Emerging planar waveguide devices for WDM and spectroscopy: Curved waveguide multiplexers and interferometer arrays

We review two emerging concepts in planar waveguide multiplexers and spectrometers. The first concept is a planar waveguide demultiplexer with the wavelength dispersive properties provided by a diffraction grating formed in the sidewall of a curved Si waveguide. The phase errors are minimized using an apodized design with a chirped grating, yielding a crosstalk below −30 dB. The device has 15 channels with a wavelength spacing of 25 nm, thus a broad operational bandwidth of 375 nm suitable for coarse wavelength division multiplexing (CWDM) applications. The layout area occupies less then 0.005 mm
²
, which is the smallest footprint yet reported for a mux/demux device of similar performance. The second concept is a Fourier-transform Mach-Zehnder interferometer array. Multi-aperture input provides a markedly increased optical throughput. Unlike conventional Fourier-transform spectrometers, the device has no moving parts. Another advantage of this device is that phase and amplitude errors can be readily measured and numerically corrected with no need for costly modifications of the waveguide physical properties. An example of the spectrometer designed for the silicon-oninsulator platform with a spectral resolution of 0.025 nm is presented.