Femtosecond Laser Inscription Method Research Articles

Spectrum design of multimode fiber Bragg gratings based on suppression of mode coupling

The direct femtosecond laser inscription method is very competitive in the fabrication of fiber Bragg gratings, but its refractive index modification region is very limited and leads to low reflectivity and wide spectra of multimode fiber gratings. In this work, the relationship between the refractive index modification region, the coupling coefficient, and the spectrum of the multimode fiber grating is investigated based on the intrinsic mechanism of mode coupling; thus the annular and eccentric elliptic refractive index modification regions are accordingly proposed to suppress mode couplings in the multimode fiber Bragg grating, so as to achieve a highly discrete spectrum and narrow spectrum of the multimode fiber Bragg grating, respectively. This work can provide a solid theoretical foundation for the spectrum design of the multimode fiber Bragg grating, which can be utilized to customize the spectrum according to application scenarios such as optical communication, fiber lasers, filtering, and sensing.

Long period grating in a multimode cyclic transparent optical polymer fiber inscribed using a femtosecond laser.

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 1560nm. 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.

All-fiber passively mode-locked ultrafast laser based on a femtosecond-laser-inscribed in-fiber Brewster device.

We report on an in-fiber Brewster device with a 45° tilted fiber grating (TFG) directly written by a plane-by-plane femtosecond laser inscription method. Up to 10dB polarization-dependent loss was achieved, proving effective polarizing functionality. Furthermore, we employ it as an in-line polarizer to successfully mode lock a fiber laser through the nonlinear polarization rotation technique. A stable soliton pulse train has been generated at 1563.64nm with a pulse width of 624fs and pulse energy of 0.42nJ. With proper polarization adjustment, the laser also can operate in a noise-like regime. The parameters of this kind of 45°-TFG can be flexibly customized owing to the high flexibility and controllability of the femtosecond laser-inscription approach. In particular, such in-fiber polarizing devices inscribed by femtosecond laser inscription without removing the fiber coating are extremely robust for fiber lasers working at a broad wavelength region including the mid-infrared.

Er/Yb Double-Clad Fiber Laser With fs-Laser Inscribed Plane-by-Plane Chirped FBG Laser Mirrors

We present an efficient monolithic fiber laser incorporating fiber Bragg grating (FBG) mirrors inscribed directly into the active fiber using a femtosecond laser. The FBGs are inscribed in an erbium/ytterbium co-doped fiber using the direct-write, plane-by-plane femtosecond laser inscription method, which allows for complete spatial control of the refractive index change in the fiber core. This approach offers a significant advantage, as we eliminate the fiber splices typically required between the active and passive fibers that would, otherwise, incorporate the FBG. The gratings were characterized in transmission and reflection, and the performance of the fiber laser was recorded when the fiber was coiled into a circular and kidney shape. The kidney shape of the fiber coil improved the laser slope efficiency, as the inner cladding of the active fiber had a circular shape. Our results show that the control of the grating inscription parameters using the plane-by-plane inscription method can result in an efficient monolithic fiber laser.

Simultaneous Measurement of Axial Strain, Bending and Torsion With a Single Fiber Bragg Grating in CYTOP Fiber

This paper presents the development of a three-dimensional (3-D) displacement sensor based on one fiber Bragg grating (FBG). In order to obtain higher sensitivity and dynamic range, the FBG is inscribed in low-loss, multimode, cyclic transparent amorphous fluoropolymers using the direct-write, plane-by-plane femtosecond laser inscription method. The proposed sensor is based on the influence of each displacement condition, namely, axial strain, torsion, and bending on the FBG reflection spectrum. Such influence is analyzed with respect to the FBG wavelength shift, reflectivity, and full width half maximum. The operation principle and theoretical background of the proposed approach is numerically analyzed by means of finite element analysis for the strain along the grating length and coupled-mode theory with a modified transfer matrix formulation for the FBG spectrum. The sensor is experimentally characterized and validated in which the results show good agreement between the applied axial strain, bending, and torsion with relative errors below 5.5%. Thus, the proposed sensor is an interesting alternative for measuring displacements in 3-D applications, such as movement analysis and the instrumentation of novel soft robots.

Fiber Bragg Gratings in CYTOP Fibers Embedded in a 3D-Printed Flexible Support for Assessment of Human⁻Robot Interaction Forces.

We developed a flexible support with embedded polymer optical fiber (POF) sensors for the assessment of human–robot interaction forces. The supports were fabricated with a three-dimensional (3D) printer, where an acrylonitrile butadiene styrene (ABS) rigid structure was used in the region of the support in which the exoskeleton was attached, whereas a thermoplastic polyurethane (TPU) flexible structure was printed in the region where the users placed their legs. In addition, fiber Bragg gratings (FBGs), inscribed in low-loss, cyclic, transparent, optical polymer (CYTOP) using the direct-write, plane-by-plane femtosecond laser inscription method, were embedded in the TPU structure. In this case, a 2-FBG array was embedded in two supports for human–robot interaction force assessment at two points on the users’ legs. Both FBG sensors were characterized with respect to temperature and force; additionally, the creep response of the polymer, where temperature influences the force sensitivity, was analyzed. Following the characterization, a compensation method for the creep and temperature influence was derived, showing relative errors below 4.5%. Such errors were lower than the ones obtained with similar sensors in previously published works. The instrumented support was attached to an exoskeleton for knee rehabilitation exercises, where the human–robot interaction forces were measured in flexion and extension cycles.

Thermal Treatments and Compensation Techniques for the Improved Response of FBG Sensors in POFs

The influence of annealing under different humidity conditions on temperature, strain, and transverse force performance of polymer optical fiber Bragg gratings (POFBG) inscribed in cyclic transparent amorphous fluoropolymers is presented. The POFBG are imprinted using direct-write, plane-by-plane femtosecond laser inscription method. Further improvement to the sensor's performance, with respect to sensitivity, linearity, and hysteresis, is obtained with a simple technique based on polymer viscoelastic response applied to the annealed POFBGs. The sensors were characterized for temperature, strain, and transverse force after annealing and results show that sensors annealed under water have the lowest hysteresis. Additionally, the temperature sensitivity increased when the sensor was annealed in low humidity conditions. Finally, for strain and transverse force, the highest sensitivity was obtained in POFBG sensors annealed under water. The compensation technique did not show a substantial decrease in hysteresis response for applied strain tests; however, for temperature and transverse force tests, the hysteresis decreased by more than two and four times, respectively.

Compensation Method for Temperature Cross-Sensitivity in Transverse Force Applications With FBG Sensors in POFs

In this paper, we investigate the influence of temperature on the transverse force response of polymer optical fiber Bragg gratings (POFBGs) inscribed in cyclic transparent fluoropolymers (CYTOPs). The gratings are imprinted in the fiber using the direct-write, plane-by-plane femtosecond laser inscription method. The temperature increase leads to a decrease in the polymer Young's modulus, which causes a sensitivity variation in the POFBG sensor for transverse force applications. The proposed technique is based on the characterization of both the sensor's response offset and the material's Young's modulus variation due to the temperature increase. Transverse force tests were performed at different temperatures (30 °C, 40 °C, 50 °C, and 60 °C) and the compensated and uncompensated responses are compared in terms of root mean squared error (RMSE). The compensated results show an RMSE lower than 3% (mean value between all tested temperatures), which is 6.7 times lower compared with the uncompensated response. In addition, the proposed compensation technique presents a maximum RMSE reduction of 16 times at 60 °C.

Polymer Optical Fiber Bragg Gratings in CYTOP Fibers for Angle Measurement with Dynamic Compensation.

This paper demonstrates the use of polymer optical fiber Bragg gratings (POFBGs) for angle measurements over a range of different oscillatory frequencies. The POFBGs are inscribed in low-loss, cyclic transparent amorphous fluoropolymers (CYTOP) and are imprinted using the direct-write, plane-by-plane femtosecond laser inscription method. As the polymer has a viscoelastic response and given that the Young’s modulus depends on the oscillatory frequency, a compensation technique for sensor frequency cross-sensitivity and hysteresis is proposed and verified. Results show that the proposed compensation technique is able to provide a root mean squared error (RMSE) reduction of 44%, and a RMSE as low as 2.20° was obtained when compared with a reference potentiometer. The hysteresis reduction provided by the proposed technique is 55%, with hysteresis <0.01. The results presented in this paper can pave the way for movement analysis with POFBG providing higher sensitivity and low hysteresis over a large range of motion frequencies.

Multi-gigahertz repetition rate ultrafast waveguide lasers mode-locked with graphene saturable absorbers

We report the development of an approach to build compact waveguide lasers that operate in the stable fundamental mode-locking regime with multigigahertz repetition rates. The approach is based on the use of depressed cladding multi- or single-mode waveguides fabricated directly in the active laser crystal using the femtosecond laser inscription method and a graphene saturable absorber. Using this approach we achieve the stable self-starting mode-locking operation of a diode-pumped waveguide Nd:YAG laser that delivers picosecond pulses at a repetition rate of up to 11.5 GHz with an average power of 12 mW at a central wavelength of 1064 nm. The saturable absorbers are formed through the chemical vapor deposition of single-layer graphene on the output coupler mirror or directly on the end facet of the laser crystal. The stable self-starting mode-locking operation is achieved by controlling the group delay dispersion in the laser cavity with an intracavity interferometer. The method developed for the creation of compact ultrashort pulse laser generators with gigahertz repetition rates can be extended further and applied for the development of compact high-repetition rate lasers that operate at a wide range of IR wavelengths.

Analysis of buried parabolic index segmented channel waveguides with z-dependent profile

We analyze, to the best of our knowledge, for the first time buried parabolic index segmented channel waveguides, with high-index segments having a z-dependent refractive index variation. Using the standard ABCD ray transfer matrices for Gaussian beam propagation, we obtain the dependence of effective index, spot-size, amplitude and phase fronts of the guided modes on various waveguide parameters. Stability conditions are obtained for various cases. The analysis is used to study segmented waveguides fabricated using femtosecond laser inscription method; the experimentally observed dependence of stability on the duty cycle of segmentation is explained.