10. Connectors, Splicing, and the Link Budget

10.1 Connector types

Hundreds of fiber connector designs have come and gone. The current dominant ones:

• SC: square push-pull. Common in older single-mode and PON installations. Relatively large.
• LC: little connector. Half the size of SC, latching like an RJ-45. The dominant connector in modern data centers. Typically used in pairs for transmit and receive.
• FC: round, threaded. Still used in test equipment and some metrology applications because the screw-on connection is repeatable.
• ST: bayonet-style. Older, mostly displaced.
• MTP/MPO: multi-fiber push-on with 12, 24, or 48 fibers in one ferrule. Used for 40 GbE, 100 GbE, and 400 GbE parallel optics, and for high-density patching in trunks.

Polish styles:

• PC (physical contact): flat-polished. Return loss (back-reflection) about -40 dB.
• UPC (ultra physical contact): convex polish. Return loss about -55 dB.
• APC (angled physical contact): 8-degree angled polish. Return loss better than -65 dB. Required for PON and DWDM systems where reflections destabilize transmitters.

APC connectors are green, UPC connectors are blue. You cannot mate an APC with a UPC: the ferrule angles do not match. This is the most common cause of unexplained loss in newly-built fiber links.

10.2 Splicing

To make a permanent join between two fibers:

• Mechanical splice: an alignment fixture clamps two cleaved fiber ends with index-matching gel. Insertion loss 0.1-0.3 dB. Quick, reusable, used for emergency repairs.
• Fusion splice: a fusion splicer aligns two cleaved fibers, then arcs an electrical discharge between two electrodes to melt the glass. The two cores fuse into one. Loss 0.02-0.10 dB. Standard for telecom.

A fusion splicer is a self-contained instrument with motorized stages, microscope cameras for alignment, an arc source, a heat shrink oven for the protective sleeve, and embedded software that estimates loss from image analysis after the splice. Modern splicers (Fujikura 90S+, Sumitomo T-72C) cost 6000 to 15000 USD and can do a splice in under 10 seconds.

10.3 Link budget

To see if a proposed link will work, you sum up all the gains and losses in dB. The basic equation:

$P_{\text{Rx}} = P_{\text{Tx}} - \alpha L - L_{\text{conn}} \cdot N_{\text{conn}} - L_{\text{splice}} \cdot N_{\text{splice}} - M$

The link works if $P_{\text{Rx}} \geq P_{\text{Rx,sens}}$ (the receiver's sensitivity). Quantities:

• $P_{\text{Tx}}$: launch power, typically 0 to 10 dBm.
• $\alpha L$: fiber loss times length.
• $L_{\text{conn}}$: connector loss (about 0.3-0.5 dB each).
• $L_{\text{splice}}$: splice loss (about 0.05 dB each).
• $M$: system margin (3-6 dB) to account for aging, temperature, and unexpected events.
• $P_{\text{Rx,sens}}$: receiver sensitivity, typically -18 dBm for 10 GbE direct detection, -25 dBm with APD, far better with coherent.

For amplified links, you also track OSNR (optical signal-to-noise ratio) because each EDFA adds ASE noise. OSNR degrades by 3 dB every time you double the number of cascaded amplifiers, so trans-oceanic links carefully manage OSNR. The required OSNR for QPSK is around 14 dB; for 16-QAM around 18 dB; for 64-QAM around 24 dB.

10.4 OTDR: optical time-domain reflectometry

An OTDR sends a short pulse into the fiber and measures the back-reflected power as a function of time. It builds a trace of loss vs. distance, with reflective peaks at connectors and breaks. From the trace you can read off:

• Total fiber length.
• Per-km attenuation.
• Connector and splice losses.
• The location of any break or macrobend.

OTDRs are the standard troubleshooting tool for fiber installations. A typical handheld OTDR (Viavi T-BERD, EXFO MaxTester) costs 5000 to 15000 USD and resolves features down to 1 meter with 50 km dynamic range.