Investigation of the fiber-to-planar waveguide coupler as a sensor for measuring the optical properties of metal films

A fiber-to-planar waveguide coupler has been investigated theoretically and experimentally as a sensor to measure the optical properties of a thin metal film. A method to get optical constants of the metal films is presented and its validity is demonstrated experimentally. Optical characteristics of the device are analyzed employing the normal-mode theory. The device reveals that the resonance sharpness as well as resonant wavelength are very sensitive to the thickness and optical constant of the metal film deposited on the planar waveguide (PWG). The couplers incorporating low loss metal claddings such as Ag and Au exhibited a strong selectivity and small throughput loss (<1 dB) at off-resonant wavelengths, while high loss metal claddings such Al and NiCr the degraded the wavelength selectivity of the couplers significantly. The measurable range of a metal film thickness reached 2-3 times of the optical skin depth of the metal.