Study On Polarization Independent Devices Based On Optical Superlattices

Optical superlattice (OSL) is a kind of artificial nonlinear material, whose second-order susceptibility ? 2 varies periodically. As a representative of the OSL, periodically poled lithium niobate (PPLN) plays an important role in nonlinear devices for its large nonlinear coefficient and mature production process, such as frequency converters (frequency doubling, sum frequency, difference frequency and parametric oscillation), optical switches, optical filters, pulse shapers, et al. However, PPLN and other frequently-used OSL are uniaxial or biaxial crystals, so the polarization state of the initial light should be fixed to satisfy the quasi-phase matching (QPM). Consequently, in order to realize efficient nonlinear process, unpolarized light should convert to polarized light before entering into the OSL. Therefore, one hand, the additional conversion makes the device more complexity, on the other hand, the initial energy will lose heavily. In this paper, we study how to realize the polarization independent frequency doubler and optical isolator, which would have important application in optical fiber systems. The main progresses are as follows:1. We propose two kinds of model to realize polarization independent frequency doubler. The first model is called depolarizing and orthogonality QPM second harmonic generation (SHG) model. When the fundamental wave is unpolarized, we use depolarizer to make the intensity of o-polarized component equals to that of e-polarized component. After passing two PPLN with orthogonality c axes, both of o-polarized component and e-polarized component are converted to second harmonic wave (SHW) by using type-0 (e-ee) phase matching condition. In this sense, the intensity of SHW is independent of the polarization state of FW. Moreover, we have experimentally proved the feasible of this model. The second model is called Cherenkov SHG model. Unpolarized FW propagate along the c axes of the crystal without birefringence effect happened. So the intensity of SHW will only proportional to the square of the intensity of FW and have nothing to do with the polarization state of FW.2. We propose three kinds of model to realize polarization independent optical isolator. The first model is based on simultaneously phase matching the first order type-I (e-oo) QPM SHG process and higher order type-0 (ee-e) QPM SHG process in an OSL with a defect inserted in an asymmetrical position, which can isolate arbitrary linearly polarized FW. This kind of isolator is all-optical scheme without applied external magnetic field and electric field. The second model is based on simultaneous polarization coupling (PC) and SHG process in an OSL with a defect inserted in an asymmetrical position, which can isolate arbitrary polarized FW in two directions. Though adjusting the external electric field and the intensity of FW, the contrast can tune from -1 to 1. The third model is based on reversed sum frequency generation in a segment OSL, which can isolate arbitrary polarized FW. Moreover, the contrast is still high when the intensity of FW is low.