# Compact Single-Frequency MOPA Using a Silica Fiber Highly Doped with Yb\({}^{3+}\)
## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Fiber**} & **Cladding/Core** & **Core/Cladding** & \multirow{2}{*}{**Core NA**} & **Number of** & **Fiber** \\  & **Absorption at 976 nm** & **Diameter** & & **Core Modes** & **Length** \\ \hline I & \textasci{19.2}/1300 dB/m & 15/125 μm & 0.082 & 2 & 1.05 m \\ II & \textasci{6.5}/1015 dB/m & 10/125 μm & 0.080 & 1 & 2.83 m \\ III & \textasci{5.7}/428 dB/m & 15/130 μm & 0.080 & 2 & 3.28 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter values of the single-frequency fiber MOPAs. lower Yb concentration, which would negatively impact compactness, and a higher core NA, which makes single-mode operation more difficult. The results of the accelerated aging are plotted in Figure 1. After 24 h, the core loss in the custom fiber I was nearly 4 times higher than in fiber II, while fiber III had a much smaller loss. The PD loss, which decreases rapidly with increasing wavelength, was estimated using [24] to be \(\sim\)80 times lower at 1064 nm (\(\sim\)2-2.5 dB/m), and \(\sim\)40 times lower at 976 nm (\(\sim\)5 dB/m). At the lower inversion used in the amplifier (14.1%), it was expected to be \(\sim\)3.5 times lower still [29], or only \(\sim\)0.6-0.7 dB/m at 1064 nm and \(\sim\)1.4 dB/m (5 dB/m divided by 3.5) at 976 nm. However, the loss at 976 nm has a much lower impact on long-term power stability than the loss at 1064 nm because the pump is mostly guided in the cladding, which does not suffer from photodarkening. The MOPA architecture is shown in Figure 2. The seed laser was a 1064-nm diode-pumped solid-state laser (Cobolt, a Hubner Group Company). It is based on a unidirectional ring-cavity design and can generate up to 3 W of linearly polarized output power with a polarization extinction ratio (PER) greater than 30 dB. The laser 3-dB linewidth, measured with a delayed self-heterodyne interferometer, was \(\sim\)150 kHz. It was kept below 2.5 W not to exceed the power rating of the tap coupler. Its output was free-space coupled into a PM isolator with a coupling efficiency of \(\sim\)85% for the 10-\(\upmu\)m-core fiber, and 90% for the 15-\(\upmu\)m-core fibers. A 1% polarization-maintaining (PM) tap coupler was spliced to the output fiber of the isolator to monitor the backward-propagating power returning from the amplifier. The 99% output fiber pigtail of the tap coupler was spliced to the PM Yb-doped fiber. The active fiber was coiled around an aluminum spool with a diameter of 8 cm, so that the two 15-\(\upmu\)m-core fibers, which are multimode at 1064 nm, carry a single mode. The spool had a U groove for better conductive cooling, and the fiber was covered by copper tape for increased heat conduction. The Yb-doped fiber was cladding-pumped in the backward direction by two wavelength-stabilized fiber-coupled 976 nm pump diode modules with a maximum power of 60 W each. The pump modules were spliced to the 105/125 \(\upmu\)m pump fibers of a high-power pump/signal (2+1)\(\times\)1 PM combiner. Finally, the output end of the Yb-doped fiber was spliced to the output fiber of the combiner. The pump power reflected by the combiner and the unwanted signal power in the cladding were efficiently removed by a 20-cm single-cladding passive fiber with the same core size as the gain fiber. Its output end was cleaved at 8\({}^{\circ}\) to avoid reflections. A high-index gel was \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Fiber** & **[Yb]** & **[AI]** & **[Ce]** & **[F]** \\ \hline I & 0.25 & 0.70 & 0.08 & 1.1 \\ IV & 0.20 & 1.1 & 0.17 & 0.78 \\ V & 0.27 & 0.92 & 0 & 1.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition of test fibers (at.%). Figure 1: Core loss induced at 600 nm by photo-darkening measured **(a)** in three fibers with similar Yb concentrations but different Ce/Yb ratios (see Table 2), and **(b)** in the three fibers used in the fiber amplifiers. placed at the pump-output end of the Yb-doped fiber (see Figure 2) to remove unabsorbed pump light from the cladding.