  Brillouin light scattering spectroscopy is a powerful technique which
incorporates several extensions such as space-, time-, phase- and wave-vector
resolution.
  Here, we report on the improvement of the wave-vector resolution by including
an electro-optical modulator. This provides a reference to calibrate the
position of the diaphragm hole which is used for wave-vector selection. The
accuracy of this calibration is only limited by the accuracy of the wave-vector
measurement itself. To demonstrate the validity of the approach the wave
vectors of dipole-dominated spin waves excited by a microstrip antenna were
measured.
