# 40 W All-Fiber Er/Yb MOPA System Using Self-Fabricated High-Power Passive Fiber Components
## 2 Experimental Setup  
### First and Second MOPA Stage: Pre-Amplifier EDFA and Medium-Power Amplifier EYDFA  
An experimental setup of the two first-MOPA-system stages is shown in Figure 1. A standard telecommunication distributed feedback (DFB) diode laser providing output power of 10 mW at 1555 nm and 1563 nm were used as a signal source. A pre-amplifier EDFA (Erbium Doped Fiber Amplifier) setup was based on an active 110-cm-long SM fiber doped with erbium ions (nLight LIEKKI Er 4/125) pumped in a backward direction (according to signal) by one SM diode laser operating at a working wavelength of 980 nm and output power of 600 mW. This stage amplified the signal up to 200 mW in the case of both the wavelengths (approximately 13 dB gain). The second stage -medium-power amplifier EYDFA (Erbium-Ytterbium Doped Fiber Amplifier) was based on a 5-meter-long Er/Yb co-doped SM-DC fiber (CorActive DCF-EY-7/128). Pumping of the active fiber was implemented in a forward direction, regarding the signal by a multimode laser diode operating at 975 nm with power of 9.3 W (JDSU 6398-L4i). The pump power was launched to the active fiber through a self-fabricated pump and signal power combiner in a configuration (\(5+1\)) \(\times\) 1, which was recently presented in Reference [36]. At the input side, this combiner consisted of five pumping ports, i.e., multimode fibers with core/clad diameters of 105/125 \(\upmu\)m (Numerical Aperture NA = 0.22) and one SM signal fiber with core/clad diameters of 9/80 \(\upmu\)m (NA = 0.13). A passive SM-DC fiber with core/clad diameters of 9/125 \(\upmu\)m (NA = 0.12/0.46) was used as the output of the combiner. The fabricated combiner provided a signal transmission efficiency at a level of 94% and a pump transmission efficiency close to 90%. In comparison, the same-class commercial pump and signal combiner (based on SM signal fibers at input and output) offered transmission efficiencies at a level of \(>\)90% for pump light and \(>\)85% for the signal [41]. The unabsorbed pump power in the active fiber was dissipated in a cladding-mode stripper (CMS). At the output of each stage of the amplifier, an optical isolator was placed and also fiber couplers (99/1% and 99.1/0.9%) were used in order to monitor output power at each stage as well as the backward stimulated Brillouin scattering (SBS) signal.  
Characteristics of signal output power versus pump power in the second medium-power MOPA stage are presented in Figure 2a. At both the wavelengths of 1555 nm and 1563 nm, the power at a level of 1.8 W was achieved (the gain at a level of 9.5 dB) with the pump power of 9 W giving a slope efficiency of 20%.  
Figure 1: Experimental setup of two first-MOPA-system stages—preamplifier EDFA (Erbium Doped Fiber Amplifier), and amplifier EYDFA (Erbium-Ytterbium Doped Fiber Amplifier).  
Figure 2b shows the optical spectra at the seed wavelengths of 1555 nm and 1563 nm, which were recorded at the output of the second medium-power amplifier MOPA stage. The optical signal to the noise ratio (OSNR) at the maximum pump power of 9 W was at a level of 50 dB and was mainly determined by the seed laser noise. There was no sign of Yb-ASE occurrence at the output because of the relatively low level of the pump power.