Impact of high UV fluences on the mechanical and sensing properties of polymer optical fibers for high strain measurements.

PMMA-based fibers are widely studied for strain measurements and show repeatable results for Fiber Bragg Gratings (FBGs) inscribed using 325 nm laser and 248 nm laser. However, there is no available material mechanical behavior characterization of the UV source impact on the fiber properties. In this manuscript, fibers are irradiated with high fluence of 325 nm and 248 nm lasers and the fibers properties are investigated using dynamic mechanical analysis and tensile strain for potential use of these fibers past the yield point. It is demonstrated that the UV sources shifted the ultimate tensile strength and changed the strain hardening behavior. Tensile strain measurements show excellent repeatability for gratings inscribed with these two sources with similar sensitivity of 1.305 nm/mɛ for FBG inscribe with 325 nm laser, and 1.345 nm/mɛ for grating written with 248 nm laser in the range 0 to 1.5 % elongation. Furthermore, tests far beyond the yield point (up to 2.8 % elongation) show that grating inscribed with lower UV wavelength exhibit hysteresis. Finally, we demonstrate that 248 nm laser fluence shall be chosen carefully whereas even high 325 nm laser fluence do not critically impact the sensor properties.

Single nanosecond-pulse production of polymeric fiber Bragg gratings for biomedical applications.

In this study, we present first-time fabrication of FBGs in all ZEONEX-based SMPOFs with a single 25 ns pulse of 248 nm UV irradiation over a 12-month period, which opens up new frontiers in optics and photonics for the effective fabrication of polymer optical fiber Bragg gratings (POFBGs), permitting mass producibility of them. POFBGs were characterized by subjecting them to various physical parameters including temperature and tensile strain. Strain responses of FBGs with similar grating strengths fabricated with 248 nm and 325 nm He-Cd laser irradiations were explored over a year to demonstrate their long-term stability and applicability. Owing to the unique features of the proposed sensing device fabricated by embedding POFBGs in silicone rubber, a good performance in the detection of human heart rate with an amplitude of 4 pm, which is 4 times higher compared to that of silica single mode fiber (SMF) was demonstrated. The response of the sensing device during a human respiration process was also explored where exhalation and inhalation were monitored and distinguished while the breath was held. These revelations signify the importance of ZEONEX-based POFBGs, which allow consistent and effective grating fabrication and are highly promising in the foreseeable future for biomedical applications.

Bragg grating inscription in PMMA optical fibers using 400-nm femtosecond pulses.

In this Letter, we report the fast growth of high quality uniform Bragg gratings in trans-4-stilbenemethanol (TS)-doped poly(methyl methacrylate) (PMMA) step-index optical fibers. Grating manufacturing was obtained using a 400 nm femtosecond pulsed laser and a 1060-nm-period uniform phase mask. For 20 mW mean laser beam power, the grating reflectivity reaches 98% in ∼60 s.

Highly reflective Bragg gratings in slightly etched step-index polymer optical fiber.

During the past few years, a strong progress has been made in the photo-writing of fiber Bragg gratings (FBGs) in polymer optical fibers (POFs), animated by the constant wish to enhance the grating reflectivity and improve the sensing performances. In this paper, we report the photo-inscription of highly reflective gratings in step-index POFs, obtained thanks to a slight etching of the cladding. We demonstrate that a cladding diameter decrease of ~12% is an ideal trade-off to produce highly reflective gratings with enhanced axial strain sensitivity, while keeping almost intact their mechanical resistance. For this, we make use of Trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. FBGs are inscribed at ~1550 nm by the scanning phase mask technique in POFs of different external diameters. Reflectivity reaching 97% is achieved for 6 mm long FBGs, compared to 25% for non-etched POFs. We also report that a cladding decrease enhances the FBG axial tension while keeping unchanged temperature and surrounding refractive index sensitivities. Finally and for the first time, a measurement is conducted in transmission with polarized light, showing that a photo-induced birefringence of 7 × 10(-6) is generated (one order of magnitude higher than the intrinsic fiber birefringence), which is similar to the one generated in silica fiber using ultra-violet laser.

Tilted Bragg gratings in step-index polymer optical fiber.

We report the first slightly tilted Bragg gratings photo-inscription in polymer optical fiber (POF). For this, we make use of trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. Tilted fiber Bragg gratings (TFBGs) are inscribed in the near-infrared wavelength range using the scanning phase mask technique with a tilted phase mask in the plane perpendicular to the laser beam direction. The transmitted amplitude spectrum evolution of a 3° TFBG is analyzed as a function of the surrounding refractive index. A maximum sensitivity close to 13 nm/RIU (refractive index unit) is obtained in the range 1.42-1.49.

Optimization of 3-hole-assisted PMMA optical fiber with double cladding for UV-induced FBG fabrication.

We propose a new hole-assisted polymer optical fiber design to eliminate the influence of dopant diffusion and to increase the UV writing efficiency in fiber Bragg grating inscription. The optical waveguide is formed inside a solid core surrounded by a ring of 3 large air holes in enhanced UV photosensitive PMMA with double-cladding. We determined a map of the single-mode and multi-mode phase transitions using a finite-element- based vectorial optical mode solver. We obtained a wide range of geometrical configuration for the single-transverse-mode (HE11) propagation in the visible. The design is optimized to operate at the minimum optical loss wavelengths of 580 nm and 770 nm.

Ultra-fast polymer optical fibre Bragg grating inscription for medical devices.

We report the extraordinary result of rapid fibre Bragg grating inscription in doped polymer optical fibres based on polymethyl methacrylate in only 7 ms, which is two orders of magnitude faster than the inscription times previously reported. This was achieved using a new dopant material, diphenyl disulphide, which was found to enable a fast, positive refractive index change using a low ultraviolet dose. These changes were investigated and found to arise from photodissociation of the diphenyl disulphide molecule and subsequent molecular reorganization. We demonstrate that gratings inscribed in these fibres can exhibit at least a 15 times higher sensitivity than silica glass fibre, despite their quick inscription times. As a demonstration of the sensitivity, we selected a highly stringent situation, namely, the monitoring of a human heartbeat and respiratory functions. These findings could permit the inscription of fibre Bragg gratings during the fibre drawing process for mass production, allowing cost-effective, single-use, in vivo sensors among other potential uses.