# dB vs. Optical Power Loss

In designing an optical link, the optical link loss budget must be calculated. This reflects the difference of power generated by the transmitter compared to the amount of light the receiver is set to receive. In calculating link loss budgets, first consider the output power of the transmitter. For example, most multimode transceivers have a minimum transmission value of -10dB; maximum receiver sensitivity is set at -17dB.  The result is that in a multimode application, with a standard transceiver at 850nm, the dB budget is -7dB. This means that up to 7dB of power can be lost in the cable plant loss and the receiver will still detect an optical signal.

The following table shows the percentage of power lost and its corresponding dB value. At -7dB, only 20% of power is received and yet the optical signal is received successfully. A theoretically perfect optical device would have no internal losses and would transmit 100% of the power, thus having 0dB.

Using the dB budget determined by the transceiver, determine the attenuation value to be used. In the example given above, there is a dB budget of -7db.  Assuming a tolerance of +/-1db, an attention value of between -6dB and -8dB is optimal.  But there is the need to eliminate false failures, so an attenuation value of -6dB comes close to maximum stress for the device under test without the fear of incurring a failure rate of 50%.

DECIBEL TO POWER CONVERSIONS
dB Power Out as a % of Power In % of Power Lost
.001 99.98% .02%
.01 99.8% .22%
.05 99% 1%
.1 98% 2%
.2 95.5% 4.5%
.3 93% 7%
.4 91% 9%
.5 89% 11%
.6 87% 13%
.7 85% 15%
.8 83% 17%
.9 81% 19%
1 79% 21%
2 63% 37%
3 50% 50%
4 40% 60%
5 32% 68%
6 25% 75%
7 20% 80%
8 16% 84%
9 12% 88%
10 10% 90%
11 8% 92%
12 6.3% 93.7%
13 5% 95%
14 4% 96%
15 3.2% 96.8%
16 2.5% 97.5%
17 2% 98%
18 1.6% 98.4%
19 1.3% 98.7%
20 1% 99%
25 0.3% 99.7%
30 0.1% 99.9%
40 0.01% 99.99%
50 0.001% 99.999%

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