ABSTRACT
The graphie shows a hypothetieallink and its corresponding link budget. Start with the transmitter output power on the left side of the chart. The typieallaunch power is -12.5 dBm. However, the transmitter LED output power can vary by ± 2 dB due to manufacturing variability ofthe LED itself. Therefore, the output power can be as high as -10.5 dBm or as low as -14.5 dBm. The block is shaded between these two values. Further transmitter variations of ± 2 dB result from the effects of temperature on the electronies and the electro-opties (e.g., LED or laser). Another potential ± 2 dB of loss is due to variations in the optieal coupling to the transmitter output. The effects of aging, typieally 1-3 dB, should be included in the system's design. The next factor involves the losses due to optieal connectors that may be in the optieal path. The graphie allows 2 dB for this factor. For this system, the loss due to the optieal fiber itself amounts to 4 dB/km of length. Multiply this value times the actuallength to determine the loss due to the fiber. considerations for temperature effects associated with most fibers usually yield ± 1 dB. The next factor, variation in loss at the receiver, requires a large-area detector to eliminate the effects of this parameter. Finally, a 3 dB safety margin should be built into all systems. At each step, any variation causes the shaded band to enlarge. On the right side of the chart the receiver has to cope with optieal inputs as high as -5.5 dBm and as low as -31.5 dBm. Or stated differently, the receiver would need an optieal loss range or optieal dynamie range of 26 dB. A discussion of the decibel is necessary to understand these link loss values. The decibel (dB) is a convenient means of comparing two powers. The loss a given link can tolerate is rated in dB. For example, a given AM video link may tolerate a maximum of 9 dB of optieal loss. How much light actually reaches the receiver? Table 10.2 describes the decibel to power conversion. According to the Table 10.2, 12% ofthe optieal power actually reaches the receiver, so 88% of the light output by the transmitter was lost somewhere along the way. If the link could tolerate 20 dB of optiealloss, only 1% of the transmitter 's optieal output would reach the receiver. To determine the amount of light reaching the receiver, take any two values that total the dB of optiealloss in question. For example, 15 dB is the total of 10 dB and 5 dB. The corresponding power out for 15 dB is 3.2% according to Table
10.2. This value is also attainable by multiplying the corresponding percent values for the two dB readings, 10 dB and 5 dB, to get the desired result, e.g. 10% times 32% is 3.2%. Thus, 3.2% ofthe light actually reaches the receiver.
