Abstract

We report the development of a tunable wide-bandwidth source with a smooth temporal pulse, based on the nonlinear mixing of two frequencies in an optical fiber. A single-longitudinal-mode pulse with a linewidth of <3 GHz was obtained by placing a 688-μm étalon in a Q-switched Nd:YLF oscillator.¹ Angle-tuning the etalon allowed continuous frequency tuning of the oscillator by ±2.5 Å about the central frequency of 10 530Å. Two modified oscillators tuned to a wavelength separation of 4.5 Å. A thinner etalon allowed a broader range of tuning, but with reduced output energy and stability; however, pulse-to-pulse energy fluctuations could be compensated for by the use of a pulse-energy regulator. The output beams were temporally and spatially overlapped, shortened to a nominally square 11-ns pulse, and then coupled into a 10-m single-mode polarization-preserving fiber with an effective mode area of 64 μm². The energy at the input and output of the fiber, the integrated output spectrum, and the temporal pulse shape of each frequency were recorded as a function of input fluence and polarization. The symmetry of the spectrum was found to be highly sensitive to the energy balance between the two input frequencies, and to the coupling efficiency and the temporal pulse shape. Time-integrated spectra indicate the energy content of the central spectral lines remained constant to within ± 10% as the input energy and the number of sidebands increased. A maximum spectral width of 140 Å (4.2 THz) FWHM was obtained by this technique (Fig. 1).

© 1995 Optical Society of America