Inscription Of Bragg Gratings Research Articles

Femtosecond laser writing of fiber Bragg gratings using the phase mask technique: a geometrical optics analysis based on the Bravais refractive index

Material modification is produced inside silica-based optical fibers of different diameters using tightly focused near-infrared (central wavelength at 800 nm) femtosecond laser pulses and the phase mask technique which is often employed for laser inscription of fiber Bragg gratings. 1st-, 2nd-, and 3rd-order phase masks designed for the operation at 800 nm are used in the experiments. The inscription is performed at different distances from the fiber’s front surface by translating the focusing cylindrical lens along the laser beam propagation direction. The results show that the material modification produced by means of the 2nd- and 3rd-order phase mask can be positioned at any predetermined distance from the fiber’s front surface. In contrast, when the 1st-order mask is used for laser writing, the maximum distance from the fiber’s front surface at which material modification can be produced is limited and determined by three main parameters: the diffraction angle of the phase mask, the refractive index of the fiber and the diameter of the fiber.

Cladding-pumped laser and amplifier for E- and S-bands based on multimode bismuth-doped graded-index fibers: toward "watt-level" output power.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

Fiber Bragg grating inscription assisted by a spatial light modulator.

In this Letter, we proposed a new technique for point-by-point fiber Bragg grating (FBG) writing in a static fiber by using a spatial light modulator to control the position of the focal point inside the fiber core. Various types of short-length FBGs (uniform, phase-shifted, and apodized) were demonstrated by this inscription technique. Moreover, the capability to tailor the transverse dimension of a grating pitch from a single point to more complex shapes, such as a wide plane covering a whole fiber core or a transverse ring, was shown.

Femtosecond Laser Plane-by-Plane Inscription of Bragg Gratings in Sapphire Fiber

Femtosecond Laser Plane-by-Plane Inscription of Bragg Gratings in Sapphire Fiber

Рефрактометрический детектор белков плазмы крови на основе наклонной волоконной брэгговской решетки с функциональным покрытием из антител

The article demonstrates the results of the development of a selective fiber optic biosensor based on tilted fiber Bragg grating. The underlying principle of the sensor involves shifts in the transmission spectrum of tilted grating, which transforms in accordance with the changes in the composition of the external environment. Optical signal propagating along the fiber couples with a tilted grating resulting in a number of cladding modes which form an evanescent field of the sensor directly interacting with the external environment. Potentially high sensitivity to the refractive index of the environment opens up prospects of using such structures as high-precision sensors of solution concentrations; however, for applications in the field of biomedicine, selective sensitivity is critically important, providing unambiguous detection of individual target components in a multicomponent composition. To ensure the selective sensitivity of the sensors presented in this work, functional coatings of antibodies against two types of human blood protein molecules were used in their design. The paper describes a technique for creating experimental samples of biosensors, involving inscription of fiber Bragg gratings and deposition of functional antibody-based coatings onto the surface of the fiber. Experimental results are presented highlighting problems occurring during detection with a single sensor. As a solution, the work proposes a differential measurement scheme involving two sensors modified against two different proteins. According to the results of studies during which fibrinogen and D-dimer were used as target proteins, the selective detection of the studied proteins in concentrations of up to 0.1 mg/ml is clearly demonstrated, which was achieved through the use of a differential measurement scheme.

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review.

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

Temperature sensing of the brain enabled by directly inscribed Bragg gratings in CYTOP polymer optical fiber implants

Brain temperature is a vital physiological parameter that has a great effect on metabolic processes, enzymatic activity, neurotransmitter function, blood flow regulation, neuroprotection, and cognitive performance. In this framework, the development of accurate and reliable brain temperature measurement tools is crucial in brain-related treatment and research, such as neurosurgery, therapeutic hypothermia, and the understanding of brain function and pathologies. Here, we developed the first large-core flexible low optical loss CYTOP polymer optical fiber (POF)-based brain temperature probe operating in the telecommunication spectral range. The temperature measurements were achieved by detecting the reflected spectrum from a fiber Bragg grating (FBG) directly inscribed at the tip of the POF using femtosecond pulses. A fluorinated ethylene propylene (FEP) tube was thermally drawn and used as a sleeve around the FBG structure to eliminate the cross-sensitivity with humidity and microstrain perturbations. The assembled POF implant has a sensitivity of 14.3 pm/°C. The local temperature of the cerebral cortex, corpus callosum, and striatum in a rat brain was measured in vivo. The results indicate that the deep brain regions have higher temperature than the top cortical ones with a relatively linear relationship between the brain structure depth and its temperature. In addition, the rectal body core temperature was detected in parallel with the brain temperature measurement to further validate the developed device. We believe that the presented CYTOP POF-based implantable temperature probe opens the way towards the development of flexible and stable tools for accurate brain temperature recording where large-core fiber-based neural devices are required.

Three-dimensional femtosecond laser inscription of type a-based high-efficiency first-order waveguide Bragg gratings

A novel type of waveguide Bragg grating (WBG) is demonstrated based on femtosecond laser-induced Type A refractive index modifications, namely based of the photochemistry of silver species in a specialty ortho-phosphate glass matrix. First-order WBGs are reported in the near-infrared and down to 736 nm in the visible. Relative transmission measurements with a 500 µm long WBGs lead to narrow-bandwidth attenuations (sub-nm spectral FWHM) from 2.29 dB to 6.25 dB for periods from 240 nm to 280 nm, respectively. The corresponding estimated backward coupling coefficients show high values from 1.66 mm-1 up to 2.69 mm-1. Additionally, we report on a true 3D helix-shaped WBG that shows an even stronger relative attenuation of 10.3 dB for a 500 µm long WBG, equivalently corresponding to a backward coupling coefficient of 3.7 mm-1. These novel results pave the way for new silver-based laser-inscribed integrated photonic devices, among which the combination of Bragg gratings to form active/passive optical resonators, but also the direct inscription of WBG at the glass interface for enhanced sensing applications.

Femtosecond laser-induced Bragg gratings in silica-based fibers for harsh environment sensing

The technique of femtosecond laser-induced inscription of fiber Bragg gratings creates a structure in the optical fiber that can be used effectively as a sensor especially when deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices can be made that are suitable for sensing applications involving high temperature, pressure, ionizing radiation, and strain. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation, and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

Spectrum collapse in a 7-core Yb-doped fiber laser with an array of fs-inscribed fiber Bragg gratings.

Femtosecond inscription of fiber Bragg gratings (FBGs) in each core of a cladding-pumped seven-core Yb-doped fiber enables efficient (≈70%) 1064-nm lasing in a robust all-fiber scheme with ≈33 W power, nearly the same for uncoupled and coupled cores. However, the output spectrum is quite different: without coupling, seven individual lines corresponding to the in-core FBG reflection spectra sum up into a broad (0.22 nm) total spectrum, whereas the multiline spectrum collapses into a single narrow line at strong coupling. The developed model shows that the coupled-core laser generates coherent superposition of supermodes at the wavelength corresponding to the geometric mean of the individual FBG spectra, whereas the generated laser line broadens, with a power (0.04-0.12 nm) like the single-core mode of a seven-times larger effective area.

Enhanced backscatter and unsaturated blue wavelength shifts in F-doped fused silica optical fibers exposed to extreme neutron radiation damage

Amorphous fused silica (a-SiO2) optical fibers, with and without inscribed Bragg gratings, were interrogated using infrared (∼1550 nm) optical backscatter reflectometry during ∼25 days of intense neutron irradiation to a fast (energy > 0.1 MeV) neutron fluence of ∼1021 n/cm2, or ∼1.5 atomic displacements per atom. The reflected light amplitudes in Ge-doped core telecommunications fiber dropped below detection limits (>15 dB attenuation) within 3 days of irradiation (∼1020 fast n/cm2). Amplitudes from a pure silica core, F-doped silica cladding fiber reached equilibrium levels ∼1.5–2 dB higher than pre-irradiation values, whereas Bragg gratings inscribed in the same fiber using a femtosecond laser (point-by-point method) suffered > 45 dB attenuation. Blue wavelength shifts were initially consistent with previous radiation-induced compaction models but increased linearly with increasing neutron fluence (no evidence of saturation) beyond 1020 n/cm2 and exceeded 0.6%, which corresponds to >1,000 °C drift if used to measure temperature changes.

Bragg Gratings in ZEONEX Microstructured Polymer Optical Fiber With 266-nm Nd:YAG Laser

We present the first fiber Bragg gratings inscription in ZEONEX-480R microstructured polymer optical fiber by employing the fourth harmonic (266 nm) of a Nd:YAG laser system. The gratings were produced in the L-band region by the phase mask method, in 270 μm diameter polymer optical fiber samples connectorized with ferrule connectors. The results of 3 produced gratings are reported, showing good stability over 99 days. The sensing response to strain, temperature and humidity was analyzed and confirmed the good stability and performance of the inscribed gratings in this polymer fiber. These results are promising from the point of view that gratings can be produced in ZEONEX-480R microstructured polymer optical fiber with the 266 nm wavelength, which is an alternative and viable cost-effective solution. Additionally, the production of gratings in this type of polymer fiber, with a relative high diameter, shows great potential for a wide variety of sensing applications.

Femtosecond writing of intra-phase-mask volume Bragg gratings.

In this Letter, we report the first, to the best of our knowledge, femtosecond inscription of volume Bragg gratings (VBGs) directly inside phase-mask substrates. This approach showcases enhanced robustness as both the interference pattern generated by the phase mask and the writing medium are inherently bonded together. The technique is employed with 266-nm femtosecond pulses loosely focused by a 400-mm focal length cylindrical mirror inside fused-silica and fused-quartz phase-mask samples. Such a long focal length reduces the aberrations induced by the refractive-index mismatch at the air/glass interface which allows to inscribe a refractive-index modulation simultaneously over a glass depth reaching 1.5 mm. A decreasing gradient of the modulation amplitude from 5.9 × 10-4 at the surface to 1 × 10-5 at a 1.5-mm depth is observed. This technique has therefore the potential of increasing significantly the inscription depth of femtosecond-written VBGs.

Femtosecond laser line-by-line tilted Bragg grating inscription in single-mode step-index TOPAS/ZEONEX polymer optical fiber.

In this Letter, we demonstrate 8°-tilted fiber Bragg grating (TFBG) inscription in single-mode step-index TOPAS/ZEONEX polymer optical fibers (POFs) using a 520 nm femtosecond laser and the line-by-line (LbL) writing technique. As a result of the tilt angle and the fiber refractive index, a large spectral range of cladding mode resonances covering 147 nm is obtained. The evolution of the transmitted spectrum is analyzed as a function of the surrounding refractive index (SRI) in a large range from 1.30 to 1.50. The cutoff cladding mode shows a refractive index sensitivity of 507 nm/RIU (refractive index unit). For single-resonance tracking near the cutoff mode, the sensitivity is at least 6 nm/RIU, depending on the exact wavelength position of the cladding modes. The main originality of our work is that it produces, for the first time, to the best of our knowledge, a TFBG in POF that operates in the refractive index range of aqueous solutions. The sensing capability for a large range of refractive index values is also relevant for (bio)chemical sensing in different media.

Beam-shaping device-free femtosecond laser plane-by-plane inscription of high-quality FBGs

Fiber Bragg gratings (FBGs) fabricated by femtosecond laser point-by-point inscription suffer from insertion loss (IL) and polarization-dependent loss (PDL) which degrade their performances in optical communication and sensing. To solve this problem, we propose a new method to realize plane-by-plane (Pl-b-Pl) inscription of high-quality fiber Bragg gratings by using high refractive index (RI) oil. The focal volume of the laser beam can be “stretched” due to the dispersive effect of the high RI oil along the transverse direction of the fiber. Compared to the reported femtosecond laser Pl-b-Pl grating inscription method, the high RI oil for Pl-b-Pl grating inscription is a simple and efficient method without additional beam-shaping devices (e.g., optical slits and spatial light modulators). Using the proposed method, a high-quality FBG can be inscribed, which has a high reflectivity of 98.1% (the Bragg resonance attenuation of 17.23 dB), and a low short-wavelength loss of 0.84 dB, and a low insertion loss of 0.23 dB. Most importantly, the PDL of the grating is only 0.12 dB.

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.

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 &gt; 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.