Fusion splicing
Joining two fibers permanently by aligning their cleaved ends and melting them together with an electric arc. A good single-mode fusion splice loses a few hundredths of a dB and reflects almost nothing, which makes it the default for any joint that never needs to be undone.
Fusion splicing joins two optical fibers permanently by melting their ends together, usually with an electric arc between two electrodes, so that light passes from one core to the other with no gap and no glass–air interface. A good fusion splice between two identical single-mode fibers loses 0.02 to 0.05 dB and reflects too little to measure, with a return loss beyond 60 to 70 dB, which makes it the lowest-loss and most reliable fiber joint; mechanical splices and connectors lose a few tenths of a dB. The fiber is stripped, cleaned and cleaved, the splicer aligns the two ends and fuses them with the arc, and the finished joint is tested and protected with a heat-shrink sleeve.
The steps of a fusion splice
- Strip. The polymer coating (250 µm or 200 µm in diameter on standard fiber) is removed over a few centimeters, leaving the 125 µm glass.
- Clean. The bare glass is wiped with isopropyl alcohol to remove coating residue, which would otherwise burn in the arc and leave bubbles.
- Cleave. A cleaver scores the fiber and breaks it under tension, leaving a flat, mirror-like end nearly perpendicular to the axis. The cleave angle is the operator's largest influence on the result, and splicers reject ends that exceed a set limit, usually 0.5 to 1°.
- Align. The splicer images both fibers from two directions and moves them until the cores (in a core-alignment splicer) or the claddings (in a cladding-alignment splicer, which holds the fibers in fixed V-grooves) line up.
- Fuse. A short cleaning arc burns off dust, then the main arc softens the glass while the fibers are pushed slightly together, and surface tension pulls the joint into a continuous fiber.
- Test. The splicer estimates the loss from its images of the finished joint and usually applies a small tension test to catch weak splices.
- Protect. The bare section is covered with a heat-shrink sleeve containing a steel or ceramic strength member, commonly 40 to 60 mm long, or recoated where space is tight.
Core-alignment splicers give the lowest losses and are used for long-haul, laboratory and specialty work; cladding-alignment and ribbon splicers, which fuse all fibers of a 12-fiber ribbon at once, trade a little loss for speed in mass deployment.
Where splice loss comes from
For two identical single-mode fibers, most of the loss is lateral offset between the cores. With Gaussian modes of radius (half the mode field diameter), an offset costs
For standard fiber at 1550 nm ( µm), an offset of 0.5 µm costs 0.04 dB and 1 µm costs 0.16 dB; at 1310 nm, where µm, the same offsets cost 0.05 and 0.21 dB. Core eccentricity, a poor cleave, dirt, and an arc that is too strong or too weak, which deforms the core or leaves bubbles, add further loss.
Between fibers with different mode field diameters, a mismatch loss remains even with perfect alignment:
| Fiber spliced to 10.4 µm MFD fiber | Mismatch loss |
|---|---|
| 9.5 µm MFD | 0.04 dB |
| 9.2 µm MFD | 0.07 dB |
| 8.6 µm MFD | 0.16 dB |
| 8.0 µm MFD | 0.30 dB |
| 6.0 µm MFD | 1.25 dB |
Splicing to small-core specialty fibers therefore costs from a few tenths of a dB to more than a dB. Holding the arc on the joint for longer lets the core dopant diffuse and expands the smaller mode gradually, which recovers part of the mismatch; mode mismatch loss gives the general overlap calculation. Cabling standards allow up to 0.3 dB per splice (ANSI/TIA-568), and a core-alignment splice of identical fibers usually lands far below that.
Measuring splice loss
The splicer's estimate is computed from the geometry it sees and misses anything it cannot image, so critical splices are checked optically. An OTDR reads the step in the backscatter trace at the splice, but a single reading is biased when the two fibers differ: a fiber with a smaller mode field captures more backscatter, so the trace level can rise across a lossy splice. For a splice from 10.4 µm to 9.2 µm MFD, whose true loss is 0.07 dB, the difference in captured backscatter alone makes the OTDR read about −0.47 dB, a "gainer", from the 10.4 µm side, and about 0.60 dB from the other. The average of the two directions removes the bias, which is why splice loss is measured from both ends. In the laboratory, the power transmitted before and after the splice gives the loss directly.
Fusion splices, mechanical splices and connectors
| Joint | Typical loss | Reflection | Undoable? |
|---|---|---|---|
| Fusion splice | 0.02 to 0.05 dB (single-mode, core alignment) | Negligible (return loss beyond 60 to 70 dB) | No |
| Mechanical splice | A few tenths of a dB | Small with index-matching gel | No |
| Connector pair | 0.1 to 0.5 dB | Return loss about 50 dB (UPC) or 60 dB or more (APC) | Yes |
A mechanical splice aligns two cleaved fibers in a precision sleeve or V-groove, with index-matching gel between them, and needs no splicer, which makes it useful for quick restoration. Connectors are used where a link must be reconfigured or tested; everything else in a permanent link is fusion spliced, since each joint adds loss and a possible reflection.
Specialty fibers
Polarization-maintaining fiber needs a splicer that also rotates the fibers until their stress rods line up, since the angular error sets the polarization extinction ratio of the joint. Photonic crystal fibers need a weak, short arc so that their air holes do not collapse. Fibers much thicker than 125 µm, such as large-mode-area fibers for high-power lasers, are spliced with filament heaters or CO₂ lasers, which heat a wider zone more evenly than an arc. Single-mode fiber can be fused to multimode fiber, but the loss depends on direction: light passes from the single-mode side into the multimode core with little loss, while in the other direction most of the multimode power cannot enter the single-mode core.
Common questions
How much loss does a fusion splice add?
Between identical single-mode fibers on a core-alignment splicer, typically 0.02 to 0.05 dB; a splice above about 0.1 dB is usually remade. Between fibers with different mode field diameters, the mismatch adds a floor that alignment cannot remove, 0.07 dB between 10.4 µm and 9.2 µm fibers.
What is the difference between fusion splicing and mechanical splicing?
A fusion splice melts the two fibers into one continuous strand and needs a fusion splicer; a mechanical splice holds two cleaved ends together in an alignment sleeve with index-matching gel. Fusion splices have lower loss and reflection and are more stable over time, while mechanical splices need only simple tools.
Why does an OTDR show a gain at a splice?
Because the two fibers scatter back different fractions of the light, usually from different mode field diameters. The trace level steps up when the fiber after the splice returns more backscatter, even though the splice itself loses power. Measuring from both ends and averaging gives the true loss.
References: A. D. Yablon, Optical Fiber Fusion Splicing (Springer, 2005). D. Marcuse, "Loss analysis of single-mode fiber splices," Bell System Technical Journal 56, 703 (1977). ANSI/TIA-568.3-D, Optical Fiber Cabling and Components Standard (2016).