High-power Fiber Laser Applications Research Articles

An advanced fabrication method for Yb3+-Doped optical fibers featuring AlPO4 core glass

Ytterbium (Yb)-doped fibers using aluminophosphosilicate (Yb-APS) host glass offer significant advantages over conventional aluminosilicate (Yb-AS) fibers, such as strong photodarkening resistance, higher RE-ion solubility and lower Numerical Aperture (NA), making them ideal for high-power laser applications. However, achieving optimal performance in Yb-APS fiber fabrication poses various challenges, including maintaining a precise Al3+/P5+ ratio (1:1), minimizing the central dip in the refractive index profile (RIP), ensuring proper dopant distribution, and reducing background loss. We introduce a new fabrication method for Yb-APS fiber to address these challenges. This approach entails first synthesizing amorphous Yb³⁺: AlPO4 particles, then incorporating them into the fiber core using a hybrid technique that merges Vapor Phase Delivery (VPD) with solution doping (SD). We extensively utilized various materials and optical characterizations, including XRD, FTIR, Raman spectroscopy, FESEM, and XPS, to fine-tune the composition of Yb³⁺: AlPO4. The optical characterization of the developed preform sample, including absorption, emission, and luminescent lifetime measurements, was conducted to assess the fabrication technique's suitability. The results confirmed the successful incorporation of Yb-ions into the preform core, with peaks consistent with reported values. The fiber's attenuation loss was measured, showing approximately 10 dB/km at 1200 nm. This indicates that the particles dissolved smoothly during preform and fiber processing, resulting in minimal scattering loss in the fiber core. The developed fiber demonstrated NA of 0.1, uniform RIP along the preform length with significant reduction of the central dip formation and improved spectral performance (∼1.7X broader bandwidth) compared with an equivalent Yb-AS fiber. These findings suggest the Yb-APS fiber's overall effectiveness and structural robustness, highlighting the potential of the proposed fabrication method for high-power fiber laser applications.

All-fiber 3 kW LP02 laser output based on long-period fiber grating for precise welding

An innovative all-fiber mode converter specifically designed for high-power fiber lasers is introduced. The mode converter relies on the coupling theory of long-period fiber gratings (LPFGs) and involves the fabrication of a LPFG, where the performance is meticulously studied by examining various fabrication parameters. The LPFG is integrated into a high-power fiber laser system to achieve efficient conversion from the LP01 mode to the LP02 mode with a conversion ratio of 90 % and a low transmission loss of only 0.088 dB. Under a 3.03 kW laser output, the LP02 fiber laser exhibits a remarkable laser-to-raman peak intensity ratio, which is 15.98 dB higher than that of the signal-mode fiber laser. Furthermore, during the welding process of a thick aluminum plate, the LP02 fiber laser demonstrates a significant increase of 1.2 times in the depth of the weld pool morphology compared to the signal-mode fiber laser, particularly evident when operating at a laser welding head speed of 100 mm/s. The mode converter with LPFG exhibits lower insertion loss, enhanced power-handling capacity, making it ideal for diverse high-power fiber laser applications with promising practical prospects.

High Power Ytterbium-Doped Fiber Lasers Employing Longitudinal Vary Core Diameter Active Fibers

Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M2~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF).

Large mode area double-layer all-solid anti-resonant fiber for high-power fiber lasers

A novel double-layer all-solid anti-resonant fiber (DL-AAF) is proposed to develop large mode area (LMA) fiber for high-power fiber laser applications. Single-mode property and mode field area scaling are investigated in simulations. Numerical results show the fiber can achieve strong suppression of higher order modes (HOMs) even with a large core size. With a core diameter of 80 μm, the loss ratio defined as the ratio of the lowest loss of HOMs to that of fundamental mode can exceed 3500, and the mode field area reaches 3030 μm2 at the wavelength of 1.064 μm. By further expanding the core size, the mode filed area can be enlarged to 7481 μm2 while the fiber is ensured to be single-mode. This is a significant enhancement in mode field area scale for the current works on LMA AAF.

Investigation of the pump coupling efficiency of a side-pumping combiner based on tapered-fused method.

Side-pumping combiner is used for pumping double-clad fiber in various fiber laser schemes. However, its coupling efficiency and temperature characteristics suffer when pumped via a large numerical aperture (NA) pump light. We investigated the method of optimizing the coupling efficiency of a (2 + 1) ×1 combiner under a large NA pump light injection. After optimization of taper ratio and length of the pump fiber and fusion area between pump and signal fiber, the coupling efficiency increased and the temperature characteristic improved, which could be useful for fabrication of a side-pumping combiner for high-power fiber laser applications.

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.

Practical, rapid, and process-on-place polymer-free cladding power stripping for high power fiber lasers.

We have studied polymer-free cladding power strippers for high-power fiber laser applications. A practical, rapid, and process-on-place chemical etching technique is presented to form surface roughness for efficient removal of the cladding light. The technique is methodically studied and performance contours are determined for frequently used 130, 250, and 400 µm double-clad fibers. The stripping efficiency and the thermal performance of the fabricated strippers are investigated with respect to such process parameters as etching time and etched fiber length. At least 15 dB attenuation with $ \lt {0.045}(^\circ {\rm C}/{\rm W})$<0.045(∘C/W) thermal slope is demonstrated for all fiber types. To show the power scalability of the proposed technique, ${\sim}{600}\;{\rm W}$∼600W power is stripped with an ultra-low thermal slope of ${\sim}{0.021}(^\circ {\rm C}/{\rm W})$∼0.021(∘C/W) over a 400 µm fiber.

Enhanced cladding pump absorption of ytterbium-doped double cladding fiber with internally modified cladding structures

We report an ytterbium-doped 20/400 µm double cladding fiber with an internally modified circular inner cladding. Through four fluorine-doped (F-doped) low refractive index quartz rods embedded into the inner cladding, the cladding pump absorption is considerably increased by 1.5 times. It is found that stimulated Raman scattering (SRS) threshold improves after using shorter fiber lengths. More than 2.1 kW laser output with good beam quality (M2 = 1.38) has been obtained with a suitable pump power injected, and the slope efficiency of the all-fiber laser oscillator is about 75.4%, when pumped at 915 nm. The present results suggest that low refractive index quartz rods embedded into the inner cladding proves to be an effective way for enhanced cladding pump absorption even with a circular inner cladding, which facilitates fiber splicing and provides a novel and robust fiber design of industry grade ytterbium-doped double cladding fibers for high power fiber laser applications.

Combined method for the fabrication of high-power cladding light stripper using a buffered oxide etchant.

Cladding light strippers (CLSs) are vital and one of the critical components for high-power fiber laser applications. In this study, we show the first studies of the formation mechanisms and optimum conditions of a CLS device using a buffered oxide etchant by a combined method of stain (wet) etching and vapor-phase etching. This high-power CLS was shown to result in a stripping performance of ∼17.2 dB at the launched power of 333W (pump limited). The thermal imaging demonstrates that the maximum temperature reached when operating the device at maximum launched power was ∼75°C. The atomic force microscopy (AFM) results show that the combined method yields crystal-like structures with the height in microscales, whereas other conventional methods give only nanoscale roughness. The method also preserves the diameter of the CLS device close to the bare fiber with about 10μm tapering leads to a high surface area to strip unwanted light, which is good for heat dissipation. The combined method possesses the outcome of two methods, including both the crystal-like structures and nanosized hillocks, resulting in high-power stripping performance and robustness.

Bend-resistant low bending loss and large mode area single-mode fiber with low NA

• We have designed a novel low bending loss and large mode area single-mode fiber with low NA. The proposed fiber can provide high leakage loss ratio (776) between the high-order modes (HOMs) and fundamental mode (FM). • The bending loss of FM and HOMs as function of number of trenches is researched in the paper. The bending loss and effective mode area will decrease as number of trench increase. Without ring and trench in core and cladding, the bending loss of FM and HOM are small and that cannot realize the single-mode operation. Loss ratio is smallest in all of fibers. • The HOMs suppression is better than the traditional step-index fibers (SIF). Furthermore, bending loss can be reduced to about 0.0074 dB/m which is lower than previous study. Effective mode area can achieve 744.49 μm 2 at 15 cm bending radius. This fiber design shows great potential in developing compact high-power fiber lasers and amplifiers. A novel design of trench in cladding with resonant ring is proposed in this paper. Performances of proposed fiber have been numerically investigated by the finite element method with a perfectly matched layer boundary. Simulation shows that the proposed fiber can provide high leakage loss ratio (776) between the high-order modes (HOMs) and fundamental mode (FM) which is beneficial to realize robust single-mode operation to enhance the transverse mode instabilities at a wavelength of 1080 nm. The HOMs suppression is better than the traditional step-index fibers (SIF). Furthermore, the bending loss can be reduced to about 0.0074 dB/m due to the trench in the cladding, and the effective mode area can achieve 744.49 μm 2 at 15 cm bend radius, which is also beneficial to improve transmission efficiency. The advantage of an all-solid structure along with a much simpler fabrication process makes our approach very suitable for realizing very large mode area fibers for high power fiber laser application.

The Influence of Fused Depth on the Side-Pumping Combiner for All-Fiber Lasers and Amplifiers

Among many types of side-pumping combiner, which have been reported recently, a fused tapered method, which is based on direct fusion of one or several tapered pump fibers to the surface of a signal fiber, seems to be a potential method because of its high coupling efficiency and the capability of handling high pump power up to kW-class. However, the fused depth is seldom taken into consideration in the numerical analysis, which is not in agreement with the real situation. We present the numerical and experimental investigation about the influence of fused depth on the performance of the side-pumping combiners for all-fiber lasers and amplifiers. The experimental results agree well with the theoretical analysis. The fused depth shows great importance and should be paid much attention when designing and developing a side-pumping combiner by the fused tapered method. We develop a fused tapered side-pumping combiner, which is successfully applied in a backward pumped fiber amplifier with a 1080 nm output power of 1.458 kW. Our results may also offer detailed suggestion and instruction of fabricating side-pumping combiner for high-power fiber laser applications.

Dielectric nanostructures with high laser damage threshold

Dielectric-based metamaterials are proposed to be the ideal candidates for low-loss, high-efficiency devices. However, to employ dielectric nanostructures for high-power applications, the dielectric material must have a high laser-induced damaged threshold (LIDT) value. In this work, we investigated the LIDT values of dielectric nanostructures for high-power fiber laser applications. Consequently, we found that the fabricated SiO2 nanostructured lens can withstand laser fluence exceeding 100 J/cm2.

Design and analysis of a modified segmented cladding fiber with large mode area

This paper proposes a novel segmented cladding fiber structure for large mode area properties. In this structure a thin ring is placed between the high index core and nonuniform cladding. It is called Single-Ring Segmented-Cladding Fiber (SR-SCF). The novel fiber offers the possibility of single-mode(SM) operation from 1µm to 1.7µm with a large core diameter. With illustrations, the fiber has a better SM operation than segmented-cladding fiber (SCF) is demonstrated. A large effective area of 1000µm2 is achieved. The SM operation with very high suppression of the higher order modes can arise by 76%. Moreover, mode spacing between the adjacent modes (LP01 and LP11) is also improved significantly. Besides, the bending property is analyzed. It is found that the fiber is insensitive to bending angle ranging from −180° to 180° at bending radius of 30cm. The proposed fiber will play an important role in developing high power fiber laser, fiber amplifier and high power delivery application.

All-solid very large mode area ytterbium-doped silica microstructured fiber based on accurate control on cladding index.

We have demonstrated a new approach for developing very large mode area silica-based microstructured Ytterbium (Yb)-doped fibers. The microstructured region acting as pump cladding around the core is composed by periodically arranged low-index Fluorine-doped silica inclusions with an extremely low filling ratio of 0.088. To the best of our knowledge, we achieved the most accurate controlling on cladding index by 1 × 10-5 via our passively doped cladding (PDC) method. Two fibers with 127μm and 50μm core diameter respectively were fabricated from the same final preform designed by this approach. It is verified that our 50μm core diameter fiber can maintain robust single mode behavior at 1064nm wavelength. The advantage of an all-solid structure along with a much simpler fabrication process makes our approach very suitable for realizing very large mode area fibers for high power fiber laser application.

An optimized configuration of large mode field area PMMA photonic crystal fiber with low bending loss: a new approach

We propose to design a ytterbium doped two large MFA fiber for high peak power fiber laser and beam delivery application. These fiber has large MFA of 2492 µm2 when kept straight and 1021 µm2 bent over 8 cm radius at a wavelength of 1550 nm. The difference between propagation loss levels of FM and HOMs extend to efficient single mode guidance in the fiber core while bent. This allows further the fiber to make compact high power laser systems. Moreover the optimizing effective MFA of straight and bend fiber can be obtained by a suitable shape of fiber core, missing air holes in the core and the bending orientation which ultimately leads to the guidance a very high optical power. In fact the bend loss on FM and HOM of straight and bend fiber can be estimated by implementing 2D FDTD technique and it is found that, FM bend loss is very small in the order of 10−2 dB/m with bend orientation and the order of 10−3 dB/m with bend radius.

Design of a Solid-Core Large-Mode-Area Bragg Fiber**Supported by the National Natural Science Foundation of China under Grant No 61405240.

We report the design and simulation results of a solid-core Bragg fiber with 3-bilayer periodic cladding. The simulation results present single mode bandgap guidance, a large effective area of ∼400 μm2 around 1.08 μm, a very low bend loss of 0.038 dB/m at 1.08 μm even under tight bend radius (R = 4 cm), and excellent beam quality. The results indicate that the proposed fiber could be a competitive solution for high-power fiber laser applications.

Design and fabrication of ytterbium-doped photonic crystal fiber with low non-linearity

We report on an ytterbium-doped photonic crystal fiber fabricated by laser sintering technology combined with a solution doping method. This novel fabrication process has never been reported to the best of our knowledge. Together with low non-linearity, this PCF combines the advantages of high pump absorption efficiency and bend-insensitive, which makes this fiber predestinated for the high power fiber laser applications. The fiber laser experiment was conducted with a simple Fabry–Perot cavity to verify the performance of the PCF. A high slope efficiency of ~70.6% was obtained from a 1.5 m-long fiber. During the experiment, no roll over was observed up to the highest power level, which was only limited by the available pump power. The experimental results reveal the enormous output power scaling potential of the PCF.

Extending mode areas of single-mode all-solid photonic bandgap fibers.

Mode area scaling of optical fiber is highly desirable for high power fiber laser applications. It is well known that incorporation of additional smaller cores in the cladding can be used to resonantly out-couple higher-order modes from a main core to suppress higher-order-mode propagation in the main core. Using a novel design with multiple coupled smaller cores in the cladding, we have successfully demonstrated a single-mode photonic bandgap fiber with record effective mode area of ~2650µm(2). Detailed numeric studies have been conducted for multiple cladding designs. For the optimal designs, the simulated minimum higher-order-mode losses are well over two orders of magnitudes higher than that of fundamental mode when expressed in dBs. To our knowledge, this is the best higher-order-mode suppression ever found in fibers with this large effective mode areas. We have also experimentally validated one of the designs. M(2)<1.08 across the transmission band was demonstrated.

Large Mode Area Multi-Trench Fiber With Delocalization of Higher Order Modes

Multi-trench fiber (MTF) is a novel large mode area fiber design for high power fiber laser applications. This fiber design allows very high suppression of the higher order modes by offering high losses and delocalizing them out of the core. MTFs allow the core refractive index to be higher than the surrounding cladding as compared to other structures such as photonic crystal fibers, photonic bandgap fibers, and Bragg fibers. This feature of MTFs dramatically reduces the complexity associated with doped fiber fabrication. The MTF design is an all-solid structure with cylindrical symmetry, which provides easy cleaving and splicing with other fibers. In this paper, we present the first experimental demonstration of the MTF. S $^{2}$ measurements indicate single mode operation with very high suppression of the higher order modes.

Alumina as a dopant in optical fiber by OVD

Alumina is an indispensable dopant in modern optical fiber technology, particularly due to its important role played in the recent developments of optical fiber amplifiers and high-power fiber lasers over the last decade or so, which have revolutionized the telecommunication systems, and the industrial laser landscapes, respectively. In this paper, Alumina-doped optical fibers advanced through the outside vapor deposition process and their key attributes in the respective applications are reviewed, with Alumina both as a sole dopant for its potential use in the low-loss transmission lines and as a co-dopant for active use in the high-brightness, high-power fiber laser applications.