Arrangement for the spectral dispersion of light; device for optical information-transmission using such arrangements; device for storing optical images using such arrangements.

The invention relates to an arrangement for spectral decomposition of light according to the preamble of claim 1. A preferred area of ​​application is optical message transmission.

For many applications in optics (eg spectral analysis or optical information processing) spectral devices are required with which multicolored (white) light of the widest possible frequency range can be broken down with the greatest possible spectral resolution and at high speed. Optical message transmission is considered as an example of the requirements set here.

Optical transmission channels for transmitting images or data in principle have a very high transmission capacity due to the very high frequency of the light. The disruptive propagation losses of light could be mastered by the development of optical fibers ("optical fibers") with very little attenuation; Since then, interest in optical communications has grown significantly. Most of the systems proposed so far use temporal sequences of light pulses that are sent via the optical fiber for message display. However, these light pulses cannot now be generated with any frequency. In the case of a monochromatic light source, the limitation of the pulse repetition frequency is determined, on the one hand, by the modulability of the light source or the switching frequency of the modulator (in both cases currently wuximal 10 Hz) and, on the other hand, by the type of propagation of the light pulses in the optical fiber: the different types of propagation one Impulses in a fiber, the so-called propagation modes, suffer differently large delays during transmission^Gs, so that pulse smearing or even mutual overlapping of pulses occurs. This last-mentioned disadvantage can be eliminated with the help of so-called single-mode fibers, in which the impulses have only one possibility of propagation. This type of fiber is expensive to manufacture, and besides other disadvantages, only lasers can be used as light sources, which in turn have the disadvantage of a limited lifespan. However, the theoretically possible transmission capacity cannot be achieved in this way either because of the limited modulability.

To increase the bandwidths, the transmission can also be carried out with white light, which is divided into a plurality of spectral channels with the aid of dispersing elements (corresponding to the frequency division multiplex method in communications technology), each of which is influenced by optical modulators. As mentioned, operating frequencies up to a maximum of 100 MHz can be achieved with the known modulators. Prisms or gratings are available as devices for spectral laying. The maximum spectral resolution of a prism is approximately<maths id="math0001"><img file="EP0031027A2_D0001.tif" /></maths>ie at a light frequency of 10¹⁵ Hz can 10⁴ Spectral range Δλ with a frequency width of 10 each¹¹ Hz are generated. A significant part of these frequency ranges remains unused due to the limited modulation frequency. The number of available data channels could be a factor<maths id="math0002"><img file="EP0031027A2_D0002.tif" /></maths> the spectral resolution could be refined.

A higher resolution than with prisms can be achieved with diffraction gratings; for a grating with a number N = 1000 grating lines and when observed in the kth order of the spectrum, e.g. B. K = 1000, the resolution is<maths id="math0003"><img file="EP0031027A2_D0003.tif" /></maths>

The number of transmission channels could therefore be increased by a factor of 100 compared to the prism.

However, the higher resolution of the grating cannot be used, since here the individual spectra of different orders overlap when illuminated with a relatively broad frequency band (white light) and when observing a spectrum in a high order and thus no pure spectral colors, but only mixed colors. In the case of grating spectrographs with high resolution, the light to be examined is therefore pre-split so that only a very narrow frequency band strikes the grating. In the case of a transmission system, pre-decomposition would in turn greatly reduce the number of usable data channels. The pre-decomposition of the light is also annoying when working with spectral analysis devices when larger frequency ranges have to be examined.

In addition to pulse transmission, other optical transmission devices have become known which aim at direct optical imaging via the optical fiber. An obvious example is the combination of a large number of optical fibers to form a light cable. The resolution of the resulting ride is then determined by the number of light fibers used.

The transmission of an image by a single optical fiber has also been proposed in the prior art. Without further aids, this type of optical transmission does not produce a sharp image in the fiber due to the dispersion of the phase velocity (which also leads to the already mentioned pulse smearing). To avoid this difficulty, the article "Parallel image transmission by a single optical fiber" in Optics letters, Volume 2, No. 5, May 1978, page 133, proposed to use an optical fiber with a specific course of the refractive index gradient and to encode the image to be transmitted spectrally and spatially. The resolution of this arrangement is low. German patent specification 1 299 902 describes an image transmission via an optical ribbon cable consisting of several optical fibers, in which dispersing elements are arranged at both ends of the transmission path. The limited spectral resolution is partially compensated for by the multiplication of the transmission fibers.

In view of this prior art, the object of the invention is to provide an arrangement for the spectral decomposition of light which supplies a very large number of very narrow-band frequency ranges which are processed in parallel; in particular, this is intended to provide an arrangement for optical message transmission in which the high theoretical transmission capacity of light can largely be used as an information carrier.

This task is solved by the invention characterized in the main claim. Applications and refinements of the invention are characterized in the further claims.

) he split the white view used for transmission into a very large (10th⁶) Number of single, very like (<maths id="math0004"><img file="EP0031027A2_D0004.tif" /></maths>= 10) Spectral ranges, two optical grids connected in series and arranged perpendicular to each other are used; the first grid generates a spectrum of relatively high order (eg 100 to 1000), the second grid generates a spectrum of relatively low order (eg 1 to 10). The combined effect of the two gratings results in a high-resolution spectral decomposition: The series connection of two gratings, which generate spectra of different orders, eliminates the overlap of the high-order spectra in a single grating. This allows the usable frequency range of a grating to be maintained without loss of resolution.

In an image plane behind the two grids there is a clear association between the geometric image point and the frequency.

In an optical transmission system with an optical fiber as the transmission medium, the coding of the image on the input side of the optical fiber takes place with the aid of two such crossed gratings; At the exit end of the fiber there are also two crossed gratings which are used for decoding and restore the intensity distribution which is present on the input side in the image plane.

If data are to be transmitted, rather than image templates, then a modulator matrix, whose individual points (light valves) can be controlled electrically, takes the place of the image.

The transmission speed of an optical transmission system designed in this way is in the tera baud range.

The achievable high resolution of crossed t-arranged grids can be used for other optical components as well as transmission systems. B. for spectrographs with high resolution, for optical data storage with high working speed or for three-dimensional television.

Exemplary embodiments of the invention will now be explained in more detail with reference to drawings.

Show it:<ul id="ul0001" list-style="none"><li>1 shows the effect of crossed gratings in the frequency plane,</li><li>2A, B, C arrangements of crossed grids with spectra and D different atomic number, which give a clear assignment of location to frequency,</li><li>3 shows the principle of an optical message transmission device with crossed gratings and optical fiber,</li><li>4 shows the basic structure of a transmission device for three-dimensional television images,</li><li>5 shows the basic structure of a transmission device for 2.5-dimensional television pictures,</li><li>6 shows the basic structure of a high-resolution real-time spectrometer, in which crossed gratings are used as dispersion elements,</li><li>7 shows the basic structure of a device for image recording at high speed, in which crossed gratings are used,</li><li>Fig. 8 shows the basic structure of a device for image recording according to Fig. 7 for the generation of volume holograms.</li></ul>

To explain the mode of operation of the invention, FIG. 1 shows the brightness distribution (spectra when illuminated with white light) generated by suitable optical elements such as lenses etc. in the frequency plane of an optical system consisting of two crossed gratings. The grids are aligned along the x and y axes. The spectra must meet the grid condition for positive interference on both grids:<maths id="math0005"><img file="EP0031027A2_D0005.tif" /></maths>(mean ^a₁, α₂ the deflection angles in the x and y directions, M, N the orders of the diffracted rays, d₁, d₂ the lattice constants and λ the light wavelength). The individual spectra arising in the xy plane are identified by their orders (N, M). The first spectrum in the X direction is identified by the index (1,0), the first spectrum in the y direction by the index (0,1), the first spectrum in the first quadrant by (1,1) etc. All Spectra lie on straight lines through the origin (zero order spectrum). The lower-order spectra are separated from each other, so that in these areas of the frequency plane there is a clear association between wavelength and geometric location. However, the extent of a lower-order spectrum is small and thus the number of definable fruity ranges Δλspectrums of higher order have a larger extent, but at the same time this extent leads to overlap of spectra of different orders. The overlap increases with increasing order. For example, x₂ the spectra of the following orders are simultaneously present on the x-axis: 4, 5, 6.

This ambiguity of the frequency assignment to a certain point on the frequency level due to the overlapping of spectra can occur not only on the x and y axis, but also on all straight lines passing through the origin of the axis cross.

With certain combinations of order parameters of the spectra, e.g. B. a spectrum of very high order in the x-direction and low order in the y-direction, areas can be found in the frequency plane in which, despite the high order, there is still no overlap. Several such spectra lying one above the other or next to one another without overlapping then define a frequency plane range in which there is a clear association between the geometric position and the light frequency. (e.g. Area B in Fig. 1.)

Such frequency ranges are now generated according to the invention by crossing two gratings which generate spectra of different orders in the x and y directions. This uniqueness of the frequency assignment to an area (eg B in FIG. 1) of the frequency level (x, y) is present if the following conditions are met:<ul id="ul0002" list-style="none"><li>- in this area no two spectra have the same atomic number ratio (N: M);</li><li>- no color (spectral range) appears twice in this area. Are the limits of area 13 in the x direction with x₀ and x₁ designated (with corresponding wavelengths λ_x0, λ_x1) the following must apply:<maths id="math0006"><img file="EP0031027A2_D0006.tif" /></maths></li></ul>

The formation of this clear assignment can be clearly illustrated by the fact that the strongly overlapping high diffraction orders on the X axis are pulled apart just enough in the y direction by the grating arranged in the Y direction with a larger grating constant and thus lower resolution relevant part of the frequency level is occupied as closely as possible with frequency points. Since the spectra of different X-order involved in the overlap are shifted in frequency with respect to one another, after the overlap has been eliminated, each point in the part of the frequency plane under consideration is clearly assigned to a specific frequency (wavelength) of the white light used for illumination. The individual frequency bands lying one above the other are essentially parallel to one another; although they are all on straight lines of origin through the zero point of the frequency plane, the differences in the slopes of these straight lines are only slight with the large value of X.

The number of raster lines (= spectra one on top of the other) in the unambiguous frequency level range depends on the highest ordinal number of the spectra and on the transmitted spectral range (x₁-x₀) (this has approximately 0.6 octaves in FIG. 1).

The areas with a clear frequency assignment to the geometric position are not limited to the vicinity of the X axis, as shown in the example. According to the same rule, other such areas can also be found in the frequency plane.

An unambiguous frequency range that arises according to this regulation is shown in FIG. 1, for example. If the grating aligned in the x direction produces a spectrum of the order n = 4, 5, 6 and the grating aligned in the y direction generates a spectrum of the order n = 1, the section of the frequency plane shown in part B of FIG. 1 results . In the x direction, the width of the frequency band is approximate<maths id="math0007"><img file="EP0031027A2_D0007.tif" /></maths>= 10^-1. In the y-direction, sections of spectra are stacked on top of one another, the order of which increases in the x-direction. The frequency range of each band is shifted from all others, so that there is a clear assignment of wavelength to qeometric location in the entire section of the frequency plane under consideration. For example, the second band is shifted from longer compared to the first band (with the spectral section from blue to green) towards longer wavelengths and covers the subsequent section from green to yellow. The same applies to frequency bands further up.

If the order of the spectrum in the y direction is chosen to be greater than n = 1, the natural order of the spectral colors is disturbed, which arises in the case n = 1 when progressing from one raster line to the next. The raster lines as components of the complete spectrum (from blue to red) are then mixed.

2 shows constructive configurations of the grids which are preferably to be used. The use of two crossed transmission gratings, as was used as the basis for the description of the principle mode of operation of the invention, is not recommended in practice, since these only provide sufficient intensities for spectra of low order. If a spectrum in a certain high order should have the maximum light intensity, so-called mirror gratings ("blazed gratinas") are available, which have a step-shaped division profile and are designed as reflection gratings.

In FIG. 2A, the light to be separated spectrally impinges from the left, via a lens 10 (optionally available), onto a first mirror grating 11, the surface and grating direction of which are oriented perpendicular to the plane of the drawing. The light diffracted at the first grating strikes a second mirror grating (12), the surface of which is also perpendicular to the plane of the drawing; the grating direction on this surface is parallel to the plane of the drawing. The light, which is spectrally split in both directions, leaves the arrangement via a lens (also optionally available).

Another possibility for producing spectra of very high order is provided by so-called step gratings according to FIG. 2B. The path difference between two interfering beams (ie the order of the interference) is determined by the height Δh of the steps at which the beams are reflected. 2B shows two crossed step gratings 20 and 21, which are each provided with collimator lenses 22, 23. The spectrally coded image of the object 25 can, for example into the inlet opening of an optical fiber 26.

A further spectral decomposition device is shown in FIG. 2C; there are two Perot-Fabry interferometers (27, 27 ') connected in series. The interferometers are slightly tilted against each other (angle δ). The particular advantage of this arrangement is that the interferometers can be designed simultaneously as a dye laser and thus serve as an optical amplification element. For this purpose, a laser-active layer (28, 28 ') is applied between the interferometer plates.

The arrangements discussed with two crossed gratings that produce different diffraction orders break down the incoming white light into a multitude (~ 10⁶) independent optical transmission channels. Each of these channels can be individually modulated and has a very small bandwidth Δν; Dispersion effects, such as occur in optical fibers, are practically without influence for these narrow frequency bands.

All frequency bands generated in this way are processed in parallel; Devices for optical data processing or for optical data transmission that make use of this spectral splitting arrangement therefore have a very high processing speed. Examples of such facilities are discussed below.

3 shows the basic structure of an optical message transmission device which works with spectral coding by means of crossed gratings. A real image 39 is to be generated from a planar object 32 via an optical fiber 38. The object 32 is illuminated with white light (from source 30 via lens 31) and the light emerging from the object is directed into a spectral unit 300 in which two crossed gratings 34 and 35 with associated converging lenses 33 and 36 are arranged. The grating 35, which is also referred to as the “y grating”, is optically closest to the entry opening 37 of the optical fiber and generates a spectrum of very high order. (The arrangement with interchanged gratings is also possible.) The grating division of the x grating 34 is spatially perpendicular to the grating division of the y grating and generates a spectrum of low order. The entrance plane 37 of the optical fiber 38 corresponds to the frequency plane of the spectral device 300. The optical fiber therefore receives only a very narrow frequency band from the different spectra of each object point, so that the frequencies occurring in the fiber are directly linked to the geometric location of the object points.

At the exit end 37 'of the optical fiber, a second spectral device 300' is arranged, which corresponds in structure to the first spectral device 300. The y-grating for generating the high-order spectrum is also optically closest to the optical fiber 38. The spectral decomposition of the frequency mixture present in the optical fiber then results in the reverse image 39 of the real image 39 of the object 32.

The number of independent data channels to be transmitted in parallel in this way is very large. (10⁶.) Each of these data channels can be modulated independently, e.g. B. with the help of an associated electrically controllable light modulator within a modulator matrix, which takes the place of the object 32. Such modulator matrices can be constructed with magneto-optical, ferroelectric or liquid crystal elements. Such electronically controllable "photocomposers" have become known from proposals for optical memories. They can be operated either in transmission or in reflection. A photo receiver matrix consisting of a corresponding number of elements is then set up at the location of the resulting image 39. With this arrangement, for example, digital data can be transmitted at very high speed (in the tera baud range).

In a modification of the arrangement according to FIG. 3, the spectral coding can also take place on the lighting side; the object is arranged behind the spectral device 300 in its frequency plane. A downstream lens collects the coded light onto the entry opening of the optical fiber.

In the case of long transmission paths, the signal may need to be refreshed. For this purpose, a broadband laser amplifier (e.g. dye laser) can be used, as is indicated in FIG. 3 under reference number 41.

The high image transmission speed of such an arrangement enables the realization of three-dimensional television. There it is necessary to view a scene from different angles and the usual frame rate of 25 frames / sec. transferred to. This problem can be solved with an arrangement according to FIG. The object 400 is illuminated with a white light source 401 and the reflected light is applied to an angle deflector 403 via an elliptical mirror 402. This angle deflector can be, for example, a mirror which can be rotated about two axes and which periodically scans the object from different angles. The resulting image, corresponding to a certain angle of view, is passed via a lens 404 to a spectral device 405 with crossed gratings and fed in parallel into an optical fiber 406. The two-dimensional images, scanned one behind the other from different angles, appear as a three-dimensional image in the receiver, which contains a synchronously operated deflection system. Intermediate images are generated at the location of the oscillating mirror and before entering the spectral decomposition arrangement.

FIG. 5 shows an alternative embodiment of a spatial television transmission system. The spectral device 501 is arranged there directly behind the white light source 500 and illuminates the object 504 via a polygon mirror 502 and an elliptical mirror 503. The spectrally split light reflected by the object is transmitted a condenser 505 into an optical fiber 506 and is broken down at the location of the receiver via a further spectral device 507. The light emerging from the spectral device is again passed through a diffuser 508 in the intermediate image plane to a polygon mirror 509 and an elliptical mirror 510 in order to generate the real image 511. This arrangement can be referred to as 2.5-dimensional image transmission, since the two polygon mirrors only allow scanning in one direction. The optical diffuser in the receiver enables full three-dimensional image reconstruction when viewed visually. Diffuser is understood here to mean a focusing screen that is effective in only one direction of the image (the vertical). A glass plate with horizontal scratches, the size, depth and distribution of which vary statistically, acts as such.

The high spectral resolution in the frequency plane that can be achieved with crossed gratings can also be used directly to set up a high-resolution real-time spectrometer. Fig. 6 shows the basic structure of such a device. The light to be examined from a source 600 is passed through a lens 601, a pinhole G02 and a collimator lens 603 to two crossed gratings 604 and 605, of which the second, the y-grating, produces a spectrum of very high resolution. In the frequency plane of this grating arrangement, a (black and white) television camera is attached, which scans the frequency plane in a grid-like manner. FIG. 6B shows the (also unambiguous) assignment of the spectral wavelengths to the scanning surface of the television camera. The video signal as a function of time thus corresponds to the spectrum in the frequency plane. This spectrometer combines the advantages of a very fast working method with high resolution and a wide working range.

If the beam deflection of the television camera is controlled in such a way that it is proportional to the wave number (1 / wavelength), a method for interferometric distance and layer thickness measurement results. This is because the interference spectrum generates a temporally periodic video signal, the layer thickness being proportional to the frequency of this change. If the video signal is fed to a frequency measuring device, the layer thickness can be adjusted extremely precisely over a large measuring range (e.g. a few hundred nanometers to a few decimeters).

Faint or infrared spectra can be made visible by switching on an image converter between the spectral device and the television camera.

Another application of spectral coding relates to a high-speed image recording method. According to FIG. 7, the light of the object 702 spectrally coded by the crossed gratings 705, 706 is imaged on a pinhole 709 and from there reaches a concave grating 710 which generates a spectrum of median order on a screen 711. The grating 710 thus spreads the spectrally coded light back in a one-dimensional direction. If a photosensitive material which moves continuously is attached in the plane 71 1, the information contained in this light can be stored with high resolution.

For recording processes that change over time, it is advantageous not to move the film perpendicularly to the splitting, but rather at a flat angle, in order to make do with a smaller film width and moderate feed speeds, similarly as in video tape technology.

A light source 712, which illuminates the recording material via a lens 713, is used for the reconstruction. In the reversal of the light path with respect to the recording, an image of the object 701 is then created again via the crossed gratings 706, 705.

A particularly high recording density can be achieved if the image is recorded in plane 711 according to the principle of volume holography. In FIG. 8, the light of the light source 800 is split into two partial beams 802 and 803 by a semi-transparent mirror 801, of which the beam 803 is shown in FIG. 7 described way is sent through the object 804 and a crossed grid arrangement 805 (here with two slightly inclined Fabry-Perot interferometers) and reaches the recording medium 808 via a pinhole 806 and a concave grid 807. The partial beam 802 represents a reference beam which is directed onto the concave grating 807 via a lens 809 and a mirror 810. It is split up there in the same way as the signal bundle 803 and directed to the same location of the recording medium 808 as the signal beam. The image and reference beam are therefore coherent at every point on the recording plane. With such an arrangement, three-dimensional images that were generated, for example, according to an arrangement according to FIG. 6 can also be stored volume-holographically. To reconstruct the image, the reference beam or a beam conjugated to it is directed onto the volume hologram.