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Meeting the OM-3 challenge

Fabricating this new generation of multimode fiber requires innovative state of the art designs and testing processes.


December 1, 2002  


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During the past 10 years multimode fiber performance has improved considerably to meet the increasing bandwidth demands of local area networks.

Multimode fiber based systems are widely used in short reach applications such as LANs due to their lower total system cost as compared to single mode fiber based systems. Transmitters used in LAN systems have evolved from light emitting diodes (LED) to 1 Gb/s, 850 nm vertical cavity surface emitting lasers (VCSELs).

And now 10 Gb/s, 850 nm VCSEL transmitters are available that enable cost effective high-speed systems by using lower cost laser technology in conjunction with a new high bandwidth 50-micron multimode fiber.

In fact, the IEEE, Fiber Channel, and OIF each specify 10 Gb/s applications using this laser optimized multimode system.

Multimode fiber design and manufacturing has evolved and improved along with the optoelectronic components. This has lead to a new generation of laser-optimized multimode fibers designed for state of the art systems.

The ISO 11801 second edition international cabling system standard describes a new 850-nm Laser Optimized multimode fiber known as OM-3. OM-3 fibers can support 10 Gb/s applications to 300 meters using low cost 850 nm VCSELs, while retaining support for lower speed legacy systems. Other 850 nm laser optimized fibers are available that support 10 Gb/s distances up to 600 meters. Successfully fabricating OM-3 and other high bandwidth multimode fibers for these new VCSEL based systems requires a world-class optical fiber manufacturing capability.

MANUFACTURING MULTIMODE FIBER

There are several processes that can be used to manufacture optical fibers. One of the most versatile is the modified chemical vapor deposition (MCVD) process. This highly stable patented process developed at Bell Labs starts with a high purity quartz tube mounted on a special glass-working lathe (See Figure 1).

A mixture of SiCl4, GeCl4, POCl3, O2 and He gases flow though the inside of the tube and a heat source is applied on the outside of the tube. The heat source converts the gases into “snow-like”, high surface area glass soot.

The soot deposits on the tube downstream of the flame. The burner traverses along the outside of the tube both creating the fine soot particles and sintering the soot into a thin layer of doped glass on the inside of the quartz tube. After the layer is deposited, the mixture of reactive gases is changed and the burner is brought back to the starting position. The above step is repeated and a subsequent layer is deposited.

This process is continued, layer-by-layer, to construct the complex core structure in the optical fiber. These MCVD layers are designed to be much thinner than the wavelength of light traveling down the fiber.

The layers are small enough relative to the wavelength of transmission, that they have no effect on the optical properties of the resulting fiber.

Once the glass is deposited, the tube is collapsed into a solid rod called a preform.

This process must be carefully controlled to assure that there are no defects in the center of the profile such as a “center dip” or a “center-line spike” in the index profile. Such defects can significantly degrade the DMD and bandwidth of a multimode fiber. For example, OFS uses a special proprietary process that greatly minimizes the formation of centerline defects.

The preform manufactured on the MCVD lathe is heated and drawn down to the accepted standard diameter of 125 microns. Each preform generates many kilometers of fiber.

This operation is performed on a draw tower. The tower has a furnace at the top to melt the glass preform. Gauges are used to measure and control the diameter of the glass fiber to 125+/-1 microns as it is pulled from the preform.

This level of control improves the robustness of the connectorization process and also helps to ensure proper coupling of the source to the fiber’s core.

Additionally, the fiber draw process must be very carefully controlled to prevent negative impacts to the fiber DMD and bandwidth.

An acrylate coating is applied during the draw process, which protects the pristine silica fiber from the environment. If a state of the art draw process is used, the resulting fiber has the same index profile as the preform it is drawn from.

MEASUREMENT

After the fiber is manufactured, each spool of fiber is measured to validate the product will meet the stringent industry and internal specifications.

These tests include the measurement of mechanical strength, geometric properties and optical properties. One key test for the laser optimized multimode fibers is high-resolution differential mode delay (HRDMD).

The HRDMD measurement serves two purposes. The first is to validate that the fiber has sufficient bandwidth for10 Gb/s systems. The second is to provide process feedback to enable accurate and precise control of the preform process. The steps taken in the MCVD process to ensure a defect free refractive index profile at the central core region and across the entire diameter of the core enable good DMD performance.

Process control and tuning the preform manufacturing process requires a thorough understanding of how MCVD and Fiber-Draw process parameters influence the fiber profile. The HRDMD measurement provides an excellent map of the modal delays that are used as inputs to the process-tuning algorithm.

The process-tuning algorithm optimizes the process parameters to minimize the modal delay. This feedback mechanism is required to manufacture 10 Gb/s fibers with superior performance.

The HRDMD process is essential for producing high performance laser optimized multimode fibers since it accurately shows defects in the fiber profile that would normally be missed by standard DMD, Over-filled bandwidth, and Restricted-Launch bandwidth measurements.

Laser Optimized Multimode Fibers such as OM-3 enable low cost 10 Gb/s, 850 nm systems while supporting lower speed legacy systems. Fabricating these high-performance, multimode optical fibers require innovative, state of the art designs and processes.

Highly optimized waveguide designs and a stable, accurate manufacturing process, such as MCVD, are required to manufacture production quantities of laser optimized multimode fibers. Data such as HRDMD from the measurement of the fiber must be fed back to the process to assure that consistent high performance is maintained.

A leading-edge fiber draw process must be used to optimize fiber performance and precisely control dimensions. A complex process control tuning method is required to adjust manufacturing process conditions to create and maintain precise fiber profiles necessary for 10 Gb/s, 850 nm optimized fiber.

Together, these optical fiber manufacturing and testing processes can produce high performance laser optimized multimode fibers that provide customers with cost effective, high bandwidth LAN systems.

David Mazzarese is a technical manager with OFS, the former optical fiber solutions division of Lucent Technologies Inc.


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