Dye with a rotating active medium in the form of a polymer matrix containing a dye

Abstract

An investigation was made of the energy characteristics and operating life of a pulse-periodic laser with a rotating polymer active medium. A deliberate selection of the ratio of the rotation frequency of the active medium to the repetition frequency of the pump pulses eliminated thermooptic distortions and increased the operating life up to 106 pulses. When the average pump power density was ~20 W/cm2 and the best dyes were used, the lasing efficiency reached 30% and the tuning range was 100 nm, which represented a considerable improvement over the corresponding characteristics of the widely used rhodamine 6G dye laser. Lasers utilizing dyes in polymer matrices combine advantages of liquid and solid-state systems, so that it is highly desirable to develop them further.18 Polymer-dye laser elements described in Ref. 1 consist of a polyurethaneacrylate (PUA) matrix, which is highly elastic at room temperature and so that it is possible to increase the optical strength of such matrices compared with glassy media. The main difficulties encountered in using polymerdye laser elements are the photochemical instability of the dye molecules in such matrices and the thermooptic effects which appear under the action of powerful light pulses. The photodecomposition of dyes in high-intensity laser radiation fields is a process which is a nonlinear function of the radiation intensity, as described in Ref. 9, where some recommendations on the selection of dyes with higher photochemical stability are also made. Thermooptic effects (thermal lenses) which appear in polymer-dye laser elements because of the heating by the pump radiation and subsequent thermal expansion are practically the same for the elastoplastic polymers and the liquids they contain. The use of polymers in the glassy state, for example, of polymethylmet hacrylate, makes it possible to reduce by an order of magnitude the thermooptic distortions, but this is at the expense of a corresponding reduction in the optical strength of the matrix. Measurements have indicated that 2.5-3 mm thick polymer-dye laser elements based on PUA are characterized by a thermooptic lens formation time amounting to hundreds of nanoseconds in the case of pump beams with diameters ~ 1 mm and the time of dispersal of such a lens to a focal length of ~ 10 m is Atd ~ 0.5 sec (detailed information on thermooptic effects in polymer matrices activated with dyes will be published separately). When a liquid active medium is used, the photochemical instability and thermooptic distortions can be minimized by circulating the liquid. In the present study we rotated, as in Refs. 2 and 3, a polymer active medium in the form of a plane-parallel disk of thickness 2-3 mm. For a pulse repetition frequency/= 25 Hz, a radius of rotation of the active region R = 11 mm, and a pump beam diameter d = 0.5 mm, the rate of rotation of the polymer matrix should be selected so as to ensure the maximum scanned area S = 2wRd and the maximum time interval Δ/ between the two successive pumpings of the same part of the disk: Δί<,Atd. These conditions are satisfied iif/n^lirR /d. At other rotation frequencies the value of//» should be selected to be irrational in order to avoid the stroboscopic effect. We shall now report the results of an experimental study of the energy characteristics and operating life of such a solid-state laser. Figure 1 shows the dependences of the lasing efficiency