Bias-free time-integrating optical correlator using a photorefractive crystal

Yu, Jeffrey W.; Hong, John

doi:10.1364/ao.24.003860

Cited by 32 publications

(3 citation statements)

References 5 publications

(3 reference statements)

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“…Timeintegration is achieved since the hologram forms with a characteristic time constant which is proportional to the incident intensity. Mathematically, the amplitude of the index grating that is formed is given by M(x,t) (1) where s (t) and s2(t) are the electrical signals driving the two AODs, V is the velocity of acoustic waves traveling in the xdirection inside the AODs, 'r is the photorefractive material response time, ? is the wavelength of light and 2 is the full angle between the two writing beams incident on the crystal.…”

Section: Photorefractive Correlatormentioning

confidence: 99%

<title>Adaptive signal processing with photorefractive systems</title>

Hong

Chang

1995

SPIE Proceedings

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Radio frequency (RF) signal processing systems that use photorefractive crystals to provide key functions are described. Some new experimental results obtained with a time integrating correlator developed to support the acoustooptic null steering processor effort [111 are presented which demonstrate the effectiveness of the photorefractive time integrating approach. We also describe a new adaptive notch filtering system which provides effective and flexible excision of narrowband interference. The system architecture makes optimal use of the unique properties of the photorefractive effect to provide a simple yet highly effective solution for the adaptive signal processing problem.The processing of radio frequency (RF) signals has been realized in many forms with the use of acoustooptic deflectors (AOD) to impress the information both spatially and temporally onto optical carriers. For the particular application of signal correlation, time integration techniques using detectors with integrative temporal response have been successful where long integration times are necessary. The conventional approach uses charge-coupled device (CCD) detector arrays where photogenerated charge is accumulated in potential wells formed for each detection element to perform the necessary integration. One of its drawbacks is the undesirable background bias that builds up in each detection element which limits both the integration time and the dynamic range of the system. These problems can be circumvented by the use of a photorefractive crystal as a holographic, time-integrating detector as has been reported [1-3J. A schematic diagram of that technique is shown in Figure 1 ; the system consists of two AODs (one for the input signal and one for the reference signal), imaging lenses, a photorefractive crystal and a linear detector array. The diffracted light from the two AODs is imaged onto the photorefractive crystal, each incident at a different angle so that a holographic grating is formed within the crystal. Timeintegration is achieved since the hologram forms with a characteristic time constant which is proportional to the incident intensity. Mathematically, the amplitude of the index grating that is formed is given bywhere s (t) and s2(t) are the electrical signals driving the two AODs, V is the velocity of acoustic waves traveling in the xdirection inside the AODs, 'r is the photorefractive material response time, ? is the wavelength of light and 2 is the full angle between the two writing beams incident on the crystal. A weak readout beam is Bragg-tuned to read out the hologram, and the diffracted output that carries the correlation information is imaged onto a linear detector array. Since the readout beam is coherent, further optical processing of the output can be performed directly if so desired. In Martin Marietta's acoustooptic null steering processor (AONSP) system, such a photorefractive correlator serves as an important link in the adaptive approach to sidelobe cancellation for a phased array antenna. The details of the ...

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Section: Photorefractive Correlatormentioning

confidence: 99%

<title>Adaptive signal processing with photorefractive systems</title>

Hong

Chang

1995

SPIE Proceedings

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“…Une autre innovation importante pour les systèmes est l'utilisation, en amont des photodétecteurs, d'une pré-intégration temporelle sur cristal BSO, qui permet d'éliminer la composante continue préjudiciable aux convoluteurs à intégration temporelle [6]. Ce dispositif est applicable à tous les modulateurs spatiaux à déformation de surface (membrane, thermoplastiques), qui permettent de soustraire un niveau ajustable de la composante continue d'éclairement, durant une intégration temporelle, pour optimiser la dynamique de modulation.…”

Section: Modulateurs Volumiques Acousto-opti-unclassified

Le point sur les modulateurs spatiaux de lumière

Lebreton

1987

Rev. Phys. Appl. (Paris)

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Les nouvelles technologies qui apparaissent aujourd'hui pour la modulation spatiale de lumière font l'objet d'une revue critique. Les divers types de modulateurs sont classés du point de vue de leur utilisation dans des architectures de processeurs optiques : modulateurs unidimensionnels pour systèmes optiques tri-dimensionnels ou à ondes guidées ; modulateurs bidimensionnels à adressage électronique ou optique ; mode d'opération analogique, binaire ou non linéaire. Une véritable révolution est observée en direction de « technologies compatibles », où matériaux et techniques de fabrication tendent à devenir très similaires dans la réalisation de modulateurs de lumière ou de circuits électroniques à haut degré d'intégration. Il est suggéré que le fossé séparant les actuels processeurs hybrides opto-électroniques et les nouvelles portes logiques « tout-optiques » sera bientôt comblé par une nouvelle génération de convertisseurs parallèles électrons-photons totalement intégrés

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“…This aspect of photorefraction, which derives from the fact that photons must be absorbed to generate the charged carriers that are responsible for setting up the gratings in the crystal, has been used to advantage in applications which require timeintegration as a processing tool. In particular, photorefractive crystals have been used in conjunction with acoustooptic devices to implement time-integrating signal correlators for RF (radio frequency) signals [6]. The correlator is a basic signal processing module that can be inserted in a more complex system such as adaptive filters [7,8], which use such devices to estimate the statistics of the signal environment.…”

Section: Discussionmentioning

confidence: 99%

Photorefractive optical information processing

Hong

Proceedings of LEOS '93

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Invited) Room B3Applications of photorefractive devices for radio frequency signa, processing problems are reviewed. The problems of signal correlation, filtering and adaptive filtering are addressed using the time integrating property of grating formation in photorefractive crystals. The dynamic range of photorefractive crystals is sufficiently large for critical applications such as adaptive signal processing. New system architectures that use photorefractive two-beam coupling to substantially simplify overall complexity are shown and demonstrated for the problems of tunable notch filtering and adaptive notch filtering. SummaryDynamic holography and its associated nonlinear optical phenomena using photorefractive crystals have been investigated for optical information processing applications ranging from volume holographic memory [ 1,2] to optical neural network implementations [3-51. The ability to record an efficient hologram in real time without the need for chemical or thermal processing and then to store it for extended periods of time is a unique property of photorefraction which holds promise for information processing tasks.

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Bias-free time-integrating optical correlator using a photorefractive crystal

Cited by 32 publications

References 5 publications

<title>Adaptive signal processing with photorefractive systems</title>

<title>Adaptive signal processing with photorefractive systems</title>

Le point sur les modulateurs spatiaux de lumière

Photorefractive optical information processing

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