  Residual disorder due to fabrication imperfections has important impact in
nanophotonics where it may degrade device performance by increasing radiation
loss or spontaneously trap light by Anderson localization. We propose and
demonstrate experimentally a method of quantifying the intrinsic amount of
disorder in state-of-the-art photonic-crystal waveguides from far-field
measurements of the Anderson-localized modes. This is achieved by comparing the
spectral range that Anderson localization is observed to numerical simulations
and the method offers sensitivity down to ~ 1 nm.
