Jürgen Petter

Self-bending of photorefractive solitons

Self-bending of photorefractive solitons is caused by diffusion in photorefractive crystals and becomes an important effect when the beam size is in the range of the charge carriers diffusion length. In this paper we present an experimental and numerical examination of the beam bending dependence on relevant parameters such as the applied electric field and the beam intensity. We demonstrate that the bending dependence on the electric field in the low saturation regime has the form of a square function at low values of the field and becomes linear for higher values. For stronger saturation the curve gets the form of a square root function. The bending dependence on the beam intensity has a maximum at defined intensity. The experimental data are compared with numerical simulations, giving a good qualitative agreement.

Guiding and dividing waves with photorefractive solitons

Abstract In this paper we show the guidance of a HeNe probe beam in photorefractive (2+1)D-solitons created by a beam of a frequency-doubled Nd:YAG laser in SBN. In the first part the development of a single soliton is shown in a time-resolved manner while the guided probe beam is found to follow exactly the movement of the soliton and even copies its shape. In the second part the guidance of a probe beam in interacting solitons is performed. When two (2+1)D-solitons propagate simultaneously different interaction scenarios can be observed, including the mutual exchange of energy. Using this effect, a guided probe beam can be divided effectively in two parts. Thus we present to our knowledge the first realization of a 1-to-2 waveguide divider using (2+1)D-solitons.

Optical control of arrays of photorefractive screening solitons

We discuss the creation of an array of 9 x 9 photorefractive spatial screening solitons in a strontium barium niobate crystal. We investigate the waveguide properties of each channel with a beam of different wavelength and find that the waveguides guide the probe beam independently. A supplementary beam is used to influence the paths of the array solitons and to effectively combine two channels by use of mutual attraction of solitons. To our knowledge this is the first all-optical control of an array of photorefractive solitons. Furthermore, we show that in principle image procession is possible with parallel propagation of photorefractive solitons.

Anisotropic waveguides induced by photorefractive (2+1)D solitons

We present theoretical and experimental investigations of the anisotropic character of the refractive-index modulation that is induced by a light beam propagating in a photorefractive strontium barium niobate crystal. Such a structure creates a so-called spatial screening soliton that is able to carry a second wave of a different wavelength and therefore can act as a waveguide. We show in numerical simulations as well as in experimental investigations the anisotropic property of refractive-index modulation. Furthermore, the noncircular shape of the induced waveguide is justified by the excitation of higher-order modes, which were found to be asymmetric in both transverse directions. Whereas in the direction perpendicular to the applied electric field the TEM01 and TEM02 modes can easily be excited, excitement of the TEM10 mode in the direction of the applied field is rather difficult. This effect can be explained by the constricted extension of the waveguide in this direction.

Bismuth tellurite - a new material for holographic memory

Abstract The photorefractive properties of Bi 2 TeO 5 crystals were measured with the wave mixing technique using the 532 nm output of a Nd:YAG laser for writing gratings. The saturation diffraction efficiency of the grating for the optimum crystal orientation and beam polarization configuration exceeded the 40%. Above a threshold write intensity, a remarkable fraction (10–20%) of the saturation diffraction efficiency remained stable for several days in the crystal. A weaker read beam of the same wavelength did not erase the grating. A two-dimensional holographic image was also written into the Bi 2 TeO 5 crystals for the first time.

Spatial optical (2+1)-dimensional scalar- and vector-solitons in saturable nonlinear media

(2+1)-dimensional optical spatial solitons have become a major field of research in nonlinear physics throughout the last decade due to their potential in adaptive optical communication technologies. With the help of photorefractive crystals that supply the required type of nonlinearity for soliton generation, we are able to demonstrate experimentally the formation, the dynamic properties, and especially the interaction of solitary waves, which were so far only known from general soliton theory. Among the complex interaction scenarios of scalar solitons, we reveal a distinct behavior denoted as anomalous interaction, which is unique in soliton-supporting systems. Further on, we realize highly parallel, light-induced waveguide configurations based on photorefractive screening solitons that give rise to technical applications towards waveguide couplers and dividers as well as all-optical information processing devices where light is controlled by light itself. Finally, we demonstrate the generation, stability and propagation dynamics of multi-component or vector solitons, multipole transverse optical structures bearing a complex geometry. In analogy to the particle-light dualism of scalar solitons, various types of vector solitons can - in a broader sense - be interpreted as molecules of light.

Optical on-line controllable filters based on photorefractive crystals

Different types of tunable and reconfigurable holographic filters based on photorefractive crystals are demonstrated experimentally. The filters exhibit an extremely narrow bandwidth (better than 0.1 nm) and allow reconfiguration or switching in a broad wavelength range and precise fast electro-optical frequency trimming. An on-line transformation of the transfer function of a narrow-band optical filter by introducing a π-shift into a holographic grating has been demonstrated for a dynamic holographic filter in BaTiO3. Some of the most promising applications of photorefractive optical filters are discussed.

Enhancing the sensitivity of an adaptive holographic interferometer using non-Bragg diffraction orders.

The signal-to-noise ratio of the output of an adaptive holographic interferometer (AHI) based on a Bi(12)TiO(20) crystal is investigated. We show experimentally that the sensitivity of an AHI using the non-Bragg orders of diffraction in a thin photorefractive material is more than an order of magnitude greater than that of an AHI employing two-wave mixing in photorefractive volume holograms.

Non-contact profiling for high precision fast asphere topology measurement

Quality control in the fabrication of high precision optics these days needs nanometer accuracy. However, the fast growing number of optics with complex aspheric shapes demands an adapted measurement method as existing metrology systems more and more reach their limits. In this contribution the authors present a unique and highly flexible approach for measuring spheric and aspheric optics with diameters from 2mm up to 420mm and with almost unlimited spheric departures. Based on a scanning point interferometer the system combines the high precision and the speed of an optical interferometer with the high form flexibility of a classical tactile scanning system. This enables the measurement of objects with steep or strongly changing slopes such as “pancake” or “gull wing” objects. The high accuracy of ±50nm over the whole surface is achieved by using a full reference concept ensuring the position control even over long scanning paths. The core of the technology is a multiwavelength interferometer (MWLI); by use of several wavelengths this sensor system allows for the measurement of objects with polished as well as with ground surfaces. Furthermore, a large absolute measurement range facilitates measuring surfaces with steps or discontinuities like diffractive structures or even segmented objects. As all the measurements can be done using one and the same system, a direct comparison is possible during production and after finishing an object. The contribution gives an insight into the functionality of the MWLI-sensor as well as into the concept of the reference system of the scanning metrology system. Furthermore, samples of application are discussed.

Measurement of the enhanced evanescent fields of integrated waveguides for optical near-field sensing

The sensitivity of an integrated optical sensing device can be enhanced by coating it with a high refractive index layer, while both incoupled intensity and spatial resolution are maintained. The potential for enhanced sensing is demonstrated using titanium indiffused waveguiding structures in LiNbO(3) coated with a TiO(2) film. To the best of our knowledge, it could be measured for the first time that the outcoupled intensity at the surface was enhanced by a factor of 12-15 while keeping the penetration depth of the evanescent field constant of the order of only a few tens of nanometers. The evanescent fields of the guided modes were measured and characterized with a scanning near-field optical microscope and are in accordance with the numerical simulations.