Amplification, OSNR & the limits of a DWDM span

A CWDM link ends where the receiver runs out of light. A DWDM link does not — amplifiers add light every 80 km — so its limit is different: noise. Every amplifier adds spontaneous emission, and after enough of them the signal is still strong but no longer clean. The metric is OSNR (optical signal-to-noise ratio); the design task is to keep it above what the modulation format needs, while staying below the power where fibre non-linearity bites and while dispersion stays manageable. This page gives the tools and numbers.

Amplifier types

Amplifier	Band	Gain	Output	Noise figure	Where
EDFA (erbium-doped fibre)	C (1530–1565 nm), L (1565–1625 nm)	15–35 dB	+17 … +23 dBm total	4.5–6 dB	booster after the mux, inline every 60–100 km, pre-amplifier before the demux
Raman (distributed)	any, set by pump wavelength (~100 nm below signal)	10–15 dB distributed in the transmission fibre	—	effective NF can be negative when combined with an EDFA	long spans, low-OSNR upgrades; high pump power (0.5–1 W) — safety
SOA	O-band and others	10–20 dB	low	7–9 dB	O-band systems, receivers (SOA-assisted receivers)
Hybrid Raman/EDFA	C/L	—	—	improved	ultra-long spans

EDFA gain is not flat: gain flattening filters and tilt control keep the 40–96 channels within ~1 dB of each other; dynamic gain control holds output constant when channels are added or dropped (transients). Pump lasers at 980/1480 nm power the erbium; they are the wear-out component of an amplifier.

OSNR

OSNR is the ratio of signal power to ASE noise power in a reference bandwidth (0.1 nm ≈ 12.5 GHz), in dB. For a chain of N identical spans:

OSNR ≈ 58 + P_ch − NF − L_span − 10·log10(N)     [dB, 0.1 nm]
  P_ch   = launch power per channel into the fibre, dBm
  NF     = amplifier noise figure, dB
  L_span = span loss, dB
  N      = number of spans (amplifiers)

Example: P_ch = +1 dBm, NF = 5.5 dB, 80 km spans at 0.22 dB/km + 2 dB of connectors = 19.6 dB, N = 8 → OSNR ≈ 58 + 1 − 5.5 − 19.6 − 9 ≈ 24.9 dB.

Format	Rate	Typical required OSNR (0.1 nm), dB	Notes
10G NRZ, no FEC	10G	18–20	legacy
10G NRZ with FEC (OTU2, GFEC)	10.7G	11–13	~6 dB coding gain
100G DP-QPSK coherent, SD-FEC	100G	12–15	very robust
200G DP-16QAM	200G	18–21	—
400G DP-16QAM (400ZR)	400G, 60 GBd	≥ 26 (specified for 400ZR)	120 km amplified reach
400G ZR+ at 8QAM / QPSK	300G / 200G	20 / 14	trade capacity for reach

Design margin: required OSNR + 2–3 dB for ageing, tilt and PMD. In the example above, 100G QPSK is comfortable and 400ZR is marginal → fewer spans, higher launch power, lower NF or Raman for the 400G case (Coherent & long haul).

Launch power and non-linearity

Raising P_ch raises OSNR by the same dB — until the fibre's Kerr non-linearity turns power into distortion:

Effect	Mechanism	Symptom	Mitigation
SPM — self-phase modulation	a channel's own intensity modulates its phase	spectral broadening, dispersion interaction	limit per-channel power
XPM — cross-phase modulation	neighbours' intensity modulates a channel's phase	crosstalk on 50 GHz grids	dispersion management, spacing
FWM — four-wave mixing	channels mix into new frequencies	ghost channels on regular grids	unequal spacing, dispersion ≠ 0 (G.655/G.652 rather than DSF)
SBS / SRS — Brillouin / Raman scattering	high power per channel / band tilt	power limits, tilt across the band	dithering, tilt control

Practical per-channel launch power: −2 … +3 dBm for coherent 100G+ on G.652, 0 … +3 dBm for 10G. The optimum is where non-linear penalty equals the OSNR gain.

Dispersion

Type	Value on G.652	Effect	Handling
Chromatic dispersion (CD)	≈ 17 ps/(nm·km) at 1550 nm	pulse spreading; 10G NRZ tolerates ~800–1000 ps/nm (≈ 60 km) uncompensated	DCF modules (dispersion-compensating fibre, adds loss), FBG compensators, or — in coherent systems — the DSP compensates tens of thousands of ps/nm electronically
Polarisation-mode dispersion (PMD)	0.1–0.5 ps/√km on modern fibre, worse on old	random, time-varying; matters at 40G+ direct detect	coherent DSP tracks it; direct-detect 40G needed low-PMD fibre
Dispersion slope	—	compensation exact at one wavelength only	slope-matched DCF

Direct-detect DWDM (10G, some 25G) therefore needs a dispersion map with DCF at amplifier sites; coherent links do not, which removed a whole layer of engineering (DWDM components).

Span design checklist

Fibre type, length and measured loss per span at 1550 nm (OTDR).
Channel count, grid and format → required OSNR and non-linear limits.
Amplifier placement: booster, inline every 60–100 km (span loss 15–25 dB), pre-amp.
Launch power per channel, tilt and transient control settings.
Dispersion map (direct detect) or DSP capability check (coherent).
Received OSNR and pre-FEC BER per channel at commissioning — the baseline for the life of the system (VDM & FEC metrics).
Protection and monitoring: OSC, per-channel power monitors, OTDR access.

Where the pluggable fits

A tunable 10G DWDM SFP+ or a 400ZR QSFP-DD is one channel of this system. Its DDM shows its own Tx and the total or per-channel Rx after the demux; the amplifiers and OSNR are invisible to it except through pre-FEC BER (VDM on coherent modules). A module reading a healthy Rx power with a bad BER is the OSNR limit showing itself (Tunable transceivers).

In CodingBox

CodingBox reads the module's side of the span: channel/wavelength, launch power, Rx power and, for coherent CMIS modules, the VDM observables (OSNR estimate, pre-FEC BER, CD, DGD, Q-factor) on DDM. Comparing these with the commissioning baseline tells whether a degradation is in the module or in the line.