Splices, Connectors, Networks, Switching, Submarine Cable

Tip

Exam Focus

Compare fusion vs mechanical splicing and name connector types (SC/LC/ST/FC); explain the OTDR ($d=vt/2$, $v=c/n$) and identify trace events; work the power budget $P_{rx}=P_{tx}-\alpha L-L_s-L_c-M$ and the rise-time budget $t_{sys}=\sqrt{\sum t_i^2}\le0.7/B$. An OTDR distance numerical and a power-budget numerical are both high-yield.

Optical Connectors

A connector is a detachable precision alignment between fibre end faces and equipment or another fibre.

Connector	Feature	Common use
SC	Push–pull square	Telecom, FTTH
LC	Small-form-factor latch	High-density panels, SFPs
ST	Bayonet twist	Older LAN systems
FC	Threaded	Test gear, high vibration
MPO/MTP	Multi-fibre ribbon	Data centres

Common fibre connectors.

Connector losses arise from lateral and angular misalignment, end separation, end-face defects, core/NA mismatch and Fresnel reflection.

Caution

Key Formula — Insertion and return loss

$$IL=10\log_{10}\!\frac{P_{in}}{P_{out}},\qquad RL=10\log_{10}\!\frac{P_{in}}{P_{r}}.$$

Low $IL$ and high $RL$ (low reflection) are desirable.

Fibre Splicing

Splicing permanently joins two fibres.

• Fusion splice: strip, clean, cleave, align, arc-fuse, protect, test. Lowest loss/reflection, high strength; needs an expensive splicer.

• Mechanical splice: cleaved ends held in an alignment fixture with index-matching gel. Quick and cheap; higher loss/reflection, for temporary restoration.

Feature	Fusion splice	Mechanical splice
Joint	Permanently fused glass	Mechanical alignment
Typical loss	Very low	Higher
Reflection	Very low	Higher
Equipment cost	High	Low
Reliability	High	Moderate
Use	Long-haul, FTTH, permanent	Temporary/quick repair

Fusion versus mechanical splicing.

Optical Time-Domain Reflectometer (OTDR)

An OTDR locates faults and measures loss/length by launching a pulse and recording Rayleigh backscatter plus Fresnel reflections versus time.

Caution

Key Formula — OTDR event distance

$$d=\frac{v\,t}{2},\qquad v=\frac{c}{n},$$

the factor 2 accounts for the round trip.

OTDR: pulse launched through a coupler; the backscatter trace shows a sloped fibre loss, a reflective connector spike, a non-reflective splice step, a bend step and a strong end/break reflection.

The dead zone after a strong reflection hides nearby events; it is reduced with a shorter pulse and a launch fibre. Longer pulses give more range but poorer resolution.

Optical Link Budgets

Power Budget

Caution

Key Formula — Power budget

$$P_{rx}=P_{tx}-\alpha L-L_s-L_c-M,\qquad P_{rx}\ge P_{sens}.$$

The power-budget staircase tracks each loss and design margin from transmitter output to the receiver-sensitivity threshold.

Power-budget staircase: transmitter power minus connector, fibre, splice, and margin losses must leave $P_{rx}$ above receiver sensitivity.

Rise-Time Budget

Caution

Key Formula — Rise-time budget

$$t_{sys}=\sqrt{t_{tx}^2+t_{fibre}^2+t_{rx}^2}\le\frac{0.7}{B}\ \text{(NRZ)}.$$

A link can pass the power budget yet fail the timing budget (ISI), so both must be checked.

Optical Networks, Switching and Submarine Cable

• PON / access networks: a passive splitter shares one feeder fibre among many subscribers (FTTH); no active elements in the outside plant.

• WDM/DWDM backbone: many wavelengths per fibre multiply capacity; optical amplifiers (EDFA) replace electrical repeaters.

• Optical switching: routes light without O/E/O conversion (wavelength/space switching, ROADMs); lowers latency and power.

• Submarine cable: inter-continental fibre with periodic optical repeaters/EDFAs and robust armouring, powered over a copper conductor.

Note

Example — OTDR distance

A reflection returns after $t=100\,\mu\mathrm{s}$; fibre index $n=1.5$.

$$v=\frac{c}{n}=\frac{3\times10^8}{1.5}=2\times10^8\,m/s,\quad d=\frac{vt}{2}=\frac{(2\times10^8)(100\times10^{-6})}{2}=\boxed{10\,km}.$$

[!NOTE]

Example — Power budget

$P_{tx}=0\,dBm$, $L=20\,km$, $\alpha=0.2\,dB/km$, five 0.1 dB splices, two 0.5 dB connectors, margin $M=3\,dB$.

$$P_{rx}=0-4-0.5-1-3=\boxed{-8.5\,dBm}.$$

Against a -18 dBm sensitivity there is 9.5 dB spare budget.