# Narrowing Linewidth of Wavelength-Swept Active Mode Locking Laser Based on Cross Gain Modulation
## 2 Experiments and Results  
Before constructing a laser cavity, we tried to confirm experimentally that the XGM works better than the DGM in a high-frequency modulation condition. Figure 1a,b show the experimental setup to measure the modulation depth of the SOA in the DGM and XGM configurations, respectively. The modulation depth of the SOA (M\({}_{\text{SOA}}\) = Ao/Ap) is newly defined as the ratio of the amplitude of the output electric signal modulation (Ao) compared to the amplitude of the pump electric signal modulation (Ap). The electrical output power from the light passing through a SOA (InPhenix, IPSAD1501) with C-band gain was measured using a 1-GHz bandwidth photodetector and an 8-GHz bandwidth electric spectrum analyzer (ESA) (Advantest, R3267). The amplified spontaneous emission (ASE) of another SOA (InPhenix, IPSAD1509) with a central wavelength of 1552.59 nm and power of 0.13 mW was used at the input port of the SOA. The modulation amplitude of the pump electric signal was set to 14 dBm and the initial frequency increased from 100 MHz to avoid the damage to the SOA due to low modulation frequency. In the DGM configuration in Figure 1a, the injected current of the SOA is directly modulated as a sinusoidal signal from a radio frequency signal generator (RF-SG) (Anritsu, 68369A/NV). A bias-T circuit was used to combine a DC current and an RF modulation signal prior to the position of the SOA. In the XGM configuration of Figure 1b, an external-cavity Littrow laser with a central wavelength of 1541.53 nm, a linewidth of \(\sim\)100 kHz, and an output power of 1.38 mW was employed as the optical pump light into a series made up of a Mach-Zehnder modulator (MZM), a circulator, and a SOA. The modulation depth of the optical pump light was controlled with the modulation depth of the pump electric signal from the RF-SG. The direction of the optical pump light from the circulator (CIR1) port 1 to port 2 was opposite to the optical amplification signal light in the SOA gain part. For both configurations shown in Figure 1a,b, it was observed that the modulation depth of the SOA decreased with increasing modulation frequency in a higher region over 100 MHz; however, the decreasing trends of modulation depth of the SOA were different for the modulation frequencies for both the configurations. Thus, the relative modulation depth of the SOA (M\({}_{\text{SOA}}\)/M\({}_{\text{i}}\)) is defined as the ratio of the modulation depth of the SOA at each modulation frequency compared to that at 0 MHz. Figure 1c shows the comparison of the relative frequency response of the modulation depth of the SOA for both the DGM and XGM cases. It is clearly shown that the frequency response of the modulation depth of the SOA decreases as the modulation frequency increases in both the DGM and XGM. The 3-dB points appeared at 462.962 and 930.509 MHz in the DGM and XGM, respectively. It means that the frequency response of the modulation depth of the SOA was enhanced by approximately two-fold by using the XGM. Figure 2 shows the experimental set-up of the wavelength-swept AML laser based on the XGM configuration. The cavity consists of a SOA (InPhenix, IPSAD1501) acting as a gain medium, two optical circulators, three polarization controllers for optimizing the polarization states of the light propagating through the single mode optical fiber, an optical isolator to block the pump light to prevent undesired lasing, and a chirped fiber Bragg grating (CFBG) (TeraXion) acting as a dispersion medium (15 ps/nm at 1.5 \(\upmu\)m) and an output coupler (reflectivity of \(\sim\)60%). A 12.5 mW external-cavity Littrow laser was used as the optical pump light. The optical pump light was modulated by the MZM (iXblue, MX-LN-20) using a sinusoidal signal from the RF-SG. The intensity-modulated optical pump light was close to 4.1 mW due to the high loss of the MZM (\(\sim\)3.5 dB) at the 1.2-GHz modulation frequency.