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Polarization insensitive second harmonic generation (SHG) is proposed in an electro-optic (EO) tunable periodically poled Lithium Niobate (PPLN). The PPLN consists of four sections. External electric fields could be selectively applied to them to induce polarization rotation between the ordinary and extraordinary waves. If the domain structure is well-designed, the fundamental wave with an arbitrary polarization state can be frequency doubled efficiently in this special PPLN.

Electromagnetically induced transparency (EIT)-like transmission is observed in an electro-optic (EO) tunable periodically poled lithium niobate (PPLN) with a central defect. When an electric field is applied, light satisfying the phase matching condition keeps its original polarization, although it experiences polarization rotation then turns back. Therefore the corresponding light always may pass though freely but with an EO tunable phase and dispersion. On the other hand, light with a neighboring wavelength is blocked. An EIT-like spectrum is thus obtained with tunable group delay. A low-voltage bulk phase shifter with over two orders of magnitude larger index change is obtained.

In recent years, photonic crystal fibre based modal interferometers have been widely investigated. Owing to the nature of Mach-Zenhder-like interference, the sensing range is limited by the sinusoidal spectrum's period. A splicing approach is proposed and demonstrated to achieve unique four-beam interference so that quadrupled sensing range is obtained without reduction in detection limit, which would be much desired in wide range refractive index, pressure and temperature sensing. As an example, a high-temperature sensor ranging from 28 to 484°C is demonstrated.