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In multimode
fibre optics An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...
, mandrel wrapping is a technique used to preferentially attenuate high-order mode power of a propagating optical signal. Consequently, if the fibre is propagating substantial energy in affected modes, the
modal distribution A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and micro ...
will be changed. A cylindrical rod wrap consists of a specified number turns of fibre on a
mandrel A mandrel, mandril, or arbor is a tapered tool against which material can be forged, pressed, stretched or shaped (e.g., a ring mandrel - also called a triblet - used by jewellers to increase the diameter of a wedding ring), or a flanged or t ...
of specified size, depending on the fibre characteristics and the desired modal distribution. It has application in optical
transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...
performance tests, to create a defined mode power distribution or to prevent multimode propagation in single mode fibre. If the launch fibre is fully filled ahead of the mandrel wrap, the higher-order
mode Mode ( meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * MO''D''E (magazine), a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is the setting fo ...
s will be stripped off, leaving only lower-order modes. If the launch fibre is underfilled, for example as a consequence of being energized by a
laser diode file:Laser diode chip.jpg, The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD or semiconductor laser or diode laser) is a semiconductor device similar to a light-emittin ...
or edge-emitting LED, there will be no effect on the mode power distribution or loss measurements. In multimode fibre, mandrel wrapping is used to eliminate the effect of "transient loss", the tendency of high-order modes to experience higher loss than lower-order modes. Numerical addition (in decibels) of the measured loss of multiple fibre segments and/or components overestimates the loss of the concatenated set if each segment or component has been measured with a full mode power distribution. In single-mode optical fibre measurements, it is used to enforce true single-mode propagation at wavelengths near or below the theoretical cutoff wavelength, at which substantial power can exist in a higher-order mode group. In this use, it is commonly termed a High Order Mode Filter (HOMF). Ultimately, the effect of mandrel wrapping on optical measurements depends on the propagating mode power distribution. An additional loss mechanism has no effect unless power is present in the affected modes.


Principle of operation

The effect of physically bending an optical fibre around a cylindrical form is to slightly modify the effective refractive index in the curved region, which locally reduces the effective mode volume of the fibre. This causes optical power in the highest order modes to become unguided, or so weakly guided as to be released into an unbound state, absorbed by the fibre coating or completely ejected from the fibre. The practical effect of mandrel wrapping is to attenuate optical power propagating in the highest-order modes. Lower-order modes are unaffected, experiencing neither increased loss nor conversion into other modes (mode mixing).


Determination of appropriate mandrel wrap conditions

The mandrel diameter and number of turns are chosen to eliminate certain modes in a reproducible way. It is empirically observed that more than 5 full 360-degree wraps creates little additional loss, so 3 to 5 turns are commonly specified. The mandrel diameter affects how far into the mode volume the modal unbinding occurs. Experimentally, one plots the transmitted power from a wrapped fibre into which a uniform modal power distribution has been excited, as a function of mandrel diameter, maintaining a constant number of turns. This reveals step-like reductions in transmitted power as the diameter decreases, where each step is the point at which the mandrel is beginning to affect the next-lower mode group. For best measurement reproducibility, one would select a diameter that is not near such a transition, although this may not be possible if measurements must be performed over a range of wavelengths. Total mode volume in a fiber is a function of wavelength, so the mandrel diameter at which the mode group transitions occur will change with wavelength.


See also

*
Mode scrambler In telecommunications, a mode scrambler or mode mixer is a device for inducing mode coupling in an optical fiber, or a device that, itself, exhibits a uniform output intensity profile independent of the input mode volume or modal excitation condit ...


External links


Fluke Networks: Using Mandrels for Testing Multimode Fiber
{{FS1037C Fiber optics