Let us now assemble these various L kinds of information about sources. detectors and fiber properties and see what kinds of communication capabilities are available. First let us calculate the range for a low bit-rate system. one capable of transmitting 106 bits per second. In order for detection to proceed with few errors the signal arriving at the detector must be 100 times larger than the detector's internal noise. If an avalanche photodetector is employed. the arriving signal must have a power level of at least 10-10 watt. For maximum range we. would choose a laser with a power output of 10-3 watt in preference to an LED. which is an order of magnitude less powerful. As we have seen. with digital coding the maximum allowed attenuation of the light passing through the fiber is a factor of 107, or 70 decibels. Since present production fibers have an attenuation of less than five decibels per kilometer. we can expect satisfactory transmission for a distance of 14 kilometers before amplification is needed. (If fibers with an attenuation of only one decibel per kilometer are perfected. the range could be stretched to 70 kilometers.) In practice it seems doubtful that fibers will ever be available with continuous lengths much greater than a few kilometers. Therefore the extra loss introduced at the junction of two fibers must be added to the loss figure. Present plug-type connectors introduce a loss of about .5 decibel. If one were to need six connectors for a 14-kilometer route. the additional loss would come to only three decibels. (The total loss could be held to 70 decibels by shortening the route by three-fifths of a kilometer.)
With the selection of a source. a detector and a fiber. what will the information- handling capacity of light-guide systems be? Since it is desirable to transmit at the highest possible bit rate. one must consider a number of factors. As we have seen. the noise level of the detector increases with the bit rate. Thus if the signal power was just adequate for transmission at 106 pulses per second. it would have to be raised by a factor of 100 for transmission at 108 bits per second. In addition. as the pulses get shorter and closer together their spreading as they travel through the fiber becomes an important limiting factor.
In order to simplify the calculation let us decide somewhat arbitrarily that the pulse-spreading will not be more than half the interval between successive pulses. In a graded-index fiber the pulsespreading due to modal dispersion (the difference in path lengths) amounts to about 10-9 second per kilometer. which means that if one attempts to transmit
109 pulses per second. the spreading is equivalent to the entire interval between pulse peaks. Therefore to maintain a separation of half an interval the signaling rate cannot exceed .5 X 109, or 5 X 108, pulses per second. This is the limit if we have a laser source. which is so nearly monochromatic that pulsespreading due to wavelength dispersion can be ignored.
If we choose an LED light source. however. wavelength dispersion becomes the limiting factor in the signaling rate. For an LED the wavelength dispersion amounts to 3.5 X 10-9 second per kilometer, a figure 3.5 times larger than that for modal dispersion. In order to hold the spreading below half the interval between successive pulses the signaling rate with an LED source must therefore be slightly less than a third of the rate permissible with a laser, or 1.4 X 108 pulses per second. Naturally as the desired transmission distance is increased the signaling rate must be proportionately reduced. For example, for a nominal transmission distance of 10 kilometers the rate for the laser source would have to be reduced tenfold to 5 X 107 pulses per second, or approximately the rate (4.47 X 107) actually selected for the Chicago installation. These simple calculations illustrate what can be achieved with today's technology and also give a feeling for the kinds of design choice that can be made among range, capacity and device complexity. Undoubtedly there will be significant improvements in the future.