Intelligent multichannel fiber optic connections

High speed data transfer problems

With the growth of the amount of information transmitted over the network, the problem of high-speed data transmission becomes more and more urgent. High-speed data transmission, as a rule, assumes the existence of a high-bandwidth communication channel between nodes. When designing a channel with a high bandwidth, both solutions based on copper electrical conductors and solutions based on optical connections can be used. Any connection consists of a transmitter that transmits a signal, and a receiver that receives a signal. The signal on the connection can be transmitted both in one and in two directions. Thus, an optical connection may consist, for example, of an optical transmitter, an optical channel and an optical receiver. In duplex mode, an optical transceiver provides both signal transmission and signal reception via separate optical fibers, which are usually in a single fiber-optic cable.



To date, the channels providing communication with a speed of more than 1 gigabit per second (1 Gb / s), the so-called “1G-connections”, are already widely used. 1G connections are well standardized (for example, there is a publicly available Gigabit Ethernet standard). Optical 1G connections are typically used to transmit data over long distances (more than 100 meters).



For high-speed data transmission channels are used that provide communication with a speed of about 10 gigabits per second (10 Gb / s), the so-called "10G-connection". When solving complex technical problems, high demands are placed on data transmission channels, which are becoming increasingly difficult to meet, especially with the help of copper electrical conductors. However, 10G connections based on copper electrical conductors are used (for example, 10GBASE-CX4 standard). 10GBASE-CX4 provides data transmission on four shielded twisted pairs in each direction (eight twisted pairs in total). This cable is quite bulky (about 10 mm in diameter) and expensive to manufacture. In addition, 10GBASE-CX4 can only be used to transfer data no more than 15 meters. A common disadvantage for all 10G connections based on copper conductors is the high level of energy consumption. For example, the standard 10GBASE-T provides data transmission at distances from 55 to 100 meters, but due to complex signal processing consumes from 8 to 15 watts per port. If we consider a standard that provides data transmission over distances of about 30 meters, then such a connection will consume at least 4 watts per port. The use of such standards with high energy consumption leads to a significant increase in the cost of service connections, and also forces developers to reduce the density of ports on the front panels. For example, energy consumption of about 8–15 watts per port limits port density to 8 pieces (or even less) in the same area on which up to 48 ports can be placed using the 1000 BASE-T standard or 1G fiber-based connection .



Thus, research of the electronics market shows that when developing channels with high bandwidth (10G), solutions based on optical connections are increasingly used. So the figure below shows a graph from a Luxtera report showing the time variation of the dependence of the range and data transfer rate on the physical data channel used. The graph shows that by 2014 there will be a complete rejection of solutions based on copper conductors in favor of solutions based on optical and hybrid compounds.

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Multichannel Fiber Optic Connections

To build high-performance data transmission systems it is proposed to use multichannel fiber-optic connections. They provide the necessary low density of optical fibers, acceptable cable sizes and solve the “duct congestion problem” problem typical of single-channel connections.



Multichannel fiber optic linkers aim to achieve high channel density with low loss and crosstalk. Crosstalk level i. the maximum force of the influence of the channels on each other is determined by the characteristics of the optical fibers, as well as the distances between them (the fibers). Obviously, the greater the distance between the optical fibers, the lower the density of the multichannel optical fiber connection. The level of loss depends on the characteristics of the optical fibers and the signal transmission distance. Thus, when designing a multi-channel fiber optic connection, it is important to consider and optimize all the above parameters.



Of the existing means of data transmission over multichannel fiber optic connections, active optical cable can be distinguished as the most common means.

Active optical cable

AOC, Electrical optical optical cable, US Patent No. 2007/0237464 A1 (October 11, 2007).



An active optical cable (AOK) includes, on the one hand, a built-in electrical connector, and on the other, an optical connector. Between them are several optical fibers that provide communication inside the optical cable. Communication can be carried out both in one and in two directions.







It is assumed that at least one of the ends of the active optical cable has an electrical connector, but the signal is transmitted over the remaining part of the cable through optical fiber. Thus, the network designer is not obliged to make a preliminary choice between the connection via copper conductors and the optical connection. Instead, it is sufficient that the network nodes have special ports that support either the connections via copper conductors or the optical connections.

The duplex mode of data transmission is realized by placing transmitting optical components (TOSA, transmit optical sub-assembly) and receiving optical components (ROSA, receive optical sub-assembly) at both ends. Chips are responsible for controlling the transmitting and receiving optical components. The microcircuits can be placed inside the TOSA and ROSA cases or can be moved out of their limits. If necessary, duplex mode can be disabled. In this case, data transmission will be carried out only in one direction (only transmitters will be placed on one end of the cable, only receivers on the other end).



When an electrical signal arrives at the corresponding terminals of the electrical connector (via the electrical port), it is converted by the laser driver and the TOSA optical-optical converter into an optical signal. The optical signal is transmitted over fiber to ROSA, where it is converted by an optical-electronic converter ROSA into the corresponding electrical signal. The received electrical signal goes to the corresponding output of the electrical connector and then to the electrical port.



The recommended signal transmission distance over an active optical cable is 30 meters. Increasing the range to 100 meters leads to a significant increase in the cost of cable.



A feature of AOK is the need for high-precision mounting and alignment of signal transmitters (lasers) and signal receivers (photosensitive photodiodes) relative to physical channels (optical fibers). The developers of such connections mainly solve the technological problems of high-precision installation, trying to accommodate as many channels as possible in small-sized buildings. As an alternative to the AOK, a data transmission technology for an intelligent multi-channel fiber optic connection (IMX) is proposed.

Intelligent multichannel fiber optic connections

RF patent №2270493, 2007



The use of IMX technology suggests that in case of partial damage to several optical fibers or displacement of optical fibers relative to the connector, the connection can be quickly restored without compromising data integrity. Thus, the compound built on the basis of the IMX technology has the property of regenerative (self-healing).







1 - activated light source; 2 - non-activated light source; 3 - emitters matrix; 4 - unused fiber opto tires; 5 - optoshin fibers involved; 6 –fibre optoshiny, in which several rays of light were directed; 7 - activated photodiodes; 8 - non-activated photodiodes; 9 - activated photo-diodes, which took the signal and the fibers 6; 10 - photodetector array.



At the inputs of the laser matrix - a source of information serves electrical pulses from the control circuit of the source, which modulate the radiation of lasers. This radiation through optoshina enters the matrix of photodiodes located in the receiver information, and activates part of the photodiodes. Activated photodiodes generate a stream of electrical pulses to the control chip of the receiver.



When connecting, the optoshin is connected to the matrices of the transmitter and the receiver rather arbitrarily, combining only the optical regions of the matrices and the optoshins by installing the ends of the optoshins into the optical connectors of the receiver and the transmitter chips. Therefore, knowing only the set of activated photodiodes of the array-receiver, it is impossible to determine which of the lasers emitted the signal activating these photodiodes. One of the basic principles of operation of the IMX is to establish a correspondence between each laser and the photodiodes activated by this laser before data transmission begins. The corresponding switching procedure should be implemented in the data link protocol. During the switching channels are determined for data transmission, i.e. a correspondence is established between the laser and the set of photodiodes activated by it.



The procedure of switching channels is performed once before the start of data transmission and does not affect the transmission rate in the future. In the event of communication failure (partial damage to a part of optical fibers or optoshine displacement relative to the receiver and transmitter arrays), it is necessary to promptly detect this violation and repeat the circuit switching procedure. In this case, the number of channels can be reduced, i.e. the bandwidth of the connection will decrease, but the connection will be restored.

Conclusion

This article is an attempt to talk about the problem of high-speed data transmission and about possible ways to solve it, in particular, about multichannel optical fiber connections.



In order to understand my place in this story, I will say that I am a member of a group of developers engaged in the implementation of the IMX technology presented here. If this topic will be interesting to readers, then I am ready to talk about our work in more detail, plus prepare notes on topics:

• hardware
• software part
• development, modeling and verification of data transfer protocols

Thank you for your attention, I hope it was interesting.