The optical power meter is a necessary test meter for testing an optical fiber communication system. It is mainly used to measure the optical power of various wavelengths at multiple measurement points of an optical link. Optical power indicates the energy of the light at a measurement point of an optical link and is an important index of the optical fiber network. When the optical power is smaller than a specified value, the optical receive end will fail to detect optical signals. In other words, the optical receive end cannot receive the signals sent from the transmit end. Hence, it is important to use the optical power meter correctly.
The following considers EXFO's PPM-350B optical power meter as an example to describe how to use an optical power meter. (Other dedicated optical power meters for PON are used in a similar way.)
The PPM-350B optical power meter can measure the optical power of various wavelengths, including 1310 nm, 1490 nm, and 1550 nm in the GPON network. Figure 1 shows the appearance of the PPM-350B optical power meter.
As shown in Figure 1, the PPM-350B optical power meter is different from common optical power meters. Specifically, the PPM-350B has a downstream input optical port and an upstream input optical port and can display the optical power of three wavelengths: 1310 nm, 1490 nm, and 1550 nm.
Figure 2 shows the common measurement points.
Maintenance engineers should also know related optical specifications on the ONT side, such as the maximum output optical power of the 1310 nm wavelength, minimum input optical power of the 1490 nm wavelength, and receiver sensitivity of the 1490 nm or 1550 nm wavelength. Table 1 lists the optical specifications on the ONT side.
Table 1 Optical specifications of optical ports on GPON ONTs
| ||||
Parameter Type
|
Wavelength (nm)
|
Unit
|
Min.
|
Max.
|
Upstream data
|
1310
|
dBm
|
+0.5
|
+5
|
Downstream data
|
1490
|
dBm
|
-27
|
-8
|
Downstream CATV
|
1550
|
dBm
|
-8
|
+2
|
To use an optical power meter, do as follows:
Connect optical fibers to optical ports correctly in upstream and downstream directions.
Turn on the power supply.
Choose the measurement unit (dB or dBm).
Perform the measurement.
Figure 3 shows the measurement interface of the optical power meter.
Optical channel loss is the total insertion loss caused by optical fibers, optical splitters, optical fiber connectors, and fiber connection points. Table 2 shows the estimation of optical channel loss in the engineering design.
Item
|
Average Loss (dB)
| |
Connection point
|
Connector
|
0.3
|
Mechanical splicing
|
0.2
| |
Fusion splicing
|
0.1
| |
Optical splitter
|
1:64
|
19.7
|
1:32
|
16.5
| |
1:16
|
13.5
| |
1:8
|
10.5
| |
1:4
|
7.2
| |
1:2
|
3.2
| |
Optical fiber (G.652)
|
1310 nm (1 km)
|
0.35
|
1490 nm (1 km)
|
0.25
| |
Optical channel loss = L x a + n1 x b + n2 x c + n3 x d + e + f (dB)
NOTE:
a indicates the average loss of an optical fiber per kilometer (unit: dB/km). L indicates the total length of the optical fiber (unit: km). The loss of patch cords and pigtail fibers used in engineering can be ignored because they are usually very short.
b indicates the loss of a fusion splicing point (unit: dB) and n1 indicates the number of fusion splicing points.
c indicates the loss of a mechanical splicing point (unit: dB) and n2 indicates the number of mechanical splicing points.
d indicates the loss of a connector (unit: dB) and n3 indicates the number of connectors.
e indicates the loss of an optical splitter (unit: dB). Only 1-level optical splitting is considered here. In the case of 2-level optical splitting, the loss of two optical splitters must be considered.
f indicates the engineering margin. Generally, the value is 3 dB.
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