# 40 W All-Fiber Er/Yb MOPA System Using Self-Fabricated High-Power Passive Fiber Components
## 2 Experimental Setup
### Setup of Third-Stage Amplifier: High-Power EYDFA
Using our self-fabricated (\(6+1\)) \(\times\) 1 power combiner guaranteed a very low pump transmission loss at a level of only 0.09 dB, which was much lower than that in the case of commercial combiners. The signal transmission loss was at a level of 1.08 dB. Even though the signal loss is larger than that in the case of an ITF combiner, we had to also take into account the number of splices:  
* 1 st additional splice as a combiner with MFA,
* 2 nd additional splice inside the MFA combined with an active fiber. Because those are LMA-type fibers, they are extremely sensitive to angular misalignments. For example, a 0.5\({}^{\circ}\) angular misalignment on a fiber with mode field diameter MFD = 6 \(\upmu\)m resulted with a small transmission loss of 0.06 dB, while in the case of MFD = 20 \(\upmu\)m, the loss increased to almost 0.7 dB [35]. Thus, it is profitable to limit the number of splices in order to avoid any additional losses. In addition, such a loss on a splice means leaking of a power, and since we are dealing with high powers, those leaks could lead to point overheating and finally damage the fibers and the setup. Figure 4: Alternative solution of the third MOPA stage construction: with commercial pump and signal power combiner (**a**) and with a backward-pumping scheme (**b**). Figure 3: Experimental setup of the third MOPA stage—high-power amplifier CYDFA. Taking all above into account, we chose one component with a higher signal-power transmission loss of only 0.23 dB, but with a lower pump-power transmission loss of 0.58 dB, under which circumstance, the number of splice was reduced from 2 to 1. We chose the forward-direction pumping scheme because of two reasons. First reason is that if we chose the backward-direction pumping scheme, additional MFAs would be needed to connect output fiber (20/125 \(\upmu\)m) with both the input MFA and an active fiber (25/300 \(\upmu\)m) (Figure 4b(I)). On the other hand, two MFAs could be replaced with one MFA connecting an SM fiber (8.2/125 \(\upmu\)m) with an LMA fiber (25/300 \(\upmu\)m) (Figure 4b(II)). However, large difference between mode field diameter could cause large losses. The second reason of choosing the forward-pumping scheme was the safe operation of the system. The backward-pumping scheme was quite dangerous, because in the case of a small change of the pump diode temperature, its operating wavelength could shift, leading to a decrease of the absorption in the active fiber. The unabsorbed pump power could reach the signal source, or in the case of the bidirectional pumping scheme, it could reach the pump diodes used for the forward pumping, resulting in damaging of the whole setup. The real level of the pump power which was reaching the active fiber was smaller than that provided by a pumping laser. Significant laser power was guided in cladding modes of the output fiber of the pumping laser. To secure safe operation conditions, a cladding-mode striper was placed on each splice connecting the laser pigtail (100/140 \(\upmu\)m) and the standard MM fiber (105/125 \(\upmu\)m) of the power combiner. A measured waste of the pump power was approximately 30% of the total pumping power, decreasing it to a level of 147 W. If we also took into account the small pump power attenuation by the fabricated combiner, the pump power reaching the active fiber was at a level of approximately 144 W. The output of the third stage was terminated with a short piece of passive LMA fiber, which was cut at an angle of 4\({}^{\circ}\) (larger than a critical angle corresponding to the NA of the used LMA fiber) in order to avoid back-reflections from the fiber end and ensure the safe operation of the system. Since the setup was designed for pure CW operation, a special care must be taken with respect to thermal issues in the system. Most of the dissipated optical power was converted into heat in the active fiber. With respect to a relatively small volume of the active fiber, the proper and effective cooling method should be used in the third high-power stage of our MOPA system.