How to calculate the optical attenuation of optical splitter?

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The optical splitter has one upstream optical interface and several downstream optical interfaces. When the optical signal is transferred from the upstream optical interface to the downstream optical interface, the optical signal strength/optical power will decrease. Do you know how much the optical splitter attenuates?

1. Attenuation calculation of optical splitter
The most important performance of the optical splitter is the different optical attenuations generated by the optical splitter under a specific splitting ratio. In the case of different splitting ratios, the optical attenuation of the optical splitter will also be different. So how to calculate the optical attenuation of the optical splitter?

  • Splitting loss: The loss caused by different splitting ratios to the optical signal is called splitting loss, and its value is -10lgK.
    Optical splitter splitting loss comparison is as follows:
Splitting ratio Splitting loss Splitting ratio Splitting loss Splitting ratio Splitting loss Splitting ratio Splitting loss
% dB % dB % dB % dB
3 15.23 25 6.02 47 3.28 69 1.61
4 13.98 26 5.85 48 3.19 70 1.55
5 13.01 27 5.69 49 3.10 71 1.49
6 12.22 28 5.53 50 3.01 72 1.43
7 11.55 29 5.38 51 2.92 73 1.37
8 10.97 30 5.23 52 2.84 74 1.31
9 10.46 31 5.09 53 2.76 75 1.25
10 10.00 32 4.95 54 2.68 76 1.19
11 9.59 33 4.81 55 2.60 77 1.14
12 9.21 34 4.69 56 2.52 78 1.08
13 8.86 35 4.56 57 2.44 79 1.02
14 8.54 36 4.44 58 2.37 80 0.97
15 8.24 37 4.32 59 2.29 81 0.92
16 7.96 38 4.20 60 2.22 82 0.86
17 7.70 39 4.09 61 2.15 83 0.81
18 7.45 40 3.98 62 2.08 84 0.76
19 7.21 41 3.87 63 2.01 85 0.71
20 6.99 42 3.77 64 1.94 86 0.66
21 6.78 43 3.67 65 1.87 87 0.60
22 6.58 44 3.57 66 1.80 88 0.56
23 6.38 45 3.47 67 1.74 89 0.51
24 6.20 46 3.37 68 1.67 90 0.46

The splitting loss is actually caused by the transfer of optical power during splitting, not the consumption of optical power, so it cannot be included in the calculation of the splitting ratio. But it must be added when calculating the total loss of the optical link.

  • Excess loss: When the optical splitter distributes the optical signal at the input end to each branch according to the pre-terminated splitting ratio, in addition to the splitting loss, there is also the loss caused by the optical splitter itself to the optical signal. This loss is called the excess loss of the optical splitter.
    The excess loss is related to the ways of the optical splitter.
Ways 2 3 4 5 6 7 8 9 10 11 12 16
excess loss(dB) 0.2 0.3 0.4 0.45 0.5 0.55 0.6 0.7 0.8 0.9 1.0 1.2
  • Connector loss: Connector losses are calculated based on the total number of connectors in the optical link. Each optical link has at least two connectors: between the optical transmitter and the optical splitter, and between the end fiber and the optical receiver. For optical splitters with a large number of outputs, connectors are usually used between the output ports and optical fibers. When optical amplifiers and multi-level optical splitters are used, the number of connectors may be more.
    The excess loss of optical splitting is caused by the optical power consumed by the optical splitter itself during optical splitting; the connector loss is also caused by the optical power consumed by itself. Therefore, these two items should have been added to the fiber loss when calculating the splitting ratio, and the total loss dB of the three should be calculated, and then the total loss power in mW should be converted, and then the splitting ratio should be calculated accordingly. The final calculation result is the most accurate.
    However, since the value of the excess loss of optical splitting and the connector loss is much smaller than the loss of optical fiber, and the values of each optical link are basically equal, these two values of each optical path are ignored when calculating the splitting ratio. The effect on the calculation result of the splitting ratio is very small. Therefore, when calculating the splitting ratio, the excess loss of splitting and the connector loss are usually ignored, and only the optical fiber loss is converted into optical power to calculate the splitting ratio. However, when calculating the total loss of the optical link, these two values must be taken into account.
  • Insertion loss: Insertion loss includes splitting loss and excess loss, namely insertion loss (dB) = -10lgk + excess loss.
    If we have listed “splitting loss” and “excess loss” separately in the design, then there is no need to use the data of “insertion loss”.
    The calculation formula for optical link (total) loss is:
    Optical link loss = fiber loss + splitting loss + splitting excess loss + connector loss (dB)
    If we need to replace “splitting loss” and “excess loss” with “insertion loss”, the calculation formula of optical link (total) loss is:
    Optical link loss = fiber loss + insertion loss + connector loss (dB)

Common splitter insertion losses are as follows:

Equal splitter Wavelength (nm) Bandwidth (nm) Coupling Ratio Insertion Loss (dB)
1×2 FBT splitter 1310 & 1490 & 1550 1310 ± 40%  & 1490 ±10% &1550 ±40 Equal ≤3.5
1×4 PLC splitter 1260-1650 ≤7.2
1×8 PLC splitter ≤10.2
1×16 PLC splitter ≤13.5
1×32 PLC splitter ≤16.6
1×64 PLC splitter ≤20
2×2 FBT splitter 1310 ± 40%  & 1490 ±10% &1550 ±40 ≤3.5
2×4 PLC splitter 1260-1650 ≤7.4
2×8 PLC splitter ≤10.6
2×16 PLC splitter ≤14
2×32 PLC splitter ≤17.2
2×64 PLC splitter ≤20.4

2. Power loss (mW) VS fiber loss (dB)
Optical fiber loss is caused by optical power consumption when optical signals are transmitted in optical fibers. The dB of fiber loss converted into the mW of single-path power loss is calculated as follows:
A single power loss = fiber loss (mW)
(A certain channel) splitting loss = -10lg K (dB)
(A certain channel) splitting ratio K = a single channel power loss / the sum of the power losses of each channel

Optical power conversion dB mW
dB-mW -25.0000 0.0032
mW-dB -30.0000 0.0010

3. Calculation of optical link loss
The optical link loss includes three parts: one is the attenuation of the optical signal strength by the optical cable; the second is the attenuation of the optical signal by various connectors in the network; the third is the attenuation of the optical signal by the devices in the network, such as optical signal, splitting loss and excess loss of the splitter.
The entire loss of the optical link can be calculated as follows: A=aL-10lgk+AC+Af.
In the formula: A is the total loss of the optical link, aL is the attenuation of the optical fiber to the transmitted optical signal, α is the optical attenuation coefficient, and L is the length of the optical link. In the design, when the wavelength of the optical signal is 1310nm, a=0.4dB/km is generally taken, and when the wavelength of the optical signal is 1550nm, a=0.25dBKm (including the splice loss) can be taken. Ac is the connector loss, and each connector can be calculated as 0.5dB. Af is the fake optical splitter additional loss.

The above is the explanation of the loss calculation of the optical splitter in the optical splitting network.

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