The AQ6331 is a newly developed portable optical spectrum analyzer (OSA) with the high performance required for 50 GHz and 100 GHz DWDM network testing, in both the C and L bands. The AQ6331 is compact, yet has all the features required for DWDM system evaluation. Further, it has excellent wavelength resolution, with accuracy and dynamic range equal to that of any conventional bench-top OSA used in research and development applications. The AQ6331 OSA is suitable both for the installation and maintenance of DWDM systems and for research and development applications.