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In a lightwave circuit using a strongly confined waveguide, higher order leaky modes must be suppressed to obtain ideal characteristics, especially in arrayed waveguide gratings (AWGs). Propagation loss for higher order leaky modes in InP-based deep-ridge waveguides was investigated by simulation and experiment. A highly sensitive loss measurement method based on optical low-coherence reflectometry was used to determine the loss, and the relationship between the loss for the higher order mode and AWG crosstalk was investigated. Optimizing the deep-ridge waveguide parameters, especially the core thickness, the refractive index of the core, and the etch depth under the core, significantly reduced the propagation loss for the higher order leaky mode. The effective elimination of the higher order modes will enable fabrication of low-crosstalk AWG routers.