# Compact Single-Frequency MOPA Using a Silica Fiber Highly Doped with Yb\({}^{3+}\)
## 3 Results and Discussion  
Figure 3 presents the output power and backward SBS power measured against pump power for all three amplifiers with gain-fiber lengths of 1.05 m (I), 2.83 m (II), and 3.28 m (III). The lengths of the combiner's fiber pigtails were shortened as much as possible (\(\sim\)48 cm) to increase the SBS threshold. The launched seed power was \(\sim\)2 W. No limit due to SBS was observed for fiber I up to the maximum pump power of 107.4 W, providing an output power as high as 81.5 W (green filled lozenges in Figure 3). Significantly lower output power was reached with fiber II, with an observed SBS threshold of 33 W, while an SBS-limited output power of 58 W was reached with the larger (15 \(\upmu\)m) core fiber III. A much shorter gain fiber would be required to increase the SBS threshold further using fiber III, but this would reduce the slope efficiency considerably. The slope efficiency with fiber I was 75%, only slightly lower than with fiber III (78%). The temperature distribution along the fiber, measured with an infrared camera, was fairly uniform along the fiber, primarily because of the use of backward pumping. The temperature was slightly higher near the pump-input end of the fiber, as expected. At outputs above 50 W, the measured temperature of fiber I was 40 \({}^{\circ}\)C due to the higher  
Figure 3: Output power (left axis) and backward SFS output power (right axis) measured versus launched pump power for fiber amplifiers utilizing 1.05 m of fiber I, 2.83 m of fiber II, or 3.28 m of fiber III. Inset is an image of the output mode profile, measured at 52 W of output power. Figure 2: Schematic of the double-clad Yb-doped MOPA fiber amplifier. Photograph shows the packaged amplifier. thermal load associated with the fiber's high absorption. The output power was limited by the available pump power, not by thermal effects. In routine runs, as mentioned earlier, the fiber was covered with copper tape, which improved heat dissipation, so the actual fiber temperature was lower than 40 \({}^{\circ}\)C. The IR camera could unfortunately not measure the actual temperature due to the presence of the tape. Although it was not the main focus of this work, handling this increased fiber temperature at higher power levels (\(\sim\)80 W) is a straightforward engineering task that should not introduce undue complications. A small amount of forced-air cooling, or passive cooling with fins, would easily reduce the temperature further. Finally, we note that at the highest output power, the power stability was better than 1%, indicating that the thermal load was insufficient to cause undue power fluctuations. For beam-quality measurements, the output beam of the amplifier was refocused with a lens to a large spot size (to avoid damage), and the beam profile was measured versus distance from the focal plane with a high-damage-threshold Nanoscan 2s pyro-electric camera from Ophir. The camera recorded a 2D image of the mode and calculated the mode's transverse dimension along the horizontal (\(x\)) and vertical (\(y\)) axis. These data points were fitted to the well-known expression of the diffraction of a Gaussian beam in air. A 2D image of the output mode is shown in the inset of Figure 3. The mode energy is confined to the fundamental mode, with a small asymmetry (the \(x\)-axis in the image is related to the spatial orientation of the cleave plane). The fits performed by the camera yielded an M\({}^{2}\) value of 1.39 in \(x\) and 1.09 in \(y\). These values are likely to be overestimated because the position of the camera was measured by hand, which increased the error in the measured diffraction curve. In these measurements, the fiber was straight, so the asymmetry cannot be attributed to a combination of fiber bending and high thermal load. The fiber output facet was cleaved at an angle, which also explains the slight asymmetry in the mode profile and in \(M^{2}\). The signal-to-noise ratio (SNR) of the MOPA amplifier using fiber I was measured with an OSA.