When testing loss in a fiber optic link, some basic principles must be kept in mind all the time.
1.The testing wavelength should always be the same as the working wavelength. Because optical fiber loss varies with light wavelength, you will get incorrect result if your measuring wavelength is different from the actual working wavelength. For example, if a system is designed for 1550nm but you test it with 1310nm light source and power meter, the result will not be correct.
2.The testing light source should be the same as the intended working lightwave equipment light source. If the system is designed for a LED source, you should test it with a LED source. If the system is designed for multimode laser light, you should use a multimode laser light source for testing. This is also true for single mode laser light source.
Fiber optic equipment used in a loss testing
In a basic loss testing setup, four types of test equipment are needed. They are the light source, the power meter, the reference patch cables and the adapter (mating sleeve).
Here are some considerations when choosing your equipment.
The light source should have the same wavelength as the operating equipment, proper mode (multimode or single mode, should be same as the operating equipment), type (LED or laser, same as the operating equipment) and proper connector.
The power meter should have the same wavelength as the light source, proper connector and calibrated.
The reference patch cables should be high quality with know loss, proper connectors and be the same type as the fiber plant being tested.