# 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  
A setup of the third-stage MOPA, which was a high-power amplifier, is presented in Figure 3. It was based on a four-meter-long active Er/Yb co-doped LMA-type DC fiber with core/clad diameters of 25/300 \(\upmu\)m (Nufern LMA-EYDF-25P/300-HE). The pump power from six multimode diode lasers (LIMO35-F100-DL915) with total available power of 210 W was launched into the active fiber in a forward direction using a self-fabricated pump and signal power combiner in a configuration \((6+1)\times 1\), presented in our work [31]. This power combiner had six pump input ports--multimode fibers with core/clad diameters of 105/125 \(\upmu\)m (NA = 0.22) and one signal LMA-type fiber with core/clad diameters of 20/130 \(\upmu\)m. At the output of the combiner, an LMA-type DC passive fiber with core/clad diameters of 25/300 \(\upmu\)m (NA = 0.09/0.46) was used, which was compatible with the active fiber used in the third MOPA stage. Our combiner provided a signal transmission efficiency at a level of 78% and a pump transmission efficiency at a very high level of approximately 98%. The output fiber from the second stage (SM, with core/clad diameters of 9/125 \(\upmu\)m) was not compatible with the input signal fiber (with core/clad diameters of 20/125 \(\upmu\)m) of the combiner, and the input fiber of the third MOPA stage. In order to overcome this issue, we used a self-fabricated MFA, which had a conventional SM fiber (SMF-28e) as an input port and an LMA-type fiber with diameters of 20/130 \(\upmu\)m as an output port, which was the same as an input signal fiber in our power combiner. This component was characterized by a signal transmission efficiency at a level of 90%, so it is comparable with commercially available MFAs [42]. At the moment when we were working on this setup, we could use the \((6+1)\times 1\) pump and signal power combiner (MMC06112571, ITF Technologies company) with 20/125 \(\upmu\)m (core/clad diameters) input signal fiber and 20/200 \(\upmu\)m (core/clad diameters) output DC fiber as a commercial alternative. This combiner was characterized with a pump transmission efficiency at a level of \(>\)93% and signal at a level of \(>\)89%. In this case, an MFA at the input was still needed not only to connect the SM fiber with 20/125 \(\upmu\)m (core/clad diameters) fiber, but also to connect an output port from the combiner (with core/clad diameters of 20/200 \(\upmu\)m) with our active fiber (with core/clad diameters of 25/300 \(\upmu\)m), as shown in Figure 4a. Such a construction was used in our previous work [43]. This configuration  
Figure 2: Characteristics of signal output power versus pump power (**a**) and optical spectra at the seed wavelength of 1555 nm and 1563 nm (**b**) recorded at the output of the second medium-power amplifier MOPA stage. of the MFA was not available in commercial offers; thus, we fabricated it by tapering the LMA fiber with core/clad diameters of 25/300 \(\upmu\)m and splicing it with the output port of the ITF Technologies combiner (with core/clad diameters of 20/200 \(\upmu\)m). The developed MFA was characterized with pump and signal transmission efficiencies at a level of approximately 92%. If we used an ITF (\(6+1\)) \(\times\) 1 combiner with transmission loss at a level of approximately 0.5 dB and a self-fabricated MFA with a transmission loss of 0.35 dB (connecting 20/125 \(\upmu\)m combiner output DC fiber with 25/300 \(\upmu\)m active fiber) in the third amplifier construction, the total signal transmission loss through those two components would be at a level of 0.85 dB. However, although both of them were propagating pump power, the combiner pump transmission losses of the combiner and the self-fabricated MFA was approximately 0.32 dB and approximately 0.35 dB, respectively, giving the pump power transmission loss of 0.67 dB in total.