Objective High-power blue-green laser light sources are widely applied in the welding of highly reflective materials (e.g., copper and its alloys), pumping Ti:sapphire lasers, deep ultraviolet laser generation, semiconductor processing, and underwater optical communication and detection. High-power blue-green lasers are mainly derived from blue semiconductor lasers, disk laser frequency doubling, and narrow-linewidth fiber laser frequency doubling. Compared to blue LDs and disc lasers, a fiber laser has the advantages of all-fiber structure, high efficiency, easy heat dissipation, and flexibility. A fiber laser with a 1-μm band kW-level near-diffraction-limit narrow linewidth and full polarization maintenance is an ideal fundamental-frequency light source for high-efficiency frequency doubling and high output power. This paper introduces a high-efficiency green laser based on single-pass frequency doubling of a polarization-maintaining fiber laser.
Methods Figure 1 shows experimental principle. Based on the master oscillator power amplifier cascaded amplification technology, we achieved a fundamental light source with an approximate output power of 1.1 kW and a beam quality at the near-diffraction-limit. The linewidth was ~20 GHz (~0.077 nm) and the polarization extinction ratio was better than 15 dB. We selected a non-critical phase-matched lithium triborate (LBO) crystal as the frequency doubling crystal cut at angles of θ=90° and φ=0°. The acceptance linewidth and matching temperature of the non-critical phase matching LBO crystal for the fundamental-frequency light were theoretically calculated. High-efficiency frequency multiplication can be obtained by optimizing the focal length of the focusing lens and the crystal control temperature.
Results and Discussions The calculated acceptance linewidths of the 40-mm and 60-mm LBO crystals were inversely proportional to crystal length. The focal length of the focusing lens and the crystal working temperature were selected as 400 mm and ~150 ℃, respectively. The difference between the experimental and theoretical optimal temperatures can mainly be explained by the incident angle of the fundamental-frequency light. When the maximum output power of the fundamental-frequency light was 1084 W, the laser output continuous green light at 610 W, and the efficiency of second-harmonic generation was 56.27% (Fig. 2). The beam-quality factor M2 of the green light was 1.05, and the output far-field spot presented a fundamental transverse-mode shape (Fig. 3).
Conclusions Based on the narrow-linewidth linear polarization fiber laser and the single-pass frequency doubling scheme, a 610 W single-mode green laser output was obtained. The efficiency of frequency doubling reached 56.27%, and the beam quality M2 was 1.05. To the best of our knowledge, we present the most efficient generation of hundreds of watts of continuous-wave green laser using the single-pass frequency doubling scheme. Furthermore, two green-light polarization beams can be combined to realize a kilowatt-level high-brightness green laser output.
