Various methods, systems and devices for optical detection and analysis of polymers, such as polynucleotides, using nanopores, e.g., for determining sequences of nucleic acids, are provided herein. In certain variations, methods and systems for determining a nucleotide sequence of a polynucleotide, which include measuring mixed FRET signals as a polynucleotide translocates through a nanopore and determining a nucleotide sequence of the polynucleotide from the mixed FRET signals, are provided.