Over the past several decades, researchers have moved from using electric currents to manipulating light waves in the near-infrared range for telecommunications applications such as high-speed 5G networks, biosensors on a chip, and driverless cars. This research area, known as integrated photonics, is fast evolving, and investigators are now exploring the shorter — visible — wavelength range to develop a broad variety of emerging applications. These include chip-scale light detection and ranging (LiDAR), augmented/virtual/mixed reality (AR/VR/MR) goggles, holographic displays, quantum information processing chips, and implantable optogenetic probes in the brain.
The one device 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. With a phase modulator, researchers can build an on-chip optical switch that channels light into different waveguide ports. With a large network of these optical switches, researchers could create sophisticated integrated optical systems that could control light propagating on a tiny chip.
But phase modulators in the visible range are very hard 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.
While both are highly transparent in the visible range, neither one provides very much tunability. 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 — embedding thousands of devices on a single microchip — have, up to now, been stymied by these bulky, energy-consuming devices.
Columbia Engineering researchers have found a solution to this problem — they’ve 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 one millimeter to 10 microns, and from tens of milliwatts for π phase tuning to below one milliwatt.
Optical resonators are structures with a high degree of symmetry, such as rings, that can cycle a beam of light many times and translate tiny refractive index changes to a large phase modulation. Resonators can operate under different conditions and so need to be used carefully. For example, if operating in the “under-coupled” or “critical coupled” regimes, a resonator will only provide limited phase modulation, and more problematically, introduce a large amplitude variation to the optical signal. The latter is a highly undesirable optical loss because accumulation of even moderate losses from individual phase modulators will prevent cascading them to form a circuit that has a sufficiently large output signal.
To achieve a complete 2π phase tuning and minimal amplitude variation, the research 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, which is primarily due to optical scattering at the nanoscale roughness on the device sidewalls.
The team developed several strategies to push the devices into the strongly over-coupled regime. The most crucial one was their invention of an adiabatic micro-ring geometry, in which the ring smoothly transitions between a narrow neck and a wide belly, which are at the opposite edges of the ring. The narrow neck of the ring facilitates the exchange of light between the bus waveguide and the micro-ring, thus enhancing the coupling strength. The ring’s wide belly reduces optical loss because the guided light interacts only with the outer sidewall, not the inner sidewall, of the widened portion of the adiabatic micro-ring, substantially reducing optical scattering at the sidewall roughness.
In a comparative study of adiabatic micro-rings and conventional micro-rings with uniform width fabricated side by side on the same chip, the team found that none of the conventional micro-rings satisfied the strong over-coupling condition — in fact, they suffered very bad optical losses — while 63% of the adiabatic micro-rings kept operating in the strongly over-coupled regime.
Their best phase modulators operating at the blue and green colors, which are the most difficult portions of the visible spectrum, have a radius of only five microns, consume power of 0.8 mW for π phase tuning, and introduce an amplitude variation of less than 10 percent. According to the researchers, no prior work has demonstrated such compact, power-efficient, and low-loss phase modulators at visible wavelengths.
The researchers note that while they are nowhere near the degree of integration of electronics, their work shrinks 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 a chip to realize much more sophisticated functions,” said Professor Nanfang Yu.
The researchers are now working 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 (e.g., ultraviolet, telecom, mid-infrared, and terahertz) and in other resonator designs beyond micro-rings.