Third-harmonic generation and blue lasing in nitrogen-doped silica fiber

Recently, efficient nonlinear UV-light generation has been demonstrated in a novel type of silica fiber with a nitrogen-doped silica core and a pure silica cladding. In this paper, we report new results obtained on these fibers. When we significantly increase the pump power by switching to the mode-locked operation, the UV spectrum at the fiber output becomes much more complicated. In short fiber pieces, we still observe only the third harmonic. In /spl sim/1-m-long pieces, new spectral components arose shifted to longer wavelengths and exceeding the third harmonic in power. The value of the shift corresponds approximately to the SRS Stokes shift in silica glass. At a further increase in fiber length, some of the Stokes components produced by the third-harmonic radiation become more intense. This is indicative of THG from the Stokes components of the pump. The most interesting effect is observed if we further increase the length of the fiber. In this case, a comparatively narrow line arises in the region 380-430 nm, depending on the experimental conditions. As follows from the far-field pattern, this radiation propagates in the fundamental mode of the fiber, in contrast to the third harmonic and its Stokes components. Its average output power far exceeds the third-harmonic output power. The appearance of this radiation in the fundamental mode is of threshold nature. This phenomenon can be interpreted as lasing on the electron transitions of nitrogen-modified oxygen vacancies. These color centers are responsible for two overlapping luminescence bands centered at 409 and 459 nm. The third harmonics produced together with the Stokes components of the third harmonic act as a peculiar kind of UV pumping for these color centers. An interesting feature of this pumping is that its characteristic absorption length amounts to several tens of centimeters, which is three orders of magnitude less than the absorption length of the original near-IR light. This allows uniform exci- ation of the color centers in a long fiber span to produce superluminescence.