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Optical Link Loss Budget

A logical way to proceed with designing a fiber optic link involves analyzing the fiber optic link power budget, also called an optical link loss budget. Figure 1 illustrates the key required optical calculations for designing a fiber optic link. A practical link must tolerate some range of optical loss. Ideally, but not always, it should work back-to-back (i.e., with the shortest possible fiber). And of course, it should work with some longer length of fiber. The designer can often adjust any or all of these variables to create a product that meets the needs of a given application.
Figure 1 — Optical Link Loss Budget

Optical Link Loss Budget
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The graphic shows a hypothetical link and its corresponding link budget. Start with the transmitter output power on the left side of the chart. The typical launch power is -12.5 dBm. However, the transmitter LED output power can vary by ±2 dB due to manufacturing variability of the 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 electronics and the electro-optics (e.g., LED or laser). Another potential ±2 dB of loss is due to variations in the optical coupling to the transmitter output. The effects of aging, typically 1-3 dB, should be included in the system's design. The next factor involves the losses due to optical connectors that may be in the optical path. The graphic allows 2 dB for this factor. For this system, the loss due to the optical fiber itself amounts to 4 dB/km of length. Multiply this value times the actual length 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 optical inputs as high as -5.5 dBm and as low as -31.5 dBm. Or stated differently, the receiver would need an optical loss range or optical dynamic 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. It is always a ratio between two numbers. For fiber optics, the ratio is usually the transmitter output power compared to the receiver input power. The equation to calculate a decibel is: dB = 10 x log10(Power1/Power2) The decibel describes all loss mechanisms in the optical path of a fiber optic link. For example, a given AM video link may tolerate a maximum of 9 dB of optical loss. How much light actually reaches the receiver? Table 1 describes the decibel to power conversion. According to the table, 12% of the optical 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 optical loss, only 1% of the transmitter's optical output would reach the receiver. To determine the amount of light reaching the receiver, take any two values that total the dB of optical loss 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 1. 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% of the light actually reaches the receiver.

Table 1 - Decibel to Power Conversion
dB Power Out as a % of Power In % of Power Lost Remarks
1 79% 21% ---
2 63% 37% ---
3 50% 50% 1/2 the power
4 40% 60% ---
5 32% 68% ---
6 25% 75% 1/4 the power
7 20% 80% 1/5 the power
8 16% 84% 1/6 the power
9 12% 88% 1/8 the power
10 10% 90% 1/10 the power
11 8% 92% 1/12 the power
12 6.3% 93.7% 1/16 the power
13 5% 95% 1/20 the power
14 4% 96% 1/25 the power
15 3.2% 96.8% 1/30 the power
16 2.5% 97.5% 1/40 the power
17 2% 98% 1/50 the power
18 1.6% 98.4% 1/60 the power
19 1.3% 98.7% 1/80 the power
20 1% 99% 1/100 the power
25 0.3% 99.7% 1/300 the power
30 0.1% 99.9% 1/1000 the power
40 0.01% 99.99% 1/10,000 the power
50 0.001% 99.999% 1/100,000 the power

When purchasing fiber optics, some customers often mistakenly feel they must over-specify the system to include transmitter optical output power, receiver sensitivity, and optical link loss budget. In most cases, the customer needs to specify only the optical link loss budget. An application that requires a 10 dB maximum optical link loss budget will operate the same using a transmitter with a 0 dBm output and a receiver with -10 dBm sensitivity or a system with a transmitter with a -10 dBm output and a receiver with -20 dBm sensitivity. By specifying only the required maximum optical loss, the most economical transmitter/receiver pair can be utilized. The only time that transmitter optical output power and receiver optical sensitivity need to be specified is when the transmitter and receiver are bought separately. In that case, the maximum optical link loss budget need not be specified.