G. Picoli

Double-phase conjugated mirror and double color pumped oscillator in photorefractive InP:Fe

We report on double‐phase conjugation at 1.06 μm wavelength in InP:Fe with applied continuous electric field. Conversion efficiencies up to 74% with a grating period of 9 μm are obtained. The dependence of this efficiency on intensity, applied field, and grating period is reported and the results are compared with numerical simulations taking into account space‐charge field nonlinearities. Double color (1.06 and 1.32 μm) pumped oscillation is also observed with conversion efficiencies higher than 30%.

Model for resonant intensity dependence of photorefractive two-wave mixing in InP:Fe

We present a new model, which incorporates both temperature and electron-hole effects, for two-beam coupling in photorefractive semiconductors under an external dc field E(0). We show that the exponential gain ? exhibits an intensity-dependent resonance. The application of this model to InP:Fe allows us to predict a value of ? near 20 cm(-1) for a thin sample at 1.06 microm with E(0) = 10 kV/cm.

High gain coherent amplification in thermally stabilized InP:Fe crystals under dc fields

After recalling the principal results of the theory of two-wave mixing gain enhancement in photorefractive InP:Fe crystals under a dc field, we report on experiments performed with several samples manufactured at CNET. For the first time, we believe, a maximum gain of 11.4 cm(-1) has been obtained under an applied dc field of 10 kV/cm in a thermally stablized crystal (T = 290 K) at lambda = 1.06 microm. In spite of a decrease in the factor n(3)r(eff)/lambda high gains (Gamma >/= 5 cm(-1)) have been achieved at lambda = 1.32microm in thick crystals. These experimental results are in good agreement with theoretical predictions.

Stabilization of photorefractive two-beam coupling in InP:Fe under high dc fields by temperature control

Abstract The application of an external dc field enhances greatly the two-wave mixing gain in photorefractive InP:Fe but the heating of the crystal prevents stable operation at high fields. We have strongly reduced the temperature variations by using a Peltier cooling element. By this way, we have observed the influence of the temperature on the gain versus pump intensity curve and we have obtained stabel operation with high gain (6 cm -1 , high fields (up to 10 kV/cm) and pump intensities at 1.06 μm as low as 10 mV/cm 2 .

Potentiality of photorefractive CdTe

Abstract For optical telecommunication networks, optical switching is now being studied. Different solutions have been proposed (integrated optics, free space switching, etc.), and reconfigurable optical interconnects, based on phase conjugation, should be one interesting method. For example, some results have been obtained with a double-phase conjugated mirror configuration, allowing reconfigurable connection between single-mode optical fibres. These phase-conjugated optics use photorefractive crystals and the first demonstration has been given using Bi12 TiO20 photorefractive crystals. In a telecommunication network, semiconductive crystals with a good efficiency in the near-IR wavelength are needed. Our first experiments were carried out with InP:Fe crystals at 1.3 μm. However, it is well known, from published studies, that II–VI materials are, in principle, more interesting for the following reasons. The electro-optic coefficient is higher (and therefore the figure of merit is higher). The solubility of dopants is higher (and therefore the space charge electric field, which modulates the refractive index of the material, may be higher). Next we tested a CdTe:V crystal and, in a two-wave mixing experiment without an external electric field, an amplification gain was observed and a high photosensitivity demonstrated at 1.3 μm. In this paper, we shall describe the photorefractive effect and explain our choice of the CdTe:V crystal, taking into account the parameters of this photorefractive effect and some other parameters required by the applications in the optical beam steering field. Next the results will be given, and finally an optical configuration using phase conjugation will be presented as an example of application.

Resonant behaviour of the temporal response of the two-wave mixing in photorefractive InP:Fe crystals under dc fields

Abstract The pump beam intensity dependent temporal response of the two-wave mixing in InP:Fe crystals under dc fields at stabilized temperature has been studied. We observed a resonant behaviour of the rise time of the photoinduced grating, like the coherent amplification gain. Numerical solutions of the coupled field and material differential equations have been performed with a time dependent fringe modulation in the sample volume. The temporal evolution of the signal beam output intensity has been described taking into account the nonlinearity effect at large modulations. Preliminary theoretical and experimental results are in rather good agreement.

Moving grating and intrinsic electron–hole resonance in two-wave mixing in photorefractive InP:Fe

The theory for two-wave mixing in photorefractive InP:Fe under a dc electric field with both electron-hole intrinsic resonance and a moving grating is developed. An extended resonance condition is predicted. The phenomenon is proved experimentally, even though a quantitative discrepancy remains between theory and experiments.

Double phase conjugate mirror and double colour pumped oscillator using band-edge photorefractivity in InP:Fe

Abstract We have observed the formation of a double phase-conjugate mirror (DPCM) and a double colour pumped oscillator (DCPO) at wavelengths near the band-edge of InP:Fe, with an external dc field of 10 kV/cm. The DCPO conversion efficiencies are respectively 63% and 26% for the couple of wavelengths 985 and 1.047 nm. These results are compared with the prediction of a calculation which takes into account the space-charge field non-linearities.

Energy transfer enhancement in photorefractive InP:Fe crystals using an auxiliary incoherent beam and a negative thermal gradient

The exponential gain of two-beam coupling in InP:Fe crystals under dc fields is a function of the pump beam intensity at a given temperature. IN this paper, the authors propose two methods to extend the sample volume working under optimum conditions. The first method is based on a negative thermal gradient and the second uses an auxiliary incoherent beam. Theoretical simulations predict an integrated gain of 15 cm{sup {minus}1} and 10 cm{sup {minus}1}, respectively (for {lambda} = 1.06 Mm, {Lambda} = 5 Mm and E{sub 0} = 10 kV/cm) in thick crystals (L = 5 mm). Preliminary experimental results confirm that high gain performances can be achieved by using these methods.

Two-Wave Mixing Gain vs Intensity Dependence in Photorefractive GaAs:EL2 in Presence of Strong Electron/Hole Competition

We studied the pump intensity dependence of the 2WM gain in a photorefractive GaAs:EL2 crystal, with a 3.3 kV/cm d.c. field, at two different wavelengths. The grating period was 37 Am and the beam ratio was 4. At 1.32 μm, the characteristics exhibits a resonant behaviour (with a maximum of 0.23 cm -1 ). As for InP:Fe, these results can be explained by considering that thermally and optically generated carriers are of different types. The curve at 1.047 Am exhibits an original feature. At low intensities the gain as the same sign than at 1.32 μm (with a maximum of 0.12 cm -1 ), but the sign changes with increasing intensity and the gain tends towards an asymptotic value of -0.15 cm -1 . Such behaviour may also be explained with the same model.