Femtosecond-laser writing of photonic structures in zinc phosphate glasses

Good quality waveguides can be fabricated in zinc phosphate glasses with O/P ratios of 3.25 using the femtosecond-laser writing technique. Compositional effects, including the addition of Al2O3 and rare-earth doping, are discussed as well as results on the fabrication of active devices.


Introduction
Focused femtosecond (fs) laser pulses can be used to permanently modify the refractive index of glasses.If the change in the refractive index of the laser-modified material is positive, the material can be easily used to create optical waveguiding structures.Unfortunately, few materials are known to exhibit this property.Phosphate glasses, which are of interest as host materials for active devices since they allow incorporation of high concentrations of rare earth ions, typically produce negative refractive index changes in the laser-exposed regions.We have recently found that good quality waveguides can be fabricated in zinc polyphosphate glass compositions with an O/P ratio close to 3.25 [1][2][3][4].This paper summarizes the effects of glass composition on fs-laser induced structural modification as well as the fabrication of active devices in zinc phosphate glasses.

Experimental methods
Glasses were prepared using standard melting techniques.A Spitfire LCX laser (Spectra-Physics) operating at 800 nm with a 1 kHz rep.rate of ~150 fs pulses was used for writing all structures in the glasses reported here.The modifications were written longitudinally through a microscope objective with an NA of 0.25 or 0.40 and a translation speed of 50 µm/s.After fs-laser modification the modified regions were tested for optical guiding and characterized with confocal fluorescence (FL) and Raman spectroscopy.Detailed descriptions about the experimental procedures can be found elsewhere [1][2][3][4].

Results and discussion
Glass compositions that were investigated are listed in Table I.Initially we investigated a number of glasses containing ZnO and P 2 O 5 with varying composition.Within this series only the glass with an O/P ratio of 3.25 produced good waveguides (see Fig. 1).In order to fabricate optical devices that are practical and reliable, it is important to use glass compositions that are stable and robust.By introducing aluminum in small quantities, <10 mol.% Al 2 O 3 , to the zinc phosphate composition the glass becomes more resilient to moisture naturally present in the atmosphere and also more chemically stable as the aluminum adds rigidity to the phosphate glass network [5,6].Waveguide writing experiments in these samples again showed that good waveguides with a positive Δn of ~ 3x10 -4 could be produced in the samples with an O/P ratio of 3.25.Raman spectroscopy of the laser-modified regions showed that the samples that produced good waveguides exhibited no (or very small) Raman spectral changes with respect to the unmodified glass compared to the samples in which no waveguiding was observed.
In the Er-Yb doped glass we have demonstrated the successful fabrication of subsurface waveguide amplifiers exhibiting internal gain of 1 dB⁄cm at 1.53 µm when pumped with 500 mW at 976 nm [3].The ZnMgP composition was used to create waveguide Bragg gratings employing a single pass technique in which a waveguide that has a periodic refractive index profile is produced.

Table I .
Glass compositions of zinc phosphate glasses Other zinc phosphate glasses in which good waveguides can be produced include Er-Yb doped glasses with O/P of 3.25 [ref] as well as 30ZnO • 30MgO • 40P 2 O 5 (ZnMgP).This glass composition is based on the original 60ZnO • 40P 2 O 5 with ½ of the ZnO replaced by MgO, and thus also has O/P of 3.25.