Pumping scheme dependent multimode laser emission from free-standing cylindrical microcavity

We report the observation of multimode laser emission from a free-standing microring cavity based on rhodamine 6G dye doped hollow poly(methyl methacrylate) optical fiber (DDHPOF) obtained by pulsed photo-excitation. Two different pumping schemes were employed to characterize DDHPOF; the stripe illumination and the spot illumination. By using spot illumination, the slope efficiency of system is enhanced by more than 3 times than that of the stripe illumination and also a red-shift in emission spectrum is observed with the pump power. When the pump power is increased beyond the threshold value, laser emission occurs with a multimode structure. From the relation between mode spacing and diameter of cylindrical cavity, the lasing action is considered to be formed by whispering gallery modes (WGMs).