  This work focuses on optical multiplexers for mid-IR broadband source in gas
sensing application. The design of multiplexer configurations based on two
types of diffraction gratings is discussed. Owing to utility of Gaussian
approximation of the field and Fourier Optics, we developed a semi-analytical
tool allowing to evaluate the responses of array waveguide grating (AWG) and
planar concave grating (PCG). This approach also opened the way to implement
numerical phase error analysis by introducing normal distribution of errors and
to study the correlation between standard deviation of phase errors and the
level of crosstalk. FTIR spectroscopy measurement results of 5.65 micrometer
AWGs, built with SiGe graded index core encapsulated in a thick Si cladding on
standard silicon substrate, are compared with the theoretical expectations. We
have also performed evaluation of AWG response at elevated temperatures, in the
range between 20oC and 41oC. The spectral shift showed linear dependence with
the temperature, which is in a good agreement with simulation predictions.
