Magneto-Optical Integrated NOCS Network

• The paper demonstrates that integrating magneto-optical and thermo-optic phase shifters on silicon enables full-duplex switching with up to 4096 routing states in an 8-port configuration.
• The architecture achieves nonreciprocal transmission via localized MO effects, ensuring channel isolation up to 16 dB and crosstalk suppression down to –18 dB.
• Leveraging advanced materials like Ce:YIG and YIG, the network offers high reconfiguration speeds, scalable integration, and potential applications in AI data centers and integrated sensing.

A magneto-optical heterogeneous integrated nonreciprocal optical circuit switching (NOCS) network is an advanced architecture for optical signal switching, routing, and isolation in large-scale silicon photonics platforms, where magneto-optical (MO) effects are harnessed to induce direction-dependent transmission properties. The haLLMark of this class of networks is the use of both reciprocal and nonreciprocal phase shifters—monolithically integrated into silicon waveguides—to enable bidirectional, full-duplex operation with channel isolation, high-speed programmability, and port reduction for scalable interconnects. These NOCS networks are distinct in combining the reconfiguration speed and functional density afforded by MO switching with the integration compatibility of silicon photonics, addressing key performance and scalability limitations in conventional OCS architectures.

1. Silicon Photonics Platform and Integrated Magneto-Optics

The NOCS network is constructed on a silicon-on-insulator (SOI) photonics platform, utilizing a Benes switching topology to realize a scalable $N \times N$ switching fabric. Each basic switch unit is a Mach-Zehnder interferometer (MZI) in which phase modulation is accomplished by two physically distinct types of phase shifters:

• Reciprocal Phase Shifter (RPS): Implemented via the thermo-optic (TO) effect; a Ti micro-heater locally modulates the refractive index in one arm of the silicon waveguide, effecting a controllable, direction-independent phase shift.
• Nonreciprocal Phase Shifter (NRPS): Integrated by monolithic deposition of Ce:YIG or YIG thin films onto patterned regions of the waveguide; these MO materials induce Faraday or Kerr nonreciprocity under an applied magnetic field produced by on-chip gold electrodes.

MO integration is spatially confined in the fabrication process to mitigate absorption losses and ensure compatibility with standard silicon processing. This heterogeneous approach provides the physical mechanism for nonreciprocal transmission: the MO segment induces different phase shifts for forward and backward propagating signals, enabling directional channel control.

2. Nonreciprocal Transmission and Circuit Switching Mechanisms

NOCS supports bidirectional full-duplex switching wherein independent forward and backward optical paths are programmed on a single waveguide. The underlying mechanism leverages:

• Push–Pull Configuration: Both reciprocal and nonreciprocal phase shifters are independently addressed to set the net phase difference between the arms of each MZI. This enables each switch unit to realize four distinct transmission states—bar or cross for forward and backward directions.
• Phase Shift Characterization: The total phase shift for each direction is governed by

$$\Phi_\mathrm{forward} = \varphi_\mathrm{R} + \varphi_\mathrm{NR}$$

$$\Phi_\mathrm{backward} = \varphi_\mathrm{R} - \varphi_\mathrm{NR}$$

where $\varphi_\mathrm{R}$ is the reciprocal phase and $\varphi_\mathrm{NR}$ is the nonreciprocal phase induced by the MO effect (which depends on the applied magnetic field and interaction length).

This mechanism decouples the switching states of forward and backward transmission, allowing for independent programming of routing matrices. Resultant switching states (CC, BB, BC, CB) determine, for example, that the MZI outputs a "bar" state in one direction and "cross" in the other, facilitating channel isolation.

3. Advantages Over Conventional OCS Architectures

The NOCS network architecture provides several substantive improvements:

• Ultra-high Reconfiguration Speed: Magneto-optical phase modulation enables nanosecond-scale switching, exceeding the response time of traditional thermo-optic-only OCS switches.
• Reduced Port Complexity via Bidirectional Channel Isolation: Full-duplex operation over shared physical connections eliminates the need for multiplexed time, wavelength, or space division, significantly reducing hardware overhead and interconnect complexity.
• Large-Scale Integration Compatibility: MO films are directly integrated into silicon photonics using industry-standard processes, supporting monolithic fabrication of large networks with high port counts.
• Flexible State Programming: In an $8$-port Benes switch, NOCS enables up to $4096$ global routing states (compared to $64$ in standard designs), meeting the endurance and flexibility demands of AI backend data interconnects and dynamic networking.

4. Applications in High-Performance Optical Networks

NOCS is particularly suitable for environments where rapid reconfiguration, simultaneous full-duplex transmission, and high-density integration are required, including:

• AI Backend Data Centers and GPU Clusters: Bidirectional channel control mitigates latency and reduces hardware requirements in collective data operations (such as all–reduce in distributed computing).
• Integrated Sensing and Communication (ISAC): NOCS supports programmable, high-bandwidth routing required in next-generation wireless communications, such as 5G/6G or optical satellite links.
• Non-multiplexed Optical Switches: Channel isolation in NOCS obviates the need for time/wavelength/space multiplexing, simplifying network architecture and scaling.

5. Technical Details and Phase Shifter Operation

Phase control is central to NOCS functionality. The phase shift imparted by the NRPS is given by a material-dependent relation:

$$\theta_{\mathrm{NR}} = \kappa \cdot H \cdot L_{\mathrm{MO}}$$

where $\kappa$ is the MO material coefficient, $H$ is the applied magnetic field, and $L_{\mathrm{MO}}$ is the length of the MO segment.

In the MZI, overall interference at the output is determined by constructive or destructive interference between the arms, with the net phase difference adjusted via the RPS and NRPS controls. Each unit switch supports four operating states, with the overall transmission matrix

$$M_{\mathrm{network}} = M_{\mathrm{MZI_1}} \otimes M_{\mathrm{MZI_2}} \otimes \dots$$

composed according to the Benes topology.

6. Experimental Demonstration and Performance Metrics

• Isolation and Loss: The NOCS prototype exhibited channel isolation ratios up to 16 dB and crosstalk levels down to –18 dB, as measured in silicon-integrated $5 \times 5$ routers (Song et al., 8 Jan 2025).
• Reconfiguration and Switching Permutations: In an $N$-port configuration, NOCS supports programmable forward and backward routing matrices, yielding high routing permutation counts (e.g., $4096$ states for $8$ ports).
• Integration Scale: MO segments are patterned with precision, and monolithic integration is achieved without introducing excessive optical loss or signal contamination.

7. Future Directions and Implications

The development of NOCS networks points toward fully integrated, reconfigurable photonic fabrics with low energy consumption and fast switching. Prospective research directions include:

• Dynamic Network Topologies: The ability to program transmission states in real time facilitates adaptive optical backbones for data centers and communication networks.
• Integration with Additional Photonic System Elements: NOCS could be combined with photonic neural networks, LiDAR, or quantum photonic circuits, extending nonreciprocal switching to broader application domains.
• Port Scalability and AI Application: Large port-count networks with full-duplex operation could drastically reduce hardware demands in AI training clusters, supporting the scale-up demanded by machine learning workloads.

In summary, magneto–optical heterogeneous integrated nonreciprocal OCS networks, as realized on silicon photonics platforms, provide a new paradigm for bidirectional, high-speed, highly integrated optical switching fabrics—overcoming limitations of traditional OCS architectures and addressing contemporary demands in high-capacity networking, machine learning, and next-generation photonic communications (Tu et al., 24 Sep 2025, Song et al., 8 Jan 2025).