Optical communication
Optical communication is the transmission of data with the help of light. The frequency spectrum ranges from visible light to the near and mid- infrared . The data is transmitted terrestrially by means of fiber optic cables in fiber optic networks. A rather new area of optical communication is optical free-space communication , in which the data is modulated onto a laser beam and transmitted over large distances through free space, i.e. not fiber-bound.
Discovery of light as a messenger
The physicist Alexander Graham Bell recognized the properties of light for the photophone he had developed early on . It is considered the first principle of optical communication and used a bundle of light rays to transmit human speech. Another form that can be added to optical data transmission or optical transmission of information is the so-called light morse , as it is used in seafaring. Similar to digital transmission, the information is transmitted in that there is a change between switched on and switched off lights.
Optical data transmission systems
Optical data transmission systems refer to data transmission systems that use light as a carrier for information. They are generally comparable with conventional transmission systems based on metal, but require additional complex technology to modulate the information on the broadband information carrier light .
history
motivation
The technology developed by Manfred Börner at Telefunken in Ulm , a "multi-level transmission system for messages represented in pulse code modulation ", was the groundbreaking development in optical communications technology . Börner's achievement consisted in recognizing the usefulness of the existing technical components such as semiconductor lasers , glass fibers and photodiodes for data transmission and combining them into a transmission system that converts analog signals in a transmitter unit into digital signals, modulates them to an optical transmitter and the resulting light pulses radiates into an optical waveguide . To make this technology interesting for wide area networks , the attenuation of the signal losses had to be compensated for by developing and installing amplifiers. This principle, designed by Manfred Börner, has prevailed up to the present day, which is why most of the technical developments in the field of optical data transmission are based on his invention.
On December 21, 1966 he applied for a patent for his invention for the Telefunken company as a "multi-stage transmission system for messages represented in pulse code modulation". It was officially recognized a year and a half later, on May 16, 1968.
On the basis of a schematic drawing created for the patent application, Börner's system can be divided into three basic components:
- Sending unit
- Transmission unit
- Amplifier and receiver unit.
Transmission technology
Börner describes the transmission technology in the patent application for his transmission system as follows:
"In the channels E1 to E300 carrier-frequency talk groups of 1200 frequency-wise juxtaposed, 4 kHz wide channels 300 separate coding levels 1, 1 ', 1" are offered. In the coding levels the message is sampled with the help of pulses and the found sampling amplitude levels in the usual way Used to represent pulse groups. These pulse groups are amplified in power levels 2, 2 ', 2 "so that they can be used to control semiconductor lasers 3, 3', 3", and each of these 300 semiconductor lasers in turn stimulates an optical waveguide 4, 4 ', 4 "of a multiple line 5."
According to this description, Börner uses the same procedure 300 times in order to be able to use 300 glass fibers that are bundled in a multi-fiber cable as an optical data transmission medium. He makes it clear that this process must be carried out separately for each individual fiber, since each fiber represents a completely independent transmission that is only bundled in the multi-fiber cable. In each of the 300 encoders, 1200, 4 kHz wide channels in the form of carrier-frequency analog signals are converted into digital binary code words using pulse code modulation. Here the carrier-frequency analog signal is sampled. It is flattened and converted into a time-discrete, i.e. graded PAM signal at certain points in time , which runs on fixed voltage levels. So that a binary code word is created from this PAM signal , these voltage levels must be assigned to individual binary code words. This step is known as quantization . The more bits are provided for the quantization, the finer the voltage levels can be divided. With z. B. 4 bits can be represented 7 levels positive and negative. The first bit defines the sign of the voltage value. During this process of digitization , a digital 4-bit long code word with zeros and ones is created from an analog signal.
So that these digital signal pulses can be modulated onto the light by means of electrical-optical converters, the pulses must be amplified in order to obtain a sufficiently strong light output for transmission. So-called E / O converters can be optical transmitters such as LEDs or laser diodes . For more complex processes, however, external modulators, which are connected downstream of the optical transmitters, are used. LEDs are used as optical transmitters for smaller transmission links, since they are much cheaper and easier to wire than semiconductor laser diodes. Their disadvantage is that LEDs are surface emitters d. H. they emit undirected light. Therefore they are not suitable for long-distance transmission with single-mode fibers . Semiconductor lasers, on the other hand, are used today in systems that require a transmission rate of more than 1 Gbit / s or when a wide area network is to be operated. They have the advantage over LEDs that they are edge emitters, so they only transmit on one oscillation level and, due to their technical functionality, can achieve shorter switching times and thus a much larger bandwidth . But lasers in particular were still a major problem at that time, they were very temperature-sensitive and therefore only had a very short service life. Much of the research in optoelectronics in the following years concentrated on the laser sector. In 1974, in a specialist lecture, Börner described the problem of service life as unsolved, typically only 1000 hours per component would be achieved. But he also emphasized that an optimization of the temperature and voltage influences could lead to success. In his patent, Börner does not specify any specific method for irradiating the laser light into the fiber. The first connector types emerged in the mid-1970s and towards the end of that decade, the first type, the F-SMA, was internationally standardized by the IEC .
Transmission medium and amplifier
In contrast to other approaches by other research groups, Börner did not use an internally mirrored waveguide and he also did not attempt to transmit optical signals over the air. He developed a system that is based on glass fibers, so-called optical waveguides. In contrast to waveguides, these can be laid relatively easily in any curves and paths and are very space-saving thanks to their small diameter of approx. 250 µm. The fiber optic cable only became of interest for information technology tasks with the invention of the laser in 1960, as it eliminates the enormous loss of light that occurs with normal light irradiation (see also numerical aperture ) in a glass fiber due to the bundled irradiation and the use of total reflection . Börner used the technical developments made up to then and combined them to form the first "optoelectronic fiber optic system". In his patent application he describes two fundamentally different types of fiber optic cables. The so-called “light wave mod fiber conductor” by Börner is better known today under the name singlemode or monomode fiber. This fiber type is still the technically highest quality and most demanding fiber type in production. The small core diameter of 9 µm ensures that no further light modes propagate in it. The second type that he lists in his patent are "lightwave intensity fibers". In these fibers, which today have a core diameter of mostly 62.5 µm or 50 µm and are referred to as multimode fibers , several modes propagate , so that the receiver receives a mixture of different wave types. These fibers generally have more impulse distortion and greater attenuation. Börner saw the purpose of these cheaper fibers in areas where amplifiers had to have a greater branch density anyway in order to create branches. There the pulse deformation would be less noticeable over short distances. Today, multimode fibers are largely only used on company premises or inside buildings.
Since attenuation and pulse deformation occur in both types of fiber, Börner developed an amplifier system. It consists of a receiver module, a regenerator and a transmitter module. The regenerator absorbs the deformed impulses , which can still be recognized as such, and creates fresh impulses from them. These are amplified again in the transmitter module and fed into the 300 fibers of the multi-fiber line with semiconductor lasers. Today's amplifiers, such as those used in intercontinental submarine cables , are exclusively purely optical amplifiers , which do not first convert the optical signals back into electrical signals, but simply refresh and reshape the optical signal. In LAN - computer networks consisting of optical fibers consist -Strecken are still amplifiers operating after Börner principle used. Since amplifiers require electrical energy, it is essential to carry them along for the energy supply, especially in large multiple lines such as submarine cables, copper wires or copper sheaths.
Reception technology
The receiving technology is not directly described by Börner. From the description of the functioning of the amplifier it can be seen that the first three components of the amplifier, the photodiode , amplifier and pulse regenerator, correspond to the receiver module. In his system, Börner assumes that a signal or the light intensity after a distance of several hundred meters without intermediate amplification is still large enough for the recipient to be able to identify it as such. This means that the pulses weakened by attenuation and deformed in width by dispersion have lost their intensity, but the signal-to-noise ratio at the pulse regenerator is still large enough to distinguish a signal from the noise of the photodiodes and amplifiers. Photodiodes are light-sensitive semiconductor elements that convert incident light into an electronic current flow. The signals arriving in the form of light pulses are converted into an electrical pulse. However, since the light intensity is only very low when receiving, weak electrical signals are generated which are refreshed by a downstream amplifier and the signal pulses are reshaped again. Börner consciously no longer mentions what happens to the still digital signals at the receiver. Depending on the type of application, the signals are digitally processed there or converted back into the original analog signals using a demodulation process , as is the case with an analog telephone connection, for example.
Development of optical communication
Optical data transmission in the form of fiber optic technology first had to distinguish itself from the tried and tested copper-based electrical system and be able to be compared with it. So it took over a decade until the fiber optic system, which was researched on a laboratory basis in 1965, was ready for the market. The copper conductor was therefore initially the first choice in the field of permanently connected data transmission for several years . Manfred Börner described this fact in an article in 1976 with the words:
"There are still some details of fiber optic communications technology that are still struggling: The coupling and splicing technology must be transferred from the laboratory stand to a technology that is adapted to a harsher [sic] working atmosphere, and components (lasers, receiving diodes) must be optimized for their respective application . The laser diode has to be further improved, especially in terms of its service life. "
From the 1960s onwards, a research race developed in various fields of optoelectronics . The two most important were probably fiber optic research itself and laser technology. In 1970, laser diodes were operated continuously at room temperature for the first time at the “ Joffe Institute ” in St. Petersburg. In the same year, Corning presented the first low-loss fiber optic cable, and shortly afterwards Charles K.Kao from ITT in England was able to develop a cable with a transmission rate of 100 Mbit / s at that time. In 1973 Corning countered with a cable of 2 dB / km attenuation, this value could be reduced to 0.7 dB / km by 1979. In 1976 an only 1.27 cm thick fiber optic cable was presented, which contained 144 individual glass fibers. An unimaginable 50,000 telephone calls could be handled over this cable for the time. It was developed by the Bell Laboratories, which were also well represented in laser research at the time . By the end of the 1970s, laser technology, with a lifespan of several thousand hours, was so advanced that the first commercial test tracks were set up all over the world . In April 1977 " General Telephone and Electronics " from California was the first company to handle telephone connections over a fiber optic with 6 Mbit / s. The first tests in Germany were carried out in 1978 by the German Federal Post Office over a distance of 4.3 km at 34 Mbit / s. From then on, optical data transmission was on the advance and just a few years later all industrial nations had installed fiber optic connections for their wide area networks. In 1986, with the last major milestone, the area of submarine cables was opened up, on October 30th a fiber optic connection through the English Channel went into operation.
State of the art in optical free space communication
Optical free space communication has already been tested for use on satellites in addition to purely terrestrial applications.
Optical inter-satellite links
- Artemis -Spot-4 (50 Mbps, Artemis since 2001)
- Artemis OICETS (50 Mbps)
- TerraSAR -X (launched 2007, 5.6 Gbps)
Downlinks from satellites
- Artemis - ESA-OGS (50 Mbps), Tenerife, Spain
- OICETS - DLR-OGS (50 Mbps), Oberpfaffenhofen, Germany
- OICETS - NICT-OGS (50 Mbps), Japan
Web links
- www.dlr.de
- Optical Communication Group DLR
- Optical Communication Group JPL
- Capanina Stratospheric Balloon - OGS (DLR) (1.25Gbps)
- Interactive representation of a coherent optical transmission
literature
- Gerhard Schiffner: Optical communication technology. Teubner, 2005, ISBN 3-519-00446-1 .
- Manfred Börner, Gert Trommer: Optical fiber. Teubner, 1989, ISBN 3-519-00116-0 .
- Volkmar Brückner: Elements of optical networks. Vieweg + Teubner Verlag, 2011, ISBN 978-3-8348-1034-2
Individual evidence
- ↑ Patent DE1254513 : Multi-stage transmission system for pulse code modulation represented messages .. Published on November 16, 1967 , inventor: Manfred Börner.
- ↑ Manfred Börner: multi-stage transmission system for information represented in pulse code modulation . English Patent No. 1 202 418, Dec 21, 1966.
- ↑ Manfred Börner: Multi-stage transmission system for messages represented in pulse code modulation ; German Patent, No. 1,254,513 , December 21, 1966.
- ↑ E / O converter. DATACOM book publisher. itwissen.info April 4, 2012.
- ↑ Manfred Börner, Stephan Maslowski: Expert lecture "Advances in fiber optic communications", 10th Technical Press Colloquium Berlin, October 17, 1974.
- ↑ Manfred Börner: Multi-stage transmission system for messages represented in pulse code modulation . German Patent, No. 1,254,513 , December 21, 1966.
- ↑ Manfred Börner: Article. In: ZS Elektronik fiber optic technology , 10/1976.
- ↑ Optical fiber in the everyday test . ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. February 23, 2012.
- ↑ Jeff Hecht: A Fiber-Optic Chronology . sff.net February 23, 2012.
