How to Use an OTDR — Step by Step
**OTDR (Optical Time-Domain Reflectometer)** is used to test a fiber link and find **fiber length, loss, splice loss, connector loss, bends, and fiber breaks**.
### 1. Prepare the Fiber
- Identify the fiber you want to test.
- Disconnect the fiber from the **OLT/ONT or any active optical equipment**.
- **Never connect an OTDR directly to a live/active fiber signal.**
### 2. Connect the OTDR
**OTDR → Launch Fiber → Fiber Link**
For more accurate testing, use a **launch cable** and, when available, a **receive cable**.
### 3. Clean the Connector
Before connecting:
- Clean the OTDR port.
- Clean the fiber connector.
- Make sure there is no dust or dirt.
### 4. Set the OTDR Parameters
Select the appropriate:
- **Wavelength:** 1310 nm / 1550 nm
- 1310 nm is commonly used for general fiber testing.
- 1550 nm is useful for detecting bends and additional attenuation.
- **Range:** Set it higher than the expected fiber length.
- **Pulse Width:**
- Short pulse → better resolution for short links.
- Long pulse → better for long-distance links.
- **Averaging Time:** Longer averaging generally gives a cleaner trace.
### 5. Start the Test
Press **Auto/Start** on the OTDR.
The OTDR sends a laser pulse into the fiber and measures the light that is reflected/scattered back.
### 6. Read the OTDR Trace
The screen will show a graph with events along the fiber.
Look for:
**Distance → Event → Loss → Reflection**
Typical events include:
- Connector
- Splice
- Splitter
- Bend
- Fiber end
- Fiber break
### 7. Find a Fiber Break
If the trace suddenly drops to the noise floor and ends, it can indicate a **fiber break or end of the fiber**.
The OTDR may show the distance, for example:
**Fiber Break = 2.35 km**
This means the suspected event is approximately **2.35 km from the OTDR**.
### 8. Check Fiber Loss
The OTDR can display the attenuation/slope of the fiber, usually in:
**dB/km**
For example:
**Fiber attenuation = 0.35 dB/km**
### 9. Check Splice/Connector Loss
An event may show something like:
**Splice Loss = 0.12 dB**
Lower loss generally indicates a better splice, provided the measurement is valid and within the instrument's limits.
### 10. Save the Test Report
After testing, save the trace/report with information such as:
- Cable name
- Fiber number
- Wavelength
- Test date
- Fiber length
- Event table
- Loss measurements
### Simple OTDR Testing Flow
**OTDR → Launch Cable → Fiber Link → Receive Cable**
**Set Wavelength → Set Range → Set Pulse Width → Start Test → Analyze Trace → Save Report**
### Important
If you are testing **FTTH/GPON**, a normal OTDR measurement requires special care around the **splitter**, because the splitter introduces substantial loss and can make downstream events difficult to resolve. Also, always use the appropriate OTDR settings and test point for your network.
