Broad-spectral-range synchronized flat-top arrayed-waveguide grating applied in a 225-channel cascaded spectrometer

Abstract

Summary form only given. There is a strong need for arrayed waveguide gratings (AWGs) with low-loss and box-like pass band over a broad spectral range. Most of the approaches proposed to implement flat-top AWGs suffer from increased insertion loss and large device size [1,2]. A Mach-Zehnder-interferometer (MZI) synchronized AWG [3] can provide a flattened pass band without introducing intrinsic loss. However, due the wavelength dependency of the directional couplers in the MZI, the achievable bandwidth is limited. We demonstrate a new pass-band flattening method by introducing 3-dB balanced couplers [4] to a MZI-synchronized AWG configuration over a broad spectral range (Fig. 1a). A low-loss cascaded AWG system is demonstrated when using this flat-top AWG as a primary filter. The lengths of the straight coupler sections were calculated as L1 = 149 μm and L2 = 34 μm, and the delay length was found to be ΔL = 0.281 μm, as summarized in Fig. 1b. The simulation and measurement result of the coupler is shown in Figs. 1c and 1d, respectively. The FSR and wavelength resolution of the primary flat-top AWG spectrometer were chosen as 90 nm and 18 nm, respectively, whereas for the secondary AWGs these values were 20.4 nm and 0.4nm, respectively. A 0.5-dB bandwidth of 12 nm and a central excess loss value of 1 dB were measured for the MZI-synchronized AWG. Electrical heaters were placed on both arms of the MZI in order to compensate the fabrication-related performance degradations. The thermal tuning effect on the transmission spectrum of one of the output channels of the MZI-synchronized AWG is shown in Fig. 1e. For the characterization of the cascaded AWG system, several of the outer and central output waveguides of each secondary AWG were measured, as shown in Fig. 1f. A central excess-loss value of 4.5 dB (1 dB from the primary AWG and 3.5 dB from the secondary AWG) and a non-adjacent crosstalk value of 30 dB were obtained. Such low-loss, broad-spectralrange AWGs are very desirable for high-density cascaded multiplexer/demultiplexer systems.