Numerical analysis of broadband optical parametrical generation in periodically poled lithium niobate and novel structure design.

摘要

The Quasi-phase-matching technique is an attractive approach to compensate phase velocity dispersion in optical parametric generation (OPG). A ferroelectric crystal of lithium niobate is often used to make QPM devices because of its large nonlinear coefficient (d33 = 27 pm/V), low cost, and wide availability. However, the maximum thickness of periodically poled lithium niobate (PPLN) samples is limited to 1mm due to its large coercive field (∼21 kV/mm) in fabrication, so the interaction aperture of PPLN is limited by its thickness. Because of its low damage fluence, this limited interaction aperture of PPLN has restricted pulsed PPLN systems to low energy operation. One way to develop high energy pulsed PPLN OPG is by increasing the interaction area with an elliptically shaped pump beam. However, this creates another problem; with an elliptical pump beam operation, the spectral band of signal and idler may be as large as 4000 cm-1 because of noncollinear phase matching. In particular, signal and idler have a high gain at degeneracy wavelength, the intended on-axis wavelength mode can't get sufficient amplification because the high gain off axis modes dominate the dynamical evolution of the system. This causes a lower efficiency for the on-axis mode and is a serious drawback to scaling up the energy by this QPM technique. By adding an injection-seeded wave into this process, the degeneracy wavelength mode can be suppressed and the resulting output signal has a narrow spectral line at the intended wavelength. However, at higher energy operation, the degeneracy wavelength mode can't be suppressed completely, and the output signal has a significant distribution of energy over a wide band of frequencies. This degrades the beam quality and lowers the conversion efficiency.; In order to understand the elliptically pumped OPG, a numerical model is needed to simulate the OPG process with a large spectral band output. In this work, I introduce a numerical model which is suitable for broadband OPG. This model has not been reported in other literature. By using this model, first, I simulate OPG process in a narrow pump beam and a wide elliptical pump beam, respectively, and verify this model by comparing the numerical results with the experimental results; then, I simulate injection-seeded OPG process and analyze some possible approaches to improve this process; finally, I introduce a novel QPM structure crystal, which can theoretically produce very high conversion efficiency and might be able to solve the noncollinear phase matching problem when it is used with elliptical pump beam operation. The quantum fluctuation of OPG output is another interesting subject in my work. I study the statistical properties of the OPG process experimentally and numerically, which helps us have a better understanding of this process and verify my numerical model in another aspect. My intention in this study is to design an OPG with a high output power, narrow spectral band and high conversion efficiency.