Octave dual-wavelength multimode interferometers for nonlinear integrated photonics

A compact dual-wavelength multimode interference (MMI) coupler based on a thin-film lithium niobate (TFLN) platform is proposed for efficient operation at both 1550 nm and its second harmonic, 775 nm. The device functions as a wavelength multiplexer and demultiplexer, which addresses the growing need for broadband and low-loss wavelength separation in nonlinear and quantum photonic circuits. Conventional wavelength division multiplexing (WDM) structures often suffer from high insertion loss and limited fabrication tolerance, particularly when handling widely separated wavelengths. Here, we present an MMI that serves as a multiplexer/demultiplexer that enables high-performance wavelength management within a single device. The structure achieves ultra-low insertion losses below 0.5 dB, high contrast exceeding 15 dB, and a broad 3-dB bandwidth of 15 nm at both wavelengths. According to the strong nonlinear and electro-optic properties of TFLN, this dual-wavelength MMI provides a low-loss alternative to traditional WDM approaches for integrated nonlinear and quantum photonic circuits.