Design, fabrication, and characterization of Deep-Etched waveguide gratings

One-dimensional (1-D) deep-etched gratings on a
specially grown AlGaAs wafer were designed and fabricated.
The gratings were fabricated using state-of-the-art electron beam
lithography and high-aspect-ratio reactive ion etching (RIE) in
order to achieve the required narrow deep air slots with good
accuracy and reproducibility. Since remarkable etch depths (up to
1.5 μm), which completely cut through the waveguide core layer,
have been attained, gratings composed of only five periods (and,
thus, shorter than 6 μm) have a bandgap larger than 100 nm. A defect
was introduced by increasing the width of the central semiconductor
tooth to create microcavities that exhibit a narrow
transmission peak (less than 7 nm) around the wavelength of
1530 nm. The transmission spectra between 1460 and 1580 nm
have been systematically measured, and the losses have been estimated
for a set of gratings, both with and without a defect,
for different periods and air slot dimensions. Numerical results
obtained via a bidirectional beam propagation code allowed the
evaluation of transmissivity, reflectivity, and diffraction losses.
By comparing experimental results with the authors’ numerical
findings, a clear picture of the role of the grating’s geometric
parameters in determining its spectral features and diffractive
losses is illustrated.