LEDs and VCSEL light sources required for Multimode Optical Fiber Signal Attenuation Testing

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Q: Why should I choose between the LED and VCSEL Optical Fiber signal attenuation tests? Which light source should I choose or specify?

A: Many fiber links in a building network building or campus can now transmit and support 10 Gbit/s Ethernet. Many links can be installed and tested by transmitting 10 Gbit/s of Ethernet traffic. Because the signaling rate supports such high-speed traffic, a good old light-emitting diode (LED) light source cannot continue to meet the needs. All network devices that support 1 Gbit/s or higher transmission rates now use laser light sources. Vertical resonator surface radiation laser or VCSEL) is a low-cost laser light source, suitable for 850 nm wavelength.

Differences between light sources

In addition to the different rate when switching these light sources, VCSEL and LEDs have many different characteristics that affect the transmission of light energy in the optical fiber link.

850 nm LEDs and 850 nm VCSEL emitting light sources are also different. The Light Emitting from the LED is a wide bundle, which is relatively uniform throughout the multi-mode fiber core. In contrast, the light emitted by the VCSEL light source is a narrow bundle, and the light is very bright in the center of the fiber core, and it quickly fades away from the center. This leads to different loss measurements. The Loss Measurement Result of LEDs is usually greater than that of VCSEL.

To confirm this difference, we used the LED and VCSEL light sources to test the 62.5 m link of the 200 μm multimode optical fiber. The difference between the measurement of LED and VCSEL Link Loss is 0.20 dB @ 850 nm. This typical result shows the difference between the time passed and the Failure decision when the budget is not sufficient.

Optical fiber link Authentication-common or default specifications and specific light source specifications

According to the TIA and ISO standards, the optical fiber link certification should include measurement of each optical fiber loss on two common wavelengths and polarity certification of the Two Optical Fiber Links. Horizontal cabling can be up to 100 meters in length and can be tested at only one wavelength.

TIA-568-B.1 standards classify the light source into 5 categories, from 1 class typical LED light source to 5 class FP laser. VCSEL is generally classified into three or four categories. This is important because the signal loss of multimode optical fiber is greater than that of the LED light source Class 1 ). Therefore, unless the work instruction requires a specific light source or provides a link deployment instruction, the standards requiring the use of the LED light source are more conservative or "worst.

The network owner or consultant can also specify other light sources. For example, when installing a laser-optimized optical fiber that supports 1 Gbit/s or 10 Gbit/s Ethernet, using the VCSEL light source instead of the LED light source to test the optical fiber link is the most appropriate choice. The network device will use VCSEL of 850 nm. The use of VCSEL light source for loss measurement can reflect the actual loss of network equipment during operation. The loss measurement with the LED light source will exaggerate the loss during the operation.

The accuracy of loss measurement is very important for high-speed networks.

The threshold value specified in the Gigabit Ethernet standard is smaller and smaller as the network speed increases. For example, a 100 μm fiber channel that supports 62.5 Mbit/s Ethernet 100BASE-FL) has a loss limit of 11 dB, however, the maximum channel loss value of the same optical fiber type is 2.38 dB for 1000BASE-SX), and the maximum length of a higher bandwidth optical fiber is 275 meters. It is important to use testing devices that can accurately measure these differences.

When using LED light sources, technicians should use appropriate reels to measure Link Loss. If no scroll is used, the loss value will be enlarged much. Scroll is not required when using VCSEL or laser light source to measure loss.

Conclusion

Test the newly installed or existing multi-mode cabling installation system to check whether the installed link has sufficient performance to upgrade the network to Gigabit Ethernet, more effective and appropriate test results can be obtained by using VCSEL and laser light sources. If you want to use a light source instead of the default LED specification to test the installation system, you can and should specify this requirement in the work instruction.

1. Development of Multimode Optical Fiber Technology and Its Network Application
2. Fluke: the most effective investment in Multi-Mode Optical Cable Attenuation Testing capabilities-transmitting reels