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Columbia develops visible light modulator that works without dimming

25 Nov 2021

Based on micro-ring resonators to reduce the size and power demands of visible spectrum phase modulator.

Integrated photonics is a rapidly-evolving industry sector, with infrared spectrum applications including high-speed 5G networks, biosensors on chip, and driverless cars.

Researchers are now exploring the visible wavelength range to develop a variety of applications: chip-scale LIDAR, AR/VR/MR goggles, holographic displays, quantum information processing chips, and implantable optogenetic probes for the brain.

The one device that is critical to all these applications in the visible range is an optical phase modulator, which controls the phase of a light wave, similar to how the phase of radio waves is modulated in wireless computer networks.

Phase modulators in the visible range are difficult to make: there are no materials that are transparent enough in the visible spectrum while also providing large tunability, either through thermo-optical or electro-optical effects. Currently, the two most suitable materials are silicon nitride and lithium niobate.

Visible-spectrum phase modulators based on these materials are thus not only large but also power-hungry: the length of individual waveguide-based modulators ranges from hundreds of microns to several millimeters, and a single modulator consumes tens of milliwatts for phase tuning. Researchers trying to achieve large-scale integration have, up to now, been limited by these bulky, energy-consuming devices.

Columbia Engineering researchers, in New York, say they have found a solution to this problem—they have developed a way based on micro-ring resonators to dramatically reduce both the size and the power consumption of a visible-spectrum phase modulator, from 1mm to 10µm, and from tens of milliwatts for π phase tuning to below one milliwatt. The work is published in Nature Photonics.

Nanfang Yu, associate professor of applied physics, co-principal investigator, commented, “It’s really hard to confine light to a spot and manipulate it without losing much of its power. We have made a breakthrough that will greatly expand the horizon of large-scale, visible-spectrum integrated photonics.”

Conventional optical phase modulators operating at visible wavelengths are based on light propagation in waveguides. Yu worked with his colleague Michal Lipson, who is an expert on integrated photonics based on silicon nitride, to develop a different approach.

“The key to our solution was to use an optical resonator and to operate it in the so-called ‘strongly over-coupled’ regime,” said Lipson, co-PI on the team and Eugene Higgins Professor of Electrical Engineering and professor of applied physics.

To achieve a complete 2π phase tuning and minimal amplitude variation, the Yu-Lipson team chose to operate a micro-ring in the strongly over-coupled regime, a condition in which the coupling strength between the micro-ring and the bus waveguide that feeds light into the ring is at least 10 times stronger than the loss of the micro-ring.

‘Compact and power efficient’

Heqing Huang, a graduate student in Yu’s lab and first author of the paper, explained, “Our best phase modulators operating at the blue and green wavelengths, which are the most difficult portion of the visible spectrum, have a radius of only five microns, consume 0.8 mW for π phase tuning, and introduce an amplitude variation of less than 10%. No prior work has demonstrated such compact, power-efficient, and low-loss phase modulators at visible wavelengths.”

The devices were designed in Yu’s lab and fabricated in the Columbia Nano Initiative cleanroom, at the Advanced Science Research Center NanoFabrication Facility at the Graduate Center of the City University of New York, and at the Cornell NanoScale Science and Technology Facility.

The researchers commented that, while they are nowhere near the degree of integration of electronics, their work is shrinking the gap between photonic and electronic switches substantially.

“If previous modulator technologies only allow for integration of 100 waveguide phase modulators given a certain chip footprint and power budget, now we can do that 100 times better and integrate 10,000 phase shifters on chip to realize much more sophisticated functions,” said Yu.

The Lipson and Yu labs are now collaborating to demonstrate visible-spectrum LIDAR consisting of large 2D arrays of phase shifters based on adiabatic micro-rings. The design strategies employed for their visible-spectrum thermo-optical devices can be applied to electro-optical modulators to reduce their footprints and drive voltages, and can be adapted in other spectral ranges.

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