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LED light source testing specialized light source is essential in testing. So why use testing fiber optic light sources? This is because the light emitted by 850nm LED and 850nm VCSEL is different. In technical terms, the emission forms of these two light sources are different. The light emission is relatively uniform across the entire cross-section of the multimode fiber core. contrary, The beam emitted by VCSEL light source is narrow, and the light it emits is concentrated at the center of the fiber core and rapidly weakens after leaving the center; It basically disappears near the boundary of the cladding layer. Two different luminescent forms result in differences in the measurement of losses. Testing is very different from single-mode testing.

Table 1 is a typical comparison with VCSEL light sources. It can be observed that the spectral width of VCSEL is much smaller than that of LD, therefore, compared to it, LD can be said to be injected with a single wavelength. At the same time, the size of the emitted light spot is different from that of the LD light source. With a larger output area, it can excite all the conduction modes of multimode fibers, while the smaller output spot of LD light sources can only excite a portion of the conduction modes. When LD is injected into different positions on the end face of multimode fiber, different mode groups are excited, which will cause significant differences in their mode dispersion and affect the transmission distance of pulses. Comparison between fiber optic LED light source and VCSEL emission spot.

Comparison of LED and VCSEL emitted light spots

Fiber Optic Light Source Link Certification - General or Default Specifications and Specific Light Source Specifications

According to TIA and ISO standards, the certification of fiber optic light source links should include measurement of fiber losses at two common wavelengths and polarity certification of two fibers in the link. The maximum length of horizontal wiring is 100 meters, and the fiber optic LED light source only needs to be tested with one wavelength.

The TIA-568-B.1 standard classifies light sources into 5 categories, ranging from Class 1 typical LED light sources to Class 5 FP lasers. VCSEL is usually classified as Class 3 or Class 4. This is important because the signal loss of fiber optic LED lighting sources is greater (or greater) than that of LED light sources (Class 1). Therefore, unless specific light source requirements are stated in the job description or link deployment instructions are provided, the standard requiring the use of fiber optic light sources represents a more conservative or 'worst-case' scenario.

The accuracy of loss measurement is crucial for high-speed networks

The loss limit specified in the Gigabit Ethernet standard becomes smaller and smaller with the increase of network speed. For example, the loss limit for a fiber channel (62.5 μ m) that supports 100 M/s Ethernet (100BASE-FL) is 11 dB, while the channel loss limit for the same fiber type is 2.38 dB (for 1000BASE-SX), and the length limit for higher bandwidth level fibers is 275 meters. It is very important to use testing equipment that can accurately measure these subtle differences.

When using fiber optic light sources, technicians should use appropriate reels to measure link losses. If a scroll is not used, the measured loss value will be greatly amplified. When using VCSEL or fiber optic LED light sources to measure losses, there is no need to use a spool.

Testing newly installed or existing multi-mode cabling installation systems to verify if the installed links have sufficient performance to upgrade the network to Gigabit Ethernet, using VCSEL and laser light sources can achieve more effective and appropriate testing results. If you wish to use a fiber optic light source instead of the default fiber optic LED light source specification to test the installation system.