An optical spectroscopy device includes a first cladding layer is positioned over a photodetector. An optical core region is over the first cladding layer where the optical core region is configured to receive a light beam. The optical core region includes a first grating having a first pitch where the first pitch is positioned to direct a first wavelength of the light beam to a first portion of the photodetector. The optical core region further includes a second grating having a second pitch where the second grating is positioned to direct a second wavelength of the light beam to a second portion of the photodetector. The first pitch is different from the second pitch, the first wavelength is different from the second wavelength, and the first portion of the photodetector is different from the second portion of the photodetector. Additionally, a second cladding layer is over the optical core region.