[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pulsed Yb-Doped Phospho-Silicate Fiber MOPA Source with 25kW Peak Power and Excellent Beam Quality

摘要： With ytterbium doped fiber (YDF) lasers and amplifiers have reached continuous wave output power of multi-kW with direct diode pumping [1] and 10kW with tandem pumping [2] in a good beam quality, today fiber lasers are becoming the laser choice for many industrial applications and processes, defense, and scientific research. One of the challenges of high average power fiber laser is to maintain a long-term stability of the output power. In particular, photodarkening (PD) is seen as a power loss in YDF gain medium that can significantly influence the operation lifetime of the device. Moreover, it was observed that the PD is related to the transverse mode instability (TMI) that can limit the output power of the laser [3]. In silica host, co-dopant phosphorous (P) is more effective than aluminium (Al) to suppress the PD effect in YDF. In addition, Yb-doped phosphosilicate (Yb-PS) fiber is found suitable for high power laser operation below 1020nm [4]. Such lasers are used as pumps in tandem pumping to reduce the heat load associated with the quantum defects in YDF lasers operating at 10kW level [4]. However, Yb-PS fiber are generally considered difficult to fabricate due to evaporation of P2O5 during the preform fabrication process, resulting in a central dip in the core refractive index profile that has detrimental effect on the output beam quality of fiber lasers. Other disadvantage of P co-doping is its smaller Yb - absorption and emission cross-sections compared to Al counterpart. To compensate for the cross-sections, higher concentrations of Yb and P (to prevent Yb ions from clustering) are needed in fiber. This contributes to a high core NA and poses a challenge to fabricate a large mode area Yb-PS fiber. Here we report an efficient Yb-PS high power laser fiber fabricated using an optimized MCVD (modified chemical vapor deposition) and all-vapor-phase chelate precursor doping technique. Double-clad fiber with a 150μm quasi-octagonal inner cladding and a 12μm core diameter was drawn with a low index polymer outer cladding. The core NA was 0.1, Fig.1 (a). The small signal absorption at the pump wavelength of ~976nm was measured as 2.5 dB/m. Initially the fiber was tested in a 4%-4% laser cavity. An output power of >100W (limited by the available pump power) and the slope efficiency of >85% were obtained. The laser emission was centered at 1066nm. We then tested the same fiber in a pico-second MOPA configuration. Fig. 1(b) shows the experimental schematic. A 4m long Yb-PS fiber was used in the final stage of the MOPA. A gain-switched diode operating at ~1035nm with 180ps pulse width and 2.95MHz repetition rate was used as a seed laser. Fig. 1(c) shows the average signal output power with respect to the absorbed pump power in the final stage of the amplifier. The output power reached 13.3W and the slope efficiency was 74%. The corresponding pulse energy and peak power was 4.5μJ and 25kW respectively. The measured beam quality (M2) was ~ 1.13, as shown in inset of Fig. 1(c). The spectra of the seed laser and at the maximum signal output power are shown in Fig. 1(d). The 3dB spectral bandwidth at the maximum output power was measured as 0.29nm compared to 0.03nm of the seed laser. The magnitude of the stimulated Raman scattering (SRS) appeared at a wavelength of ~1080nm was 30dB lower than the output signal – see Fig. 1(d). In conclusion, we have demonstrated that an all-vapor-phase chelate doping technique holds great potential to fabricate LMA Yb-doped phosphosilicate fibers with diffraction limited beam for high power lasers.