Optical feedback structures

All these waveguiding films lack one important feature that would be necessary for true lasing They do not have a resonator for optical feedback that would lock the optical modes traveling in the gain direction. The thin-film waveguide confines the optical modes in one direction (in the vertical), but in the other two dimensions the modes have translational and rotational symmetry. The incorporation of resonator structures into the thin films in order to get true organic solid-state lasers will be described next. 

As has been pointed out above, a laser basically consists of an active material and a resonator. The latter enables the build-up of certain resonant modes and essentially determines the lasing characteristics. In most conventional devices, the optical feedback is provided by an external cavity with two end mirrors forming the resonator. With the advent of polymers as active materials, various new feedback structures were invented. Initially, a microcavity resonator device of the type shown schematically in Fig. 6.13 a was employed [48]. 

As mentioned above, the gain medium of every kind of laser is associated with an optical resonator, whose structure will be detailed below. The simplest Fabry-Perot-type resonator comprises only two mirrors, between which the gain medium is situated. The feedback structure imposes two basic properties upon the oscillating laser field. It defines not only the allowed resonant frequencies within the gain medium s emission spectrum but also the spatial characteristics of the laser beam. 

Fig. 11. Schematic of edge-emitting laser diodes where the arrows represent the direction of laser emission and H represents the active region (a) standard structure with cleaved facets for mirrors and (b) distributed feedback (DFB) laser that employs coherent reflection from a grating to generate optical...

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