Single-fibre CWDM, OADMs & band splitters

The basic CWDM link uses two fibres and a mux/demux pair at each end. Two variations squeeze more out of scarce fibre: single-fibre (bidirectional) CWDM, where each channel travels one way and its partner the other on the same strand, and optical add/drop multiplexers (OADM), which let intermediate sites pick off one or two channels from a passing trunk. Both are passive and cheap; both introduce their own loss arithmetic and pairing rules.

Single-fibre CWDM

On one fibre every wavelength is used in one direction only. A link therefore consumes two channels — one per direction — and the modules at the two ends are a pair: Tx λ1 / Rx λ2 at site A, Tx λ2 / Rx λ1 at site B.

Element	Dual-fibre CWDM	Single-fibre CWDM
Channels per 1G/10G link	1	2
Max links on 18 channels	18 (over 2 fibres)	9 (over 1 fibre)
Modules	identical CWDM modules at both ends (same λ)	paired: Tx/Rx wavelengths swapped between ends
Mux/demux	separate mux and demux, or a duplex unit	one single-fibre mux/demux per end with a common port; channel ports are duplex (Tx λn, Rx λm)
Reflections	tolerated	must be low — a Tx reflection lands in the local Rx filter path; APC connectors or good UPC and no open ports

Typical pairings follow the vendor's mux design (e.g. 1270↔1290, 1310↔1330 … or odd↔even). Mixing two modules of the same wavelength at both ends gives a dark link on both sides (Physical mismatches). BiDi SFPs (1310/1490, 1270/1330) are the two-channel special case of the same idea (CWDM transceivers).

OADM

An OADM sits inline on a trunk and, for one or more selected wavelengths, drops the channel to a local port and adds the local transmitter back onto the trunk in the same wavelength slot; all other channels pass through (express).

OADM type	Channels dropped/added	Express loss (per pass)	Drop/add loss
1-channel	1 (dual-fibre: 1 λ; single-fibre: 1 pair)	0.8–1.2 dB	1.0–1.5 dB
2-channel	2	1.0–1.5 dB	1.0–1.5 dB
4-channel	4	1.5–2.0 dB	1.2–1.8 dB
East/West (two-direction)	as above, on both trunk directions	as above per direction	—

Cascading N OADMs on a bus or ring costs N × express loss for the channels that pass all of them — the far channels must carry the budget of every intermediate site (Mux/demux & link budget).

Topologies

Topology	Description	Notes
Point-to-point	mux/demux at both ends	the baseline
Bus (linear add/drop)	trunk from a hub through sites A, B, C, each with an OADM	channel budgets differ per site; plan wavelengths so distant sites get the low-loss ones
Ring	trunk closes back to the hub; East/West OADMs at each site	protection: each site can reach the hub both ways with 1+1 modules or a switch-over
Hub-and-spoke over one trunk	1-channel OADMs per remote site	mobile backhaul, campus

Deployment patterns and faults: CWDM deployment.

Band splitters and overlays

A 1310/1550 band splitter (WDM coupler) separates the O-band from the C/L-band with 0.5–1 dB loss; it lets an existing 1310 nm link coexist with a CWDM or DWDM overlay in the upper band (1470–1610 nm), or a legacy 1550 nm link with new O-band channels. Combinations to watch:

CWDM upper band + DWDM — DWDM C-band (1530–1565 nm) occupies CWDM channels 1530 and 1550; a "1550 skip" CWDM mux leaves that slot free for a DWDM overlay.
CWDM + PON — GPON's 1490/1310 nm and XGS-PON's 1577/1270 nm collide with CWDM channels; coexistence needs a dedicated filter plan (PON wavelengths).
1383 nm water peak — channels 1370–1410 need low-water-peak fibre (G.652.D).

Channel table: CWDM channel plan; bands: Wavelength bands.

Budget arithmetic for an add/drop site

Example: 10G CWDM, hub → site C via OADMs at A and B, 35 km total.

Item	Loss, dB
Hub mux (insert)	1.5
Fibre 35 km at 1550 nm band (0.25 dB/km)	8.8
OADM at A, express	1.0
OADM at B, express	1.0
OADM at C, drop	1.2
Connectors (8 pairs × 0.4)	3.2
Splices (12 × 0.1)	1.2
Total	17.9
10G CWDM 40 km module: Tx 0 dBm min, Rx sensitivity −16/−18 dBm	margin 0 … 2 dB — marginal
Same with 80 km-class (APD, Tx +1, sens −24) modules	margin ≈ 7 dB — comfortable, but check overload at sites A/B (short paths)

Rules: give the farthest site the channels with the lowest fibre loss (1550–1610 nm, mind dispersion at 10G), use APD modules for long express paths and attenuate nearby sites, and record per-channel Rx at commissioning (Rx power & link budget).

Faults specific to these designs

Symptom	Cause
Single-fibre link dark both ways	same-wavelength modules at both ends; pair mismatch
Single-fibre link with errors, fine on dual fibre	reflections from an open/dirty connector into the local receiver
One add/drop site dark, others fine	wrong OADM channel, module wavelength ≠ OADM slot
Far site marginal after adding an OADM	express loss added to every downstream channel — re-run the budget
Errors only on 1610/1590 nm channels at 10G	dispersion penalty on the longest wavelengths (Mux/demux & link budget)

In CodingBox

The module's wavelength field is the only proof of its channel — read it, do not trust the colour label (Check transceiver). For single-fibre pairs, read both modules and confirm the Tx wavelengths differ and match the mux port plan before they leave the bench.