Polygonal Maxwell’s fisheye lens via transformation optics as multimode waveguide crossing

Abstract

Multimode waveguide crossings are crucial components for novel mode-division-multiplexing systems. One of the challenges of multimode waveguide routing in MDM systems is decreasing the inter-mode crosstalk and mode leakage of waveguide crossings. In this work, we present the intersections of three and four waveguides based on polygonal Maxwell's fisheye lens via transformation optics. The designed lenses are implemented by mapping their refractive index to the thickness of guiding Si layer. The three-dimensional finite-difference time-domain simulations are used to evaluate the performance of the proposed 3×3 and 4×4 crossings. The footprint of the 3×3 and 4×4 waveguide star crossings are 18.6×18.6 and 27.5×27.5 µm 2 , respectively. For both waveguide crossings, the intermodal crosstalk in the output port is lower than -22dB while the crosstalk to other ports is lower than -37dB for TE0, TE1, and TE2 modes. The insertion losses for these modes are lower than 0.5dB in a bandwidth of 415nm covering the whole optical telecommunication bands.

Coupling Si3N4 waveguide to SOI waveguide using transformation optics

“…The ray-tracing calculations indicate that the truncation of the lens has a limited effect on the performance of the lens as long as the incident rays are within the left and right edges of the lens. GRIN lenses have been implemented by multilayer structures [37][38][39], varying the guiding layer's thickness [40][41][42], and GPC [43,44]. We implement the truncated lens with a GPC structure as shown in Fig.…”

Silicon-based multimode waveguide crossings

“…Multimode waveguide crossings are the essential components in high-density and large-scale on-chip MDM optical network. Over the past decades, many schemes have been proposed and experimentally demonstrated including conventional MMI coupler [33][34][35], Maxwell's fisheye lens [36][37][38] and inverse-designed MMI coupler [39]. The demonstrated devices with the high performance of compact footprint, low insertion loss and low crosstalk will show great potential in practical applications for high-density and large-scale MDM systems.…”

“…Besides, to implement three or more modes simultaneously, the compact asymmetric subwavelength Y-junction is introduced to achieve a multimode waveguide crossing [35]. The proposed [36]. (b) Multimode waveguide crossing using transformation optics-based Maxwell's fisheye lens [37].…”

“…Transformation optics offers a new route to manipulate light propagation via spatial coordinate transformations to design and realize many marvellous optical devices, including hyperlenses [44,45], optical invisibility cloaks [46,47] and on-chip integrated optical devices [48][49][50]. Recently, the Maxwell's fisheye lens (MFL) based on transformation optics (TO) designing has attracted a considerable attention to design on-chip integrated single-mode or multimode waveguide crossings [36][37][38]51]. Maxwell's fisheye lens is a cylindrical or spherical lens with a gradient index (GRIN) profile, which could focus rays emanating from a point on the boundary of the lens to a conjugate point of the diagonally opposite side.…”

“…In [36], Badri et al theoretically designed and numerically evaluated a 3 × 3 and 4 × 4 multimode waveguide crossings based on polygonal Maxwell's fisheye lens. As a proof of concept, a 4 × 4 multimode waveguide crossing based on truncated octagonal MFL is designed and demonstrated, as shown in figure 2(a).…”