DWDM Optical Power Balancing: Attenuator Calculation

A DWDM link is only as good as its power balance. Each wavelength is an independent channel sharing one fibre and a chain of amplifiers, and if one channel arrives 8 dB hotter than its neighbour, either the hot channel drives the amplifier into saturation or the cold channel loses OSNR and starts producing errors that only appear at a specific wavelength. Doing this arithmetic on paper before the deployment team arrives on site is what separates a commissioning visit from a week of amplifier tuning.

The components and why most of them are passive

A DWDM system multiplexes many wavelengths into one fibre: a multiplexer at the transmitting end, a demultiplexer at the receiving end, and optical amplifiers in between in a variety of gain and output-power ratings. Transponders at each end convert the ingress signal — often a 1310 nm or 1550 nm SDH/fibre-channel wavelength — to the ITU grid wavelength the system uses, then convert back at the far end. Because most of the optical chain is passive, DWDM is cheaper than an equivalent SDH-based approach: fewer fibre pairs, no per-channel regeneration.

Terminology you need before the numbers

• Insertion loss: dB removed by a passive component (Mux, DeMux, connector).
• Gain: dB added by an amplifier. An amplifier is characterised by both a gain figure and a maximum output power — for example gain 16 dB with maximum output power 20 dBm.
• Receive sensitivity: the lowest power at which the receiver still works within spec. APD-type receivers are often specified for best operation at around −14 dBm.
• Channel power: per-wavelength power, which is what must be matched. The total power of N equal channels is channel power + 10 log N.

The 10 log N relationship is where most manual calculations go wrong: going from 1 to 5 equal channels raises total power by 10 log 5 ≈ 7 dB, not by a factor of 5.

The 4 dB rule

Keep the optical power difference between any two wavelengths in the system below 4 dB. The smaller the spread, the better: an uneven channel plan forces compromise amplifier settings and often leads to per-channel attenuators being retrofitted in the field, which is exactly what a pre-deployment calculation exists to avoid.

Worked example: balancing a partially populated 32-wave system

Design targets, taken from the published example:

Work the link forwards. Transponder at −3 dBm, Mux costs 7 dB, so the multiplexed input to the booster is −10 dBm per channel; the booster adds 16 dB, so the launch per channel is +6 dBm and the total launch power for five channels is +6 + 10 log 5 ≈ +13 dBm — comfortably under the booster's 20 dBm ceiling. The 20 km span costs 20 dB, leaving the pre-amplifier input at roughly −7 dBm total; the pre-amplifier adds 14 dB and is capped at 12 dBm output, so it operates below its ceiling. The DeMux then removes 7 dB.

The useful inversion in the published example works backwards from the receiver: with the DeMux output at −3 dBm for five channels, the per-channel level at the DeMux output is −3 − 7 = −10 dBm. The target is −14 dBm, so the system is 4 dB too hot per channel.

The fix, and where to put it

Two options were evaluated:

1. Insert a 4 dB attenuator at the input of every transponder receive card — five attenuators, five places for a mistake during installation.
2. Insert a single 4 dB attenuator at the input of the DeMux — one component, one location.

Option 2 is the recommendation, and it generalises: attenuate the common path wherever the imbalance is common to all channels. Keep per-channel attenuators only for correcting individual wavelength differences, and place them at the point where you can measure per-channel power, which is after the DeMux.

Pre-deployment checklist

• Compute per-channel power at every named point: transponder out, Mux in/out, amplifier in/out, span out, DeMux out, receiver in.
• Check the result against both the receiver sensitivity floor and the amplifier maximum output ceiling — overload damages receivers just as surely as under-power.
• Verify the maximum inter-channel spread is under 4 dB at the receiver.
• Remember the 10 log N term whenever the channel count in a partially populated system is expected to grow; a link balanced for 5 channels must be re-checked at the final channel count.
• Plan the attenuation as a common element of the optical path so field commissioning needs only minor adjustment.

For the passive layer that feeds a DWDM system, see MPO/MTP fibre polarity and loss budget, and for a transport technology that coexists with DWDM in the same facilities, see FlexE flexible Ethernet explained.

原文链接：https://blog.apnic.net/2024/12/10/optical-power-balancing-in-dwdm/