A compact arrayed-waveguide grating with a locally enhanced optical confinement structure using trenches filled with low-refractive index materials

We have reduced the minimum bending radius of a silica waveguide from 2 mm to 200 μm by burying low-refractive index material along both sides of the core. The local lateral relative refractive index difference (Δ) was increased to 9.05%. Moreover, we fabricated both an 8-channel, 100-GHz channel-spacing arrayed-waveguide grating (AWG) and an 8-channel, 12.5-GHz channel-spacing AWG using variations of these new compact bend structures. Compared with conventional AWGs, the relative chip-sizes of these devices were reduced to about 1/2 and 1/4, respectively. Introduction Planar lightwave circuits (PLC) are useful devices for realizing large capacity and high-speed photonic networks. In particular, arrayed-waveguide gratings (AWGs) are key devices that can multi/demultiplex a number of wavelength channels with low loss. Optical communication requires multifunctional small-sized PLCs. However, dense integration is difficult because the minimum bend radii of conventional silica waveguides with high (H)-relative refractive index differences (Δ) of 0.75 % and with super-high (SH)-Δ values of 1.5% are 5 mm and 2 mm, respectively. Semiconductor AWGs [1] and Si photonic wire AWGs [2] with large Δ have been reported. However, the coupling losses of these devices with single mode fibers are large without the use of mode-conversion interfaces. Air trench bend (ATB) technology, which consists of a “cladding taper” and a pair of air trenches along the waveguide bend, can reduce the bend radius, maintaining low loss [3], [4]. We have previously fabricated air-trenched AWGs by this method [5]. However, such trenches can be contaminated by dust and by impurities, and therefore they do not have stable characteristics. On the other hand, we have proposed a locally-enhanced optical confinement structure using trenches filled with low-refractive index resin along the both sides of the core, and we applied this structure to AWGs with H-Δ [6], [7]. In this paper, we have fabricated both a 90degree bend-type AWG and a triple-arrowheadtype AWG using locally-enhanced optical confinement structures with SH-Δ waveguides. The sizes of these AWGs were reduced to about 1/2 and 1/4 of those of conventional devices, respectively. Waveguide Design We used a two-dimensional beam propagation method (2DBPM) to optimize the taper angle of the transition region between a conventional single mode straight waveguide and a narrow-width core sandwiched between low-refractive index material, as shown in Fig. 2.