Graham R. Atkins

Experimental and theoretical analysis of the resonant nonlinearity in ytterbium-doped fiber

Experimental measurements are described characterizing the nonlinear index change over the range from 500 to 1550 nm induced in an ytterbium (Yb/sup 3+/)-doped twin-core fiber by a 980 nm pump. At 1550 nm, a phase change of /spl pi/ is induced with as little as 14 mW of pump power for a signal loss of only 0.2 dB, By allowing the doped fiber to lase and observing the associated clamping of the induced phase change, we show that a digital nonlinear response can be achieved in which a constant, pump-power-insensitive, phase change is induced for all pump powers above a certain threshold. This lasing induced clamping of the phase change also demonstrates that the nonlinear effect is population dependent as opposed to thermal. The pump-induced phase change is observed to increase for shorter signal wavelengths, which suggests that the effect is due principally to pump-induced changes in the strong ultraviolet (UV) absorptions of Yb/sup 3+/. This observation is accurately predicted by a theoretical analysis that takes into account absorptions in both infrared and ultraviolet regions. This analysis shows that Yb/sup 3+/ may be suitable for low-power all-optical switching applications in both 1300 and 1550 nm telecommunications windows when the speed of response is not a critical parameter.

Optical properties of an ormosil system comprising methyl- and phenyl-substituted silica

Abstract We report an organically modified silica (ormosil) system in which the refractive index can be varied from 1.40 to 1.55 by modifying the silica backbone with phenyl and methyl groups, which increase and decrease the refractive index, respectively. By changing the quantities of the phenyl- and methyl-modified precursors in the sol–gel mixture, the refractive index can be varied while keeping the organic fraction of the material constant. Crack-free, dried films up to 15-μm thick have been coated onto silicon and glass substrates, with potential application to optical waveguides for planar integrated circuits. The contributions of SiO–H and C–H absorptions to the attenuation in the communications windows are discussed, along with factors affecting the homogeneity and cracking tendency of the ormosils.

Photodarkening in Tb 3+ -doped phosphosilicate and germanosilicate optical fibers

The presence of Tb(3+) is found to sensitize greatly phosphosilicate and germanosilicate optical fibers to photodarkening when they are exposed to 488-nm light. The darkening is a three-photon process and may involve the photoionization of Tb(3+) to Tb(4+). This sensitivity raises the possibility of side writing refractive-index gratings into silica-based optical fibers with 488-nm light.

Intermediate excited-state absorption in erbium-doped fiber strongly pumped at 980 nm

We report the observation of fluorescence at 530 to 550 nm from erbium-doped germanosilicate fiber under strong pumping in the 980-nm band. The observed nonlinear dependence on pump power is shown to arise from a sequential two-photon absorption from the ground state to the (4)F(7/2) state through the short-lived (4)I(11/2) intermediate state. Rate-equation modeling shows that the pump power dependence of this intermediate excited-state absorption is influenced by population cycling induced by amplified spontaneous emission in the fiber from the metastable (4)I(13/2) state.

Nonlinear phase changes at 1310 nm and 1545 nm observed far from resonance in diode pumped ytterbium doped fiber

Resonant nonlinearities at 1310 mm and 1545 nm have been observed in diode pumped ytterbium doped fiber, with greater than /spl pi/ phase changes observed with 18 mW launched pump power. These signal wavelengths lie far from the ytterbium absorption peak centred at 980 nn, and can be explained by considering the effect of strong absorptions in the UV. It has been confirmed that these phase changes are not due to thermal effects in the optically pumped fiber.

Direct laser writing of self-developed waveguides in benzyldimethylketal-doped sol-gel hybrid glass

The Australian Photonics Cooperative Research Center in Australia and Ministry of Information and Communication and the Brain Korea21 project in Korea financially supported this study. B-S.B. acknowledges the Australian Research Council for the award of and IREX fellowship.

Preparation and spectroscopy of anhydrous borosilicate sols and their application to thin films

Abstract Borosilicate sols with various boron:silicon ratios were prepared from an anhydrous boric acid (H 3 BO 3 )–ethanol (C 2 H 5 OH)–tetraethyl orthosilicate (TEOS) mixture, and dip-coated onto microscope slide substrates under N 2 protection. Transparent crack-free films were obtained when the molar ratio of boric acid to TEOS was between 0.66:1 and 1.33:1. NMR ( 29 Si and 11 B) and IR spectroscopic techniques were used to infer the formation of borosiloxane (B–O–Si) linkages in the anhydrous sols, and their susceptibility to hydrolysis by atmospheric moisture was observed.

Writing and visualization of low-threshold type II Bragg gratings in stressed optical fibers

Type II phase gratings were produced by the use of holographic side writing in high-birefringence optical fibers with UV fluences of 0.06 J/cm(2) over 10 times lower than that previously reported for standard fibers. The grating growth, transmission and reflection spectra, temperature response, short-wavelength light ejection, and high-resolution confocal microscopy images are reported. Diffraction theory is used to interpret the grating microstructure revealed by confocal microscopy. Each period of the grating is shown to consist of a plate of oriented cracks, and arguments relating to the arrangement of the cracks and crack growth are linked to the observed grating-growth dynamics.

A SPECTROSCOPIC STUDY OF AN ANHYDROUS TETRAETHYL ORTHOSILICATE-BORIC ACID-ETHANOL SYSTEM

Hydrolysis and condensation reactions in an anhydrous sol-gel system comprising tetraethyl orthosilicate (TEOS, Si(OC2H5)4), boric acid (H3BO3) and ethanol have been studied using X-ray diffraction and NMR spectroscopy, and similarities and differences with the more traditional aqueous system are discussed. Boric acid is able to hydrolyse TEOS directly, and subsequent condensation reactions form borosiloxane (=B–O–Si≡) and siloxane (≡Si–O–Si≡) linkages. In an aqueous system, borosiloxanes are unstable to hydrolysis and are formed only upon heat treatment of the gel, a restriction avoided with anhydrous conditions. The anhydrous mixture is stable indefinitely against gelation, but can be readily gelled by addition of NaOH in ethanol. This system may be useful for preparing borosilicate glasses at lower temperatures with good homogeneity.

High-isolation demultiplexing in bend-tuned twin-core fiber

By applying a cantilever bend to a twin-core optical fiber wavelength division demultiplexer, isolations of >30 dB have been achieved between channels at 1.33-1.54 /spl mu/m. These bend-tuned TCFs exhibit /spl ges/20 dB isolation over a bandwidth of 32 and 21 nm in the two wavelength windows, respectively. This is significantly better than that typically offered by commercially available, single-stage, tapered coupler demultiplexers. The bend-tuning mechanism is demonstrated experimentally and described theoretically using a coupled local-mode analysis.