Efficient frequency doubling of a cw Q-switched and mode-locked Nd:YAG laser has been observed in commercial single-mode optical glass fibers. Pulses of duration ~55 psec and intensities as high as ~0.55 kW were produced at 0.53 μm. The maximum peak power-conversion efficiency measured was ~3%. The frequency-doubled light generated in the glass fibers was sufficient to pump a commercial Rh6G dye laser with ~19% efficiency at 570 nm.

Dye laser pumped by Nd:YAG laser pulses frequency doubled in a glass optical fiber.

We numerically examine the dynamics of stimulated Raman scattering combined with self-phase modulation for intense 100-psec-long input pulses in a dispersive single-mode optical fiber. Linear dispersion is treated in a first-order approximation by introducing group-velocity mismatch between pump and Stokes pulses. It is shown how this mismatch combined with pump depletion leads to asymmetric distortions of both pulse shapes and frequency chirps. The resulting delay of the Stokes peak is found to reach higher values for lower input pump powers. For sufficiently long fibers the emerging pump pulse has a one-peak structure, in contrast to the two-peak structure in the dispersionless case. The existence of an optimum fiber length to obtain the shortest Stokes pulse for a given input pulse is shown.

Numerical study on combined stimulated Raman scattering and self-phase modulation in optical fibers influenced by walk-off between pump and Stokes pulses

We have investigated experimentally the influence of stimulated Raman scattering, under conditions of pulse walk-off, on the propagation of mode-locked 1.06-μm pulses in single-mode, polarization-preserving fibers and on their subsequent pulse compression. Intense stimulated Raman scattering preferentially depletes (and steepens) the leading edge of 1.06-μm pump pulses and clamps the pump pulse energy. Subsequent self-phase modulation of the reshaped pump leads to nonsymmetric spectral broadening and to a nonlinear chirp. Nevertheless, high-quality compressed pulses may be obtained by using an asymmetric spectral window to select a subset of the broadened spectrum. The best compression is achieved when the downshifted spectral components, corresponding to the portion of the pump pulses most severely depleted by stimulated Raman scattering, are selected. Such pulse compression performed under conditions of strong Raman conversion is highly stabilized compared with conventional pulse compression.

Self-phase modulation and optical pulse compression influenced by stimulated Raman scattering in fibers

Strong infrared optical signals at the wavelengths 0.991 and 1.149 microm were produced as a result of phase-matched four-photon mixing when Nd:YAG laser light was coupled into a <4-m-long piece of single-mode glass fiber. Stimulated gain was observed, and laser action was obtained with the fiber in an external cavity.

Four-photon fiber laser.

Pump pulse depletion has been examined for high peak power (∼ 100 kW) 90 ps pulses from a Q -switched and mode-locked Nd:YAG laser in 4 m of single-mode optical fiber. Fragmentation of the fundamental and low Stokes Raman orders has been observed for this cascading process. Pulse compression using an interferometric technique has been carried out, obtaining compression ratios of up to times five, on these fragments which exhibited a negative frequency chirp.

Picosecond stimulated Raman generation, pump pulse fragmentation, and fragment compression in single-mode optical fibers

Stimulated Raman scattering of Q -switched and mode-locked Nd:YAG laser pulses at 1.06 μm in a 120-m-long single-mode fiber has been investigated experimentally. A Photochron IIA Synchrosan streak camera with picosecond resolution was used to give time resolved detection of the first four order Stokes-Raman pulses at 1.12, 1.18, 1.24, and 1.31 μm. It has been shown that in traversing the fiber the fundamental and the first two order Stokes were substantially shortened and that the Stokes pulses were asymmetrical with sharper leading edges.

B. Stoltz

Papers

Streak camera investigation of Raman pulse generation and propagation in an optical fiber

An ultrafast differentiator and its use in picosecond laser pulse measurement