Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-01-16
2002-04-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S275000, C252S586000, C544S069000
Reexamination Certificate
active
06366388
ABSTRACT:
DESCRIPTION
The invention relates to a light modulator having a photochromic layer, which can be activated optically by control light, for modulating signal light, and at least one optically transparent substrate for the photochromic layer.
BACKGROUND OF THE INVENTION
Light modulators of the aforementioned type are also denoted as optically addressable, spatial light modulators. Although the photochromic layer is “addressed” optically only two-dimensionally and not three-dimensionally, it is usual to talk of a spatial light modulator instead of a planar one. Such light modulators are denoted below as OASLMs.
The photochromic layer serves the purpose of transmitting or conveying information from the control light to the signal light. In the event of irradiation with control light of a predetermined first optical wavelength, the photochromic layer reacts at the site of irradiation with a change in specific optical properties—in particular with a change in the optical absorptivity—for signal light of a predetermined second optical wavelength. For example, the control light can be used to project an intensity contrast image onto the photochromic layer, which then reacts with a setting, corresponding to the contrast image, of its absorptivity as regards the signal light beyond the area irradiated by the control light. If the photochromic layer thus activated optically by control light is irradiated with signal light, the signal light emerging from the photochromic layer has a modulation corresponding to the absorption contrast pattern. Information from the control light can therefore be transmitted onto the signal light in a planar fashion in a way which varies with time. The signal light striking the photochromic layer can be an extended light bundle which simultaneously covers the entire light entry area of the photochromic layer. A corresponding statement holds for the control light. However, it is also possible to “write” the relevant information into the photochromic layer with the aid of a deflectable control light beam. Likewise, it is also possible for the purpose of “reading” or “erasing” the information to make use of a signal light beam or “erasing light beam” which, for example, scans the photochromic layer by row or by column.
A multiplicity of photochromic materials come into consideration for such applications. An overview of the essential photochromic material classes, their best known representatives and their properties is to be found in H. Dürr, H. Bouas-Laurent, “Photochromism—Molecules and Systems”, Studies in Organic Chemistry, Elsevier, Vol. 40, 1990. In addition to other photochromic materials such as, for example, synthetic inorganic and organic photochromics, bacteriorhodopsin in the form of a-purple membrane, denoted below as BR, is a particularly interesting material for forming the photochromic layer. Purple membrane is the form used for the naturally occurring two-dimensional crystalline form of bacteriorhodopsin. The design of the so-called purple membrane from lipids and bacteriorhodopsin is described in numerous examples in the literature. D. Oesterhelt et al., Quart. Rev. Biophys., 24 (1991) 425-478 may be cited by way of example as a reference.
It is, inter alia, the following five reasons which render bacteriorhodopsin particularly suitable for the application outlined.
(i) BR is distinguished by a very efficient photochemical reaction with several photoactive states which render it possible to implement “writing” and “erasing” photochemically.
(ii) BR has a particularly high reversibility, and this predisposes it to dynamic use.
(iii) The specific absorptions of the long lived states of BR, and also the difference in refractive index between these states are very high, and so good modulation of the signal light is achieved.
(iv) Bacteriorhodopsin has a strongly anisotropic chromophore and is therefore suitable for polarization-selective modulation.
(v) Apart from the wild type of BR, there is currently available a whole series of variants of BR produced using gene technology and having altered amino acid sequences and/or variants, which contain as chromophores molecules differing chemically from retinylidene radical and have other spectral and/or other photokinetic properties than the wild type, for example different absorption properties and/or substantially longer lived photointermediates.
The material group specified in (v) is denoted below as BR variants. The term bacteriorhodopsin or BR is used in such a way that either the wild type of bacteriorhodopsin or one of the BR variants is understood thereby. Furthermore, the term bacteriorhodopsin or BR is used both for monomeric BR and for BR in the form of purple membranes. BR variants may be obtained with various methods. An overview of known methods for producing mutated bacteriorhodopsins and BR analogs, which are typified by the presence of chromophore groups differing from the retinylidene radical occurring in the wild type, is given in N. Vsevolodov, “Biomolecular electronics—an introduction via photosensitive proteins” (1988), Birkhaüser, Boston, Chapter 3. Typical BR variants of technical interest which are obtained by modifying the amino acid composition of wild type BR are those with a lengthened lifetime of the so-called M state, for example those in which the aspartic acid at position 96 has been replaced or removed or displaced in its position by removal of other amino acids, or those with a high probability of the formation of 9-cis-retinal, for example those in which the aspartic acid at position 85 has been replaced or removed or displaced in its position by removal of other amino acids. Typical technically interesting BR variants which are produced by replacing the retinylidene radical occurring in wild type BR by analog molecules are, for example, 4-ketoretinal and dihydroretinal (Sheves et al., Biochem., 24, 1985, 1260-1265). It may be pointed out expressly that a combination of modifications of the amino acid composition and replacement of the chromophore group is also understood by the term BR variants.
Said possibilities and properties of BR are known to the person skilled in the art and have also influenced applications of BR in various optical information processing techniques.
The optically active component is formed by the BR layer in OASLMs. The optical modulation is based on the fact that bacteriorhodopsin can be converted from the initial state B (maximum absorption at approximately 570 nm) by irradiation of light of wavelength &lgr;
B
into at least one other spectrally different state. The longest lived state of the photocycle of wild type BR is usually denoted as M state (maximum absorption at approximately 410 nm). Light of wavelength &lgr;
M
can be used to convert said state photochemically into the initial state B. Consequently, light in the wavelength region of &lgr;
B
can be varied and/or controlled, or else vice versa by simultaneous illumination of the BR layer with light in the wavelength region of &lgr;
M
, using the BR layer as a mediator.
The degree of modulation depends in this case on the magnitude of the photochromic optical absorption changes which is caused by the irradiation of light in the BR layer, on the quantum yield of the phototransformations B
M and the intensities and wavelengths of the two optical irradiations. Because of the polarization-sensitive photoreaction of the BR, the relative position of the polarization states of the two wavelengths or wavelength regions likewise plays a role in the level of the modulation. Furthermore, the local refraction index, which can likewise be used for modulation purposes, is modulated in a manner proportional to the absorption modulation.
OASLMs have been known for a long time as active optical components in beam paths for the purpose of optical processing of images and information, and are used to control and/or to modulate the amplitude, the phase and, if appropriate, also the polarization of a spatially extended lightwave field as a function of the intensity of a control light source.
An overvi
Choi William
Epps Georgia
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