The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of LaO and NaO can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of LaO in the guiding region with a slope of 8·10(mol.%). The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial LaO content of the glass. Maximum Δn values >10 for samples with just 5.5 mol. % of LaO have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.

The local modification of the composition of glasses by high repetition femtosecond laser irradiation is an attractive method for producing photonic devices. Recently, the successful production of waveguides with a refractive index contrast (Δn) above 10−2 by fs-laser writing has been demonstrated in phosphate glasses containing La2O3 and K2O modifiers. This large index contrast has been related to a local enrichment in lanthanum in the light guiding region accompanied by a depletion in potassium. In this work, we have studied the influence of the initial glass composition on the performance of waveguides that are produced by fs-laser induced element redistribution (FLIER) in phosphate-based samples with different La and K concentrations. We have analyzed the contribution to the electronic polarizability of the different glass constituents based on refractive index measurements of the untreated samples, and used it to estimate the expected index contrast caused by the experimentally measured local compositional changes in laser written guiding structures. These estimated values have been compared to experimental ones that are derived from near field images of the guided modes with an excellent agreement. Therefore, we have developed a method to estimate before-hand the expected index contrast in fs-laser written waveguides via FLIER for a given glass composition. The obtained results stress the importance of considering the contribution to the polarizability of all the moving species when computing the expected refractive index changes that are caused by FLIER processes.

No abstract

The measurement of the refractive index of optical waveguides is a difficult task that involves different methods, among which those based on the refracted near field determination (RNF) are likely the ones providing the best resolution. Still, most such methods lack spectral resolution, which impedes accessing the index dispersion of the waveguide, an essential parameter for many applications. In this work, the refractive index of channel waveguides produced by fs‐laser induced ion‐migration in a P2O5‐La2O3‐K2O‐based glass is measured by imaging ellipsometry. Along with EDX compositional maps and guiding performance, the dispersion and refractive index maps of several waveguides are measured. The results confirm that, in this glass, waveguides are formed due to an enrichment in La in the topmost part of the laser‐excited region which is accompanied by the cross migration of K toward the region underneath. Interestingly, the index contrast of the waveguides shows a wavelength‐independent behavior for wavelengths above ≈600 nm. This indicates that in the compositional range analyzed, La3+ ions linearly contribute to the glass polarizability due to the relatively large mass of La3+ ions and the relatively small size of the isolated La‐polyhedra accommodated in the phosphate glass network.

Here in we present the fabrication and performance of waveguide tapers produced by femtosecond laser induced element redistribution in modified phosphate glasses. More particularly, it is demonstrated that by controlling the scan velocity during the writing process it is possible to adequately tune both the size of the modified area and the refractive index contrast to produce waveguides that can cope with mode field diameters in the range of 7-16 µm. In addition, we fabricated tapered structures through the induction of an acceleration in the laser scanning velocity, resulting in a device that can efficiently convert a wide range of mode fields. The fine control achieved over the index contrast in the range of 10 -3 and 10 -2 would allow the production of a wide variety of tapers that could potentially be used to couple numerous photonic devices.