A short and broadband silicon asymmetric Y-junction two-mode (de)multiplexer is proposed and simulated. An adiabaticity parameter suitable for high index-contrast silicon waveguides is defined. The fast quasiadiabatic dynamics protocol is used to homogeneously distribute adiabaticity over the device length. This protocol limits the power coupled into the unwanted waveguide eigenmode under a fixed value along the propagation. A 16 μm long mode (de)multiplexer with crosstalk < -34 dB is obtained. Simulations show that the optical bandwidth is as large as 300 nm (1400 nm ~ 1700 nm). The design is also fabrication tolerant.

In recent years, the concept of shortcuts to adiabaticity (STA), originally developed for speeding up slow adiabatic state evolution in quantum systems, has found numerous applications in guided-wave optics. Optical waveguides, enabled by the advanced fabrication technologies, provide an ideal platform to implement the STA protocols in terms of geometry variations; moreoever, STA has enabled the development of short and robust waveguide components, with applications in beam couplers, beam splitters, mode converters, mode (de)multiplexers, and polarization manipulation devices. Concepts such as counterdiabatic driving, invariant-based inverse engineering, fast-forward approach, or fast quasiadiabatic dynamics, have been shown to provide shortcuts to adiabatic mode evolution in optical waveguides, resulting in compact functional devices with large bandwidth and fabrication tolerance. Novel devices have recently been fabricated following years of theoretical efforts, showing that STA have emerged as a new paradigm in optical waveguides. In this work, we discuss the major STA protocols for applications in optical waveguides and illustrate the shortcuts with device examples.

We report a 2 × 2 broadband and fabrication tolerant mode-evolution-based 3 dB coupler based on silicon-oninsulator rib waveguides. The operating principle of the coupler is based on the adiabatic evolution of local eigenmodes. The key element of the device is an adiabatically tapered mode evolution region, which converts two dissimilar waveguides into two identical waveguides. Contrary to conventional designs using a linear taper function where the device adiabaticity is uneven during evolution, we use the fast quasiadiabatic approach to homogenize the adiabaticity of the device, leading to a shortcut to adiabaticity. Devices with an optimized taper region of 26.3 μm are designed and fabricated in a complementary metal-oxide-semiconductor compatible process with 193 nm deep ultraviolet lithography. The measured devices exhibit a broadband 3 dB 0.5 dB splitting within a bandwidth of 100 nm, uniformly across a 200-mm wafer, showing good tolerance against fabrication variations.

We propose an ultrashort and broadband silicon mode-conversion polarization splitter-rotator (PSR) consisting of a taper and a Y-junction both designed by the fast quasiadiabatic dynamics (FAQUAD). The FAQUAD is used to homogeneously distribute adiabaticity over the length of the PSR, providing shortcut to adiabaticity at a shorter device length. The total length of the silicon PSR is 39.2 μm. For a wavelength range from 1.5 μm to 1.6 μm, the PSR exhibits a good performance with > 88% transmission and > 11.4 dB extinction ratio (ER). Simulations also show that the designed devices have good fabrication tolerance.

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