Low loss Type II regenerative Bragg gratings made with ultrafast radiation

Nanoscale morphology and thermal properties of low insertion loss fiber Bragg gratings produced using the phase mask technique and a single femtosecond laser pulse

Fiber Bragg gratings with a very low insertion loss are inscribed using the phase mask technique and a single infrared (800 nm) femtosecond laser pulse. The morphology of the resultant light-induced structural changes in the Ge-doped silica fiber (SMF-28) is analyzed using scanning electron microscopy. The electron microscopy images reveal that each Bragg grating period incorporates an elongated micropore embedded in a region of homogeneous material modification. The Bragg wavelength drift and reflectivity of fiber Bragg gratings produced with single pulses having the same energy but different duration (80 fs and 350 fs) are monitored for 1000 hours in the course of isothermal annealing at 1000°C. The annealing data demonstrate that both the isothermal Bragg wavelength drift and the decrease in the reflectivity of the fiber Bragg gratings under test are statistically slower for the 350 fs inscription pulses.

Thermal Stability of Type II Modifications Inscribed by Femtosecond Laser in a Fiber Drawn from a 3D Printed Preform

Fiber drawing from a 3D printed perform was recently discussed to go beyond the limitations of conventional optical fiber manufacturing in terms of structure and materials. In this work, the photosensitivity of silica optical fibers to femtosecond laser light, and fabricated by 3D printing a preform, is investigated. The writing kinetics and the thermal performance of Type II modifications are studied by varying the laser pulse energy and investigating the birefringence response of the femtosecond (fs)-laser written structures. Compared with a conventional telecom single mode fiber (SMF28), the fiber made by 3D printing is found to have similar writing kinetics and thermal performance. Additionally, the thermal stability of the imprinted fs-laser induced nanostructures is investigated based on the Rayleigh–Plesset equation, describing a model of nanopores dissolution underpinning Type II modifications with thermal annealing.

Ultra-high-temperature resistant distributed Bragg reflector fiber laser based on type II-IR fiber Bragg gratings

We demonstrate a distributed Bragg reflector fiber laser that is capable of long-term operation at ultra-high temperatures. To form the laser cavity, a piece of Er-doped fiber is fusion spliced to a pair of type II-IR gratings, which are written using a femtosecond laser with a phase mask. Saturated gratings with different reflectivities are fabricated by varying the position of the grating region relative to the fiber core center. An eccentric grating with a relatively low reflectivity is chosen as the laser output coupler, while a regular grating with a higher reflectivity is used as the laser's high-reflection reflector. After an annealing process, the laser performance is tested at high temperatures. The results show that the laser can operate with a stable output wavelength and no output power degradation at high temperatures up to 1000°C.

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 °C) 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 °C 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 °C) up to 1400 °C 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 °C 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.

Spectral degradation of Type II π-phase-shifted fiber Bragg gratings due to femtosecond laser induced loss

Narrowband high-temperature stable fiber Bragg gratings (FBGs) can be made by introducing a π-phase shift in the middle of a Type II periodic grating structure. This creates a passband inside the wavelength rejection band. During the inscription of Type II Bragg gratings broadband, optical loss is induced in the fiber core as a result of interaction between the inscription beam and the silica host. The amount of broadband loss will determine the passband's spectral characteristics (bandwidth and loss).

High-temperature stable π-phase-shifted fiber Bragg gratings inscribed using infrared femtosecond pulses and a phase mask

Type II π-phase-shifted Bragg gratings stable up to ~1000°C are written inside a standard single mode silica optical fiber (SMF-28) with infrared femtosecond pulses and a special phase mask. Inscription through the protective polyimide fiber coating is also demonstrated. The birefringence of the Bragg gratings and, as a result, the polarization dependence of their spectra are strongly affected by the femtosecond laser polarization. Using optimized writing conditions, the full width at half maximum of the π-phase-shifted passband feature can be ~30 pm in transmission, while the polarization-dependent shift of its central wavelength can be less than 8 pm, for a 7 mm long grating structure. This makes such gratings a unique tool for high-resolution measurements of temperature, load and vibration in extreme temperature environments.

Extreme Environment Sensing Using Femtosecond Laser-Inscribed Fiber Bragg Gratings

The femtosecond laser-induced fiber Bragg grating is an effective sensor technology that can be deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices suitable for high temperature, pressure, ionizing radiation and strain sensor applications are possible. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

Observations from direct UV-written, non-hydrogen-loaded, thermally regenerated Bragg gratings in double-clad photosensitive fiber

In this Letter, experimental evidence is provided for an enhanced thermal sensitivity for a double thermal regeneration feature in fiber Bragg gratings fabricated by direct ultraviolet (UV) writing. Here 47 gratings of varying fluence and wavelength were written along a double-clad, germanium-doped core fiber. Subsequently thermal processing without hydrogen loading the fiber was performed and thermal treatment was carried out in a pure oxygen environment. Thermal sensitivity for the double regeneration increased from 13.6±0.3  pm/°C to 21.3±0.2  pm/°C. Furthermore, one of the highest nominal fluence gratings, #45, exhibited a regeneration factor of 1.73.

Regenerated volume gratings in PMMA after femtosecond laser writing

Self-regeneration of volume gratings recorded inside polymethyl methacrylate (PMMA) after 70-100 days is demonstrated. First, volume gratings were made inside PMMA by femtosecond laser writing. The diffraction efficiency of the gratings reached the maximum-was regenerated-following an initial slow decrease within the first several days after the fabrication. Time-lapse measurements of the diffraction efficiency in both top-plane (as laser written) and side-plane illumination were used to monitor changes of the diffraction efficiency. The final efficiency was reaching values well in excess of the as-fabricated efficiencies, with a maximum diffraction efficiency of 90% for the side readout illumination. The regenerated volume grating is a possible candidate to achieve high diffraction efficiency in PMMA. The regeneration is consistent with the α-relaxation of the polymer structure and oxidation of the dangling bonds.

Self-organized nanostructure formation during femtosecond-laser inscription of fiber Bragg gratings

Periodic planar nanostructures are found in Type II-IR Bragg gratings produced in SMF-28 fiber by side-illuminating it with infrared femtosecond-laser pulses through a phase mask. The planar nanostructures are aligned perpendicular to the laser polarization, as demonstrated using scanning electron microscopy analysis of cleaved fiber samples. Dark field optical microscopy is employed for real-time monitoring of structural changes occurring inside the fiber during the inscription process.