# L-Band Erbium Fiber Laser with Tunable and Narrow Sub-kHz Linewidth Output
## 3 Results and Discussion  
First, the initial amplified spontaneous emission (ASE) curve of L-band EDFA measured was in the wavelength scope from 1553 to 1563 nm, as exhibited in Figure 2a. In the observation, the L-band ASE spectrum seemed relatively flat. The 6 dB bandwidth (BW) of the ASE spectrum was in the range from 1568.0 to 1609.5 nm (BW \(\cong\) 41.5 nm). The 6 dB ASE bandwidth was close to the operative gain range of L-band EDFA (1568 to 1604 nm). When we initially adjusted the passband of TBF, the output wavelength of the proposed EDF laser configuration was generated from 1558.0 nm. Hence, all the output wavelengths could be achieved in a range from 1558.0 to 1618.0 nm, when the five wavelengths of 1558.0, 1573.0, 1588.0, 1603.0, and 1618.0 nm were selected for representation, respectively, as shown in Figure 2b. The wavelength excited at 1558.0 nm could not effectively suppress the ASE background noise after the wavelength of 1568 nm, as seen in Figure 2b. This is due to less gain competition on either side of the effective gain distribution of the presented EDF laser. Figure 1: Optical setup of first presented L-band EDF ring laser. PC: polarization controller; OC: optical circulator; EDF: erbium-doped fiber; FMR: fiber mirror reflector; CPR: 10:90 and 1 \(\times\) 2 optical coupler; TBF: tunable bandpass filter; SMF: single-mode fiber; OSA: optical spectrum analyzer; EDFA: erbium-doped fiber amplifier. This is why the high ASE background noise near the 1568 nm wavelength could not be suppressed. As the tuning wavelength gradually shifted to longer wavelengths, the ASE background noise could be more completely suppressed, as also exhibited in Figure 2b. Figure 3 also presents the optical signal-to-noise ratio (OSNR) and output power detected for this EDF ring laser at different lasing wavelengths, from 1558.0 to 1618.0 nm. The output power and OSNR of the EDF laser were between \(-\)14.73 and 0.78 dBm and 46.72 and 68.44 dB, respectively, within an available wavelength-tuning bandwidth of 60 nm. The largest and lowest output power of 0.78 and \(-\)14.73 dBm was at the lasing wavelength of 1578.0 and 1618.0 nm with OSNR of 64.39 and 54.50 dB, respectively. However, the minimum OSNR of 46.72 dB was at the output wavelength of 1558.0 nm. Moreover, a flat power output profile with 1.01 dB power fluctuation [AP = 1.01 dB (P\({}_{\mathrm{out}}\) = \(-\)0.23-0.78 dBm)] could also be reached over a useful bandwidth from 1568.0 to 1598.0 nm (BW = 30 nm) and the corresponding OSNR was between 61.66 and 68.44 dB, as displayed in Figure 3. Due to the flattened ASE and gain distribution of the original L-band EDFA, the presented EDF laser could also reach a flat power output bandwidth of 30 nm. In general, to achieve flattened power output operation of the fiber laser, adjusting the pump power of the laser diode (LD) of EDFA and using the corresponding optical filter were the common methods. This also means that the proposed laser does not require other controls and operations to achieve flat power output. Figure 2: (**a**) Measured initial ASE spectrum of EDFA in the L-band range. (**b**) Observed optical output spectra in a wavelength range of 1558.0 to 1618.0 nm. In a previous investigation, the unpumped EDF-SA of 2 m long causes the auto-tracking filter with an ultra-narrow bandwidth effect, filtering and suppressing the dense longitudinal modes [18]. Therefore, the most longitudinal modes are removed by this SA-induced filter effect. We also applied a short length of 100 m SMF in the laser architecture to produce Rayleigh backscattering (RB) injection to narrow the laser linewidth. Essentially, when the wavelength of the laser wavelength is larger than the particle's radius, the RB occurs [18; 24]. The reflected RB light and lasing wavelength enter the EDFA simultaneously. Then, the amplified lasing wavelength can also generate the RB lightwave again with a higher Rayleigh coefficient to narrow the lasing wavelength. Moreover, the RB light enters the EDFA thousands of times to be excited and amplified.