Tunable add-drop filter using an active whispering gallery mode microcavity

TL;DR

This paper demonstrates a tunable, low-crosstalk add-drop filter using an active whispering gallery mode microcavity with gain medium, achieving significant improvements in quality factor and efficiency.

Contribution

Introducing an active gain medium in a whispering gallery mode resonator enables loss compensation, tunability, and reduced crosstalk in add-drop filters.

Findings

24-fold enhancement in intrinsic quality factor

3.5-fold increase in drop efficiency

35 MHz bandwidth tunability

Abstract

An add-drop filter (ADF) fabricated using a whispering gallery mode resonator has different crosstalks for add and drop functions due to non-zero intrinsic losses of the resonator. Here, we show that introducing gain medium in the resonator and optically pumping it below the lasing threshold not only allows loss compensation to achieve similar and lower crosstalks but also tunability in bandwidth and add-drop efficiency. For an active ADF fabricated using an erbium-ytterbium co-doped microsphere, we achieved 24-fold enhancement in the intrinsic quality factor, 3.5-fold increase in drop efficiency, bandwidth tunability of 35 MHz and a crosstalk of only 2%.