Cascaded second-order processes for frequency-shifting in planar Ti : LiNbO3 waveguides

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 (2 omega) 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-indiffused LiNbO3 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 modelling of the fabrication process, the waveguide behavior could be predicted directly from the fabrication parameters. A converted signal at 1.100 mu m was obtained from a pump at 1.104 mu m and a signal at 1.108 mu m 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.