Compact and Ultra-Broadband Silicon Photonic Adiabatic Directional Coupler Using Rib Waveguides

An ultra-broadband and compact 3-dB power splitter has been designed and fabricated using an adiabatic rib silicon waveguide on the silicon-on-insulator (SOI) platform. This device primarily consists of two tapered rib waveguides to facilitate mode evolution for the transverse electric (TE) mode. The results demonstrate that the proposed device can achieve a 150 nm bandwidth in the wavelength range from 1450 nm to 1600 nm for 3-dB power splitting. In addition, the mode evolution footprint is significantly reduced to $78~\mu \text{m}$ with an excess loss of less than 0.18 dB. This proposed device offers a notable trade-off of device performance for power splitters. With multiple performance metrics, the rib waveguide-based adiabatic directional coupler exhibits significant potential for application in high-density photonic integrated chips (PICs).

I. Introduction

The silicon-on-insulator (SOI) is an attractive platform for photonic integrated chips (PICs) due to its complementary-metal-oxide semiconductor (CMOS) compatibility and significant optical field confinement [1]. The power splitter engineered on SOI is widely used as a building block in PICs, which is desired to be broadband, compact, low-loss and easily fabricated. In multiple device/system-level designs, power splitters have been extensively adopted in silicon photonics, such as reconfigurable wavelength division multiplexing (WDM) [2], optical switches [3], modulators [4], and optical neural networks [5]. Several types of architectures have been proposed to achieve desired power splitting including multimode (MMI) couplers, Y-junction couplers, directional couplers (DCs), etc. MMI couplers are common options, but the devices usually suffer from narrow bandwidth and excess loss (EL) in the operating band [6], [7]. The Y-junction couplers can also be used for power splitters due to their compact footprint, but the loss caused by mode mismatch cannot be ignored when junction angle is not small enough [8]. Low-loss and compact DCs are widely used in PICs, but they are wavelength sensitive due to mode dispersion [9]. Several approaches for improving wavelength insensitivity have been proposed. Adiabatic directional coupler (ADC) has been demonstrated to break the bandwidth limitation of conventional DCs [10]. The strip waveguide-based ADC could achieve 100 nm bandwidth [11]. However, the device footprint reaches , which is unconducive for large-scale integration. Asymmetric DCs have been proposed with an asymmetry introduced in coupling region [12], [13]. A relatively broad bandwidth can be obtained, but asymmetric DCs usually suffer from a limit to extend the bandwidth further. Ultrabroadband and compact subwavelength grating-assisted DCs (SWG-DCs) have attracted a lot of attention, but they require a high fabrication accuracy [14], [15]. Although these power splitters can meet some performance metrics, it is still challenging to realize a splitter with broadband, compact footprint, low loss, and large fabrication tolerance simultaneously. Through the high coupling strength as well as minor overlap between the optical field and waveguide sidewalls, a great tradeoff of the above performance metrics could be achieved with adiabatic rib waveguides.