347875.fig.003a
(a)
347875.fig.003b
(b)
347875.fig.003c
(c)
Figure 3: The “walk-off” angles as function of frequency in an active (pumped) NRM. The dashed line in all the plots shows the frequencies at which . (a) The blue line is the spatial “walk-off” angle with the pumping rate of  s−1 at . Both losses and the “walk-off” within the gain window are precisely canceled. (b) The angles obtained at the pumping rate of  s−1. The group velocities and phase velocities are strictly antiparallel in this case ( ) at different when (at  s−1). (c) The angles obtained at the pumping rate of  s−1. Even though at this pumping rate the NRM is turned into a light-amplifying medium, the spatial “walk-off” reappears and depends on the value of when , which may lead to degradation of the optical performance of an NRM.