Continuously spacing-tunable dual-wavelength erbium-doped fiber laser based on polarization-dependent in-line multimode-single-mode-multimode fiber filter

Abstract

A continuously spacing-tunable dual-wavelength erbium-doped all-fiber laser is proposed and experimentally demonstrated in this paper. The key component of the laser is a novel polarization-maintained multimode-single-mode-multimode fiber interference filter, which is composed of two single-mode-multimode-single-mode fiber mode converters with a polarization-maintaining fiber sandwiched between them. As the polarization-maintaining fiber gives rise to a polarization-dependent phase difference, the fiber filter shows good polarization-dependent characteristics in interference filtering. Based on the mode interference and polarization control, the good wavelength tuning results are obtained in experiment. An optimized length of 1.3 mm for multimode fiber and 1.5 mm for polarization-maintaining fiber are adopted based on the theoretical and experimental analysis. When the phase difference between the lasing in the fast axial direction and that in the slow axial direction is <inline-formula><tex-math id="M1">\begin{document}$ {\text{π}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190297_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20190297_M1.png"/></alternatives></inline-formula>, the peaks and valleys in the transmission spectrum of the fiber filter correspond exactly to the two orthogonal polarization states. In the test, when the pump power is 50 mW, a high-quality dual-wavelength lasing output (at 1544.82 nm and 1545.61 nm) is observed to have side-mode suppression ratio better than 45 dB, the wavelength spacing of 0.8 nm, the peak power difference of less than 1 dB, and the output keeps stable with a small power fluctuation less than 0.7 dB. By adjusting the polarization controller in the ring cavity, two different dual-wavelength outputs can be obtained, which are corresponding to tuning Ⅰ and tuning Ⅱ. In tuning Ⅰ, a dual-wavelength lasing output in a wavelength spacing tuning range of 0−1.2 nm can be obtained within 1 dB peak power difference, correspondingly, 0−1.6 nm tuning range within 10 dB peak power difference. In tuning Ⅱ, with continuously adjusting the polarization controller, the short wavelength signal of the dual-wavelength output stops resonating, and simultaneously another wavelength signal near 1547.8 nm is excited, the switching is continuous and the system remains dual-wavelength output. In tuning Ⅱ, a maximum tuning range of 1.6−3 nm is obtained. In both of tuning Ⅰ and tuning Ⅱ, a 0−3 nm continuously spacing-tunable dual-wavelength output is obtained, all of which keep stable single-polarization operation. The test results show that the polarization state of the dual-wavelength lasing varies with tuning; a maximum polarization extinction ratio of 35 dB is obtained as the two wavelengths are orthogonally polarized.