915 nm pumping kilowatt fiber oscillator with high optical-to-optical efficiency | Scientific Reports

Scientific Reports volume 14, Article number: 26331 (2024) Cite this article

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We demonstrate an all-fiber oscillator with high optical-to-optical efficiency using laser diodes working at 915nm as the pump sources to reduce the demand for thermal management. When the output power of the bidirectional pumped oscillator is 1.16kW, the optical-to-optical efficiency is as high as 75.4%. In this working state, the output characteristics of the oscillator are observed. The Raman suppression ratio is 41.23dB and the beam quality factor Mx2 = 1.14, My2 = 1.29. Analyzing the time trajectory of the highest power output, there are no significant characteristic peaks in the time domain signal and the frequency domain signal, which indicates that the oscillator has good stability.Improving the output characteristics of the 915nm pumping lasers has a positive significance for the application of non-strict ambient temperature control.

Fiber lasers are widely used in industrial fields such as laser cutting, laser welding, and laser drilling due to their advantages of compact structure, high efficiency, and high stability1,2. Since the output power of ytterbium-doped(Yb-doped) fiber lasers exceeded the kilowatt for the first time in 20043, the single fiber output power of Yb-doped fiber lasers has continuously increased to tens of kilowatts4,5,6,7,8,9,10. The research on 1-kilowatt fiber lasers is mainly focused on the early stages, usually using pump sources working at 97×nm and active fibers with large core diameters. In 2004, Jeong et al. reported a 1.36kW fiber laser using large core diameter fibers and 975nm pumping4. In 2011, Yan et al. reported 1.1kW of output power based on monolithic fiber laser with MOPA structure. The pump wavelength is 975nm and the optical-to-optical efficiency is surround 66.5%11. In 2012, Xiao et al. present a 1kW output power single mode fiber oscillator operating at a wavelength of 91×nm, utilizing Yb-doped large mode area double-clad fiber with core and inner cladding diameters of 20/400µm. Notably, the pump sources in this study exhibits a typical power conversion efficiency of 30% and the slope efficiency of the output laser is 71%12. In the same year, Yu et al. achieved a 1.2kW single-mode laser output with an optical-to-optical efficiency of 60%. The laser diodes(LDs) as pump sources operate at 915 ± 10nm and electro-to-optical efficiency less than 54%13. In 2010, the concept of mode instability threshold was proposed14. And since then, there has been an increasing amount of research on mode stability15,16,17,18,19. When the output power in the fiber laser system reaches a certain threshold, mode coupling or mode competition will occur between different modes in the fiber, leading to the deterioration of beam quality, usually resulting in a decrease in output efficiency. In recent years, research on fiber lasers has mainly focused on achieving high-power fiber lasers, pursuing breakthrough power, better beam quality, and longer-term reliable operation20. For industrial processing fields with kilowatt power requirements, higher brightness, higher optical-to-optical efficiency, and lower cost are also important.

Unlike most previous studies on 1-kilowatt fiber lasers, we selected laser diodes with a working wavelength of 915nm as the pump sources and provided energy to the Yb-doped fiber in a bidirectional pumping manner. The Yb-doped fiber with core/cladding diameters of 14/250µm, which reduces the number of transmission modes that can be supported within the core. Studies has shown that the bidirectional pumping method can provide higher mode instability threshold for fiber amplifiers21. It is worth mentioning that according to the absorption characteristics of Yb-doped fiber, the absorption cross section at the wavelength of 915 nm is smaller and wider. The smaller absorption cross section reduces the thermal load in the gain fiber, also bringing about strong gain saturation, thereby improving the mode instability threshold22,23,24. In addition, according to the temperature drift characteristics of the laser diode, the output wavelength of diodes will change with the temperature, and when the output wavelength changes in a range near 915nm, the laser can still maintain a good working condition. This has a very positive significance for industrial applications. Therefore, it is meaningful to study the output characteristics of fiber lasers pumped at 915nm. Generally, in order to obtain laser output with good mode stability, the winding radius of the Yb-doped fiber can be appropriately adjusted to increase the loss of the higher-order modes and increase the threshold of mode instability25,26. However, the loss of higher-order modes will also lead to an increase in system energy loss and a decrease in optical-to-optical efficiency. Meanwhile, due to the absorption cross section near the 915nm being much smaller than that of 976nm, the optical-to-optical efficiency of the entire system is significantly lower than that of laser systems with 976nm as the pump wavelength. In our experiment, when the maximum output power is achieved, the optical-to-optical efficiency of the oscillator system can reach 75.4%. The measurement of its beam quality factor and time trajectory indicates that the oscillator system has good mode stability in this operating state.

Our experimental configuration utilized 9 LDs operating at 915nm through bidirectional pumping as shown in Fig. 1, and two (6 + 1)×1 combiners connected the LDs and two fiber Bragg gratings(FBGs), with a high reflectivity(HR) FBG having a reflectivity of 99.5% and an output coupled(OC) FBG having a reflectivity of 10%. We selected a Yb-doped double-clad fiber with core diameter and inner cladding diameter of 14/250µm (Core Attenuation ≤ 20.0dB/km at 1200nm.Cladding Absorption = 0.80 ± 0.10dB/m at 915nm). And after our testing, the length of the gain fiber was finally determined to be 25m. Subsequently, the output from the cavity passed through the cladding stripper through a cladding pump stripper (CPS), and the laser working at 1080nm was output through quartz block head(QBH).

In our experiment, we achieved the highest efficiency of this oscillator by optimizing the fiber length, adjusting the winding radius, and finally coiling the gain fiber in a runway shape. During the experiment, we primarily employed the spectrum analyzer (Yokogawa AQ6370D), calibrated optical power meter (Coherent PM-USB PM3K), oscilloscope (Tektronix TDS3032C) and beam quality analyzer(WinCamD Series) to assess the output characteristics of this oscillator.

Experimental setup. LD laser diode operating at 915nm, HR-FBG high reflectivity fiber Bragg grating, OC-FBG output coupler fiber Bragg grating, YDF Yb-doped fiber, CPS cladding pump stripper, QBH quartz block head.

On the basis of the above structure, we analyzed the output characteristics of the oscillator. By continuously optimizing the gain fiber length and adjusting the winding radius, high optical-to-optical efficiency have been achieved. According to the formula for curvature loss of optical fibers, the winding radius of Yb-doped fibers can affect the loss of different modes within the fiber core27. When reducing the winding radius, higher-order modes have greater bending losses compared to lower order modes. This is a process of mode selection. When the high-order mode loss is large, the mode inside the fiber tends to approach a single fundamental mode, and the beam quality improves. In our experiment, according to our test results, the minimum turning radius during the winding process of the 25 m long Yb-doped fiber is ultimately controlled at around 5cm. Enable the oscillator to achieve high conversion efficiency while outputting good beam quality. Figure 2 shows the evolution of the optical-to-optical efficiency of this oscillator. In order to reduce errors during the measurement process, the results shown in the Fig. 2 are the average of two measurement values. When the output power up to 871W, optical-to-optical efficiency increases to 74.1%. Continue to increase the output power, the optical-to-optical efficiency has not decreased and continues to improve. When the pump power is 1538W, the maximum output power of the oscillator reaches 1160W, and the optical-to-optical efficiency is as high as 75.4%. This also indicates that the system has excellent stability during the process of power increase.

Output power evolution and optical-to-optical efficiency.

Observing the output spectral characteristics, the spectral evolution process under bidirectional pumping conditions is shown in Fig. 3. It can be observed that as the output power increases, the spectral shape remains almost unchanged while the spectral linewidths gradually widens. The Yb-doped fiber with diameters of 14/250µm used in the experiment has a smaller effective mode area, which leads to an increase in power density inside the fiber and the significant enhancement of nonlinear effects such as self phase modulation as the output power gradually amplifies, further widening the nonlinear effects of the output spectra28,29. In addition, as the oscillator power gradually increases, multiple factors such as nonlinear effects, gain saturation, and thermal effects can lead to changes in spectral linewidths. When reaching the output power of 1011W, weak Raman spectrum can be observed around wavelength 1135nm. At this point, the Raman suppression ratio is 47.39dB. There is a threshold for stimulated Raman scattering, which is related to the pump light intensity. When the pump power reaches the threshold of stimulated Raman scattering, the light field transfers part of the energy to the nonlinear medium to generate stimulated Raman scattering30. When the output power reaches 1160W, the Raman suppression ratio is 41.23dB.

Spectral evolution process and Raman threshold spectra.

To further observe the output spectral characteristics, Fig. 4 shows the center wavelengths of the output spectra at different powers, as well as the spectral linewidths of 3dB, 10dB, and 20dB. It can be observed that as the output power gradually increases, the center wavelengths of the oscillator’s output spectra remain within the range of 1079.9–1080.3nm. Observing the linewidth changes of 3dB, 10dB, and 20dB, we found that the overall linewidths increase linearly with the increase of output power. It is worth mentioning that after the output power increased to 732W, the phenomenon of spectral linewidths broadening at 3dB and 10dB become slow. After the output power reaches 1011W, the spectral linewidths tends to narrow.

(a) Spectral center wavelengths, (b) Spectral linewidths.

To verify the mode stability of the system, the beam quality factor measured at the highest output power in the experiment is Mx2 = 1.14, My2 = 1.29, which means a good single-mode output characteristic. Figure 5 shows the beam quality factors Mx2 and My2 of the oscillator at different output powers, as well as the energy distribution of its laser spot. When the maximum output power is reached, the value of Mx2 and My2 actually decreases slightly. According to the shape of the laser spots, it can be observed that the spot at this time is more round than other spots at low power.

Beam quality factor evolution with output power.

In recent years, more and more people have characterized mode stability by analyzing the time trajectory and Fourier spectrum of oscillator output31. In order to further observe the output working state of the oscillator system, we also verified the stability of the system from a time domain perspective. The time-domain spectrum of the 1160W power stable output of the oscillator was measured. Figure 6 shows the time-domain spectrum and frequency-domain spectrum under the highest power stable operating state. It can be seen that both time-domain and frequency-domain signals are very stable throughout the entire process, without generating periodic signals or other characteristic peaks. This indicates that the system is in a stable working state and has not reached the mode instability threshold at the highest power output.

(a) Frequency domain intensity, (b) Time domain intensity.

In this paper, a nearly diffraction limited all-fiber oscillator with high efficiency is reported and we have fully demonstrate the advantages of 915nm laser diodes as pump sources in oscillators. By optimizing the length and winding radius of the gain fiber, the laser can achieve kilowatt-level output power with high efficiency and high beam quality nearly 1080nm, which has good time-domain characteristics. When the pump power is 1538W, the output optical power is 1160W, which the optical-to-optical efficiency can reach 75.4%. The oscillator can be widely used in the industrial field because of its good output characteristics. Overall, this study also demonstrates the potential of 915nm-pumped all-fiber oscillators for high-power, high-efficiency fiber laser systems. The advantages of 915nm pumping will give lasers the opportunity to face more diverse industrial applications. In the future, we hope to achieve further breakthroughs in output power on the basis of ensuring high optical-to-optical efficiency and high stability.

The datasets used and analysed during the current study available from the corresponding author on reasonable request.

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This work was supported by National key research and development program “Information Photonics Technology” Key Project (Grant No. 2022YFB2804302).

Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen, 518055, China

Xin Chen, Yide Yang, Ping Su & Jianshe Ma

Department of Precision Instruments, Tsinghua University, Beijing, 100084, China

Mali Gong

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X.C. designed the experiments, X.C. and Y.Y. performed the experiments, X.C. analysed the results. X.C. wrote the manuscript. M.G., P.S. and J.M. revised the manuscript. All authors reviewed the manuscript.

Correspondence to Jianshe Ma.

The authors declare no competing interests.

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Chen, X., Yang, Y., Gong, M. et al. 915 nm pumping kilowatt fiber oscillator with high optical-to-optical efficiency. Sci Rep 14, 26331 (2024). https://doi.org/10.1038/s41598-024-77317-6

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Received: 13 June 2024

Accepted: 21 October 2024

Published: 01 November 2024

DOI: https://doi.org/10.1038/s41598-024-77317-6

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