Fabrication Of Long-period Gratings Research Articles

Wavelength and sensitivity tunable long period gratings fabricated in fluid-cladding microfibers

We report the fabrication of long period gratings in fluid-cladding microfibers by directly focusing a femtosecond laser beam on the microfibers surface to induce periodical modification a long one side of the microfibers. A long period grating is fabricated in a water-cladding microfiber with a diameter of ~ 5 μm, which demonstrates a resonant attenuation of 28.53 dB at wavelength of 1588.1 nm with 10 pitches. When water cladding is changed to be refractive index oil of n= 1.33 and alcohol solution with concentration of 5%, the resonance wavelength shifts to 1575.1 nm with resonant attenuation of 24.91 dB and 1594.1 nm with resonant attenuation of 35.9 dB, respectively. The long period grating demonstrates different temperature sensitivities of–0.524 nm/°C,–0.767 nm/°C and–1.316 nm/°C for water, alcohol solution and refractive index oil cladding microfibers, respectively, which means the alterable liquid cladding allows the availability of tunable wavelength and sensitivity. The fluid-cladding protects the microfibers from external disturbance and contamination and allows more flexibility in controlling the transmission property and sensing characteristics of long period gratings, which can be used as fiber devices and sensors for chemical, biological, and environmental applications.

Arc-Induced Long Period Gratings in Erbium-Doped Fiber

In this paper, we report about the fabrication of long period gratings (LPGs) directly into an Erbium-doped fiber, by using the electric arc discharge technique. The attention is focused on the writing process and the resulting properties, by considering gratings with different periods and measuring their spectra over a wide wavelength range. The LPGs show high order attenuation bands with tunable resonance wavelengths and depths up to 30 dB, while the lengths of the final devices are in range 20-45 mm. The polarization-dependent loss of these LPGs is also measured, for the first time in this kind of fiber. As further novelty, the influence of surrounding refractive index, applied strain, and temperature is investigated and discussed in comparative manner. Based on the achieved results, this fabrication procedure can be adapted to a specific application, for example in optical communications, signal processing, and sensing fields.

The effect of UV irradiation duty cycle on the 2nd harmonic coupling efficiency in optical fiber long period gratings

Long period gratings (LPGs) as a sensing platform have the potential for multi-parameter measurement through the utilization of 2nd order coupling resonance bands. Although the current literature has produced LPGs with 2nd order resonance bands, the fabrication parameters required to generate these features have not been elucidated. Here, using UV irradiation via the point-by-point method, it is shown that by varying the duty cycle it is possible to fabricate LPGs that exhibit 2nd, 3rd, and 4th order coupling resonance bands. Fabrication of LPGs with a 25% or 75% duty cycle produces distinct 2nd order resonance bands, which are not observed when a 50% duty cycle is adopted.

Fabrication of Long Period Gratings by Periodically Removing the Coating of Cladding-Etched Single Mode Optical Fiber Towards Optical Fiber Sensor Development.

Here, we present a novel method to fabricate long period gratings using standard single mode optical fibers (SMF). These optical devices were fabricated in a three-step process, which consisted of etching the SMF, then coating it with a thin-film and, the final step, which involved removing sections of the coating periodically by laser ablation. Tin dioxide was chosen as the material for this study and it was sputtered using a pulsed DC sputtering system. Theoretical simulations were performed in order to select the appropriate parameters for the experiments. The responses of two different devices to different external refractive indices was studied, and the maximum sensitivity obtained was 6430 nm/RIU for external refractive indices ranging from 1.37 to 1.39.

Arc-Induced Long Period Gratings from Standard to Polarization-Maintaining and Photonic Crystal Fibers.

In this work, we report about our recent results concerning the fabrication of Long Period Grating (LPG) sensors in several optical fibers, through the Electric Arc Discharge (EAD) technique. In particular, the following silica fibers with both different dopants and geometrical structures are considered: standard Ge-doped, photosensitive B/Ge codoped, P-doped, pure-silica core with F-doped cladding, Panda type Polarization-maintaining, and Hollow core Photonic crystal fiber. An adaptive platform was developed and the appropriate “recipe” was identified for each fiber, in terms of both arc discharge parameters and setup arrangement, for manufacturing LPGs with strong and narrow attenuation bands, low insertion losses, and short length. As the fabricated devices have appealing features from the application point of view, the sensitivity characteristics towards changes in different external perturbations (i.e., surrounding refractive index, temperature, and strain) are investigated and compared, highlighting the effects of different fiber composition and structure.

Arc-Induced Long Period Gratings in Phosphorus-Doped Fiber

Phosphorus-doped fibers have been reported and discussed in extensive works, as well as the radiation effects on their guiding features in different spectral ranges. However, the assessment of a technique enabling the fabrication of grating devices in this fiber is still an open issue. Here, for the first time to the best of our knowledge, we report on the fabrication of long period gratings (LPGs) in P-doped optical fiber by means of electric arc discharge technique (EAD). LPGs having periods of $500~\mu \text{m}$ and of $700~\mu \text{m}$ were presented, achieving the coupling up to LP06 cladding mode, attenuation bands with depth up to 35 dB, power losses lower than 0.5 dB, and devices length less than 20 mm. The results being also compared with numerical simulations. Moreover, the sensitivity characteristics to surrounding refractive index (SRI) and temperature changes of LPGs in P-doped fibers were also investigated. The SRI sensitivity of LP06 for SRI = 1.437 was about −478 nm/RIU, slightly higher than in standard fiber. Whereas, differently from standard Ge-doped fibers, a negative temperature response was found (−116 pm/°C) opening the possibility of new applications, for example for temperature compensation solutions.

Periodic micro-structures in optical microfibers induced by Plateau-Rayleigh instability and its applications

A periodic micro-structure on optical microfibers induced by Plateau-Rayleigh instability (PRI) was investigated and a potential application for long period gratings (LPGs) fabrication was given. The linear relation between the average periods of micro-structures and the diameters of optical microfibers was demonstrated first. By brushing a glass rod with a Teflon droplet suspended at the end tip along microfibers, a continuous film of Teflon was formed at once, then the film broke up into a series of periodic droplets due to PRI. Periodic Teflon nodes were left after the evaporation of the solvent. A LPG structure based on polymer was finally formed by this method on a microfiber with a diameter of 5.5 μm. An attenuation transmission dip of 15 dB around 1447 nm was achieved. Investigation of the strain and temperature response characteristics of the grating presented a strain sensitivity of -2.5 pm/με and a temperature sensitivity of -157 pm/°C. The technique proposed here provides a versatile technique for polymer-based LPGs fabrication. Benefiting from the high sensitivities, LPGs based on numerous polymers fabricated in this way could have potential applications in optical and biological sensing.

Spatially Arrayed Long Period Gratings in Multicore Fiber by Programmable Electrical Arc Discharge

Based on electrical arc discharges mechanisms, we inscribed spatially arrayed long period gratings (LPGs) into a heterogeneous seven-core fiber. The LPG fabrication platform was built upon the commercial fiber fusion splicer with a self-developed real-time control software. With the help of homemade fan-in/fan-out multiplexer, the high-quality grating spectra in seven cores are measured. The largest resonant dip of -42 dB and minimum insertion loss of 0.5 dB are achieved in the central core. To demonstrate potentials of the spatially diversified gratings, we measured the strain and temperature simultaneously with the spatial response matrix to eliminate the cross sensitivity. Accurate sensing results have been achieved with relative errors less than 4%.

Experimental Study of the Refractive Index Sensitivity in Arc-induced Long Period Gratings

This paper presents an experimental study of the sensitivity characteristics to the surrounding refractive index (SRI) in arc-induced long period gratings (LPGs), in order to outline their dependence over the fabrication parameters. Several LPGs were fabricated, with spectral features that are appealing for chemical sensing applications, e.g., negligible power losses, bands depth greater than 20 dB, and total length smaller than 35 mm. In addition, low spatial periods Λ, which are close to the limit of the arc-based fabrication technique, were selected in order to excite high-order cladding modes. In particular, the period was chosen in range 350–500 μm to focus attention on the same cladding modes for the gratings under investigation. Accordingly, a wide experimental analysis was then carried out to investigate the dependence of the sensitivity to SRI changes on the period in order to derive design criteria for LPG fabrication. In the wavelength range 1100–1700 nm and taking into consideration the attenuation bands related to the LP05 and LP06 cladding modes, an SRI sensitivity enhancement up to one order of magnitude can be achieved by proper selection of the grating period.

Arc-Induced Long Period Gratings in Polarization-Maintaining Panda Fiber

In this letter, we report about the fabrication of long period gratings (LPGs) in a polarization-maintaining Panda optical fiber, by means of the electric arc discharge technique. This is the first time that LPGs are arc-induced in a polarization maintaining fiber. The dependence of the LPG’s spectral features on the input light polarization alignment is illustrated, by considering the two principal polarization states, i.e., along the fast and slow axes of the Panda fiber. Moreover, an investigation about the surrounding refractive index sensitivity, as a function of the two principal polarization states, is also reported with interesting results.

Induced nanoscale changes with low temperature annealing inside composite optical fibres probed by strain-insensitive long period gratings

Nanoscale glass milling with mild thermal annealing is proposed and shown to occur within a tri-material composite optical fibre. Evidence of glass relaxation with annealing is inferred directly through measured diameter changes in the core and inner cladding of the long period gratings (LPGs) (Δϕcore = (0.13 ± 0.05) μm; Δϕinner cladding = -(0.5 ± 0.2) μm) using scanning electron microscopy (SEM) before and after annealing at T ~300 °C for 1 hour. Large reductions in the magnitudes of both the temperature and strain coefficients are observed after annealing. The temperature sensitivity drops from dλ/dT = -(111.2 ± 2.4) pm/K to dλ/dT = (1.3 ± 0.3) pm/K and the strain sensitivity decreases from dλ/de = -(0.11 ± 0.13) pm/μe to dλ/de = (0.02 ± 0.11) pm/μe. The fabrication of LPGs using 193 nm radiation is shown to produce measurable increases in the core dimensions within tri-material composite fibres whereas no changes are observed under similar conditions for commercial bi-material single mode fibre.

Fabrication of long-period fiber gratings through periodic ablation using a focused CO_2-laser beam

Fabrication of long period gratings in optical fibers through periodic ablation using a focused CO2 laser beam has been studied. During the thermal ablation process most of the energy is absorbed a ...

Inscription of long period gratings using an ultraviolet laser beam in the diffusion-doped microstructured polymer optical fiber.

We show that diffusion of azobenzene from the solution in methanol into a cladding of a polymer fiber facilitates fabrication of long period gratings by the use of a He-Cd focused laser beam. We have measured a diffusion rate into PMMA cladding of the microstructured fibers annealed in advance at different temperatures and showed that the diffusion rate is strongly affected by temperature treatment of the fiber. We have also investigated an impact of the azobenzene diffusion on fiber spectral loss and cladding surface quality. Furthermore, we have examined a temporal stability of the fabricated long period gratings and their response to temperature and tensile strain.

Characterization of refractive index change and fabrication of long period gratings in pure silica fiber by femtosecond laser radiation

Abstract Ultrafast laser induced refractive index (RI) change in the core of a standard telecommunication fiber is quantified using the spectral shift of an in-fiber Bragg grating (FBG) based Fabry–Perot cavity. Measured RI change is used to design and then fabricate long period grating (LPG) in pure silica core single mode fiber (SMF) employing identical laser irradiation conditions used in core index characterization. A core length of 100 µm within the 10 mm long cavity structure is scanned with ultrafast laser pulses, and the corresponding spectral shift is used to calculate index modification. The index change of 0.000449 found in characterization process is used to simulate the LPG in pure silica fiber. Identical index modulation written in pure silica fiber by femtosecond laser radiation provides a rejection band that is in good agreement with the simulation results. The fabricated LPG sensors are also characterized for ambient temperature and RI.

Fabrication and Characterization of Long-Period Gratings in Hollow Core Fibers by Electric Arc Discharge

Recently, the fabrication of long-period gratings (LPGs) in hollow-core air-silica photonic bandgap fibers by means of pressure assisted electrode arc discharge (EAD) technique have been presented. The EAD procedure properly combined with air pressure inside fiber holes enables the localized modification of hole size and shape in both core and cladding region avoiding holes collapsing. LPGs are fabricated with a step-by-step approach by periodically repeated EAD treatment. In this paper, the role of pressure inside the fiber holes as well as the effect of the grating pitch on the transmitted spectra have been experimentally investigated to achieve the design criteria of novel hollow core devices. An appropriate perturbation of fiber structure (core and/or cladding) may change the field profile of the fiber modes and cause light coupling from the fundamental mode to higher order modes. Here, the experimental demonstration of LPG prototypes with different features exhibiting attenuation bands with depth up to 12 dB are reported. Finally, the resonant wavelength dependence on local temperature and strain changes are experimentally investigated. We believe that the fabrication of LPGs-based devices in hollow core optical fibers enable new functionalities hitherto not possible.

Fabrication of fiber optic long period gratings operating at the phase matching turning point using an ultraviolet laser.

It is known that optical fiber long period gratings (LPGs) exhibit their highest sensitivity to environmental perturbation when the period is such that the phase matching condition is satisfied at its turning point. The reproducible fabrication of LPGs with parameters satisfying this condition requires high resolution control over the properties of the grating. The performance of an LPG fabrication system based on the point-by-point UV exposure approach is analyzed in this paper, and the control of factors influencing reproducibility, including period, duty cycle, and the environment in which the device is fabricated, is explored.

Long-Period Gratings in Hollow Core Fibers by Pressure-Assisted Arc Discharge Technique

We report on the fabrication of long-period gratings (LPGs) in hollow-core air-silica photonic bandgap fibers by using the pressure-assisted electrode arc discharge (EAD) technique. The EAD procedure combined with pressure actuation inside fiber holes enables the modification of hole size and shape in both core and cladding region avoiding holes collapsing and thus acts as a useful tool to impress effective refractive index modulation leading to low loss gratings. Periodically repeated EAD treatments permit the fabrication of LPG-based devices in hollow core optical fibers enabling new functionalities hitherto not possible. Here, the experimental demonstration of LPG prototyping with different characteristics exhibiting attenuation bands with depth up to 12 dB are reported.

Design and Fabrication of Novel Broadband Long-Period Fiber Gratings Using Synthesis Techniques

We present results of novel broadband long-period gratings (LPGs) that were fabricated axially symmetric in single-mode fiber with a carbon-dioxide laser. The discrete layer-peeling (DLP) technique and a genetic algorithm (GA) were used to design the LPGs. Numerical simulations were used to indicate the performance and absolute error of reconstructing a complex spectral profile for a particular synthesis technique. We found that the DLP technique has the highest performance and executes in the least amount of time. However, a GA could not efficiently synthesize an LPG, but produced a refractive index change profile that can be implemented using a common LPG fabrication system. It is shown that the experimental results obtained with the GA are superior to that obtained with the DLP technique.

Long Period Gratings in Random Hole Optical Fibers for Refractive Index Sensing

We have demonstrated the fabrication of long period gratings in random hole optical fibers. The long period gratings are fabricated by a point-by-point technique using a CO2 laser. The gratings with a periodicity of 450 μm are fabricated and a maximum coupling efficiency of −9.81 dB has been achieved. Sensing of different refractive indices in the surrounding mediums is demonstrated by applying standard liquids with refractive indices from 1.400 to 1.440 to the long period grating.

Fabrication of Long-Period Gratings by Femtosecond Laser-Induced Filling of Air-Holes in Photonic Crystal Fibers

A new method for fabricating long-period gratings (LPGs) in photonic crystal fibers (PCFs) is presented. A tightly focused femtosecond infrared laser beam scans across a PCF periodically, resulting in periodic filling of cladding air-holes through a micro-explosion and redeposition process and forming an LPG. Strong resonance of ~26 dB has been demonstrated with a grating length of <;5 mm. An annealed LPG demonstrated stable operation at 800°C for over 30 h.