Fiber patch cord (jumper)

A fiber patch cord, also called a jumper, is a length of optical fiber, usually 0.5 m to a few tens of meters, protected by a buffer and a jacket and terminated with a connector at each end. It joins a transceiver to a patch panel, a laser to a power meter, or one rack to the next, and it is the most common fiber component on any optical bench. A good single-mode cord adds about 0.1–0.3 dB of insertion loss at each mated connector pair; the glass in a 2 m cord at 0.2 dB/km loses 0.0004 dB, so the connectors set the performance.

Construction and variants

The fiber, with its 250 µm acrylate coating, is usually given a 900 µm tight buffer, surrounded by aramid strength yarn and enclosed in a round jacket, commonly 2.0 or 3.0 mm in diameter. A simplex cord carries one fiber; a duplex cord carries two side by side for the transmit and receive fibers of a link, and MPO cords carry 8 to 24 or more.

The fiber type is the first choice. Single-mode fiber, 9/125 µm, carries telecom and most laboratory work at 1310 and 1550 nm. Multimode fiber, 50/125 or 62.5/125 µm, serves short 850 nm links. Jacket colors commonly signal the type: yellow for single-mode, orange for older OM1 and OM2 multimode, and aqua for laser-optimized OM3 and OM4. These are conventions, and the printed legend on the jacket is the reliable record.

The connector body (LC, SC, FC, ST, MPO) is compared in fiber connector types, and a cord may carry a different body at each end, such as FC to LC. The end-face polish is an independent choice, described in PC, UPC and APC connectors: UPC connectors are commonly blue and APC connectors green.

Polarization-maintaining cords use PM fiber with one of its stress axes keyed to the connector, usually the slow axis, and are specified by the polarization extinction ratio they preserve.

Loss and reflection budget

Each mated pair is a loss and a reflection. A path from a laser through three mated pairs at 0.3 dB each loses 0.9 dB, leaving 81% of the power, before any component is counted. The optical return loss of the path is set by the worst reflection in it: an open flat-polished connector reflects about 3.4% of the light, a return loss of 14.7 dB, which is enough to disturb a narrow-linewidth laser. Joints that never need to be undone are fused instead, since a fusion splice loses a few hundredths of a dB.

Measurement

Insertion loss of a cord is measured with a stable source and an optical power meter, referencing the power through a known-good test cord and then adding the cord under test; the procedure and the choice of reference method are in Measuring Insertion Loss and Return Loss. Return loss is measured with a reflectometer or an OTDR.

Patch cords and pigtails

A pigtail is a fiber with a connector on one end only; the other end is bare and is spliced to a cable or attached permanently to a device during pigtailing. Cutting a patch cord in half gives two pigtails, a common way of making them.

Pitfalls

• Mixing APC and UPC. The two polishes fit the same adapter, but the angled and flat end faces meet away from the core, leaving an air gap that costs several dB, reflects strongly and can chip the ferrules.
• Dirty end faces. A particle a few micrometers across on a single-mode core can exceed the whole loss allowance for a joint, and mating transfers it to the other side. The procedure is in Fiber Connector Inspection and Cleaning.
• Tight bends. Standard single-mode fiber is routed no tighter than about 30 mm radius; below that, bend loss rises steeply, and more so at 1550 nm than at 1310 nm. Bend-insensitive fibers tolerate much tighter routing.
• Wrong fiber for the transceiver. A single-mode transceiver run over legacy multimode cable needs a mode-conditioning patch cord, which launches the single-mode light off the axis of the multimode core so that a controlled set of modes is excited.

Common questions

Can an APC connector be mated to a UPC connector?

It should not be. The end faces touch off the core, which costs several dB, gives poor return loss and can damage both ferrules. Hybrid cords with APC on one end and UPC on the other connect the two kinds of equipment.

How much loss should a single-mode patch cord have?

About 0.1–0.3 dB per mated connector pair when the end faces are clean and the polish matches. A cord that measures 1 dB or more usually has a dirty or damaged end face.

What do the jacket colors mean?

Commonly, yellow is single-mode, orange is OM1 or OM2 multimode and aqua is OM3 or OM4. Connector housings add blue for UPC and green for APC.

References: G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019); D. Derickson (ed.), Fiber Optic Test and Measurement (Prentice Hall, 1998).