their widespread use, direct comparisons of G.652 and G.655 fibers for polarization-encoded QKD applications remain scarce. This study addresses this gap through field trials conducted on metropolitan fiber networks in Italy's Veneto region, comparing the two fiber types under identical conditions.

The tests utilized a 19-km fiber link between Treviso and Venice-Mestre, operated by Retelit S.p.A. The parallel installation of both fiber types allowed for controlled comparison of environmental effects. Key characteristics:

• G.655 fiber: Part of a 36-core cable with 94% active data transmission
• G.652 fiber: Part of a 72-core cable with only 8% occupancy

The study employed ThinkQuantum srl's QuKy system implementing the BB84 protocol with polarization encoding. Tests included:

• 24-hour "dark fiber" operation on both fiber types
• Coexistence tests with classical communication signals
• Optical Time Domain Reflectometer (OTDR) measurements

The 24-hour continuous operation yielded significant results:

• Key generation rate: G.655 fiber demonstrated superior performance
• Quantum Bit Error Rate (QBER): Both fibers maintained acceptable levels, with G.652 showing marginally better results
• Classical signal interference: Manageable through proper power control and filtering
• Channel stability: Both fibers maintained consistent performance throughout testing

The study confirms both fiber types can support metropolitan QKD networks, with G.655 offering advantages in key generation rate while G.652 provides slightly better error performance. The successful coexistence tests demonstrate QKD's viability in operational networks carrying classical traffic.

Additional factors influencing QKD performance include:

• Polarization mode dispersion (PMD) characteristics
• Nonlinear effects at high power levels
• Environmental stress and temperature variations
• Post-processing algorithms for key distillation

Future research should explore:

• Longer-distance QKD implementations
• Enhanced integration with classical networks
• Development of novel QKD protocols
• Practical deployment in financial and government sectors

This study provides valuable insights for network operators planning QKD deployment, demonstrating that existing fiber infrastructure can support next-generation quantum-secured communications.