Spectroscopic sensing with resonant cavity-enhanced photodetector arrays

Resonant cavity-enhanced photodiodes (RCE-PD) have previously been studied for the narrow spectral responses that can be achieved. A narrow response is useful for spectral sensing of a specific absorption line without interference from neighbouring absorption lines. We have designed and fabricated an array of photodiodes that each have a slightly shifted resonance wavelength; so combined photocurrent measurements from all the pixels can be used to not only monitor a single absorption line, but also monitor all absorption lines in a region of the infrared simultaneously. The RCE-PD array concept allows for many substances to be identified and measured, offering much more versatility than a measurement from a single-wavelength RCE-PD. The shift in resonance wavelength is created by inducing a thickness gradient in the epitaxial layers across the wafer. Using this technique, a fabricated 2-inch wafer showed a resonance shift between 1.9 mu m and 2.5 mu m. A 1D array of pixels was created with a shift in the resonance of 4nm per pixel. Light from a monochromator was used to test the ability of the array to determine the wavelength of the light. For four closely spaced wavelengths, an accuracy of +/- 2nm was seen.