# A 20 W, Less-Than-1-kHz linewidth linearly Polarized All-Fiber Laser
## 2 Experimental Setup  
The setup of the experiment for the Er\({}^{3+}\):Yb\({}^{3+}\)-co-doped fiber amplifier (EYDFA) system is shown in Figure 1. A linearly polarized narrow-linewidth laser of 1550 nm with the linewidth of 0.4 kHz was used as the seed source. The amplifier consisted of a preamplifier, a boost amplifier, a power amplifier. The preamplifier was based on 1 m-long Er\({}^{3+}\)-doped single-mode polarization-maintaining fiber with core diameter of 7 \(\upmu\)m and clad diameter of 125 \(\upmu\)m, core-pumped by aa wavelength-stabilized 976 nm laser diode (LD) through a polarization-maintaining wavelength division multiplex (WDM). The fiber pump absorption coefficient was 24 dB/m at the wavelength of 976 nm. The wavelength of the pump LD was stabilized at 976 nm in order to match the peak absorption of the active fiber. The boost amplifier was based on 6 m-long double-cladding Er\({}^{3+}\):Yb\({}^{3+}\)-co-doped polarization-maintaining active fiber with core diameter of 6 \(\upmu\)m and inner clad diameter of 125 \(\upmu\)m, whose absorption coefficient was 0.8 dB/m, cladding-pumped by 915 nm LD through a \((2+1)\times 1\) pump combiner. The power amplifier was based on 4 m-long cladding-pumped large core Er\({}^{3+}\):Yb\({}^{3+}\) -co-doped polarization-maintaining active fiber with core diameter of 25 \(\upmu\)m and inner clad diameter of 300 \(\upmu\)m. The fiber core was an octagon preform in order to increase pump absorption; the numerical aperture (NA) of the core and the inner clad were 0.09 and 0.46, respectively. The pump absorption coefficient was 2.9 dB/m at the wavelength of 915 nm. The amplifier could boost the signal to 20 W at the 75 W pump power of 915 nm through a \((6+1)\times 1\) polarization-maintaining pump combiner. All components were placed on the optical platform, while the power amplification section was placed on a water-cooled platform for active cooling. Each amplifier was connected through a polarization-maintaining optical fiber isolator (ISO). Between the amplifiers, ISOs were used to avoid the reflected laser, unabsorbed pump light and the amplified spontaneous emission (ASE) and to prevent the frontal stage components from being damaged by backscattering light. To achieve a compact all-fiber structure, fusion welding connection was employed in the proposed fiber laser. The output power was measured by a digital power meter with a photodiode sensor head. A delayed self-heterodyne was used to measure laser linewidth [20; 21]. Figure 2 presents the diagram of the linewidth measurement system. The laser was split into two paths by a splitter in a Mach-Zehnder (MZ) fiber interferometer. One beam was delayed by the single-mode optical fiber. The other beam was directed through an acousto-optic modulator (AOM) frequency shifter operating at 150 MHz. The two beams were recombined by a coupler to generate a beat signal and transported into a photodetector to produce the power spectrum, whose shape and width were related to the laser linewidth. When the delayed length was much longer than the laser coherent length, the power spectrum was the laser spectrum lineshape, fitting the power spectrum and measuring 20 dB in width corresponding to the linewidth of the laser. When the laser linewidth became narrower and narrower, hundreds of kilometers of delayed fiber were needed to detect the sub-kilohertz laser linewidth. However, the broadening of the laser linewidth could be avoided when the ultra-long delayed fiber was used in the self-heterodyne [22]. To decrease the linewidth broadening induced by the ultra-long delayed fiber, the short-delayed heterodyne method should be used. If the delay time is less than the laser's coherence time, there will be a delta function peak and coherent envelopes in the power spectrum [23]. In this case, the laser linewidth can be measured by the amplitude difference comparison of the coherent envelope method [24].