ABSTRACT
After discussing the intricacies of video transmission over fiber, we will now look at data transmission where fiber excels because of high bandwidth, low loss, and EMI/RFI/lightning immunity. These characteristics are more important in some applications than in others. In security applications, EMI/RFI/lightning immunity is the paramount consideration. For very long distance applications, low loss prompts the choice for fiber. For highspeed applications, such as gigabit data rates between computers, the high bandwidth determines the use of fiber. After the initially deciding to use fiber optics for a data transmission requirement, other decisions must follow such as: • Operating wavelength • Opticalloss budget • Fiber type • Fiber size • Optical connector type DIGITAL DATA TRANSMISSION Whereas analog transmission over fiber often uses a carrier transmission, digital data transmission typically uses base band transmission. This means that a logic 0 is sent as a low light level (or the light may be completely off) and a logic 1 is sent as a high light level. Often, employing some sort of data co ding (e.g., 4B5B, 8B10B, etc.) guarantees minimum transition density. This coding scheme alleviates the difficulty in designing a fiber optic data link that includes true DC or steady-state data rates requires this data coding. Low data rates (typically < 1 MHz) accommodate true DC, but achieving this becomes increasingly difficult at high er data rates, mainly because of the relatively poor DC performance of amplifiers capable of high frequencies. True DC response is difficult because negative light does not exist. Light is an unbalanced transmission media. When transmitting data using an electrical signal on copper cable, a balanced signal provides optimum signal fidelity, especially at higher data rates. A logic 0 might be sent using a -1.0 Volt level, while a logic 1 might be sent using a + 1.0 Volt level. This is a balanced, symmetrical transmission scheme. Recovering the data requires comparing the received signal to a threshold of 0.0 Volts regardless of how much the signal is attenuated during transmission. For instance, if the signal was attenuated by a factor of ten, the received voltage levels would be + 0.1 Volts and -0.1 Volts. A threshold of 0.0 Volts would still be perfectly centered allowing optimum data recovery. DC data transmission with light is not quite so easy. Since negative light cannot be generated, the best one can do is use a zero light level for a logic 0 and a high light level for a logic 1. This makes recovering the data very challenging at the receiver. The attenuation of light through the fiber requires an adjustment ofthe receiver threshold to 50% ofthe peak received light. However, the receiver cannot determine the peak if a steady logic 0 is sent. To correct this problem, set the logic threshold to 5% of the largest peak signal. This leads to varying duty cycle distortion as the input light level changes. DC coupled data links also cannot tolerate as much opticalloss as an AC coupled data link.
