The precision, versatility and broad bandwidth of frequency combs are the basis of many different applications from the
microwave via the millimeter and THz up to the optical range of the electromagnetic spectrum. Optical frequency combs
can be used for the new definition of physical constants, for high-precision metrology and spectroscopy and for ultrahigh
bitrate data communications, for instance. Besides the stability and the bandwidth, the most important parameters of
a frequency comb are the free spectral range ,as well as the linewidth and amplitude of the single comb lines. A
conventional grating based optical spectrometer can easily measure the bandwidth of the comb. However, it fails for the
measurement of all other comb parameters, if the comb is generated by a mode-locked fiber laser for instance.
Here we present a proof-of-concept setup for an optical spectrometer with a resolution in the kHz-range and first
measurements of the free spectral range, linewidth and amplitude of a comb source. The spectrometer is based on the
combination of optical heterodyning with the polarization pulling effect of stimulated Brillouin scattering. As we will
discuss, the maximum possible resolution is only restricted by the linewidth and stability of the used reference laser.
Thus due to the stability of our laser used as local oscillator, our setup has a maximum resolution of around 5 kHz or 40
attometer, corresponding to 11 orders of magnitude compared to the center frequency of the comb of around 190 THz.