Femtosecond laser inscribed fiber Bragg gratings based on precise spatial apodization

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

Fibre cladding filters through femtosecond laser inscription

We inscribe several in-cladding-fibre filters using the same key femtosecond laser parameters, via an “inscribe and step”, plane-by-plane (Pl-by-Pl) approach, leading to ultra-compact waveguides and Mach Zehnders (MZs) that can support functional, integrated fibre Bragg gratings (FBGs)

All-in-Fiber Cladding Interferometric and Bragg Grating Components Made via Plane-by-Plane Femtosecond Laser Inscription

We introduce a method of inscribing in-fiber devices using a femtosecond laser that is applicable to crucial components, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs), embedded waveguide Bragg gratings (WBGs), and waveguide Fabry-Perot cavities using the same key femtosecond laser parameters, via an 'inscribe and step,' plane-by-plane approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fiber Bragg gratings; the unique sensing characteristics of the filters are maintained and provide complementary measure and information. This ensures a single inscription process, offering reliability and repetition in component manufacture, as the basic conditions to inscribe the here-demonstrated elements are common. We characterize CWG-WBG devices for axial strain, bend, and response to refractive index. The MZI-WBG is exposed to temperature and humidity excursions, confirming that the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in the development of compact, smart optical fiber sensors

Fibre cladding interferometers and Bragg gratings made via plane by plane femtosecond laser inscription

We present an extremely flexible femtosecond (fs) laser inscription method, applicable to the development of critical filtering and wave-guiding components in optical fibres. We inscribe in-fibre devices, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs) and embedded waveguide Bragg gratings (WBGs) using the same key femtosecond laser parameters, via an "inscribe and step", plane-by-plane (Pl-by-Pl) approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fibre Bragg gratings (FBGs); the unique sensing characteristics of the filters are maintained and provide complementary measurand information. The flexibility and control in waveguide/grating fabrication leads to sensing device customization, e.g.Tailored bend sensing. We characterize CWG-WBG devices for their bend response, whereas the MZI-WBG is exposed to temperature and humidity excursions, confirming the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in developing compact, smart optical fibre sensors

Temperature and Humidity Sensitivity of Polymer Optical Fibre Sensors Tuned by Pre-Strain

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

Flexible direct write inscription of tilted fibre Bragg gratings using a femtosecond laser

A flexible, plane-by-plane, direct-write, femtosecond-laser inscription method for tailored, tilted fibre Bragg gratings (TFBGs) is presented. We characterize 10th order gratings in the C-to-O bands and their refractometric sensitivity with grating order

Sensing capabilities of higher order cladding modes

In this work, 5-mm long TFBGs were inscribed in photosensitive single-mode optical fiber using the direct writing plane-by-plane femtosecond laser inscription method; a flexible inscription approach that enables absolute control of the grating period, length, angle, width and depth of the grating planes. This new fabrication method brings important differences compared to classical inscription methods. Firstly, these gratings exhibit very low photo-induced birefringence (measured ∼8pm) and as we rely on a direct writing process, the tilt angle of the inscribed grating does not affect the Bragg wavelength, allowing for precise positioning. In addition, this method enables the high order grating production, allowing a behavioral study of higher order cladding modes located at lower wavelengths in the 1200-1600 nm range. 8th order gratings were produced with cladding and Bragg mode resonances in the C+L bands. The temperature and strain sensitivities were measured for both the Bragg and higher order cladding modes, yielding an exceptional performance. The higher order modes exhibit a negative axial strain, up to-1.99nm (more than two times higher than the standard Bragg peaks) and a solid temperature sensitivity of 10.25 pm/°C: At the same time, for the designed order cladding modes (of the 8th) the refractive index sensitivity is measured at 22 nm/RIU

All-in-fiber fabrication of cladding devices and components using femtosecond laser pulses

Femtosecond (fs) lasers are well suited for high-resolution inscription in transparent materials of all types, and in particular optical fibres. The 'inscribe and step', Plane-by-Plane (Pl-by-Pl) inscription method enables the fabrication of components, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs) and embedded waveguide Bragg gratings (MZI-FBGs), using the same key femtosecond laser parameters. The cladding waveguides structures were inscribed in such proximity for the fiber core that allowed evanescent coupling to occur. Having this operational principle, the compound cladding sensor has minimal effect on fiber strength, leading to a robust sensing device. Furthermore, the versatile femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fibre Bragg gratings (FBGs). This method ensures a single inscription process, offering reliability and repetition in component manufacturing. This hybrid configuration can measure multiple parameters using the same demodulation equipment, with very high sensitivity while minimizing cross sensitivity issues. We focus on the response to changes in temperature, strain, bend, and relative humidity of the developed components