Inscription Of Fiber Bragg Gratings Research Articles

Optimized femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss.

We propose and experimentally demonstrate a femtosecond laser plane-by-plane (Pl-b-Pl) technology for inscription of high-quality fiber Bragg gratings (FBGs). The spherical aberration (SA) was introduced to elongate the focal volume, and then combined with the scanning process, an expanded rectangular refractive index modification (RIM) region can be achieved. Such RIM regions exhibit a length of 15 µm and a width of 14 µm. Note that it consists of a negative region and a positive region. We have systematically studied the influence of the overlap between the RIM region and fiber core on the spectrum of FBG. After optimizing, the core of a conventional single-mode fiber (SMF) is covered completely by using the positive RIM region, resulting in a significant enhancement of the coupling strength coefficient (i.e., 3177.6 m-1). A 500 µm long FBG assembled by using these RIM regions can achieve a high reflectivity of 95.83%. Moreover, the cladding mode resonances in transmission spectrum are suppressed thoroughly, since the localized effect in RIM region was avoided. In addition, this FBG exhibits a high birefringence of 2.13 × 10-4. Therefore, the proposed fabrication method can be used to inscribe high-quality FBGs that could be used in many fields such as communication, fiber laser, polarization-selective filtering and multi-parameter sensing.

Thermal diffusion coupling mechanism and its application of discrete waveguide

For discrete optical systems integrated into optical fibers, the optical fields of the individual waveguides are coupled and correlated with each other. This paper studies how to adjust the refractive index of discrete waveguides by thermal diffusion, so as to enhance the coupling between discrete waveguides, and also constructs the discrete waveguide thermally diffused model and the thermally diffused coupling model of twin-core and three-core fibers. The multicore fiber is heated different times by a hydrogen-oxygen flame, and the outgoing light field at the end face of the optical fiber is monitored at the same time. Then, the three-dimensional refractive index measurement results of the thermally diffused multicore fiber verify the feasibility of thermal diffusion technology to change the refractive index of discrete waveguides for coupling. Thermal diffusion technology can be used to fabricate multicore fiber couplers. By combining multicore fiber and core-by-core inscribed fiber Bragg grating technology and by using thermal diffusion technology, the single-channel sensing measurement can be realized. The method of changing the refractive index of discrete waveguides through thermal diffusion has the advantages of high integration, high stability, and mass fabrication. The research on the thermal diffusion of discrete waveguides can improve the application potential of multicore fiber sensing systems, and promote the broad application of discrete waveguide structure optical fiber in the fields of optical communication, optical sensing, biomedicine, and artificial intelligence.

Integrated fiber-optic sensor based on inscription of FBG in seven-core fiber for curvature and temperature measurements

An integrated fiber-optic sensor with cross-sensitivity elimination effect is proposed and experimented for simultaneous measurements of curvature and temperature. The sensor structure mainly consists of a section of seven-core fiber (SCF) that fiber Bragg gratings (FBGs) were inscribed in each fiber core by phase-mask technique. Since the FBG in the central-core of SCF is only sensitive to temperature, and others in six outer-cores have same temperature responses and different sensitivity for curvature, the sensor can simultaneously measure temperature and curvature through analyzing the wavelength shifts of central-core FBG and every-two cores of FBG. The experiment results show that the maximum curvature sensitivity reaches 0.39 nm/m−1 in the curvature range of 2.53 m−1–8.62 m−1, and temperature response sensitivity is 9.97 pm/°C, which is consistent with ordinary single-mode FBG in temperature range of 30 °C–70 °C. Furthermore, the verified experiment for simultaneous measurement are performed, the central-core FBG can be used as a temperature calibration during the curvature measurement. The relative errors of curvature and temperature are 2.22 % and 2.58 %, respectively. The SCF-based FBG sensor has the advantages of small dimension, easy fabrication, better repeatability, and potential application values in many fields such as machine structure motion monitoring.

Fatigue Performance of Type I and Type II Fibre Bragg Gratings Fabricated by Femtosecond Laser Inscription through the Coating.

Strain sensing technology using fibre Bragg grating (FBG) sensors is an attractive capability for aerospace structural health monitoring (SHM) and assessment because they offer resistance to harsh environments, low maintenance, and potential for high density and high strain sensing. The development of FBG inscription techniques through the fibre polymer coating using infrared (IR) lasers has overcome the mechanical weaknesses introduced by removal of the fibre coating, which is typically required for conventional UV laser inscription of FBGs. Type I and Type II femtosecond gratings are fabricated using through-coating inscription techniques, but the higher laser energy used for Type II gratings damages the glass fibre core, impacting mechanical performance. This paper investigates the fatigue performance of Type I and Type II through-coating FBG sensors with different fibre geometries and photosensitisation approaches to evaluate their overall reliability and durability, with a view to assess their performance for potential use in civil and defence SHM applications. The fatigue performance of FBG sensors was assessed under high-strain and high-frequency mechanical loading conditions by using a custom-designed electro-dynamically actuated loading assembly. In addition, pre- and post-fatigue microscopic analyses and high-resolution reflection spectrum characterisation were conducted to investigate the failure regions of the fibres and the effect of fatigue loading on reflection spectrum features. As expected, Type I gratings had a significantly higher fatigue life compared to Type II gratings. However, Type II gratings performed significantly better than conventional UV laser-inscribed FBGs and electrical foil strain gauges. Type II gratings withstand higher temperatures, and are therefore more suitable for application in harsh environments.

Random Laser Based on Ytterbium-Doped Fiber with a Bragg Grating Array as the Source of Continuous-Wave 976 nm Wavelength Radiation

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG.

High-Quality Fiber Bragg Gratings Inscribed by Femtosecond Laser Point-by-Point Technology.

We experimentally studied the inscription of fiber Bragg gratings by using femtosecond (fs) laser point-by-point (PbP) technology. The effects of the focusing geometry, grating order, laser energy and grating length on the spectral characteristics of the PbP FBG were investigated. After optimizing these parameters, a high-quality first-order PbP FBG with a reflectivity > 99.9% (i.e., Bragg resonance attenuation of 37.7 dB) and insertion loss (IL) of 0.03 dB was successfully created. Moreover, taking advantage of the excellent flexibility of the fs laser PbP technology, high-quality FBGs with various Bragg wavelengths ranging from 856 to 1902.6 nm were inscribed. Furthermore, wavelength-division-multiplexed (WDM) FBG arrays consisting of 10 FBGs were rapidly constructed. Additionally, a Fabry-Perot cavity was realized by using two high-quality FBGs, and its birefringence could be reduced from 3.04 × 10-5 to 1.77 × 10-6 by using a slit beam shaping-assisted femtosecond laser PbP technology. Therefore, such high-quality FBGs are promising to improve the performance of optical fiber sensors, lasers and communication devices.

Femtosecond inscription of large-area fiber Bragg gratings for high-power cladding pump reflection.

A new, to the best of our knowledge, method for inscribing fiber Bragg gratings inside a fiber's cladding based on the motorized rotation of the fiber is reported. By minimizing the aberrations induced by the fiber curvature on the femtosecond writing beam, this technique based on a phase mask allows to cover large transverse areas of a standard high-power fiber's cladding. With this approach, a first-order Bragg grating was inscribed in the pure-silica inner cladding of a 20/400-µm fiber. It was then implemented as a pump reflector at the end of a 36-m-long Yb-doped fiber laser reaching 600 W of output power, confirming the power handling capabilities of such a component. Comparison of the laser performances with and without the pump reflector showcases its great potential for increasing pump absorption inside cladding-pumped fiber lasers, which paves the way for significantly reducing their active fiber length.

Point-by-point inscribed sapphire parallel fiber Bragg gratings in a fully multimode system for multiplexed high-temperature sensing.

We study the point-by-point inscription of sapphire parallel fiber Bragg gratings (sapphire pFBGs) in a fully multimode system. A parallel FBG is shown to be critical in enabling detectable and reliable high-order grating signals. The impacts of modal volume, spatial coherence, and grating location on reflectivity are examined. Three cascaded seventh-order pFBGs are fabricated in one sapphire fiber for wavelength multiplexed temperature sensing. Using a low-cost, fully multimode 850-nm interrogator, reliable measurement up to 1500°C is demonstrated.

Selective fiber Bragg grating inscription in multicore fiber by femtosecond laser and phase mask

An effective and stable method for selective fiber Bragg gratings (FBGs) inscription in multicore fibers (MCFs) using a femtosecond laser and phase mask is presented. The nonlinear photoluminescence imaging technique is adopted for visually displaying the position of the femtosecond laser focus-line inside the MCF, which helps to guarantee that the laser focal-line is precisely positioned in a preset fiber core during the inscription process. FBGs with different wavelengths are successfully inscribed into each core of a seven-core fiber (SCF). The FBGs show good consistency with a side-mode suppression ratio more than 27 dB. In addition, the largest difference of the peak intensity in the reflection spectrum between different FBGs is less than 1.4 dB and the 3-dB bandwidth for the seven FBGs is less than 0.3 nm. Confirmatory experiments show that the FBGs in the SCF have perfect directional bending response. This method provides an effective scheme for selective FBGs inscription in a multicore fiber.

Femtosecond laser point-by-point inscription of helical-sampled fiber Bragg gratings

In this Letter, a helical-sampled fiber Bragg gratings (HSFBGs) fabrication using a femtosecond laser point-by-point (PBP) technique is proposed. The unique helical structure generates sampled gratings owing to its periodicity. A simple, single-step method for inscription of the sampled gratings is described. The effects of geometrical parameters, including length of grating, helical diameter, helical pitch, and off-axis distance on the resonances are studied, and a series of comb-like spectra are obtained.

Tilted Fiber Bragg Grating Inscription in Boron Co-Doped Photosensitive Optical Fiber Using 266 nm Solid State Laser Pulses

Tilted Fiber Bragg Grating Inscription in Boron Co-Doped Photosensitive Optical Fiber Using 266 nm Solid State Laser Pulses

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers.

In this paper, the inscription of 2-mm-long fiber Bragg gratings (FBGs) on benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a femtosecond laser and a point-by-point FBG inscription technique is reported. The highest reflectivity of approximately 99% is obtained with a pulse energy of 68.5 nJ, showing a large refractive index modulation amplitude of 7.2 × 10-4. Afterwards, grating stabilities at room and higher temperatures of up to 80°C are investigated.

Development of an Optic Fiber-Based Torque Sensor With a Torsion-Translation Conversion Flexure

This work proposes a miniature and high-sensitivity torque sensor that mainly consists of a torque-sensitive flexure and one tightly suspended optic fiber with an inscribed Fiber Bragg Grating (FBG) element for application in minimally invasive surgery robots and multi-finger robotic hands. The proposed flexure can achieve a torsion-translation conversion with a linear measurement range and a solid resistance to moment loadings. It is implemented based on the hybrid design among the specially designed hyperboloid, hollow cruciform, and cross-beam structures. The mounted FBG fiber has been configured at the flexure’s central line with its two ends glued. This tight suspension configuration can improve resolution and sensitivity and avoid FBG chirping compared to the direct FBG-pasting methods. The flexure translational motion overlaps with the FBG fiber suspension direction, generating a large and uniform strain distribution on the suspended FBG. Design optimization has been performed based on finite element modeling (FEM) simulation for structural investigation and sensor sensitivity enhancement. The prototyped sensor achieved a high resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.295 {\mathrm {~N}} \cdot {\mathrm {mm}}$ </tex-math></inline-formula> within [−600, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$600 {\mathrm {~N}} \cdot {\mathrm {mm}}$ </tex-math></inline-formula> ] and exhibited excellent linearity with a small linearity error of 1.24%. The dynamic experiments demonstrated small crosstalk values of 1.6% and 3.4% for Mx and My moments. Furthermore, the proposed design supports convenient customization, and it can also achieve a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.81 {\mathrm {~N}} \cdot {\mathrm {mm}}$ </tex-math></inline-formula> within [−2000, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2000 {\mathrm {~N}} \cdot {\mathrm {mm}}$ </tex-math></inline-formula> ] by structural modification.

Line Position-Dependent Effect in Line-by-Line Inscribed Fiber Bragg Gratings.

Line-by-line direct writing by femtosecond laser has been proved to be a simple and effective method for the fabrication of low-loss fiber Bragg gratings (FBGs), and is more flexible compared with the traditional ultraviolet exposure method. In this paper, the line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs have been studied, to the best of our knowledge, for the first time. Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling, in which the core-guided fundamental mode would couple to the cladding-guided LP0n and LP1n simultaneously. By aligning the line positions across the core region, the first apodized line-by-line FBG was achieved. This work enriches the theories of line-by-line FBGs and provides an inscription guidance to meet different application requirements.

High-Peak Power Frequency Modulation Pulse Generation in Cascaded Fiber Configurations with Inscribed Fiber Bragg Grating Arrays

We explored the dynamics of frequency-modulated (FM) pulses in a cascaded fiber configuration comprising one active and one passive optical fiber with multiple fiber Bragg gratings (FBGs) of different periods inscribed over the fiber configuration length. We present a theoretical formalism to describe the mechanisms of the FM pulse amplification and pulse compression in such fiber cascades resulting in peak powers up to ~0.7 MW. In combination with the decreasing dispersion fibers, the considered cascade configuration enables pico- and sub-picosecond pulse trains with a sub-terahertz repetition rate and sub-kW peak power generated directly from the continuous optical signal.

Experimental Approach for Complex Optical Fiber Angular Orientation and Twist Measurement

Applications associated with complex, structured optical fibers having no circular symmetry often require information about their specific angular orientation. Such cases include, for example, splicing, sophisticated mode coupling, selective fiber Bragg gratings inscription, or direction‐sensitive sensing. Besides the external angular orientation of the fiber, additionally an intrinsic twist of the internal fiber structure has been identified as a critical parameter for applications. Such an intrinsic fiber twist might occur as a side effect of the fiber drawing process, or it may have been introduced intentionally as a special fiber property. In all such cases, a measurement of the local orientation along the fiber is highly desirable. A method for such measurements based on the analysis of the refractive–diffractive scattering patterns is presented. By comparing such measured patterns with reference patterns from modeling or from reference measurements, the orientation of the fiber can be identified with high accuracy and without measuring or reconstructing the detailed cross section of a fiber. The method, therefore, provides an approach for quick identification of a local angular fiber orientation status.

High-precision point-by-point fiber Bragg grating inscription

The paper presents a novel multi-pass point-by-point method for high-precision inscription of fiber Bragg gratings with a femtosecond laser radiation. The dynamics of the gratings formation is demonstrated in comparison with the classical method of UV-radiation inscription. It is shown that due to the feasibility of the inscription dynamics control, it is possible to fabricate high-precision Bragg gratings with specified parameters and a high degree of reproducibility. The proposed technique is demonstrated to be a convenient tool for studying the processes occurring in glass during multiphoton absorption of laser radiation.

Femtosecond inscription of phase-shifted fiber Bragg gratings using defocusing and phase-mask movement

Two slightly shifted wavelength Bragg gratings are inscribed one over the other in a single mode fiber, by moving the phase-mask (PM) between two positions. The inscription setup includes a NIR femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. A first fiber Bragg grating (FBG) is inscribed, while the second overlapping FBG is inscribed only after a slight movement of the PM, enabling a slight wavelength shift. The transmission spectrum of the complex structure is like that of a phase-shifted fiber Bragg grating (PSFBG), while the inscription process is very fast and simple compared to other standard methods. A high-quality PSFBG with two −22 dB transmission dips, and a central 16 dB transmission peak with a 30 pm transmission bandwidth at 3 dB is achieved. It is also observed that this PSFBG structure is birefringent. Finally, the temperature and strain sensitivities of the PSFBG are experimentally measured and are 10.8 pm/°C and 0.8 pm/µɛ, respectively.

Highly Dense FBG Temperature Sensor Assisted with Deep Learning Algorithms.

In this paper, we demonstrate the application of deep neural networks (DNNs) for processing the reflectance spectrum from a fiberoptic temperature sensor composed of densely inscribed fiber bragg gratings (FBG). Such sensors are commonly avoided in practice since close arrangement of short FBGs results in distortion of the spectrum caused by mutual interference between gratings. In our work the temperature sensor contained 50 FBGs with the length of 0.95 mm, edge-to-edge distance of 0.05 mm and arranged in the 1500–1600 nm spectral range. Instead of solving the direct peak detection problem for distorted signal, we applied DNNs to predict temperature distribution from entire reflectance spectrum registered by the sensor. We propose an experimental calibration setup where the dense FBG sensor is located close to an array of sparse FBG sensors. The goal of DNNs is to predict the positions of the reflectance peaks of the reference sparse FBG sensors from the reflectance spectrum of the dense FBG sensor. We show that a convolution neural network is able to predict the positions of FBG reflectance peaks of sparse sensors with mean absolute error of 7.8 pm that is slightly higher than the hardware reused interrogator equal to 5 pm. We believe that dense FBG sensors assisted with DNNs have a high potential to increase spatial resolution and also extend the length of a fiber optical sensors.

Rectangular single-mode polymer optical fiber for femtosecond laser inscription of FBGs

In this study, a novel rectangular polymer single-mode optical fiber for femtosecond (fs) laser-inscribed fiber Bragg gratings (FBGs) is proposed and demonstrated. The cylindrical geometry of the widely used circular fiber elongates the fs laser beam along the fiber axis, resulting in reduced laser intensity and requiring index-matching oil immersion during FBG inscription. However, the flat geometry and negligible surface roughness of the featured fiber significantly diminish this lensing distortion and eliminate the need for oil immersion, thereby resulting in optimal focusing of the laser beam, permitting direct and efficient inscription of FBGs within the optical fiber. The core and cladding of the rectangular fiber were fabricated using two different grades of ZEONEX, a cyclo olefin polymer, which have slightly different refractive indices. The similar glass transition temperature for core and cladding simplifies the fiber drawing process, and a rectangular single-mode optical fiber with dimensions of 213 μm × 160 μm and core diameter of 9.4 μm was fabricated using an in-house fiber drawing facility. A second harmonic (520 nm) fs laser beam was used to successfully inscribe a 2-mm-long FBG in the rectangular fiber within a few seconds with a point-by-point technique. The inscription of a single FBG leads to the excitation of higher order FBG peaks at 866.8 and 1511.3 nm, corresponding to widely used wavelength bands in fiber optic sensing. The strain and temperature sensitivities of the FBG were measured to be 7.31 nm/ % ε ( 0.731 pm / με ) and 10 pm/°C, and 12.95 nm / % ε ( 1.29 pm / με ) and 15 pm/°C at 866.8 nm and 1511.3 nm, respectively.