for Fiber Optics (Page 2)

 "Overfilled Light Sources"

  Let's get specific and talk about what an "overfilled Light Source" is, exactly.  As we told you on page one, virtually all field-portable LED Light Sources and multimode fiber Test Adapters currently used with premises cable test sets are overfilled Light Sources.  "Overfilled Light Sources" launch both Low-order modes (tightly coupled) and High-order (loosely coupled) light modes into a multimode fiber.  Low-order modes (tightly coupled modes) consist of low angles relative to the core and are confined to the inner regions of the core of a fiber.  High-order modes (loosely coupled modes) have high angles relative to the core and travel throughout the entire region of the core, including the outermost region of the core next to (and sometimes are launched into) the cladding.   Figure-2 shows an over filled Light Source launching both high order modes and low order modes into a multimode fiber.      

Figure-2 (Click on the image to enlarge)

An overfilled Light Source will launch both tightly coupled and loosely coupled modes into a multimode fiber.  The tightly coupled modes will be transported by a multimode fiber link with acceptable (specified in the TIA/EIA-568-B Standard) fiber attenuation and component loss.  On the other hand, loosely coupled modes will experience excessive loss at connections, splices, fiber bends, and even bends that meet or exceed minimum TIA/EIA bend radius specifications.

  Low-order modes that meet the TIA/EIA-568-B Standard specifications, including maximum fiber attenuation, connector loss, splice loss, and minimum fiber bend radius, are referred to as tightly coupled modes.  The Mandrel Wrap Mode Filter removes the High-order modes (loosely coupled modes) while allowing Low-order modes (tightly coupled modes) to pass down the multimode fiber.  

The Light Source Debate

  For many years, test equipment manufacturers have debated the advantages of light source types used for measuring insertion loss on multimode fiber links.  The advocates of overfilled LED-based Light Sources argued that these types of Light Sources provide the truest measure of insertion loss because transceivers used in 10 Mb/s Ethernet, 100 Mb/s Ethernet, and other LED-based LAN systems are overfilled.  The advocates of filtered LED Light Sources  (these are basically overfilled LED Light Sources that have been filtered to remove loosely coupled modes either by internal "beam optics" or other devices) argued that they provide lower and more consistent loss readings than the unfiltered LED Light Sources.  A third group of laser-based  test equipment manufacturers recently proposed using VCSELs (vertical cavity surface emitting lasers) for multimode insertion loss testing, arguing that the "SX" (850nm) and "LX" (1300 nm) versions of Gigabit Ethernet are currently using  VCSEL and lasers respectively. 

Figure-3 (Click on the image to enlarge)

Four Light Source types have been proposed for measuring insertion loss on multimode fiber links:  3a) is an overfilled LED Light Source, 3b) is an overfilled LED Light Source with a mode filter (also known as a "controlled launch source"), 3c) is a Vertical Cavity Surface Emitting Laser (VCSEL), and 3d) is a standard Laser  

  This debate ended with two parts of the TIA/EIA-568-B Standard that deals with fiber, TIA/EIA-568B.3 (published in April, 2000) and TIA/EIA-568-B.1.  These new Standards require that insertion loss tests of multimode fiber links shall be performed using an overfilled Light Source, such as an LED, with a Mandrel Wrap Mode Filter located on the transmit jumper.  This set-up is also referred to as a "controlled launch source".  TIA/EIA requires the use of a filtered LED or equivalent Light Source for multimode testing for the following reasons:

1. All multimode fiber attenuation and component insertion loss specifications are based upon the use of a filtered LED or equivalent) Light Source
2. TIA/EIA wants contractors to be able to use their existing 850/1300 nm hand-held LED Light Sources, or multimode fiber Test Adapters, to test 50 and 62.5 µm multimode fiber links for any application including Gigabit Ethernet and 10 Gigabit Ethernet.
3. VCSELs of different designs can generate different mixes of loosely and tightly coupled modes into the fiber.  Therefore, VCSELs in general, produce less repeatable loss measurement results than the filtered LED Light Sources.
4. Lasers significantly under fill multimode fiber cores.  Therefore, any loss measurements made with lasers may not detect poorly mated connectors and other link problems.   (See Figure-4)

 Figure-4

A filtered LED Light Source compared to a laser Light Source.  Notice the problems associated with detecting poorly mated connectors and other link problems when using a laser Light Source due to the problem of "under filling" the multimode core.   

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