Indoor high-speed optical cable (Fluke): how to test the network based on laser technology

The installation of protocol-based (for example, Gigabit Ethernet) networks boosts the rapid increase in the use of optical cables in Lan, which can meet the increasing bandwidth and transmission speed requirements. Multimode Optical fiber cables, especially for Gigabit Networks, and single-mode optical fiber cables as high-speed transmission media began to be laid in large quantities in buildings. Therefore, in order to get a fast data transmission rate, the LED light source is replaced by a laser light source. Because the laser light source can be used in multi-mode optical fiber cables or in single-mode optical fiber cables, the test requirements have changed. Engineers who install and maintain optical fiber cables or optical fiber networks must learn the new optical fiber testing process and learn how to choose the right testing instrument for accurate measurement results.

Optical Fiber Transmission Basics

After learning about the differences between laser and LED light sources and the differences between single-mode and multi-mode optical fiber cables, you will know why high performance is required. A high-speed optical fiber cable network needs to properly select the appropriate testing instrument.

Laser and LED

The laser emitting power is very high, and the emitted light is very concentrated, while the light emitted by the LED is usually scattered and the power is low. Compared with LEDs, a laser can send high-speed pulses, which is also the main reason for using a laser in high-speed networks. For Single-Mode Optical Fiber Links, the Fabry-Perot Laser light source is usually used, while the new VCSEL (Vertical Cavity Surface Emitting Laser) Laser is used in the Multi-Mode Optical Fiber short-wave long gigabit network.

Multimode and Single-Mode Optical Cables

The main physical difference between a single-mode optical fiber cable and a multi-mode optical fiber cable is the size of the cable core. Multimode Optical fiber cables have two core sizes (50.0 μm and 62.5 μm), and the rated size of single-mode optical fiber cables is 9.0 μm. Multi-Mode Optical Cables allow optical signals to be transmitted in multiple paths (or modes), while single-mode optical cables allow only one path to be transmitted, just like their names. (See)

High mode and Low Mode

In a single-mode optical cable, because there is only one mode, it is called "fundamental" or "Low Mode", it is transmitted in the center of the cable core. On the contrary, multimode optical cables contain multiple modes. Those restricted in the core center of the cable are called the "Low Mode", while those that are transmitted outside the core of the cable are called the "high mode ". This difference is very important for Optical Loss testing, because the higher order mode is more prone to optical cable bending, resulting in attenuation.

Laser and LED are also different in actual work. Because the energy of the laser light source is concentrated in the center of the optical cable, only the low mode is actually used in Multimode Optical Cable. However, LED generation is called overflow emission because it is fully filled with the entire optical cable used in all modes, including low mode and high mode. (See)

Therefore, multimode optical fiber cables are more susceptible to attenuation than laser light sources, such as VCSEL light sources. This is also one of the main reasons why VCSEL laser sources are selected as high-speed network applications.

Now we know the main differences between the light source and the optical cable. Let's discuss the correct method for testing the laser-based network. First, let's take a look at the single-mode optical cable.

Test single-mode laser network

Single-mode optical fiber cables are transmitted using a laser light source. Therefore, the laser light source should be used for pull testing. But what if you already have a 1300nm LED light source? Can you use it to test single-mode optical fiber cables? The answer is that you may be able to use it, but it is strongly recommended not to use it because this method is subject to many restrictions. First, there is a considerable loss between the laser source (62.5 μm multi-mode optical fiber cable) and the network cable (9.0 μm. This type of optical cable core does not match to form a loss of about-20 dB. See the description of the mismatch of Optical Cable Size in. Therefore, the actual distance you test is limited.

But more importantly, if you only perform a loss test at the wavelength of 1300nm, you will miss the most important part of the measurement at the wavelength of 1550nm. When testing a single-mode optical cable, the characteristics of the tested optical cable link at any possible wavelength should be correctly evaluated. If you use LEDs to test single-mode optical fiber cables as discussed above, you can only know the performance of the optical fiber cable link at 1300nm. However, because a single-mode optical fiber cable may use nm for transmission throughout its life, it is very important to test at two wavelengths. It is particularly important to test at 1550nm, because the 1550nm wavelength is more susceptible to the loss caused by the completeness than the 1310nm wavelength. (See table 1 ). If the installed optical cable network is only tested at 1310nm, you may still be severely affected by bending, which cannot be found at the 1310nm wavelength. If the network application uses a wavelength of 1550nm, problems may occur, because the attenuation of 1550nm wavelength is more susceptible to bending.

Table 1

Test 11.5 and 18mm cylinders are severe. They do not simulate the actual application, but describe the relationship between wavelength and loss, which may not be caused during installation.

All the above instructions prove that it is important to follow the correct testing method for single-mode optical fiber cables. All in all, it is fundamental to test the network based on a single-mode optical fiber cable using a laser light source and testing at 1310nm and 1550nm wavelengths.

Testing of Gigabit Ethernet multi-mode optical fiber cables

Many networks based on Multimode Optical fiber cables are running Gigabit Ethernet, and the allowable Loss Test indicators are much stricter than 10 or 100 M optical fiber cables. Therefore, the testing method and accuracy are more critical. For Gigabit Networks, VCSEL laser light sources are used for optical transmission at nm and conventional Fabry-Perot laser light sources are transmitted at nm. (Because the nm vcsel laser light source has not yet been commercialized ).

It is very important to use a light source of the same type as the actual light source when testing a gigabit network multi-mode optical fiber cable. Therefore, laser is the best choice. Why? Because the light sent by the LED is very wide and the energy is divergent, it is filled with the entire multimode optical fiber cable and there are many higher modes than the laser light source. These high-order models are more susceptible to bending and cause great attenuation. In addition, if there is a mismatch between the optical fiber and the optical fiber, (the non-uniformity is certainly there), the optical fiber at the receiving end cannot receive all the energy. On the contrary, the concentrated laser light source is not so sensitive to non-matching (unmatched connections. (SEE ).

Therefore, the loss of LED is much greater than that of VCSEL light source (due to bending and connector loss ). (See table 2 ).

The result is that unqualified results are likely to be obtained when LED testing is used (especially when Gigabit Ethernet standards are strictly required), while the actual optical cable link is a qualified link. Unqualified test results lead to a large amount of time to check for faults that may not exist at all. The usage of the gigabit network in the Multi-Mode Optical Cable 1310nm is the same.

Conclusion

The installation of single-mode optical fiber cables and multi-mode optical fiber cables for applications supporting Gigabit Ethernet is rapidly increasing. These high-speed networks use different laser sources as transmission sources. Perform optical fiber link authentication tests on the use of the basic optical fiber cables of the network, switches, and light sources of the same type as other network devices to ensure that the Optical Fiber Links can meet the requirements of network applications.

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