Fig. 4: Adiabatic mode converters with the advanced matching of arbitrary modes.

All images (LED, laser) in a and b have the same frame size of 30 × 30 µm. Scale bars are 10 µm. a Left: circular–Rectangular mode converter that couples single-mode fibers (circular shape mode) to rectangular shape waveguides (elliptical shape mode). A waveguide with length of 1.32 cm was measured to have ultra-low total loss of 0.59 dB (coupling + propagation). The coupling loss was calculated to be 0.19 dB (95.7% light coupled). Middle: LED-illuminated microscopic images demonstrating classic multiscan technique is not capable to fabricate these high-quality mode converters while SPIM-WGs technique enables this capability. About 1550-nm laser-mode profiles are presented for SPIM-WGs. Right: plots of intensity versus distance for corresponding SPIM-WGs circular and elliptical mode profiles. Plotting lines are indicated in mode profiles. b Circular-ppKTP mode converter that couples single-mode fibers to ppKTP waveguides. A waveguide with length of 1.38 cm was measured at 1550 nm wavelength to have total loss of 0.65 dB (coupling + propagation). The coupling loss was 0.24 dB (94.6% light coupled). c Circular–rectangular TE01 mode converter that converts between Gaussian circular mode with rectangular TE01 mode (also called LP11 mode in waveguide theory). A waveguide with length of 1.27 cm was measured at 785 nm wavelength to have ultra-low total loss of 0.78 dB (coupling + propagation). The coupling loss was calculated to be as low as 0.2 dB (95.5% light coupled). d Circular–circular TE01 mode converter that converts between Gaussian circular mode with circular TE01 mode or a hollow ring shape intensity distribution. A waveguide with length of 1.27 cm was measured at 785 nm wavelength to have total loss of 0.73 dB (coupling + propagation). The coupling loss was calculated to be 0.21 dB (95.3% light coupled). e Top: COMSOL simulated field distribution of designed circular shape mode profile to match a single-mode fiber at 1550 nm. Middle: COMSOL simulated field distribution of designed mode profile to match ppKTP waveguide at 1550 nm. Bottom: designed refractive index profile for matching ppKTP waveguide mode. f Top: COMSOL simulated field distribution of designed circular shape mode profile to match a single-mode fiber at 785 nm. Middle: COMSOL simulated field distribution of designed rectangular TE01 mode profile at 785 nm. Bottom: COMSOL simulated field distribution of designed circular TE01 mode profile at 785 nm