On‐Chip Multi‐Mode Manipulation via 2D Refractive‐Index Perturbation on a Waveguide

Abstract

illustrate our design methodology. Previously, little progress was made on multimode conversion from multiple low-order modes to high-order modes except a recent demonstration with two particular pairs of modes using an inverse design. [32] The key challenge of realizing multi-mode conversion is to achieve equally high conversion efficiencies for multiple mode pairs. Due to the significant discrepancy of the phase matching conditions between different pairs of modes, multi-mode conversion is difficult to realize using conventional phase matching techniques based on periodic perturbations. [16-18] In the following, we demonstrate a scalable and computationally efficient method for low-loss multi-mode conversion using a quasi 2D metastructure on a silicon waveguide. Shallow hexagonal trenches etched on the silicon waveguide provide sufficient refractive-index perturbations and introduce low excess losses. A segmented index profile in the transverse direction is designed based on coupled mode theory to maximize target mode coupling coefficients, while a quasi-periodic refractiveindex variation along the propagation direction is optimized by particle swarm optimization (PSO) algorithm to achieve approximately equal conversion efficiencies for multiple pairs of modes. As a proof-of-concept experiment, we demonstrate a multi-mode converter that can realize the simultaneous conversion processes from TE i modes to TE i+3 (i = 0, 1, and 2) modes. The length of the multi-mode converter is 16.2 µm. For the TE 0to-TE 3 , TE 1-to-TE 4 , and TE 2-to-TE 5 mode conversion processes, the measured insertion losses are 0.4, 1.0, and 0.5 dB, respectively, and the corresponding crosstalk values are below −15.5, −16.5, and −14.1 dB, respectively, at 1538 nm. We also verify the scalability of the device with more mode pairs by simulations. The proposed method shows several advantages: 1) the design is computationally efficient relative to other approaches, for example, inverse design, which may take tens of hours; [32,33] 2) the insertion loss of the device is low by using shallow etching of optimized 2D patterns on the surface of the silicon waveguide; 3) the device is tolerant to fabrication errors as the device performance is insensitive to certain structural parameter changes; 4) the design methodology is scalable toward higherorder modes and more mode pairs. The propagation of optical field in a perturbed dielectric structure can be approximately described by the coupled mode theory. Attributed to the index perturbations on a silicon multimode waveguide, energy can be coupled from one waveguide mode to other modes, and the amplitude of each mode along Generation and manipulation of optical modes are of general interest to the photonics community. Mode conversion is an essential requirement in modedivision multiplexed systems. In this paper, a scalable method to simultaneously manipulate multiple waveguide modes on chip is proposed. As one experimental demonstration, simultaneous multi-mode conversion processes have been achieved o...

Optical meta-waveguides for integrated photonics and beyond

“…• By saddling subwavelength architectures on top of dielectric waveguides, exquisite control over the electromagnetic fields is attainable to facilitate versatile couplers [36][37][38][39][40][41][42][43][44] , compact polarization-or wavelength-routers [48][49][50] , on-chip structured light generators 35,[45][46][47][77][78][79][80][81] , integrated mode convertors 51,[82][83][84][85][86][87][88][89] , sensors [90][91][92][93] and nonlinear devices 52,94,95 (discussed in Section "Dielectric waveguide-integrated meta-structures").…”

“…The structures can be metal or dielectric materials on top of as-fabricated waveguides after deposition and lift-off 35,37,[48][49][50][51][52] . Alternatively, these subwavelength features (either fully 38,39,47,[54][55][56][63][64][65][66][67][68] or partially etched 45,78,[84][85][86][87][88][89] ) itself can be part of the waveguides defined by lithography. These devices are application-wise more appealing, for they are promising for mass-production by lithographic or imprintingbased nanopatterning 12,[137][138][139] .…”

“…The most significant distinction of meta-waveguides from their non-subwavelength equivalents like photonic crystal waveguides is c-t Brief zoology of meta-waveguides. A plethora of applications are prototyped by the playground of meta-waveguides, including, for instance, metasurface-patterned dielectric waveguides [35][36][37][38][39][40][41][42][43][44][50][51][52][53]82 , forward-designed metamaterial waveguides [54][55][56][83][84][85][86][87][88][89] , metasurface-or metamaterial-hybrid optical fibers [74][75][76][96][97][98][99][100][101][102][103][104][105][106][107][108][109][110] , meta-structures-assisted plasmonic waveguides [70][71][72][73]…”

“…i, j Waveguide mode convertor based on shallow 84 or fully etched 87 metasurface perturbations. k Simultaneous multimode convertor 89 . l Efficient coupler 38 .…”

Subwavelength silicon photonics for on-chip mode-manipulation

“…Furthermore, there are some irregular structures introduced with very tiny feature sizes (e.g., < 80 nm [199,203]), which increases the fabrication difficulty. As an alternative, metamaterial or metasurface waveguides have been utilized for realizing on-chip higher-order mode converters [205][206][207][208][209][210][211][212][213].…”