Testing PON optical loss starts with a power meter that reads receive power at the ONU, then an OTDR to locate any break or splice loss along the fiber. Most PON faults trace back to the optical link — a bent fiber, a dirty connector, a bad fusion splice, or a miscalculated splitter loss — so measuring light budget is the fastest way to find out why a passive optical network stops performing. A power meter and an OTDR between them pin down roughly 80–90% of field problems.
A quick OLT test with a power meter tells you whether receive power sits in the normal band before you blame the equipment.
GPON Class B+ ONUs should read between −8 dBm and −27 dBm; the closer to −27, the thinner your margin.
An OTDR finds the break or the big loss point, but always disconnect the OLT and ONU first so you don't burn their optics.
At the ONU side or the downstream port of a fiber splitter, connect a power meter and select the right wavelength. For GPON and XGSPON the downstream is 1490 nm and the upstream is 1310 nm; to check downstream receive power you read 1490 nm. A GPON Class B+ ONU has a receive sensitivity near −28 dBm and saturates around −8 dBm, so a healthy reading lands between −8 dBm and −27 dBm. In a passive optical network this single number decides whether each gpon onu can stay online, so read it before swapping any hardware.
Tip: Clean the fiber connector before you measure. A dirty ferrule looks exactly like a link fault and will make you chase a problem that isn't there.
The power meter only answers one question — is the light I am receiving correct? It does not tell you where the fault sits. That is the job of the OTDR.
An OTDR fires a pulse into the fiber and reads the reflection curve back. A step in the curve is a fusion or connector loss, a sharp reflection spike is a mating connector, and a curve that drops straight to the floor is a break. Run the test from the central office or from the suspected segment.
Warning: Disconnect the OLT and the gpon onu before launching the OTDR. The OTDR pulse can damage the optical ports of live equipment if they stay connected.
This is where the test earns its keep: once the power meter says the budget is wrong, the OTDR walks you to the exact meter where the loss happens.
| Item | Reference | Note |
|---|---|---|
| GPON Class B+ receive power | −8 to −27 dBm | Closer to −27 means thinner margin |
| 1:64 splitter insertion loss | ≈ 20 dB | Includes the splitter and even splitting |
| Single-mode fiber attenuation | ≈ 0.36 dB/km | At the 1490 nm window |
| Mating connector loss | ≤ 0.5 dB each | A dirty connector pushes this up fast |
Can I still run the link at −30 dBm receive power? You are at the edge of GPON Class B+ sensitivity with zero margin — any temperature swing or slightly dirty connector drops you offline. Check your split ratio and fiber distance, or move to a higher-power class such as Class C+.
Should I buy a power meter or an OTDR first? Buy the power meter first. It is cheap, used daily, and instantly tells you whether receive power is inside the threshold. The OTDR is expensive and only comes out to locate a break. The two together are the most efficient field kit.
Power reads fine but the link still drops — why? The upstream direction (GPON 1310 nm) may have its own issue, or the problem is gpon onu power supply or data configuration. A clean optical link does not guarantee the whole service is up.
Run an OLT test by reading receive power with a power meter first (−8 to −27 dBm for GPON Class B+), then use an OTDR to find the break or the large loss point. Clean the connector before you draw any conclusion and you will avoid the majority of false faults.
Customer Manager at Shenzhen Rayin Technology Co., Ltd.
