Why OM2 Multimode Fiber Has More Fiber Loss Than OM1?

The OM1 and OM2 fiber have been widely adopted and deployed in enterprise networks and campus networks and small office buildings throughout 80’s, 90’s and into the early 2000’s. During the process of usage, users are gradually noticed that OM2 fiber are more easily damaged compared to OM1. Due to OM1 and OM2 are completely two different fibers, it is hard to compare them at some extend, especially in terms of parameter. In this article, we would clarify the reasons from the aspect of diameter core, modal bandwidth, and modal dispersion.

OM2 Fiber

Figure1: OM2 Multimode Fiber

Core Diameter

Core diameter is the key factor that affects the fiber loss. OM1 has 62.5um (micron) core diameter while OM2 has 50 um (micron) core diameter. A smaller 50μm core reduces the coupling input optical power based on the led light source, and thereby reduces the number of connectors allowed in the link and shortens the distance supported by the power limit. Due to the small core diameter and numerical aperture of 50/125μm multimode fiber, it is not conducive to efficiently coupling the fiber with the LED. And that is the one of the reasons why OM2 optical fiber has more fiber loss than OM1. Besides, the low-cost LED was generally adopted for OM2 as a light source rather than expensive ld with a purpose to minimize the cost of LAN systems. The low output power of LED leads to that the divergence angle is much larger than ld, and thus the connector loss is heavy.

OM1&OM2 Multimoder-FIber-Core-Diameters

Figure2: Core diameter of OM1&OM2 MMF

Modal dispersion

Modal dispersion is basically the way the light travels more “efficiently” with the lasers/leds. When an optical signal is transmitted over multimode fibers, it spreads into multiple modal components as it enters the medium and travels along the path. These components need to be recombined in a timely manner at the other end of the transmission link in order to be recognized and processed by the detection subsystem converting the optical signal into an electrical signal. The larger the size of the core, the easier it is for modes to be spread, and the harder it is for them to be recombined. Smaller core diameter eliminates modal dispersion and enables tremendous transmission capacity over very long distances. The number of transmission modes of 50μm multimode fiber is about 1/2.5 of the 62.5μm multimode fiber. You can get up to 10 times the speed in 50 micron rather than 62.5 just because of the effectiveness.

Modal despersion of OM1 and OM2 fiber

Figure3: Modal dispersion in OM1&OM2 MMF

Modal Bandwidth

Pulse spreading occurs due to Modal Dispersion or Differential Mode Delay (DMD). Pulse spreading limits Bandwidth. The main property of a multimode fiber is its capability to transmit a certain amount of information over a certain distance. This property is known as “modal bandwidth” and is expressed in MHz*km. The mode dispersion of the OM2 multimode fiber is effectively reduced, which results in a significant increase in bandwidth. OM1 supports between 160 and 200 MHz*km while OM2 supports between 400 and 500 MHz*km.

Conclusion

This article mainly discussed the reason why OM2 has more fiber loss than OM1, and that lies in three factors—diameter core, modal bandwidth, and modal dispersion. You can use both OM1 and OM2 fiber cable in the same types of networks, although OM2 cable is recommended for premise applications: backbone, horizontal, and intrabuilding connections, and should be considered especially for any new construction and installations. If you ever patch OM1 and OM2 fiber cable together, it is a kind of like putting two diffrent water pipes together. The data will work pretty good from 50 to the 62.5, but 62.5 to 50 it will have lots of loss. You can patch them to a GBIC or adaptor to do the conversion properly.

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