The Interaction Of Acousto-optic, Electro-optic And Nonlinear Optic Effects In Periodically Poled LiNbO₃

We study the acousto-optic (AO) tunable quasi-phase-matched (QPM) second harmonic generation (SHG) in periodically poled lithium niobate (PPLN). We also propose a collinear electro-optical (EO) modulator that may regulate the RGB primary lights simultaneously. Based on PPLN with a defect, three kinds of optical diodes are designed with the help of EO and AO tuning. Finally a kind of multi-section EO tuning PPLN is investigated showing the great advantage of polarization insensitive frequency conversion.1. AO tunable SHG is proposed in PPLN. The acoustic wave could either be induced from an external transducer or self-generated in PPLN driving with a cross-field radio frequency field. The reciprocal vector of PPLN compensates the SHG wave-vector mismatch when QPM condition is satisfied, while phonons with suitable frequencies may affect it by scattering photons to different polarization state. The QPM SHG and AO polarization rotation are coupled together. Second harmonic waves' intensities, polarization states and even phases thus could be manipulated instantly through AO interaction. 2. A three-section PPLN is proposed to collinearly modulate the RGB primary lights simultaneously. The period of each section is designed to rotate the polarization of the chosen light when there is a proper external DC electric field along the y axis. We also discuss the condition when there is a defect in the structure.3. We theoretically propose several broadband optical isolators based on second-harmonic generation in a PPLN with an embedded defect. An external DC electric field along the z axis or y axis is applied to modulate the sample's refractive index periodically. Complete optical isolation always could be reached with the help of an external field. We also propose a bidirectional AO tunable optical diode based on PPLN with defect. An acoustic wave propagates together with the light beam, which affects the nonlinear optical processes in PPLN. The fundamental wave only may travel toward a single direction while the opposite way is isolated. However, the acoustic wave could adjust the contrast of optical isolation from -1 to 1. A direction-optional operation is thus realized.4. Polarization independent second harmonic generation (SHG) and sum frequency generation (SFG) are proposed in an EO tunable PPLN. The PPLN consists of four sections. External electric field could be selectively applied to them to induce polarization rotation between the ordinary and extraordinary waves. If the domain structure is well designed, light with arbitrary polarization states could be frequency converted. The applications in single photon detection and optical communications are discussed.