Programmable optical filters based on feed-forward photonic meshes

We demonstrate an integrated photonic circuit based on feed-forward photonic meshes that may be reconfigured to perform a wide range of spectral filter functions in a programmable way. We investigate a subset of the available filter functions, demonstrating that a $N=4$ input triangular mesh with $M=3$ layers may be operated via self-configuration algorithms to filter M arbitrary wavelengths from a given input spectrum. The tunable nature of the architecture enables preconfigured filter functions to be swept in the spectral domain continuously over the free spectral range of the device. This removes any strict requirements between the design parameters of the architecture and the center wavelength of a desired filter function. With this architecture, we experimentally demonstrate arbitrary wavelength rejection filters with contrasts as deep as 40 dB. Further, by intentionally selecting the center wavelengths of each filter function to lie along a wavelength grid defined by $\Delta \lambda ={\lambda }_{\mathrm{fsr}}/N$ we demonstrate deep wavelength division demultiplexing (DWDM) with inter-channel crosstalk between -25 dB and -40 dB. Unlike typical DWDM systems, in this architecture the center wavelength of each channel is not fixed at fabrication and instead may be programmatically defined. This device demonstrates advantages over typical methods for DWDM, Raman spectroscopy, and correlation spectroscopy as well as other applications.