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Black polyimide (PI) film is a promising polymer material that has been widely applied in the electronic circuit industry and aerospace. However, the addition of black additives shows a trade-off effect on the mechanical and electrical properties of PI films. Herein, we propose a novel preparation method to eliminate the above trade-off effect. It combines pyromellitic dianhydride (PMDA) and biphenyl dianhydride (BPDA) as monomers in polycondensation. The flame retardants are doped to minimize the addition of black additives. In this way, after optimization of the preparation conditions, the obtained films are observed to possess outstanding mechanical (tensile strength of 165.2 MPa, elongation at break of 22.1%, elastic modulus of 4587.4 MPa, and puncture strength of 521.3 N/mm), electrical (dielectric strength of 160.0 kV/mm) and optical properties (transmittance of 0.20%). A mechanistic study by microscopic tests and theoretical calculations shows that, with thermal decomposition, the flame retardants could form a dispersed carbon deposit. The dispersed carbon deposit and carbon additive synergistically reduce the light transmittance. With this method, the addition of black additive decreases from over 5 to 0.5 wt %. PMDA and BPDA regulate the rigidity of the polymer chains and the intermolecular interactions, thereby improving the mechanical and dielectric strength. This method has also been scaled up for industrial production without sacrificing the stability and physicochemical properties of the film.

We propose, analyze and demonstrate a side-coupled Fabry-Perot (F-P) resonator filter composed of two dissimilar waveguides with π phase-shifted Bragg grating. The resonance circuit is formed by the mode conversion between the fundamental transverse electric (TE0) modes of the two waveguides. It can realize a wide free spectral range (FSR) or an FSR-free response with a single strong resonance at the central wavelength of mode conversion. In addition, compared with the conventional F-P resonator, only one transverse mode in the proposed resonator can be coupled to the bus waveguide. Thus, it has some advantages including no intrinsic reflection to the bus waveguide and low resonance loss. The experimental results on silicon-on-insulator (SOI) platform have shown that the extinction ratio, full width at half maximum (FWHM) and quality factor (Q-factor) of the proposed resonator are 52.5 dB, 0.125 nm and 1.24 × 104, respectively. The fabrication error and add-drop filters based on the proposed resonator are discussed then. The proposed resonator is believed to benefit the practical applications of optical filters and sensors.