College of Physics and Electronics, Shandong Normal University, Jinan 250014, China;
College of Physics and Electronics, Shandong Normal University, Jinan 250014, China;
Communication College, Shandong Normal University, Jinan 250014, China;
College of Physics and Electronics, Shandong Normal University, Jinan 250014, China;
College of Physics and Electronics, Shandong Normal University, Jinan 250014, China;
spontaneously generated coherence; Doppler broadening; lasing without inversion; gain;