L.J. Poyntz-Wright

Generation of permanent optically induced second-order nonlinearities in optical fibers by poling

It is shown that large permanent enhancements in second-order optical nonlinearity can be induced in germanosilicate fibers (both pure and codoped with phosphorus) by application of a transverse dc electric poling field in the presence of high-intensity light. The macroscopic inversion symmetry of the core material is broken by the excitation and alignment of defect centers. Significant frequency doubling results despite the absence of phase matching. The saturation (with both increasing dc field and increasing intensity) of this effect is investigated.

Nonlinear transmission and color-center dynamics in germanosilicate fibers at 420–540 nm

We report evidence in support of the view that induced loss and nonlinear transmission in pure germanosilicate fibers at blue-green wavelengths are governed by the formation (through two-photon absorption), spontaneous and stimulated transformation, and bleaching (through single-photon events) of Ge(1), Ge(2), and Ge(3) color centers. Using a tunable pulsed-dye laser, the excitation spectrum of the induced absorption, its spectral attenuation, and the effects of Ge concentration and thermal annealing are investigated.

Frequency doubling, absorption, and grating formation in glass fibers: effective defects or defective effects?

The present understanding of colour centres in germanosilicate glass fibres and the diverse effects attributed to colour centre activity are reviewed. Drawing on a wide range of up-to-date research results an attempt is made to piece together as far as possible a unified picture of the defect processes behind second harmonic generation nonlinear transmission and photorefractive grating formation in optical fibres.

Tunable holographic second-harmonic generators in high-birefringence optical fibers

The formation of efficient holographic second-harmonic generators in high-birefringence phosphorus-doped germanosilicate fibers is reported. The influence of optical polarization on the nonlinear writing and read-out processes is explored. Fiber birefringence permits phase-matched second-harmonic conversion at wavelengths within +/-125 cm(-1) (+/-14 nm) of the writing wavelength (1.064 microm).

Optically induced creation, transformation, and organization of defects and color centers in optical fibers

Over the past five years, a color-center model for the dynamics of the absorption induced in germanosilicate fibers upon exposure to blue/green light has been under development at Southampton. This model is introduced and its predictions used for the first time to test our proposed Kramers-Kronig mechanism for the concurrent refractive index changes induced in the visible and the infrared. It is found that the predicted color-center population changes in the UV are insufficient to explain these refractive index changes. A possible alternative model, based on density changes in the glass triggered by color-center formation, is assessed experimentally and analytically. The implications of this result to photonically driven self-organization in fibers is briefly assessed, and reference made to recent experimental results.

Second-harmonic generation using gratings optically written by mode interference in poled optical fibers.

Second-order nonlinear susceptibility gratings are induced in optical fibers by mode interference of high-intensity blue light in the presence of an external dc electric poling field. As a result, efficient second-harmonic generation can be obtained for any infrared design wavelength.