With the advance of optical communication and silicon photonics, dispersion compensators based on chirped waveguide Bragg gratings (CWBG) have developed rapidly in recent years. Nowadays, the longest CWBG has reached 20.11 cm with the largest group delay and lowest propagation loss. However, its group delay is nonlinear due to linear width variation. Thus, the extra third-order dispersion existed and is hard to use for second-order dispersion compensation. In this work, we propose and experimentally demonstrate, for the first time, an on-chip circulator-free dispersion compensator based on silicon nitride (Si3N4) CWBG with large and linear group delay. The dispersion device comprises spiral multi-mode antisymmetric index-chirped waveguide Bragg gratings and an asymmetric directional coupler on an 800-nm-thick low-loss Si3N4 platform. It utilizes two mode conversions to avoid the optical fiber circulator. The width variation function is modified to achieve the linear variation of the effective refraction index while the period remains unchanged. At last, the CWBG-based dispersion compensator is applied in a time-lens system to realize frequency-totime mapping. This integrated device has great potential for diverse applications such as long-haul transmission links, pulse compression, and pulse shaping.
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
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