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.