PMC

(a) The transmission of the metal-core circular cavity coupled with a waveguide, where the black-line inset shows the detailed structure, and the color insets show the magnetic-field H_z distributions at the transmission peaks of P_cir1 (λ = 852 nm) and P_cir2 (λ = 526 nm); (b,c) the |H| distributions of dipole eigenmodes, and (d,e) the |H| distributions of quadrupole eigenmodes, for the metal-core circular cavity without coupling with a waveguide.

Proposed 4-port quarter-mode substrate-integrated waveguide (QMSIW) cavity resonator developed from the circular full substrate-integrated waveguide (SIW) cavity resonator: (a) microstrip line (MSL)-fed circular SIW cavity resonator, (b) coaxial-fed QMSIW cavity resonator, and (c) 4-port QMSIW resonator and corresponding electric field (E-field) distribution at 4.5 GHz.

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

Experimental characteristics of measured straight dielectric waveguides for the mode. (a) Experimental setup including VNA (1), extension modules with WR-15 output (2), WR-15-to-circular waveguide transition (3), circular waveguide-to-dielectric waveguide transition (4) and circular dielectric waveguide under test (5). (b) Transition from circular waveguide to dielectric waveguide. (c) Measured and simulated S-parameters for solid waveguide SL with a length of 306 mm in back-to-back configuration. (d,e) Measured and simulated attenuation and group delay of the solid (SL) and hollow waveguides (HL1 and HL2), respectively. An equivalent setup of this configuration was also simulated by Ansys HFSS. Solid and dashed lines are measured and simulated results, respectively.