# L-Band Erbium Fiber Laser with Tunable and Narrow Sub-kHz Linewidth Output
## 3 Results and Discussion
Therefore, the output linewidth of the EDF laser is compressed and narrowed to achieve stable SLM and sub-kHz linewidth output. As mentioned above, the cascaded unpumped EDF-based mode-filter effect and SMF-induced RB feedback injection can also decrease the MLM oscillations and narrow linewidth performance. Therefore, we confirmed the SLM performance of the proposed first EDF laser system utilizing the delayed self-homodyne measurement. The trial setup was constructed by a dual-arm Mach-Zehnder interferometer (MZI) with two 1 \(\times\) 2 and 50:50 CPR, a PC, and a 73 km single mode fiber (SMF) as a delay line, as illustrated in Figure 4. And the self-homodyne setup was without using acousto-optic modulator (AOM). As seen in Figure 4, each selected output wavelength enters the MZI setup and then the 10 GHz PIN photodiode (PD) to generate a beat signal. Then, the beat signal is converted to an electrical signal through the PD and observed using a 3 GHz BW electrical spectrum analyzer (ESA). Hence, Figure 5a shows the electrical output spectrum of the lasing wavelengths from 1558.0 to 1618.0 nm detected at 5 nm measurement intervals in the frequency range from 0 to 1000 MHz through the ESA. Here, we did not observe any longitudinal mode oscillations within the 1000 MHz RF bandwidth at the thirteen selected wavelengths, as seen in Figure 5a. The experiment results show that the demonstrated EDF ring laser architecture can lead to the SLM output. In addition, when we reduced the observing bandwidth to 80 MHz, the detected electrical spectra of the thirteen wavelengths also remained unchanged at stable SLM operation, as shown in Figure 5b. During a 40 min monitoring period, the measurement results also remained the same as above. Figure 3: Measured output power and OSNR of the demonstrated first EDF laser over the wavelengths from 1558.0 to 1618.0 nm. Then, we employed the delayed self-heterodyne method through the same MZI setup to measure the linewidth of the first EDF laser, as displayed in Figure 4. In the measurement, a 55 MHz shift frequency (\(f_{\text{shift}}\)) was applied to the acousto-optic modulator (AOM) to generate the beat signal for linewidth detection. The PC was applied to control and attain the optimal beat signal performance for the experiment. Primarily, a lasing wavelength of 1618.0 nm was utilized for self-heterodyne setup for measurement. The electrical linewidth spectrum measured at the wavelength of 1618.0 nm at a center frequency of 55 MHz is shown as the red square symbol at the top of Figure 6 through the ESA. To realize the definite laser linewidth at 1618.0 nm wavelength, a Lorentzian fitting curve can be applied based on the measurement results. Thus, a 3 dB Lorentzian linewidth of 313 Hz was obtained at 1618.0 nm wavelength and plotted in the solid line at the top of Figure 5 through profile fitting  
1618 nm wavelength was maintained at 313 Hz during 40 min of monitoring. Next, we also measured and obtained the corresponding 3 dB Lorentzian linewidth of other output wavelenghts via the same measurement over the full tunability bandwidth from 1558.0 to 1618.0 nm. Furthermore, the measured Lorentzian linewidths from 1558.0 to 1618.0 nm with 5 nm wavelength intervals were all at 313 Hz in the blue square symbols at the bottom of Figure 5. We confirmed that this EDF laser architecture can achieve a linewidth of 313 Hz within the effective wavelength output range. Therefore, the above results indicate that the cascaded EDF-based SA filter and feedback RB injection effect inside a fiber cavity can acquire stable SLM output and narrow sub-kHz linewidth. In addition, the steady output of each generated wavelength in the EDF laser structure is also a noteworthy cause. To realize the real instabilities of the first designed EDF laser configuration, we also applied thirteen selected output wavelengths with a 5 nm tuning step in the tunability bandwidth from 1558.0 to 1618.0 nm.