Fig. 1. (a) Schematic of the proposed coherency-broken cascaded cladding-modulated Bragg grating filter. The incident fundamental mode (TE0) converts into the backward-propagating first-order mode (TE1) as it passes through each section of the grating. The adjacent sections of the grating are mode-dependent attenuators composed of tapers, single-mode waveguides, and bends, which efficiently radiate and dissipate the backward-propagating TE1 mode. Top left: electric field intensity distribution of TE0 and TE1 modes of the bimodal waveguide. Bottom right: schematic representation of the propagation characteristics of TE0 and TE1 modes in the mode-dependent attenuator. TE1 mode experiences significantly higher propagation loss than TE0 mode. (b) Schematic of the asymmetric Bragg grating geometry. (c) Diagram illustrating the effect of phase error on the grating operation of single-section Bragg grating and coherency-broken cascaded gratings.

Fig. 2. (a) Microscope image of one of the fabricated coherency-broken cascaded grating filters comprising seven Bragg grating filters. Scanning electron microscope image of (b) single-mode waveguide bend and (c) cladding-modulated grating filter.

Fig. 3. Measured transmission spectra of single-section Bragg filter with (a) different gap widths ranging from 80 to 110 nm, and (b) single-section filters with different grating lengths ranging from 400 to 2000 μm while gap G equals 90 nm. The red solid line in (b) indicates the rejection of the air cladding grating, and the blue solid line indicates the grating covered by SU-8.

Fig. 4. Measured transmission spectra of (a) single-section 2000-μm-long Bragg filter without artificial imperfections, (b) single-section 2000-μm-long Bragg filters with artificial imperfections, and (c) two sections of cascaded 1000-μm-long Bragg filters with artificial imperfections. The insets of (b) show the grating structure in three different cases, where the introduced imperfections are denoted by red dots.

Fig. 5. (a) Measured transmission spectra of 2000  μm×1, 500  μm×4, 250  μm×8, 125  μm×16, and 100  μm×20 gratings (left and bottom axes). The blue plot (right and top axes) summarizes the change of the rejection as the number of sections increases. The total grating length is fixed at 2000 μm. (b) Comparison of the rejection of single-section (red square) and coherency-broken cascaded (blue circle) strategy as a function of the filter length. (c) Measured transmission spectrum of the coherency-broken cascaded filter consisting of seven 400 μm Bragg grating sections.