A passive filter that removes selected wavelengths from a fibre and inserts others, passing the rest through untouched.
What a Node Actually Does
An OADM sits mid-span and performs three operations at once: it drops the wavelengths that terminate at this site, adds the wavelengths that originate here, and expresses everything else through untouched. Physically it is a small stack of thin-film filters, each reflecting one channel and passing the rest.
That is what turns a point-to-point CWDM or DWDM link into a network. A ring where every site takes its own channels and forwards the others is the classic metro architecture, and it needs an OADM at each node.
The Loss Budget Is the Design
A single-channel OADM typically costs no more than 0.8 dB on the express path and 0.6 dB on the add or drop leg; a two-channel unit around 1.0 dB express. Add the two connector pairs at each node and roughly 1.5 dB per node is the number to plan with.
That accumulates. Eight nodes at 1.5 dB is 12 dB of insertion loss before any fibre is counted, which on a 25G CWDM budget of about 18 dB on a long-reach part leaves very little. Node count, not distance, is usually what caps a passive ring, and the link budget calculator is the place to find that out rather than at turn-up.
Isolation, and Why It Matters More Than It Sounds
Adjacent-channel isolation of 30 dB and non-adjacent of 40 dB are typical. Those numbers set how much of a neighbouring channel leaks into a drop port, and on a ring the leakage from every upstream node adds up while the wanted signal keeps losing 1.5 dB per hop.
It is the mechanism behind the classic ring fault: a link that works fine on a short chord and errors on the long way round, at a receive power that looks entirely healthy. Crosstalk, not loss, is what has changed.
Fixed, Reconfigurable, and How to Order
A fixed OADM is passive glass. No power, no software, nothing to configure, nothing to fail, and no way to change the channel it drops without swapping the module. A ROADM replaces the filters with wavelength-selective switches and lets a planner re-route wavelengths remotely, at several times the cost and with a power feed to maintain.
When specifying a fixed unit, settle three things: dual-fibre or single-fibre working, whether a monitor port is wanted (about 0.3 dB per direction), and which channels the site will want in three years rather than today. See our CWDM OADM and DWDM OADM modules.
OADM reference
| Function | drop chosen channels, add others, express the rest |
|---|---|
| Express loss, 1 channel | typically ≤ 0.8 dB |
| Express loss, 2 channels | typically ≤ 1.0 dB |
| Add or drop path loss | typically ≤ 0.6 dB for one channel |
| Adjacent-channel isolation | ≥ 30 dB |
| Non-adjacent isolation | ≥ 40 dB |
| DWDM channel passband | about ±0.11 nm on a 100 GHz filter |
| Monitor port | adds about 0.3 dB per direction |
| Reconfigurable version | ROADM, using wavelength-selective switches |
Related Terms
CWDM, DWDM, EDFA, OSNR and insertion loss.