Programmable Optical Filters Based on Feed-Forward Photonic Meshes

Abstract

We demonstrate an integrated photonic circuit based on feed forward photonic meshes that can be programmed and reconfigured to perform arbitrary spectral filter functions. 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 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 swept or reordered arbitrarily. This device demonstrates advantages over typical methods for DWDM, Raman spectroscopy, and correlation spectroscopy as well as other applications.

Figures (6)

• Figure 1: (Top) Schematic diagram of a four-channel programmable spectral filter. (Bottom) Microscope image of the fabricated photonic integrated circuit. The photonic circuit is comprised of three subcircuits: a power splitter, an array of waveguide delay lines, and a photonic mesh. Here the power splitting subcircuit has been implemented via a two-stage binary tree of balanced MZIs. The array of $N$ waveguide delay lines introduces the phase shifts $\phi_p$ to each respective waveguide. The array of waveguide delay lines has been designed with a uniform increment $\Delta{L} =$ 740 $\upmu$m to introduce a linear phase tilt. The photonic mesh applies a programmable matrix transformation $O$, described by matrix elements $O_{pq}$. We implement this subcircuit as an $N=4, M=3$ triangular mesh capable of implementing any linear unitary $4\times4$ matrix transformation.
• Figure 2: Calibration data of the $\theta_1$ phase shifter at a central wavelength $\lambda_0 =$1550 nm. The $\theta_1$ phase shifter controls the split ratio to the $p = 2$ and $p=3$ input waveguides of the photonic mesh and has no impact on the $p = 1$ and $p=0$ input waveguides. We observe the characteristic sinusoidal transmission of a balanced MZI and measure a $P_\pi$ of 18 mW.
• Figure 3: Simulated (Left) and measured (Right) output spectra of a $N=4$, $M=1$ spectral filter which has been programmed via self-configuration. The output spectra demonstrate that light at the central angular frequency $\omega_0$ corresponding to a central wavelength $\lambda_0 =$ 1550 nm has been collected to the $q=3$ output waveguide and has been completely rejected from the remaining outputs. We measure rejection of the central wavelength from the remaining output channels with over 35 dB of contrast and a free spectral range of 0.78 nm.
• Figure 4: Simulated (Left) and measured (Right) output spectra of a $N=4$, $M=1$ spectral filter while applying an additional phase profile $e^{-j2\pi\gamma{p}}$ for different values of $\gamma$. By adjusting the value of $\gamma$ continuously between 0 and 1, a preconfigured filter function may be tuned continuously across an FSR.
• Figure 5: (Top Left) Simulated output spectra of a $N=4$, $M=3$ spectral filter where each layer of the mesh has been programmed via self configuration to reject a frequency associated with the nulls of each other layer of the mesh. The resulting spectra produces high transmission for a single channel of the generated frequency grid while maintaining high rejection each other channel. (Top Right) The chosen wavelengths of interest are $\lambda_0$ = 1550 nm, $\lambda_1$ = 1550.19 nm, and $\lambda_2$ = 1550.38 nm. (Bottom Left) The wavelengths of interest have been reordered here as $\lambda_0$ = 1550 nm, $\lambda_1$ = 1550.38 nm, and $\lambda_2$ = 1550.57 nm to demonstrate that the devices capacity to route any wavelength channel to any output. (Bottom Right) Measured transient response of output waveguide 1 when switching between two preconfigured filter functions.