Self-organized quadratic nonlinear optical phenomena in optical fibers: a critical review of up-to-day models and prognoses on further development of the research

The first observation of efficient second-harmonic generation (SHG) in germanium-phosphorous-doped optical fibers when passing a strong Nd:YAG laser radiation through the fiber for about ten hours has inspired many efforts to explain the origin of self-organized growth and freezing-in of second-order susceptibility grating. The preparation of fibers for SHG with an external SH seeding is usually considered. In this paper models for the origin of the initial self-seeded SH radiation are reviewed