Easy-to-Tune All-Fiber Bandpass Filter

Breck Hitz

Tunable bandpass filters are crucial components in numerous photonic applications, including telecommunications. All-fiber filters eliminate the need to couple into bulky and lossy external components, and many approaches to all-fiber tunable filters have been explored by researchers worldwide. Now scientists at Tianjin University and at FiberHome Telecommunication Technologies Co. Ltd., in Wuhan, both in China, have demonstrated an all-fiber tunable bandpass filter that avoids the complex tuning mechanisms — e.g., acousto-optic, electro-optic, temperature or pressure — of previous all-fiber filters.

The bandwidth and center frequency of the new filter are set merely by changing the bend radii of a pair of fibers. The bend in one fiber sets the long-wavelength cutoff, and the bend in the other sets the short-wavelength cutoff (Figure 1).

Figure 1. Scientists spliced two fibers in series with each other, so that the bend radius of one fiber set the short-wavelength cutoff and the bend radius of the other set the long-wavelength cutoff. SC PBGFs = solid-core photonic bandgap fibers; SM = single mode. Images reprinted with permission of IEEE Photonics Technology Letters.

The fibers in question are a solid-core photonic bandgap fiber and a concentric-ring, one-dimensional Bragg reflection fiber (Figure 2). The former is composed of high-index silica-doped rods embedded in a pure-silica fiber, with a defect at the center serving as the low-index core.

Figure 2. The two fibers that were spliced together to create a tunable bandpass filter were a solid-core photonic bandgap fiber (left) and a concentric-ring, one-dimensional Bragg reflection fiber (right).

The Bragg fiber is a one-dimensional bandgap fiber whose 1.5-μm core is surrounded by a cladding of alternating high- and low-index concentric rings. Light is confined to the core by Bragg reflection from the rings but, because the core index is higher than the average cladding index, total internal reflection also plays a role. The small core of this fiber seems to be mismatched to the much larger core of the solid-core photonic bandgap fiber. But because the core is so small, most of the electric fields of the core-propagating modes extend into the concentric cladding layers so the modes of the two fibers are nicely matched. The measured loss at the splice between them is only ∼0.25 dB.

The transmission spectra of the two fibers can be altered by coiling them to different radii (Figure 3). The short-wavelength cutoff of the solid-core photonic bandgap fiber moves smoothly from ∼950 to ∼1100 nm as the fiber is bent from a straight line to a 13-cm-diameter bend. And the long-wavelength cutoff of the Bragg fiber moves from ∼1100 nm to ∼1400 nm as its bend is loosened from a 3-cm diameter to a straight line.

Figure 3. The fibers’ transmission spectra are dependent on their bend radii.

By splicing the two fibers in series and adjusting the bend radii of both, the scientists tuned the filter’s bandwidth and center wavelength (Figure 4). The filter’s insertion loss was ∼5 dB at 1100 nm, but they think that can be reduced with straightforward improvements, including reducing each fiber’s length from the 1.6 m used in this demonstration.

Figure 4. By adjusting the bend radii of the two fibers, the scientists could tune the filter’s bandwidth between 50 and 250 nm (left). Likewise, they could tune the center wavelength from ∼1011 nm to ∼1120 nm (right). SC PBGFs = solid-core photonic bandgap fibers.

IEEE Photonics Technology Letters, April 15, 2008, pp. 581-583.