Thermo-Optic Modulator Achieves 80dB Extinction Ratio for Silicon Nitride Photonics

Researchers are tackling a critical bottleneck in silicon nitride integrated photonics , achieving efficient optical modulation. Dmitriy Serkin, Kirill Buzaverov, and Aleksandr Baburin, all from Shukhov Labs at Quantum Park, Bauman Moscow State Technical University, alongside Sergeev et al., present a novel thermo-optic modulator demonstrating an ultra-high extinction ratio exceeding 80 dB. This breakthrough is significant because it addresses the need for wide frequency response, high contrast, and low power consumption, paving the way for advancements in diverse fields such as quantum technologies, LiDAR, and biotechnology , all built upon extremely low-loss silicon nitride circuits with a propagation loss of just 0.058 dB/cm.

Low-power silicon nitride modulator exceeds 80dB

The research team fabricated a novel integrated phase shifter incorporating isolation trenches, operating within the crucial C-band of optical communication. This performance represents a substantial advancement over existing technologies and unlocks new possibilities for high-speed optical control. This exceptionally low loss is paramount for maintaining signal integrity over long distances and enabling complex photonic circuits. Researchers employed a direct waveguide design with a parallel heating element, ensuring stable operation irrespective of the optical signal’s direction, and carefully optimised the heater geometry to maximise efficiency.
The incorporation of isolation trenches effectively minimises thermal crosstalk, allowing for precise and rapid phase control. Experiments confirm the modulator’s ability to dynamically tune the phase of light, a fundamental requirement for interferometric structures, delay lines, switches, and other essential components in information processing systems. The work opens avenues for advancements in quantum technologies, high-performance computing, LiDAR systems, and biotechnological applications. A detailed comparison with existing phase-shifting technologies, including MEMS, ferroelectrics, and plasma-dispersion methods, highlights the advantages of this thermo-optic approach.

While MEMS devices offer low power consumption, they suffer from fabrication complexity and high driving voltages. Ferroelectric modulators boast fast speeds but are hampered by high insertion loss and integration difficulties. This new modulator strikes a favourable balance between fabrication simplicity, optical performance, and reliability, positioning it as a promising candidate for future photonic integrated circuits.

Silicon nitride thermo-optic shifter design and optimisation

This represents a significant advancement over conventional thermo-optic phase shifters, which typically require over 100mW and exhibit bandwidths in the order of kHz. They fabricated unbalanced Mach-Zehnder interferometers (MZIs) with optimized phase shifters to comprehensively characterize both static and dynamic properties. The study pioneered the use of isolation trenches in the SiO2 cladding, reducing the π-phase shift power consumption from an initial 195mW down to 65mW, a substantial improvement in energy efficiency. Both phase shifter designs exhibited a 12kHz bandwidth (−3 dB cutoff) with 10-90% rise and fall times of less than 35μs, demonstrating rapid switching capabilities.

The team measured an extinction ratio of over 80 dB in MZI-based amplitude modulators for both designs, confirming high-contrast modulation performance. The phase shifter’s structure integrates a single-strip titanium heater on a silicon substrate with a 220nm-thick silicon nitride layer, and the thermally induced phase shift is governed by the equation Δφ= 2πL λ dneff dT ΔT. A two-dimensional multi-physics model, developed using the Finite Element Method (FEM), integrated electrothermal and optical physics to simulate the device’s response to electrical power. This allowed for systematic evaluation of geometric parameters affecting the power required for a π-phase shift, calculated as Pπ= λCwgρS 2η dT dneff, and highlighted the importance of minimizing heat leakage to maximize power efficiency. This innovative methodology enabled the creation of highly efficient and scalable phase shifters for advanced photonic integrated circuits.

Low-loss silicon nitride modulator with high Q-factor

Researchers meticulously optimized the phase shifter design for the C-band using both thermal and electromagnetic simulations, enabling minimized power consumption on their integrated photonic platform. This exceptional performance unlocks potential applications in programmable notch filters and optical phased arrays, systems that do not require modulation at microwave frequencies. The phase shifter incorporates a single strip titanium heater with isolation trenches fabricated on a silicon substrate with 220nm-thick stoichiometric silicon nitride. Measurements confirm that the unbalanced Mach-Zehnder interferometers (MZIs) exhibit a 10-90% rise and fall time of less than 35μs, indicating rapid switching capabilities.

Data shows that the initial π-phase shift required 195mW, but the introduction of isolation trenches in the SiO2 cladding successfully reduced power consumption to 65mW. The team’s two-dimensional multi-physics model, utilizing the Finite Element Method (FEM), integrated electrothermal and optical physics to simulate device response to electrical power. This allowed for systematic evaluation of geometric parameters affecting the power needed for a π-phase shift. Table 2 details the physical properties of the materials used, including silicon (density 2.33g/sm3, specific heat 0.711 J/(g·K)), silicon dioxide (density 2.20g/sm3, specific heat 0.709 J/(g·K)), and titanium (density 4.50g/sm3, specific heat 0.523 J/(g·K)).

Further analysis demonstrated that the performance of the thermo-optic phase shifter is largely determined by the heat transfer coefficient and is independent of device length. Simulations revealed the impact of varying heater cladding thickness (hclad) and waveguide width (wh) on power consumption and heater temperature, with optimized parameters identified to minimize energy usage. Specifically, the study showed that a waveguide width of 2μm and a heater cladding thickness of 1μm yielded optimal results.

Low-loss silicon nitride phase shifter demonstrated promising results

Scientists have developed a compact and power-efficient silicon nitride thermo-optic phase shifter operating within the C-band, demonstrating a key advancement in integrated photonics. This research establishes a promising platform for reconfigurable photonic circuits with potential applications spanning quantum technologies, high-performance computing, surveying, navigation, and medical diagnostics. The integration of isolation trenches significantly reduces power consumption, achieving a Pπ of 65mW compared to 195mW in devices without these trenches, and enhances overall performance. Future work could focus on optimising the fabrication process to further minimise these losses and explore the scalability of this technology for larger, more complex photonic integrated circuits. The demonstrated low loss and efficient modulation suggest potential for mass production and wider adoption in various technological fields. This achievement represents a substantial step towards realising practical, high-performance photonic systems.