Large group delay based on spiral tapered antisymmetric Bragg grating waveguide

With advance of microwave systems, the optical delay line based on chirped Bragg grating waveguides (CBGWs) have attracted much attention in recent years. However, the loss limits the length of CBGW to achieve larger group delay (GD). In this work, we propose and experimentally demonstrate a novel circulator-free CBGW with low loss and large GD. This CBGW device consists of a 20.11-cm long spiral tapered antisymmetric Bragg grating waveguide (STABGW) and an asymmetric directional coupler (ADC), and is fabricated on a low-loss 800-nm-height silicon nitride platform. The CBGW is realized by linearly increasing the width of Bragg grating waveguide along the length, and its period keeps the same. In our design, the widths of STABGW at two ports are 1.8 and 2.2 μm, respectively and the period is 435 nm. The minimum radius of the waveguide wrapped into Archimedean spiral is 600 μm. The length of ADC is 25 μm, and the widths of two parallel waveguides are 2.3 and 1 μm, respectively, with a 300-nm gap. The experimental results show that a total GD of 2852 ps within the bandwidth of 23 nm is realized. The propagation loss in STABGW is 0.15 dB/cm, and the total insertion loss of the device is 5.4 dB at the wavelength of 1550 nm. The GD is the largest amount achieved by CBGW reported. This integrated device has great potential for diverse applications such as dispersion compensation, all-optical signal processing, and nonlinear optics