Results and Discussions For the SMSP-MSF with a regular hexagonal lattice, with the increase in the diameter d₁₁ of the gold-coated hole (Fig. 2) or the decrease in the thickness t of the gold layer (Fig. 4), the modal effective index of SPP mode increases gradually. With the increase in the diameter d₁₁ of the gold-coated hole or that in the thickness t of the gold layer, the peak value on the loss curve for the x-polarized core mode increases first and then decreases (Fig. 3 and Fig. 5). After optimizing the structural parameters for each defect core and each gold-coated hole, six resonant wavelengths are evenly distributed in the whole interested band where the x-polarized core mode, defect core mode, and SPP mode resonate. The superposition of the six loss peaks forms a relatively flat and ultra-wide single-mode single-polarization transmission band. For the SMSP-MSF with a square lattice, the x-axis is the direction of the fast axis of the core but the slow axis of the defect cores. Even when the modal effective indexes of the x-polarized core mode and defect core mode have similar values, the difference between the modal refractive indexes of the y-polarized core mode and defect core mode is still very large. Therefore, the confinement loss ratio and polarization extinction ratio between the x-polarized and the y-polarized core modes are quite large in the whole interested wavelength range (Fig. 10).

The conventional single-mode single-polarization micro-structured fibers (SMSP-MSFs) achieve their SMSP property by coupling the unwanted polarized core mode to the cladding defect mode. This design method has contradicted the requirement on the number of air-hole layers outside the defect core, which further leads to the tradeoff between the confinement loss ratio Γ and polarization extinction ratio. In this paper, a novel kind of SMSP-MSFs based on a "coupling-coupling-absorption" mechanism is proposed for the first time to the best of our knowledge. In order to construct the proposed SMSP-MSFs, some gold-coated holes are introduced outside the cladding. The energy in unwanted polarized core mode is firstly transferred to the gold-coated areas through double coupling, namely, coupling from core to defect and coupling from defect to gold-coated area. Then, it is strongly absorbed in the gold-coated area by surface plasmon resonance. As a result, SMSP-MSFs with broadband single-mode single-polarization transmission can be achieved.

Based on the above mechanisms, two broadband SMSP-MSFs are proposed. The first SMSP-MSF has a regular hexagonal lattice. The birefringence is introduced in the core by symmetrically enlarging two air holes around the core. In the cladding, air holes at the middle of every side of the fourth hexagonal air hole ring are reduced to different diameters to form six defect cladding cores. Besides, six air holes with different sizes are drilled outside the cladding, and then they are coated with a layer of gold of different thicknesses. By optimizing the structural parameters of the SMSP-MSF, the energy of the x-polarized core mode can be coupled to the gold's surface plasmon polariton (SPP) mode through the defect core mode at multiple wavelengths, due to the resonance among those modes. Then, the energy is strongly absorbed by the SPP mode, which results in a great increase in the lossof the x-polarized core mode. For this SMSP-MSF, the influences of the gold-coated hole's diameter and the gold layer's thickness on the position of the resonant wavelength and the confinement lossof the x-polarized core mode are studied respectively by the full vector finite element method. The second SMSP-MSF has a square lattice with one rectangular inner core and two rectangular defect cores. While the long sides of the core and the defect core are placed along the y and x axes, respectively, the directions of the fast axes of the core and the defect core are perpendicular to each other. When the modal effective index of the x-polarized core mode (which is its fast axis) is adjusted to have a similar value with that of the x-polarized defect core mode (which is its slow axis), the difference between the value of the modal effective indexes of the y-polarized core mode and y-polarized defect mode is still very large. This guarantees strong energy coupling between the core mode and defect mode in the x-polarized direction but very weak coupling between the core mode and defect mode in the y-polarized direction. After the energy in the x-polarized defect core mode is coupled to and absorbed by the SPP mode in the gold-coated air holes, an SMSP-MSF whose x-polarized core mode has large confinement loss but y-polarized core mode can transmit with low confinement loss can be achieved. For this SMSP-MSF, confinement losses for both polarized core modes are numerically computed around 1.55 μm by using the full vector finite element method. The modal field distribution of both polarized core modes is studied. The single-mode single-polarization bandwidth of this SMSP-MSF is analyzed.

In this paper, two novel ultra-broadband SMSP-MSFs are proposed for the first time based on mode coupling mechanism and surface plasmon resonance effect. The influences of the structural parameters of the SMSP-MSFs on both polarized modes' confinement loss are studied. For the SMSP-MSF with a regular hexagonal lattice, the coupling of x-polarized core mode, defect core mode, and SPP mode at multiple wavelengths is realized by introducing six different defect cores in the cladding and constructing six different gold-coated areas outside the cladding. The x-polarized core mode, defect core mode, and the SPP modes resonate simultaneously at 1.320 μm, 1.388 μm, 1.440 μm, 1.490 μm, 1.570 μm, and 1.640 μm. The energy in the x-polarized core mode is transferred out and absorbed by the SPP mode in a wide wavelength band efficiently. This results in an SMSP-MSF with a single-mode single-polarization wavelength band of larger than 380 nm. For the SMSP-MSF with a square lattice, the fast axis of the core and the defect core are perpendicular to each other. By this mechanism, strong coupling between the x-polarized core mode and x-polarized defect core mode, as well as weak coupling betweenthe y-polarized core mode and y-polarized defect core mode, can be achieved at the same time. After optimization of the structural parameters, the x-polarized core mode shows a confinement loss of 113.57 dB/m at 1.55 μm, while the y-polarized core mode's confinement loss is only 2.54×10^-4 dB/m. The confinement loss ratio and polarization extinction ratio of the square-latticed SMSP-MSF can reach as high as 5×10⁵和113 dB, respectively. The two SMSP-MSFs proposed in this paper can be used in the areas of fiber sensors, fiber optic gyroscopes, in-line polarizers, coherent optical communication systems, etc.