PHOTONIC BANDGAP FIBERS: All-solid bandgap fiber has low loss, high bandwidth

A new type of all-solid photonic-bandgap fiber (PBGF) with low loss and high bandwidth has been developed by researchers at Nanyang Technological University (Singapore) and Yangtze Optical Fiber and Cable Company (Wuhan, China).¹ While the lowest loss reported for other all-solid PBGFs is approximately 20 dB/km, this new fiber has a loss on the order of 2 dB/km—a factor-of-ten improvement. In addition to improved stability and easier splicing compared to holey PBGFs, the all-solid fiber also has very low bend sensitivity and a bandwidth greater than 700 nm.

The decreased loss and improved bend sensitivity of this fiber is attributed to an index-depressed layer around the high-index rods that comprise the fiber cladding. The preform for the PBGF is fabricated by first creating a number of cladding rods that consist of a central high-index germanium-doped silica with a quasi-parabolic index profile surrounded by a fluorine-doped index-depressed layer, which is then overclad with a pure silica jacket (see figure). The rod preform is etched and drawn into smaller canes. These are then stacked in a hexagonal pattern and the core formed by replacing the central doped cane with a pure silica one. The stack is then jacketed with silica and drawn to a final fiber diameter of 175 µm; for the drawn PBGF, the lattice spacing for the hexagonal array is approximately 7.23 µm.

Spectral transmission through the all-solid PBGF is measured using a broadband supercontinuum source based on photonic-crystal fiber. The broadband source is coupled into a standard single-mode fiber that is then butt-coupled to the PBGF. An optical-spectrum analyzer measures the output between 600 and 1750 nm from a 5 m length of PBGF, revealing a transmission spectrum with first and second bandgaps at 820 to 1720 nm and 530 to 790 nm, respectively, corresponding to the low-loss transmission windows of the fiber.

Low bending loss

The cutback technique was used to measure the attenuation spectrum of a 490‑m-long sample of the PBGF. Broadband transmission was observed from 880 to 1600 nm, corresponding to the first bandgap-a bandwidth of greater than 700 nm. Within this wavelength range, the minimum loss of approximately 2 dB/km was recorded at a wavelength of 1310 nm; however, the minimum attenuation can be shifted to other wavelengths (such as 1550 nm) by scaling the fiber diameter or lattice spacing. Because the floor of the bandgaps is efficiently deepened by introducing the index-depressed layer in the photonic-crystal rod structures of the fiber cladding, the enlarged index mismatch of the guided core mode and the edge of the bandgaps serve to reduce the bend loss of this fiber. In comparison to a step-index single-mode fiber with a bend loss of 54.3 dB/m for an 8 mm bend radius, the PBGF only has a bend loss of 1.8 dB/m for the same bend radius.

“All-solid PBGF is a new member of the PBGF family,” says researcher Guobin Ren from Nanyang Technological University. “The all-solid PBGFs are particularly interesting, as they present chromatic dispersion and spectral-filtering properties in an all-solid version that is relatively easy to fabricate and splice compared with air-core PBGFs. The loss of our PBGF is adequately low to realize a lot of practical applications such as Bragg-grating inscription and active doped PBGFs. Next, we will optimize the fiber design and fabrication for further improvement of the attenuation performance at 1550 nm.”

REFERENCE

1. G.Ren et al., Optics Lett. 32(9) 1023 (May 1, 2007).