Scanning FP Interferometer 159 2.2.4 Optical Spectrum Analyzer Using the Combination of Grating and FPI In the last two sections, we have discussed OSAs based on diffractive gratings and FPIs. A grating-based OSA has wide wavelength coverage, but the spectral resolution is limited by the groove-line density of the grating and the size of the collimated optical beam on the grating. A desktop OSA can usually provide 0.05 nm spectral resolution, and a miniature OSA using photodiode array pro- vides about 0.25 nm resolution in a 1550 nm wavelength window. On the other hand, a scanning FPI-based OSA can have much higher spectral resolution by using long cavity length. However, since the transfer function of an FPI is peri- odic, the spectral coverage is limited to a free spectral range. The ratio between the wavelength coverage and spectral resolution is equal to the finesse. To achieve both high spectral resolution and wide wavelength coverage, we can design an OSA that combines an FPI and a grating. The operation principle of the high-resolution OSA using the combination of an FPI and a grating is illustrated in Figure 2.2.13. The optical signal is transformed into discrete narrowband slices by the FPI with the wavelength separation equal to FSR between each other. After the transmission grating, each wavelength slice is dispersed into a light beam at a certain spatial angle with the angular width of Df. Theoretically, Df is determined by the convolution between spectral bandwidth