Recently, in optical communications, 2 μm is expected to be the fourth window in the near-infrared after 850 nm, 1310 nm, and 1550 nm. The research on the devices, which could be applied to 1.26-1.675 μm and 2 μm bands with large broadband and low loss, follows the high-capacity communication development in the future. As part of the photonic integrated circuits (PICs), the optical power splitter is widely employed in high-capacity communication and interconnection scenes. It supports the design of wavelength division multiplexing (WDM) system, modulator, optical switch, and other devices. As a key device for beam splitting and beam merging, the optical power splitter requires compact structure, broadband, and low loss. The power splitter with breakthrough bandwidth will play a cornerstone role in the ultra-broadband systems on chip, which can cover the full communication bandwidth of O-, E-, S-, C-, L-, U- bands (1.26-1.675 μm) and 2 μm band with low loss.

According to the equivalent medium theory (EMT), the refractive index of the dielectric that does not exist in nature can be obtained by designing the subwavelength structure, which brings high flexibility to the PIC design. With the progress in micro-nano fabrication, subwavelength structures are adopted to yield better performance. Many kinds of devices have achieved large bandwidths by introducing subwavelength structures, including polarization beam splitter, polarization rotator, phase shifter, and fiber-chip edge coupler. Inspired by this, we propose a 3 dB power splitter on the silicon-on-insulator (SOI) platform. Subwavelength grating (SWG) structures with adjustable refractive index are introduced to realize large bandwidths. Then the optical power splitter is fabricated on an SOI wafer using electron beam lithography and dry etching processes. Additionally, a vertical coupling test rig is set up to test the loss of the TE mode and the beam splitting ratio in the bands from 1496.8 nm to 1600 nm.

As shown in Fig. 1, a novel structure is utilized to replace the traditional sharp-corner design in the traditional Y-branch, which radically solves the problems of actual processing in sharp corner. The light wave can be separated evenly and smoothly by the combination of adiabatic taper structure, SWG structure, and branch waveguides. The ultra-large bandwidth (1.26 μm-2.02 μm) and ultra-compact size can be obtained, which is shown in Fig. 2. Considering a large process tolerance of ±15 nm, the bandwidth of 760 nm is realized, with the excess loss (EL) less than 0.54 dB and the size of the beam splitting area 5 μm. The fabrication process and results are shown in Figs. 3 and 4 respectively. After preparation on an SOI wafer by electron beam lithography and dry etching processes, the processing error of the SWG structure is no more than ±10 nm when observed under the scanning electron microscope (SEM). As shown in Fig. 5, a vertical coupling test rig is set up to test the loss of the TE mode and the beam splitting ratio. In Figs. 6 and 7, the results show that the loss is about 0.3 dB in the bands from 1496.8 nm to 1600 nm, which is consistent with the simulation results, and the splitting ratio error is within 5% in the same waveband.

A compact, broadband, and low-loss silicon optical power splitter is designed through the equivalent medium theory. The 3D FDTD method is leveraged to optimize the device performance and analyze the fabrication tolerance. The subwavelength grating structure with an adjustable refractive index is introduced to the design. Sharp corner design in the traditional Y branch is replaced by a novel SWG structure, which solves the problems in the actual machining of sharp corner. Under the large process tolerance of ±15 nm, the TE mode loss is less than 0.54 dB in the band of 1.26 μm to 2.02 μm (760 nm bandwidth). For the first time, the full communication bandwidth covering O-, E-, S-, C-, L-, U-bands (1.26 µm-1.675 µm) and 2 µm band with low loss is realized in the simulation. The effective length of the beam splitting region is only 5 µm, which greatly reduces the effective device size and realizes a compact device design. A single optical power splitter is proven to achieve low loss coverage of full communication bandwidth under small size and large process tolerance. It is of significance to design communication systems with large capacity on the chip. Meanwhile, the optical power splitter processing has been completed through the operations of spin coating photoresist, electron beam exposure, laser direct writing, development, fixing, and dry etching. The processing size error is less than 10 nm, which proves the structure processing feasibility. The vertical coupling test system is established to complete the performance test of the optical power splitter. The excess loss is about 0.3 dB at 1496.8-1600 nm bands, and the spectral ratio is between 45% and 55%. The measured results in this band are close to the simulation to realize the design goal.