Objective Laser is used as light energy and light tool in the manufacturing industry. Its characteristics such as including laser power, beam quality, and control of light, are not only the standards to measure performance of laser manufacturing system, but also the bases for selecting laser manufacturing system. Therefore, both laser power and beam quality must be considered seriously. Laser power is used as an index the processing capacity of the laser manufacturing system. Beam quality limits processing method, transmission distance, focal spot size, and processing quality. Under the condition of high average power pumping, thermal lens effect is generated, as the pump light is tightly concentrated at the laser gain medium. In the meantime, the spherical aberration of the thermal lens strongly affects the beam quality. Thus, how to improve beam quality, enhance the conversion efficiency, and shorten pulse duration are very important. In order to reduce the influence of thermal lens effect on beam quality of the solid-state laser, an Innoslab amplifier of spherical-aberration self-compensation based on ZEMAX physical optical simulation is proposed. In present study, we demonstrate a fiber-solid hybrid system based on an Innoslab amplifier with an all fiber laser and a double-passing amplifier as the seed source. This system combines the advantages of fiber laser, such as high beam quality, high electro-optical efficiency, and easy to obtain mode-locked narrow pulse, and those of solid laser, such as able to reduce the nonlinear effect and sustain high peak power.
Methods In this paper, we proposed a fiber-solid hybrid Innoslab picosecond amplifier with high beam quality and high repetition rate. The seed consisted of an all fiber laser and a double-passing end-pumped amplifier. During the experiment, an eight-passing Innoslab amplifier based on the spherical-aberration self-compensation theory was designed by using the sequence mode of ZEMAX software. The seed light realized eight-passing amplification in the slab crystal. Higher extraction efficiency was obtained by increasing the overlap area of seed light and pump light in the gain medium. In order to reduce thermal lens effect in the slab crystal thickness direction and its influence, two measures were taken. The first one was to connect the crystal with the heat sink by double-sides welding method, so that air holes in the welding layer were avoided, the flatness of the indium layer was improved, and the crystal surface was uniformly dissipated. The second one was to suppress the thermal distortion in the slab crystal thickness direction with spherical-aberration self-compensation, so that the beam quality degradation caused by the thermal lens effect was overcame.
Results and Discussions In order to better understand the phase variation of the light beams with positive spherical aberration, we used ZEMAX software to simulate the process according to the principles of geometrical optics (Fig. 7). If the two identical thermal lenses were symmetrically placed about the focus of the laser beam, the degradation of the beam quality caused by the first thermal lens can be compensated by the second thermal lens. Fig. 8 shows the Innoslab amplifier based on spherical aberration compensation theory. To reduce thermal lens effect in the slab crystal thickness direction and its influence, the method was to connect the crystal with the heat sink by double-sides welding method, so that air holes in the welding layer were avoided, the flatness of the indium layer was improved and the crystal surface was uniformly dissipated (Fig. 11). The output power from the all fiber laser was 2 W. The laser received further amplification in the double-passing end-pumped amplifier and the gain was dramatically enhanced by the Innoslab amplifier. Maximum output power of 28.4 W was achieved at a solid-state pump power of 117 W (Fig. 14). Meanwhile, the beam quality was well preserved with M2 factor of 1.3 by the Innoslab amplification structure which was favorable to the spherical-aberration compensation (Fig. 17). Figure 16 shows the spectral width of the Innoslab amplifier was narrowed to 0.21 nm, indicating remarkable gain narrowing effect. It can be seen that the gain narrowing effect broadened the pulse width of the double-passing amplifier to 10.6 ps (Fig. 10).
Conclusions In present study, we designed a picosecond fiber-solid hybrid Innoslab amplifier which was seeded by an all fiber laser and a double-passing end-pumped amplifier. The number of amplifiers and the gain were numerically simulated, and the Innoslab cavity was designed based on thermal lens effect compensation. The seed light realized eight-passing amplification in the slab crystal. The seed light injected into the Innoslab amplifier with average output power of 4.5 W and the repetition rate of 18 MHz. As a result, a laser pulse with average power of 28.4 W and pulse width of 10.6 ps was achieved under the pump power of 117 W, corresponding to an optical-to-optical efficiency of 20.4%. The beam quality factors M2 were 1.33 in the horizontal direction and 1.24 in the vertical direction. This system, which combined the advantages of the all fiber amplifier and the solid-state laser amplifier, enabled high repetition rate, and good beam quality with high gain picosecond pulses. It made significant contributions to many applications such as material micro-processing, laser ranging, and laser detection.
