A fabrication technique developed by researchers at the University of Oxford enabled the rapid production of waveguides in a chip with precisely controlled 3D cross sections that exhibited changing behavior along the waveguide. The waveguides were demonstrated with very low losses and show promise for the design of photonic and/or quantum chips. Among the basic components of PICs are micron-scale optical waveguides. The elements are analogous to the semiconductor diodes of conventional electronic integrated circuits. Due to limitations in fabrication, the optical waveguides are limited to two-dimensional square, elliptic, and circular cross-section architectures. Current options are limited in the production of these waveguides to demonstrate both low loss and precise 3D cross-section variation. Spherical phase-induced multicore waveguides with varying cross-sections, enabling mode conversion. A multi-institutional research team showed SPIM-WGs that provided the capabilities of optical mode conversion for any arbitrary shapes, limited only by the diffraction limited size of the fabricating laser focus. The technology showed promise for photonic and/or quantum chip design. Courtesy of Bangshan Sun. The current work, conducted with scientists from Imperial College London, yielded a technology called spherical phase-induced multicore waveguides (SPIM-WGs). In the approach, optical waveguides with continuously variable 3D cross sections are efficiently fabricated onto a chip. Based on adaptive optics, scientists used the method to produce variable cross sections including circular, square, annular, and many other complex shapes. The precision in controlling the cross section in each axis could be reduced to hundreds of nanometers. For a single waveguide, the shape of cross section was shown to vary along the waveguide itself. During the precise change in morphology, the waveguide exhibited very low transmission losses of about -0.14dB/cm, translating to a loss of only 3% of the optical power when transmitting 1 cm through the chip. Experimental results further showed that the extra transmission loss caused by cross section variation was almost negligible. Traditional silica-on-silicon methods take about a month or more to produce waveguides. Comparatively, SPIM-WGs are produced in a matter of minutes. In theory, SPIM-WGs could provide the capabilities of optical mode conversion for any arbitrary shapes, limited only by the diffraction limited size of the fabricating laser focus. SPIM-WGs were demonstrated to easily convert between Gaussian light modes and elliptical light modes, which appear in a range of optoelectronic chips. Among the most important applications in mode conversion is between pp-KTP waveguides and single-mode fiber, for bridging quantum light sources and quantum chips. Currently, the pp-KTP waveguide in a quantum light source must be directly connected to a single-mode fiber, which loses about 25% to 30% of the light intensity. If the mode conversion waveguide made by SPIM-WGs is used for the bridging, it is expected that the light intensity loss can be reduced below 10%. This would provide a significant boost in efficiency for most quantum chips. Additionally, based on the functionality of mode conversion, SPIM-WGs can be connected to a single-mode fiber with coupling efficiency up to 95%. This allows SPIM-WG devices to be easily combined with most existing photonic devices. The researchers also found that waveguides with rectangular cross sections twisted at 90º controlled the polarization of light, holding promise for a variety of photonic and quantum applications. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-022-00907-4).