Ultrashort and efficient adiabatic waveguide taper based on thin flat focusing lenses

Abstract

A new structure is reported, which realizes the flat focusing by introducing the silicon subwavelength slits into the waveguide. The subwavelength silicon-air slits, with variable widths to match the phase compensation, makes possible to focus a plane wave. The flat lens proposed here demonstrates relatively high power gain at the focal point or two focal points. By using such a design, we demonstrate a grating coupler with an ultrashort taper of 22.5-μm to connect a 10-μm-wide input waveguide and a 0.5-μm-wide output waveguide, achieving a transmission up to nearly 95.4% numerically in the communication band. The length of which is one-twentieth of that for the traditional taper. To our best knowledge, this work is the first demonstration of an ultrashort taper based on flat lens, which significantly improves the integration of the photonics integrated circuits, and indicates an effective solution for potential applications in compactly integrated micro/nano optical devices.

Design of polarization-insensitive high-visibility silicon-on-insulator quantum interferometer

“…In particular, when MMI height and width reach 320 nm and 1.58 μ m, the effective refractive index for TE 00 , TE 01 , TM 00 , TM 01 modes reach 3.0361, 2.9160, 2.6271 and 2.5069, respectively, which enables Δ n eff (TE) = 0.1201 being most approximate to Δ n eff (TM) = 0.1203. Note that by using an unique height of 320 nm, the resulting polarization-independent beat length and the capable minimal MMI length are only 6.44 μ m and 2.4 μ m, respectively, while by using conventional height values of 220 nm 27 , 250 nm 28 and 340 nm 20 , there remains a challenge to trade off Δ n eff for both polarizations involving MMI width as the only degree of freedom. As MMI height heavily determined by SOI wafers is often fixed in actual cases, the optimized value of 320 nm is mainly concerned in the following discussions.…”

On-chip arbitrary-mode spot size conversion

“…Using on-chip components in analogy with freespace devices, such as lenses, exhibits enormous advantages for intuitional control on trajectory and wavefront. For example, focusing lens has been adopted in fundamentalmode spot size converters (SSCs) [7][8][9][10]. Eaton lens has been demonstrated to bend the light trajectory on chip (waveguide bend) [11,12], and Maxwell fish eye lens's aberration-free imaging property has been utilized to build multimode waveguide (MWG) crossings [13,14].…”

Ultrashort waveguide tapers based on Luneburg lens

“…An adiabatic taper with a length of 120 µm has been reported to connect a 12 µm-wide waveguide to a 0.5 µm one with a coupling loss lower than 0.07 dB [15]. An adiabatic 22.5 µm-long taper based on a flat lens has been proposed to connect a 10 µm-wide waveguide to a 0.5 µm one with coupling loss lower than 0.45 dB in a bandwidth of 60 nm [16]. A taper using a quadratic sinusoidal function has been designed to connect a 10 µm-wide waveguide to a 1 µm one with a length of 19.5 µm in silicon nitride (SiN) platform.…”

“…A hollow tapered waveguide has been introduced to reduce the lateral width of the waveguide from 15 µm to 0.3 µm at a length of 60 µm while the coupling loss is lower than 2.0 [20]. The theoretical results of [15][16][17][18][19][20] are reported in this section. Tapers based on transformation optics (TO) have also been proposed, however, the implementation of these designs require anisotropic metamaterials [21][22][23].…”