Powerful single-frequency lasers are required for many scientific and engineering undertakings, such as gravity-wave detection and coherent combination of multiple beams. Such sources also have potential as very high power transmitters in telecommunications systems. Researchers at the University of Southampton in the UK have demonstrated a single-frequency, single-mode, large-core fiber master oscillator power amplifier laser system capable of generating 125 W of output that can be continuously tuned from 1546 to 1566 nm. This power level is significantly higher than has been obtained previously from such a source.Figure 1. The three-stage master oscillator power amplifier system consists of a diode-laser master oscillator, a fiberpreamplifier incorporating an erbium-doped fiber amplifier (EDFA), and a double-clad, Er:Yb codoped fiber power amplifier. Images ©OSA. The system comprises a tunable diode laser as the master oscillator, a commercial fiber preamplifier and a fiber power amplifier (Figure 1). An external-cavity diode laser from Santec Corp. of Komaki, Japan, generates 10 mW at a single frequency, tunable between 1530 and 1620 nm, which is the input to a fiber preamplifier supplied by SPI Lasers plc of Southampton, UK.The Er-doped fiber preamplifier boosts the signal to 1.8 W and feeds it into the power amplifier. Dichroic mirrors and beamsplitters on either side of the amplifier allow the pump radiation to be inserted into and removed from the beam path, and extract other wavelengths from the beam path.The power amplifier consists of 10 m of coiled, double-clad, Er:Yb codoped, 30-µm-core fiber. It is pumped with up to 470 W, launched into the cladding from a 975-nm diode laser stack. The Er:Yb codoping is necessary to obtain efficient absorption of the 975-nm light: The Yb absorbs efficiently at that wavelength and subsequently transfers the absorbed energy to the erbium, which lases in the 1540- to 1570-nm region.Figure 2. The output of the power amplifier is essentially the same for all input wavelengths between 1546 and 1566 nm. At the optimal 1563-nm wavelength, the researchers have obtained a maximum output of 151 W. The output at this level is 40 dB above the background amplified spontaneous emission (inset). The output power varies almost linearly with pump power across the 20-nm tuning range, but there is a minor discontinuity in the slope efficiency at the ∼100-W output level (Figure 2). The researchers attribute this to ytterbium emission at 1060 nm, a loss mechanism that would eventually limit the maximum power from the laser. At the optimum wavelength of 1563 nm, they achieved an output power of 151 W from 473 W of launched pump power. This peak output is 40 dB above the background level from amplified spontaneous emission (Figure 2, inset).Despite the large core of the power amplifier, the quality of the output beam is nearly diffraction-limited, with an M2 value of 1.1. Although the fiber is coiled, bend losses to high-order modes are not significant for high-numerical-aperture fibers like this one, so the beam quality cannot be attributed to mode-filtering by bend losses. Rather, the researchers attribute it to the high quality of the beam that is launched into the power amplifier. The input from the preamplifier excited only the fundamental mode of the power amplifier, and that mode propagated the 10-m length of the fiber without coupling to other modes.The researchers also finessed another issue with high-power fiber lasers in that they observed no output degradation from stimulated Brillouin scattering. The 10-m amplifier produces up to 150 W of output from its ~400-µm2-mode-area core, a combination of power density and fiber length that should produce significant Brillouin gain. The researchers believe that the longitudinal thermal gradient in the fiber caused by the high powers propagating in the fiber broadens the Brillouin gain along the fiber’s length. As a result, the Brillouin gain generated from the narrowband signal is diminished sufficiently to prevent stimulated Brillouin scattering from reaching threshold. They have observed no degradation, even at the highest output powers.An issue they have not finessed is parasitic emission from the ytterbium codopant in the fiber. They have cleaved the ends of the fiber to prevent lasing at the 1060-nm ytterbium wavelength, but even without feedback, the amplified spontaneous emission at that wavelength usurps a significant portion of the energy absorbed by the ytterbium. The researchers have observed broadband emission in excess of 70 W from the ytterbium (Figure 3). They suspect that the high ytterbium emission results from reduced coupling from the ytterbium to the erbium, a problem that might be unique to the particular fiber they employed.Figure 3. The laser’s output is limited by broadband amplified spontaneous emission from ytterbium in the 1000- to 1100-nm region. Alternately, the parasitic emission could result from too-slow depopulation of the upper laser level in the erbium, which in turn could force population into the upper ytterbium level. They plan to address this issue in future investigations.