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International Conference on Laser, Optical Technology, and Applications (LOTA)

Design of low-loss polarization-insensitive MMI based on silicon nitride

Guan-Zhong Huang, Guowei Zhang, Wei Chen, Fang‐Xiang Wang, Jia-Lin Chen, Jinlu Liu, Bingjie Xu, Zheng‐Fu Han

Published Dec 16, 2025• 0 citations
Publisher

Abstract

Integrated quantum key distribution (QKD) has achieved substantial advancements across different material platforms. Among these, silicon nitride has attracted widespread attention due to its low loss and compact size. However, silicon nitride exhibits pronounced polarization correlation, which makes it susceptible to unpredictable polarization state rotations induced by the fiber birefringence effect within the channel, thus affecting the normal operation of quantum processes. In this work, we design and fabricate the fundamental devices in QKD systems, namely 1×2 and 2×2 multimode interferometers (MMIs) with low polarization sensitivity and loss. We also improve the splitting balance via parameter optimization based on the particle swarm optimization (PSO) algorithm. Simulation results indicate that the 1×2 (2×2) MMI exhibits a loss of about 0.1 (0.2) dB, along with 0.001 (0.09) dB polarization-dependent loss (PDL) and a splitting ratio nearly 50:50 (±0.3%) at 1550nm, respectively. Test results for the fabricated devices demonstrate that the loss of both MMI types is confined to less than 0.25 dB, their PDL is kept within 0.2 dB, and splitting ratio imbalance is within 1% and exhibits temperature robustness. This research explores compact polarization-insensitive devices via a novel method, thereby laying the foundation for the development of integrated quantum information processing.

DOI
10.1117/12.3091397

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Integrated quantum key distribution (QKD) has achieved substantial advancements across different material platforms. Among these, silicon nitride has attracted widespread attention due to its low loss and compact size. However, silicon nitride exhibits pronounced polarization correlation, which makes it susceptible to unpredictable polarization state rotations induced by the fiber birefringence effect within the channel, thus affecting the normal operation of quantum processes. In this work, we design and fabricate the fundamental devices in QKD systems, namely 1×2 and 2×2 multimode interferometers (MMIs) with low polarization sensitivity and loss. We also improve the splitting balance via parameter optimization based on the particle swarm optimization (PSO) algorithm. Simulation results indicate that the 1×2 (2×2) MMI exhibits a loss of about 0.1 (0.2) dB, along with 0.001 (0.09) dB polarization-dependent loss (PDL) and a splitting ratio nearly 50:50 (±0.3%) at 1550nm, respectively. Test results for the fabricated devices demonstrate that the loss of both MMI types is confined to less than 0.25 dB, their PDL is kept within 0.2 dB, and splitting ratio imbalance is within 1% and exhibits temperature robustness. This research explores compact polarization-insensitive devices via a novel method, thereby laying the foundation for the development of integrated quantum information processing.
We based on integrated silicon-on-insulator platforms design the key components of an on-chip interferometer, beam splitter and directional coupler included, valid in high-visibility interference at telecommunication wavelengths. Special attention is given to the equal-proportion beam splitting and directional coupling, which is achieved by carefully designing the geometric dimension of multi-mode interferometer structure. The proposed interferometer facilitates low loss, broad operating bandwidth, anticipated large tolerance on size variation induced in fabrication procedures, based on a particular wafer with silicon layer thickness of 320 nm. The most highlight property of polarization-insensitive, enables the path-selective qubits generation for bi-polarization that further makes possible quantum key distribution using high dimensional protocols. We numerically demonstrate interference at 1550 nm with visibilities of 99.50% and 93.99% for transverse-electric and transverse-magnetic polarization, respectively, revealing that the proposed interferometer structure is well capable of on-chip optical control especially in quantum optics regime.
Silicon-based polarization-encoding quantum key distribution (QKD) has been widely studied, owing to its low cost and robustness. However, prior studies have utilized off-chip devices to demodulate the quantum states or perform polarization compensation, given the difficulty of fabricating polarized independent components on the chip. In this paper, we propose a fully chip-based decoder for polarization-encoding QKD. The chip realizes a polarization state analyzer and compensates for the BB84 protocol without requiring additional hardware. It is based on a polarization-to-path conversion method that uses a polarization splitter-rotator. The chip was fabricated using a standard silicon photonics foundry; it has a compact design and is suitable for mass production. In the experimental stability test, an average quantum bit error rate of 0.56%0.56\%0.56% was achieved through continuous operation for 10 h without any polarization feedback. Furthermore, using the developed feedback algorithm, the chip enabled the automatic compensation of the fiber polarization drift, which was emulated by a random fiber polarization scrambler. In the case of the QKD demonstration, we obtained a finite-key secret rate of 240 bps over a fiber spool of 100 km. This study represents an important step toward the integrated, practical, and large-scale deployment of QKD systems.
Programmable waveguide circuits are crucial building blocks for integrated spectrometric applications and quantum photonic information processing. Amongst the dielectric material platforms in integrated photonics, silicon nitride stands out with highly attractive properties such as a large bandgap energy and a moderately high index contrast. This allows low propagation losses in a wide spectral range while simultaneously allowing a dense packing of components. The aforementioned properties, together with the inherent phase stability and phase programmability achievable in silicon nitride, enable the creation of complex photonic circuits. In this thesis we describe and demonstrate densely integrated programmable photonic circuits based on silicon nitride waveguides for wavelength metrology and quantum information processing. We concentrate on reconfigurable photonic integrated circuits based on silicon nitride waveguides with low-loss propagation, to explore interference in the spectral and temporal domain for advanced applications. We investigated two types of integrated interferometric devices featuring low loss in combination with programmability for classical and quantum photonic processing. The first is simple tunable microring resonator circuits in combination with neural network data processing for the analysis of classical light in the spectral domain as wavelength meter. The second is a complex tunable network of waveguide interferometers for controlling quantum correlations (coincidences) between single photons. Exploiting the long-term interferometric stability, low propagation loss and tight optical confinement of integrated silicon nitride waveguides, we have shown complex reconfigurable optical circuits both for classical and quantum photonic processing. For future development of integrated programmable photonic processors, various challenges need to be addressed such as compact and low-power phase shifters, a further increase of the component density and lower the propagation losses.
Abstract Quantum communication has extremely high security and is a development direction of future communication networks. In this paper, an on-chip path, off-chip polarization encoded quantum key distribution (QKD) chip is designed and fabricated using advanced silicon photonics fabrication technology. Using a special two-dimensional grating, we can convert light of different path states into corresponding polarization states and propagate in fiber. The performance of the basic optics, waveguides, gratings, and Mach–Zehnder interferometers on the chip is characterized. In the demonstrated QKD experiment, the quantum bit error rate is as low as 1.08%. It provides an important device for the future development of advanced integrated quantum information.
Precise temporal delay generation is a key requirement for asymmetric Mach-Zehnder interferometers (aMZIs) used in high-speed quantum key distribution (QKD) receivers. In this work, a compact integrated aMZI architecture based on a silicon nitride on silicon dioxide (Si3N4/SiO2) photonic platform is presented. A 3-dB directional coupler enabling accurate 50:50 power splitting at the 1550 nm telecommunication wavelength is designed and optimized. The coupling length is initially estimated from the effective index difference between the even and odd supermodes and subsequently refined using three-dimensional eigenmode expansion (EME) simulations. The optimized structure employs single-mode Si3N4 waveguides with a cross section of 1 um x 0.4 um, providing a group index close to 2 and enabling accurate delay engineering. Spectral analysis demonstrates stable 3-dB power splitting across the C-band with insertion loss below 0.5 dB and negligible power imbalance, indicating high transmission efficiency and structural symmetry. An integrated Si3N4 aMZI delay line providing a 500 ps optical delay, corresponding to a 2 GHz free spectral range (FSR), is further demonstrated. Simulations show nearly constant group delay across the 190-200 THz frequency range with sub-10 ps variation and a smooth, near-linear phase response. These characteristics enable interference visibility above 0.99, corresponding to an estimated quantum bit error rate (QBER) below 0.5 percent for gigahertz-rate time-bin QKD systems. The wideband linear phase behavior also indicates compatibility with wavelength division multiplexed (WDM) QKD architectures. The results confirm that the proposed Si3N4 integrated aMZI provides a low-loss, dispersion-controlled, and spectrally stable delay solution suitable for scalable chip-scale quantum photonic receivers.
Silicon photonic technologies exploit silicon as an optical material to build a wide array of devices and systems. The flexibility, modularity, and stability of silicon photonic systems provide unprecedented opportunities for next-generation quantum key distribution (QKD) and quantum networking. This doctoral thesis focuses on the design, simulation, fabrication, and experiment of QKD chips based on silicon photonics. Crucial components for QKD chips are designed and fabricated, including the grating coupler, polarization beam splitter, polarization rotator, modulator, photodetector, ring resonator, and interferometer. Using these components, chip-based continuous variable quantum key distribution (CV-QKD) is demonstrated for the first time. The chip-based CV-QKD system is capable of producing a secret key rate of 9.3 kbits/s (under collective attack) at a distance of 100 km in fiber. The successful demonstration of a single chip-based QKD offers new possibilities for low-cost, scalable and portable quantum networks.