We describe an experiment in which a supercontinuum spectrum is generated by exciting the third-order mode of a
highly nonlinear photonic crystal fiber (PCF). Our experiment consists of launching a train of femtosecond pulses into a
45-cm-long span of a PCF by means of an offset pumping technique that can selectively excite higher-order modes. For
input wavelengths below 810 nm, the fiber was found to allow for the propagation of higher-order modes. When exciting
the third-order mode we were able to generate an almost purely visible supercontinuum even with pulse energies below
100 pJ. Although the spectrum broadens on the short-wavelength side down to the blue region, no components at
wavelengths larger than the pump wavelength were observed. The mechanism behind the spectral broadening is mainly
ruled by soliton propagation leading to the generation of a blue-shifted dispersive wave. The fact that higher-order modes
have a cut-off wavelength plays a fundamental role that accounts for the observed asymmetry of spectral broadening.
Our experimental results are compared with the numerical solutions of the nonlinear Schrödinger equation. Good
agreement between experimental and numerical results is found.
Wavelength conversion is a key function in wavelength- division multiplexing. Frequency-shifting can be obtained through cascaded second-order nonlinear processes: a pump at (omega) is coupled into the waveguide, second harmonic is generated and made to interact with a coupled signal at (omega) -(Delta) (omega) so as to obtain a converted signal at (omega) + (Delta) (omega) via difference frequency generation. For practical applications, it is essential to achieve a good control in waveguide fabrication so as to be able to design a frequency-shifting device for specific pump and signal frequencies. In this work we report frequency- shifting based on cascaded second-order nonlinear processes obtained in simple planar Ti-undiffused LiBnO<SUB>3</SUB> waveguides, where phase-matching is achieved by birefringence. A Y-cut planar waveguide, 17mm long, was fabricated by diffusing a 290-angstrom-thick titanium layer for 6 hours at a temperature of 1000 degrees C. Thanks to a good modeling of the fabrication process, the waveguide behavior could be predicted directly from the fabrication parameters. A converted signal at 1.100 micrometers was obtained from a pump at 1.104 micrometers and a signal at 1.108 micrometers at a working temperature of 85 degrees C. The phenomenon was observed with a reasonable efficiency and was highly reproducible. The experimental results were in very good agreement with the expected ones.