A wavelength interrogator employing tapered hollow waveguides and a low-cost silicon board camera

We report on a multi-channel wavelength interrogator for the 850 nm wavelength region, constructed by coupling an integrated array of multi-mode, tapered hollow waveguides to a low-cost silicon-based image sensor. The waveguides are clad by omnidirectional Bragg reflectors, such that guided light is radiated in an out-of-plane direction near and at cutoff. Wavelength shifts were extracted using a simple centroid detection algorithm applied to the terminal cutoff point. This concept combines the small size of a Fabry Perot filter with the dispersive property of a diffraction grating. By imaging multiple tapered waveguides onto a single image sensor with each waveguide coupled to a different fiber, simultaneous extraction of wavelength shifts from several wavelength-multiplexed sensors can be achieved in a very compact package. The prototype described provides resolution on the order of 5 pm and can accommodate similar to 20 sensors spaced by 5 nm on each of the 4 fiber input channels. Enhanced capacity and performance are anticipated through future improvements in waveguide materials and the use of more advanced image processing algorithms.