Three-dimensional long-period waveguide gratings for mode-division-multiplexing applications

Abstract

We propose a three-dimensional (3D) long-period grating structure that has a controllable grating width and depth and can be formed at any chosen position on the surface of a waveguide core with a single photolithography process. The process relies on the partial etching of small structures on the surface of a polymer waveguide through a waveguide mask with narrow apertures that define the grating pattern. The 3D grating structure allows the design of mode converters for any nondegenerate guided modes of a waveguide, regardless of their symmetry properties, and thus relaxes the design constraint of conventional two-dimensional waveguide gratings. To show the flexibility of the 3D grating structure, we present several mode converters fabricated with this structure. The mode-conversion efficiencies achieved are higher than 90% at the resonance wavelengths. In addition, we demonstrate a three-mode multiplexer by integrating a grating-based mode converter with two asymmetric directional couplers. The proposed grating structure together with the fabrication process can greatly facilitate the development of grating-based devices, especially for MDM applications.

Diffraction-less propagation beyond the sub-wavelength regime: a new type of nanophotonic waveguide

“…State-of-the-art sub-wavelength grating waveguides rely on periods shorter than half of the wavelength to suppress diffraction effects 9,10 . On the other hand, detrimental scattering loss has been partially mitigated in long-period gratings by implementing very weak corrugations 20,24 . We propose a radically different approach to suppress diffraction effects in strongly-corrugated long-period gratings, based on the engineering of the phase-matching conditions.…”

“…However, this strategy fails when (high index) silicon waveguides and strong perturbations are considered. Indeed, silicon-on-insulator (SOI) mode converters based on long-period gratings require very weak corrugations and few-microns-long devices to minimize radiation loss 20 .…”

Thermo-Optically Controlled Vertical Waveguide Directional Couplers for Mode-Selective Switching

“…The mode-symmetry issue in the realization of mode-selective couplers has been analyzed in detail with fiber couplers [18]. One approach to solving this problem is to use a mode rotator to change the symmetry property of the problematic mode (e.g., to convert the LP 11b mode into the LP 11a mode) [7], [8]. The mode rotator can be a carefully trenched waveguide [7] or an asymmetric long-period waveguide grating [8].…”

“…One approach to solving this problem is to use a mode rotator to change the symmetry property of the problematic mode (e.g., to convert the LP 11b mode into the LP 11a mode) [7], [8]. The mode rotator can be a carefully trenched waveguide [7] or an asymmetric long-period waveguide grating [8]. The other approach is to use three-dimensional (3D) structures [9]- [17], such as fiber lanterns [9], combined horizontal and vertical directional couplers [10]- [14], all vertical directional couplers [15], 3D waveguide branches [16], and non-planar directional couplers [17].…”

Design and Fabrication of Broadband Polymer Mode (De)Multiplexer Using a Direct Inscribing Method

“…One of the main concerns of the MDM is the design and fabrication of mode (de)multiplexers which enable the selective excitation and detection of the individual modes. Several solutions have been demonstrated such as multi-plane light conversion (MPLC) [5], free-space optics [6], photonics lanterns [7], [8], asymmetric Y-junctions [9], long-period gratings [10], [11], multimode interference (MMI) couplers [12], asymmetric directional couplers (ADCs) [13]- [15], and tapered mode-selective couplers [16]. Among them, the ADC is a promising candidate due to their good scalability and flexibility to be able to selectively excite the higher-order modes.…”