Fabrication Of Long-period Fiber Gratings Research Articles

High-sensitive refractive index sensor based on the long-period gratings inscribed in the tapered fiber at dispersion turning point

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the tapered single-mode fiber by CO2 laser. The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters have been studied theoretically and experimentally. When the waist diameter of the tapered fiber decreased from 90 μm to 22 μm, the slope of the phase-matching curve changed dramatically. The refractive index sensitivity can be increased by inscribing the LPFGs in the tapered fiber around the dispersion turning point. A maximum refractive index sensitivity of 21702.78 nm/RIU has been achieved in the range of 1.445–1.457 for the LPFG inscribed in the tapered fiber with a waist diameter of 50 μm. This kind of LPFGs inscribing in the tapered fiber may be a good candidate for the development of refractive index based chemical and biological sensors.

High-efficiency broadband third-order OAM mode converter based on a multi-period preset-twist long-period fiber grating.

All-fiber mode converters for generating orbital angular momentum (OAM) beams have many applications in optical communications, optical sensing and lasers. Currently, it is a great challenge to use a long-period fiber grating (LPFG) to broadband excite high-order OAM modes above the second-order. Here, we demonstrate a preset-twist LPFG fabrication method, which introduces asymmetry in the refractive index modulation area, for efficient generation of third-order modes. Through optimization, the generation of third-order OAM modes with 99.55% conversion efficiency, 0.81 dB insertion loss, and over 99% purity is achieved with only 40 pitch number. In addition, a multi-period preset-twist LPFG is proposed and demonstrated to achieve the excitation of broadband third-order mode with conversion efficiency of more than 99%, insertion loss of less than 1 dB, and mode purity of more than 90%. The 15 dB bandwidth (96.8% conversion efficiency) of the LPFG is 109 nm in the wavelength range from 1475 nm to 1584 nm, and the 20 dB bandwidth (99% conversion efficiency) of the LPFG is 92 nm from 1488 nm to 1580 nm. To the best of our knowledge, this is the first time to generate efficient and broadband third-order mode using a long-period fiber grating.

Long-Period Fiber Gratings Coated with Poly(ethylene glycol) as Relative Humidity Sensors

Relative humidity is an important parameter in controlled environments and is typically monitored using low-cost electrochemical sensors with low resolution and accuracy. This kind of sensors cannot not be implemented in harsh or explosive environments (as in pyrotechnic facilities) due to electrical discharges, or in marine structures where the oxidation of the sensing probe materials changes the sensing response). In these cases, fiber optic sensors can provide solutions due to their intrinsic properties, such as immunity to electromagnetic interference and resistance in harsh environments. This work presents preliminary results regarding the steps of the fabrication of Long-Period Fiber Gratings, the coating processes with a thin layer of poly(ethylene glycol) (PEG) and its sensing performance to relative humidity, displaying a from 60 to 100%sensitivity of 0.6 nm/%RH in the range of 80 to 100%RH.

Efficient generation of Second-order beam Based on angular modulated Long-period fiber grating

Higher-order mode generation using long-period fiber grating (LPFG) has a lot of advantages such as simple structure and compatibility with optical fiber system. However, due to the large effective refractive index difference and very small cross-coupling coefficient between fundamental core mode and higher-order core modes in a few-mode fiber, it is difficult to achieve a high coupling efficiency and low insertion loss at the same time. In this paper, we demonstrated a simple model to illustrate the relationship between refractive index (RI) modulation and coupling coefficient in a CO2 laser LPFG fabrication system. Theoretical analysis shows that changing the RI modulation area from semicircle to sector with certain angle can significantly enhance the mode coupling between LP01 and LP21 modes. We also proposed an easy way who can achieve an angular modulation to fabricate LPFG which could generate second-order modes with a high efficiency as well as a low insertion loss. Experimental results are in good agreement with theoretical analysis. Finally, we get a LPFG mode converter who has a conversion efficiency of over 99% and an insertion loss of only 0.27 dB. Furthermore, the CO2 laser has almost no damage on the fiber structure. This fabrication method and analyzation model has a great guiding significance to regular the coupling coefficient and realize higher-order mode excitation in the future.

Raman suppression in 5 kW fiber amplifier using long period fiber grating fabricated by CO2 laser

Long period fiber grating (LPFG) can be used as a band rejection component, providing a potential effective way for Stimulated Raman scattering (SRS) suppression in high power fiber laser. Here, an improved CO2 laser scanning system with a galvanometer is built up for fabrication of LPFGs, which greatly improves the speed and consistency of grating inscribing. The characteristics of the manufactured LPFG is measured in detail, then a direct laser diode pumped 5 kW-level fiber amplifier system is established to test the validity of the LPFGs for Raman suppression for the first time. Experimental results show that the SRS suppression ratio is about 14 dB at 5.1 kW with the LPFG being inserted between the seed and the amplifier stage, meanwhile the effective output signal laser power is increased. In contrast to the ultraviolet laser exposed band rejection components, such as chirped and tilted fiber Bragg grating (CTFBGs), this type of LPFG does not need photosensitive fibers, annealing treatment and has a much lower thermal effect, which is very useful for high-power fiber lasers applications.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part I: Fabrication, Characterization and Mechanisms of Formation.

In this work, we reviewed the most important achievements of INESC TEC related to the fabrication of long-period fiber gratings using the electric arc technique. We focused on the fabrication setup, the type of fiber used, and the effect of the fabrication parameters on the gratings’ transmission spectra. The theory was presented, as well as a discussion on the mechanisms responsible for the formation of the gratings, supported by the measurement of the temperature reached by the fiber during an electric arc discharge.

Mode Selective Conversion Enabled by the Long-Period Gratings Inscribed in Elliptical Core Few-Mode Fiber

We demonstrate the fabrication of long-period fiber gratings (LPFGs) in an elliptical core few-mode fiber (e-FMF) by CO2 laser. The mode conversion from fundamental mode to high-order modes (LP11a, LP11b, and LP21a mode) can be realized by the LPFGs with high efficiency of more than 95%. The effect of bending, torsion, temperature, and strain on the resonance wavelength of the e-FMF based LPFG mode converters are experimentally investigated. The temperature and strain sensitivities of the LPFGs are characterized to be −36 pm/°C and −4.9 pm/μϵ, and the LPFGs are found to be insensitive to the bending and torsion. Due to the existence of trench in the e-FMF, the higher-order LP02 and LP31a mode can also be stimulated by the e-FMF based LPFGs with a grating period of 515 μm and 400 μm, respectively. The proposed e-FMF based LPFGs could have a promising prospect in the application of fiber sensing and mode division multiplexing optical communication system.

Design of an Edge Filter Based on a Phase-Only Modulated Long-Period Fiber Grating

An efficient edge filter with an ultrabroad dynamic range is first proposed and numerically demonstrated, which is achieved by using a phase-only modulated long-period fiber grating (LPG). As a typical design example, an edge filter with a linear dynamic range of ∼50 nm in wavelength and ∼85% in transmission loss has been successfully obtained, which are the best results reported to date among those of the fiber grating based edge filters. Fabrication tolerance analyses for the designed filter have also been performed. The simulation results show that the designed filter can be practically fabricated according to the position accuracy (±1.5 μ m) of the current LPG's fabrication platform.

Fabrication of long-period fiber gratings using end-coupled ultrafast laser pulses.

We propose a novel technique for the fabrication of long-period fiber gratings (LPFGs) using ultrafast laser pulses coupled into opposite ends of an optical fiber. The pulses counter-propagate in the fiber and induce a permanent refractive index change where they overlap. Based on absorption rate measurements, it was confirmed that the pulses were partially absorbed at the overlapping position, with a maximum absorption rate of 6%. Furthermore, a LPFG with a smooth transmission spectrum and low background loss was successfully fabricated using this technique.

Recent progress in design and fabrication of novel long-period fiber grating

Long-period fiber grating (LPFG) is a kind of wide-range transmission passive photonic device with extensive applications in the field of fiber communication and fiber sensing. In this review, from the angle of refractive index spatial modulation, we extract three characteristic parameters of LPFG:grating period length, index modulated depth and normal orientation of grating plane, and classify LPFG as two types:uniform LPFG (none of these three parameters changes) and nonuniform LPFG (at least one of them changes), and analyze the deficiency of LPFG, including larger size than fiber Bragg grating, no reflection peak, too large bandwidth, polarization loss from single-side exposure, etc. We define the concept of novel LPFG (NLPFG) as the LPFGs based on general LPFG but having new structures and new characters by importing new factors from different aspects, like grating formed mechanism, grating structure, making material, processing technique, application performance, etc. Then we point out that the research significance of NLPFG lies in improving and exploring its real usable property, and making it practical by overcoming the defects of general LPFG in structure, property and application. We expound new techniques of LPFG fabrication, such as multi-exposure, apodized exposure, outfield action, coating and filling, fiber incised and welded, multi-dimensional modulation, and show some NLPFG examples written with these techniques. We build the spatial model of NLPFG to expand the refraction index modulation region from only fiber core to both core and cladding, and to correctly mark the direction of grating plane with tilted angle and azimuth angle. On this basis, we propose the design theory of NLPFG by adding those two angles into the coupling mode coefficient and solving the coupling mode equation. We also expound three different NLPFG design processes, as the direct design to start from given factors of grating, the reserve design to calculate the factors back from expected function or spectrum, and the direct-reserve design combined by them. Meanwhile, we introduce some typical design methods of NLPFG, like geometrical structure changed method, materials changed method, medium coated and embedded method, etc. In addition, we review the recent fabrication and typical application of NLPFG, then introduce different LPFG devices based on excentric core LPFG, multi-core LPFG, few-mode LPFG, stagger LPFG, mismatched LPFG, over-melted LPFG, phase-shift LPFG, tuning LPFG, coupled LPFG and cascaded LPFG, and show their sensing applications in strain, twisting, bending, temperature, displacement, gas concentration and biology. Finally, we provide a developing prospect of the research on NLPFG and give three possible means to improve the research, as innovating new gating structures, exploring new design methods and developing new fabrication techniques.

Mode converter based on the long-period fiber gratings written in the two-mode fiber.

We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO2 laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP01 mode and LP11 mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

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 ...

Femtosecond Laser Inscription of Long-Period Fiber Gratings in a Polarization-Maintaining Fiber

The method of femtosecond laser inscription by slit-apertured beam is applied for fabrication of long-period fiber gratings in polarization-maintaining Panda-type fiber. Polarization properties as well as ambient refractive index sensitivity are investigated for the inscribed gratings. It is shown that the method allows us to create high-quality element for the use as spectral filter of high-power fiber laser as well as high-temperature sensor.

Refractive index sensitivity of nano-film coated long-period fiber gratings.

We demonstrate the fabrication of long-period fiber gratings (LPFGs) coated with high index nano-film using the atomic layer deposition (ALD) technology. Higher index sensitivity can be achieved in the transition region of the coated LPFGs. For the LPFG coated by nano-film with a thickness of 100 nm, the high index sensitivity of 3000 nm/RIU and the expanded index sensitive range are obtained. The grating contrast of the over-coupled LPFGs and conventional LPFGs are measured and the over-coupled gratings are found to have a higher contrast in the transition region. The cladding modes transition is observed experimentally with increasing surrounding index using an infrared camera. The theoretical model of the hybrid modes in four-layer cylindrical waveguide is proposed for numerical simulation. The experimental results are well consistent with theoretical analysis.

Fabrication of long-period fiber gratings using low-pressure mercury lamp: Shortening of exposure time

Long-period fiber gratings were fabricated by exposure to a low-pressure mercury lamp instead of using a laser or a fine positioning system. A hydrogen-loaded Ge–B-codoped fiber was used. Shortening of the exposure time was studied by increasing the grating length. For a grating period of 212 µm, a maximum loss of 20 dB was obtained with a grating length of 30 mm for an exposure time of 12 h. This time was 2/3 that in the case of a grating length of 20 mm. Shortening of the exposure time to 2/3 was also achieved with the grating period of 460 µm, and a 10 dB loss was obtained by exposure for 4.5 h. Fabrication after the outdiffusion of hydrogen was also studied. With a hydrogen concentration of 2/3 of the above experiments, the exposure time increased 1.5 times, and the resonance wavelengths were shorter than those with higher hydrogen concentrations. The growth rate of the refractive index modulation was 4.3 × 10−5/h peak to peak for a grating period of 212 µm. Temperature and strain sensitivities were also measured. Both the temperature and strain sensitivities of a grating period of 212 µm were higher than those of a grating period of 460 µm.

Effects of average index variation in apodized long-period fiber gratings

We analyze the effects of average index variation on the transmission characteristics of an index-apodized long-period fiber grating (LPFG) by the transfer matrix method and study how these effects depend on the grating length, the grating profile, the modal dispersion factor, and the duty cycle of the index modulation. Apart from shifting the resonance wavelength and modifying the rejection band, average index variation can give rise to significant side lobes that may appear on the short-wavelength or long-wavelength side of the rejection band, depending on the signs of the average index change and the modal dispersion factor. Our results provide general guidance for the writing of LPFGs for the minimization of side lobes. Our analysis compares well with published experimental results and should be useful for the design and fabrication of LPFGs.

Fabrication of long-period fiber gratings by CO2 laser in fiber under tension

We demonstrate experimentally fabrication of CO2 laser written long-period fiber gratings (LPFGs) in single-mode fiber (SMF) under tension. New transmission dips can be generated due to the frozen-in stress in the fiber under tension. Dynamics analysis of laser writing process is performed to study the mechanism of the grating inscription. Experimental results show that the wavelength shift of new generated resonance dips can shift towards the longer or shorter wavelength with the increase of laser scanning cycles for gratings with different pitches.

Long-period fiber gratings fabricated by use of defocused CO_2 laser beam for polarization-dependent loss enhancement

An alternative of the long-period fiber grating (LPFG) fabrication method has been developed in this work by use of defocused CO2 laser pulse scanning, which is achieved by moving the laser beam focus plane slightly away from the fiber cladding surface. By increasing the distance shift from the cladding surface, the area with refractive index change also increases on the side with laser irradiation, which leads to an apparent mode field profile distortion at the resonant wavelength of the LPFG. Such a distortion in the mode field profile significantly increases the polarization-dependent loss of the LPFG. Moreover, for the cascaded LPFGs fabricated by use of this technique, the maximum polarization-dependent loss obtained can be largely increased from 7 to 17.8 dB, with a maximum dip in the wavelength change of 0.45 nm.

Measurement of CO_2-laser-irradiation-induced refractive index modulation in single-mode fiber toward long-period fiber grating design and fabrication

We report a new method to measure the CO(2)-laser-irradiation-induced refractive index modulation in the core of a single-mode optical fiber for the purpose of design and fabrication of long-period fiber gratings (LPFGs) without applying tension. Using an optical fiber Fabry-Perot interferometer, the laser-induced axial refractive index perturbation was measured. We found that the CO(2)-laser-irradiation-induced refractive index change in the fiber core had a negative value and that the magnitude was a sensitive function of the laser exposure time following almost a linear relation. Under the assumption of a Gaussian-shaped refractive index modulation profile and based on the first two terms of Fourier series approximation, the measured refractive index perturbations were used to simulate the LPFG transmission spectra. LPFGs with the same laser exposure parameters were fabricated without applying tension, and their spectra were compared with those obtained by simulations.

Fabrication of long-period fiber gratings by twisting a standard single-mode fiber

A new method of fabrication of long-period fiber gratings by twisting of a standard single-mode fiber at high temperature is presented. The method relies on the fact that there always exists some core-cladding eccentricity in the optical fiber. Therefore, when the fiber is twisted, its core follows a helicoidal path inside the cladding. The transmission spectrum of the helicoidal long-period fiber grating that is produced contains several dips that correspond to resonances with the fiber cladding modes.