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Abstract
An LP11-mode output all-fiber laser was presented, utilizing long-period fiber gratings (LPFGs) and polarization-maintaining optical fiber (PMF). The LPFG was designed and fabricated, achieving a 90.56% efficiency in LP01 to LP11 mode conversion. Furthermore, the transmission stability of LP11-mode in the PMF was also explored, with the spatial mode overlap ratio exceeding 0.95. Ultimately, the high-power polarization-maintaining (PM) fiber laser, capable of the LP11 mode output, was constructed, with the output power of 600 W and the beam quality M2 of 2.84. During the process of welding a thick Al-plate, the LP11 fiber laser exhibits a notable 1.88 times greater depth of fusion compared to the commercial single-mode fiber laser, when operating at the laser welding head speed of 100 mm/s. For applications demanding non-circular symmetric high-order modes, this research holds substantial potential for widespread adoption within the field of industrial processing.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber lasers are regarded as outstanding representatives of the new generation of solid-state lasers, being widely recognized for their exceptional beam quality, highly stable output, excellent optoelectronic conversion efficiency, low maintenance costs, and compact design [1,2]. In recent years, significant progress has been achieved in the technology of fiber lasers, with output power reaching kilowatt and even megawatt levels [3]. This rapid advancement has resulted in the widespread application of fiber lasers in various fields, including fiber optics communication, industrial processing, medical, and defense industries. Particularly in the realm of industrial processing, significant advantages have been demonstrated by fiber lasers. In comparison to traditional processing techniques, they are characterized by being contactless and having fewer consumables consumed, which has led to their extensive utilization in cutting, welding, engraving, and other processes [4,5]. This has not only improved production efficiency but has also reduced costs, thus leading to the continuous expansion of the market in the industrial and manufacturing sector.
Despite the common trend in laser technology of power increment, a notable shift is observed towards the implementation of intelligent beam intensity control. This trend has been evidenced in multiple fields, including laser welding [6], additive manufacturing [7], and laser hardening [8]. Numerous applications of the beam shaping have been successfully deployed, leading to a transformation of the laser processing landscape. In laser processing applications, the optimization of beam shape is met with a distinct challenge. The interaction between the material and the laser is influenced by the settings of numerous process parameters [9]. The enhancement of laser cutting and welding applications is widely acknowledged to be significantly achieved through the adjustment of beam characteristics [10].
Furthermore, it has been suggested by research that process efficiency can be enhanced by the utilization of multi-beam modes [11], elliptical beams [12], tunable beams [13], annular beams [14], or dynamic beam shaping methods [15–18]. Although these beam shaping techniques have been proven to enhance laser cutting performance, their potential for industrial applications is not considered ideal due to the requirements for hardware complexity. Therefore, currently, beam shaping solutions employing a full-fiber structure are still regarded as the preferred choice in high-power fiber lasers. This approach allows the structural stability of fiber lasers to be maintained while fully capitalizing on the advantages of fiber technology. In the method for converting mode selection within the full-fiber structure, several common approaches are encompassed, including fiber coupler conversion, fiber Bragg grating mode selection, and long-period fiber grating (LPFG) conversion [19,20]. Among these, LPFG conversion is widely adopted due to its customizability and stability.
In traditional few-mode circular-core optical fibers, it is commonly believed, based on the weakly guiding approximation, that only a few low-order modes are supported, such as the LP01 and LP11 modes. However, more modes are supported by these fibers due to differences in polarization direction. Thus, even in a typical two-mode fiber, at least six different fiber modes are supported. These modes are associated with slight differences in propagation speeds, and when multiple modes coexist in the same fiber, changes occur in the cross-sectional intensity distribution of high-order modes along the fiber's length. Furthermore, these modes can be influenced by variations in the fiber's surrounding environment. Minor environmental changes can result in small phase differences among nearly identical eigenmodes, further leading to variations in the cross-sectional intensity distribution of high-order modes. This environmental sensitivity limits the practical applications of high-order modes in some fiber devices. The instability of high-order modes presents a challenge that restricts their practical utilization in fiber devices [21–23].
Considering the above, an all-fiber structure LP11 fiber laser was proposed in this paper. Efficient optical energy conversion from LP01 to LP11 modes was achieved while maintaining the transmission stability of the LP11 mode. First, mode conversion from LP01 to LP11 modes was accomplished using a LPFG based on the designed periodic structure. The exceptionally low insertion loss guaranteed the laser power of 600 W or even higher power through the LPFG. Subsequently, the polarization-maintaining optical fiber (PMF) fiber and the fiber laser were designed to guide only a specific fiber mode, thereby preserving the spatial stability of the LP11 mode. Finally, comparative tests were carried out to explore the application capabilities of the fiber laser. In comparison to conventional single-mode fiber lasers, it demonstrated a processing depth advantage that was 1.88 times greater. The all-fiber structure LP11 fiber laser not only permitted efficient energy conversion but also offered an exceptional spatial mode overlap ratio, making it suitable for various applications, including laser processing, optical communication, and optical sensing.
2. Simulation of the LPFG and the PMF
Various methods exist for achieving mode conversion from LP01 mode to higher-order modes. These conversions can be accomplished through techniques such as offset core fusion splicing, optical fiber taper couplers, spatial light modulators, and LPFGs. In this paper, the utilization of a LPFG for the conversion of the LP01 mode to the LP11 mode in a high-power all-fiber laser was considered. The LPFG was preferred due to high mode conversion efficiency, low insertion loss, and high power-handling capabilities.
Similar to regular optical fiber gratings, the central resonance wavelength of a LPFG can be calculated using the following formula.
The equation above clearly illustrates that the resonant wavelength of a LPFG is determined by both the grating period and the effective refractive index of the optical fiber mode. In other words, when a LPFG fulfills the phase-matching condition, it facilitates the transformation between the fundamental mode and higher-order modes within the fiber. Furthermore, the efficiency of mode conversion in the LPFG can be quantified using the cross-coupling rate formula.
According to the equation provided above, the relationship between grating length and cross-coupling efficiency can be simulated and calculated. As depicted in Fig. 1, it can be noted that, as the grating length is increased, the conversion efficiency is initially enhanced and subsequently diminishes. This phenomenon is explicable in terms of the coupling and over-coupling between modes. When a certain grating length threshold is surpassed, the energy of the LP01 mode is shifted into the LP11 mode, and with further lengthening, the energy of the LP11 mode is redirected back into the LP01 mode.
Fig. 1. The relationship between cross-coupling efficiency and grating length for LPFG at a Wavelength of 1064 nm.
Numerous studies have already been conducted, demonstrating mode conversion from LP01 to LP11 using long-period fiber gratings. However, most of these research findings have primarily focused on applications in the fields of communication and sensing, resulting in limited power-handling capabilities. In contrast, this study focuses on the LPFG with a comprehensive consideration of mode conversion efficiency, insertion losses, power-handling capacity and other parameters. With the aim of enabling the application in high-power industrial fiber lasers, it is imperative to attain a kilowatt-level power capability for the LPFG.
The instability of the LP11 mode, which can adversely affect the consistency of fiber laser processing applications, is associated with certain issues. Two orthogonal polarization modes are supported by the core of a single-mode fiber (SMF), and they exhibit the same spatial intensity distribution, known as the LP01 mode under the weakly guiding approximation. The following four higher-order modes, denoted as the LP11 mode, are supported by the core of a dual-mode fiber. These modes have nearly identical propagation velocities and cross-sectional optical intensity distributions. Therefore, a dual-mode fiber supports six modes, including the three modes depicted in Fig. 2, along with their cross-polarization counterparts.
Fig. 2. The guided modes in optical fibers. (a) LP01 mode; (b) and (c) LP11 modes. The red circles indicate the core regions.
In a circular core optical fiber, the LP11 mode is comprised of the TE01, TM01, even HE21 and odd HE21 modes, each with slightly different propagation velocities. Consequently, when more than one mode is simultaneously propagated in a multimode fiber, the intensity distribution of the fiber modes is varied along the fiber's length. Additionally, changes in the cross-sectional intensity distribution of the output mode field with variations in the environment are caused by the differential phase shift among the four eigenmodes of the LP11 mode. The practicality of optical fiber devices is limited by the instability of the LP11 mode. However, during the investigation of the PMF, it was observed that two polarization eigenmodes exhibit different propagation velocities. In this scenario, the intensity distribution of the second-order mode is uniquely defined and remains stable.
PMFs come in a variety of types, among which the panda-type PMF has gained popularity due to its numerous advantages. It achieves polarization maintenance by creating a locally symmetric structure with stress regions located on both sides of the fiber core, leading to a significant radial expansion force due to thermal expansion within the stress application region. This increases the difference in propagation constants between the two orthogonal polarization modes, effectively eliminating mode coupling and thereby achieving polarization state preservation.
The simulation model of the panda-type PMF is depicted in Fig. 3(a), and the parameters of the simulation model were configured based on the dimensional measurement diagram presented in Fig. 3(b). For the simulation, the core of the optical fiber was characterized by a diameter of a = 25 µm, and the cladding had a diameter denoted as b = 400 µm. Within the optical fiber, the stress region was characterized by a diameter of c = 105 µm. The distance from the core's center to the stress region was set as d = 90 µm.
Fig. 3. The simulation model (a), the actual dimensions (b) and the stress distribution (c) of the panda-type PMF.
The optical fiber material had a Young's modulus of E = 78 GPa, a Poisson's ratio of Nu = 0.17, and thermal expansion coefficients for the core, cladding, and stress region, denoted as acore = 1.06e-6 K^-1, aclad = 5.4e-7 K^-1, and astress = 2.15e-6 K^-1, respectively. The melting temperature of the optical fiber was designated as Tm = 1000 °C, while the ambient temperature was denoted as T0 = 20 °C. The refractive indices for the core, cladding, and stress region in the absence of stress were Ncore = 1.454, and Nclad = Nstress = 1.45. The photoelastic coefficients of the optical fiber material were specified as A = 7.572e-13 m2/N and B = 4.1878e-12 m2/N, with the relative photoelastic coefficient being expressed as C = A-B. A geometric model has been established based on the aforementioned specifications, and material properties have been assigned to each region. In accordance with solid mechanics, the simulated result of the stress distribution in the PMF is depicted in Fig. 3(c).
Then, by employing the finite element method for numerical calculations, as illustrated in Fig. 4, the effective refractive indices of the LP11E and LP11O modes are presented. It can be calculated that the effective refractive index difference between the LP11E and LP11O modes in the optical fiber being utilized is approximately 4e-4. Therefore, the effective mitigation of mode coupling between the LP11E and LP11O modes during the transmission of polarized laser light is enabled by the utilization of this polarization-maintaining optical fiber.
Fig. 4. The simulation results of the mode field distribution in the PMF. (a) The LP11E mode; (b) The LP11O mode.
The simulation analysis was conducted on the effective refractive indices of LP01 mode along the fast axis, LP01 mode along the slow axis, LP11O mode, and LP11E mode in the PMF, as depicted in Fig. 5, Wwhere, LP01f represents the effective refractive index of LP01 mode along the fast axis, and LP01s represents the effective refractive index of LP01 mode along the slow axis.
Fig. 5. The relationship between the effective refractive index and the wavelength for different fiber modes in the PMF.
Through formula (1), the relationship between the central resonant wavelength and the grating period for the mode converter can be calculated. As depicted in Fig. 6(b), the two curves of the LP01f-LP11O and LP01s-LP11E mode converters are remarkably similar. Upon closer examination of the curves around 1064 nm, as depicted in Fig. 6(a), it is evident that, when the grating period is 2425 um, the central resonant wavelength of the LP01f-LP11O mode converter occurs in the short-wavelength direction, while the central resonant wavelength of the LP01s-LP11E mode converter appears in the long-wavelength direction.
Fig. 6. The relationship between the resonant wavelength of the mode converters and the grating period. (a) The simulation wavelength is from 1060 nm to 1070 nm; (b) The simulation wavelength is from 600 nm to 1700nm.
3. Fabrication and performance of the LPFG
3.1 Preparation platform for LPFGs
The creation of LFPGs with a tapered design was made by the FSM-100P + fiber fusion splicer. The preparation platform for fabricating LPFGs is depicted in Fig. 7. In contrast to conventional fiber fusion splicers, not only are the ZL and ZR motors controlled by this fusion splicer, but also the optical fiber is periodically repositioned by the SWEEP motor. The entire range of motion for the fusion splicer is ±18 mm, with a movement precision of down to 0.01 µm. This advancement significantly enhances the length and precision of LPFG manufacturing. Furthermore, this fusion splicer is equipped with program editing capabilities, which are highly advantageous for LPFG development.
Fig. 7. The preparation platform for fabricating LPFGs. SMF: single-mode optical fiber; MFA: Mode Field Adapters; CPS: Cladding power stripper.
The method of fabricating LPFGs using the FSM-100P + fusion splicer is preferred over traditional electric arc discharge techniques for several reasons. Firstly, the need for additional moving platforms, counterweights, or control computers is eliminated by this approach. Secondly, fiber alignment is ensured, and high stability is exhibited by it. Therefore, the FSM-100P + fusion splicer was ultimately chosen for the production of LPFGs. Compactness, integration, stability, flexibility, and portability are among the benefits offered by this technology. Naturally, similar to traditional fabrication techniques, the tapered shape is achieved by controlling parameters such as discharge power, discharge time, and movement speed.
In the production of LPFGs, the utilization of a supercontinuum light source, such as the NKT Phonics Superk Compact, and a spectrometer, such as the Yokogawa AQ6370C, can be employed to monitor the transmission spectrum of LPFGs. Traditional LPFGs are typically used to couple core modes with cladding modes and are employed for the detection of changes in the external environment. When the transmission spectrum is observed, techniques like matching liquids and high refractive index gels can be employed to remove the cladding light after the LPFG. This allows for clear observation of the transmission peaks in the spectrometer.
However, the LPFG developed in this study are designed for coupling core modes, and it is necessary to connect a SMF with a single cladding layer to the optical path of the fabricated LPFG. This approach introduces losses in higher-order modes, enabling the observation of resonance peaks during the product fabrication process in the spectrometer.
Using this preparation platform, the sample of the LPFG was successfully fabricated, as shown in Fig. 8. By adjusting the motion parameters of the optical fiber fixture and the discharge parameters of the electric spark rod, it is possible to effectively control the fabrication process of LPFGs.
3.2 Performance of the PM LPFG
In the experiment, an optical fiber with a core diameter of 25 µm, a clad diameter of 400 µm, and a numerical aperture (NA) of 0.11, was used to fabricate the LPFG. The appropriate tapering parameters were determined based on the simulation results, and the fused tapering method was utilized to fabricate the LPFG, resulting in the conversion of LP01-LP11 modes. The transmission spectrum of the LPFG was monitored using a spectrometer. To enhance the clarity of the LP01-LP11 mode conversion process, an SMF cladding filter was fabricated at the fiber output end, leading to an increase in the loss of the converted LP11 mode. This allowed the acquisition of the transmission spectrum of the LPFG.
For the fabrication of the LP01-LP11 mode converter, the grating period was set to 1980µm. In the spectral observations during the manufacturing process of LPFG, as shown in Fig. 9 (a), the presence of two loss peaks in the LP01-LP11 mode conversion were revealed. This phenomenon can be attributed to the results obtained from refractive index measurements on actual optical fibers, which revealed distinct but closely spaced effective refractive indices for the LP11E and LP11O modes, thus generating closely spaced loss peaks and impacting the bandwidth of the transmission spectrum. Upon comparing the simulation analysis with the experimental results, there were some disparities. This is primarily attributed to the fact that the index distribution of the optical fiber was an ideal step function in the simulation. However, in practical testing, some minor irregularities were observed in the index distribution of the core due to limitations in the manufacturing process.
Fig. 9. The transmittance spectra of the LP01-LP11 mode converter. (a) the grating length equaled 13.86 mm; (b) the grating length equaled 19.8 mm.
When the grating length equaled 19.8 mm, the maximum coupling efficiency was achieved. The measured transmission spectrum for LP01-LP11 mode conversion is depicted in Fig. 9(b), exhibits a resonant center wavelength of 1063.4 nm and a full width at half maxima (FWHM) of 72.2 nm. The loss peak measured -10.25 dB, indicating a 90.56% mode conversion efficiency from the LP01 mode to the LP11 mode.
Utilizing a low-power, narrowband light source at 1064 nm, the optical beam quality analyzer equipment was utilized for the assessment of the input and output beam qualities of the LP01-LP11 mode converter. As depicted in Fig. 10, the beam quality M2 of the input source was measured at 1.2. Following transmission through the LPFG, a change in the output beam quality M2 to 2.9 was attained. The mode field distribution of the focal spot indicates successful mode conversion from LP01 to LP11.
The phase changes of transmission modes within optical fibers can be disrupted by various factors, such as environmental temperature and optical fiber jitter. Consequently, the coupling between different modes can be affected. This is why widespread applications in the field of optical fiber sensing are found for LPFGs. However, in the realm of industrial laser technology, these influences can be detrimental, potentially causing severe disruptions to the consistency of optical fiber laser processing. To quantitatively investigate the spatial mode stability properties of the LP11 laser under construction, the spatial mode overlap ratio between the output mode field distributions of the laser and the ideal mode field distribution can be determined through the equation of overlap integration.
The PM LPFG was utilized within a PM fiber laser for the achievement of mode conversion from LP01 to LP11. Two fiber holders were securely attached to the translation fiber. Subsequently, the relative angles of the holders were adjusted to introduce a twist to the fiber. When the translation fiber was twisted, the spatial mode overlap ratio for the output mode consistently remained above 0.95, as depicted in Fig. 11. This minor degradation is primarily attributed to the incomplete excitation of the LP01 mode into the LP11 mode, which results in a phase difference between the LP01 mode and the LP11 mode during transmission.
Fig. 11. The relationship between the spatial mode overlap ratio of the PM LP11 laser output mode and the twist angle of the output fiber.
When the LPFG was utilized in a randomly polarized fiber laser, the output fiber mode was still transformed from LP01 to LP11. However, when the output fiber was twisted, significant fluctuations were observed in the spatial mode overlap ratio, with a decrease to 0.6, as depicted in Fig. 12. Owing to the severe mode coupling occurring between degenerate modes in commercially available circular optical fibers, the stable transmission of the LP11 mode is unattainable within such fiber.
Fig. 12. The relationship between the spatial mode overlap ratio of the randomly polarized LP11 laser output mode and the twist angle of the output fiber.
4. Construction and application of the fiber laser
4.1 Experimental setup of the PM fiber laser
To ensure the practicality of the LP11 mode, it is crucial to maintain its beam stability. Based on the previous analysis, firstly, it is necessary to construct a single-mode laser that is propagated into a PMF. Subsequently, the LP11 mode is translated from the LP01 mode through the use of a LPFG. This will guarantee the stability of the beam.
Firstly, a PM fiber laser was constructed employing a master oscillator power amplifier (MOPA) structure, as depicted in Fig. 13. This structure encompasses several critical optical components, including the seed source, the pump source, the pump combiner, the gain fiber, and so on. The seed source generates low-power laser, which is gradually amplified through three amplification modules, eventually reaching a power level of 600 W.
Fig. 13. The optical path structure of the PM fiber laser for outputting the LP11 mode. PM: Polarization-maintaining; YDF: Yb- doped fiber; CPS1 and CPS2: Cladding power stripper; MFA: Mode field adapter; LPFG: Long-period fiber grating; QBH: Quartz block head.
Once a stable PM LP01 fiber laser has been obtained, the LP01 mode was converted into the LP11 mode using a LPFG. To achieve high-power output, all these fiber components were placed on a water-cooled platform to ensure the appropriate temperature range. The excess cladding laser and pump laser were filtered out by cladding power strippers (CPS). Finally, the laser was directed through a quartz block head (QBH) for characteristic measurements.
4.2 Performance of the PM fiber laser with the PM LPFG
Assembled with a master oscillator power amplifier (MOPA) structure, a PM fiber laser has been designed with a maximum output power of 600 W. The structure comprises key fiber components, including the seed source, pump source, pump combiner and so on, as depicted in Fig. 13. Furthermore, apart from the pump fiber, all fiber types used in the laser system were panda-type PMFs.
First, the seed source possessed excellent PM characteristics and a narrow bandwidth, thereby laying the groundwork for the subsequent high-power laser output. After the seed source, a CPS was employed to eliminate unabsorbed pump light and cladding light escaping from the fiber core. The CPS was produced by chemically etching the surface of the fiber cladding, creating irregular pits on the cladding surface. This results in the laser leaking out at the cladding interface where it does not satisfy the total internal reflection condition. Additionally, an 8° cleave angle was implemented at the fiber's output end to forestall end-face reflections. Through experimentation, an output power of 10 W was achieved, featuring a central wavelength of 1063.628 nm.
In the amplifier stage, the higher pump power was required. To effectively mitigate nonlinear phenomena, the method of backward direction pumping was employed for the amplifier stage. Using three 976 nm pump lasers with a combined pump power of 739.3 W, the pump lasers were coupled into the Yb-doped optical fiber through a (6 + 1)x1 pump combiner. The core diameter of the Yb-doped optical fiber is 20 µm, and the NA is 0.065. The length of the Yb-doped fiber was 16 meters, and the absorption coefficient of the fiber at 976 nm was 1.2 dB/m. A mode-matching adapter (MFA) was used between the seed source and the amplifier stage. The MFA utilizes optical fiber tapering, with meticulous matching of the core diameter of the PM 20/400 fiber and the CPS1. Utilizing a PM fusion splicer, the stress regions of both fibers were accurately aligned during the fusion splicing process. This approach significantly improves the transmission efficiency of the LP01 mode while minimizing the excitation of higher-order modes. After the fabricated LPFG was connected to the amplifier, the CPS was utilized to eliminate the laser leakage into the cladding. The leakage can be attributed to two primary factors: firstly, during the conversion of the LP01 core mode to the LP11 mode, there were also conversions from the LP11 mode to other modes, including higher-order modes and the fundamental mode; secondly, additional leakage was incurred during the fusion splicing process of the optical fiber device.
Finally, the fiber components were securely mounted on a water-cooled plate for effective heat dissipation, with the utilization of the QBH output. The length of the translation fiber in the QBH was 10 meters, which facilitates flexible industrial processing applications. Following high-power testing, an output power of 600.1 W is achieved, featuring a central wavelength of 1063.636 nm and an FWHM of 0.104 nm, as depicted in Fig. 14 (a). In the amplifier stage, the input power of the pump, the input power of the seed laser, and the output power of the laser were independently measured using power meters. The seed laser power injected from the seed source into the amplifier was 10 W, and the amplified laser, having traversed the LPFG, CPS, and QBH, attained a maximum output power of 600.1 W. As illustrated in Fig. 14 (b), an 80.11% slope efficiency was realized, with an optical-to-optical conversion efficiency of 79.82%.
Fig. 14. The measurement of the laser power. (a) The output spectrum of the amplifier stage; (b) The slope efficiency and optical-to-optical conversion efficiency of the amplifier stage.
Due to effective heat dissipation and careful fiber fusion, the temperature of the LPFG was maintained at a very low level. Power variations were recorded for 30 minutes at 600 W, as depicted in Fig. 15. During this period, the laser exhibited a maximum power of 602.73 W, a minimum power of 595.34 W, and an average power of 600.65 W, with a standard deviation of 1.15 W. The laser's beam quality was tested using Primes LQM + at 600 W. As shown in Fig. 16, a symmetric two-lobe distribution was observed in the mode field, with an M2 factor of 2.84.
Fig. 16. The beam quality of the laser under high-power testing conditions. (a) Intensity distribution in focus plane;(b) The relationship between spot radius and measurement position.
4.3 Applications of the PM fiber laser
In the analysis of factors affecting the quality of welding in the laser welding process, a multitude of interconnected factors is taken into consideration [24]. Firstly, the welding outcome is significantly influenced by the utilization of high-power and low-power lasers in laser welding. Higher power lasers generate increased power density within the welding zone, signifying more energy focalization and consequent rapid heat accumulation. Conversely, lower power lasers under similar conditions produce shallower melt pools and smaller weld sizes. The swift heat accumulation from high-power lasers can lead to an increase in spattering during welding, potentially posing challenges to both welding quality and process stability.
Additionally, the defocus distance is also a critical factor influencing the quality of welding. Upon adjustment of the defocus distance, the spot size of the laser beam changes. For instance, with a standard Gaussian beam, adjusting the defocus distance causes the spot to enlarge, resulting in the dispersion of the central energy of the focal point into other areas. This alteration in the spot directly impacts the energy transfer and welding characteristics during the welding process, necessitating consideration and adaptation.
Furthermore, changes in processing speed significantly impact the final welding outcome. Decreasing the processing speed leads to increased heat accumulation, consequently resulting in deeper weld pools and larger surface radii of the weld. Fine-tuning such parameters is crucial for controlling the welding outcome, especially in applications requiring precise welding control, such as in the welding of miniature parts or the production of high-precision components.
Hence, for comparative analysis of morphological characteristics of melt pools, both the LP01 laser and the LP11 laser were employed to weld a thick Al-plate under the same processing speed (100 mm/s) and power (600 W) settings. Welding was conducted at the focal point, representing a zero focal distance, to showcase the beam characteristics, while a fast-processing speed was utilized to prevent excessive heat accumulation.
As depicted in Fig. 17, the comparative analysis highlights the differences in melt pool morphology observed when using LP01 and LP11 lasers for laser welding. In Fig. 17(a) and (b), due to the higher energy at the center of the LP01 laser with lower energy on both sides, it formed a ‘V'-shaped molten pool. The width of the melt was 1.27 mm, with a depth of 0.95 mm. As shown in Fig. 17(c) and (d), when the LP11 laser's twin-lobed peak line was perpendicular to the processing direction, it generated ‘U'-shaped molten pools due to the formation of heating areas in two directions. The width of the melt was 1.48 mm, with a depth of 1.85 mm. The LP11 fiber laser displayed a notable 1.88 times greater depth of fusion compared to the LP01 fiber laser. And the ‘U'-shaped molten pool possessed a higher aspect ratio. Under equivalent depths or similar weld bead diameters, it offers a larger bonding area. Furthermore, by adjusting the laser direction to align the twin-lobed peak line of the LP11 laser parallel to the processing direction, as shown in Fig. 17(e) and (f), the width of the melt was 1.29 mm, with a depth of 1.49 mm. In the welding process, the same weld point is effectively welded twice. Although it also forms a ‘V'-shaped molten pool, it attains a deeper depth compared to the LP01 laser.
Fig. 17. The correspondence between the direction of the laser beam spot and the welding molten pool. The LP01 laser beam spot (a) and its molten pool morphology (b); when aligned with the laser processing direction, the LP11 laser beam spot (c) and its molten pool morphology (d); when perpendicular to the laser beam processing direction, the LP11 laser beam spot (e) and its molten pool morphology (f).
5. Conclusion
In summary, we presented the design of a fiber laser capable of generating the LP11 mode, utilizing the LPFG and PMF. Through theoretical simulation and experimental analysis, the LPFG was fabricated with a grating period of 1980µm and a grating length of 19.8 mm, resulting in a mode conversion efficiency of 90.56% from LP01 mode to LP11 mode. Simultaneously, the stable transmission of the LP11 mode was achieved using the PMF, with the spatial mode overlap ratio exceeding 0.95. Finally, a high-power PM fiber laser was assembled, yielding an output power of 600.1 W at a central wavelength of 1063.636 nm, a FWHM of 0.104 nm and an M2 factor of 2.84. Utilizing the LP11 fiber laser for welding a thick aluminum plate, under identical processing speed (100 mm/s) and power (600 W) settings, results in the formation of ‘U'-shaped molten pools, characterized by a width of 1.48 mm and a depth of 1.85 mm. The methods proposed aim to enhance mode stability, particularly for non-circular symmetric modes, contributing to improved consistency in industrial processing applications.
Funding
National Key Research and Development Program of China (2023YFB3610700).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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