In this Letter, we report a flexible, plane-by-plane direct write inscription method for the development of tailored, tilted fiber Bragg gratings using a femtosecond laser. Compared to ultraviolet or femtosecond laser inscription based on the phase mask, interferometric, or point-by-point methods, the presented approach is far more flexible and offers several advantages. Laser inscription is made through the fiber coating, while the grating planes are controlled to minimize birefringence, with precise control over the wavelength location and strength of cladding modes. Tenth-order gratings were produced in the $C+L$ bands so that higher-order gratings could be studied at shorter wavelengths. In particular, we show that the refractometric sensitivity depends on the grating order, ranging from $\sim 28\mathrm{nm}/\text{refractive}$ index unit (RIU) at $\sim 1510\mathrm{nm}$ to $\sim 13\mathrm{nm}/\mathrm{RIU}$ at $\sim 1260\mathrm{nm}$.

In this Letter, we report, to the best of our knowledge, the first inscription of long period gratings (LPGs) in a multimode cyclic transparent optical polymer (CYTOP) fiber using a femtosecond laser inscription method. The LPG was inscribed directly in the center of the fiber core, tailored for operation at 1560 nm. The CYTOP-LPG was characterized in transmission, and its response for relative humidity and temperature was measured. The humidity measurements, to the best our knowledge, are the first for a POF-LPG, whereas the temperature sensitivity is significantly higher than reported in other works. In addition, dynamic mechanical measurements were performed comparing the mechanical characteristics of the laser exposed sections of the polymer fiber, where the LPG was inscribed, with the unexposed regions.

Surface plasmon resonance excitation with optical fiber gratings has been typically studied in aqueous solutions. This work describes the procedure to excite a plasmon wave in gaseous media and perform refractive index measurements in these environments. Grating photo-inscription with 193 nm excimer laser radiation allows us to obtain slightly tilted fiber Bragg gratings exhibiting a cladding mode resonance comb along several hundreds of nanometers. Their refractive index sensitive range extends from gases to liquids, so operation in both media is compared. We demonstrate that the thickness of the metal coating required for surface plasmon excitation in gases is roughly one third of the one usually used for liquids. The developed platforms exhibit a temperature insensitive response of 78 nm/RIU when tested with different gases.

Polymer optical fibre Bragg grating (POFBG) sensors are of high interest due to their enhanced fracture toughness, flexibility in bending, and sensitivity in stress and pressure monitoring applications compared to silica-based sensors. The POFBG sensors can also detect humidity due to the hydrophilic nature of some polymers. However, multi-parameter sensing can cause cross-sensitivity issues in certain applications if the temperature and humidity measurements are not adequately compensated. In this work, we demonstrate the possibility of selectively tuning sensors’ temperature and humidity sensitivities to the desired level by applying a certain amount of fibre pre-strain. The temperature sensitivity of POFBG sensors fabricated in perfluoropolymers (CYTOP) can be selectively tuned from positive to negative values, having the option for insensitivity in specific temperature ranges depending on the amount of the applied pre-strain. The humidity sensitivity of sensors can also be changed from positive values to insensitivity. The importance of thermal annealing treatment of POFBG sensors for improved repeatability in temperature measurements is also reported. An array of 4 multiplexed POFBGs was fabricated, and each sensor was pre-strained accordingly to demonstrate the possibility of having targeted temperature and humidity sensitivities along the same fibre.

Plane-by-plane femtosecond laser fabricated apodized fiber Bragg gratings (FBG) are demonstrated for the first time, to the best of our knowledge. The method reported in this work provides a fully customizable and controlled inscription that can realize any desired apodized profile. By using this flexibility, we experimentally demonstrate four different apodization profiles (Gaussian, Hamming, New, Nuttall). These profiles were chosen to evaluate their performance with regard to the sidelobe suppression ratio (SLSR). Usually, a higher reflectivity of a grating fabricated with a femtosecond laser will result in a greater difficulty to achieve a controlled apodization profile due to the nature of the material modification. Therefore, the goal of this work is to fabricate high-reflectivity FBGs without sacrificing the SLSR and provide a direct comparison with apodized low-reflectivity FBGs. In our weak apodized FBGs, we also consider the background noise introduced during the femtosecond (fs)-laser inscription process which is fundamental when multiplexing FBGs within a narrow wavelength window.