On the existence of nanogratings in commercial oxide glasses

The ability to induce nanogratings using a femtosecond laser in common oxide glasses is investigated experimentally. A simple and general viscosity-based approach is subsequently employed to predict their existence in glass

On the Formation of Nanogratings in Commercial Oxide Glasses by Femtosecond Laser Direct Writing

Nanogratings (NGs) are self-assembled subwavelength and birefringent nanostructures created by femtosecond laser direct writing (FLDW) in glass, which are of high interest for photonics, sensing, five-dimensional (5D) optical data storage, or microfluidics applications. In this work, NG formation windows were investigated in nine commercial glasses and as a function of glass viscosity and chemical composition. The NG windows were studied in an energy—frequency laser parameter landscape and characterized by polarizing optical microscopy and scanning electron microscopy (SEM). Pure silica glass (Suprasil) exhibits the largest NG window, whereas alkali borosilicate glasses (7059 and BK7) present the smallest one. Moreover, the NG formation windows progressively reduced in the following order: ULE, GeO2, B33, AF32, and Eagle XG. The NG formation window in glasses was found to decrease with the increase of alkali and alkaline earth content and was correlated to the temperature dependence of the viscosity in these glasses. This work provides guidelines to the formation of NGs in commercial oxide glasses by FLDW

Polarization-oriented LiNbO3 nanocrystals by femtosecond laser irradiation in LiO2–Nb2O5–SiO2–B2O3 glasses

This work investigates the role of a B2O3 addition (up to 21 mole %) into a lithium niobium silicate glass matrix, focusing on the orientational dependency of second harmonic generation (SHG), induced after femtosecond laser irradiation. We detected the sharp emission of light at 515 nm, characteristic of SHG, in both static and scanning configurations, using pulse energy, repetition rate, and laser polarization as varying parameters. Among the results to highlight, the SHG signature appears within a few seconds in highly doped B2O3 glass, i.e., one order of magnitude smaller than in B2O3-free glass. Additionally, we found that the orientability of the polar axis of LiNbO3 nanocrystals by writing laser polarization can be obtained in glasses when SiO2 is substituted with B2O3. These preliminary results open the door to the fabrication of crystal / glass based photonic devices with lower laser power deposited and much faster crystallization kinetics

Towards a Rationalization of Ultrafast Laser-Induced Crystallization in Lithium Niobium Borosilicate Glasses: The Key Role of The Scanning Speed

Femtosecond (fs)-laser direct writing is a powerful technique to enable a large variety of integrated photonic functions in glass materials. One possible way to achieve functionalization is through highly localized and controlled crystallization inside the glass volume, for example by precipitating nanocrystals with second-order susceptibility (frequency converters, optical modulators), and/or with larger refractive indices with respect to their glass matrices (graded index or diffractive lenses, waveguides, gratings). In this paper, this is achieved through fs-laser-induced crystallization of LiNbO3 nonlinear crystals inside two different glass matrices: a silicate (mol%: 33Li2O-33Nb2O5-34SiO2, labeled as LNS) and a borosilicate (mol%:33Li2O-33Nb2O5-13SiO2-21B2O3, labeled as LNSB). More specifically, we investigate the effect of laser scanning speed on the crystallization kinetics, as it is a valuable parameter for glass laser processing. The impact of scanning energy and speed on the fabrication of oriented nanocrystals and nanogratings during fs-laser irradiation is studied. Fs-laser direct writing of crystallized lines in both LNS and LNSB glass is investigated using both optical and electron microscopy techniques. Among the main findings to highlight, we observed the possibility to maintain crystallization during scanning at speeds ~ 5 times higher in LNSB relative to LNS (up to ~ 600 μm/s in our experimental conditions). We found a speed regime where lines exhibited a large polarization-controlled retardance response (up to 200 nm in LNSB), which is attributed to the texturation of the crystal/glass phase separation with a low scattering level. These characteristics are regarded as assets for future elaboration methods and designs of photonic devices involving crystallization. Finally, by using temperature and irradiation time variations along the main laser parameters (pulse energy, pulse repetition rate, scanning speed), we propose an explanation on the origin of 1) crystallization limitation upon scanning speed, 2) laser track width variation with respect to scanning speed, and 3) narrowing of the nanogratings volume but not the heat-affected volume

Overview of High Temperature Fibre Bragg Gratings and Potential Improvement Using Highly Doped Aluminosilicate Glass Optical Fibres

International audienceIn this paper, various types of high temperature fibre Bragg gratings (FBGs) are reviewed, including recent results and advancements in the field. The main motivation of this review is to highlight the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) near-infrared (IR) radiation in fibres fabricated using non-conventional techniques, such as the Molten Core Method (MCM). As a demonstration to this, an yttrium aluminosilicate (YAS) core and pure silica cladding glass optical fibre is fabricated and investigated after being irradiated by fs laser within the Type II regime. The familiar formation of nanogratings inside both core and cladding regions are identified and studied using birefringence measurements and scanning electron microscopy (SEM). The thermal stability of the type II modifications is then investigated through isochronal annealing experiments (up to T = 1100°C; time steps, t = 30 min). For the YAS core composition, the measured birefringence does not decrease when tested up to 1000°C, while for the SiO 2 cladding and under the same conditions its value decreased by ~ 30%. These results suggest that inscription of such "Type II fs-IR" modifications in YAS fibres could be employed to make FBGs with high thermal stability. This opens the door toward the fabrication of a new range of "FBGs host fibres" suitable for ultra-high temperature operation

Volume nanogratings inscribed by ultrafast IR laser in alumino-borosilicate glasses

Self-assembled nanogratings, inscribed by femtosecond laser writing in volume, are demonstrated in multicomponent alkali and alkaline earth containing alumino-borosilicate glasses. The laser beam pulse duration, pulse energy, and polarization, were varied to probe the nanogratings existence as a function of laser parameters. Moreover, laser-polarization dependent form birefringence, characteristic of nanogratings, was monitored through retardance measurements using polarized light microscopy. Glass composition was found to drastically impact the formation of nanogratings. For a sodium alumino-borosilicate glass, a maximum retardance of 168 nm (at 800 fs and 1000 nJ) could be measured. The effect of composition is discussed based on SiO2 content, B2O3/Al2O3 ratio, and the Type II processing window is found to decrease as both (Na2O+ CaO)/Al2O3 and B2O3/Al2O3 ratios increase. Finally, an interpretation in the ability to form nanogratings from a glass viscosity viewpoint, and its dependency with respect to the temperature, is demonstrated. This work is brought into comparison with previously published data on commercial glasses, which further indicates the strong link between nanogratings formation, glass chemistry, and viscosity

Molten Core Fabrication of Intrinsically Low Nonlinearity Glass Optical Fibers

Optical nonlinearities limit scaling to higher output powers in modern fiber-based laser systems. Paramount amongst these parasitic phenomena are stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and nonlinear refractive index (n2)-related wave-mixing phenomena (e.g., self-phase modulation, SPM, four-wave mixing, FWM). In order to mitigate these effects, the fiber community has largely focused on the development of micro-structured large mode area (LMA) fibers whereby the fiber geometry is engineered to spread the optical power over a larger effective area. In addition to increasing the resultant complexity and cost of these fibers, such LMA designs introduce new parasitic phenomena, such as transverse mode instability (TMI), which presently serves as the dominant limitation in power-scaling. This dissertation explores a different approach for mitigating these nonlinearities in optical fiber lasers; namely attacking the aforementioned effects at their fundamental origin, i.e., the material through which the light propagates. Indeed, the Brillouin gain coefficient (BGC), the Raman gain coefficient (RGC), the thermo-optic coefficient (TOC) and the nonlinear refractive index (n2) are all intrinsic material properties that respectively drive SBS, SRS, TMI and wave-mixing phenomena. Though less well studied within the fiber laser community, such a materials approach offers a powerful yet simpler way to address nonlinearities. Chapter I investigates the thermodynamic origins of light scattering and provides insight into the prime material properties that drive optical nonlinearities. Chapters II and III offer an overview of how these (and other) properties can be measured and modeled in multicomponent glass systems, considering both bulk or fiber geometries. In Chapter IV, a materials road map for binary and ternary glass material systems is provided to identify which compositions should be of specific focus for the development of intrinsically low optical nonlinearity optical fibers. These four Chapters have been adapted from a series of published journal articles1 entitled “A unified materials approach to mitigating optical nonlinearities in optical fiber” [1]–[4]. In Chapter V, the fabrication of oxyfluoride-core silica-cladding optical fibers using the molten core method is described and their core glass compositions and structures investigated. The thermodynamics and kinetics of fluoride-oxide reactions are also studied, and insights on the dominant mechanisms that drive the fluoride-oxide reactions during fiber processing are discussed. In Chapter VI, optical properties that drive optical nonlinearities are studied, and their relationships with glass compositions investigated. Oxyfluoride fibers exhibiting concomitant reductions of 6-9 dB in BGC, 0.5-1.5 dB in RGC, and 1.2-3.2 dB in TOC, relative to conventional silica fibers, as well as reduced linear and nonlinear refractive indices, are reported. Spectroscopic properties of active Yb-doped fibers are also considered, and suggest enhanced laser performance and higher lasing efficiencies for these systems compared to conventional silica fibers. Finally, Chapter VII portrays the current challenges and the future perspectives of these oxyfluoride-core silica-cladding glass optical fibers, along with approaches to overcome them

Impact of Glass Free Volume on Femtosecond Laser-Written Nanograting Formation in Silica Glass

International audienceIn this study, we investigate the effects of densification through high pressure and temperature (up to 5 GPa, 1000 °C) in the making of nanogratings in pure silica glass, inscribed with femtosecond laser. The latter were monitored through retardance measurements using polarized optical microscopy, and their internal structure was observed under scanning electron microscopy. We reveal the difficulty in making nanogratings in densified silica glasses. Based on this observation, we propose that free volume may be a key precursor to initiate nanograting formation

An overview of the thermal erasure mechanisms of femtosecond laser induced nanogratings in silica glass

International audienceThe Type II modifications induced by IR femtosecond (fs) laser are used in many optical devices due to their excellent thermal stability at high temperatures (typically> 800 °C). The characteristic feature of Type II modifications is the formation of nanogratings, which can easily be detected through birefringence measurements. However, the measured birefringence is an aggregate value of multiple contributions that include form birefringence, stress-induced birefringence due to permanent volume changes, and point defects. In this work, we investigate the thermal erasure kinetics for each one of these contributions in silica glass. Firstly, we irradiate silica glass samples with a fs-laser using different conditions (polarization, energy). Secondly, we perform accelerated aging experiments to evaluate the stability of the laser-induced modifications, including defects, densification, stress field and porous nanogratings. Finally, the aforementioned contributions to the thermal stability of the nanogratings are identified and discussed using spectroscopic techniques (Raman and Rayleigh scattering, UV-Vis absorption) and electron microscopy. Moreover, porous nanogratings erasure kinetic is simulated using the Rayleigh-Plesset (R-P) equation. This work provides a valuable framework in the realization of silica glass-based optical devices operating at high temperatures (>>800 °C) by 1) evidencing the effect of annealing on each erasure mechanism and 2) providing information on the optical response (mainly the birefringence) upon annealing

An overview of the thermal erasure mechanisms of femtosecond laser induced nanogratings in silica glass

International audienceThe Type II modifications induced by IR femtosecond (fs) laser are used in many optical devices due to their excellent thermal stability at high temperatures (typically> 800 °C). The characteristic feature of Type II modifications is the formation of nanogratings, which can easily be detected through birefringence measurements. However, the measured birefringence is an aggregate value of multiple contributions that include form birefringence, stress-induced birefringence due to permanent volume changes, and point defects. In this work, we investigate the thermal erasure kinetics for each one of these contributions in silica glass. Firstly, we irradiate silica glass samples with a fs-laser using different conditions (polarization, energy). Secondly, we perform accelerated aging experiments to evaluate the stability of the laser-induced modifications, including defects, densification, stress field and porous nanogratings. Finally, the aforementioned contributions to the thermal stability of the nanogratings are identified and discussed using spectroscopic techniques (Raman and Rayleigh scattering, UV-Vis absorption) and electron microscopy. Moreover, porous nanogratings erasure kinetic is simulated using the Rayleigh-Plesset (R-P) equation. This work provides a valuable framework in the realization of silica glass-based optical devices operating at high temperatures (>>800 °C) by 1) evidencing the effect of annealing on each erasure mechanism and 2) providing information on the optical response (mainly the birefringence) upon annealing