# L-Band Erbium Fiber Laser with Tunable and Narrow Sub-kHz Linewidth Output
## 4 Second EDF Laser Experiment  
In the next experiment, we swapped the placement of the EDF SA filter and SMF-induced feedback RB injection to test the output quality based on the experimental setup of Figure 1. This second laser configuration meant that the lasing wavelength passes through the 2 m EDF, 100 m SMF and then enters the FRM for round-trip propagation. In the experiment, the second EDF laser configuration achieves the same wavelength-adjusting bandwidth from 1558.0 to 1618.0 nm as the first laser. Figure 8 exhibits the detected optical spectrum of the second fiber laser at the selected wavelength of 1558.0, 1573.0, 1588.0, 1603.0, and 1618.0 nm through the OSA, respectively. Moreover, the observed output spectra and features of the second EDF laser were similar to those in Figure 2. In the same way, we also observed that the lasing wavelength at 1558.0 nm had high ASE noise after the wavelength of 1568 nm, as shown in Figure 8. To understand the output characteristics of the second EDF laser, the output power, OSNR, 3 dB linewidth, and instability over the effective operation, bandwidths were also executed and discussed using the same measurement method. Figure 8: Observed output spectra of second proposed EDF laser over the whole wavelength-tuning range from 1558.0 to 1618.0 nm. Figure 7: Measured fluctuations in output power and center wavelength in the first proposed EDF laser are in the full tuning range through 40 min observation. First, the output power and the OSNR of the second proposed EDF laser were measured in the ranges of \(-\)14.17 to 1.74 dBm and 49.09 to 68.71 dB, as exhibited in Figures 9a and 9b, respectively. The second fiber laser system could also reach a flat power output scale [\(\Delta\)P = 1.1 dB (P\({}_{\mathrm{out}}\) = 0.64-1.74 dBm)] over a wavelength bandwidth from 1568.0 to 1598.0 nm (BW = 30 nm) with the corresponding OSNR between 61.72 and 68.71 dB, as exhibited in Figure 9a,b. The above results were also similar to those obtained using the first EDF laser. Moreover, it was also verified that the second fiber laser has stable SLM output through self-homodyne setup over the entire tuning bandwidth through the delayed self-homodyne observation. And the SLM output results obtained were the same as for the first EDF laser. Then, to understand the obtained linewidth of the second fiber laser, the self-heterodyne and Lorentzian curves were applied for measurement. We also used the 1618.0 nm wavelength for the linewidth measurement via the self-heterodyne first. Figure 10a shows the measured linewidth profile and Lorentzian curve of the electrical spectrum when the shift frequency (\(f_{\mathrm{shift}}\)) of 55 MHz was added to AOM. Thus, the 3 dB Lorentzian linewidth of 313 Hz could be obtained through fitting. In the observation, the other 3 dB linewidths of the second laser were all 313 Hz under the Lorentzian fit, as displayed in Figure 10b. As stated above, the first and second EDF laser configurations had the same 3 dB linewidth of 313 Hz. However, there were some differences in other output characteristics. Next, we also performed and discussed the output stabilities of power and center wavelength in the second EDF laser over the wavelength-adjusting scale from 1558.0 to 1618.0 nm during a short-term monitoring of 40 min, when the same thirteen output wavelengths were employed. As seen in Figure 11, the fluctuations in output power and center wavelength observed were between 0.07 (at 1573, 1608, and 1618 nm) and 0.22 dB (at 1603 nm) and 0.008 (at 1563, 1568, 1583, 1593, 1598, 1608 and 1613 nm) and 0.024 nm (at 1558 and 1618 nm), respectively, during the 40 min measurement period. Figure 9: (a) Detected output power and (b) OSNR of the second EDF laser in the whole tuning range, respectively. In this study, the corresponding output features of the first and second EDF lasers are displayed in Table 1 for comparison over the achievable wavelength bandwidth. As seen in Table 1, the obtained 3 dB linewidths of the first and second EDF lasers were all the same.