Cascaded optical microspectrometer based on additive dispersion planar gratings

We demonstrate that planar reflective gratings can be used to design a cascaded microspectrometer that is capable of processing optical signals with different spectroscopic signatures. The filter is based on a double-grating additive dispersion architecture. The first planar grating is used to multiplex single-mode signals from the 1310-nm band into the fiber and weakly demultiplex the band around 1550 nm from the fiber. The second grating doubles the dispersion of the first grating, improves the rejection of stray light, and produces a box-like spectral response around 1550 nm at the output. The device was fabricated using a standard silica-on-silicon process with a refractive index contrast of 0.82% and has a remarkably small footprint of only 0.29 cm². Experimental measurements of the device show Gaussian-shaped spectral response at 1310 nm with a 1-dB bandwidth of 21 nm and box-like transmission at 1550 nm with a bandwidth of 33 nm. The insertion losses for the two channels were measured to be -4.7 and -5.8 dB, respectively. To the best of our knowledge, this is the first demonstration of an integrated planar lightwave circuit that is based on multiple reflective gratings. Applications of the microspectrometer for biophotonics, spectroscopy, and telecommunications are discussed.