# Narrowing Linewidth of Wavelength-Swept Active Mode Locking Laser Based on Cross Gain Modulation
## 2 Experiments and Results
Thus, the mode-locking orders, \(N\), of 13, 17, 21, 25 correspond to the modulation frequencies, \(f_{m}\), of 707.85, 925.65, 1143.45, 1361.25 MHz, respectively. We compared the linewidths of the XGM and DGM by applying the same RF signals with a different mode-locking order into the SOA directly or into the MZM for the same main cavity with the same \(FSR_{0}\). In the DGM condition with the mode-locking frequencies higher than 707.85 nm for the 13 th mode-locking order, it was observed that the linewidth tends to become broader as the frequency increases. However, the mode locking does not occur properly in  
Figure 2: Experimental setup of the wavelength-swept active mode locking (AML) laser based on XGM. BOA: booster optical amplifier; ISO: optical isolator; CFBG: chirped fiber Bragg grating; PC: polarization controller; AFG: arbitrary function generator. higher modulation frequency ranges because the relative frequency response of the modulation depth of the SOA deteriorates in the high mode-locking frequencies, as shown in Figure 1c. On the contrary, in the case of the XGM, the linewidth gradually decreased as the modulation frequency increased over 707.85 MHz. The linewidth, defined as a 3-dB spectral width, was 0.119 nm for the XGM, but it was 1.291 nm for DGM, at a modulation frequency of 1361.25 MHz of the 25 th mode-locking order. Inset of Figure 3b shows the relationship between the linewidth and modulation frequency under the XGM condition. Thus, we can experimentally confirm that the enhanced frequency response of the modulation depth of the SOA by using the XGM was indeed helpful to achieve a narrower linewidth of the AML laser output. Figure 4a shows the tuning of the output spectra of the wavelength-swept AML laser. For a cavity length of 10.27 m and the corresponding \(FSR_{0}\) of 19.89 MHz, the modulation frequencies around 850 MHz are matched to the mode-locking order of 43. The intensity of the optical pump light measured at port 2 of CIR1 was 7.2 mW for an injection current of 301.9 mA. As the modulation frequency, fm, increased from 844.77 to 855.3 MHz, the lasing wavelength \(\lambda\)m shifted from 1551.05 to 1589.45 nm. The sensitivity was 3.64 nm/MHz. Undesired lasing at 1590.9 nm was also observed, which was caused by the optical pump light reflected from the SOA. The intensity difference between the signal light and optical pump light was around 18 dB. The linewidth and intensity of the laser varied depending on the wavelength, but there was no significant difference within the tuning range. The signal to noise ratio (SNR) was measured over 35 dB in most of tuning region. As shown in Figure 4b, the average linewidth of the laser was 0.112 nm and the narrowest linewidth was 0.086 nm at the central wavelength of 1559.7 nm for the modulation frequency of 847.11 MHz. Figure 4b also shows the linearity of the wavelength-swept AML laser based on the XGM with the R-squared value of 0.99997. To measure the dynamic characteristics of the XGM, the spectrum was measured while sweeping the modulation frequency. For high linearity and a high duty cycle, a negative saw-tooth waveform from an arbitrary function generator (AFG) (Tektronix, AFG3022C) was externally applied to a frequency modulation (FM) port of the RF-SG to sweep the modulation frequency. Figure 5a shows the peak-hold  
Figure 4: **(a)** Static spectra of the wavelength-swept AML laser based on the XGM. **(b)** Mode locked wavelength (left y axis) and linewidth (right y axis) versus modulation frequency. Figure 3: Comparison of the output spectra at the 13 rd, 17 th, 21 st, and 25 th orders of mode locking of **(a)** DGM and **(b)** XGM. Inset: Relationship between the linewidth and modulation frequency of XGM. spectra at a sweeping rate at 100 kHz, and the 6-dB tuning bandwidth was measured to be 41.25 nm; this result is in good agreement with the 3-dB optical bandwidth of the SOA, which was \(\sim\)45 nm. Figure 5b shows the temporal series of the laser output, measured with 350 MHz photodetector (PD) (Thorlabs, PDB430C-AC).