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A WDM/DWDM model for AWGs and Echelle Gratings

Channel broadening using MMI input waveguides


Epipprop combined with FIMMPROP can be used to design single-mode devices with broadened wavelength tolerance using Multimode Interferometer (MMI) input and output waveguides. The MMI allows one to maintain coupling from a single-mode-in to a single-mode-out in the wavelength division multiplexor. This facilitates further light processing downstream which a multimode output waveguide would preclude.

A device will usually suffer from a shift in its channel position that can be a result of imperfections in the slab waveguide, material properties, doping, temperature variation or multiple other causes. As such it is common to want to broaden a channel response such that there is a wavelength tolerance built into the design. One approach is to use a multi-section MMI input waveguide and this can be modelled using Epipprop with FIMMPROP.

You can see below the response from devices without MMIs on the left-hand side and with MMIs on the right-hand side; the channel broadening is clearly visible.

Spectra with and without broadening

Spectra of the echelle grating (left) without broadening and (right) with broadening

The first device has 2um wide shallow etch input and output waveguides. The second device uses an MMI on the input waveguide, with a 3.5um wide waveguide section before the free propagation region. The 1dB-cutoff-width of the peak has more than doubled, going from 1.2nm to 2.5nm, at the expense of an extra 2dB insertion loss. 

You can see below the field profile of the MMI created in EPIPPROP and modelled in FIMMPROP. The length of the second section has been tuned in length to the produce a double image of the input waveguide, seen on the left. 

Spectra with and without broadening

Intensity profile in the MMI waveguide (left) cross-section view at the end facet (right) top view