Abstract:
A telecommunication system having frequency-dividing optical components where light pulses having different frequencies are coupled out of a optical fiber by fiber grating and/or photonic crystals and imaged by focusing elements outside of the optical fiber. The fiber grating for different frequencies can be used in a single period or in different periods disposed one after the other. The photonic crystals can be used at the optical fiber extremity or etched in a channel or trench in a glass fiber. Delay elements are added to ensure that different frequency light pulses are imaged simultaneously in a given and desired time relation as required for further parallel processing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a telecommunication system having frequency-dividing optical components for parallel processing of optical pulses, and more particularly to the use of fiber gratings and photonic-crystals for the spatial distribution of the frequency-coded optical pulses. 
     RELATED TECHNOLOGY 
     Optical telecommunications nearly always take place through a sequence of individual, binary-coded light pulses. Since transmission frequencies are already advancing today into ranges which no longer permit electronic data processing, and especially not the complex encoding and decoding required of secret communications, there is a considerable need for optical elements which can read chronological bit sequences into a single- or multi-dimensional spatial areal, and then process them further in an optically parallel manner. An optical, parallel processor is capable of simultaneously transforming a large quantity of binary or analog signals, arranged as an image or a pattern and, thus, works considerably faster than an electronic computer. When working with areals (areas) of 1000×1000 optical points (pixels), a parallel processing of 10 6  signals can be readily achieved and serves as an exceptional time saver for certain numerical operations, such as a Fourier transformation. Since Fourier transformations comprise an essential component of machine pattern recognition, it is precisely the encoding and decoding of communications that could be realized easily and very quickly in terms of optics. 
     It is known that electro-optical components which read chronological pulse sequences into spatial areals have a Brownian tube type design. At the present time, as shown in German Patent No. 196 09 234.5 (H. Koops, filed March 1996), which is not necessarily prior art to the present invention, it is only in micro-tubes that electron beams are able to be deflected quickly enough to feed signals in the multi-gigahertz range. 
     Another method encodes the individual, optical pulses optionally with the aid of light polarization. The first, third, fifth, etc. of each odd-numbered pulse is polarized, for example, vertically, and all even-numbered pulses are polarized horizontally-linearly. The even and odd pulses are then able to be separated locally in each case with the aid of a polarizing beam splitter. Cascading renders possible a greater degree of separation. The advantage of this method is that the separating element- the beam splitter, is purely passive. After the pulses have been electro-optically polarization-encoded, for example, there is no longer a need for an active switching operation. Obviously, the drawback of the method is the small number of only two parallel channels per cascade stage. 
     SUMMARY OF THE INVENTION 
     The present invention is characterized by individual, successive optical pulses being frequency-encoded instead of polarization-encoded. Since the light frequency within an optical telecommunication window can be easily altered by 100 nm, and on the other hand, since semiconductor lasers are able to be detuned by several nanometers by varying the applied voltage, it is possible, in principle, for different frequencies to be assigned within a broad range to optical pulses. To this end, a plurality of semiconductor lasers having different center-of-mass frequencies should be electrically switchable with respect to their radiation frequencies. The resulting optical pulses having different frequencies are then binary-encoded for telecommunication purposes and are fed into the transmitting glass fiber. Thus, a sequence of pulses, the first having the frequency υ 1 , is impressed upon the communication. Here, for example, it may be that υ 1 &lt;υ 2 &lt;υ 3  &lt; - - - &lt;υ i &lt;υ i+1 &lt; - - - &lt;υ N . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the use of a passive optical component to divide light at the extremity of an optical fiber, according to an embodiment of the present invention. 
     FIGS. 2 a  and  2   b  show the use of diagonally arranged fiber gratings to couple the light out of the optical fiber, according to two embodiments of the present invention. 
     FIG. 3 shows the fiber gratings arranged in a spiral form, according to an embodiment of the present invention. 
     FIG. 4 shows the use of photonic crystals etched into a glass optical fiber for frequency-dividing of light, according to an embodiment of the present invention. 
     FIGS. 5 a  and  5   b  show the delay between optical pulses of different frequencies and the use of a delay element to ensure that the optical pulses are imaged simultaneously on a matrix, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention uses a passive, optical component which is able to divide light spectrally-locally to distribute the optical pulses within a spatial area. Referring to FIG. 1, an optical component  1  of this kind is any prism-based or grating-based spectrograph (but also includes the two-beam and multi-beam interferometers) incorporated into or at the end of an optical fiber  2  carrying a frequency encoded digital or analog message  3  as an optical pulse for imaging on a linear or fiber areal  4 . Also, fiber gratings can be used as spectrographs for the spatial distribution of the frequency-coded optical pulses. Such gratings are known for example from U.S. Pat. No. 5,546,481 to Meltz et al., which patent is hereby expressly incorporated by reference herein. 
     As shown in FIGS. 2 a  and  2   b , blazed gratings  5  are arranged diagonally, in order to direct the light out of the optical fiber. By using suitable focusing elements  6 , such as anamorphotic lenses as shown in German Patent Application No. 196 30 705 A1 (July 1996, published March 1997) to H. Koops, which is hereby incorporated by reference herein, the beams of one single frequency (color) are able to be focused on a linear areal  4 , in a punctiform manner. Each frequency υ i  has a different focal point, all situated, for example, on one line parallel to the optical fiber  2 . 
     With regard to lenses  6 , these can be constructed as well directly on the optical fiber. For example, German Patent Application No. 197 13 374.6 (filed March 1997) to Koops et al., hereby incorporated by reference herein, shows a method of fashioning such lenses. In an optical fiber having a blazed fiber-Bragg grating, a lens can be placed on the cladding of the fiber. The lenses to be used are mounted on the cylindrical fiber cladding surfaces and can be manufactured with the aid of vapor deposition technology, corpuscular beam lithography characterized by a high depth of focus, and with the aid of X-ray lithography using intensity-modulated masks. The lens can also be constructed by means of polymerization, i.e., through beam polymerization of monomeric materials adsorbed or condensed on the surface, with the aid of the light diffracted out of the fiber. In this context, to define the lens profile, the supplying of material should be controlled through a slotted mask. 
     Referring to FIG. 2 b , in another specific embodiment, blazed fiber gratings  5  are comprised of a plurality of gratings of different periods disposed one after the other in the optical fiber  2 . Each grating is positioned to couple light of a single frequency out of the optical fiber  2  and to emit that light. In this manner, in the same way as in FIG. 2 a , linear arrays of the individual light pulses are able to be formed by means of focusing elements  6  outside of the optical fiber  2 . 
     Referring to FIG. 3, in another specific embodiment, individual blazed fiber gratings  5  are arranged in a spiral form in the optical fiber  2 . By this means and in conjunction with suitable focusing elements  6  (here not shown but understood to be similar to those in the other Figures), the pulses are able to be arranged on two-dimensional areals  4 , such as a screen, by rows and columns. Each spiral turn of the grating group corresponds more or less to one row where the columns are situated side-by-side. Adjacent row positions correspond to gratings situated one behind the other in the optical fiber  2 , with a slightly different osculating plane. Subjacent column positions correspond to gratings lying directly one behind the other in the spiral turns. In place of the spiral-shaped grating configuration, the optical fibers  2  are also able to be coiled or twisted (not shown) and, thus, achieve the same effect of an areal light-pulse array. 
     As shown in FIG. 4, in place of light-generated optical fiber gratings, the frequency-dividing elements are also able to be replaced by photonic-crystals  7 . Photonic crystals are crystals having lattice constants of a few hundred nanometers, which, in contrast to the fiber gratings described above, comprise fewer individual elements (grating components), since the differences in their refractive index are far greater than those of the fiber gratings. Moreover, resonance effects enhance their efficiency. Photonic crystals and their fabrication are discussed in H. Koops, “Photonic crystals built by three-dimensional additive lithography enable integrated optic of high density,” SPIE, vol. 2849/29 (Denver 1996), which article is herein expressly incorporated by reference. The photonic crystals, as with the fiber gratings, may be fashioned as frequency-selective reflectors, prisms, or beam-splitters. 
     The photonic crystals may be used at the glass-fiber extremities (as shown in FIG.  1 ), or, as shown in FIG. 4, they are placed in small, etched channels or trenches  8  in the glass optical fiber  2 . Such small trenches or channels  8  in the optical fiber  2  may be fabricated for example as in German Patent Application No. 197 13 371.1 (filed March 1997) to H. Koops et al., entitled “Wavelength Decoupling out of D-Profile Fibers Using Photonic Crystals” which is also incorporated by reference herein. In this patent, light is conducted in a D-profile fiber just underneath the surface of the fiber. A slit a few micrometers wide is cut in this fiber surface by means of lithography and dry etching or wet-chemical etching, or by means of laser or ion ablation. Then a photonic crystal is placed exactly in the path of the light by means of additive, three-dimensional lithography and, because of the crystal&#39;s selective effect on transmitted light, it enables a small portion of the spectrum to be coupled into the fiber or decoupled out of the fiber. This light from a small spectral range can be decoupled laterally out of the fiber, since photonic crystal media either permit the passage of light or conduct it exclusively inside the matter, provided that the light has a specific wavelength. In this manner, because of the special configuration of the crystal, a portion of the spectrum can be reflected by less than 90° out of the fiber. With the aid of a three-dimensionally constructed lens, the light can also be diverted into a continuing fiber. 
     Thus, from the channel or trenches, the photonic crystals diffract the light of the frequency υ i  out of the fiber  2 . Suitable lattice constants of the photonic crystals  7 , situated one behind the other in pits (cut-outs)  8  in the glass optical fiber  2 , enable the various frequencies and, thus, light pulses to be coupled out of the fiber  2  and imaged by means of focusing elements on an areal  4  or to be coupled into other waveguides or detectors. 
     With respect to FIGS. 5 a  and  5   b , when the frequency-dividing elements discussed above are used, the telecommunications transmission takes place with individual light pulse sequences. Each of these sequences comprises of a number of optical pulses, which are spatially separated from one another by the new element and are projected, for example, onto a screen. As one can readily see, referring to FIG. 5 a , the pulses reach the screen  4  simultaneously only at certain screen positions. As a delay mechanism for some of the individual light pulse sequences, the screen  4  can be tilted or, in some instances, curved so that all pulses are displayed simultaneously through illumination. In the case of a flat two-dimensional areal, once curved, it takes on a what is referred to herein as a three-dimensional shape. 
     Rather than using a tilted or curved screen (or in conjunction therewith), a delay can also ensue when the screen  4  is coated with a fluorescent or phosphorescent substance, which phosphoresces until all pulses of one pulse sequence have arrived. To avoid a strong, undesired afterglow, which permits the individual pulse sequences to overlap, electrical or electro-optical switching elements should be used to separate the individual sequences. 
     Referring to FIG. 5 b , in place of a screen, the individual pulses can be collected by glass fibers  9  downstream from the frequency-dividing element and be imaged on a matrix  10 . Each individual glass fiber  9  must serve as a delay distance (delay interval) for the pulses that it collects, so that the pulses of one frequency sequence are simultaneously imaged on the matrix  10 . Detectors, which take the wide-band property of the pulse sequence into account, are required for the further optical processing. Other delay elements may include air gaps or glass or gradient index prisms. 
     Instead of glass fibers  9 , a direct use of detectors is also possible. Each pulse of one sequence is detected separately, and the delay until further parallel electronic processing ensues in the electric domain.