Researchers at the National Institute of Standards and Technology (NIST) have developed a chip-scale optical isolator that achieves 30 dB peak isolation without using magnetic materials. The device is built on a silicon nitride and thin-film lithium niobate platform and is designed to work with standard semiconductor manufacturing processes.
The magnet-free isolator achieves 30.6 dB ± 0.1 dB peak isolation at 789.7 nanometres. It maintains more than 24 dB isolation across a 30-nanometre bandwidth spanning 770 nm to 800 nm and can isolate two lasers simultaneously with more than 20 dB isolation within a 10-nanometre window, without heater adjustment.
Optical isolators work like one-way valves for light. They allow forward-propagating photons to pass while blocking backward reflections that can destabilize lasers, corrupt signals, and introduce noise in sensitive photonic integrated circuits.
Conventional optical isolators typically rely on the Faraday effect and bulk magneto-optical materials. These materials are difficult to integrate with standard silicon-based microfabrication.
The NIST team instead uses radio-frequency electro-optic modulation to create synthetic motion. The approach uses time-periodic phase modulation to cancel backward-propagating light while allowing forward light to pass.
At its core is a four-channel Mach-Zehnder modulator that splits incoming light into four parallel interferometric paths. Each path is phase-modulated by radio-frequency waves applied through a gold coplanar waveguide beneath a 150-nanometre thin-film lithium niobate layer, bonded onto 350-nanometre silicon nitride waveguides.
The device uses a 6.5 GHz base frequency, with a third harmonic at 19.5 GHz. About 32 mW of RF power is delivered to the device, significantly below the watt-level drive levels required by earlier traveling-wave isolator approaches.
Six chromium microheaters tune the power balance and relative phase between the four channels. This allows the system to maintain the required interference conditions.
The measured forward-to-backward peak-spectrum power ratio reached 30 dB, with maximum isolation of 30.6 dB ± 0.1 dB at 789.7 nm. The researchers also measured an excess insertion loss of 0.023 dB ± 0.018 dB under fixed heater bias, a variation that is statistically indistinguishable from zero.
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The 770-800 nm operating range covers important rubidium-87 transitions, including the D2 line at 780.2 nm, the D1 line at 794.8 nm, and the two-photon transition at 778.1 nm, as well as potassium-39 resonances.
These wavelengths are relevant to chip-scale atomic clocks, quantum sensors, laser cooling systems, and atomic spectroscopy. The technology could also be adapted to telecommunications wavelengths by redesigning the passive splitters and combiners.
The isolator uses heterogeneous integration, with a thin-film lithium niobate layer bonded onto silicon nitride. A 100-millimeter wafer can accommodate nine chips, each 22 by 20 millimeters, with 171 isolators in total.
The NIST team said the approach provides a route towards mass-produced photonic integrated circuits using standard semiconductor infrastructure. The work was developed through the NIST-on-a-Chip program, funded by the US Department of Commerce through awards 70NANB23H104 and 70NANB23H105.