High-order mode suppression in double-clad optical fibers by adding absorbing inclusions.

We proposed and experimentally demonstrated a technique for the suppression of unwanted modes in double-clad fibers with a high core-to-clad diameter ratio by introducing high-index absorbing inclusions into the first cladding of the fibers. These inclusions disturb the shape of undesirable modes, and a noticeable part of the power becomes localized inside the inclusion, resulting in an increase in the propagation loss of these modes. Two fiber designs were studied and realized: one with cylindrical symmetry and an absorbing high-index ring as the inclusion and another with high-index absorbing rods inserted around the fiber core.

Low-loss hybrid fiber with zero dispersion wavelength shifted to 1 µm.

We proposed and investigated a novel type of all-glass hybrid fiber where light is confined in the low-index core due to both total internal reflection and coherent Fresnel reflection (a photonic bandgap mechanism). The hybrid mode has an anomalous dispersion of 13 ps/(nm km) at 1064 nm and low loss (~6 dB/km), and it can be easily excited by splicing with a single-mode step-index fiber. The compression of positively chirped 8 ps pulses down to 330 fs was demonstrated with the fabricated hybrid fiber.

Very-large-mode-area photonic bandgap Bragg fiber polarizing in a wide spectral range.

A design of a polarizing all-glass Bragg fiber with a microstructure core has been proposed for the first time. This design provides suppression of high-order modes and of one of the polarization states of the fundamental mode. The polarizing fiber was fabricated by a new, simple method based on a combination of the modified chemical vapor deposition (MCVD) process and the rod-in-tube technique.

Effect of the AlPO4 join on the pump-to-signal conversion efficiency in heavily Er-doped fibers.

Heavily Er-doped fibers (EDFs) based on P(2)O(5)-Al(2)O(3)-SiO(2) (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a AlPO(4) join having a structure similar to that of SiO(2) glass and a refractive index below it. It is found that the Er(3+) absorption and emission spectra in the PAS EDFs are defined by the dopant (Al(2)O(3) or P(2)O(5)) present in excess and are close to those of the corresponding binary glass (Al(2)O(3)-SiO(2) or P(2)O(5)-SiO(2)).

Polarization-maintaining photonic bandgap Bragg fiber.

The possibility of fabricating a polarization-maintaining Bragg fiber has been studied. It is shown that violation of the cylindrical symmetry of a Bragg mirror in most cases results in a sharp increase in optical loss, which is caused by resonance transmission through the Bragg mirror at wavelengths near the cutoffs of the modes of the high-index rings with a nonzero azimuthal index. It is shown that placing stress-applied parts or air holes inside the Bragg fiber core allows one to avoid this effect.

High-power photonic-bandgap fiber laser.

An original architecture of an active fiber allowing a nearly diffraction-limited beam to be produced is demonstrated. The active medium is a double-clad large-mode-area photonic-bandgap fiber consisting of a 10,000 ppm by weight Yb(3+)-doped core surrounded by an alternation of high- and low-index layers constituting a cylindrical photonic crystal. The periodic cladding allows the robust propagation of a approximately 200 microm(2) fundamental mode and efficiently discriminates against the high-order modes.

Simple nonlinear interferometer-based all-optical thresholder and its applications for optical CDMA.

We present an experimental demonstration of an ultrafast all-optical thresholder based on a nonlinear Sagnac interferometer. The proposed design is intended for operation at very small nonlinear phase shifts. Therefore, it requires an in-loop nonlinearity lower than for the classical nonlinear loop mirror scheme.

Highly dispersive large mode area photonic bandgap fiber.

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length.