In multimode fiber optics, 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 will be changed.

A cylindrical rod wrap consists of a specified number turns of fiber on a mandrel

Mandrel

A mandrel is one of the following:* an object used to shape machined work.* a tool component that grips or clamps materials to be machined.* a tool component that can be used to grip other moving tool components.- Variants :...

 of specified size, depending on the fibre characteristics and the desired modal distribution. It has application in optical transmission

Transmission (telecommunications)

Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...

 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

Transverse mode

A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...

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

Laser diode

The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...

 or edge-emitting LED

Light-emitting diode

A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

, 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 fiber 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.

External links

• Fluke Networks: Using Mandrels for Testing Multimode Fiber