40% Slope efficiency in a waveguide dye laser using a random active layer

Solid-state dye laser based on plastic materials exhibits minimal cost, easy fabrication and flexibility. The solid dye laser combined with distributed feedback (DFB) waveguide (waveguide dye laser) has been attractive research subject in laser spectroscopy, because of its broadband tunability, narrow-band lasing and lower threshold. However, its insufficient absorption for pump energy limits frequently the optical conversion efficiency from pump energy to laser energy. Use of thin film containing nano-size particles as well as laser dye (random active layer) is a promising solution to improve the efficiency. Until now, we have demonstrated a waveguide dye laser including a thin active random layer. The slope efficiency reached up to 12.0 % [1]. In our previous report, there was little discussion concerning device parameters. In this paper, we report a full characterization of the device parameters. And the slope efficiency of 38.5 % and threshold of <1 μJ have achieved. Figure 1 shows our bulky DFB dye laser structure. A p(MMA_0.95:HEMA_0.05) co-polymer containing Rhodamine-6G dye as well as SiO₂ nano-particles (random active layer) was fabricated on a PMMA substrate. Its thickness was ∼400 μm. The concentration of dye was 4 mM. The diameter of the SiO ₂ particles was ∼10 nm, and its refractive index was ∼1.46. A p(MMA_0.9:HEMA_0.1) co-polymer film (over-coating layer) was laminated on the random active layer by the spin-coating method. The thickness of the over-coating layer was ∼3 μm. The DFB index grating was fabricated by UV exposure on this over-coating layer by using a frequency quadrupled Q-switched pulsed Nd:YAG laser (266nm). The refractive indexes of the substrate, the matrix of the random active layer and the over-coating layer were 1.4893, 1.4918, and 1.4944. respectively. The DFB dye laser was transversely pumped by a frequency-doubled Nd:YAG laser with a pulse duration of 0.5 ns at 100 Hz. Pump size was 250 μm × 1.5 cm. The laser output energy and the lasing spectrum were shown in Fig. 2. The laser operated at 592.6 nm with FWHM of 0.1 nm. This output wavelength can be determined by the DFB structure. The slope efficiency of 38.5 % and threshold energy of 0.96 μJ were obtained, respectively. This slope efficiency is three times higher than that of our previous waveguide dye laser. The improvement of laser performance is due to the increase of the absorbed pump energy, the homogeneous pumping and the enhancement of stimulated emission in the active media by scattering effect. So. it was found that our DFB dye laser device has advantages of higher gain and lower threshold, and its lasing wavelength can be controlled by the recoded index grating in the over-coating layer. In conclusion, we demonstrated a low-threshold, highly efficient distributed feedback dye laser using an active random scattering media. The slope efficiency of 38.5 % and the threshold energy of 0.96μJ were achieved, respectively. This device has the ability that can be applied for the laser spectroscopy.