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

Abstract

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.

Accurate Measurements of a Wavelength Drift in High-Temperature Silica-Fiber Bragg Gratings

“…For the long-term stability measurements, several identical π-phase-shifted Type II FBGs [15] were fabricated in acrylate-coated SMF-28 single mode fiber using a third order phase mask and Fourier-transform-limited 80 fs pulses from a regeneratively amplified Ti:sapphire laser operating at λ = 800 nm with 1 mJ pulse energy as described in detail in [14]. It is well known that FBGs at a high constant temperature experience a Bragg wavelength shift which can range from a few picometers to hundreds of picometers depending on the annealing time [14]. We chose π-phase-shifted Type II FBGs for our experiments in order to allow measurements of spectral shifts with a picometer precision, since these gratings have a very narrow pass-band feature in the center of their resonance spectra (see Figure 1).…”

“…In an attempt to shed light on the origin of the wavelength drift and eliminate some of the aforementioned drawbacks present in the previous research, we embarked on a systematic study of the long-term drifts in packaged π-phase-shifted femtosecond laser written gratings that promise an improved wavelength stability [14].…”

Fiber-Optic Bragg Gratings for Temperature and Pressure Measurements in Isotope Production Targets for Nuclear Medicine

“…Their classification differs depending on their FBG inscription technique and fiber design. We also note that in order to improve accuracy in the wavelength measurements, the FBGs under consideration were π-phase-shifted FBGs which have a very narrow passband feature in their transmission/reflection spectra [15]. In the following text, the π-phase-shifted FBGs will be referred to simply as "FBGs", for brevity.…”

Sensing Properties of Thinned Phase-shifted Fiber Bragg Gratings

“…(9)(10)(11) The transmission peak undergoes a certain amount of shift with a change in the external environment, which is utilized to measure temperature, strain, and magnetic field. (12)(13)(14)(15)(16) Because of its very narrow transmission peak, a PS-FBG has a high resolution. To improve the sensing performance, the diameter of a PS-FBG needs to be decreased to expose the evanescent field to the surrounding environment.…”