  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%.
