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Frequency doubling of picosecond pulses in periodically poled D-shape silica fibre
V.Pruneri and P.G.Kazansky
D-shape silica fibres have been periodically poled at elevated temperature by applying high voltage via a patterned electrode deposited on the plane side. The resulting nonlinear grating of 25μm period, uniform over the whole 1.8cm length, has been used for quasi-phase-matched second harmonic generation. With a mode-locked laser as fundamental source blue powers up to ~76μW have been generated at ~422nm with an average conversion efficiency of ~0.22%.
Electric-field thermal poling has allowed to produce permanent second-order nonlinearities in glasses, such as silica [1, 2, 3]. These nonlinearities can be exploited to realize electro-optic and nonlinear optical devices, including modulators, switches, frequency doublers and optical parametric amplifiers/oscillators. As we recently pointed out [4, 5], compared with nonlinear crystals waveguides, such as lithium niobate (LN) and potassium titanyl phosphate (KTP), poled silica fibres offer greater bandwidths availability (more than one order of magnitude for the same device length) because of their lower dispersion. Therefore the relatively low value of the nonlinear coefficient could be compensated by increasing the length of the device, thus achieving the same efficiency as for LN and KTP without altering the frequency stability.
In the case of pulsed applications the group velocity mismatch (GVM) between pulses at different frequency determines the device length. The walk-off due to GVM prevents the spatial quadratic growth of the second harmonic signal and conversion efficiency (although they still increase with length) and produces spreading and distortion of the pulses . Thus the device length should be the result of a good trade-off between high conversion efficiency and low pulse-spreading. The low GVM in silica fibres (more than one order of magnitude less than in LN and KTP ), combined with the high optical damage threshold, makes poled silica a suitable material for pulsed frequency conversion since the relatively low nonlinear coefficient (deff) can be compensated by extended interaction lengths (leff) and high peak intensities (Ip), so that the figure of merit deff2leff2Ip for the conversion efficiency is maintained high. Here we report the first demonstration of efficient quasi-phase-matched (QPM) second harmonic (SH) generation to the blue of picosecond pulses in thermally poled fibres. The content of this paper represents a great improvement in comparison with our previous results [4, 5] in terms of nonlinearity and conversion efficiency indicating that periodically poled silica fibre is an emerging and promising material for nonlinear guided-wave frequency conversion.
The D-shape fibre used in the experiments had a numerical aperture of 0.09, core and outer diameters of 5.8μm and ~150μm and distance between plane surface and core region of 5μm. Poling was achieved by applying continuous voltage of ~5kV, directly across the fibre using metal electrodes, at ~270°C for ~10 minutes in vacuo. The main difference with refs. and  is that in this work the patterned aluminum electrode of 25μm period was directly fabricated on the plane face of the D-shape fibre, rather than on a separate glass support which was afterwards pressed against the fibre. This allowed to realize more uniform gratings, ~3 times longer and with an increase of ~5 times in effective nonlinear coefficient, thus >200 times in conversion efficiency for the same fundamental intensity. To fabricate the patterned Al electrode (fig. l), we initially placed the fibre on a planar silicon substrate and then used ordinary photolithography for patterning. During poling the patterned electrode was the anode and the curved face of the fibre, in contact with a metal substrate which was grounded, was the cathode. After poling the aluminum electrode was removed by etching. The length of the grating was chosen to be 1.8 cm which is close to the ratio (τ/GVM) ~ 1.6cm at around 840nm, between the pulse duration (τ) of ~2.2 past and the GVM of ~ 0.2ps/mm. This ratio is the length over which the walk-off of the interacting pulses takes place and gives a good conversion efficiency without significant pulse lengthening.
Figure 1 Aluminium pattern (25μm period and 1.8cm long) on the plane face of the D-shape fibre.
Before carrying out the picosecond experiment we optically assessed the sample using a tunable Ti:sapphire laser as fundamental source. The quasi-phase-matching curve (SH power against fundamental wavelength) is shown in fig.2. The bandwidth (FWHM) of ~0.42nm agrees well with the calculated value for a perfect nonlinear periodic structure of the same length indicating the good uniformity of the grating over the whole length.
Figure 2. Quasi-phase-matching curve: second harmonic power against fundamental wavelength.
The maximum cw blue power produced at 422.3 nm was ~1.1μW corresponding to a fundamental power of 150mW. From these measurements one can easily calculate [5, 8] an effective nonlinear coefficient of ~1.5·10−2 pm/V (which includes the overlap of the modes with the poled region and the 1/π reduction factor associated with first-order quasi-phase-matching). Although ~5 times greater than in previous work on QPM silica [4, 5], the effective nonlinear coefficient is still more than 10 times lower than the limit of ~2.2·10−1 pm/V that we have estimated from measurements on uniformly poled silica fibres . The reasons of this degradation are probably the spreading of the poled regions, which produce mark-to-space ratio far from the optimum 50/50 and lower depth of nonlinear modulation, as well as the presence of regions completely poled or unpoled rather than periodically poled.
In the pulsed experiment the fundamental source was a tunable mode-locked Ti:sapphire laser which produced ~2.2ps pulses at a repetition rate of 76MHz. The average SH power as a function of the average fundamental power, at the QPM fundamental wavelength of 844.6nm, is shown in fig.3. The maximum average SH power was ~76μW corresponding to an average fundamental power of 35mW (~200W peak power). Therefore the maximum conversion efficiency was ~0.22% which corresponds to a normalized conversion efficiency with respect to the average fundamental power of ~6.3%/W.
Figure 3. Average second harmonic power against average fundamental power in mode-locked regime.
Our results show that periodically poled silica fibres have become a promising medium for second-order nonlinear frequency conversion. Considering that the effective nonlinear coefficient can still be improved >10 times (corresponding potential increase in conversion efficiency >100), great progress is expected from optimization of the poling conditions. In particular we will also consider second-order nonlinear processes, such as frequency doubling and frequency conversion at longer wavelengths, which would require longer grating periods with associated reduced spreading of the poled regions, thus less demanding fabrication.
We would like to thank L. Dong for fabricating the D-shape fibre used in the experiments.
The Optoelectronics Research Centre is an Interdisciplinary Research Centre, partly supported by a grant from the U.K. EPSRC.
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Electronics Letters (1997) Vol.33(4) pp.318-319
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