We present a novel coding-based technique for remote monitoring of passive optical networks which exploits the wavelength-to-time mapping effect of broadband short pulses in a highly dispersive medium. In this scheme, each encoder consists in a single FBG written at the same central wavelength but each one having a unique spectral bandwidth. The ultra-compactness and low-cost of the encoders allow them to be potentially integrated inside the customers' terminal. We experimentally demonstrate the feasibility of the proposed method for the supervision of current and future high-density networks.