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Variable Optical Attenuator induced Crosstalk in Photonic Integrated Circuit based Quantum Key Distribution Transmitter
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- Issue Date
- 2026-08
- Citation
- International Conference on Quantum Cryptography (QCrypt) 2026, pp.1-1
- Language
- English
- Type
- Conference Paper
- Abstract
- Photonic integrated circuits (PICs) provide a compact and scalable platform for practical quantum key distribution (QKD), enabling the integration of multiple optical functions required for quantum state preparation [1, 2]. While PIC can provide compact structure integrating a variety of optical components in small space, the compact structure can cause that final output state can be sensitive to unintended optical coupling between the densely integrated optical components. In this work, we investigate inter-channel crosstalk induced by the VOA array in the PIC-based QKD transmitter which is recently reported in [3] as a free-space reference-frame-independent QKD transmitter including six laser diodes, six variable optical attenuators (VOAs), and a polarization beam combiner into a C Form-factor Pluggable 2 (CFP2) module. Each VOA is implemented using a 2 × 2 Mach–Zehnder interferometer (MZI), although only one input port and one output port are connected to the functional optical path between the laser-diode array and the polarization beam combiner. The remaining ports are redundant from the viewpoint of QKD transmitter operation, but they are inherently present because of the MZI structure. When a VOA is driven to provide high attenuation, only a small fraction of the optical signal is delivered to the intended output port, while the remaining optical power is routed toward the unused output port. We observe that this residual light can couple into adjacent VOA channels as stray light, even in the absence of a designed waveguide connection. Consequently, although the transmitter is intended to activate only a single path in a given time slot, unintended neighboring paths may also activate adjacent path, resulting in a mixture of polarization components at the transmitter output. The aforementioned effect directly distorts the generated polarization states. As the VOA driving current is varied to control the attenuation and mean photon number, the measured Stokes parameters deviate from the target polarization states, and the deviation becomes more pronounced at higher attenuation. Therefore, the usable attenuation range of the VOA in a QKD transmitter is not determined solely by its standalone attenuation performance, but is instead limited by the resulting polarization-state distortion. This finding reveals that VOA-induced inter-channel crosstalk is a critical design constraint for densely integrated PIC-based QKD transmitters. Our results suggest that future designs should suppress stray-light coupling from redundant MZI ports increasing physical and optical isolation between adjacent VOAs, or redesigning the attenuation structure to avoid unused high-power output paths. These considerations are essential for simultaneously achieving high attenuation of VOA and high fidelity of generated polarization state in PIC-based QKD systems.
- KSP Keywords
- Attenuation performance, Beam combiner, Channel crosstalk, Critical design, Design constraints, Driving current, High Power, High-fidelity, Integrated circuit(IC), Laser diodes, Mean photon number