Configurable Add-Drop Filter With Stable Insertion Loss for X-Band Microwave Photonics

An add-drop filter (ADF) with configurable center wavelength and bandwidth was demonstrated in a tapered fiber (TF) coupled microsphere. The center wavelength was optimized utilizing the thermal effect by heating the microsphere with a control light injected from the handle of the microsphere and the bandwidth was tuned by changing of the coupling gap based on the ideal quasi-critical coupling (QCC). In the experiment, the center wavelength could be optimized from 1551.04 nm to 1551.13 nm (over 10 GHz) covering the range of X-band of microwave, with the control light power tuned between 7.41 mW to 8.51 mW. As a typical bandwidth tuning result, the band-pass peak could be tuned from 127.5–2529.7 MHz in the Drop port while the band-rejected notch could be tuned from 54.4–2681.6 MHz in the Through port, the coupling efficiency changing at the center wavelength of 1551.04 nm was less than 5%. The calculations and discussions on the range of the stable coupling efficiency when the center wavelength tuned were also presented. The demonstrated filter gains advantages of stable coupling efficiency and tunable narrow bandwidth, which will be helpful for its application in microwave photonics, narrow linewidth lasers, nonlinear optics and so on.