Fabrication Of Bragg Gratings Research Articles

Fabrication of high temperature chirped FBG by thermal stretching of regenerated FBG

This paper presents an innovative method of high temperature sustainable chirped fiber Bragg grating (FBG) fabrication from the uniform regenerated FBG or their combination written by a single uniform phase mask. A thermal stretching technique is employed that modifies the grating structure of uniform FBGs, resulting in chirped FBGs capable of withstanding high temperatures without compromising their sensing capabilities. Thermal regeneration of uniform FBG was carried out at 850oC by applying step annealing schedule. Thermal stretching of single regenerated FBG and dual regenerated FBG was carried out for the fabrication of high temperature chirped FBG. For both approaches, the FBG was stretched by applying strain ~1200 µε along the length of the grating. In the single FBG approach, the chirped grating with reflection intensity 4500 (A.U.) and bandwidth ~ 7.1nm was fabricated by applying thermal gradient from 860oC to 900oC across the length of strained thermally regenerated FBG. In double grating approach, the chirped FBG with reflection intensity 6000 (A.U.) and bandwidth 12.7nm was fabricated. The fabricated chirped FBG can sustain temperature upto 900 oC. This innovative method not only extends the operating range of chirped FBGs to high temperatures but also opens up new possibilities for fabricating chirped FBGs of longer length and bandwidth using low-cost uniform FBGs.

Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 °C–570 °C for 10–120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the near-infrared and visible bands.

Bragg grating in a flexible and stretchable coreless polymer optical fiber.

In this Letter, we propose and experimentally demonstrate fiber Bragg grating (FBG) fabrication in a flexible and stretchable coreless polymer optical fiber. The flexible polymer optical fiber is prepared with polydimethylsiloxane (PDMS). Femtosecond laser direct writing and slit beam shaping are used to form periodic grating structures in the fiber. The fabricated FBG exhibits a large strain measurement range and a blueshift response to temperature. Moreover, it offers low humidity sensitivity due to its low permeability toward water vapor. Taking advantage of the unique sensing performances of the PDMS fiber, the proposed FBG has considerable advantages over the traditional silica FBG devices for strain and temperature sensing.

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.

Bragg gratings and BIO-Bragg-gratings in tapered optical fibers

Tapered optical fibres are well-established devices for different applications, in order to exploit nonlinear effects, manage dispersion, excite azimuthal resonances in microresonators (so-called Whispering Gallery Modes). Also, the micro- or nanofibres guide optical-fields with large evanescent fields. In this talk, we will focus on the combination of tapers and Bragg gratings to perform novel optical devices. We will present two applications: the first, the fabrication of single-mode Bragg gratings in commercial multimode or few-mode tapered fibres by means of UV-photoinscription. The result is a grating that exhibits a single-mode reflection band and low insertion loss. The second application is the fabrication of Bio Bragg Gratings in micrometric tapers of single-mode fibres. In this case, the Bragg Grating is formed by a periodic pattern of biomolecules microstamped on the surface of the taper waist. As the molecules binds to its specific antibody, the reflectivity of this Bio Bragg Grating will increase, allowing quantification of the antibody concentration.

Ultrafast Laser Processing of Optical Fibers for Sensing Applications.

A review of recent progress in the use of infrared femtosecond lasers to fabricate optical fiber sensors that incorporate fiber Bragg gratings (FBG) and random fiber gratings (RFG) is presented. The important advancements in femtosecond laser writing based on the phase mask technique now allow through-the-coating (TTC) fabrication of Bragg gratings in ultra-thin fiber filaments, tilted fiber Bragg gratings, and 1000 °C-resistant fiber Bragg gratings with very strong cladding modes. As an example, through-the-coating femtosecond laser writing is used to manufacture distributed fiber Bragg grating sensor arrays for oil pipeline leak detection. The plane-by-plane femtosecond laser writing technique used for the inscription of random fiber gratings is also reviewed and novel applications of the resultant devices in distributed temperature sensing, fiber lasers and fiber laser sensors are discussed.

Fabrication of PDMS-based volume Bragg gratings by stitching of femtosecond laser filament

This work demonstrates the fabrication of poly-(dimethylsiloxane) (PDMS)-based volume Bragg gratings (VBGs) by stitching of femtosecond laser filament. The refractive index change induced by femtosecond laser pulses is low (orders of 10−4 to 10−3) in PDMS. Therefore, the VBG thickness is the key parameter to fabricate high-efficiency VBGs in PDMS. With the intention of increasing diffraction efficiency, this paper presents fabrication of multi-layered PDMS-based VBGs. We produce VBGs with 10 μm line pitch using a femtosecond pulse laser (100 fs pulse width, 1 kHz repetition frequency). The modified zone thickness can be controlled by arranging the relative displacement of filament at different depths and the distances between layers. High-efficiency VBGs were created by depth multiplexing of 350 μm long filamentary modification in PDMS. Diffraction efficiency of 87.2% was realized by forming 0.63 mm grating thickness.

Inscription of Bragg gratings in undoped PMMA mPOF with Nd:YAG laser at 266 nm wavelength.

We present the first Bragg gratings fabricated in two, three and five rings undoped PMMA microstructured polymer optical fibres (mPOFs) with relative low cost 266 nm Nd:YAG laser in the 850 nm region. The fibers were connectorised with commercial ferrules for easy coupling with silica patch cables. Temperature, humidity and strain sensitivities are measured and also the impact of ring structure and the diameter of POF on the characterization measurements are studied for potential applications. We also analyzed the effect of the number of hexagonal rings structure in gratings fabrication, noticing that larger number of rings lead to more difficulties to obtain strong gratings, where we consider this performance due to the scattering effects. We demonstrate Bragg gratings fabrication in 5-rings structure mPOF after 6 min by using 266 nm Nd:YAG laser whereas no Bragg gratings have been fabricated so far using 325 nm He-Cd laser system. Up to 30 dB relative reflected power gratings are obtained in two rings mPOF, showing good time stability and promising results for undoped mPOF applications.

Wide Range Refractive Index Measurement Based on Off-Axis Tilted Fiber Bragg Gratings Fabricated Using Femtosecond Laser

In this paper, we propose an off-axis tilted fiber Bragg grating (OTFBG) in which the grating structure is limited to one side of the fiber core cross section. The tilted grating planes are fabricated using a femtosecond laser with a phase mask. The wavelength range of the OTFBG cladding mode resonance could be twice that of a conventional tilted fiber Bragg grating with the same tilt angle. The resonance intensity of the OTFBG core mode is significantly enhanced. We also fabricate a serial-double-angle OTFBG (SOTFBG) and a parallel-double- angle OTFBG (POTFBG). The wavelength ranges of their cladding mode resonances are effectively extended. The SOTFBG and POTFBG with tilt angles of 5° and 10° possess a wavelength range of cladding mode resonances of approximately 120 nm. They are capable of measuring the surrounding refractive index (SRI) with a refractive index (RI) sensible range from 1.24 to 1.45. A significantly wide range of cladding mode resonances is achieved by an SOTFBG with tilt angles of 10° and 15°, which is as wide as 215 nm. The RI sensible range of this SOTFBG is estimated to be from approximately 1.00 to 1.41, thereby making it suitable for SRI measurements in both gaseous media and aqueous solutions.

Fabrication of high-precision reflective volume Bragg gratings.

Reflecting Bragg gratings (RBGs) recorded in photo-thermo-refractive (PTR) glasses have been widely used in narrowing and stabilization of the laser emission spectrum. As the center wavelength of RBGs determines the final output wavelength of lasers, it is necessary to carefully control the center wavelength of RBGs during the fabrication process. In this paper, the fabrication process of high-precision RBGs was investigated. We developed a two-step method and demonstrated both theoretically and experimentally that it is effective and can be used to guide the fabrication process of high-precision RBGs. The experimental results show that the center wavelength of the fabricated RBG deviates from the target center wavelength within ±10 pm.

Point-by-Point Femtosecond-Laser Inscription of 2-<italic>μ</italic>m-Wavelength-Band FBG Through Fiber Coating

We propose a point-by-point inscription method for fiber Bragg grating (FBG) fabrication using a femtosecond pulsed laser focused through a fiber polyimide coating, realizing a 2- μ m-wavelength-band FBG. A femtosecond laser acts as the inscription laser, focused by a 63× oil lens. A first-order FBG was fabricated with a grating interval of 700 nm. The 15- μ W pulsed laser was focused directly on the fiber core through the polyimide coating and cladding. A first-order FBG for 2004.8-nm light with more than 56% reflectances was fabricated. High wavelength sensitivity and linearity of the FBG were demonstrated by temperature and strain-sensing testing. The first-order FBG of 2004.8 and 2000.2 nm resonant wavelengths could be realized simultaneously through parallel inscription method in 3000 μ m long grating area.

Line-by-line fiber Bragg grating fabrication by femtosecond laser radiation

We report on the fabrication of second order and fourth order fiber Bragg gratings (FBGs) in standard single mode telecom fiber through the polymer coating made line-by-line technique using femtosecond laser radiation. Measurement of spectral characteristics were performed in the reflection mode of the input signal of tunable laser.

Deterministic fabrication of circular Bragg gratings coupled to single quantum emitters via the combination of in-situ optical lithography and electron-beam lithography

In the present work, we investigate the coupling of deterministically pre-selected In(Ga)As/GaAs quantum dots (QDs) to low Q circular Bragg grating cavities by employing a combination of state-of-the-art low-temperature in-situ optical lithography and electron-beam lithography. The spatial overlap between the cavity mode and quantum emitter is ensured through the accurate determination of the QD position via precise interferometric position readout. Simultaneously, the high precision of the electron-beam lithography is exploited for the cavity fabrication. In order to optimize the spectral overlap, prior to cavity fabrication, finite-difference time-domain simulations are performed to estimate the spectral position of the cavity mode. A Purcell factor of 2 together with an increased count rate is reported for a deterministically positioned cavity where the emission line is detuned by 3.9 nm with respect to the cavity mode. This non-negligible Purcell enhancement for large detunings and, thus, the large range where this can be achieved points towards the possibility of using the cavity for the simultaneous enhancement of spectrally distinct transitions from the same quantum emitter located spatially in the mode maximum. Furthermore, investigations on the bending of the cavity membrane and the effects on the cavity mode and QD emission are presented.

Fiber Bragg grating fabrication by femtosecond laser radiation

The paper presents the results of fiber Bragg gratings fabrication by femtosecond laser radiation using point-by-point and line-by-line inscription methods. The approach makes it possible to fabricate the second and higher diffraction orders fiber Bragg gratings, which can be used as sensitive elements of fiber-optic sensors.

Multi-wavelength FBG based on thermal diffusion and phase mask techniques

A short multi-wavelength fiber Bragg grating (MW-FBG) fabrication method is proposed and demonstrated. The MW-FBG fabrication method includes making expanded core in the photosensitive single mode fiber (P-SMF) by the discharge-based thermal diffusion technique and writing FBGs on the pre-processed P-SMF through ultraviolet exposure technique. The effect of the arc discharge is to make the dopant in the core of the P-SMF diffuse to the cladding, which can reduce the refractive index of the core at the discharged region. As a result, a single exposure through one phase mask can produce a FBG with multiple resonant wavelengths. The strain and temperature responses of a two-wavelength FBG are experimentally investigated. Results show that the multiple wavelengths of the FBG have almost the same responses to the strain or temperature. The strain sensitivities of the two wavelengths of the FBG are 0.9025 pm/με and 0.9101 pm/με, respectively, and the temperature sensitivities are 10.55 pm/°C and 10.33 pm/°C, respectively. Such MW-FBGs have potential applications in the case of small size and high spatial resolution measurement.

Phase-shifted fiber Bragg gratings fabrication method

In this paper, we showed a technique for the formation of phase-shifted fiber Bragg gratings. The analysis of the efficiency of the obtained structures was carried out with the help of an optical spectrum analyzer with the subsequent calculation of the full width at half maximum of a bandpass in reflection spectrum. Fiber Bragg gratings inscription was performed on the Talbot interferometer, the KrF excimer laser system was used as a UV radiation source, and a phase shift was introduced by the electrical discharge of an arc fusion splicer. The experimental dependence between the length of the obtained structure and the full width at half maximum of a bandpass in reflection spectrum of a phase-shifted grating was presented, the value of the last parameter varies in the range from 24 pm to 96 pm.

Micro-scale fiber-optic force sensor fabricated using direct laser writing and calibrated using machine learning.

Fiber-optic sensors have numerous existing and emerging applications spanning areas from industrial process monitoring to medical diagnosis. Two of the most common fiber sensors are based on the fabrication of Bragg gratings or Fabry-Perot etalons. While these techniques offer a large array of sensing targets, their utility can be limited by the difficulties involved in fabricating forward viewing probes (Bragg gratings) and in obtaining sufficient signal-to-noise ratios (Fabry-Perot systems). In this article we present a micro-scale fiber-optic force sensor produced using direct laser writing (DLW). The fabrication entails a single-step process that can be undertaken in a reliable and repeatable manner using a commercial DLW system. The sensor is made of a series of thin plates (i.e. Fabry-Perot etalons), which are supported by springs that compress under an applied force. At the proximal end of the fiber, the interferometric changes that are induced as the sensor is compressed are read out using reflectance spectroscopy, and the resulting spectral changes are calibrated with respect to applied force. This calibration is performed using either singular value decomposition (SVD) followed by linear regression or artificial neural networks. We describe the design and optimization of this device, with a particular focus on the data analysis required for calibration. Finally, we demonstrate proof-of-concept force sensing over the range 0-50 μN, with a measurement error of approximately 1.5 μN.

Fabrication of tunable chirped mPOF Bragg gratings using a uniform phase mask.

We demonstrate chirped Bragg gratings fabrication in doped microstructured tapered polymer fibers by using a uniform phase mask. The use of high photosensitive benzyl dimethyl ketal (BDK) doped core fiber allows to obtain chirped Bragg gratings by means of a single krypton fluoride laser pulse. The stability of the gratings has been confirmed and the strain and temperature sensitivity measurements demonstrate their tunable properties. Finally, different tapered profiles have been implemented in order to show the potentiality of this fabrication technique in polymer optical fibers.

Bragg resonance in microfiber realized by two-photon polymerization.

A new method for microfiber Bragg gratings (μ-FBGs) fabrication by means of two-photon polymerization in photosensitive resin is reported. Such polymerized μ-FBGs were cured along with the surface of microfibers without any damage or distortion to the substrate. The laser intensity was optimized to improve the spectral properties of the polymerized gratings. The refractive index measurement was performed and the maximum sensitivity obtained is ~207 nm/RIU at the refractive index value of 1.440 with the fiber diameter being 1.7 μm. This work opens a new idea for optical structure integration and further optical functionality integration.

Fabrication of Bragg Gratings in Random Air-Line Clad Microstructured Optical Fiber

Fiber Bragg gratings (FBGs) are written using an infrared (IR) femtosecond laser and a phase mask into pure silica microstructured optical fiber having a cladding region comprised of random air-lines (RAL). Scanning electron microscopy measurements of the IR irradiated RAL fiber samples shows the presence of subwavelength nanograting structures associated with thermally stable type II Bragg gratings. High temperature annealing testing shows that although the RAL FBGs have a high degree of stability even at 1000 °C, they are less stable than similar FBGs made in single mode telecom fiber (SMF-28).