Abstract

Difference-frequency mixing (DFM), in which a pump at frequency ω_p is mixed with a signal at frequency ω_s to generate an idler at frequency ω_i = ω_p - ω_s, is useful for generating infrared radiation. By mixing the outputs of two infrared lasers, one can generate coherent infrared radiation¹,² for spectroscopic applications such as molecular sensing³; one can also apply DFM to wavelength conversion in wavelength-division-multiplexed (WDM) communication systems.⁴,⁵ Optical parametric generation (OPG) and optical parametric oscillation (OPO)⁶, in which a strong pump is mixed with vacuum noise input, are useful for generating widely tunable infrared radiation from a single pump laser. DFM in bulk nonlinear crystals has been demonstrated with AgGaS₂ and AgGaSe₂⁷ and with periodically-poled LiNbO₃ (PPLN)³, but with low conversion efficiencies. Waveguide confinement has been shown to increase single-pass conversion efficiencies for second harmonic generation by factors up to 1000, as compared with bulk. However guided-wave DFM has, to date, failed to demonstrate comparable improvements to the bulk, primarily because of difficulties in coupling of the pump radiation. In this paper, we address the mode launching problem using integrated mode coupling structures, including mode filters, adiabatic tapers and directional couplers. Using these coupling components, we have demonstrated efficient guided-wave DFM in periodically-poled LiNbO₃ and guided-wave OPG.

© 1998 Optical Society of America