Abstract

The discovery by Osterberg and Margulis of efficient (>5%) conversion of 1.06-μm light to the second harmonic (0.532 μm) in a P₂O₅:GeO₂:SiO₂ glass fiber has resulted in an interesting yet unexplained physical effect. We believe that the defect responsible for this effect is a clathrate structure or interstitial trapping of a GeO molecule. The maximum doping of ~10% mol of GeO₂ into the SiO₂ network may result in extra GeO molecules, which are interstitially trapped during preform fabrication. The GeO molecule has an inter-nuclear separation of 1.786 and 1.650 Å in its ground (A¹∑⁺)and excited (X¹∑⁺) states, respectively. GeO is rotationally frozen in its A¹∑⁺ state, while multiphoton optical excitation of the X¹∑⁺ state (~2600 Å) by three photons (2ω + ω + ω) at 1.06 μm allows the GeO guest molecule to reorient itself. The threshold for preparation is believed to be due to the onset of a parametric instability in vibrational Raman scattering (~1 GW/ cm²). The symmetry of the diffusional reorientation process of the GeO molecule is broken by its dependence on the 2ω field strength, which is in turn coupled to the local ensemble orientations of the GeO molecules. This process results in an asymmetric angular random walk, which preferentially self-organizes the fiber to maximize its doubling efficiency. Phosphorous oxygen hole centers are two-photon pumped at 1.06 μm and emit photons between 2 and 2.5 eV helping to initiate the reorientation.

© 1988 Optical Society of America