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Is multimode fiber a better choice than single mode fiber?

DATE: 2012-06-06 | SOURCE: HOPE | Click:

Well, the short answer is YES for premises and local area networks.

Single mode and multimode fibers have different network applications, because they have very different optical-performance attributes.

In the design of premises and local area networks the decisive factor is the overall cost of all components. The large core of multimode fibers (50um or 62.5um) has distinct advantages: it enables low loss connection and facilitates fiber-to-fiber and fiber-to-transceiver alignment. This makes it best suited to premises and local area network applications.

Premises and LAN networks include three major components: fibers, switch equipment and optical transceivers. Since switch equipment are independent of fiber type, transceiver cost has the greatest impact on fiber optic network budget.

So now let’s take a closer look at the four major types of optical transmitters and their difference in cost and optical performance.

DFB(Distributed Feedback) laser: DFB lasers are the most expensive lasers compared to the other three type of lasers. Its high-power and highly focused beam makes efficient power coupling into the 9um small core of single mode fiber. DFB lasers are most suitable for long haul single mode fiber applications. 

FP(Fabry-Perot) laser: FP lasers are also expensive compared to LED and VCSEL light source. Its high-power and highly focused light beam also make it most suitable for long distance single mode fiber communication applications.

LED: LEDs have low launch power and broad spectrum. Its large numerical aperture and low power level make it unsuitable for single mode fiber applications. But it traditionary offers an inexpensive solution for short-reach network applications, though now it is fading out of favor in premises networks due to its limited bandwidth.

VCSEL: Vertical-Cavity, Surface-Emitting Lasers (VCSEL) do have a low numerical aperture which makes coupling into 9um single mode fiber possible. But since it has a large active area (15um) and low source power, the efficiency of coupling VCSEL lasers into single mode fibers is very low. So VCSEL has become the sweetheart for short-reach multimode fiber premises network applications.

The standardized 1,310-nm FP and DFB laser transmitters, required for transmission over single-mode fiber, are restricted by their inherent composition to a lower data rate boundary of 1,000 Mbps. By contrast, low-cost 850-nm VCSELs that are suitable only for transmission over multimode fiber have a broad data-rate operating range–from 10 Mbps up to 10 Gbps. Therefore, the combination of multimode fiber and 850-nm VCSELs constitutes a low-cost solution that facilitates the current needs of a low data rate network, while also future proofing that network by providing a clear migration path from 10 Mbps to 10 Gbps on the same fiber.

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