Photonics integrated circuitry would benefit considerably from the ability to
arbitrarily control waveguide cross-sections with high precision and low loss,
in order to provide more degrees of freedom in manipulating propagating light.
Here, we report on a new optical-fibres-compatible glass waveguide by
femtosecond laser writing, namely spherical phase induced multi-core waveguide
(SPIM-WG), which addresses this challenging task with three dimensional on-chip
light control. Precise deformation of cross-sections is achievable along the
waveguide, with shapes and sizes finely controllable of high resolution in both
horizontal and vertical transversal directions. We observed that these
waveguides have high refractive index contrast of 0.017, low propagation loss
of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single mode
fibre. SPIM-WG devices were easily fabricated that were able to perform on-chip
beam rotation through varying angles, or manipulate polarization state of
propagating light for target wavelengths. We also demonstrated SPIM-WG mode
converters that provide arbitrary adiabatic mode conversion with high
efficiency between symmetric and asymmetric non-uniform modes; examples include
circular, elliptical modes and asymmetric modes from ppKTP waveguides which are
generally applied in frequency conversion and quantum light sources. Created
inside optical glass, these waveguides and devices have the capability to
operate across ultra-broad bands from visible to infrared wavelengths. The
compatibility with optical fibre also paves the way toward packaged photonic
integrated circuitry, which usually needs input and output fibre connections.