  The field dependence of the electron spin resonance in a helimagnet,
LiCu_2O_2, was investigated for the first time. In the paramagnetic state a
broad resonance line was observed corresponding to a g-factor of 2.3. In the
critical regime around the paramagnetic to helimagnetic phase transition the
resonance broadens and shifts to higher frequencies. A narrow signal is
recovered at low temperature, corresponding to a spin gap of 1.5 meV in zero
field. A comprehensive model of the magnons is presented, using exchange
parameters from neutron scattering (T. Masuda et al,
  Phys. Rev. B 72, 014405), and the spin anisotropy determined here. The role
of the quantum fluctuations is discussed.
