Automatic tuning of silicon photonic ring filters

Silicon ring resonators make excellent tunable filters, but they don't stay where you put them. Fabrication variation puts every ring at a slightly different wavelength, and temperature moves it by roughly 0.07–0.08 nm per kelvin, which is about a linewidth for every couple of degrees. A chip with several rings needs an automatic way to find each resonance and hold it there. This note describes, at a high level, how I did that for the reconfigurable filters in my Ph.D. work.

The hardware

Each ring has a thermal phase shifter (a resistive heater) and a small monitor tap that sends part of the ring's light to a photodiode. A 16-channel, 16-bit DAC board that I designed drives the heaters, and a microcontroller reads the photodiodes, runs the search, and steps the tunable laser to the wavelengths of interest.

How the search works

1. Coarse search. Sweep each ring's heater across a wide range and find the setting where its monitor reads highest, which is where the ring sits on resonance.
2. Fine search. Sweep a narrow window around that point with smaller steps.
3. Iterate. Heat from one ring shifts its neighbours, so the whole set is re-tuned in several passes, each with a narrower window, until every ring stops moving.

Each photodiode reading combines many samples through a median filter, so a single noisy sample cannot pull a ring off resonance. When a monitor signal is too weak to trust, the ring keeps its last good setting instead of jumping somewhere new.

What it enabled

The same loop configured band-pass and notch responses on demand and kept them stable during measurements. The results are in my papers on automated tuning of silicon photonic filters and on tunable mm-wave photonic receiver front ends, listed on the Publications page.

More details

This page is an overview. For details on the hardware, the algorithm, or the measured results, email me at engramyrady@gmail.com.