Mode scrambler

A mode scrambler is a device that perturbs an optical fiber's geometry (typically by bending or by clamping in a controlled pattern) to couple light among the fiber's guided modes, so that the power distribution at its output no longer depends on how the source launched the light. Followed by a mode filter that strips the high-order modes, it approximates the equilibrium mode distribution (EMD) that a long fiber would reach on its own. The goal is to remove the dependence of measurement results on the source-fiber launch conditions.

Why mode scrambling is necessary. Multimode optical fibers support hundreds to thousands of guided modes. The power distribution among these modes depends sensitively on launch conditions:

• A small core beam launched into a multimode fiber initially excites only a few low-order modes
• A divergent overfilled launch excites all modes
• Different launch conditions produce dramatically different propagation behavior

For repeatable fiber measurements (loss, bandwidth, attenuation), the modal power distribution must be standardized. The mode scrambler does this by providing controlled perturbation that makes the modal distribution independent of the input launch condition.

How scramblers are built. Most scramblers bend the fiber. Microbend designs press it between corrugated plates, so that the periodic perturbation couples neighboring modes (the same perturbation causes microbend loss when it couples guided light into radiation modes). Macrobend designs route it through a series of tight loops, which couple modes and also radiate the highest-order ones.

The equilibrium mode distribution. Far enough along a multimode fiber, the relative power in its modes stops changing with length and no longer depends on the launch. That equilibrium is set by two effects:

• Mode coupling, caused by random microbends and imperfections in the fiber
• Differential mode attenuation: high-order modes lose more power, mostly to bending

In modern graded-index fiber, mode coupling is weak and the natural equilibrium may not form within the length of a typical link. A scrambler followed by a mode filter produces a distribution close to it within a short length of fiber.

Test fiber considerations.

For repeatable testing of multimode fiber (OM1, OM2, OM3, OM4, OM5), the launch side of the measurement setup typically includes:

1. Launch cord: a test reference cord of the same fiber type as the fiber under test, carrying the source's light
2. Mode scrambler: removes the dependence on the source
3. Mode filter (5 turns on a mandrel of about 17–25 mm diameter, set by fiber core size and jacket): strips high-order and leaky modes
4. Cladding-mode stripper: removes light guided in the cladding, which would otherwise reach the detector

Current standards for installed cabling specify the result rather than the hardware: the launch must meet an encircled-flux template at the output of the launch cord, and a scrambler and mandrel alone do not guarantee that (measuring insertion loss and return loss covers the multimode case).

Standards specifying mode scrambling and launch conditions.

• TIA-455-54 (FOTP-54): requirements for a mode scrambler that produces an overfilled launch
• TIA-455-78: attenuation measurement of optical fiber, including the cutback method; the TIA adoption of IEC 60793-1-40
• IEC 61280-4-1 and TIA-526-14: attenuation of installed multimode cabling; their current editions specify the launch as encircled flux
• ANSI/TIA-568: cabling standard; its test requirements give the mandrel diameters

Modal launch alternatives. Mode scrambling produces a specific modal distribution. Other launch conditions exist for specific purposes:

Polarization scrambling in single-mode fiber. Single-mode fiber (SMF) supports only the LP01 mode, in two polarizations, so there is no modal distribution to redistribute. The related single-mode device is the polarization scrambler, which varies the polarization state quickly enough that its average over the measurement or response time is nearly unpolarized; fiber-squeezer scramblers do it by mechanical perturbation, much like a mode scrambler. In long amplified WDM links, a scrambler at the transmitter that runs faster than the amplifiers can respond suppresses polarization hole burning in the amplifiers. In the lab, scrambling the input while recording the maximum and minimum output power is one way to measure polarization-dependent loss.

Common errors in fiber measurement without proper mode scrambling.

1. Reported loss varies by source: measurements with LED vs laser sources give very different loss values without mode conditioning
2. Source-dependent bandwidth: VCSEL-launched fiber gives different bandwidth than overfilled launch. This is real (it's the bandwidth-vs-launch condition variation in OM3/OM4 fiber) but must be specified
3. Factory and field results disagree: factory measurements made with one set of test cords may not match field measurements of the installed cable made with different cords

Scramblers are made for 50/125 μm (OM2 to OM5) and 62.5/125 μm (OM1) fiber and are used at 850 nm and 1300 nm, the multimode test wavelengths.

References: Saleh & Teich, Fundamentals of Photonics, 2nd ed. (Wiley, 2007), Ch. 9 (fiber optics); TIA-455-54 for mode scrambler requirements; IEC 61280-4-1 and TIA-526-14 for installed-cabling loss measurement; IEC 60793-2-10 and ITU-T G.651.1 for multimode fiber specifications.