Abstract

Nonlinear frequency conversion in waveguides has been investigated since the first days of integrated optics, but practical applications were hampered by a lack of suitable pump lasers and the often inconvenient phase-matching characteristics of the available waveguides. Recent developments in high power (>100 mW) single-mode diode lasers and quasi-phase-matching techniques have led to renewed interest in waveguide frequency conversion. Periodic modulation of the optical properties of a nonlinear medium, quasi-phase-matching, is an attractive technique as it decouples phase-matching from birefringence and thus allows any interaction within the transparency range of the medium using any component of the nonlinear susceptibility tensor. The most efficient form of quasi-phase-matching involves periodic reversal of the sign of the nonlinear susceptibility (χ⁽²⁾) of the medium, with a period equal to an odd multiple of the coherence length of the interaction. In ferroelectrics, such a periodic sign reversal in χ⁽²⁾ can be accomplished by periodic reversals in the orientation of the spontaneous polarization. Periodic incorporation of dopants has been found to induce domain reversal in several ferroelectrics, e.g. Ti or Li in LiNbO₃, H in LiTaO₃, and Rb and Ba in KTP, leading to rapid progress in device demonstrations in these material systems. [1–5] Several milliwatts of blue light generated by quasi-phase-matched (QPM) frequency doubling have been demonstrated in LiNbO₃[3],[6], Ktp [4], and LiTaO₃ waveguides [5], as has QPM difference frequency generation of 2.1 μm radiation in LiNbO₃ waveguides. [7]

© 1991 Optical Society of America