Widely tunable integrated filter/receiver with apodized grating-assisted codirectional coupler

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In recent WDM communication systems, a wavelength-selective filter or detector is required at receiver end to pick up desired channels from incoming data streams. In addition to the wavelength-selectivity, one also desires these devices to have wavelength tunability such that the entire system is reconfigurable. Filters and receivers based on arrayed waveguide grating (AWG) structures have been developed. Even though AWG devices demonstrated good side-lobe suppression ratios (SSRs), they are not tunable. Devices based on vertically stacked grating-assisted codirectional couplers (GACC) have therefore attracted great attention due to the wide and fast wavelength tunability. However, the response of a one-stage GACC device with a uniform coupling strength across the entire filter section is a sinc<SUP>2</SUP>-like function which limits its SSR to less than 9 dB. It is well known that there is roughly a Fourier transform relationship between the coupling coefficient distribution and the filter response. Therefore, apodizing the coupling strength along the filter can effectively suppress the side-lobes. In this paper, we report on the apodization of the coupling strength in the GACC filter by varying the grating duty ratio. Using this technique in an integrated wavelength-selective receiver, we demonstrate a device with a 40 nm tuning range and a 16 dB SSR. A schematic drawing of the integrated apodized GACC receiver is shown in Figure 1. The device includes a semiconductor optical amplifier (SOA), a two-stage GACC optical filter, and a waveguide photodetector on InP/InGaAsP based materials. The operating principle of the device is explained as the following: Input light is first coupled into the top waveguide and is amplified the SOA. It is then filtered by two apodized GACC filters in series. At the wavelength where phase matching condition is satisfied, light will be coupled down to the bottom waveguide and then back up to the top waveguide. Finally, it will be detected by the waveguide photodetector. The middle of the top waveguide is broken by a 30 degree tilted etched groove. The tilted groove deflects any uncoupled light laterally to avoid multiple reflections of unwanted signals to the detector.