LC-filled PC fibers form tunable bandpass filter

Two short sections of optical fiber that combine to become an electrically tunable bandpass filter have been fabricated by scientists at the Technical University of Denmark (Lyngby, Denmark) and NKT Photonics (Birkerød, Denmark). The 10 mm long sections are each large-mode-area solid-core photonic-crystal (PC) fibers with their holes filled with a fluid of liquid-crystal (LC) molecules; the hole diameters and the LC type are different for each section. The two sections are placed serially and butt-coupled in a silicon v-groove containing gold electrodes along the sides of the groove; two single-mode fibers are also held in the ends of the groove and butt-coupled to the sections to couple light in and out of the PC-fiber arrangement.

Light from a supercontinuum-fiber source is coupled into the device. One LC type is a longpass filter and the other a shortpass filter; the combination produces a bandpass filter transmitting over the 1520 to 1680 nm range. When a driving voltage variable between 90 and 120 V is applied to one LC and no voltage to the other, the shortpass edge of the filter can be tuned over a 36 nm bandwidth, with the longpass edge immobile. A similar process applied conversely to the fiber sections produces a longpass tuning range of 12 nm with no shortpass change. Contact Lei Wei at lewe@fotonik.dtu.dk.

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