Optofluidic tuning of multimode interference fiber filters

We report on the optofluidic tuning of MMI-based bandpass filters. It is well known that MMI devices exhibit their highest sensitivity when their diameter (D) is modified, since they have a D² wavelength dependence. In order to increase the MMF diameter we use a special fiber, called No-Core fiber, which is basically a MMF with a diameter of 125 μm with air as the cover. Therefore, when this No-Core fiber is immersed in liquids with different refractive indexes, as a result of the Goes-Hänchen shift the effective width (fundamental mode width) of the No-Core fiber is increased, and thus the peak wavelength is tuned. A tunability of almost 40 nm in going from air (n=1.333) to ethylene glycol (n=1.434) was easily obtained, with a minimum change in peak transmission, contrast, and bandwidth. Moreover, since replacing the entire liquid can be difficult, the device was placed vertically and the liquid was covering the No-Core fiber in small steps. This provided similar amount of tuning as before, but a more controllable tuning mechanism.