In the optical fiber transmission system, the optical signal will be attenuated with the extension of the transmission distance due to factors such as nodes or pollutants in the system. This is what we often call optical fiber loss. Optical fiber loss directly affects the transmission distance and performance of the optical fiber system. Therefore, we should always pay attention to the optical fiber loss of the optical fiber link in the transmission system. This tutorial will introduce in detail the fiber loss test process of single-mode fiber and multi-mode fiber.

 

Tool preparation for fiber loss test

 

1 light source

 

LED light sources and laser light sources are commonly used light sources in fiber loss testing. The LED light source is a light source with a light emitting diode (LED) as the luminous body. The price is relatively low, and it is more suitable for multi-mode fiber optical fiber loss testing applications. Compared with laser light sources, LED light sources have wider bandwidth and lower output power, and their center wavelength is generally 850 nm or 1300 nm.

 

The laser light source is an electric light source that uses excited particles to emit light under the action of stimulated radiation. The structure is more complicated, so the price is higher. The output power is about 10 times that of the LED light source. The center wavelength is 850 nm, 1310 nm or 1550 nm, which is suitable for single-mode fiber loss testing applications.

 

2 Optical power meter

 

With the continuous development of photoelectric detection technology, PIN photodetectors are more and more used in optical power meters. There are three main types of PIN photodetectors in optical power meters: Si-PIN, Ge-PIN and InGaAs-PIN . Among them, the operating wavelength of the Si-PIN optical power meter is 850 nm, and the operating wavelengths of the Ge-PIN optical power meter and InGaAs-PIN optical power meter are 850 nm, 1310 nm and 1550 nm. Based on this, the price of Si-PIN optical power meter is low. In general, Ge-PIN optical power meters and InGaAs-PIN optical power meters are commonly used in most fiber loss testing applications.

 

3 Fiber optic patch cords and adapters

 

Fiber optic jumpers and connectors are tools used to connect the optical power meter and light source to the link under test. It should be noted that the core diameter and cladding diameter of the fiber jumper must be the same as the fiber under test, and the fiber jumper should not be too long or too short, generally 1-3m.

 

In addition, before starting the test, use a microscope to check whether the connectors on the fiber jumper are physically damaged. The importance of this step cannot be ignored, because physical damage on the connector will directly affect the results of the fiber loss test. Not only that, because the connectors are divided into FC, LC, SC, ST and PC, UPC and APC, we should choose the connector type that matches the test equipment.

 

The choice of the adapter is also crucial, because the alignment mechanism of the adapter has an important influence on the test results. Some poor-quality adapters cannot accurately align the fibers in the two connectors after multiple uses, causing additional fiber loss. Here we recommend you to use high-quality adapters made of zirconia ceramics. In addition, the number of uses of the adapter should be less than 100 times to obtain accurate test results.

 

The steps of fiber loss test need to be understood: fiber loss measurement measures the difference in power. For example, suppose we want to test the fiber loss from point A to point B (as shown in the figure below), then there may be Fusion splices or mechanical splices, but these are not the issues we are concerned about when doing fiber loss testing. In addition. Before performing the fiber loss test, determine the working wavelength of the light source, then turn on the light source and keep it in this state for about 10 minutes to stabilize the emitted optical signal; then follow the steps below to complete the loss test:

 

Step 1: Measure the power when there is no optical cable to be tested (set the reference value), that is, connect the light source and the optical power meter through the fiber jumper and adapter described above, measure the optical power value at this time (such as -10dBm), and Use this value as the reference value (the reference value key on the optical power meter must be pressed at this time, and the optical power meter screen displays 0dB after pressing).

 

Step 2: Disconnect the jumper wire and the adapter in the picture above, and keep the connection between the jumper wire and the light source and the optical power meter still, and the light source and the optical power meter cannot be turned off.

 

Step 3: Connect the free ends of the jumpers on both sides to the fiber to be tested, as shown in the figure below. The value displayed on the screen of the optical power meter is the loss value of the optical fiber link under test (for example, 3dB). This is the end result that engineers want.

 

Because optical fiber links have great advantages in meeting people's needs for high bandwidth, and the use of optical fibers is becoming wider and wider, it is necessary to master the skills of optical fiber link testing whether it is wiring construction personnel or network maintenance personnel. In addition, choosing high-quality optical fiber loss test tools is also an important guarantee to ensure the accuracy of the test results.