High-power Fiber Laser Research Articles

Using a High-Power Fibre Laser to Cut Concrete

Concrete cutting at construction sites causes problems such as noise, vibration, and dust. In particular, during the demolition and renovation work on buildings in urban areas, protection against noise, vibration, dust, etc., is needed. Concrete cutting using a CO2 laser was investigated 20 years ago. However, this method had never used because the equipment is difficult to carry. In this study, we used a portable fibre laser, which is convenient to carry. Two types of concretes with different strengths were prepared for the experiment. High-strength concrete has never been used in similar research before. High-strength concrete is just only used for skyscrapers because of its high quality and costs. Furthermore, it has already been used for skyscrapers in Japan. It is for this reason that we chose to use it in this study. Irradiation measurements were conducted under various conditions using laser powers of 6 and 9 kW. It was confirmed that the cutting effectiveness of CO2 and fibre lasers was approximately identical for concretes with a thickness of 200 mm. Furthermore, the cutting effectiveness for the two concretes with different densities was almost the same. However, the situation after cutting was different because the vitrification of the cutting and glass formation progressed in low-density concrete and an explosion phenomenon occurred in high-density concrete, simultaneously. This study suggests that laser concrete cutting can be used as a solution when noise and dust are major problems.

2.3 W monolithic fiber laser operating in the visible

We report, to the best of our knowledge, the first monolithic visible fiber laser pumped by a pigtailed diode. The robust cavity design proposed is based on a highly reflective fiber Bragg grating spliced to a double-clad praseodymium-doped fiber. The laser signal generated at 635.5 nm is single-mode, has a FWHM bandwidth of 0.16 nm, and reaches a maximum cw output power of 2.3 W. This demonstration breaks ground for the development of reliable high-power visible fiber lasers.

Segmented Cladding Fiber With a High-Index Ring in Core for Wideband Single-Mode Operation in Any Bending Orientation

A resonant-ring-assisted large mode area segmented cladding fiber (RR-SCF) with a high-index ring in the core is proposed. Numerical investigations of the effects of the high-index ring on the leakage losses of guiding modes, mode area, and electric field distribution of FM of the proposed fiber show the high-index ring not only increases the ratio of the loss between the fundamental mode (FM) and the higher-order modes (HOMs) but also improves the electric field distribution of the FM to be flat. By taking these advantages of the high-index-ring-assisted core, the effective single-mode (SM) operation is achieved in the proposed fiber bent in any orientation, which makes up the defect of the conventional RR-SCF that the SM output is affected by the bending orientation. Meanwhile, the mode area is enlarged. Furthermore, the effects of the heat load, duty cycle, bending, and operation wavelength on the fiber performance are analyzed. The results show single-mode operation in all bending orientations with a mode area larger than 824 μm2 can be achieved at a tight bending radius of 15 cm to 16 cm when the operating wavelength ranges from 1.06 μm to 1.76 μm. This fiber shows great potential in high-power fiber lasers.

Resonant-ring assisted large mode area segmented cladding fiber with high-index rings in core

This paper proposes a modified segmented cladding fiber with two-layer high-index rings in the core. The inner high-index ring helps to increase the loss ratio between the higher-order modes (HOMs) and the fundamental mode (FM) to achieve single-mode operation. The outer high-index ring is beneficial to improve the waist of the electric field distribution of the FM to enlarge the mode area. The simulation results show thanks to the joint defect of these two-layer high-index rings, the defect of the RR-SCF that cannot provide single-mode operation in all bending orientations is made up. Effective single-mode operation is achieved at a tight bending radius of 15 cm when the operating wavelength ranges from 1.06 μm to 1.58 μm. The mode area of the fiber operating at 1.06 μm is markedly enlarged to 835 μm2. This new fiber shows great potential in compact high-power fiber lasers and amplifiers.

Design of High-Power Radiation-Balanced Silica Fiber Lasers With a Doped Core and Cladding

A model of laser cooling in a fiber with a doped cladding shows that a radiation-balanced fiber laser (RBFL) made of silica can produce substantial output powers. Bidirectional pumping is found to reduce the average temperature of the laser, enabling higher output powers while maintaining radiation-balanced operation. For a large-mode-area silica fiber doped with Yb in the core and cladding, simulations predict that output powers as large as 115 W can be achieved by bidirectionally pumping the doped cladding. This is not only slightly higher than with a Yb-doped ZBLAN fiber with the same dimensions, but the temperature gradient in silica is also about half as large. Since silica is the most common host in fiber lasers, these predictions are very promising for the near-term realization of practical RBFLs.

Brillouin gain spectrum characterization in Ge-doped large-mode-area fibers* *Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant No. 2018B090904001), the National Natural Science Foundation of China (Grant Nos. 61805261, 61405202, and 61705243), and the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No. 2020252).

The dependence of Brillouin gain spectrum (BGS) characteristics, including the Brillouin frequency shift (BFS) and the BGS bandwidth, on germanium concentration in large-mode-area Ge-doped passive fibers is investigated theoretically and experimentally. The simulation results show that the BFS is inversely proportional to GeO2 concentration, and the BGS bandwidth initially increases with the augment of GeO2 concentration, and then decreases. The BGSs of four fibers with core diameters of 10μm and 20μm for different GeO2 concentrations are compared experimentally. Experimental results demonstrate that with the same core diameter, the variations of BFS and BGS bandwidths with GeO2 concentration accord with the simulation results. Additionally, the BGS characteristics of three large-mode-area passive fibers with diameters of 10μm, 25μm, and 30μm are measured, which confirm that the increasing of the fiber diameters will cause the BGS bandwidth to broaden. We believe that these results can provide valuable references for modulating the high-power narrow-linewidth fiber lasers and Brillouin fiber amplifiers.

Large-mode-area all-solid anti-resonant fiber with single-mode operation for high-power fiber lasers.

We investigate the feasibility of applying an anti-resonant guiding mechanism in an all-solid anti-resonant fiber (AS-ARF) to achieve a large mode area (LMA) and single mode for high-power fiber laser applications. A novel, to the best of our knowledge, AS-ARF with nonuniform rods is proposed to enhance the single-mode property and enlarge the mode area. The numerical results show that the core diameter can expand to 57, 80, and 100 µm at the wavelengths of 1.064, 1.55, and 2 µm, respectively. The loss ratio of the lowest loss of higher-order modes to the loss of the fundamental mode can exceed 1000, 550, and 860 at the wavelength of 1.064, 1.55, and 2 µm; thus, robust single-mode operation can be ensured. Besides, the fiber can also be adapted to bent condition under certain heat load. These indicate that the proposed AS-ARF with nonuniform rods is a great candidate as an LMA fiber for high-power fiber lasers.

Mode coupling in large-mode-area double-cladding chirped and tilted fiber Bragg gratings

Recently, chirped and tilted fiber Bragg gratings (CTFBGs) have received great attention because they can realize suppression of stimulated Raman scattering (SRS) in high-power fiber lasers. In this study, the possible coupling between the core modes and cladding modes in CTFBGs inscribed in large-mode-area double-cladding fibers is investigated for the first time. Theoretical results show that the coupling between the LP11 mode and cladding modes would destroy the transmission spectra envelope only considering the coupling of LP01 for single-mode CTFBGs, which will degenerate the SRS suppression performance. This was confirmed experimentally by measuring the spectral response under different mode excitations. A reliable method is demonstrated to ease the LP11-excitation-induced spectral deterioration by choosing an appropriate chirp rate for the inscription of CTFBGs, which is useful for improving the Raman suppression effect of large-mode-area double-cladding CTFBGs in high-power fiber lasers.

Determination and Analysis of Line-Shape Induced Enhancement of Stimulated Brillouin Scattering in Noise Broadened, Narrow Linewidth, High Power Fiber Lasers

We investigate the origin of line-shape induced enhancement of stimulated Brillouin scattering (SBS) in narrow linewidth, noise broadened, high-power fiber lasers. A polarization-maintaining seed laser with continuously tunable linewidth (single frequency to >10 GHz), based on white noise modulation was developed for this study. With increasing linewidths, a substantial difference in SBS thresholds was observed depending on the end termination utilized. This observation can be explained by the line-broadened source, having significant power in the Stokes frequency at larger linewidths, seeding the SBS process. Here, SBS threshold for the system terminated with an anti-reflection coated delivery cable is compared with a simple angle cleaved end termination. The influence of end termination on SBS threshold becomes significant with increased linewidths, showing >20% difference in output power between the two cases at ~10 GHz linewidth. The experiments are complemented by simulations, analyzing relative contributions of Rayleigh scattering and fiber end-facet reflections to SBS. At larger linewidths, due to substantial overlap between laser line-shape and SBS Stokes, with low end-facet reflectivity, Rayleigh is the dominant mechanism, which gives way to end-facet reflections with increasing reflectivity. The Rayleigh contribution is negligible at smaller linewidths, and end-facet reflectivity has a weaker influence than with larger linewidths.

Progress on mode field distribution and characterization technology of the optical fiber laser

In fields of practical applications such as the optical fiber communications, optical fiber lasers, and optical fiber sensinIn fields of practical applications such as the optical fiber communication, the optical fiber laser, and the optical fiber sensing, it is necessary to focus on the mode problems in optical fibers. Mode division multiplexing is an effective method to improve the information capacity of optical communication. Interference between modes is the basic method for the most of optical fiber sensing. Mode controlling technique is one of the key technologies for beam quality control of high-power fiber lasers. Therefore, research on the theory of optical fiber modes, modes’ generation, modes’ conversion, and mode characterization technology are of great significance and practical application value. In this paper, we discuss the mode and beam quality of the optical fiber, analyze the methods of multiple modes’ generation and conversion, and summarize the mode characterization methods by means of incoherent, coherent and low-coherence measurement. Currently, the fiber mode characterization is a hot research topic. Among a variety of characterization methods, the spatial and spectral imaging method (S2) and the spatial and spectral double Fourier transform method (F2) have significant advantages. Without knowing the geometric parameters of the fiber in advance, we can obtain characteristics such as the mode field distribution, the mode power ratio, and the group time delay. Results indicate that the F2 method is better for characterizing mode field distributions of high-power fiber lasers.

19 × 1 high power pump combiner with large input core diameter

In this paper, we investigate a 19 × 1 tapered fiber bundle (TFB) pump combiner for high power fiber lasers theoretically and experimentally, based on the twist method. Theoretically, we analyze the influence of the taper length, taper ratio and twist angle on the performance of a 19 × 1 TFB combiner by using the beam propagation method. Then, a 19 × 1 all-fiber combiner is fabricated experimentally, with input fibers and output fiber having core/clad diameter of 220/242 μm (numerical aperture (NA) = 0.22) and 30/600 μm (NA = 0.06/0.46) respectively. The transmission efficiency of each port is higher than 98.6% and the average efficiency of all 19 ports is 99.3%. The combiner achieved an output power of 1.5 kW with transmission efficiency of 99.3% when tested with four ports of 976 nm laser diode. To our best knowledge, this is the first time that 220/242 μm (NA = 0.22) multimode fibers act as the pump fibers of a 19 × 1 end-pumping combiner.

Research on key technologies for 2μm band of thulium-doped fiber laser

The gain material of 2μm wavelength thulium-doped fiber laser is a thulium-doped fiber doped with rare earth element thulium. Rare earth doped optical fiber as gain medium is the basic structure of optical fiber laser. This type of laser is mainly composed of pump source, coupler, gain optical fiber doped with rare earth elements, cavity and other optical devices. The pump source is generally a combination of one or several high-power laser diodes. The light emitted by the pump source enters the gain optical fiber doped with rare earth elements through a certain pumping mode. The inversion of the number of particles occurs when the doped optical fibers absorb photons, and then the light waves stimulated from the doped optical fibers are emitted by the laser after the oscillation. Since the connection structure and parameters between optical fibers and optical components can be maintained stable over a long period of time, thulium-doped lasers do not need to be maintained regularly and are based on output power. They have smaller volume and weight, lighter weight and longer life than solid-state lasers. However, in general, the power of a 2μm wavelength thumbium-doped fiber laser is not too high, while a high-power 2μm wavelength thumbium-doped fiber laser can meet people’s needs and play a greater role. Therefore, improving the power of 2μm wavelength thumbium-doped fiber laser is an important direction for the development of thumbium-doped fiber laser. The energy level structure of thulium ion is analyzed, the doping concentration of thulium ion is studied, and the method to increase the power of a 2μm wavelength thulium-doped fiber laser is given.

Laser transformation hardening of various steel grades using different laser types

Laser surface hardening has become a promising replacement technique for conventional heat treatment. The laser is a heat source, deployed with optics to modify the surface along with the desired geometry. Laser density and its interaction time can be controlled and varied depending upon the final requirement at the surface. Increased hardness, diffusion-less phase transformation, self-quenching, uniform case depth over the entire scanned area are the reasons for laser-assisted hardening to become very popular and successful. Another reason for its success is that there is no bulk transformation of the material; only selected regions are subjected to laser irradiation for better surface improvements. This paper is a precis of various investigations done with a gas type CO2 laser, a solid-state Nd: YAG laser, high-power diode laser and fiber laser (only Continuous Wave mode) over a variety of steel grades. Laser wavelength, power intensity, transverse/scanning speed, focal plane distance, surrounding atmosphere, pre-treated surface and surface absorptivity are parameters involved in laser surface hardening technique. By varying these independent variables, the desired surface property is achieved on the substrate subjected. A significant analysis of various experimental investigations done in this area is provided. Particular reference is made to surface hardening using different lasers as mentioned above. Although laser hardening is a potential technique for enhancing the surface properties, multi-track hardening/back tempering of the large surface area is still a concern and needs some attention. The main reason is a decrease in hardness in the overlap region. Various studies indicate that a single-track laser hardened region results in 3- or 4-times higher hardness than that of the original or base metal hardness. In contrast, in the overlapped region, the hardness value decreases, i.e., the difference between the hardness values at the single-track hardened region and an overlapped region is around 100–400 HV. This article reviews the chronological development of laser transformation hardening method explicitly by outlining the effect of process parameters on hardened depth and width, hardening influence in wear and corrosion resistance, numerical simulation and experimental validation techniques. Particular reference is made to CW (continuous wave) type laser hardening.

Activation of oxygen molecules by 1070 nm laser radiation in aerated solvents

Population of the chemically active singlet 1Δg(0) state of molecular oxygen occurring due to direct laser excitation of the 1Δg(1)←3Σg-(0) transition has been observed for the first time, to the best of our knowledge, in oxygen molecules dissolved in organic solvents saturated with air under natural conditions (room temperature and normal atmospheric pressure). The data were obtained in 1 cm spectrophotometric cells due to the application of a set of high-power IR fiber and diode lasers. The rate of laser generation of the singlet (1Δg(0)) states in oxygen molecules was monitored by a chemical trapping method. It was found that the action spectra of singlet oxygen generation have one distinct band with a maximum at 1070 nm and half-width of ∼10nm. The absorption coefficients at 1070 nm were shown to be 100-110-fold lower than those at the main oxygen absorption peak (1273 nm) corresponding to the 1Δg(0)←3Σg-(0) transition. Under excitation at 810-1061 nm, very low trapping rates were observed, which did not depend on excitation wavelengths being probably caused by thermal effects. There was no reliable increase in the trapping rate under irradiation at 810 and 920 nm corresponding to the 1Δg(2,3)←3Σg-(0) transitions. This fact suggests that absorbance corresponding to these transitions is much lower than that at 1070 nm. The obtained results are important for both spectroscopy of oxygen and mechanistic studies of biological and therapeutic action of laser radiation.

A feasibility study on high-power fiber laser cutting of thick CFRP laminates using single-pass strategy

High-power fiber laser as an emerging technology has been developed for machining composite sheets (less than 5.0 mm thick) with relatively high efficiency, while very limited works are reported when using fiber laser to cut thick CFRP plates. In order to further explore the cutting capability of fiber laser system without sacrifice of efficiency, a feasibility study on high-power fiber laser cutting of thick CFRP laminates using single-pass strategy was carried out in this work. Surface morphology and cross-section characteristics were analyzed for further understanding laser-CFRP composite interaction/removal mechanisms. Various thermal defects were qualified to better understand the influence of processing parameters on machining quality. Results showed that relatively high level of laser power (4800 W) and cutting speed (2000 mm/min) were preferred to effectively improve surface quality and cut through samples during laser cutting process, while excessive laser energy damaged CFRP composites with severe cracks and delamination mainly due to high level of thermal stress. CFRP laminate with a thickness of 10.0 mm could be successfully cut through when using high-power fiber laser and single-pass strategy, the threshold of energy per unit length was about 270 J/mm. Experimental results from this work can provide significant benchmark data for further industrial applications.

An Eye-Safe, SBS-Free Coherent Fiber Laser LIDAR Transmitter with Millijoule Energy and High Average Power

We report on an eye-safe, transform-limited, millijoule energy, and high average power fiber laser. The high gain and short length of the NP phosphate-glass fibers enable the SBS-free operation with kW level peak power. The output energy is up to 1.3 mJ, and the average power is up to 23 W at an 18 kHz repetition rate with 600 ns pulses (peak power > 2.1 kW). The PER is ≈16 dB and the M2 of the beam is 1.33 × 1.18. The coherent LIDAR Figure Of Merit (FOM) is 174 mJ*sqrt(Hz), which to our knowledge is the highest reported for a fiber laser. We also report 0.75 mJ energy and >3.7 kW peak power with down to 200 ns pulses and up to 1.21 mJ energy with a 3–5 kHz repetition rate operation of the current system.

＞500 W passively-cooled fiber cladding light stripper

To realize of passive-cooling high power fiber cladding light stripper, it is important to optimize the thermal management of both the fiber and the package. By using Teflon capillaries to make segmental etching configuration on fiber, using copper as the package material, and optimizing the package structure through finite element thermal simulations, cladding light stripper capable of handling 500 W power was designed and fabricated. It was experimentally verified that the stripping efficiency reached 23.7 dB and the temperature increase rate on the bare fiber of cladding light stripper was as low as 0.007 ℃/W. In addition, at 540 W of power injection, cladding light stripper could work continuously if mounted on water-cooled cold plate, and could work for 50 s each time if mounted on cold plate filled with phase-change material, with the maximum temperature of package being 58.7 ℃ and 80 ℃ respectively. The researches and results could provide valuable information to the design and development of high power fiber lasers.

Realization of in Situ Fiber-Core Temperature Measurement in a Kilowatt-Level Fiber Laser Oscillator: Design and Optimization of the Method Based on OFDR

High-power fiber lasers have been widely used in various industrial manufacturing and military defense applications. During the development of fiber lasers in the past decades, the thermal effect has always been one of the biggest obstacles. It is crucial to study the temperature characteristics and overcome the thermal restrictions for a better output performance. With the systematic design and optimization in this article, optical frequency domain reflectometry (OFDR) can achieve in situ distributed temperature measurement of the fiber core in high-power fiber lasers. This allows a better study of the temperature characteristics and thermal effects. The fiber-core distributed temperature of a kilowatt-level fiber oscillator is first demonstrated based on this method here. The splicing point between the high reflectivity fiber Bragg grating (HR-FBG) and gain fiber withstands the highest temperature, reaching 101.6 °C at a 1.47 kW output. In addition, the temperature of the gain fiber gradually decreases from 92 °C to 30 °C along the pumping direction. The internal temperatures of the combiner and HR-FBG are also measured to evaluate their performances in the high-power regime. The temperature distributions in the experiment agree well with the theoretical simulation.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Abstract Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot. At present, research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser. However, the suppression of spectral broadening, caused by self-phase modulation or four-wave mixing, is still a challenging attribute to the close distance between the broadened laser and signal laser. If using a traditional fiber grating with only one stopband to suppress the spectral broadening, the signal power will be stripped simultaneously. Confronting this challenge, we propose a novel method based on phase-shifted long-period fiber grating (PS-LPFG) to suppress spectral broadening in a high-power fiber master oscillator power amplifier (MOPA) laser system in this paper. A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique. The resonant wavelength of the fabricated PS-LPFG is 1080 nm, the full width at half maximum of the passband is 5.48 nm, and stopband extinction exceeds 90%. To evaluate the performance of the PS-LPFG, the grating is inserted into the seed of a kilowatt-level continuous-wave MOPA system. Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%, providing an effective and flexible way for optimizing the output linewidth of high-power fiber MOPA laser systems.

Novel constant-cladding tapered-core ytterbium-doped fiber for high-power fiber laser oscillator

Abstract Power scaling based on traditional ytterbium-doped fibers (YDFs) is limited by optical nonlinear effects and transverse mode instability (TMI) in high-power fiber lasers. Here, we propose a novel long tapered fiber with a constant cladding and tapered core (CCTC) along its axis direction. The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering (SRS) threshold and suppress higher-order mode resonance in the laser cavity. The CCTC YDF was fabricated successfully with a modified chemical vapor deposition (MCVD) method combined with solution doping technology, which has a cladding diameter of 400 μm and a varying core with a diameter of ~24 μm at both ends and ~31 μm in the middle. To test the performance of the CCTC fiber during high-power operation, an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally. As a result, a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%, although the TMI effect was observed at an output power of ~3.12 kW. The measured beam quality (M2 factor) was ~1.7, and no sign of the Raman component was observed in the spectrum. We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects, and further power scaling is promising by optimizing the structure of the YDF.