Optical delay lines (ODLs) have been widely used in data synchronization, wave shaping and phased array. Compared to fiber components or bulk optics ODLs, integrated ODLs feature reduced cost, size, weight and power consumption. Practical applications call for continuously tunable ODLs with broad operating bandwidth, low insertion loss and high group delay linearity. In this paper, we propose and experimentally demonstrate an integrated low-loss high-linearity ODL based on cascaded multimode waveguide Bragg gratings (WBGs). By coupling the input fundamental mode into the reflected TE2 mode, the proposed cascaded second-order WBGs achieved 104 ps group delay over a bandwidth of 7 nm, and the insertion loss of each stage of WBG is about 0.65 dB. Moreover, the application of multimode WBGs and mode demultiplexer allows easy separation of forward and backward propagating wave without introduce extra loss. Our design enables further application as a modular on-chip delay line and can be used in various on-chip systems.

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