Fiber lasers are widely used in industry, national defense, and military fields owing to their advantages of high electro-optical efficiency, compact structure, and excellent stability, and Yb-doped fiber (YDF) lasers are a crucial fiber laser direction. Compared with 25 μm/400 μm fiber, 20 μm/400 μm YDF is less expensive and exhibits easier mode-controlling. Hence, 20 μm/400 μm YDF lasers have been widely applied and developed rapidly. Here, we successfully prepare high-quality 20 μm/400 μm YDF by improving the YDF preform fabrication technique. A master oscillator power amplifier (MOPA) system with the domestic 20 μm/ 400 μm double-clad YDF realizes the highest output laser power of 4 kW without the transverse mode instability (TMI) by adopting the backward pumping method.
Using a homemade 20 μm/400 μm double-clad YDF, we construct a MOPA system to test the property of the fiber at high power. The seed has a center wavelength of 1064 nm and output power of 19 W. A mode field adapter with a 10 μm/130 μm input fiber and 20 μm/400 μm output fiber is adopted to connect the seed and the chirped and tilted fiber Bragg grating which is utilized to suppress the stimulated Raman scattering (SRS) signal. We adopt a cladding power stripper (CPS) before the main amplifier to discard the residual pump power from the backward pump sources. The active fiber of the amplifier is a 15-m-long 20 μm/400 μm YDF and coiled with a 9-cm diameter to suppress the TMI effect. The pump sources comprising five 976-nm-LD pump modules are injected into the 20 μm/400 μm YDF using a (6+1)×1 signal-pump combiner with a 25 μm/400 μm signal fiber. The maximum output power of each pump module is 1030 W. A CPS is utilized before the quartz block holder to discard the higher-mode in laser signal. After passing through the system, the output power of the seed decreases to 8 W. The beam quality of the output laser is measured at different output powers, and the TMI effect is assessed according to time-frequency characteristics. The continuous laser output power at the 4-kW level is recorded to examine the long term stability of the laser system.
A laser output power of 4015 W with a slope efficiency of 81% is obtained . At the highest power, the beam quality factor (M2) is approximately 1.39. The output laser spectrum has a center wavelength of 1064 nm and broadens as the output laser power increases. When the output power reaches 2875 W, the SRS effect emerges. The 3-dB linewidth of spectrum at the highest output power is 1.3 nm, and the Raman suppression ratio reaches 31 dB. Here, TMI does not emerge. In addition, the continuous stable operation at 4 kW is realized for 1830 s. The obtained result indicates that the maximum variation of output power is 2.5% (Fig.4).
Based on the homemade 20 μm/400 μm double-clad Yb-doped fiber, we construct a master oscillator power amplifier system to test the property of the fiber at high power. The output laser power reaches 4 kW with a center wavelength of 1064 nm and a slope efficiency of 81%. At the highest power, the M2 is 1.39 and the Raman suppression ratio is better than 30 dB. In the future, we will optimize the structure of the laser system and increase the pump power to further study the property of homemade 20 μm/400 μm Yb-doped fiber.
