Conjugated polymer semiconductors combine the processing and mechanical characteristics of plastics with the desirable optical and electronic properties of semiconductors. The aim of the research reported in this thesis was to investigate the evolution of the optical gain properties through three generations of electroluminescent fluorene-based polymers. Detailed optical, optoelectrical and gain characterisations were carried out on a range of different electroluminescent polyfluorene-based polymers. It was discovered that not all of the polymers were gain media as some were unable to give ASE. SC006 was found to be the most intriguing material among the rest of the tested polymers; this third generation polymer was found to be a non ASE material while achieving a high PLQE of 96% with 1.3ns-long excited state lifetime. Therefore it was evident that optimised highly efficient light emitting conjugate polymers for PLEDs are not necessarily effective optical gain media, and high steady state PLQE and long excited state lifetime are insufficient for good optical gain properties. Furthermore, in order to investigate the ASE quenching mechanism in SC006, a series of solvatochromism studies were carried out on this polymer. The time-resolved PL characteristics were compared between polymers of second and third generations. The combination of intermolecular and intramolecular energy transfer process was found to be responsible for the ASE quenching. Moreover, the effects of the differences in Yamamoto and Suzuki synthesis routes on optical gain properties of the first generation statistical and alternating copolymers were investigated and were found to be insignificant. Finally, the application of the gain quenching mechanism was demonstrated by an optical switching process performed on a polymer DFB laser. This enabled complete control over the laser emission from the polymer laser, thus achieving a minimum of a thirty fold reduction in the visible light output in the presence of a control pulse.