Edge Coupler Links All-Solid Photonic-Bandgap Fibers

Breck Hitz

Researchers at Nanyang Technological University and at the Institute for InfoComm Research, both in Singapore, have fabricated what they believe is the first photonic-bandgap fiber coupler. Couplers are crucial to almost all applications of optical fiber, but previous couplers for holey fibers had been unable to avoid contamination of the airholes, which introduces excess loss to the coupler.

Figure 1. Shown are an optical micrograph and a scanning electron micrograph of the fiber cross section. The holes of the photonic bandgap fiber were filled with glass whose average refractive index was higher than that of the surrounding glass. The fiber’s core is the small lattice defect at the center of the fiber. Images reprinted with permission of Optics Letters.

Holey fibers, sometimes called microstructured optical fibers, depend on a network of longitudinal holes along the length of the fiber to confine light to its core. There are two broad categories of holey fibers, index-guided and bandgap-guided. In index-guided fibers, a network of airholes in the cladding surrounding the core lowers the effective index of the cladding, so that light is confined in the high-index core just as it is in a normal (non-holey) optical fiber.

Figure 2. The scientists epoxied the fiber into a groove in a silica block and polished the block to remove most of the cladding from a raised section of the fiber.

The physics is different in bandgap-guided fibers. The periodicity of the holes creates a photonic bandgap in the cladding, analogous to the electronic bandgap in a semiconductor. Light in a given wavelength range cannot propagate in the cladding and, hence, is confined to the core. In a bandgap-guided fiber, the refractive index of the core can be less than that of the cladding.

The most common technique for fabricating couplers with conventional fibers is to polish the sides of two fibers almost to their cores and to press the polished sides together so light is evanescently coupled from one core to the other. This technique is less successful with holey fibers because the side-polishing exposes airholes, which are then easily contaminated.

Figure 3. The researchers positioned the two silica blocks one atop the other, so that the side-polished fibers in each block were aligned (left). By displacing one block longitudinally relative to the other, they could tune the degree of coupling between the fibers (right). (PBGF = photonic bandgap fiber.)

The researchers finessed this difficulty with holey fibers in which the holes themselves were solid, having a higher index than the glass of the cladding surrounding them (Figure 1). The periodicity of the holes nonetheless created a photonic bandgap, but now the fiber could be side-polished without exposing hollow airholes to contamination.

The researchers machined a 250-μm-wide groove in each of two silica blocks and epoxied a 175-μm-diameter fiber into each groove so that a section of the fiber approximately 12 mm in length was raised slightly in the groove. They polished the blocks so that the raised sections of the fibers were polished almost to their cores (Figure 2).

Figure 4. The amount of light coupled from one fiber into the other could be smoothly tuned by adjusting the longitudinal displacement of one fiber relative to the other.

After positioning the two blocks next to each other — so the side-polished fibers were pressed together — they injected a thin film of index-matching fluid between the blocks to eliminate index discontinuities at any air gaps and to lubricate the blocks as they slid back and forth against each other.

By sliding the blocks back and forth longitudinally, the scientists tuned the amount of light coupled from one fiber to the other (Figure 3). Defining the coupling ratio as the ratio of power emerging from one fiber to the sum of the powers emerging from both fibers, they observed a maximum coupling of 92.5 percent at 1550 nm and could tune the coupling smoothly by adjusting the position of one block relative to the other (Figure 4).

Optics Letters, Nov. 1, 2007, pp 3059-3061.