Inscription Method Research Articles

Spectrum design of multimode fiber Bragg gratings based on suppression of mode coupling

The direct femtosecond laser inscription method is very competitive in the fabrication of fiber Bragg gratings, but its refractive index modification region is very limited and leads to low reflectivity and wide spectra of multimode fiber gratings. In this work, the relationship between the refractive index modification region, the coupling coefficient, and the spectrum of the multimode fiber grating is investigated based on the intrinsic mechanism of mode coupling; thus the annular and eccentric elliptic refractive index modification regions are accordingly proposed to suppress mode couplings in the multimode fiber Bragg grating, so as to achieve a highly discrete spectrum and narrow spectrum of the multimode fiber Bragg grating, respectively. This work can provide a solid theoretical foundation for the spectrum design of the multimode fiber Bragg grating, which can be utilized to customize the spectrum according to application scenarios such as optical communication, fiber lasers, filtering, and sensing.

Femtosecond laser direct writing large-area fiber Bragg grating based on diaphragm shaping

We propose and demonstrate a new method of direct writing large-area fiber Bragg grating by femtosecond laser through the coating. By adding an adjustable diaphragm before the focusing objective, we can precisely control the length of the refractive index modulation line along the femtosecond laser incident direction up to 29.1 µm. In combination with femtosecond laser scanning fabrication technology, a uniform refractive index modulation plane can be inscribed in the fiber in a single scanning. Based on the plane-by-plane inscription method, we have fabricated a high-quality high-reflectivity fiber Bragg grating and a chirped fiber Bragg grating on 20/400 double-clad fiber core. The reflectivity of both gratings is greater than 99%, and the insertion loss is as low as 0.165 dB and 0.162 dB, respectively. The thermal slope of chirped fiber Bragg grating without any refrigeration is 0.088 °C/W and there is no obvious temperature increase when using the water cooling. Therefore, the fabrication method of large-area fiber Bragg grating based on diaphragm shaping can efficiently fabricate high-quality fiber Bragg grating in the large core diameter fiber, which has an important application prospect in high-power all-fiber oscillators, especially all-fiber oscillators in special wavebands.

Demand-side adjustable resources unified data model study

With the large-scale grid integration of new energy sources, the power relationship between source and demand side is becoming more and more complex, the flexibility requirements of the distribution network are increasing, and new requirements for system safety and reliability are put forward, and it can’t meet the peak regulating requirements of grid by relying solely on source side to cope with variable loads. In view of this, to make the grid operation flexibility improved by using an adjustable load capacity, the study first constructs a load classifying method on the foundation of fuzzy style K-plane clustering method to understand the interaction information of the load devices. Then the adjustable value of the load is analyzed from the demand response and standby perspectives, respectively, and an improved dynamic time-bending-based source-load similarity inscription method is proposed, which aims to unify the multiple load information obtained by clustering. The proposed clustering algorithm takes the highest value of 0.096 for the Davies-Bouldin index index, which is 0.012 and 0.014 higher than the K-plane clustering algorithm and the K-means algorithm, respectively. In addition, the load demand response regulation with the improved dynamic time bending method has a higher capacity for new energy consumption than the variance method, with a difference of 2.0 kW. This indicates that using load regulation to consume new energy to exploit the load curve will stimulate the active participation of customer-side load in maintaining power balance between the electricity consumption side and the demand side, and form an interest community between power supplying and demanding sides.

Antiresonant hollow-core fiber Bragg grating design.

Fiber Bragg gratings (FBGs) inscribed in hollow-core fibers hold a potential to revolutionize the field of gas photonics by enhancing the performance and versatility of hollow-core fiber-based matter cells. By effectively transforming these cells into cavities, FBGs can significantly extend the effective length of light-matter interactions. Traditional FBG inscription methods cannot be extended to hollow-core fibers, because light in the fundamental mode is predominantly confined to the hollow region where an index change cannot be induced. In this Letter, we propose a bi-thickness dual-ring hollow-core antiresonant fiber (DRHCF) design that achieves substantial overlap between the fundamental mode and cladding glass in a well-controlled manner, ensuring a strong FBG response with a minimal insertion loss. Through detailed numerical investigations, we demonstrate the feasibility of creating a high reflection FBG in the DRHCF using standard FBG inscription techniques. The proposed device is expected to have a length of <1 cm and the insertion loss of <0.3 dB, including splice loss.

Highly cascaded first-order sapphire optical fiber Bragg gratings fabricated by a femtosecond laser.

This Letter reports an innovative technique for fabricating large-scale, highly cascaded first-order sapphire optical fiber Bragg gratings (FBGs) using a femtosecond laser-assisted point-by-point inscription method. For the first time, to the best of our knowledge, this study successfully demonstrates a distributed array of 10 FBGs within highly multimode sapphire crystal fiber, made possible by employing a high-power laser technique to generate larger reflectors with a Gaussian intensity profile. These first-order FBGs offer advantages such as enhanced reflectivity, shorter fabrication time, and simplified spectral characteristics, making them easier to interpret compared with high-order FBGs. The FBGs' resilience and effectiveness are analyzed by subjecting them to temperature tests, proving their capacity for accurate temperature monitoring up to 1500°C-a testament to their suitability for harsh environments. This novel approach broadens the scope for sensing and communication applications in sapphire fibers, particularly under challenging conditions. The novelty of our work lies in successfully overcoming the limitations of previous designs by integrating a cascade of 10 FBGs in sapphire fibers, thereby enhancing multiplexing capabilities, minimizing overlapping of FBG peaks, and ensuring reliable temperature monitoring in industries and applications with thermal gradients.

A passively Q-switched multi-wavelength erbium-doped fiber laser modulated by 12° tilted fiber Bragg grating

As a promising small-angle TFBG, the 12° tilted fiber Bragg grating (TFBG) has been successfully used in biochemical sensors, fiber optic anemometers, and continuously tunable photon fraction time differentiators. On the other hand, multi-wavelength Q-switched fiber lasers have widely potential and practical applications in various fields, such as laser processing, laser marking, medical diagnosis, military applications, and so on. In this paper, first, a 12° TFBG is prepared by femtosecond laser direct inscription method (FSL-DIM), then, a pulse multi-wavelength erbium-doped fiber laser (EDFL) passively Q-switched by the as-prepared 12° TFBG is exclusively and systematically demonstrated for the first time. When the pump power is raised from 100 mW to 500 mW, the repetition rate is boosted from 51.64 kHz to 149.4 kHz resulting in a repetition rate range about 97.76 kHz, while the pulse duration is compressed from 3.928 μs to 1.033 μs. The average output power monotonically increases up to the maximum value of 6.357 mW corresponding to the maximum pulse energy of 42.55 nJ. The signal-to-noise ratio (SNR) is measured to be 53 dB, which indicates a high stability. The experimental results are comparable to other Q-switched multi-wavelength EDFLs reported so far, and during the experiment, the 12° TFBG exhibits excellent stability and reliability. As far as we can explain, the multi-wavelength Q-switched operation obtained in this paper benefits from the filtering effect and polarization dependent loss (PDL) effect of the 12° TFBG. The work not only provides a reliable reference about the effective-SA performance of 12° TFBG, but also enriches the studies of passively Q-switched lasers based on different kinds of modulators.

Encrypted optical fiber tag based on encoded fiber Bragg grating array

Optical fibers are typically used in telecommunications services for data transmission, where the use of fiber tags is essential to distinguish between the different transmission fibers or channels and thus ensure the working functionality of the communication system. Traditional physical entity marking methods for fiber labeling are bulky, easily confused, and, most importantly, the label information can be accessed easily by all potential users. This work proposes an encrypted optical fiber tag based on an encoded fiber Bragg grating (FBG) array that is fabricated using a point-by-point femtosecond laser pulse chain inscription method. Gratings with different resonant wavelengths and reflectivities are realized by adjusting the grating period and the refractive index modulations. It is demonstrated that a binary data sequence carried by a fiber tag can be inscribed into the fiber core in the form of an FBG array, and the tag data can be encrypted through appropriate design of the spatial distributions of the FBGs with various reflection wavelengths and reflectivities. The proposed fiber tag technology can be used for applications in port identification, encrypted data storage, and transmission in fiber networks.

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.

Weak reflectivity measurements of large-mode-area FBGs by a scale composed of a fiber Fabry-Perot interferometer.

In a previous study, we proposed a measuring method for the reflectivity of weak-reflection large-mode-area fiber Bragg gratings by using scale gratings. We experimentally found that the interference between two scale fiber Bragg gratings (FBGs) is beneficial for increasing reflectivity scales, which can improve the measurement accuracy. Therefore, in this study, we designed and fabricated FBG-based Fabry-Perot cavities (FBG-FP) in single-mode fibers by two inscription methods, namely ultraviolet (UV) laser exposure and femtosecond-laser direct writing. Then, a large-mode-area double-clad (LMA-DC) FBG of weak reflectivity was measured by these two scales, and the experimental results show that the Bragg resonance reflectivity is less than 4.28% and 1.14% ∼ 2.28%, respectively. This method of measuring the weak grating reflectivity based on FBG-FP scales is convenient, efficient, and accurate. It is also worth mentioning that the method of femtosecond-laser direct writing eliminates the period limitation of the phase mask, thereby expanding the measurement wavelength range of FBGs. In the future, with the improvement of fiber grating fabrication technology, it is expected that more accurate results can be obtained.

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.

Two dimensional gradient-index beam shapers fabricated using ultrafast laser inscription.

In this paper gradient-index beam shapers are fabricated using the ultrafast laser inscription method. This method enables the fabrication of two-dimensional refractive index profiles inside silica glass, resulting in highly robust and compact beam shapers. The magnitude of this refractive index change can be tailored by adjusting the laser pulse energy, enabling arbitrary two-dimensional refractive index profiles to be manufactured. The process is then demonstrated by fabricating planar waveguides with quadratic index profiles that predictably resize Gaussian beams. Then a more complex two-dimensional refractive index profile is fabricated to transform an input Gaussian beam into a super-Gaussian (flat-top) beam.

Multi-wavelength random fiber laser based on a tilted parallel inscribed apodized fiber Bragg grating array.

We propose and demonstrate a multi-wavelength random fiber laser (RFL) based on a novel, to the best of our knowledge, compact apodized fiber Bragg grating array (AFBGA). The AFBGA is fabricated by a femtosecond laser with the point-by-point tilted parallel inscription method. The characteristics of the AFBGA can be flexibly controlled in the inscription process. Hybrid erbium-Raman gain is used in the RFL and reduces the lasing threshold to sub-watt level. Stable emissions at two to six wavelengths are achieved with the corresponding AFBGAs, and more wavelengths are expected with higher pump power and AFBGAs containing more channels. A thermo-electric cooler is employed to improve the stability of the RFL, and the maximum wavelength and power fluctuations of a three-wavelength RFL are 64 pm and 0.35 dB, respectively. With flexible AFBGA fabrication and simple structure, the proposed RFL enriches the choice of multi-wavelength devices and has significant potential in practical applications.

Laser-induced galfenol embedded multi-layer graphene-oxide in solution

The proposed work demonstrates the direct synthesis of nanomaterial-embedded laser-induced few-layer graphene-oxide by directly ablating galfenol in a water-based solution for the first time. Laser-induced multilayer graphene-oxide (GO) embedded with galfenol (gallium–iron alloy) nanoparticles (NPs) is created through a method of direct laser inscription of bulk galfenol in deionized (DI) water with femtosecond laser ablation. The NP-embedded GO is achieved by irradiating a near-infrared (near-IR) femtosecond laser at 1040 nm on a bulk galfenol material submerged in a solution comprising DI water and a small concentration (5%/wt.) of polyvinylpyrrolidone followed by a second ablation in pure DI water. Results show nanoparticles with a mean diameter of ∼30 nm embedded in GO sheets with visible folds spaced at ∼0.63 nm. The composition of iron and gallium shifts by less than 2% during the laser ablation process, and the few-layer GO sheets exhibit similar Raman peaks to bulk graphite.

Temperature-Insensitive Curvature Sensor With Plane-by-Plane Inscription of Off-Center Tilted Bragg Gratings in CYTOP Fibers

We demonstrate of an off-centered tilted fiber Bragg grating (FBG) inscribed in cyclic transparent optical polymer (CYTOP) fiber as a curvature sensor. The gratings were inscribed using a femtosecond (fs) laser with the direct write, plane-by-plane method due to its high flexibility and suitability for customization, particularly for the development of a single-peak spectrum in multimode CYTOP fiber. The grating is applied to curvature sensing, where the spectral features, namely the wavelength shift and reflected optical power, are analyzed with respect to the curvature angle on the optical fiber. The off-center and tilted FBG response as a function of the angle is compared with a centered and uniform FBG inscribed in CYTOP fiber using the same inscription method and laser parameters. Results show a significant improvement for the off-center grating when the wavelength shift information is considered and lower optical power sensitivity as a function of the angle with respect to the centered FBG. In addition, the temperature sensitivity differences of the two gratings, enables the development of temperature-insensitive angle sensor using a single off-center tilted FBG.

Fabrication and Sensing Application of Phase Shifted Bragg Grating Sensors.

As a special kind of Bragg grating, phase-shifted fiber Bragg grating (PS-FBG) has attracted extensive attention because of its extremely narrow transmission window and excellent sensing performance. The main purpose of this manuscript is to discuss the PS-FBG with special sensing characteristics and explore the influence of different inscription technologies on the sensing characteristics of PS-FBG by comparing the existing inscription methods. The sensing characteristics, advantages and disadvantages of PS-FBG with different structures are analyzed.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses.

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.

Fabrication of cascaded wideband LPFGs by CO2 laser and Raman suppression in a 5 kW one-stage MOPA fiber laser

Long period fiber Bragg grating (LPFG) has been proved to be an effective component for stimulated Raman scattering (SRS) suppression in high power fiber lasers. The rejection bandwidth has significant influence on the Raman suppression effect, usually the wider the better. In this paper, a simple cascaded inscription method for achieving wideband LPFGs, to the best of our knowledge, is proposed and experimentally validated for the first time. The influence of grating periods on the transmission spectrum was simulated, and then a wideband cascaded LPFG was designed and fabricated by CO2 laser. To test its performance, a 5-kW level oscillator one-stage power amplification fiber laser was established and several comparative experiments were done. The validity of the cascaded LPFG has been demonstrated by the better Raman suppression effect. This work provides a simple and effective method for obtaining wideband LPFGs, which have many other applications besides Raman suppression, such as mode conversion in fiber lasers.

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.

High Reflectivity Mid-Infrared Fiber Bragg Grating by Femtosecond Laser Direct Inscription Method

High Reflectivity Mid-Infrared Fiber Bragg Grating by Femtosecond Laser Direct Inscription Method

Design and fabrication of wideband chirped tilted fiber Bragg gratings

Chirped and tilted fiber Bragg gratings (CTFBGs) have been proved to be effective components for Raman suppression in fiber lasers. However, research shows that the rejection bandwidth of CTFBGs is a great limitation for Raman suppression effect, especially at high power level. In this paper, a simple tandem inscription method for achieving wideband CTFBGs (WB-CTFBGs), to the best of our knowledge, is proposed and experimentally validated for the first time. The influence of several key factors including tilt angle, grating period, and chirp rate of phase mask, on the transmission spectrum is studied in detail by simulation. According to the simulation results, a WB-CTFBG is designed and fabricated. The Raman suppression effect in a 5 kW level master-oscillator-power-amplification (MOPA) fiber laser is compared with that of normal CTFBGs. The validity of the proposed inscription technique has been demonstrated by the better Raman suppression effect. This work provides a simple and effective method for fabrication of WB-CTFBGs, which is significant for Raman suppression in high-power fiber laser systems.