G. Bonfrate

Greater than 20%-efficient frequency doubling of 1532-nm nanosecond pulses in quasi-phase-matched germanosilicate optical fibers

We fabricated second-order nonlinear gratings in D-shaped germanosilicate fibers, using thermal poling and periodic electrodes defined by standard photolithography. These gratings, which are up to 75 mm long, were used for efficient quasi-phase-matched frequency doubling of 1.532-mum nanosecond pulses from a high-power erbium-doped fiber amplifier. Average second-harmonic powers as high as 6.8 mW and peak powers greater than 1.2 kW at 766 nm were generated, with average and peak conversion efficiencies as high as 21% and 30%, respectively.

Parametric fluorescence in periodically poled silica fibers

We report the observation of quasiphase matched parametric fluorescence from a periodically poled silica fiber. A pair-photon production rate of more than 100 MHz around 1532 nm was achieved in second-order nonlinear gratings for 300 mW of pump power at 766 nm. These results are very promising for the realization of reliable all-fiber single-photon sources for quantum cryptography systems and metrology applications.

Thermal poling of silica in air and under vacuum: The influence of charge transport on second harmonic generation

A comparison between thermal poling of silica in air and in vacuum is reported. It is shown that the second-order susceptibility and thickness of the nonlinear layer as well as their time evolution are highly dependent on the surrounding poling atmosphere. In the vacuum case a charge distribution (under the anode) more complex and broader than that for the air case has also been revealed by laser induced pressure pulse measurements. A multiple charge carrier model can explain the formation and evolution of the depletion region under the anode. The findings are relevant to achieve improved nonlinearities in fiber and waveguide devices.

Efficient frequency doubling of 1.5 /spl mu/m femtosecond laser pulses in quasi-phase-matched optical fibers

Summary form only given. We report on the fabrication of second order nonlinear gratings up to 7.5 cm long for quasi phase matched (QPM) SHG of 1.5 /spl mu/m and their use for ultrashort pulsed frequency conversion. It is shown that periodically poled glass fibres (PPGF) are very promising for QPM-SHG and parametric processes involving highpower laser sources.

High second-order optical nonlinearities in thermally poled sol-gel silica

Silica glass samples prepared by a sol-gel process, the mixture of silica sol for gelation and colloidal silica particles, have been thermally poled in vacuum by continuous high voltage (8-12 kV) at elevated temperature (280°C). High second-order nonlinearities (>1pm/V), located under the anodic surface, have been measured and the values are higher than those obtained in fixed silica glass, poled under the same conditions. A model for thermal poling is suggested which explains the experimental results in sol-gel silica and the difference between thermal poling of sol-gel and fused silica. The granular structure and the boundaries of sol-gel silica seem to play a major role in establishing the electrical properties of the depletion layer during and after poling

Periodic UV erasure of the nonlinearity for quasi-phase-matching in optical fibers

Summary form only given. Summmary form only given. Recently more than 20% efficient second-harmonic generation (SHG) and parametric fluorescence from periodically poled optical germanosilicate fibers were reported, opening new prospects for the realization of second-order nonlinear optical processes in all-fiber devices. In both cases the fiber samples were thermally poled, subjected to a periodic electric field (periodic thermal poling, PTP), generated by the anodic electrode which had been photolitographically defined on the flat side of a D-shape optical fiber. The field periodicity induced a periodic second-order nonlinearity (/spl chi//sup (2)/), necessary for quasi-phase-matching (QPM). We report on a new technique, periodic UV erasure of second-order nonlinearity (PUESN), which presents a few important advantages over PTP: device lengths of /spl sim/1 m or more can be obtained by scanning the UV beam through and amplitude mask over the uniformly poled fiber; much shorter periods can be achieved by using a phase mask to create UV interference fringes, thus making in principle possible more complicated and finer patterns for example for backward parametric interactions; also a fairly complex photolitography process can be skipped altogether.

A spectral characterisation method for /spl chi//sup z/ gratings

Summary form only given. In a widely used technique for the characterisation of second-order nonlinear gratings the measurement of the quasi-phase-matching (QPM) curve for second-harmonic generation (SHG) is achieved by continuously varying either sample temperature or fundamental (F) wavelength. This curve reveals the information concerning the length and the regularity of the nonlinear grating. This technique exhibit some drawbacks, especially for long gratings. A temperature gradient, practically unavoidable over a long grating, leads to a chirp in the period or the need for very fine scan step in the tuning of the fundamental wavelength are some of these drawbacks. To overcome these constraints, we propose a method that does not need tuning of any parameter and involves short laser pulses and relatively long nonlinear gratings. The method is applied to the case of SHG of ultra-short pulses in QPM structures without depletion of the pump.

Highly efficient generation of 1 /spl mu/m femtosecond pulses in an optical fibre pumped at 1.5 /spl mu/m

Summary form only given. We report experimental results on frequency conversion in germanosilicate fibres under high power pump excitation at 1.5 /spl mu/m in the femtosecond regime. Signals at around 1 /spl mu/m were generated with high powers (>20 mW) and conversion efficiencies (>10%).

>20%-efficient pulsed frequency doubling of 1532 nm in periodically poled silica fibres

Summary form only given. The recent discovery that thermal poling can produce a permanent and large second-order nonlinearity in silica has made it possible to implement Quasi-phase-Matching (QPM) in glass waveguide/fibres. Despite having a lower effective nonlinear coefficient (d/sub eff/) than crystal waveguides, periodically poled silica fibres (PPSF) and waveguides (PPSWV) can offer longer interaction length (L) for the same bandwidth and higher damage intensity threshold (I), thus keeping high values for the waveguide efficiency-factor d/sub eff//sup 2/L/sup 2/I. In addition they offer high transparency, low cost and straightforward integrability. These features make PPSF/PPSWV ideal for a wide range of QPM processes, such as frequency conversion of fibre lasers and difference frequency generation as a means for wavelength conversion in telecommunications.

Quasi-phase-matched parametric fluorescence in poled silica fibres

We report the first observation of parametric fluorescence from a periodically-poled silica fibre. The achieved pair-photon production rate resulted in more than 100 MHz at 1532 nm for 300 mW of pump power at 766 nm