# Tunable and Switchable Erbium-Doped Fiber Laser Using a Multimode-Fiber Based Filter
## 3 Experimental Results and Discussions  
Based on the configuration in Figure 1, we set up our experiments to investigate how the MMF-based PC could essentially control the laser operating wavelengths and their performance. The laser spectra are measured by an Agilent optical spectrum analyzer (OSA). When the paddles of the MMF-based PC are properly adjusted, the proposed laser could operate in a single- and dual-wavelength regime under invariable pumping power at room temperature. Figure 2 plots the tuning characteristics of the erbium-doped fiber laser at a pump power of 130 mW. The laser can be continuously tuned from 1554.96 nm to 1564.25 nm, with a tuning range of 9.3 nm. For operating at 1554.96 nm, the measured laser linewidth and the side mode suppression ratio (SMSR) are about 0.249 nm and 45.1 dB, respectively. The length of the MMF is 1 m. When using 1.4 m long of the same MMF, the free spectral range (FSR) is 6.4 nm [23]. As the FSR of the MMF filter is inversely proportional to the length of the MMF, the estimated FSR is about 9 nm at the C band with consideration of the two lowest dominant modes, which suggests the available tuning limit. To the best of our knowledge, this is the largest tuning range using one polarization controller as the tuning element for an MMF-based mode beating filter. Note that reducing the length of the MMF could increase the FSR of the filter, consequently, the tuning range, which is, however, limited by the paddle size of the PC. At the same pumping power, when properly adjusting the paddles of the PC, the proposed laser could switch to the dual-wavelength output mode. Figure 3 shows the laser spectra of three sets of dual wavelength operations with different wavelength spacing. The power differences of the two peaks for the lines with blue, red, and orange color are 0.19, 0.53, and 0.35 dB, respectively, indicating a relatively good power uniformity. The dual wavelength spacing could be adjustable. The largest wavelength spacing is 7.65 nm, with one peak at the C-band, 1557.936 nm, and the other at the L-band, 1565.582 nm; the measured linewidth of the first peak is 0.104 nm and that of the second one is 0.131 nm. It is worth pointing out that at the same pumping power with suitable adjusting of the paddles of the PC, for the dual wavelength operating mode, the dual-wavelength laser could be tunable. The tunability of the dual wavelength laser spectra is plotted in Figure 4. The first peak can be tuned from wavelength, 1560.815 to 1562.054 nm; while tuning the first peak wavelength, the wavelength spacing varies. The smallest wavelength spacing is 1.46 nm, where the two laser peaks are located at 1561.884 and 1563.343 nm with a linewidth of 0.125 and 0.133 nm, respectively. The largest wavelength spacing is 3.08 nm, where the two oscillating wavelengths are 1560.815 and 1563.893 nm and their linewidths are 0.136 and 0.114 nm, respectively. The minimum and the maximum power variation for the two peaks in Figure 4 are 0.02 dB and 1.14 dB, respectively. The feature of tuning the dual-wavelength could be attributed to the change of the additional phase shift and the contrast of the two peaks. Figure 3: Dual-wavelength output spectra. Figure 2: Superimposed laser spectra of the tunable erbium-doped fiber lasers. of the filter by adjusting the PC, which could also excite different polarized LP mode components, consequently, to control the balance of the gain and loss in the cavity for different wavelengths. In the experiment, the stability of the laser operating at single and dual wavelengths is also tested. The laser output spectra are repeatedly scanned at the interval of 1 min for fifteen min, as shown in Figure 5a,b. For operating at the single-wavelength mode with a wavelength of 1559.03 nm, the fluctuations of the peak power and the laser wavelength are 0.51 dB and 0.09 nm as indicated in Figure 5c. For the case of the dual-wavelength mode at 1558.15 and 1565.38 nm, as shown in Figure 5d, the wavelength spacing of the dual wavelength is 7.23 nm.