ایمپلیفیکیشن، OSNR اور DWDM اسپین کی حدود

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

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.

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:

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

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

1. Fibre type, length and measured loss per span at 1550 nm (OTDR).
2. Channel count, grid and format → required OSNR and non-linear limits.
3. Amplifier placement: booster, inline every 60–100 km (span loss 15–25 dB), pre-amp.
4. Launch power per channel, tilt and transient control settings.
5. Dispersion map (direct detect) or DSP capability check (coherent).
6. Received OSNR and pre-FEC BER per channel at commissioning — the baseline for the life of the system (VDM & FEC metrics).
7. 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.

Chromatic dispersion, PMD and dispersion compensation on the spans these amplifiers bridge: Dispersion and bandwidth.

Why launch power cannot simply be raised to buy OSNR — SPM, XPM, FWM, Brillouin and Raman limits, the optimum power per channel and mitigation: Non-linear effects.

The infrastructure view of amplification — EDFA, Raman, SOA and OEO compared, where they sit, what operating an amplified line demands: Amplifiers & regeneration.