Fiber lasers feature sound beam quality, compact structure, and flexible transmission. In recent years, they have developed rapidly and have been widely applied in industrial fields, such as metal cutting, remote welding, 3D cutting, and laser marking. With the rapid development of high-power and high-brightness laser diode (LD), and fabrication technologies of double clad fibers, the output power of fiber lasers continues to increase. In addition to gaining fibers as the core raw material of fiber lasers, the performance indicators and research progress of fiber passive devices, including but not limited to fiber Bragg gratings, cladding optical filters, fiber end caps, end pump/signal combiners, and signal combiners, are also closely related to the development of fiber lasers. The cladding light filter is employed to filter the cladding light by breaking the total reflection condition of the outer boundary of the fiber cladding, which causes the cladding light to be transmitted outside the cladding through refraction, scattering, or absorption effects. The fiber end cap is a high-power device designed for processing the output end face of high-power fiber lasers and amplifiers. By expanding the core of the output fiber to reduce the optical power density at the output end, the fiber end face is protected from damage. Additionally, an anti-reflective film is applied to the output surface of the glass cone rod to avoid the back light from ruining the laser or amplifier. The function of the end pump/signal combiner is to efficiently couple several pump beams and one signal beam into the double clad fiber spontaneously. Therefore, the fusion loss and signal quality deterioration rate are two important optical performance indicators for this combiner. The signal combiner is designed to combine multiple medium power fiber lasers to obtain higher-power fiber laser output, with the advantages of compact structure, high reliability, low cost, and sound stability.

We introduce the latest research progress of the high-energy laser research team at the National University of Defense Technology (NUDT) on key passive devices adopted in high-power fiber lasers. 2 kW bidirectional cladding optical filter, 30 kW fiber end cap, end pump/signal combiner with low beam quality degradation rate, and 20 kW signal combiner with high beam quality are mainly included.

1) Cladding power filter: In 2022, the NUDT research team designed a new preparation scheme for a weak-strong-weak cladding power filter that can achieve bidirectional filtering. The weakest textured area on both sides of the double clad fiber is 1 cm, and then it enters the medium textured area by 3 cm on both sides. Finally, the strongest textured area is located in the middle area, with a length of 3 cm. The fabricated cladding power filter is tested under 2051 W input power, of which the temperature rise rate is 3.5 ℃/kW and the filtration efficiency is 20.1 dB.

2) Fiber end cap: The NUDT research team designed and built an end cap fusion system in 2014. The heating source is a hydrogen oxygen flame, and an automatic alignment fusion system is designed to achieve the fusion of different shapes of end caps and optical fibers. Based on this system, the development of 3 kW single mode fiber end caps and 6 kW multimode fiber end caps was achieved in 2015. Based on the key technologies and specially designed fiber end caps, the current fabricated end caps have been successfully applied in a 30 kW high-power fiber laser system.

3) End pump/signal combiner: By changing the fusion position of the pump arm on the signal arm, the NUDT research team improves the pump coupling efficiency of the side pump combiner under high-brightness fiber laser pumping. The optimized combiner single arm passes the pump power test of 2737 W, with a coupling efficiency greater than 99% and a temperature rise coefficient of only 6.5 °C/kW. In 2015, the NUDT research team reported a type of end face signal pump combiner that utilized heating and core expansion technology to reduce signal insertion loss. Through this technology, the combiner's pass rate was increased from 51% to 94%. In 2019, the NUDT research team utilized multi-mode field adaptive structures to achieve a transformation of core size from 10 to 50 μm. In 2022, the NUDT research team employed the corrosion-threading method to improve the performance of the signal pump combiner. The combiner prepared through this scheme maintains sound signal beam quality, the signal degradation ratio of which is only 2.2% for 25/400 μm fiber and 5.1% for 50/400 μm fiber.

4) Signal combiner: In 2018, the NUDT research team developed a 7×1 signal power combiner with the maximum output power of 14 kW and the beam quality of 5.37. To further improve the beam quality of the combined laser, the key preparation process has been optimized to produce a signal power combiner that changes the numerical aperture of the fiber core from 0.22 to 0.12. In 2019, based on a self-developed 3×1 signal combiner with an output signal fiber of 50/400 μm (NA=0.12/0.46), the combined beam quality M² was optimized to about 3.5 under 6 kW output power. In 2021, based on a self-developed 4×1 signal combiner with an output signal fiber of 50/400 μm (NA=0.12/0.46), the maximum output power was about 12 kW with the beam quality M² of less than 4, which is the ever-reported optimal beam quality for synthetic lasers greater than 10 kW. In 2022, the NUDT research team fabricated a 7×1 signal power combiner and achieved more than 20 kW output power with M² less than 4.5, which is the highest reported power in similar synthetic laser systems in the size of the 50 μm fiber core.

The NUDT research team has been engaged in research on high-power fiber optic devices for over a decade. Some of the developed devices have reached international advanced performance indicators and applied as core fiber optic passive devices in multiple major equipment and scientific research tasks. This review provides a detailed introduction to the latest research progress of high-power fiber optic passive devices, including key process methods, technical difficulties, and some considerations for the future development of fiber optic devices. There are two potential directions of fiber optic passive devices, containing further improvement in beam quality retention characteristics and implementation of integrated fiber optic devices.