Toward low-loss mid-infrared Ga2O3-BaO-GeO2 optical fibers

The development of efficient and compact photonic systems in support of mid-infrared integrated optics is currently facing several challenges. To date, most mid-infrared glass-based devices are employing fluoride or chalcogenide glasses (FCGs). Although the commercialization of FCGs-based optical devices has rapidly grown during the last decade, their development is rather cumbersome due to either poor crystallization and hygroscopicity resilience or poor mechanical-thermal properties of the FCGs. To overcome these issues, the parallel development of heavy-metal oxide optical fiber from the barium-germanium-gallium oxide vitreous system (BGG) has revealed a promising alternative. However, over 30 years of fiber fabrication optimization, the final missing step of drawing BGG fibers with acceptable losses for meters-long active and passive optical devices had not yet been reached. In this article, we first identify the three most important factors that prevent the fabrication of low-loss BGG fibers i.e., surface quality, volumic striae and glass thermal-darkening. Each of the three factors is then addressed in setting up a protocol enabling the fabrication of low-loss optical fibers from gallium-rich BGG glass compositions. Accordingly, to the best of our knowledge, we report the lowest losses ever measured in a BGG glass fiber i.e., down to 200 dB km-1 at 1350 nm.

Understanding the effect of wet etching on damage resistance of surface scratches

Fused silica optics often exhibit surface scratches after polishing that radically reduce their damage resistance at the wavelength of 351 nm in the nanosecond regime. Consequently, chemical treatments after polishing are often used to increase the damage threshold and ensure a safe operation of these optics in large fusion-scale laser facilities. Here, we investigate the reasons for such an improvement. We study the effect of an HF-based wet etching on scratch morphology and propose a simple analytic model to reflect scratch widening during etching. We also use a finite element model to evaluate the effect of the morphological modification induced by etching on the electric field distribution in the vicinity of the scratch. We evidence that this improvement of the scratch damage resistance is due to a reduction of the electric field enhancement. This conclusion is supported by secondary electron microscopy (SEM) imaging of damage sites initiated on scratches after chemical treatment.

Effects of deep wet etching in HF/HNO3 and KOH solutions on the laser damage resistance and surface quality of fused silica optics at 351 nm

We investigate the interest of deep wet etching with HF/HNO₃ or KOH solutions as a final step after polishing to improve fused silica optics laser damage resistance at the wavelength of 351 nm. This comparison is carried out on scratches engineered on high damage threshold polished fused silica optics. We evidence that both KOH and HF/HNO₃ solutions are efficient to passivate scratches and thus improve their damage threshold up to the level of the polished surface. The effect of these wet etchings on surface roughness and aspect is also studied. We show that KOH solution exhibit better overall surface quality that HF/HNO₃ solution in the tested conditions. Given the safety difficulties associated with the processing with HF, KOH solution appears as a pertinent alternative to HF deep wet etching.

Enhancement of nanograting formation assisted by silver ions in a sodium gallophosphate glass

We investigated the influence of silver ions during the direct femtosecond laser-induced formation of nanogratings (NGs) at the surface of a tailored sodium gallophosphate glass. We observed that the silver ions had a remarkable influence because the silver-containing glass showed (1) lower fluence thresholds for the formation of the NGs; (2) much smoother NG shapes; and (3) a bifunctional behavior because fluorescence from laser-induced silver clusters occurs prior to the formation of the NGs. We demonstrate for the first time, to our knowledge, the formation of NGs assisted by noble metal ions, such as ions playing a kind of catalytic-like role that enhances and improves the NG formation and its incubation process. Our innovative approach provides promising potential for further improvements in processes for NG formation.

Evidence of a green luminescence band related to surface flaws in high purity silica glass

Using luminescence confocal microscopy under 325 nm laser excitation, we explore the populations of defects existing in or at the vicinity of macroscopic surface flaws in fused silica. We report our luminescence results on two types of surface flaws: laser damage and indentation on fused silica polished surfaces. Luminescence cartographies are made to show the spatial distribution of each kind of defect. Three bands, centered at 1.89 eV, 2.75 eV and 2.25 eV are evidenced on laser damage and indentations. The band centered at 2.25 eV was not previously reported in photo luminescence experiments on indentations and pristine silica, for excitation wavelengths of 325 nm or larger. The luminescent objects, expected to be trapped in sub-surface micro-cracks, are possibly involved in the first step of the laser damage mechanism when fused silica is enlightened at 351 nm laser in nanosecond regime.

Impact of storage induced outgassing organic contamination on laser induced damage of silica optics at 351 nm

The impact of storage conditions on laser induced damage density at 351 nm on bare fused polished silica samples has been studied. Intentionally outgassing of polypropylene pieces on silica samples was done. We evidenced an important increase of laser induced damage density on contaminated samples demonstrating that storage could limit optics lifetime performances. Atomic Force Microscopy (AFM) and Gas Chromatography -Mass Spectrometry (GC-MS) have been used to identify the potential causes of this effect. It shows that a small quantity of organic contamination deposited on silica surface is responsible for this degradation. Various hypotheses are proposed to explain the damage mechanism. The more likely hypothesis is a coupling between surface defects of optics and organic contaminants.

Laser induced damage of fused silica polished optics due to a droplet forming organic contaminant

We report on the effect of organic molecular contamination on single shot laser induced damage density at the wavelength of 351 nm, with a 3 ns pulse length. Specific contamination experiments were made with dioctylphthalate (DOP) in liquid or gaseous phase, on the surface of fused silica polished samples, bare or solgel coated. Systematic laser induced damage was observed only in the case of liquid phase contamination. Different chemical and morphological characterization methods were used to identify and understand the damage process. We demonstrate that the contaminant morphology, rather than its physicochemical nature, can be responsible for the decrease of laser induced damage threshold of optics.

Large second-harmonic generation of thermally poled sodium borophosphate glasses

Second harmonic generation (SHG) has been obtained in a rich in sodium niobium orophosphate glass by a thermal poling treatment. The thermally poled glass SHG signal has been studied through an original analysis of both transmitted and reflected polarized Maker-fringe patterns. Therefore, the second order nonlinear optical (NLO) efficiency was estimated from the simulation of the Maker-fringe patterns with a stepwise decreasing profile from the anode surface. A reproducible chi(2) susceptibility value as high as 5.0 +/-0.3 pm/V was achieved at the anode side. The nonlinear layer, found to be sodium-depleted up to 5 microm deep inside the anode side, identical to the simulated nonlinear zone thickness, indicates a complex space-charge-migration/ nonlinear glass matrix response process.

Structural modification on silica glass surface induced by thermal poling for second harmonic generation

O-K edge XANES spectroscopy evidences structural modification induced by thermal poling treatment in surfaces of bulk Herasil silica glass presenting second harmonic generation. Considering model silicon dioxide clusters, calculations based on full multiple scattering approach have been performed in order to explain accurately the differences observed on XANES spectra at different stage of the poling treatment. These structural modifications on extreme surface affect both network and defects by breaking Si-O-Si bridging bonds. Despite of the formation of bridging bond occurring during the thermal depoling -which erases the SHG inside the glass-, the initial structure of the unpoled sample is not reproduced.