Waveguide Reveals Singularities

ENSCHEDE, Netherlands -- The optical equivalent of scanning tunneling electron microscopy promises to aid the design and analysis of photonic devices in telecommunications and integrated optics. Called a heterodyne interferometric photon scanning tunneling microscope, the instrument is reported to be the first to observe phase singularities in an optical waveguide.
The researchers from the Mesa⁺ Research Institute and department of applied physics at the University of Twente have turned a photon scanning tunneling microscope into one branch of a Mach-Zehnder interferometer. Typically, a fiber optic probe would scan in a raster pattern 10 to 20 nm over the surface of an optical waveguide, within the waveguide's evanescent field, so that some of its field could tunnel into the fiber and become detectable photons. The Dutch team, however, split the laser beam before injecting it into the waveguide, which produced a reference beam with which the tunneled signal could be mixed.
"The result is a microscope that maps optical field amplitude, phase and waveguide topography, all simultaneously, with nanometric resolution," said Niek F. van Hulst, chairman of the optical techniques group at the university. In contrast, photon scanning tunneling microscopes provide information only about the intensity of the light in the waveguide. Van Hulst co-authored a report describing the technique in the July 10 Physical Review Letters.
As a demonstration, the researchers investigated the phase evolution of light in a 4-nm-high, 3-µm-wide Si₃N₄ channel. They recombined the tunneled and reference signals from a laser in a 50/50 fiber coupler and collected the interference signal with a photomultiplier tube. In agreement with theory, singularities appeared where the TE₀₀, TE₀₁ and TM₀₀ modes interfered and the summed phase was undefined.
"These singularities, though never visualized before, are omnipresent in all multimodal waveguides," van Hulst said. The team also has observed mode conversion in Y junctions and mode splitters, quasi-interference of mutually perpendicularly polarized transverse electric and magnetic modes in waveguide channels, and whispering gallery modes in cylindrical microcavities.
"The characterization and optimization of integrated optical devices generally relies on input/output measurements and comparison with computer simulation," he explained. "This microscope gives a direct, detailed look inside integrated components." Specifically, he noted, it may allow engineers to determine the origin of attenuation and crosstalk in wavelength division multiplexing systems.