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We develop a phenomenological model of the double-phase-conjugate mirror grating formation based on geometrical considerations. We show that the grating develops from one lateral side of the interaction zone towards the other side. This model is confirmed by the behaviour of different configurations of a 2-zone double-phase-conjugate mirror. Finally, we obtained stable operation in InP:Fe, this made it possible to implement a 1×8 switch between single-mode fibres at telecom wavelength with 12% global efficiency and 200 ms reconfiguration time.

We present a polarization-independent double phase conjugate mirror (DPCM) working at telecommunication wavelength 1.32 μm. To do this, InP:Fe photorefractive crystals are used. A 2Z-DPCM configuration is realized in order to eliminate conical diffraction and to reduce the instabilities. With this device, the conjugation efficiency remains almost unchanged when the polarization state of the incident beam varies. Dynamic measurements demonstrate that the device is well-adapted to relatively slow variations of the polarization, induced by thermal variations for example.

We describe the principle and present preliminary results obtained on a dynamic bandpass filter using a 4-f free space imaging system with demultiplexing optics in one arm of a Michelson interferometer. Attenuation and blocking are performed by delaying the channel optical paths thanks to a reconfigurable nano-polymer dispersed liquid crystal phase modulator array. Our system is intrinsically polarization insensitive.

A finite difference method is used to analyze the behavior of photorefractive InP:Fe at high fringe contrast under externally applied field. The evolution with increasing fringe contrast m of the electron‐hole resonance, which occurs in the presence of a continuous field, is studied. As m increases, the field tends to concentrate in a small zone and reaches very large local values. On the other hand, the resonance loses height and widens. For illuminations closed to the resonance, the two‐wave mixing gain presents the usual aspect of a decreasing function of m. However, for other illuminations, it can be larger at a small pump‐to‐probe ratio than at a large one. When an ac field technique is used, nonlinearities drastically reduce the two‐wave mixing gain, even at small fringe contrast. As m increases, the space‐charge field tends to take a square shape. Even for rather small m values (0.1), the gain presents a maximum near the dc field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the dominant process for the grating formation, the applied field limits the amplitude of the space‐charge field, independently of an optimized dopant concentration which allows high gain in the small m approximation.

How the fanning can act as a seed in the formation of the semilinear phase conjugate mirror and of the two facing double phase conjugate mirror was studied. These phase conjugate mirrors were obtained in BTO crystals using a square shape electric field and the conjugation quality was high enough for efficient reinjection of the reflected beam into a monomode fibre. Performances of the device could be predicted from DPCM measurements.

The Double Phase Conjugated Mirror (DPCM)1 and the Double Colour Pumped Oscillator (DCPO)2 are particularly interesting devices for reconfigurable optical interconnects2,3,4,5. In a DPCM two mutually incoherent pump beams (at nearly identical wavelengths) incident on the opposite faces of a photorefractive (PR) crystal share a common grating which diffracts them into the phase conjugated of the opposite one. When the wavelength difference of the interacting beams is important (larger than the grating selectivity), they can still induce a common grating (DCPO). Recently DPCM has been observed in InP:Fe6 and GaAs:EL27 at 1.06 μm and DCPO has been achieved in InP:Fe using 1.06 and 1.32 μm wavelengths8.

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.

Active splitting between up to several hundreds outputs is shown to be feasible using cascaded liquid crystal beam shifters. Performances of 1/spl times/4 and 1/spl times/16 devices are reported.

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%.

Double Phase Conjugate Minors (DPCM) are of great interest for the realization of free-space interconnections between single mode optical fibers. However, an alteration of the polarization of the incident beams, which can be easily induced by mechanical constraints on standard fibers, leads to a dramatic reduction of the conjugation efficiency. We present here a polarization-independent DPCM. The polarization insensitive experimental set-up is described in figure 1. A biréfringent calcite crystal is inserted at the output of the monomode fiber. Thus, two orthogonally polarized components are spatially separated. The polarization of the component that would not contribute to the formation of the DPCM in a conventional configuration is rotated in a liquid crystal cell. Both components are then directed into the InP:Fe sample with a lens. The pump beam results from the interference of the two components. Our experiment is performed by manipulating the polarization of only one pump beam to demonstrate the principle of the polarization independent DPCM. Obviously, the other pump beam could be obtained applying the same method. In order to eliminate conical diffraction and instabilities, this elementary set-up was incorporated in a 2Z-DPCM configuration. Experiments were carried out with an InP:Fe photorefractive crystal at the telecommunication wavelength of 1.32 μm. The diffraction efficiency and the depletion rate were measured for different polarization directions of the incident beam labeled A. The results are illustrated in figure 2. The averaged values of the depletion rate and the diffraction efficiency are 50% and 10% respectively. It is worth noting that these results are comparable to those obtained with a classical configuration. The relative variations of the diffraction efficiency are less than 8% when the polarization state of the incident beam varies. Keeping in mind that the relative variations can reach nearly 100% in a conventional DPCM, our device can be considered stable.

The Double Phase Conjugate Mirror (DPCM) [1] is particularly attractive for the realisation of free space bi-directional optical links, owing to the self aligning properties of phase conjugation[2].

Free space optical switches are very attractive because of their high-density interconnections, transparency to signal format and bit rate. Recently, multistage optical switches using liquid crystal (LC) cells have been demonstrated at the telecommunication wavelengths [1] [2] [3], By alternately cascading polarisation controller arrays (LC cells) and routing elements (bireffingent plates, polarising beam splitters, prisms, diffractive optical elements), a N input N output optical switch can be realized. To create an optical switch with low insertion loss and good crosstalk, we must resolve some technical issues such as the development of low-loss and high- contrast polarisation controllers.

Liquid crystals are frequently used as active holographic elements for beam deflection applications. Thanks to their small response time, ferroelectric liquid crystals are mostly studied for telecom applications. However, for routing applications, the reconfiguration rate is not a key parameter, on the other hand, nematic liquid crystals make it possible to produce phase ramps and therefore high efficiencies. [1 2 3 4 5 6-7]

The photorefractive effect is now extensively studied in semiconductors, in particular for applications to optical communications.

Fe-doped InP is a promising photorefractive material offering the possibility of implementing optical processing functions at IR wavelengths compatible with semiconductor lasers and optical communication applications.

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.

The interest of Double Phase Conjugated Mirror1 (DPCM) for various optical interconnects schemes has been emphasized by several authors2,3,4 but experimental demonstrations were restricted to visible wavelengths. We present here high 2WM gain performances obtained at 1.06 and 1.32 µm with InP:Fe crystals grown at CNET LANNION and report on the observation of DPCM at 1.06 µm in the same material.

Modest performance of photorefractive III-V semiconductors is expected in experiments such as two wave mixing (2WM) in the absence of external field, due to their low electro-optic coefficient. Nevertheless we have recently shown experimentally and theoretically that InP:Fe exhibits an average gain higher than 10 cm-1 (with maximum of 20 cm-1 in agreement with theory) [1,2]. This gain can be high enough to obtain the Double Phase Conjugated Mirror (DPCM) phenomena at 1.06 μm with a sample no longer than 5 mm [2]. The underlying physics of such high gain is a synergy of hole and electron emissions giving rise to a new mechanism of resonance [1], which will be named here hydrodynamic resonance.

In order to improve the properties of photorefractive (PR) sillenite crystals, different dopants are currently being investigated. In this paper, we present some of the characteristics of two cooper doped Bi12GeO20 (BGO) crystals.

Two-wave mixing in photorefractive InP:Fe crystals is revisited in terms of an externally applied dc field combined with the moving-grating technique. We show that the disagreement between theory and experiment that was previously observed in the literature can be reconciled by an examination of the influence of both the absorption along the propagation axis and the nonlinearities that occur at relatively large fringe modulations. A finite-difference method is used to demonstrate the dependence of these nonlinearities on the average incident intensity.

The velocity dependence of the associative ionisation cross section sigma AI of Ne atoms by He(31P) atoms of very short lifetime is measured in the relative velocity range 1450< nu r

High resolution laser spectroscopy coupled to a sensitive method of detection via mass analysis of He + 2 ions produced in He(5 3 P) + He(1 1 S) collisions, is used to obtain the fine and hyperfine spectra of the ultra-violet He 2 3 S → 5 3 P transition. A cw tunable UV radiation around 294.5 nm is generated by intracavity frequency doubling a Rhodamine 6G single mode ring dye laser using an ADA crystal. Both spectra enable fine and hyperfine structures to be determined within a few MHz. The magnetic dipole coupling constant A of the 5 3 P term of 3 He is found to be -4326 ± 9 MHz (-0.1443 ± 0.0003 cm -1 ).

The effect of absorption on double phase conjugate mirror (DPCM) in photorefractive materials is studied. An analytical condition for the threshold is derived; in particular the 2WM gain Γ and the absorption α must satisfy γ>2α. The diffraction efficiency above the threshold is numerically evaluated taking into account the saturation of Γ at large fringe modulation. Theoretical predictions are compared with the results obtained with a BGO:Cu crystal at 632.8 nm.

We report a c.w. intracavity, frequency doubled, rhodamine B dye laser using an RDP crystal 5 cm long. Generated uv powers up to 15 mW are obtained in a single direction with a linewidth of 2 GHz over the range 314–318 nm. A conversion efficiency of about 1.25 × 10-3 W-1 is measured.

End preparation appears to be one of the most delicate operations during fibre splicing. While, in a mechanical splice, fibres are polished, a cleaving tool is usually used for fusion splicing. A common way of testing fibre breaks is interferometry : the interference pattern perfectly characterises a fibre end. However patterns are so different from one sample to another that comparison remains a difficult task. We propose a simplified method for the measurement of break angle as well as for the evaluation of surface quality : a collimated light beam is directed parallel to the fibre axis onto the cut where it is reflected and diffracted. A diffraction spot, which characterises the fibre end, forms in the infinite plane. The position of the spot gives the break angle with an accuracy of 10'. The size of the diffraction pattern is proportional to the amplitude of the surface defects. The experimental setup has been used for testing cleaving tool efficiency and checking fibre ends before splicing. The technique is convenient enough to give rise to a field control device. A PIN photodiode will be set in the centre of the diffraction image plane. The diffraction pattern is central, small and bright for a good break, the photodiode will then receive maximum, flux. The spot extends with a poor quality break so that the small size photodiode receives less flux. A quick selection of fibre breaks can be made by eliminating samples for which the flux received by the photodiode is less than a minimum value.

The energy level diagram of substitutional vanadium in CdTe (and also Cd 0.96 Zn 0.04 Te) has been investigated by using a variety of optical and thermal spectroscopy techniques: optical absorption, photoconductivity, photoluminescence, deep level optical spectroscopy, DLTS and thermally stimulated current (PICTS). This study has led to set the V 2+ /V 3+ level at 0.95 eV below the conduction band with an optical photoionization threshold at 1.03 eV. Two-wave mixing experiments were used to evaluate the photorefractive properties. Gain coefficients in excess of 10 cm -1 have been measured at λ=1.32 μm with applying a 40 Hz AC electric field