Tunneling of optical pulses at 1.5 µm wavelength through double-barrier periodic fiber Bragg gratings is experimentally investigated. Tunneling time measurements as a function of barrier distance show that, far from the resonances of the structure, the transit time is paradoxically short, implying Superluminal propagation, and almost independent of the distance between the barriers. These results are in agreement with theoretical predictions based on phase time analysis and also provide an experimental evidence, in the optical context, of the analogous phenomenon expected in Quantum Mechanics for non-resonant Superluminal tunneling of particles across two successive potential barriers. [Attention is called, in particular, to our last Figure].

We report on the experimental observation of superluminal tunneling of picosecond optical pulses in a periodic fiber Bragg grating. Optical pulses of 380-ps duration, generated by an externally-modulated single-frequency erbium-ytterbium laser operating near 1.5-microm wavelength, were propagated at a group velocity greater than approximately 1.97 times the speed of light in vacuum across a 2-cm long fiber grating. Owing to the very large ratio between the thickness of the barrier (2 cm) and the wavelength of probing optical pulses ( approximately 1.5 microm), our experiment allows for the observation of superluminal tunneling in the optical region by direct optoelectronic time-domain measurements.

In this paper, we will review the state-of-the-art of LiNbO3 based integrated electro-optic modulators and will show how micro-structuring techniques such as etching, domain inversion and thin film processing can be used to realize new configurations which can take the performance to unprecedented levels. In particular, we will review recent results on the use of domain inversion on a micron scale and we report on the fabrication of a chirp-free modulator having ∼ 2 V switching voltage and bandwidth of 15 GHz designed by placing the waveguide arms of the Mach-Zehnder interferometer in opposite domain oriented regions. We also review some of the new modulation formats (e.g. DQPSK) that can represent an application development of the presented micro-structured devices. Finally, we address the issue of the integration of the modulator chip in a transmitter board comprising tunable laser, bias-control electronics and RF driver. The requirements of integration can even push further the reduction in size of modulator chips, thus making more crucial the use of micro-and nano-structuring techniques.Mach-Zehnder modulator exploiting domain inversion for ultralow voltage operation.

A theoretical and experimental analysis of group velocity reduction in periodic superstructure Bragg gratings is presented. Experimental demonstration of group velocity reduction of sub-nanosecond pulses at the wavelength of optical communications is reported using a Moiré fiber grating.

Domain walls in periodically poled ferroelectric KTiOPO4 and LiNbO3 crystals are observed by making use of second-harmonic (SH) generation enhancement in the transition regions between neighboring domains. SH images of domain walls obtained with various samples for different polarization configurations are presented. The SH generation enhancement is found especially pronounced for the polarization of the SH radiation being perpendicular to the domain walls. The origin and selection rules for the contrast in SH images of domain walls are discussed. The results obtained suggest that the domain walls produce a deteriorating effect on SH generation by quasiphase matching.