With the advent of the industrialization 3.0 era, a 1.5 μm laser has been focused due to its "eye-safe" and low atmospheric transmission loss, which can be widely used in lidar, laser ranging, optical fiber communication and other fields. An erbium-doped fiber (EDF) with a small absorption cross section at 900-1000 nm is difficult to be used for high-power laser amplification. Moreover, concentration quenching is triggered with the increase of the concentration of erbium ions. Previous reports found that ytterbium ions in the erbium-ytterbium co-doped fibers could effectively absorb pump light and transfer energy to erbium ions. Therefore, the pump conversion efficiency and output power are greatly enhanced. Moreover, the number of ytterbium ions in an erbium-ytterbium co-doped fiber is much higher than that of erbium ions, so that the concentration quenching effect is effectively inhibited. In view of these, an erbium-ytterbium co-doped fiber has been the main gain medium for a 1.5 μm laser. A large mode area erbium-ytterbium co-doped fiber is fabricated based on the modified chemical vapor deposition (MCVD) process and the solution doping technology, and its laser amplification performance is also investigated.
In the paper, an erbium-ytterbium co-doped fiber is prepared by MCVD combined with solution doping. The silica rude is prepared through corrosion, deposition, liquid phase doping, drying, vitrification, collapse, burning and other processes in the existing MCVD machine. In the gas phase doping process, the reverse deposition is utilized to improve the content of P2O5 in the fiber core. And the doping of P promotes the energy transfer from Yb3+ to Er3+ . Finally, the fiber prefabricated rod is matched with a suitable octagonal quartz sleeve and pulled into a specific size of fiber. A two-stage all-fiber main oscillation power amplification (MOPA) platform is set up to test the laser performance of the fiber. The MOPA structure contains a 1550 nm continuous laser as seed light, and the 915 nm and 940 nm lasers as pump light.
Two erbium-ytterbium co-doped fibers of 10 μm/130 μm (EYDF1) and 25 μm/300 μm (EYDF2) are prepared. The cross section and refractive index profile of the fiber are shown in Figs. 2 and 3, respectively. The core for EYDF1 has a diameter of 10.12 μm, the cladding diameter is 128.25 μm, and the relative refractive index step numerical aperture (NA) of the core is measured to be 0.10. For the EYDF2, the core has a diameter of 25.01 μm, the cladding diameter is 292.09 μm, and the relative refractive index step NA is measured to be 0.12. The fiber cladding absorption coefficient is measured to be 4.50 dB/m at 915 nm for EYDF1 and 2.85 dB/m at 940 nm for EYDF2. For the laser experiment, the seed light power is measured to be 1.13 W. Through the 6.8 m long EYDF1, the signal power is pre-amplified to 5.25 W, and its spectrum is shown in Fig. 5(a). After pre-amplification, the central spectrum is stable at 1550 nm without obvious stray light such as amplified spontaneous radiation(ASE). As the scattered light is collected during the spectral test, the optical signal-to-noise ratio (OSNR) is only 17 dB. The output power versus pump power at the pre-amplification stage is plotted in Fig. 5(b), and the slope efficiency is 38.6%. The signal is coupled into an 8 m long EYDF2 through the MFA and the (6+ 1) ×1 forward pump combiner after pre-amplification, and the remaining signal power is measured to be 2.9 W after EYDF2. The pump light is coupled into the large mode area erbium-ytterbium co-doped fiber by the (6+ 1) ×1 pump combiner. The optical spectrum at 61.7 W is shown in Fig. 6(a). The 1 μm and 1.5 μm ASEs are too small to observe, and the laser signal-to-noise ratio is measured to be 45 dB. The variation of 1.5 μm laser output power with pump power is shown in Fig. 6(b). The maximum output power is measured to be 61.7 W, and the slope efficiency of 42% is achieved. With the increase of pump power, the maximum optical efficiency is measured to be 42.7% when pump power is 35.5 W. The optical-to-optical efficiency begins to decrease when the pump power is increased to 100 W, which is caused by the increase of the backward 1 μm ASE power. With the increase of pump power, the spectral quality is good, and the output power increases steadily. Therefore, the output power and slope efficiency can be further improved by the increase of pump power and the optimization of fiber length in the future.
Large mode area erbium-ytterbium co-doped fibers are successfully fabricated by the MCVD process combined with solution doping. A large mode area erbium-ytterbium co-doped fiber amplification system is established. The 1550 nm laser power of 61.7 W is achieved with a slope efficiency of 42%. It is verified that our large mode area erbium-ytterbium co-doped fiber has a good amplification performance for a 1.5 μm laser.
