|Using Mandrels for Testing Multimode Fiber
A characteristic of multimode fiber is the simultaneous transmission of light along multiple modes, or paths. Some of these modes are located near the center of the fiber core while other modes are closer to the cladding interface. The innermost "low-order" modes are relatively stable as compared to the "high-order" modes.
High-order modes are susceptible to extinction due to fiber bending, connections and normal transmission. This multimode fiber characteristic has implications with regards to fiber testing. To achieve repeatable, consistent optical power measurements, it is desirable to use a light source that excites only the relatively stable low-order modes of the fiber under test.
To obtain a low-order mode launch you can either use:
The light emitted from a VCSEL source is concentrated within a small spot located near the center of the fiber core, exciting only low-order modes. VCSELs emitting at 850nm are common, especially since the introduction of Gigabit fiber networks. An LED source emits light over an area that is larger than the typical multimode fiber core. When the core is "overfilled" with light from an LED, both low and high-order modes are excited. You can alter the launch condition by adding a special conditioning cable to the LED source. The conditioning cable removes the unwanted high-order modes so only low-order modes are launched into the fiber under test.
Extinguish high-order modes with fiber mandrels
By adding a special conditioning cable to an LED source, only the low-order modes will be excited when the launch cable is connected to the fiber under test. It is simple and inexpensive to build a special LED mode conditioning cable. The simplest method of removing or stripping off, the high-order modes is to tightly bend the launch cable around a round mandrel, or rod. The tight bends extinguish the high-order modes so that only low-order modes emanate from the launch cable.
ANSI/TIA-568-C.0 mandrel guidelines
The mandrel diameter and number of mandrel wraps is specified in clause 220.127.116.11. The diameter varies depending upon the fiber core size and launch cable diameter.
Field-test instruments for multimode fiber cabling shall meet the requirements of TIA-526-14-A. The light source shall meet the launch requirements of ANSI/TIA-455-78B. This launch condition can be achieved either within the field-test instrument or by use of an external mandrel wrap with a Category 1 light source. When using a mandrel wrap, the source reference jumper should be wrapped in five non-overlapping turns around a smooth round mandrel (rod) during the reference calibration of the source to the detector and for all loss measurements. The mandrel diameter size is dependent upon the fiber core size and shall be chosen as specified in table 3.
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