The Difference Between Types of Fiber Optic Cable

What Are The Three Types Of Fiber Cable Available In Enterprise Networks Today?

There are three type of fiber cable in Enterprise networks today – Multimode, Singlemode, and Laser-Optimized Multimode. Which fiber cable is better? The answer depends on the parameters of the network: the applications the network will need to support over the next few years and the length of the links. It also depends on whether you are evaluating a new installation or upgrading from an installed base.

 

Historically there were three types of fiber cables commonly used in cabling systems: 62.5/125 µm multimode fiber (OM1), 50/125 µm multimode fiber (OM2), and singlemode fiber (OS1 or OS2). The other type of 50/125 µm fiber, optimized for low-cost 850 nm laser applications (OM3 or 4), in now probably the most common specified in cabling and LAN application standards worldwide.

 

The main performance difference lies in the fibers’ bandwidth, or information-carrying capacity, and in the power-coupling efficiency to light-emitting-diode (LED) sources. Bandwidth is actually specified as a bandwidth-distance product with units of MHz-km, and as the data rate goes up (MHz), the distance that data can be transmitted (km) at that rate goes down. Thus, a higher fiber bandwidth can enable you to transmit at higher data rates or for longer distances.

 

But while fiber bandwidth is important in determining link length and data rate, transmitter and receiver characteristics also play a critical role. Any statements on the distance capabilities of a particular fiber cable type must be made in the context of the full suite of specifications for a given application.

 

EXTENDING THE CAPABILITIES OF OPTICAL FIBER

There are multiple ways to extend the capability of the different types of fiber cable, some of which optical fiber standards have not yet make the most of:

 

Copper based-LANs for example use multi-level coding which increases transmission capacity and uses less bandwidth. This technique has yet to be used widely on multimode fiber cables.

 

Fiber cables can also take advantage of wavelength division multiplexing (WDM), which uses different colours/wavelengths of light across the same fiber to provide more channels.

Parallel transmission is another way of increasing link speeds, with multiple fibers used to transmit data. Also, devices such as short wavelength lasers and vertical cavity surface emitting lasers (VCSELs), are capable of providing cost-effective gigabit-rate data links over multimode fiber.

 

As network speeds continue to evolve ever higher, these new technologies and approaches will continue to be developed and deployed.

 

HOW DO YOU COMPARE MULTIMODE FIBER TYPES?

How fiber is qualified and tested should be one of the first questions asked in any situation. The bandwidth of a fiber is always specified in MHz-km and at specific wavelengths (i.e. 850 nm); however, test methods differ.

 

Historically, multimode fiber was tested and bandwidth specified using the OFL (Overfilled Launch) method. This method was optimized for use with LEDs. But as the gigabit networking era kicked in, lasers (VCSELs) were needed to transmit speeds above 1 Gbps, so a new test method was required called DMD (Differential Mode Delay).

 

In the DMD process, a laser is used to transmit pulses across the entire fiber core. As each of these pulses is received by a high-speed detector at the far end, the pulse delay is plotted and the DMD is calculated. This process is automated and covers all laser launch modes.

 

It is important to note that “laser” bandwidth, also referred to as Effective Modal Bandwidth (EMB), is NOT the same as “overfilled” bandwidth (OFL). For instance, 50 micron multimode fiber with an OFL bandwidth of 500 MHz-km at 850 nm does not automatically equate to a laser bandwidth of 500 MHz-km; that can only be proven by laser testing.

 

The standard DMD measurement process involves scanning the output from a singlemode fiber across the core of the sample multimode fiber core in radial launch positions separated by incremental steps of 2 µm. Some DMD testing facilities use a more precise laser and extract even higher resolution information by reducing the step size to 1 µm, effectively doubling the number of scanning positions. It has been shown that this ‘High Resolution DMD’ provides greater assurance of adequate bandwidth for a wider set of fibers and laser launch conditions. As vendors look for looser laser specifications to reduce cost for 10G, 40G and 100G optoelectronics, HRDMD will become more important.