Radially and azimuthally polarized nanosecond Yb-doped fiber MOPA system incorporating temporal shaping.

We report an Yb-doped fiber master-oscillator power-amplifier (MOPA) system with the capability of selectively generating doughnut-shaped radially and azimuthally polarized beams with user-defined temporal pulse shapes. The desired output polarization was generated with the aid of a nanograting spatially variant half-waveplate (S-waveplate). The latter was used to convert the linearly polarized fundamental (LP01) mode output from the preamplification stages to a doughnut-shaped radially polarized beam prior to the power amplifier stage. A maximum output pulse energy of ∼860 µJ was achieved for ∼100 ns pulses at 25 kHz with user-defined pulse shape for both radial and azimuthal polarization states. The polarization purity and beam propagation factor (M2) were measured to be >12 dB and 2.2, respectively.

Revealing the nanoparticles aspect ratio in the glass-metal nanocomposites irradiated with femtosecond laser.

We studied a femtosecond laser shaping of silver nanoparticles embedded in soda-lime glass. Comparing experimental absorption spectra with the modeling based on Maxwell Garnett approximation modified for spheroidal inclusions, we obtained the mean aspect ratio of the re-shaped silver nanoparticles as a function of the laser fluence. We demonstrated that under our experimental conditions the spherical shape of silver nanoparticles changed to a prolate spheroid with the aspect ratio as high as 3.5 at the laser fluence of 0.6 J/cm2. The developed approach can be employed to control the anisotropy of the glass-metal composites.

Cladding-pumped ytterbium-doped fiber laser with radially polarized output.

A simple technique for directly generating a radially polarized output beam from a cladding-pumped ytterbium-doped fiber laser is reported. Our approach is based on the use of a nanograting spatially variant waveplate as an intracavity polarization-controlling element. The laser yielded ~32 W of output power (limited by available pump power) with a radially polarized TM (01)-mode output beam at 1040 nm with a corresponding slope efficiency of 66% and a polarization purity of 95%. The beam-propagation factor (M(2)) was measured to be ~1.9-2.1.

Poled-fiber source of broadband polarization-entangled photon pairs.

We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 µm telecom band. Even with signal-idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong-Ou-Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement.

Noncollinear Maker's fringe measurements of second-order nonlinear optical layers.

A novel technique for characterizing thin-film second-order nonlinearities with submicrometer resolution for the film's depth is proposed. This method is substantially a variation of the classic one-beam Maker's fringe technique and uses the second harmonic generated by two noncollinear fundamental beams. Compared with that for the one-beam case, this configuration reduces the coherence length of the process, thus increasing the resolution for the nonlinear depth measurements. The technique has been implemented on thermally poled silica samples, revealing the initial growth of the nonlinear region.

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.

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses.

We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500 degrees C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique will be useful for fabrication of fiber gratings with a superior aging characteristic.

Effect of poling conditions on second-harmonic generation in fused silica.

A systematic study of the effects of poling time and applied voltage on second-harmonic (SH) generation in thermally poled silica glass reveals that the SH signal is proportional to the square of the applied voltage and that the speed of the poling process increases with the applied voltage. Prior treatment of the samples is found to affect the poling process, and the optimum poling conditions are observed to depend on the poling atmosphere. The mechanism of thermal poling is discussed in the light of these new results.

Blue-light generation by quasi-phase-matched frequency doubling in thermally poled optical fibers.

Periodically patterned second-order nonlinearities are created in optical fibers by thermal poling in vacuo. Cw quasi-phase-matched frequency conversion to the blue is demonstrated. The measured bandwidth, 0.78 nm cm, is close to the theoretical prediction.

High second-order nonlinearities in poled silicate fibers.

Effective quadratic nonlinearities as high as 0.2 pm/V are reported for the first time to our knowledge in poled germanosilicate fibers. This value is ~200 times higher than previously achieved in these fibers. The presence of Ge is found to enhance the efficacy of both thermal (in combination with OH doping) and electron-beam poling in silica.

High second-order nonlinearities induced in lead silicate glass by electron-beam irradiation.

A new technique for inducing a large permanent second-order susceptibility in lead silicate glass is reported. The procedure involves implanting electrons by irradiating the glass with an electron beam. Second-order nonlinearities chi((2)) as high as 0.7 pm/V are obtained.

Photoinduced second-harmonic generation in gamma-ray-irradiated optical fibers.

The effect of gamma-ray irradiation on photoinduced second-harmonic generation in optical fibers is investigated for the first time to our knowledge. A significant increase (by a factor of 20) in the conversion efficiency of gamma-ray-irradiated optical fibers is found. An explanation of the experimental results is suggested, which is based on the photovoltaic mechanism of the photoinduced second-harmonic generation.