Ultra-fast fiber Bragg grating inscription in CYTOP polymer optical fibers using phase mask and 400 nm femtosecond laser

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.

Gamma-radiation enhancement of sensing properties of FBGs in a few-mode polymer CYTOP fiber

We investigate the effect of γ-radiation on temperature (T) and relative humidity (RH) sensitivities of polymer perfluorinated fiber Bragg gratings (FBGs). To this aim, different γ-radiation doses (80, 120, 160, and 520 kGy) were applied to a set of FBGs. We show that irradiated FBGs demonstrate an RH sensitivity rise with the received dose: from 13.3 pm/%RH for a pristine FBG up to 56.8 pm/%RH for a 520-kGy dose at 30℃. In contrast, T sensitivity decreases with radiation dose with a subsequent change of sign from positive to negative. Therefore, by experimental interpolation, T sensitivity can be eliminated at around a 160-kGy dose. This opens the possibility of designing an RH sensor with enhanced sensitivity, which at the same time is insensitive to T.

Properties of Fiber Bragg Grating in CYTOP Fiber Response to Temperature, Humidity, and Strain Using Factorial Design

The characteristics of fiber Bragg grating (FBG) in cyclic transparent fluoropolymer (CYTOP) optical fiber have attracted more and more attention in recent years. However, different results of the FBG response to environmental parameters are reported. This work presents a three-variable two-level factorial experimental method to investigate the FBG response to temperature, humidity, and strain in CYTOP fiber. Two uniform FBGs are inscribed separately in CYTOP fiber with and without over-clad. With only eight measuring points, the interactions among three variable parameters are computed and the parameter sensitivities and cross-sensitivities are estimated. Similar temperature and strain sensitivities were found for both gratings, whereas significant cross-sensitivity between humidity and temperature was present only in FBG inscribed in CYTOP fiber with over-clad.

Enhanced refractometer for aqueous solutions based on perfluorinated polymer optical fibres

The use of the new CYTOP (Cyclized Transparent Optical Polymer) fibres for the inscription of optical structures and the detection of different parameters has started to gain importance in the past decade. This work presents the design, simulation and manufacture of a CYTOP-based surrounding refractive index sensor for aqueous solutions, given its high sensitivity in the range 1.315 - 1.333 (at 1550 nm wavelength). The structure is based on a bent and polished fibre (in order to increase its sensitivity), the polished area being the surface on which a diffraction grating is inscribed with a femtosecond laser. The interaction of the field propagated by the fibre with the grating causes diffraction of certain orders towards the outside, depending, among other things, on the refractive index of the fluid. In addition to a maximum sensitivity of -208.8 nm/RIU and a remarkable insensitivity to temperature, it offers a spectral fingerprint of each sensed fluid.

Highly Sensitive Surface Plasmon Resonance Humidity Sensor Based on a Polyvinyl-Alcohol-Coated Polymer Optical Fiber

A surface-plasmon-resonance-based fiber device is proposed for highly sensitive relative humidity (RH) sensing and human breath monitoring. The device is fabricated by using a polyvinyl alcohol (PVA) film and gold coating on the flat surface of a side-polished polymer optical fiber. The thickness and refractive index of the PVA coating are sensitive to environmental humidity, and thus the resonant wavelength of the proposed device exhibits a redshift as the RH increases. Experimental results demonstrate an average sensitivity of 4.98 nm/RH% across an ambient RH ranging from 40% to 90%. In particular, the sensor exhibits a linear response between 75% and 90% RH, with a sensitivity of 10.15 nm/RH%. The device is suitable for human breath tests and shows an average wavelength shift of up to 228.20 nm, which is 10 times larger than that of a silica-fiber-based humidity sensor. The corresponding response and recovery times are determined to be 0.44 s and 0.86 s, respectively. The proposed sensor has significant potential for a variety of practical applications, such as intensive care and human health analysis.

Femtosecond laser inscribed helical sapphire fiber Bragg gratings

This Letter reports a novel helical sapphire fiber Bragg grating (HSFBG) in a single crystal sapphire fiber with diameter of 60 µm fabricated by a 515 nm femtosecond laser. Due to the large refractive index modulation region and high structural symmetry of the HSFBGs, high-reflectivity and high-quality spectra can be prepared and additionally have good bending resistance. The spectral properties of HSFBGs with different helical diameters are studied. When the helical diameter is 30 µm, the reflectivity of HSFBG is 40%, the full width at half-maximum is 1.56 nm, and the signal-to-noise ratio is 16 dB. For the HSFBG bending test, the minimum bending radius is 5 mm, which can still maintain relatively good spectral quality. In addition, the HSFBG array with different periods has been successfully cascaded in a sapphire fiber. The experimental results of the HSFBG high-temperature test show that this HSFBG can work reliably at 1600°C, and the temperature sensitivity in the high-temperature range can reach 35.55 pm/°C. This HSFBG can be used in high-temperature and harsh environments, such as metal smelting and aeroengine structural health monitoring.