Amplifiers and regeneration: EDFA, Raman, SOA, OEO

A transceiver pair carries a signal as far as its power budget and dispersion tolerance allow — 10 km for LR, 80 km for ZR, 120 km for an amplified 400ZR. Beyond that, something in the line must restore what the fibre took: an optical amplifier that boosts every wavelength at once without touching the data, or a regenerator that receives the signal electrically and transmits it anew. This page covers when the budget ends, the amplifier types and their numbers, where they sit on the line, how the budget turns into an OSNR budget once amplifiers are present, what operating an amplified line demands, and how to choose between amplification, regeneration and a coherent module.

When the transceiver's budget ends

Technology	Unamplified reach	What extends it
10G LR / ER / ZR	10 / 40 / 80 km	dispersion compensation and an EDFA to 100–150 km; DWDM line systems beyond
100G LR4 / ER4 (direct detect)	10 / 40 km	none — switch to coherent for longer spans
100G–400G coherent pluggables (ZR, ZR+)	40 km (400ZR unamplified, ≤ 11 dB)	one EDFA pair to 120 km (ZR), amplifier chains to 1 000+ km (ZR+, OpenZR+) (Coherent long haul)
10G DWDM over a mux	80 km per span with EDFAs	chains of EDFAs with dispersion compensation modules to 400–600 km, then regeneration
Coherent transponders	80–120 km per span	EDFA and Raman chains, ROADMs, 1 000–3 000 km per regeneration section
PON, class C+	20 km, 1:32–1:64	reach extenders (ITU-T G.984.6) to 60 km or larger splits
Multimode	100–400 m	never amplified — a switch or media converter regenerates

Amplifier types

Type	Principle	Band	Gain and output	Noise figure	Notes
EDFA (erbium-doped fibre amplifier)	a few metres of erbium-doped fibre pumped at 980 or 1480 nm; the signal stimulates emission	C-band 1 530–1 565 nm; L-band variants 1 565–1 625 nm	15–35 dB gain; +17 … +23 dBm total output, boosters to +27 dBm	4–6 dB	the workhorse; single-channel or multichannel; gain-flattening filters, variable gain, transient control for channel add/drop (DWDM components)
Raman (distributed)	a pump about 100 nm below the signal (≈ 1 450 nm for C-band) is launched into the transmission fibre, usually backward; the fibre itself amplifies over its first 20–40 km	any band the pump allows	10–15 dB of distributed gain	effective noise figure lower than an EDFA's — 3–5 dB better OSNR per span	pumps +27 … +30 dBm, Class 4 — connector cleanliness and APR are life-critical (Safety & handling); hybrid Raman-EDFA for long or repeaterless spans (300+ km)
SOA (semiconductor optical amplifier)	a laser chip without mirrors; electrically pumped	any, including the O-band at 1 310 nm	10–20 dB; +10 … +13 dBm	7–9 dB	fast, small, cheap; PON reach extenders and burst-mode; pattern-dependent gain limits use in DWDM
Remote optically pumped amplifier (ROPA)	an unpowered erbium coil in the line, pumped from the terminal through the fibre	C-band	10–15 dB	—	repeaterless submarine and desert spans
3R regenerator (OEO)	receive, retime, reshape, retransmit — two transponders back to back	per channel	resets OSNR, dispersion, PMD and non-linear distortion	—	expensive per channel and adds latency; needed where amplification alone no longer reaches the required OSNR
2R regenerator, media converter	re-amplify and reshape electrically without a new clock	per channel	resets power and eye	—	small links; a switch port is the everyday regenerator (Transport & access equipment)

Where amplifiers sit on the line

Position	Purpose	Typical values
Booster (post-amplifier) after the multiplexer at the transmit terminal	raise the combined channels to the launch power	+17 … +20 dBm total, 0 … +3 dBm per channel
Inline (line amplifier) every span	compensate the span loss	span 80–120 km, 20–28 dB loss; gain set equal to it; mid-stage access for a dispersion compensation module in legacy 10G lines
Pre-amplifier before the demultiplexer at the receive terminal	lift weak channels above the receiver's noise	−25 dBm per channel in, −5 dBm out; improves effective sensitivity by 10 dB and more
Distributed Raman at the receive end of a span	lower the effective noise figure, stretch the span	20–40 km of distributed gain
Amplifier site	a powered hut or cabinet with batteries, cooling, the OSC terminal and monitoring	every 80–120 km on long routes (Fibre network topologies)

From a power budget to an OSNR budget

An amplifier adds amplified spontaneous emission — noise — with every stage. Once amplifiers are in the line, the receiver is limited by the optical signal-to-noise ratio, not by power.

Formula	Meaning
OSNR_span [dB, 0.1 nm] ≈ 58 + P_channel [dBm] − L_span [dB] − NF [dB]	the OSNR after one span and one amplifier
OSNR_total ≈ OSNR_span − 10 · log₁₀ N	N identical spans
Required OSNR: 10G NRZ ≈ 15–18 dB; 100G DP-QPSK with soft-decision FEC ≈ 12–15 dB; 400ZR ≈ 26 dB; 400G 16QAM ZR+ ≈ 22–24 dB	what the receiver needs

Example: 0 dBm per channel, 22 dB spans, 5 dB noise figure → 31 dB per span; three spans → 26.2 dB, the limit of 400ZR; ten spans → 21 dB, comfortable for 100G DP-QPSK. Raising the channel power buys OSNR until non-linear effects take it back — the optimum is usually 0 … +3 dBm per channel on G.652 (Non-linear effects, Amplification & OSNR).

Operating an amplified line

Item	Practice
Optical safety	Class 3B/4 at every amplifier output and Raman pump; automatic power reduction (APR, ITU-T G.664) shuts the pumps on loss of signal; never disconnect a live high-power connector; hazard labels at every site
Connector cleanliness	at +20 dBm and above a contaminated endface burns and can start a fibre fuse; inspect before every mating (Endface inspection & cleaning)
Reflections	isolators inside the amplifiers, APC connectors, ORL ≥ 24 dB on the line; a strong reflection into an EDFA makes it lase (Reflections & return loss)
Pump monitoring	pump current at constant output is the ageing indicator; spare pump cards or whole amplifiers
Gain and tilt	gain-flattening filters and dynamic gain equalization keep 40–96 channels within 1–2 dB; tilt grows with Raman and span count
Channel add and drop	transient control prevents surviving channels from surging when others disappear; add channels per procedure
Optical supervisory channel (OSC)	a separate wavelength, typically 1 510 or 1 610 nm, terminated at every amplifier site, carrying management, alarms and span-loss telemetry (Monitoring & management)
Power and environment	−48 V DC with 4–8 hours of battery, cooling, door and temperature sensors at unmanned sites
Fibre characterization first	span loss, ORL, dispersion, PMD and effective area decide the amplifier plan (Fibre characterization)

Choosing between amplification, regeneration and coherent

Situation	Choice
10G single channel, 80–100 km	a 10G ZR module, or LR/ER with an EDFA and dispersion compensation
10G DWDM, 200–400 km, legacy	EDFA chain with dispersion compensation modules; consider replacing with coherent
100G, 80–120 km	400ZR/100G ZR pluggable with one EDFA pair — the simplest modern answer
100G–400G, 500–2 000 km	coherent transponders or ZR+ with EDFA chains and ROADMs; regeneration only where OSNR runs out
Repeaterless 250–400 km	high-power boosters, Raman, ROPA, coherent
PON beyond 20 km or large splits	reach extender or class C++/E2 optics (ODN classes)
Campus multimode beyond 400 m	a switch or media converter as regenerator, or single-mode fibre
Any link with an OSNR shortfall the amplifiers cannot fix	3R regeneration

In CodingBox

The modules that feed and terminate an amplified line are still pluggables, and CodingBox reads what matters at the amplifier's input: launch power, exact wavelength or DWDM channel, tunable range, power class. A coherent module also reports OSNR, chromatic dispersion and pre-FEC BER through VDM — the line system's health as the receiver sees it — and CodingBox shows which of these observables the module advertises (Tunable transceivers, VDM & FEC metrics, Check transceiver).