Propagation Modes in Periodically Poled Lithium Niobate Waveguides Exploiting Cascaded Second Order Nonlinearity

The improvement of cascaded second-order nonlinearity ≤(2) : ≤(2) effects in a quasiphase-matching (QPM) structure constituted by a periodically poled lithium niobate (PPLN) waveguide is obtained utilizing an integrated metal mirror placed after an additional waveguide segment. The resulting double-pass device, which can be employed for wavelength conversion, is refined by choosing the most advantageous length of the segment waveguide and by taking into account both the thermal and electro-optic effects. In the ideal case of perfect QPM, for a grating length z G4 = 25 mm, the calculated double-pass maximum efficiency is eta dp = 22.86 dB, which is almost the triple of eta sp = 8.20 dB, calculated for the single-pass version (without mirror) for the same value of the pump and signal powers.     

Use of metallic and dielectric films for hollow fibers

The problem of transmission of CO₂ laser radiation through hollow fibers and waveguides was studied theoretically and confirmed experimentally. The transmission of the laser radiation through metal and metal-dielectric tubes was measured and compared with the theoretical data based on a ray model solution. This makes possible the investigation of the transmission of the CO₂ radiation through waveguides when the internal wall is covered with a metal or a metal- dielectric film. It was shown theoretically and proved experimentally that the transmission of the CO₂ radiation is possible even through bent waveguides.