Optical properties of a fs laser-created sphere inside a CYTOP fiber by Mueller polarimetry.

Optical elements embedded in an optical fiber can be used to shape and modulate the light transmitted within. We consistently observe, via Mueller polarimetry, that the optical properties of a femtosecond (fs) laser-created spherical cavity within a perfluorinated fiber exhibit predictable patterns. Specifically, linear birefringence is always induced at the periphery of the cavity, with its value showing a bell-shape distribution. The peak value of LB showed an increase correlating with the laser fluence and power, but its FWHM remains unchanged. Furthermore, it is important to highlight that when the cavity is disrupted, forming a channel to the fiber's surface, a negative LB is observed at the cavity's periphery, with a value reaching up to -0.4 rad. These optical phenomena may pique the interest of engineering and technical fields, potentially inspiring innovative approaches in optical fiber technology and its associated applications.

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.

Femtosecond laser direct writing multilayer chiral waveplates with minimal linear birefringence.

Chirality transfer from femtosecond laser direct writing in achiral transparent materials mainly originates from the interplay between anisotropic nanogratings and mechanical stress with non-parallel and non-perpendicular (oblique) neutral axes. Yet, the laser fabrication simultaneously induces non-negligible linear birefringence. For precise manipulation of circular polarization properties, as well as to unlock the full functionality, we report here a geometry-inspired multilayer method for direct writing of chiral waveplates with minimal linear birefringence. We perform a theoretical analysis of both circular and linear properties response for different multilayer configurations and achieve strong circular birefringence of up to -2.25 rad with an extinction ratio of circular birefringence to total linear birefringence of up to 5.5 dB at 550 nm. Our strategy enables the precise control of circular properties and provides a facile platform for chiral device exploration with almost no linear property existence.

Upper temperature limit for nanograting survival in oxide glasses.

The thermal stability of self-assembled porous nanogratings inscribed by an infrared femtosecond (fs) laser in five commercial glasses (BK7, soda lime, 7059, AF32, and Eagle XG) is monitored using step isochronal annealing experiments. Their erasure, ascertained by retardance measurements and attributed to the collapse of nanopores, is well predicted from the Rayleigh-Plesset (R-P) equation. This finding is thus employed to theoretically predict the erasure of nanogratings in the context of any time-temperature process (e.g., thermal annealing, laser irradiation process). For example, in silica glass (Suprasil CG) and using a simplified form of the R-P equation, nanogratings composed of 50 nm will erase within ∼30m i n, ∼1µs, and ∼30n s at temperatures of ∼1250∘ C, 2675°C, and 3100°C, respectively. Such conclusions are expected to provide guidelines to imprint nanogratings in oxide glasses (for instance, in the choice of laser parameters) or to design appropriate thermal annealing protocols for temperature sensing.

Upper temperature limit for nanograting survival in oxide glasses: publisher's note.

This publisher's note contains corrections to Appl. Opt.62, 6794 (2023)APOPAI0003-693510.1364/AO.496351.

Lifetime prediction of nanogratings inscribed by a femtosecond laser in silica glass.

This paper is dedicated to the lifetime prediction of Type II modifications (i.e., nanogratings) written in silica glass using an infrared femtosecond laser. Herein we report accelerated aging experiments of such nanogratings through the monitoring of their characteristic linear birefringence signature. Based on the master curve formalism, we demonstrate that these laser-induced nanostructures can survive for 200 hours at 1100°C. Under the reported processing conditions and after a dedicated passivation treatment, the estimated lifetime of the birefringent optical elements is beyond 10 years at 800°C with a minor erasure of 7%.

Accurate modeling of radiation-induced absorption in Er-Al-doped silica fibers exposed to high-energy ionizing radiations.

We offer here an accurate quantitative model of the RIA (radiation-induced absorption) at low dose-rate (below 1 kGy) that experience the most common erbium-doped fibers (Ge-Al-Er-doped silica) under radiations. It addresses the degradation mechanisms of the glass fiber, especially the influence of its doping elements versus its sensitivity to radiations. Moreover, it depends mainly on macroscopic quantities coming from literature or experiments. For these two reasons, it is a reliable and efficient tool for the engineering of erbium-doped fibers (erbium-free fibers too) exposed to ionizing radiations and is validated in this paper by comparing the modelisation results to RIA experiments on 14 Er-doped optical fiber samples, in which composition changes a lot from one sample to another (in the range 0-25%wt for Ge, 0-10%wt for Al and 0-1500ppm for Er).

Helical distributed feedback fiber Bragg gratings and rocking filters in a 3D printed preform-drawn fiber.

Using induced UV attenuation across a twisted fiber asymmetric core drawn from a 3D printed preform, linear fiber Bragg gratings (FBGs) are produced on one side of the core. By removing the twist, a helical grating with a period matching the twist rate is produced. Balancing the rate with the polarization beat length in a form birefringent fiber allows the production of a combined rocking filter and FBG device with tunable properties. Direct observation of the fiber grating dispersion within the rocking filter rejection band is possible.

Impact of Al2O3 doping on Bi active center photobleaching in Bi/Er-codoped fibers.

In this Letter, the impact of Al2O3 doping on the Bi active center (BAC) photobleaching is investigated in Bi/Er-codoped fibers (BEDFs). By measuring the evolution of emission attributed to the BAC associated with silica (BAC-Si) at ∼1400nm, the linear relationship between the ratio of unbleached/bleached part (γUB/γB) and 830 nm irradiation intensity (P830) was revealed in the log-log plot. The experimental results demonstrate that Al2O3 doping or its induced defects could be one key factor exaggerating the BAC photobleaching in BEDFs.

Thermal Stability of Type II Modifications by IR Femtosecond Laser in Silica-based Glasses.

Femtosecond (fs) laser written fiber Bragg gratings (FBGs) are excellent candidates for ultra-high temperature (>800 ºC) monitoring. More specifically, Type II modifications in silicate glass fibers, characterized by the formation of self-organized birefringent nanostructures, are known to exhibit remarkable thermal stability around 1000 ºC for several hours. However, to date there is no clear understanding on how both laser writing parameters and glass composition impact the overall thermal stability of these fiber-based sensors. In this context, this work investigates thermal stability of Type II modifications in various conventional glass systems (including pure silica glasses with various Cl and OH contents, GeO2-SiO2 binary glasses, TiO2- and B2O3-doped commercial glasses) and with varying laser parameters (writing speed, pulse energy). In order to monitor thermal stability, isochronal annealing experiments (Δt⁓ 30 min, ΔT⁓ 50 ºC) up to 1400 ºC were performed on the irradiated samples, along with quantitative retardance measurements. Among the findings to highlight, it was established that ppm levels of Cl and OH can drastically reduce thermal stability (by about 200 ºC in this study). Moreover, GeO2 doping up to 17 mole% only has a limited impact on thermal stability. Finally, the relationships between glass viscosity, dopants/impurities, and thermal stability, are discussed.

BAC activation by thermal quenching in bismuth/erbium codoped fiber.

We have investigated the thermal quenching effect on the bismuth active center (BAC) in a Bi/Er co-doped fiber (BEDF). The effects from varying quenching conditions are studied and discussed. We report, for the first time to our knowledge, a significant BAC activation achieved by thermal quenching. We observed that the peak luminescence at ∼1405 nm of the BAC associated with silica (BAC-Si) could be enhanced more than two times by thermal quenching. The experimental results indicate that thermal quenching could be an effective way for BAC activation of bismuth-doped fibers.

Stress-induced optical waveguides written by an ultrafast laser in Nd3+, Y3+ co-doped SrF2 crystals.

In this paper, we report on the ultrafast laser-induced birefringence, refractive index changes, and enhanced photoluminescence properties in the volume of neodymium (Nd), yttrium (Y) co-doped strontium fluoride (SrF2) and Nd, Y co-doped calcium fluoride (CaF2) crystals. The optical waveguides written with 500 kHz repetition rate provided lowest propagation loss of 1.63±0.21 dB cm-1 for transverse magnetic (TM) polarization at 632.8 nm in Nd,Y:SrF2 crystal. The measured retardance can be interpreted by stress-induced birefringence related to the permanent volume expansion, photo induced by a non-spherical irradiated zone. The absorption, steady-state, and time-resolved photoluminescence properties are also carried out in and out of the laser irradiated zone, enabling the local changes of the Nd and Y network in Nd,Y:SrF2, as well as well-preserved Nd fluorescence in the written optical waveguides.

Pulse energy dependence of refractive index change in lithium niobium silicate glass during femtosecond laser direct writing.

Femtosecond laser-induced refractive index changes in lithium niobium silicate glass were explored at high repetition rate (300 fs, 500 kHz) by polarized light microscopy, full-wave retardation plate, quantitative birefringence microscopy, and digital holographic microscopy. We found three regimes on energy increase. The first one corresponds to isotropic negative refractive index change (for pulse energy ranging 0.4-0.8 µJ/pulse, 0.6 NA, 5µm/s, 650µm focusing depth in the glass). The second one (0.8-1.2 µJ/pulse) corresponds to birefringence with well-defined slow axis orientation. The third one (above 1.2 µJ/pulse) is related to birefringence direction fluctuation. Interestingly, these regimes are consistent with crystallization ones. In addition, an asymmetric orientational writing effect has been detected on birefringence. These topics extend the possibility of controlling refractive index change in multi-component glasses.

Study of femtosecond laser-induced circular optical properties in silica by Mueller matrix spectropolarimetry.

Transmission Mueller-matrix spectroscopic ellipsometry is applied to femtosecond laser-induced modifications in silica glass in the spectral range of 450-1000 nm. Within a type II regime, the modifications exhibit not only circular dichroism, but also circular birefringence. We suggest that the laser polarization orientation with respect to pulse front tilt determines the amplitude and the sign of the circular properties. By using differential decomposition of the Mueller matrix, optical rotation is revealed for the first time, to the best of our knowledge. A maximum value of the effective optical activity of 143°/mm at 550 nm is found.

Form birefringence induced in multicomponent glass by femtosecond laser direct writing.

We demonstrate a new kind of form birefringence in lithium niobium silicate glass induced by femtosecond laser direct writing. By combining electron backscatter diffraction and transmission electron microscopy, we reveal a self-assembled nanostructure consisting of periodic phase change: nonlinear optical nanocrystals embedded in a network of "walls" in a vitreous phase. These "walls" are aligned perpendicular to the laser polarization direction. This self-organized nanostructure may successfully explain the origin of the laser-induced birefringence in this multicomponent glass quite differently from pure silica. These findings highlight a spectacular modification of glass, and enable construction of a high contrast three-dimensional refractive index and birefringent structures at the micrometer scale in multicomponent glasses.

Femtosecond laser written nanostructures in Ge-doped glasses.

We report on nanostructures induced by femtosecond laser pulses in the bulk of Germanium-doped silica glasses. For studying structural properties of the nanostructure constituents small-angle x-ray scattering and SEM served to map pore size, filling factor and periodicity. Our results show that with increasing the Ge doping concentration, the aspect ratio (transverse to inscribing laser) of nanometric pores rises while they arrange in a smaller period in contrast to nanogratings in pristine fused silica. Consequently, higher optical retardance can be obtained demonstrating the pronounced glass decomposition due to the changing network structure.

Radiation hardening of sol gel-derived silica fiber preforms through fictive temperature reduction.

The impact of fictive temperature (Tf) on the evolution of point defects and optical attenuation in non-doped and Er3+-doped sol-gel silica glasses was studied and compared to Suprasil F300 and Infrasil 301 glasses before and after γ-irradiation. To this aim, sol-gel optical fiber preforms have been fabricated by the densification of erbium salt-soaked nanoporous silica xerogels through the polymeric sol-gel technique. These γ-irradiated fiber preforms have been characterized by FTIR, UV-vis-NIR absorption spectroscopy, electron paramagnetic resonance, and photoluminescence measurements. We showed that a decrease in the glass fictive temperature leads to a decrease in the glass disorder and strained bonds. This mainly results in a lower defect generation rate and thus less radiation-induced attenuation in the UV-vis range. Furthermore, it was found that γ-radiation "hardness" is higher in Er3+-doped sol-gel silica compared to un-doped sol-gel silica and standard synthetic silica glasses. The present work demonstrates an effective strategy to improve the radiation resistance of optical fiber preforms and glasses through glass fictive temperature reduction.

Influence of photo-inscription conditions on the radiation-response of fiber Bragg gratings.

We compared the sensitivity to X-rays of several fiber Bragg gratings (FBGs) written in the standard telecommunication fiber Corning SMF28 with different techniques. Standard gratings were manufactured with phase-mask and UV lasers, continuum wave (cw) at 244 nm or pulsed in the nanosecond domain at 248 nm, in a pre-hydrogenated fiber. Others gratings were written by exposures to a femtosecond IR-laser (800 nm), with both phase-mask and point by point techniques. The response of these FBGs was studied under X-rays at room temperature and 100°C, by highlighting their similarities and differences. Independently of the inscription technique, the two types of fs-FBGs have showed no big difference up to 1 MGy(SiO(2)) dose. A discussion on the causes of the radiation-induced peak change is also reported.

One-step photoinscription of asymmetrically oriented fresnoite-type crystals in glass by ultrafast laser.

Oriented fresnoite-type crystals (Sr(2)TiSi(2)O(8)) were photoinduced directly in bulk silica-based glass by femtosecond laser irradiation at high repetition rate (typ. 300 kHz). Unlike related results obtained from other researchers, asymmetrical polar-axis orientation of those nonlinear crystals in transverse direction of the cross section has been demonstrated by electron backscattered diffraction and micro-second-harmonic generation (SHG). The nonlinear optical property of laser lines has been further characterized by SHG measurement. We found that the preferential directions of the polar axis were in the laser motion direction with a small dispersion in part of the heated volume. The other part of the crystallized volume shows an axis perpendicular to the writing direction. The mechanism of asymmetric orientation of femtosecond-laser-induced crystallization also is discussed.

Usable Analytical Expressions for Temperature Distribution Induced by Ultrafast Laser Pulses in Dielectric Solids.

This paper focuses on the critical role of temperature in ultrafast direct laser writing processes, where temperature changes can trigger or exclusively drive certain transformations, such as phase transitions. It is important to consider both the temporal dynamics and spatial temperature distribution for the effective control of material modifications. We present analytical expressions for temperature variations induced by multi-pulse absorption, applicable to pulse durations significantly shorter than nanoseconds within a spherical energy source. The objective is to provide easy-to-use expressions to facilitate engineering tasks. Specifically, the expressions are shown to depend on just two parameters: the initial temperature at the center denoted as T00 and a factor Rτ representing the ratio of the pulse period τp to the diffusion time τd. We show that temperature, oscillating between Tmax and Tmin, reaches a steady state and we calculate the least number of pulses required to reach the steady state. The paper defines the occurrence of heat accumulation precisely and elucidates that a temperature increase does not accompany systematically heat accumulation but depends on a set of laser parameters. It also highlights the temporal differences in temperature at the focus compared to areas outside the focus. Furthermore, the study suggests circumstances under which averaging the temperature over the pulse period can provide an even simpler approach. This work is instrumental in comprehending the diverse temperature effects observed in various experiments and in preparing for experimental setup. It also aids in determining whether temperature plays a role in the processes of direct laser writing. Toward the end of the paper, several application examples are provided.