How to Measure Fiber Loss with Optical Power Meter and Light Source

How to measure fiber loss with optical power meter and light source? What is optical power? Simply put, optical power is the "brightness" or "intensity" of light. In optical fiber networks, the units of optical power are often expressed in milliwatts (mw) and decibel milliwatts (dbm). The relationship is: 1mw=0dbm, that is to say, 2mw=3dbm, 10*lgmw is the dbm value. It should be noted that decibel milliwatts less than 1mw are negative values.

In addition to measuring optical power, optical power meters can also be used with light sources to measure optical loss. So, what is optical loss? How to measure optical loss with an optical power meter?

What is optical loss?

Fiber loss refers to the loss of light energy when light propagates in the fiber. These losses are mainly caused by the absorption of fiber materials, the conversion of light energy to heat energy and the scattering of light. We also call this fiber loss "light attenuation". Optical fiber attenuation is the attenuation per unit length of optical fiber, and the unit is dB/km. When connecting two optical fibers, there will be loss inside any connector or joint. We call this loss "insertion loss", which is the attenuation caused by the insertion of the device. The insertion loss is closely related to the quality of the fiber optic connector and the splicing state of the fiber. In addition, since the optical fiber is inserted into the multiplexer, loss is also generated, so the insertion loss can also refer to the loss in the multiplexer.

How to measure fiber loss with optical power meter and light source?

Fiber loss is the difference between the power when light is coupled from the transmitting end to the fiber and the power when the light reaches the receiving end. To measure fiber loss, not only an optical power meter but also a light source are required. Generally speaking, when measuring the fiber loss of multimode fiber, you need to use 850/1300nm LED light source, and when measuring the fiber loss of single mode fiber, you need to use 1310/1550nm laser light source. In addition, in order to accurately measure the fiber loss, it is necessary to make the test environment and conditions of the fiber and connectors similar to the actual working conditions, that is, to select a suitable light source and optical cable, and the optical cable has a standard transmission power, and there is no influence on the power of the light source. Influence.

At present, there are mainly two methods for measuring fiber loss: one-end loss measurement method and two-end loss measurement method. The single-ended loss measurement method uses only the launch cable, while the double-ended loss measurement method uses a receive cable connected to the power meter in addition to the launch cable. Single-ended loss measurements measure the loss due to the connection of the connector to the launch cable as well as the loss within the fiber, splice, or other connector. Therefore, each connector can be tested individually, which is the best way to measure fiber optic patch cords; the double-ended loss measurement method can measure the loss in both connectors and all fibers, including the loss between the connector and the splice.

In fiber optic measurement applications, in addition to using optical power meters and light sources, you also need to use launch cables, adapters, visible light fault locators or light trackers, and clean inspection kits.

Four key factors of optical power meter

There are various types of optical power meters, and now the hand-held optical power meter is more commonly used. This type of optical power meter is easy to carry and is suitable for use in various optical fiber testing environments. In addition, when purchasing an optical power meter, we should also consider various factors such as application, wavelength, interface, and measurement range:

(1) Application

Ordinary optical power meters can generally test the absolute optical power of ordinary optical fiber links, but in some special applications, such as passive optical network (PON), coarse wavelength division multiplexing (CWDM) systems, etc., it is necessary to use PON optical power Power meter and CWDM optical power meter, so as to quickly and accurately measure the optical power of the corresponding line.

(2) Wavelength range

Each optical power meter has a certain working wavelength range, and generally between 800nm and 1700nm. If we want to measure the optical power of the line more accurately, we need to calibrate the wavelength of the optical power meter before measurement to make it consistent with the working wavelength of the optical fiber equipment. Most of the optical fiber devices now work at 850nm, 1310nm or 1550nm, so it is best to ensure that the wavelength range can cover these three wavelengths when purchasing optical power.

(3) Power measurement range

The power range that the optical power meter can measure also has an important impact on the accuracy of the measurement results. Generally speaking, the power measurement range of optical power for common applications should be between -70dBm to +30dBm.

(4) Interface type

When measuring optical power, it is usually necessary to use a fiber jumper to connect the optical power meter and the link under test. At this time, the interface type of the optical power meter must be consistent with the connector interface of the fiber jumper. In the past, the interface of the optical power meter was fixed. For example, many optical power meters only had one FC interface, which was very inconvenient to use; now, some optical power meters adopt the design of a common interface, and some are equipped with MPO/ MTP multi-core interface is much more flexible in measurement.