# A 20 W, Less-Than-1-kHz linewidth linearly Polarized All-Fiber Laser
## 3 Experimental Results and Analyses  
In the experiment, the seed laser was a distributed feedback (DFB) fiber laser with an output power of 1 mW and a linewidth of 0.4 kHz. The spectrum of the seed laser was measured by an optical spectrum analyzer (YOKOGAWA, AQ6375), as shown in Figure 3. The central wavelength of the seed laser was 1549.6 nm, and the SNR of the laser signal was greater than 47 dB. First, we used two stages of amplifiers to enlarge the power of the seeder to 1 W. In the preamplifier, the 1 mW seeder could be boosted to 50 mW. The output spectrum of the first preamplifier was amplified spontaneous emission (ASE)-free. The boost amplifier could boost the first-class signal power to 1 W at the pump LD power of 6.5 W. Figure 4 shows the output spectrum of the second amplifier, where the SNR was 45 dB. The spectral envelope observed around 1560 nm is the noise amplification of the seeder. Figure 4: Output spectrum of the second preamplifier. Figure 3: Output spectrum of the seed signal. The last stage was the power amplifier; the maximum output power of 20 W was obtained from the power amplifier stage under the launched pump power of 75 W, with no occurrence of saturation effect, and the corresponding optical efficiency was 26.7%. In order to increase the nonlinear effect threshold in the fiber and reduce the ASE noise, the length of the gain fiber was reduced, so that the amplification efficiency was relatively low. Figure 5 shows the signal-output power of the last power amplifier as a function of the launched pump power. The yellow line is the laser-output power without the isolator, and the blue line is the laser-output power after the isolator is added. The isolator insertion loss in the last level was 0.8 dB. Because of the insertion loss of the isolator, the output power was reduced after laser input. The spectrum and power of the output signal were monitored to observe whether the SBS occurred. Figure 6a shows the output spectrum of the power amplifier at the maximum output power. In the process of power amplification, the noise near 1560 nm was amplified at the same time, and the SNR was still more than 40 dB. The linewidth was achieved by the delayed self-heterodyne method with the analysis of the frequency spectrum. A fiber-coupled InGaAs-biased detector was used to detect the beat signal. The frequency spectrum of the beat signal was obtained with a spectrum analyzer (Agilent E4447A, Santa Clara, CA, USA). Figure 6b shows the actual recorded power spectrum at the maximum output power of the laser measured with 25 km delayed fiber. The delta function peak of the power spectrum and the coherent envelopes appearing on the two wings are caused by the interference of the two beat frequency beams and indicate that the coherence length of the laser was greater than the delayed fiber of 25 km. In order to accurately obtain the laser linewidth, the amplitude difference comparison of the coherent envelope method was used to measure the laser linewidth [24]. Since a long delayed fiber broadens the laser linewidth and a short delayed fiber leads to inaccurate coherent envelope in self-heterodyne measurements, the length of the delayed fiber should be chosen properly, such as 3 km, 4 km, or 5 km [24]. The laser linewidth was calculated through the value of contrast difference between the first sideband near the center frequency peak and the trough of the coherent envelope of the power spectrum. Figure 7 shows the actual recorded power spectrum of a 3 km delay length. The value of contrast difference between the first sideband near the center frequency peak and the trough was 16.3 dB. Therefore, the laser linewidth was calculated to be 0.88 kHz by the relationship between the laser linewidth and the contrast difference of the second peak and the second trough [24]. By observing the spectrum and linewidth broadening, the nonlinear effect does not appear obvious. The increase of the linewidth was mainly attributed to the increase of incoherent components, such as the noise of the seeder near 1560 nm.