Fabrication of a Counter-Directional Pump/Signal Combiner with Built-In Mode Field Adapter

Abstract

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic.

1. Introduction

Fiber lasers have found extensive use across diverse domains, including medical, industrial processing, and national defense, owing to their excellent beam quality, high conversion efficiency, easy heat management, and flexible operation [1,2,3,4]. The pump/signal combiner (PSC) plays a crucial role in efficiently coupling the pump light into the double-cladding fiber and transmitting the signal light. It is one of the most critical components in fiber lasers [5]. Currently, the PSC can be categorized into two types: side-pumping combiners and end-pumping combiners. The side-pumping combiner is known for effectively maintaining the signal beam quality during the fabrication process because there is no interference to the signal fiber. Nevertheless, the high-power output of lasers based on side-pump combiners is restricted by the limited number of pump arms [6]. End-pumping combiners, which rely on tapered fused bundle (TFB) technology, are commonly employed due to their high-power capability and high transmission rate.

The quality of signal light transmission is a serious problem for the end-pumping combiner due to the complex manipulations that signal fiber must undergo. The degradation of signal light transmission quality in the end-pumping combiner is primarily attributed to the loss of splicing [7]. Liu conducted an analysis to examine the impact of lateral core offset and angular misalignment on the transmission quality of the signal light during the splicing process between the TFB and the output fiber [8]. Li suggested utilizing the M² value, which exhibits greater sensitivity towards the axial offset value, as the feedback alignment parameter [9]. The mode field mismatch is another major reason for the deterioration of the signal light transmission quality [10]. The researchers fabricated PSCs with built-in mode field adapters (MFA) via a thermally expanded core (TEC), fiber tapering, and intermediate fiber [11,12,13]. Nevertheless, these PSCs are mainly used to solve the mode field mismatch of single-mode fiber (SMF) and large-mode-area (LMA) fiber in the co-directional pumping, and there are no reports of counter-directional PSC with a built-in MFA. In contrast, the signal fiber of the co-directional PSC only requires the transmission of a low-power seed light, while the signal fiber of the counter-directional PSC requires the transmission of an amplified high-power laser [14]. Moreover, since counter-directional pumping is an effective means to improve the thresholds of transverse mode instability (TMI) and nonlinear effects, it is obvious that the counter-directional PSC must possess the capability to endure the injection of the pump light at greater power levels [15]. Therefore, the development of a counter-directional PSC with a built-in MFA that can achieve high coupling efficiency of the signal light and pump light, with low beam quality degeneration, presents a significant challenge.

TMI and nonlinear effects are the bottlenecks for the further improvement of fiber laser power [16,17]. With regard to the fiber, the nonlinearity suppression (preferring large core size) and TMI mitigation (preferring relatively small core size) could hardly be simultaneously optimized, owing to the contradicted core size requirements [18]. In order to achieve a more optimal equilibrium between TMI and nonlinear effects in high-power fiber laser systems, the implementation of a few-mode fiber (20/400 μm, NA = 0.06) as the gain fiber to suppress TMI and a larger core multimode fiber (50/400 μm, NA = 0.12) as the delivery fiber to mitigate stimulated Raman scattering (SRS) is employed. The counter-directional PSC as a device connecting the gain fiber and the delivery fiber will therefore have a mode field mismatch. In order to address this issue, a (6 + 1) × 1 counter-directional PSC with a built-in MFA is constructed in this paper. Through numerical simulation and experimental verification, the influence of taper length and taper ratio on signal light transmission quality are studied. With the help of an M² analyzer for in-line alignment and splicing, the signal transmission rate of the combiner is more than 98.5% and the beam quality is well maintained. The combiner exhibits an average pump coupling efficiency of 98.5% and a temperature rise coefficient below 10 °C/kW without active cooling. In addition, an integrated device is constructed by fabricating a cladding light stripper at the end of the signal fiber of the combiner and fusing it with the endcap, which is applied in a narrow linewidth fiber laser system.

2. Theoretical Analysis

Firstly, the transmission of signal light in the PSC is simulated by Rsoft, which is based on the beam propagation method. Figure 1 illustrates the schematic representation of the simulation structure. The signal fiber corroded by hydrofluoric acid is situated at the center of the TFB, consisting of six pump fibers and a glass tube. The TFB is gradually reduced in diameter to match the cladding diameter of the output fiber, hence enhancing the efficiency of pump transmission. In the proposed scheme, the core/cladding diameter of the pump fiber of the combiner is 220/242 μm (NA = 0.22), the signal fiber is 50/400 μm (NA = 0.12/0.46), and the output fiber is 25/400 μm (NA = 0.065/0.46). D₀ is the core diameter of the tapered signal fiber’s waist.

By fiber tapering, the MFD of the signal fiber is decreased to match the MFD of the output fiber. The primary principle that underlies the physical tapering process is the maintenance of a constant refractive index, while simultaneously reducing both the core diameter and cladding diameter in proportion. As a result, the numerical aperture (NA) of the core remains consistent throughout the tapered fiber. Figure 2 presents the numerical computation of the correlation between MFD and core diameter for fibers possessing varying NAs. It is evident that fibers exhibiting a smaller NA will possess a larger MFD when their core diameters are identical. The MFD of the fiber with a core diameter of 25 μm and NA = 0.065 is equivalent to that of the fiber with a core diameter of 28 μm and NA = 0.12.

The taper length will have an impact on the efficiency of signal light transmission. Figure 3 illustrates the relationship between the proportion of fundamental mode and the length of the taper, while keeping the waist area at a fixed length of 10 mm. Additionally, the fiber is injected with the fundamental mode, while varying the taper ratios. TR is defined as the diameter ratio of the original fiber to the taper waist. It can be seen that for fibers with the same TRs, when the taper length is less than 5 mm, the proportion of the fundamental mode decreases significantly with the decrease in the taper length. When the taper length increases to a certain value, the proportion of the fundamental mode gradually tends to be stable. Comparing fibers with different TRs, it is evident that the fundamental mode exhibits the highest proportion, reaching a maximum of 99%, when the mode field diameter (MFD) is matched. However, the proportion decreases when the MFD is mismatched. Furthermore, the proportion of fundamental modes exhibits oscillatory behavior around a stable value when the taper length is varied. As the degree of mode field mismatch grows, there is a corresponding increase in the amplitude of oscillation. The reason for this is that the mode field mismatch causes the higher order modes to be stimulated in the large-core multimode signal fiber. As a result, multimode interference occurs during transmission, causing periodic fluctuations in the proportion of the fundamental mode [19]. The taper length of the TFB is determined to be 10 mm, while the TR is fixed at 1.79, based on the simulation results.

3. Experimental Setup and Discussion

3.1. Fabrication Process of the PSC

According to the results of numerical simulation, it is suggested to reduce the diameter of the signal fiber core to 28 μm to improve the quality of signal transmission. To achieve this goal, the following scheme is adopted: Initially, a total of six pump fibers are conjoined in a pre-tapered configuration, forming a hollow fiber bundle with an annular structure. Subsequently, the etched central signal fiber is introduced into the cavity of the aforementioned hollow TFB. Following this, the TFB, comprising the central signal fiber and the pump fibers, is further tapered until it reaches the diameter equivalent to the cladding of the output fiber. Through the corrosion of the signal fiber and the pre-tapering of the pump fibers, it is possible to fabricate the PSC with various fiber types.

The specific manufacturing operation of (6 + 1) × 1 PSC with a built-in MFA has the following four steps.

1. The tapering of pump fibers. Firstly, the fluorine-doped glass tube with the inner/outer diameters of 850/1000 µm is pre-tapered to slightly larger than 726/854 μm so that the fibers can be arranged neatly and closely in a hexagonal shape in the tube. Secondly, with the help of alcohol and the seven-hole tube, as shown in the Figure 4a, seven pump fibers are bunched and inserted into the pre-tapered tube, and then, the fiber in the middle hole is pulled out separately. Thirdly, the fiber bundle, which consists of fibers and the tube, is tapered down to 715 μm. In this case, the six pump fibers are fused firmly together to form a TFB with a hole in the middle, as depicted in Figure 4b.

2. The etching of the input signal fiber. The hydrofluoric acid (HF) etching technique is frequently employed for the purpose of reducing the diameter of fiber cladding. This method is preferred due to its nondamaging effect on the fiber core throughout the etching process. The cladding diameter of signal fiber is etchded to slightly less than the size of the middle hole in the TFB (about 202 μm) and inserted into the TFB through the middle hole of the seven-hole tube.

3. The tapering and cleaving. The TFB is tapered down to 400 μm, which corresponds to the dimensions of the output fiber cladding. Then, the waist region of the TFB is cleaved by a large-core fiber cleaving system to achieve a smooth cleaving cross-section. The cross-section image observed under the microscope is shown in Figure 4c.

4. The in-line alignment splicing. The TFB is spliced to the output fiber (25/400 μm, NA = 0.065/0.46) by using the fiber fusion splicer. Given that the center of the signal core and the fiber bundle may not coincide perfectly, as shown in Figure 4d, it is imperative to perform manual alignment prior to splicing in order to minimize the axial offset value. The utilization of an M² analyzer is employed for the purpose of alignment splicing, as depicted in the schematic design presented in Figure 5. In this experimental setup, the output fiber of the combiner and the tail fiber of a nearly single-mode laser source is connected through an MFA with cladding light stripping function. In order to mitigate the potential impact of excessive laser power on the alignment process, the power of the laser source is set at about 1 milliwatt. The other end of the output fiber and the cleaved TFB are securely positioned within a fusion splicer to enable manual alignment adjustments. The output pattern from the signal fiber of the TFB is sent to the M² analyzer, which uses the second moment assessment method to measure the beam quality after each manual adjustment. The M² value after the completion of alignment is 1.25, as shown in Figure 6a, and the M² value after the splicing is 1.23, as shown in Figure 6b. An end face interval exists between the output fiber and the cleaved TFB during the process of in-line alignment. The difference in the refractive index between the fiber and air can cause a slight degradation in beam quality. The beam quality after splicing is better than the beam quality after alignment, hence suggesting the rationality of the splicing parameter setting.

In addition, we have manufactured MFAs with an input fiber of 25/400 μm and output fiber of 50/400 μm by in-line splicing. The experimental setup is similar to that in Figure 5: the input fiber of the MFA is spliced with a milliwatt single-mode laser source. The other end of the input fiber and the tapered output fiber are positioned within the fusion splicer. The output pattern from the output fiber of the MFA is sent to the M² analyzer.

The measured results in

Table 1. Beam quality test results of MFA with different TRs.

TR	D0	M2 after Alignment	M2 after Splicing
1.67	30 μm	1.27	1.3
1.79	28 μm	1.22	1.24
2	25 μm	1.33	1.36

illustrate a comparison of beam quality between MFA manufactured using output fibers with varied TRs, all with a taper length of 10 mm. It can be found that the beam quality of MFA after alignment is the best when TR is 1.79, followed by TR at 1.67 and TR at 2, which is consistent with the simulation results. Additionally, it has been observed that the beam quality after the alignment is superior to that observed after the splicing. This is because the tapering of output fiber leads to a mismatch between the cladding diameters of the input fiber and the output fiber, and fibers with different diameters require different heating areas and heating powers when fusion splicing, which leads to difficulties in guaranteeing fusion strength and beam quality at the same time.

3.2. Tests of the PSC

During the pump coupling efficiency test, a laser diode (Reci, DAB1200, with wavelengths of 915 and 976 nm) is connected through splicing to each of the six pump ports of the combiner. Each of the six pump ports exhibits a pump coupling efficiency that is above 98%, while the average pump coupling efficiency of the combiner is recorded at 98.5%. During the experimental phase, it is noticed that the maximum temperature point in the combiner is located at the edge of the signal fiber’s coating. This phenomenon is mostly attributed to the leakage of pump power from the coating. In the event that active cooling measures are not implemented, this temperature threshold has a coefficient lower than 10 °C/kW. It is important to note that this is the temperature rise measured by the suspension of the combiner before encapsulation, which can be reduced to less than 5 °C/kW by covering the low refractive index with UV glue at the coating edge during encapsulation of the combiner. The test results validate the exceptional efficiency of the pump coupling and heat management performance of the combiner, hence confirming its appropriateness for high-power fiber laser applications.

The signal performance of the combiner is tested with a 3 kW fiber oscillator. The output pigtail of the fiber oscillator is 25/400 um, NA = 0.065/0.46, while the pigtail of the endcap is 50/400 um, NA = 0.12/0.46. The output patten of the laser is tested through the endcap, whose M² value is 1.52, as shown in Figure 7a. After passing through an MFA with an input fiber of 25/400 μm and output fiber of 50/400 μm, the M² value of the signal light improves to 1.45, as shown in Figure 7b, and the measured transmission rate is more than 98%. Furthermore, when the signal light passes through the MFA and a PSC which is fabricated by the method described in this paper, with a signal fiber and output fiber of 50/400 μm, the M² value of the signal light is 1.51, as shown in Figure 7c, and the measured transmission rate is 97%. The beam quality degradation ratio of the PSC is only 4.1%, which indicates that the beam quality degradation of the PSC based on active alignment is negligible. After passing through the PSC with a built-in MFA, the M² value of the signal light is 1.38, as shown in Figure 7d, and the measured transmission rate is 98.5%. By reducing the number of splicing points, the PSC with a built-in MFA improves both the beam quality by 10% and the signal passing rate by 1.5%, compared to fusing MFA before PSC. It can also be found that the ability to maintain beam quality of the PSC with a built-in MFA is superior to that of a conventional MFA. The reason for this is the equivalent cladding formed by the pump fibers and the tube avoids splicing loss caused by cladding diameter mismatch.

3.3. Fabrication of Integrated Device Based on PSC and Application of Amplifier

An integrated device is created by incorporating a cladding light stripper (CLS) and fusion splicing an endcap at the end of the signal output fiber of the PSC with a built-in MFA. This methodology results in a decrease in the number of splicing points and efficient reduction in the length of the delivery fiber, leading to improved compactness and stability of the fiber laser system. The fabricated integrated device is applied to a narrow linewidth laser system with simple one-stage MOPA structure. The structure of the amplification stage of the system is depicted in Figure 8. The seed is a composite cavity structure narrow linewidth fiber oscillator in which external feedback is introduced into the cavity by a long passive fiber with the end of a flat angle [20]. The output power of the seed is about 20 W, and both the fiber and the device of the seed use 20/400 μm double-cladding fiber. The pump sources are eight wavelength-stabilized laser diode (WS LD) modules working at 976 nm, with each LD providing a maximum pumping power of ∼1100 W. CLS 1 is used to strip the backward pump light of the amplification stage. A 12.5 m long Ytterbium-doped fiber (YDF, 20/400 μm, NA = 0.065/0.46) is utilized here as the gain fiber, which is placed on the water-cooling plate. The absorption coefficient of the YDF is about 1.2 dB/m at 976 nm. The integrated device based on the combiner is spliced with the YDF to assume the tasks of pump light injection, mode field adaptation, cladding light stripping, signal light transmission, and output. The output delivery fiber length is deliberately regulated to a certain value of 1.2 m in order to effectively attenuate the Raman effect during the transmission of the laser. In order to achieve stable and safe working of the combiner under high-power conditions, the combiner is affixed onto the water-cooling plate using thermal silicone grease, and the cooling temperature is set as 20 °C.

Figure 9 displays the outcomes according to power and efficiency. The relationship between the output power of the amplifier and the pump power can be observed to demonstrate a linear increase. At an injection pump power of 7030 W, the output power achieved its maximum value of 5840 W, with all six backward LDs running at their maximum power capability. The performance of the amplifier demonstrates a comparatively diminished efficiency when the output power falls below 1.5 kW. The primary cause of this phenomenon can be attributed to the lack of stability exhibited by the pump diodes when operating at low power levels, as well as the potential inaccuracies associated with the measurements obtained from the power meter. However, as the injection current is augmented and the pump laser diodes attain a condition of stability, the slope efficiency of the amplifier stabilizes at 83.1%. This stability in slope efficiency is maintained due to the high efficiency of the pump coupling and signal transmission of the homemade combiner.

The spectrum of the amplifier at different output powers is depicted in Figure 10a, which was obtained by measurements conducted using an optical spectrum analyzer with a resolution of 0.02 nm. The amplifier’s output wavelength is centered at 1080 nm, with a 3 dB bandwidth that progressively increases from 0.44 nm, when operating at 3126 W, to 0.88 nm, while operating at 5840 W. The Raman light is seen to be 31 dB lower than the signal light when the output power reaches its maximum value of 5840 W, and there is no detection of any higher order Raman light. The variation in beam quality in the narrow linewidth system with power is illustrated in Figure 10b. It can be seen that with the increase in power, the beam quality of the system is well maintained. Figure 10c displays the temporal signals and corresponding Fourier spectra of the dumped cladding light. The presence of a frequency component between 0 and 5 kHz can be observed as a result of the dynamic interaction between fundamental and higher order modes. This observation suggests that the power of the system is approaching the threshold of TMI and a slight TMI occurs.

Table 2. The output performances of a narrow linewidth system with different PSCs.

Output Fiber	Signal Fiber	Output Power	Efficiency	M2	Signal to Raman Ratio
25/400 μm	25/400 μm	5500 W	82.7%	1.49	28 dB
50/400 μm	50/400 μm	5800 W	82.5%	1.74	32.3 dB
25/400 μm	50/400 μm	5840 W	83.1%	1.55	31 dB

presents a comparative analysis of the output characteristics of narrow linewidth systems operating at about 5800 W, while utilizing various PSCs. The utilization of PSC with a fiber core diameter of 25 μm has shown the ability to maintain high beam quality. The M² value at 5.5 kW is less than 1.5. However, as the power continues to increase, the proportion of Raman light increases rapidly, causing beam quality degradation, and the M² value is greater than 2 at 5.7 kW. The utilization of a PSC with a fiber core diameter of 50 μm results in an increased signal to Raman ratio. However, the beam quality is degraded as a consequence of the mode field mismatch. The PSC with a built-in MFA has the capability to retain a higher signal to Raman ratio while still maintaining outputs with excellent beam quality. When comparing the efficiency of systems utilizing different PSCs, it is observed that the system employing the PSC with a built-in MFA exhibits a higher optical-to-optical conversion efficiency. This can be attributed to the superior pump and signal light coupling efficiency of the combiner. As detailed in the present study, the PSC with a built-in MFA demonstrates the extensive potential for application in high-power fiber laser systems.

4. Conclusions

In conclusion, this study introduces a novel method for (6 + 1) × 1 counter-directional PSC with a built-in MFA, utilizing the pre-tapering of pump fibers and corrosion of the signal fiber. The combiner possesses the capability to effectively resolve the problem of mode field mismatch that occurs between the gain fiber and the delivery fiber. The degradation of the signal beam quality in the combiner is negligible thanks to the use of feedback alignment in splicing. The combiner exhibits an average pump coupling efficiency of 98.5% and a temperature rise coefficient less than 10 °C/kW without active cooling. Additionally, a nonsplicing point integration device based on the combiner is produced and utilized in a narrow linewidth one-stage MOPA laser system with 5840 W output power, proving the great potential of this kind of combiner for high-power laser applications.