In designing an optical link, the optical link loss budget must be calculated. This reflects the difference of the power generated by the transmitter compared to the amount of light, the receiver is set to receive. In calculating the link loss budgets, Consider first 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 applications, with a standard transceiver at 850nm, the dB budget is-7dB. That this means up to 7dB of power cans be lost in the cable plant loss and the receiver will of 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 devices have no internal losses Would and Would transmits 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 budget of-7db dB. Assuming a tolerance of + /-1db, the attention value of between-6dB and-8dB is optimal. But there is the need to eliminate false failures, so the attenuation value of-6dB comes close to the maximum stress for the device under test without the fear of incurring a failure rate of 50%.
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 devices have no internal losses Would and Would transmits 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 budget of-7db dB. Assuming a tolerance of + /-1db, the attention value of between-6dB and-8dB is optimal. But there is the need to eliminate false failures, so the attenuation value of-6dB comes close to the 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 | ||