Silicon-Photonic Dense 8-Channel Multiplexer Using Mach-Zehnder Interferometer Filters with Nested Ring Resonators

One of the major applications of silicon photonics is the all-optical filtering and multiplexing of spectral data channels [1,2]. Key performance metrics of such filters include a uniform passband, sharp spectral transitions and strong out-of-band rejection. Layouts based on cascaded stages of Mach-Zehnder interferometers (MZIs), ring resonators and multimode-interference couplers may accommodate a narrow free spectral range (FSR) [3]. Narrowband filters mandate comparatively long waveguide paths, on the order of mm and even cm. Fabrication tolerances in the width and height of long waveguides introduce an uncertainty in the accumulated phase delays along each optical path. The proper function of devices critically relies on known phase relations. Hence, post-fabrication trimming of phase delays is a necessity. This challenge becomes more severe with narrowing of the FSR. Active tuning of phase delays is based on the driving of current through metallic heaters and/or p-i-n junctions [4,5]. These solutions increase the complexity of device fabrication and operation, and require continuous feedback and control. One-time permanent post-fabrication trimming is advantageous when the operation temperature of the device is sufficiently stabilized.

We propose and demonstrate an integrated 8- channel dense wavelength division multiplexer in the silicon-on-insulator platform. The device layout comprises of 7 stages in a tree topology. Each stage is based on an unbalanced MZI with a nested ring resonator. The transfer function of each stage is that of an auto-regressive moving-average filter, characterized by a uniform passband, strong out-of-band rejection and sharp spectral transitions. The passband of each channel is extremely narrow: only 0.135 nm (16 GHz), and the worst-case crosstalk among channels is -22 dB. Individual phase delays within the device were trimmed through selective photo-removal of an upper cladding layer of photo-sensitive chalcogenide glass.