This paper, “State-of-the-art of photorefractive holographic interferometry and potentialities for space applications," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
In Lippmann photography, the interference of the image with its reflection onto a mirror in contact with the photographic
emulsion allows, for each pixel of the image, the recording of Bragg gratings. Removing the mirror, processing the plate
and reading out these Bragg gratings with a white light source diffracts the very colours used for recording and thus
reproduces the images in colours. Using Lippmann photography as a data storage technique was proposed in the 1960th:
for a given pixel, and to each recording wavelength is associated one bit of data, several bits being recorded at the same
pixel. In this paper, we revisit this data storage technique and we propose and demonstrate an homodyne detection to
improve the efficiency of Lippmann data storages. The proposed homodyne geometry also presents the advantage to
simplify the architecture: the Lippmann mirror required for recording is kept in place for data retrieving. Such an
homodyne readout could also be applied to enhance the detected signals in other holographic approaches.
We have investigated a three-dimensionally periodic (3D) photonic crystal structure based on an epitaxial periodic GaAs/Al0.93Ga0.07As multilayer structure that was designed for non-linear optical interactions. The 3D photonic crystal structure consisted of a two-dimensionally periodic planar photonic crystal hole pattern etched into the one-dimensionally periodic multilayer structure designed for a centre wavelength of λ = 1.6 μm. Numerical simulations on the 3D PhC structure have shown that it should exhibit slow group velocity modal features near the edge of the photonic bandgap.
Interferometric measurements for in-vivo imaging of biological tissues are strongly sensitive to the related speckle decorrelation time tc, whose effect is to reduce the contrast of the speckle pattern at the exit of the sample and thus blur detection. Though
acousto-optic imaging is a well suited technique for the case of
thick tissues, it has been shown that an acquisition rate in the
1-10kHz range is required for a good efficiency. We have previously built for this purpose an holographic setup that combines a fast but large area single photodetector and a photorefractive crystal, in order to measure a real-time acousto-optic signal by the so-called self-adaptive wavefront holography technique. In such a configuration, one critical point is the time response tPR of the
photorefractive effect, which depends on the photorefractive
configuration of the setup as well as the light intensity within
the crystal. We have developed an original in situ method that determines this time in measuring the acousto-optic response through a combination of an amplitude modulation of the ultrasound and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. This offers the possibility to adapt the response of the setup to the decorrelation time of the medium under study, and also to have a measurement of τc.
A new concept of acousto-optical imaging is emerging based on an interferometric setup containing a photorefractive crystal as the
recombination plate, and a single detector. This wavefront-adaptive holography technique is promising since the measurements are made in real-time with a high flux collection and at a high rate, faster than the speckle decorrelation time. We present here a detailed model that describes correctly the measured signal, whether in a temporal phase or amplitude modulation of the ultrasound.
The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction limited, linearly polarized beam by a self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO3). Up to 11.6W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency.
Many industrial applications require high power semiconductor laser sources emitting beams of good quality. However, the emission of a free running high-power broad-area semiconductor laser contains many lateral modes that explains its poor beam quality and low brightness. One of the techniques to improve beam quality consists in placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. Such a technique has proved its efficiency to produce a nearly diffraction limited beam at least for low pumping current level. For higher pumping currents, its spatial quality is deteriorated by the oscillation of higher order extended cavity modes. Using numerical simulations, we demonstrate that the insertion of a photorefractive crystal inside a broad-area laser diode extended cavity should extend the laser operating single mode range.
Photorefractive crystals offer many advantages over other classical holographic recording media. They allow fast in-situ processable holographic recording as well as indefinite reusability. Moreover high signal-to-noise ratio can be reached as well as high resolutions. We present the results obtained by pulsed holographic interferometry obtained with photorefractive crystals. First experiments showing the potentiality of photorefractive crystals for such applications will be reviewed. Some limitations were pointed out and led us to consider new developments under the frame of an European-funded project. The PHIFE European project (Pulsed Holographic Interferometer for the analysis of Fast Events) aims at developing a full field measurement system at a high repetition rate, based on a double-pulse YAG Q-switch laser. This device will be used for the analysis of vibrations and for aerodynamic studies. Different crystals are considered which are adapted to the fundamental line of the YAG (AsGa or CdTe crystals) or after frequency doubling at 532 nm (Bi12SiO20). Different crystal configurations and recording geometries are compared. Also different innovative phase quantification techniques are studied and take into account some special properties of the PRCs. We present the results obtained so far in the first development phases of the project.
The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction limited, linearly polarized beam by a self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO3). Up to 11.6W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency.
We present a new holographic vibrometer based on dynamic holograms written in a GaAs crystal. It measures the instantaneous velocity of a vibrating target even with a rough surface. It is adapted to the measurement of high amplitude (as high as some mm) low frequency (until some 10 kHz) vibrations.
We will present the basic properties of photorefractive crystals and show how they can be used efficientl in holograpic interferometry experiments. We then will present some holographic systems and their numerous applications. With continuous illumination, we will show classical non destructive testing (defect detection), displacement metrology, vibration mode shape visualization, as well as a study for the use of photorefractive crystals in microgravity monitoring of fluid (FSL) experiment aboard the International Space Station). With pulsed illumination we will present the possibilities of the photorefractive crystals in vibration measurement (and will introduce the ongoing EC-funded PHIFE research project). The purpose of the paper is to unmystify photorefractive crystals and to show that they could be a good alternative to traditional speckle-based techniques for highly demanding applications. For that we will highlight the highest resolution achieved as well as the high temporal dynamics of the holographic recording.
Dramatic reduction of the number of oscillating modes in CW and pulsed lasers has been observed using a photorefractive crystal placed inside the cavity. Reflective Bragg gratings recorded by the standing waves inside the crystal act with the output coupler as a self-adapted Fabry-Perot filter. Its spectral characteristics enhance mode competitions which can lead to a single mode operation.
In this paper we describe the realization and the operation of a high capacity optoelectronic neural network implementing a classification of vectors through a Kohonen topological map. The setup uses volume holographic interconnects inside a photorefractive crystal to implement the neurons. We show that the system work and is able to classify several tens of vectors.
We present a characterize theoretically and experimentally a photorefractive velocimeter. This device, based on two wave mixing in a rapid photorefractive crystal, measures the instantaneous velocity of a vibrating target. It is particularly adapted to the measurement of high amplitude (as high as some mm) low frequency (until some kHz) vibrations. Instantaneous velocity as high as 25 mm.s-1 are expected to be measured with common photorefractive semiconductors and CW lasers.
It is well known that the conservation of energy in an optical system can be described by an invariant (intensity of the optical wave) during propagation. We identically show that reciprocity can be described by an invariant. This invariant is the overlap integral of two counterpropagating waves, that stays constant in every place of a reciprocal optical system. Applied to the double phase conjugate mirror (DPCM) this invariant can be rewritten as an equality between the ratio of the conjugation fidelity of the two ports of the DPCM and the ratio of the transmission in intensity of the two beams. If restricted to a plane wave case this relation becomes the well known equality of the diffraction efficiencies in both directions. We have implemented an experimental set-up that allows to show that the double phase conjugate mirror is reciprocal what confirms all the above discussion. We use the same set-up to measure the conjugation fidelity of the DPCM.
The wall free growth of CdTe by the Bridgman technique under microgravity was performed. This phenomenon called Detached or Dewetting growth was studied in the flight mission STS- 95. At this mission two CdTe crystals were grown co-doped with vanadium and zinc. Identical samples were grown with different ampoule designs. The effect of Detached or Dewetting growth was observed by analysis of crystal surface and roughness by optical and mechanical methods. All results of the crystals grown under microgravity are compared to the earth grown reference samples. Surface differences can be found between the (mu) g and the 1g samples. The surface roughness measurements demonstrate that the detaching was partially successful.
Laser-based ultrasonic systems for non destructive testing and evaluation rely on the coherent detection of phase modulation imprinted on the beam by the vibrating surface. As this detection principle requires an exact superposition of the wave issued from the target with a plane reference wave, these devices only operate with a plane wave signal beam. This constraint strongly limits the spread of laser ultrasonic systems in industry. We will show how the use of dynamic holographic materials allows to implement adaptive interferometers with a large light gathering power. Two different systems that have recently led to industrial developments, will be presented and compared. Requirements for the dynamic material to be used will be discussed. Performances for ultrasound detection will be finally given.
It is now well known that rhodium doped barium titanate (BaTiO3:Rh) exhibits a significant photorefractive response at near infrared wavelengths .We studied and characterized this crystal at 1 .06 jtm by two-wave mixing experiments. In a 45°-cut crystal with a low absorption (0.1 cm1), we measured a photorefractive gain F of 23 cm1 with cw illumination and 16.6 cm1 with nanosecond illumination. Using spectroscopic determinations of the photorefractive sites 2(Rh3+, Rh4+, RhS+), we demonstrated that the photorefractive properties of BaTiO3:Rh are well described by a three charge state model '. Internal parameters of the material were derived using these experimental characterizations which allowed to accurately predict its performances at 1 .06 tm. Comparative characterizations of several BaTiO3 :Rh samples proved that this material is now well reproducible. Reproducibility, high photorefractive gain, low absorption and accurate theoretical description make BaTiO3:Rh a good candidate for realization and optimization of non linear functions like optical phase conjugation. The application we are interested in, is the dynamic wavefront correction of nanosecond Nd:YAG master-oscillator power amplifier (MOPA) laser sources. We implemented a ring self-pumped phase conjugate mirror using a BaTiO3:Rh crystal . This geometry brings several advantages. The threshold in terms of "gain X interaction length" product, is low (F1=2) 6 This phase conjugate mirror does not require a source of long coherence length 7and the gratings involved in the four wave mixing process can be well controled. Moreover the phase conjugate beam can be efficiently selected among the backscattered light by inserting optical elements in the ring 8,9• Such a self-pumped mirror is self-starting and the four-wave mixing process is initiated by the beam-fanning. To avoid spurious internal oscillations in total reflection on the crystal faces that initially developed in several of our experiments and prevented phase conjugation we optimized the geometry of the crystal. Roof-cut, 45° orientation of the c axis and antireflection coatings of the crystal suppressed these internal oscillations and efficient phase conjugation was demonstrated.
Photorefractive rhodium doped barium titanate (BaTiO3:Rh) is now well known for its significant response at near infrared wavelengths .We studied and characterized this crystal at 1.06 j.tm. By twowave mixing experiments in a 45°-cut crystal, we measured a maximum photorefractive gain F of 23 cm1 with cw illumination and 16.6 cm1 with nanosecond illumination, together with a low absorption (0. 1 cm1). Using spectroscopic determinations of the photorefractive sites 2 (Rh3, Rh4, Rh5), we showed that the photorefractive properties of BaTiO3 Rh could be well described by a three charge state model .This enabled to determine the internal parameters of the material using experimental characterizations and to accurately predict its performances at 1 .06 tm. Comparative characterizations of several BaTiO3:Rh samples proved that this material is now well reproducible, which is of prime importance for applications. Reproducibility, high photorefractive gain, low absorption and accurate theoretical description make BaTiO3:Rh a good candidate for realization and optimization of non linear functions like optical phase conjugation. The application we are interested in, is the dynamic wavefront correction of nanosecond Nd:YAG master-oscillator power-amplifier (MOPA) laser sources.
KEYWORDS: Spatial light modulators, Holograms, Image processing, Signal processing, Signal to noise ratio, Holography, Modulation, Multiplexing, Optical storage, Data storage
SLMs' finite contrast causes noise which bounds the number of storable holograms. We present a method to reduce this noise and increase the storage capacity.
We present and describe different techniques based on the photorefractive effect that are used for the optical detection of ultrasonic signals. These techniques use the wavefront adaptation properties of the photorefractive effect. They are: the photorefractive beam combiner, the double phase conjugate heterodyne detection and the adaptive photodetector based on non steady state photoelectromotive force. Their respective advantages and drawbacks, are overviewed. We insist on the latest development and performances obtained with the photorefractive beam combiner that seems to use the most promising technique for the detection of ultrasonic signals on rough surfaces. We show that a sensor with near optimum sensitivity can be developed with the same photorefractive crystal at different wavelength in the range of 1 micrometer to 1.55 micrometer.
We present and describe two techniques used for optical detection of ultrasonic signals based on the photorefractive effect. These techniques used the wavefront adaptation properties of the photorefractive effect. In the photorefractive beam combiner, a local oscillator matched to the signal wavefront is created, leading to an homodyne detection system having a large etendue. In the double phase conjugate heterodyne detection system the signal beam wavefront is cleaned by a double phase conjugate mirror and transformed in a plane wave that is sent on a classical heterodyne detection system. Both systems are characterized and used to detect ultrasound.
Light induced changes of optical properties are in close relation with photoelectrical properties of semiconductors. From the dynamics of nonlinear optical response, not only electronic parameters of crystals but also the channels of energy relaxation, role of defects and mechanisms of light interaction with matter may be studied. Light diffraction on laser induced transient gratings has proved to be a very powerful technique for these fundamental swdies of semiconductors as well as for their applications in opto- and microelectronics. Photorefractive Ill-V and il-VI semiconducting compounds reveal optical nonlinearities of different origins under excitation by short las pulses or by CW lasers [1,2]. The contributions to refractive index modulation by free carriers and by space charge electric fields coexist at pulse excitation while solely linear electrooptic effect is responsible for optical gain in a quasi-stationary case of excitation. Carrier plasma kinetic coefficients, as ambipolar and monopolar mobiities, carrier concentrations and lifetimes, defect concentration and other parameters may be obtained from light diffraction on transient grating experiments [3-7]. In addition, at short pulse excitation a number of novel effects have been obsved: light induced space-charge (SC) field enhancement [8], fast oscillations of diffraction efficiency and multiexponential grating decay [9,10]. These effects probably arise from simultaneous presence of two refractive index modulation mechanisms and their interaction. Indeed, the diffusive decay of free carrier grating leads to the build-up of space charge electric fields, which in turn involve electron and hole sal separation, screening of space charge fields, and affect the decay processes of electronic gratings. A significant step towards the understanding of the temporal behavior of nonlinearities involved as well as possibilities of those optical techniques for semiconductor research may be obtained by help of a detail analysis of carrier and field dynamics in different experimental situations.
After a rapid introduction that indicates the main reasons for the renewal of interest for photorefractive holographic memories, we will first present and compare both the coding techniques and recording procedures used for storage of superimposed images. Potentialities and limitations that are relevant to existing photorefractive crystals will be then discussed. Refreshing procedures that allow to operate the dynamic memory without loss of information will be also described. We will then devote the major part of the article to discussing applications we envision for photorefractive holographic memories.
We proposed to periodically refresh the images stored in dynamic holographic memories by using an opto-electronic feedback loop. Readout of the images without loss of information is experimentally demonstrated in a photorefractive LiNbO3 sample.
After a rapid introduction that presents the main reasons for the renewal of interest for photorefractive holographic memories, we first discuss their potentialities and limitations. We then continue by a presentation and analysis of novel techniques that permit considerable facilitation of the memory operation and improve its capabilities. In conclusion, we indicate envisioned applications for photorefractive holographic memories.
We initially study a peak of the photorefractive two-wave mixing gain which appears for a particular frequency of a sinusoidal applied electric field. The optimization of the process by applying periodic pulsed electric fields leads to a considerable rise of the two-wave mixing gain. The authors demonstrate that the gain reached with this technique is higher than the maximum gain obtained with usual enhancement techniques. Effectively, with a Bi12GeO20:Fe sample, a gain of 10 cm-1 is obtained, compared to 2 cm-1 in the same crystal with the other techniques.
Energy transfer between nanosecond light pulses interacting in a semi-insulating InP crystal is analyzed. Optically induced charge redistribution in the iron impurity states are shown to be the source of both photorefractive and absorption nonlinearities.
optical wave mixing in nonlinear media such as photorefractive crystals, semiconductors and
liquid crystals have been vigorously studied in the past few years (1-4). Such researches are motived
either by potential applications like amplification and phase conjugation, or by investigation on the
physics of such effects.
Here we report on the generation of conjugate wavefronts by degenerate forward and backward
four wave mixing in some GaAs samples. We perform our experiment with a Nd-YAG laser
emitting pulses of eight nanosecond duration. The semi-insulating GaAs sample used exhibits a large
concentration of EL2 defects which present a good photoionisation cross section at 1.06 micron.
The nonlinearity we observed is due to a hole and electron grating photogenerated from the EL2
defects. This is quite different from direct band to band absorption. Indeed, the hole and electron
distributions are coupled through the concentration of ionized EL2+ and neutral EL2° defect.
Nevertheless, as we will show,at high energy the densities of free electrons and holes tend to be
equal due to an evolution of the respective concentration of EL2+ and EL20. This plasma of holes
and electrons modifies the plasma frequency. The modulation of the index is then derived from the
DrUde Lorentz model.
We first present our experimental results .We show that most of them can be explained by the
kinetics of the carriers photogenerated from deep levels. Analytical and numerical results are
presented. From the numerical simulation, with these numerical results and semi analytical solutions
of the wave propagation equations, one then predicts the reflectivity of our multiwave mixing
experiment.
optical wave mixing in nonlinear media such as photorefractive crystals, semiconductors and
liquid crystals have been vigorously studied in the past few years (1-4). Such researches are motived
either by potential applications like amplification and phase conjugation, or by investigation on the
physics of such effects.
Here we report on the generation of conjugate wavefronts by degenerate forward and backward
four wave mixing in some GaAs samples. We perform our experiment with a Nd-YAG laser
emitting pulses of eight nanosecond duration. The semi-insulating GaAs sample used exhibits a large
concentration of EL2 defects which present a good photoionisation cross section at 1.06 micron.
The nonlinearity we observed is due to a hole and electron grating photogenerated from the EL2
defects. This is quite different from direct band to band absorption. Indeed, the hole and electron
distributions are coupled through the concentration of ionized EL2+ and neutral EL2° defect.
Nevertheless, as we will show,at high energy the densities of free electrons and holes tend to be
equal due to an evolution of the respective concentration of EL2+ and EL20. This plasma of holes
and electrons modifies the plasma frequency. The modulation of the index is then derived from the
DrUde Lorentz model.
We first present our experimental results .We show that most of them can be explained by the
kinetics of the carriers photogenerated from deep levels. Analytical and numerical results are
presented. From the numerical simulation, with these numerical results and semi analytical solutions
of the wave propagation equations, one then predicts the reflectivity of our multiwave mixing
experiment.
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