P. Bernasconi

Fast, reconfigurable light-induced waveguides

Fast and reconfigurable one-dimensional waveguides are produced by interband photorefraction. The index barriers are induced by ultraviolet light. The guiding of a red laser beam with a full width at half-maximum of 15 mum is demonstrated. Buildup and decay times of the waveguide in pure KNbO(3) are of the order of 100 mus and 10 ms, respectively. The intensity of the guided light has no influence on the guiding properties over the range from 4 mW/cm(2) to 200 W/cm(2) . By reconfiguration of the waveguide, deflection angles of as much as 1.75 deg inside the crystal are achieved.

High-resolution, high-speed photorefractive incoherent-to-coherent optical converter

We demonstrate a photorefractive incoherent-to-coherent optical converter driven by ultraviolet light that provides a 35-mus response time and an optical resolution of 124 line pairs/mm. The device, implemented in KNbO(3) , operates with a modulating intensity of 85 mW/cm(2) , which corresponds to an optical switching energy per bit of 0.5 pJ. A conversion rate of the order of 90 Gbits/(s cm(2)) is achieved. The conversion between the ultraviolet light and the visible laser beam at lambda=532 nm occurs through anisotropic Bragg diffraction at a modulated interband photorefractive grating. Our device has a better optical resolution and conversion rate than optically addressed solid-state spatial light modulators based on the photorefractive effect and multiple quantum wells, and it is also faster than devices based on liquid crystals.

Self-pumped and incoherent phase conjugation in Fe-doped KNbO 3

Self-pumped phase-conjugated reflectivities in KNbO3 crystals doped with Fe have been optimized in specially cut crystals. Without using external cavities, we have obtained reflectivities of as high as 60%. With the pump beams derived from the self-pumped phase-conjugation process, degenerate four-wave mixing buildup time is 6 s, with a reflectivity of 30%. For the first time, the double incoherent phase conjugation in KNbO3 is also reported with reflectivities of as high as 65% and buildup times of 10 s. We present what are, to our knowledge, the first experiments on quantitative investigations of the fidelity of double phase-conjugate mirrors by interferometric methods that showed the high quality of the phase-aberration correction achieved in Fe:KNbO3.

Stoichiometric LiTaO3 for ultraviolet photorefraction

Abstract The near ultraviolet photorefractive properties of nominally undoped congruent and stoichiometric lithium tantalate crystals are compared. Nearly stoichiometric crystals show enhanced photorefractive two-wave mixing gain, faster response, and a drastically reduced photogalvanic field as compared to congruent crystals

Photorefractive grating fixing in KNbO3 by ferroelectric domains

We report observations of photorefractive grating fixing in potassium niobate at room temperature by means of ferroelectric domains. A grating is recorded while the crystal is simultaneously depoled. By repoling the crystal, the presence of a latent grating is revealed. This fixed grating diffracts light with an efficiency which increases when an electric field is subsequently applied and when it is uniformly illuminated. We explain these observations qualitatively using a previously developed model that involves charge compensation by domain walls, and a new aspect that invokes the creation of new trap sites at the ferroelectric domain walls.

Phase conjugation in optical communication links with photorefractive Fe:KNbO3

Abstract Self-aligning of optical links and good wavefront matching in heterodyne detection can be achieved through the use of phase conjugation. We demonstrate an optical self-aligning link between two mutually incoherent laser sources using photorefractive Fe:KNbO3 as a double incoherent phase conjugate mirror. The performance of a heterodyne detection system for coherent optical communication based on double incoherent phase conjugation and four-wave mixing phase conjugation is also presented.

Interband Photorefraction for High Resolution Incoherent to Coherent Optical Converters

Unlike for the conventional photorefractwe effects. in inrerhond photorefraction the photocarierr a i r excited by with pnotons whose energy exceeds the mdtcrid energy baodgap. Because of the higher absorprm ( a IO cm-1, the charge phoioexciration process is much more efficient and given the possibility to gcnerare a space-charge eiectric field by rnadulating!he density of the free churger v~hich are nor trapped into midgap energy Ievdi The advantages are farter response limes combined with a higher sensitivity. Time resolved fovr awe-mixing cxperimentr i n pure KNbO3 crystals have shown that even at low intensities of fcq mWlcm. rei[pme limes in the p range can be achieved wiih B consequent hologriiphx smsitiity cf the order of few cmVJ [ i ] . Recent erperimeiits af uitraviaiei (UV) light induced intieband photorefraction in KNbO; showed that the gratings have a .ayered s t r ~ c t u r e [ 2 ] . The grating, s h o w d a layered structure. Close to die illuminated surface. the phororefractirc grating is genernted by a rpatielly modulated charge densit) in the bands while, deeper m the ctystal, the holographs grating is created by charger stored in dccp rrsps These two giarmg compments h a w different amplitude. thickness. timeresponse. and may bc mutually phase shifted by n. The experimental parameters have k e n adjusted to oprinuze the performances of an high ie ioi~~t ion I terband Phoiarefractiw Incoherent tn Coherent Optical Contener (IPICOC) operating at U wnvelengthr. As shown in the scheme. IWO la% heans interfere inside a thin sample 147 bni) of pure KNbO,. The charge rnodula!ion a~h ich oiiginater the graring is then partially deleted hy the spatially modulated illununation coming from an Hg mc-Pomp. Thc resulting grating E finally reeduut by a coherent visible heam which does not afiect the interband grating. Through the Brigg diffraction procesa, thc informalion canted by the incoherent wave is transfened. contrast inverted, 10 the read-out h e m . The mentation of the cvstal axes can be chosen tu allow isotsopic as wel l as anisotropic Braggdifiraction. In this latter configaratiun ure obtained a resolution in the crysial as hieh as 120 loimm limted bv the i m a z n g aystein used to pro~cit he incoherent light onto the crys ta l The response time was oi =30 ~5 for a total LiV iorensiry of 100 U r n W l c m 2 und the diffraction 103 ,m

Fidelity in double phase-conjugate mirrors with Fe-doped KNbO/sub 3/

Multiple high-Stokes and anti-Stokes orders are frequently observed in stimulated Raman scattering (SRS). In the vast majority of cases, the higher orders are the result of parametric Pwave processes occurring under phase-matched conditions. This phase matching requires that the various orders are generated at widely differing angles and the resulting multifrequency beam is not focusable to a single spot. The much more interesting case is where higher orders are generated with a phase mismatch but are collinear. Such a beam may have application to laser fusion.z The rotational transitions in gases have been extensively used in Raman amplification, and, recently, multiple components of comparable amplitude spanning a broad bandwidth have been generated using both H, and N2 gas.g Previously, it has been thought that it would not be possible to switch on bandwidth on a timescale much shorter than the dephasing time of the medium, Tz, and that this delay would severely limit the usefulness of the generated specExperiment has tended to support this conclusion! In recent modeling we have discovered how to optimize ultrabroad bandwidth generation in collinear SRS? We shall report on modeling that predicts the generation of more than 40 Stokes orders having intensity greater than 10% of the peak. Figure 1 shows output generated after propagation through a Raman cell of length 200 cm. This figure is calculated from the temporal distributions of intensity, I A.(T) I , of the Raman lines. For a more quantitative picture, one may calculate the time-integrated spectrum, log,, .L I A,(T) I d7. This is shown in Fig. 2 for the same set of parameters. The experimental realization of just a fraction of the predicted bandwidth would be a very significant result. In addition to this, we shall demonstrate that, with an appropriate choice of experimental parameters, it is possible to switch on ultrabroad bandwidths essentially instantaneously in the local time frame (see Fig. 3). The central motivation for this research is the optimization of collisional absorption in high-energy laser-target

A comparative study on the clamped linear electro-optic effect in organic and inorganic crystals

Experiments were carried out to assess both power throughput and nearest neighbour cross talk. Three detector apertures were used: 250,400, and 500 pm. Power focused onto these apertures, normalised to the total emitted optical power ranges from -8.3 dB (aperture 250 pm) to -6.7 dB (aperture 500 pm). Corresponding cross talk values (i.e., the cross talk to signal ratio) vary between -13.7 dB and -10.2 dB. These values indicate that such LED-arrays can be used in the interconnect situation envisaged. Figure 2 gives an idea of the accuracy for lateral alignment required for efficient source-detector coupling. Power incident on detector aperture is given as a function of lateral misalignment in the image plane. 3 dB spotsizes were measured using a CCD-camera based imaging system, as a function of propagation distance. Measurement results are presented in Fig. 3, and are compared with ray tracing. Good agreement between theory and modelling can be seen. As a conclusion, arrays of LEDs with integrated diffractive focusing lenses have been realised. The power transfer to a detector array, as well as the cross talk and alignment tolerance, agrees reasonably well with the theoretical predictions and were found to be suitable for boardto-board interconnect. *Department of Electronics and Information Systems (ELIS), Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium of a mechanically unclamped inorganic ferroelectric crystal (BaTi03). A comparative study on the clamped linear electro-optic effect in organic and ., inorganic crystals