With different interactions between material and femtosecond lasers, two-dimensional (2D) and three-dimensional (3D) waveguide couplers, whose separation distances are fabricated in z-cut lithium niobate crystal by femtosecond laser writing, are reported. Experimentally and numerically, it is shown from results that the guidance is only propagating along TM polarization due to the Type I modification and holds equal splitting ratios, which are the same as power splitters at 632.8 nm. The propagation losses of 2D and 3D waveguide couplers exhibit better transmission properties than those of the previously reported Type I Y-junction waveguide splitters.

Separating lights into different paths according to the polarization states while keeping their respective path’s polarizations with high purification is keen for polarization multiplex in optical communications. Metallic nanowire gratings with multi-slits in a period are proposed to achieve polarized beam splitters (PBSs) in reflection and diffraction. The setting of multi-slits largely reduces the reflection of photons with a transverse magnetific field via the plasmonic waveguiding effect, which leads to highly polarized output lights with extinction ratio larger than 20 dB in each channel. The proposed reflection/diffraction PBSs enrich the approaches to control the polarization states with the advantages of wide incident angles and flexible beam splitting angles.

The miniaturization of polarization beam splitters (PBSs) is vital for ultradense chip-scale photonic integrated circuits. However, the small PBSs based on complex hybrid plasmonic structures exhibit large fabrication difficulties or high insertion losses. Here, by designing a bending multimode plasmonic waveguide, an ultrabroadband on-chip plasmonic PBS with low insertion losses is numerically and experimentally realized. The multimode plasmonic waveguide, consisting of a metal strip with a V-shaped groove on the metal surface, supports the symmetric and antisymmetric surface plasmon polariton (SPP) waveguide modes in nature. Due to the different field confinements of the two SPP waveguide modes, which result in different bending losses, the two incident SPP waveguide modes of orthogonal polarization states are efficiently split in the bending multimode plasmonic waveguide. The numerical simulations show that the operation bandwidth of the proposed PBS is as large as 430 nm because there is no resonance or interference effect in the splitting process. Compared with the complex hybrid plasmonic structure, the simple bending multimode plasmonic waveguide is much easier to fabricate. In the experiment, a broadband (Δλ≈120 nm) and low-insertion-loss (<3 dB with a minimum insertion loss of 0.7 dB) PBS is demonstrated by using the strongly confined waveguide modes as the incident sources in the bending multimode plasmonic waveguide.

In this paper, we have proposed a hybrid optical wavelength demultiplexer and power combiner for a hybrid time- and wavelength-division multiplexing (TWDM) passive optical network (PON), i.e., a single passive optical device that functions as a 1×N wavelength demultiplexer for distributing the downstream signal in multiple wavelengths from the optical line terminal (OLT) to the N optical network units (ONUs), and simultaneously as an N×1 power combiner for collecting the upstream signal in the same wavelength from the N ONUs to the OLT. Through a design example of a 32 channel hybrid optical wavelength demultiplexer and power combiner on the silicon-on-insulator platform, our numerical simulation result shows that the insertion loss and adjacent channel crosstalk of the downstream wavelength demultiplexer are as low as 4.6 and 16.3 dB, respectively, while the insertion loss and channel non-uniformity of the upstream power combiner can reach 3.5 and 2.1 dB, respectively. The proposed structure can readily be extended to other material platforms such as the silica-based planar lightwave circuit. Its fabrication process is fully compatible with standard clean-room technologies such as photo-lithography and etching, without any complicated and/or costly approach involved.