The present invention relates to optical spectrum analysis of an optical signal.
Optical spectrum analysis serves to analyze an unknown spectrum to determine the power of certain wavelengths of the spectrum. For such an analysis it is normally used an optical spectrum analyzer (OSA) which typically comprises a pivotable grating. The grating serves to disperse the light under measurement and so to select a certain wavelength of the spectrum. By pivoting the grating the spectrum can be analyzed. To allocate a certain wavelength to a certain pivot angle it is necessary to calibrate the OSA in advance. For such a calibration sharp laser lines, gas absorption lines or reflection lines of fiber Bragg gratings or Fabry-Perot filters with a known wavelength are usually used.
The resolution bandwidth (RBW) of such an OSA is substantially given by the formula RBW=λ/(n N), with λ being the wavelength, N being the number of grating passes, and n being the number of illuminated grating lines. In known OSAs, such as the 8614×A of Agilent Technologies Inc., a Delaware Corporation, Palo Alto, Calif. 94303, USA, the RBW can be around 40 pm. Generally, there are two principle ways to enhance the RBW of such an OSA. First, to increase the number of the illuminated lines, i.e., Increase the size of the grating, or second, to increase the number of grating passes, i.e., increase complexity.