Bragg order effect on ultrashort pulse generation by distributed feedback dye lasers under subnanosecond excitation: Numerical study

Abstract

A comparative numerical study of the 1st- and 2nd-order distributed feedback (DFB) dye lasers pumped with 0.5 ns, 532 nm laser pulses is performed. A dynamic model of the gain-coupled DFB dye laser based on a coupled-wave theory and taking into account Bragg diffraction order is used. Due to the periodical distribution of the pump field and stimulated emission intensities along the excited dye volume, the rate equations of the population densities are turned into the equations of the Fourier coefficients. To simulate the DFB lasing, coupled-wave equations of optical fields are used. Effect of the pumping intensity on the temporal and energy characteristics of DFB dye lasers is studied depending on the dye concentration and the length of the excited dye volume. Influence of the lasing seed type (constant or noise-like) on the simulation results is also analyzed. A special emphasis is on the characteristics of single ultrashort (US) picosecond pulses generated by the relaxation oscillations mechanism. The results obtained indicate that for both types of DFB dye lasers the temporal course of the output intensity is governed by the pumping strength. Depending on the excess above the threshold (high or low), either US pulse trains or single US pulses are generated. The influence of the active medium parameters on their characteristics has been studied in detail. In addition, it was established that unlike the 1st-order DFB lasing, where the simulation results are independent of the seed type and smooth US pulses are always obtained, for the 2nd-order regime, to get smooth rather than structural single US pulses, a noise-like seed should be used. However, the most important finding is that the changeover from the 1st to the 2nd order leads to ∼1.1–3.2 (∼1.13–1.46) times reduction in duration, ∼1.4–3.4 (∼1.4–3.0)-fold increase in energy and ∼2.8–9.7 (∼1.5–4.8)-fold growth in peak power of single US pulses for the case of constant (noise-like) lasing seed. Hence, the use of the 2nd-order regime represents a simple and convenient way to improve the energy characteristics of single US pulses of DFB dye lasers generated by the relaxation oscillations mechanism.