  We present design and realization of an ultra-broadband optical spectrometer
capable of measuring the spectral intensity of multi-octave-spanning light
sources on a single-pulse basis with a dynamic range of up to 8 orders of
magnitude. The instrument is optimized for the characterization of the temporal
structure of femtosecond long electron bunches by analyzing the emitted
coherent transition radiation (CTR) spectra. The spectrometer operates within
the spectral range of 250nm to 11.35$\mu$m, corresponding to 5.5 optical
octaves. This is achieved by dividing the signal beam into three spectral
groups, each analyzed by a dedicated spectrometer and detector unit. The
complete instrument was characterized with regard to wavelength, relative
spectral sensitivity, and absolute photo-metric sensitivity, always accounting
for the light polarization and comparing different calibration methods.
Finally, the capability of the spectrometer is demonstrated with a CTR
measurement of a laser wakefield accelerated electron bunch, enabling to
determine temporal pulse structures at unprecedented resolution.
