S.L. Semjonov

Low-loss singlemode large mode area all-silica photonic bandgap fiber.

We describe the design and characterization of solid core large mode area bandgap fibers exhibiting low propagation loss and low bend loss. The fibers have been prepared by modified chemical vapor deposition process. The bandgap guidance obtained thanks to a 3-bilayer periodic cladding is assisted by a very slight index step (5.10-4) in the solid core. The propagation loss reaches a few dB/km and is found to be close to material loss.

Single-mode all-silica photonic bandgap fiber with 20-μm mode-field diameter

An all-silica photonic bandgap fiber with a cladding index difference of approximately 2 % and diameter-to-pitch ratio (d/Λ) of 0.12 was fabricated and studied. To our knowledge, this is the first report on the properties of photonic bandgap fiber with such a small d/Λ. The fiber is single-mode in the fundamental bandgap. The mode field diameter in the 1000-1200 nm wavelength range is 19-20 μm. The minimum loss in the same range is 20 dB/km for a 30-cm bending diameter. In our opinion, all-silica photonic bandgap fiber can serve as a potential candidate for achieving single-mode propagation with a large mode area.

Broadband dispersion-compensating fiber for high-bit-rate transmission network use.

The optimum refractive-index profile and drawing temperature were investigated so as to maximize the figure of merit for multicladding broadband dispersion-compensating fibers. Based on the results of the investigation, the authors have fabricated a highly bend-resistant fiber with a 92.6-ps/(nm dB) figure of merit using the modified chemical-vapor deposition method for dispersion compensation in the 1.5-1.6-µm wavelength region. The manufactured dispersion compensator does not suffer bend loss at 1.55 µm for curvatures of radia of 6.3 and 3.3 cm, and it has a 1.1-dB/km bend loss at a curvature of radius of 1.6 cm. Codoping the germanium silicate core with fluorine diminishes the optical loss down to 0.70 dB/km at a 1.55-µm wavelength.

Mechanisms of optical losses in Bi:SiO 2 glass fibers

The mechanisms of optical losses in bismuth-doped silica glass (Bi:SiO(2)) and fibers were studied. It was found that in the fibers of this composition the up-conversion processes occur even at bismuth concentrations lower than 0.02 at.%. Bi:SiO(2) core holey fiber drawn under oxidizing conditions was investigated. The absorption spectrum of this fiber has no bands of the bismuth infrared active center. Annealing of this fiber under reducing conditions leads to the formation of the IR absorption bands of the bismuth active center (BAC) and to the simultaneous growth of background losses. Under the realized annealing conditions (argon atmosphere, T(max) = 1100°C, duration 30 min) the BAC concentration reaches its maximum and begins to decrease in the process of excessive Bi reduction, while the background losses only increase. It was shown that the cause of these background losses is the absorption of light by nanoparticles of metallic bismuth formed in bismuth-doped glasses as a result of reduction of a part of the bismuth ions to Bi(0) and their following aggregation. The growth of background losses occurs owing to the increase of the concentration and the size of the metallic bismuth nanoparticles.

Broadband wavelength conversion in a germanosilicate-core photonic crystal fiber

A photonic crystal fiber with a germanosilicate core having a nonlinear coefficient of 40 (W km)(-1) near the single dispersion zero at 1.09 microm is fabricated and studied. Broadband parametric wavelength conversion of the Ti:sapphire laser output tunable at 0.8 microm to the 1.55 microm band is obtained at 1.064 microm cw pump. The tuning of the converted signal in the 300 nm range was first realized without variation of the pump wavelength.

On the origin of excess loss in highly GeO/sub 2/-doped single-mode MCVD fibers

Index profile grading and elimination of the central dip have yielded the lowest loss ever reported for modified chemical vapor deposition-made highly GeO/sub 2/-doped single-mode fibers (1.33 dB/km at 1550 nm for /spl Delta/=2.9%). Investigation of the angular distribution of scattering has shown that much of the optical loss in such fibers is due to anomalous scattering. This scattering arises in the region of the central dip in the refractive index profile and at the core-cladding interface.

Phosphate-core silica-clad Er/Yb-doped optical fiber and cladding pumped laser

We present a composite optical fiber with a Er/Yb co-doped phosphate-glass core in a silica glass cladding as well as cladding pumped laser. The fabrication process, optical properties, and lasing parameters are described. The slope efficiency under 980 nm cladding pumping reached 39% with respect to the absorbed pump power and 28% with respect to the coupled pump power. Due to high doping level of the phosphate core optimal length was several times shorter than that of silica core fibers.

Mechanical reliability of polymer-coated and hermetically coated optical fibers based on proof testing

Proof testing is the principal technique for revealing the largest fiber flaws that reduce fiber strength below some definite level. At the same time the proof test does not guarantee long-term (25-50 yr) reliability of optical fibers due to the growth of surface defects during proof stress unloading. In the present paper we analyze the possibility of reducing unloading effects by means of shortening the unloading time or proof testing under inert conditions. We have also estimated the reliability of hermetically coated proof-tested optical fibers and obtained simple relations that take into consideration the proof test and service conditions when evaluating the fiber reliability.

Reduction of bend loss in large-mode-area bragg fibres

The delivery or generation of high power in optical fibre requires the increase of the core size to increase the threshold of nonlinear effects and the damage threshold. However the bend loss strongly limits the increase of the effective area (Aeff). All-solid photonic bandgap fibres are attractive for the delivery of power since they can be made singlemode whatever the core diameter is. Moreover the silica core can be doped with rare-earth ions. A Bragg fibre is a bandgap fibre composed of a low index core surrounded by N concentric layers of high and low index. We have fabricated Large Mode Area Bragg fibres by the MCVD process. These Bragg fibres present a ratio Aeff/λ2 close to 500. A first Bragg fibre, defined by N = 3 and an index contrast between the cladding layers Δn = 0.01, exhibits a measured critical bend radius Rc close to 16 cm (bend loss equal to 3 dB/m). Increasing the index contrast Δn leads to a tighter field confinement. The field distribution of the guided mode strongly decays in the periodic cladding and is thus less sensitive to bending. We propose here the design of an improved Bragg fibre with a very large index contrast Δn = 0.035 which leads to a dramatic reduction of the bend loss. The critical bend radius was measured to be lower than 3 cm. This fibre is less bend sensitive than an equivalent solid core fibre, either a step-index fibre or a photonic crystal fibre.

High-beam quality, high-efficiency laser based on fiber with heavily Yb(3+)-doped phosphate core and silica cladding.

We have fabricated and tested a composite fiber with an Yb(3+)-doped phosphate glass core and silica cladding. Oscillation with a slope efficiency of 74% was achieved using core pumping at 976 nm with fiber lengths of 48-90 mm in a simple laser configuration, where the cavity was formed by a high-reflectivity Bragg grating and the cleaved fiber end. The measured M(2) factors were as low as 1.05-1.22 even though the fiber was multimode at the lasing wavelength.