(a) The multiwavelength spectrum based on IDT at SOA current of 125 mA. (b) Magnified view within dashed lines of (a). 

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... this experiment, we used a PMF length of 26.6 m to determine 0.2 nm of channel spacing. The two PCs are always adjusted to obtain the highest extinction ratio (ER) of spectrum for all experimental results. The proposed laser structure has a lasing threshold around 110 mA and its performance is recorded beyond this threshold value. Fig. 2 shows the flattest multi- wavelength spectrum at 125 mA of SOA current and 10% of throughput port (TP). The lasing lines count is relative to the vertical bandwidth range. As shown in Fig. 2(a), 285 and 153 lines are counted within 7 dB and 3 dB bandwidth, respectively. Each laser line has an ER of 16 dB over wavelengths as the multi- ...

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... for all experimental results. The proposed laser structure has a lasing threshold around 110 mA and its performance is recorded beyond this threshold value. Fig. 2 shows the flattest multi- wavelength spectrum at 125 mA of SOA current and 10% of throughput port (TP). The lasing lines count is relative to the vertical bandwidth range. As shown in Fig. 2(a), 285 and 153 lines are counted within 7 dB and 3 dB bandwidth, respectively. Each laser line has an ER of 16 dB over wavelengths as the multi- wavelength pattern follows the amplified spontaneous emission of the SOA. Fig. 2(b) is the magnified view of the multiwavelength fiber laser at 2 nm span, as to view clearer lines of the 0.2 nm ...

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... current and 10% of throughput port (TP). The lasing lines count is relative to the vertical bandwidth range. As shown in Fig. 2(a), 285 and 153 lines are counted within 7 dB and 3 dB bandwidth, respectively. Each laser line has an ER of 16 dB over wavelengths as the multi- wavelength pattern follows the amplified spontaneous emission of the SOA. Fig. 2(b) is the magnified view of the multiwavelength fiber laser at 2 nm span, as to view clearer lines of the 0.2 nm channel spacing. The 3 dB linewidth of each lasing line is measured at around 0.11 nm, which is attributed to the inhomo- geneous gain broadening of ...

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... 0 is the wave number in vacuum, dn r / dn c is the nonlinear change in refractive index, n 0 c is the equilibrium of excess carrier density, G is the optical confinement factor and L a is the length of the SOA’s active region [19]. Using typical values of dn r / dn c 1⁄4 À 1.2 Â 10 À 25 m 3 , n 0 c 1⁄4 4.8 Â 10 22 m À 3 , G 1⁄4 0.3 m and L a 1⁄4 250 Â 10 À 6 m, the value of nonlinear phase shift at À 0.557 p is obtained. According to the Eq. (4), we can obtain a working state of either IDT or IDL at different value of y and y by adjusting PCs. In this experiment, we used a PMF length of 26.6 m to determine 0.2 nm of channel spacing. The two PCs are always adjusted to obtain the highest extinction ratio (ER) of spectrum for all experimental results. The proposed laser structure has a lasing threshold around 110 mA and its performance is recorded beyond this threshold value. Fig. 2 shows the flattest multiwavelength spectrum at 125 mA of SOA current and 10% of throughput port (TP). The lasing lines count is relative to the vertical bandwidth range. As shown in Fig. 2(a), 285 and 153 lines are counted within 7 dB and 3 dB bandwidth, respectively. Each laser line has an ER of 16 dB over wavelengths as the multiwavelength pattern follows the amplified spontaneous emission of the SOA. Fig. 2(b) is the magnified view of the multiwavelength fiber laser at 2 nm span, as to view clearer lines of the 0.2 nm channel spacing. The 3 dB linewidth of each lasing line is measured at around 0.11 nm, which is attributed to the inhomogeneous gain broadening of SOA. To investigate the intensity influence to the multiwavelength flatness, we increased the SOA current with a fixed TP ratio as depicted in Fig. 3. As seen in Fig. 3(b) and (c), the multiwavelength flatness is decreased when the SOA current is biased from 225 mA to 325 mA, as compared to the SOA current of 125 mA shown in Fig. 3(a). On the other hand, as the SOA current increased from 225 mA to 325 mA, the output peak power is also increased to À 39 dBm and À 33 dBm, respectively. The multiwavelength flatness is determined from the difference between the highest to the lowest peak lines as indicated in the magnified spectrum as depicted in Fig. 3(d). From our measurement, the difference of peak lines at SOA current of 225 mA and 325 mA are 3 dB and 7 dB, respectively. Therefore, as the peak power difference is lower, the flatness is better. In multiwavelength fiber laser based on IDT, when the intensity is increased, the transmission is high. At high transmission, the cavity loss is low and affecting the multiwavelength flatness. However, according to [6], cavity loss increment is crucial to obtain a flat multiwavelength spectrum. From the experiment, we proved that the flat spectrum is obtained at low intensity which the cavity loss is high. It is important to note that the PCs have to be adjusted to a specific PS in order to obtain the working state of IDT, because the IDT mechanism can only be induced at a certain PS [15]. In other PS, the intensity increases as the transmission decreases, which is the working state of IDL. On the other hand, we also observed that the lasing lines were maintained at different SOA currents. This is due to the inhomogeneous gain broadening within the SOA and its combination with PDI, PC1 and PC2. We also investigated the effect of intensity in the cavity on the laser performance by changing the value of TP as illustrated in Fig. 4. At fixed SOA current of 125 mA, we changed the TP from 10% to 90% as to increase the intensity in the ring cavity. The 90% of TP contributed to approximately 0.5 dB of insertion loss and has higher intensity in the cavity but lower output power as shown in Fig. 4(b). From the figure, the multiwavelength flatness is fairly flat as compared with the use of 10% of TP. However, to prove that the intensity in the cavity is affecting multiwavelength flatness, the ER of each line against its center wavelength is plotted for a clearer flatness observation, as shown in Fig. 4(c). From the figure, the use of 10% TP is seen flatter than using 90% TP. This can be verified from the measurement of ER difference between the highest and the lowest. In our measurement, for the 10% and 90% TP, the ER discrepancies are 0.51 dB and 2.62 dB, respectively. Hence, the use of 90% TP deteriorates the quality of ER flatness for each line. Owing to the IDT, lower intensity in the cavity is required so that the cavity loss in the cavity is high. The high loss is important to obtain a flat multiwavelength spectrum [6], as depicted in Fig. 4(a). Based on these findings, SOA current adjustment gives more impact on the flatness as compared with the use of different TP ratios. Instead of adjusting PC to increase the multiwavelength flatness as reported in [14], we have demonstrated that a multiwavelength flatness can be increased by using a low intensity in the cavity, due to the working state of IDT. Note that, our multiwavelength fiber laser has higher ER and lasing lines compared to the IDT-based SOA multiwavelength fiber laser as reported in [15]. The polarizer is then removed from the setup to check its effect to the multiwavelength generation. No multiwavelength generation is obtained, using the same setting of SOA current and TP ratio, as shown in Fig. 5 even with PCs adjustment. With polarizer, the multiwavelength spectrum is greatly flat at 16 dB of ER with each lasing line has nearly equal peak intensity over lasing wavelengths range. The polarizer proved to be a significant device for the generation of multiwavelength fiber laser. The use of polarizer at a different device combination in a ring cavity contributed to a slightly dissimilar function. As reported in [20], the polarizer worked as a tunable comb filter and mode locker in a ring cavity. In our case, the use of polarizer in a combination of polarization devices and SOA (SOA-PC1-PC2-PDI) is crucial to induce the IDT mechanism in the ring cavity setup [3]. Thus, with the polarizer in the setup, the IDT mechanism can be induced as stable multiwavelength fiber laser can be generated at a room temperature. Stability of the multiwavelength fiber laser is then tested in 100 min time frame. As depicts in Fig. 6, the scanned spectra show no significant peak power dithering. Three lasing lines of 1530.4 nm, 1530.6 nm and 1530.8 nm are then selected and their peak power variations against time are observed for every 3 min as can be seen in Fig. 7. Based on the finding, there is no sign of large laser fluctuation as the laser dithering is lower than 0.2 dB in 60 min. The stability tests are clearly show that the fiber laser has a stable operational work at room temperature and can be used in the application of multiwavelength fiber laser. In conclusion, we have experimentally demonstrated the influence of intensity in the cavity to the multiwavelength flatness. The multiwavelength fiber laser is based on IDT mechanism in con- junction with SOA and Lyot filter. A flat multiwavelength spectrum is obtained from the contribution of high cavity loss caused by the use of low intensity in the ring cavity. Owing to IDT, the high cavity loss is obtained from a low intensity of oscillating lasers, and it is the key to a stable and flat multiwavelength generation. The multiwavelength flatness is decreased when the intensity in the cavity is increased through the adjustment of either SOA current or TP ratio. However, the SOA current adjustment has more influence to the flatness as compared to TP ratio change. With the removal of polarizer from the setup, no multiwavelength spectrum can be generated as the polarizer is a crucial component to induce the IDT ...