WDM inside the module: filters, AWGs, BiDi, wavelength control

Many modules are small WDM systems in their own right. An LR4 or FR4 module multiplexes four lasers onto one fibre and demultiplexes four wavelengths at the other end; a BiDi or PON module sends and receives on one fibre through a filter; a DWDM module holds its wavelength to a few gigahertz for years. This page covers the optics that make that possible — the filters and waveguide multiplexers, the BiDi splitter, the coolers and wavelength lockers — and the losses and failure modes they add.

Where WDM lives inside modules

Module type	Internal WDM	Wavelengths	Fibre port
40GBASE-LR4, 100GBASE-LR4/ER4	4-channel LAN-WDM mux + demux	1295.56 / 1300.05 / 1304.58 / 1309.14 nm (800 GHz spacing)	duplex LC
100G CWDM4, 400G FR4, 800G 2×FR4	4-channel CWDM mux + demux	1271 / 1291 / 1311 / 1331 nm (20 nm spacing)	duplex LC
40G/100G SWDM4	4-channel short-wave mux on MMF	850 / 880 / 910 / 940 nm	duplex LC, OM4/OM5
BiDi (1G–100G), PON ONU/OLT	2- or 3-port filter (BOSA)	1310/1490, 1270/1330, 1490/1310/1550…	single LC or SC/APC
400G BiDi (SR4.2)	2 wavelengths per fibre on 4 pairs	850 / 910 nm	MPO
DWDM fixed / tunable	no mux inside; precise single wavelength	ITU grid, 100/50 GHz	duplex LC — mux is external
PSM4, DR4, SR4	none — parallel fibres	one wavelength	MPO

Channel plans and bands: Wavelength bands & channel plans.

Multiplexer technologies

Technology	How it works	Insertion loss (per channel, mux or demux)	Notes
Thin-film filter (TFF) block — "Z-block"	dielectric interference filters glued to a glass block; light zig-zags, each filter passes one band and reflects the rest	1–1.5 dB	robust, athermal, the workhorse of LR4/CWDM4/FR4 TOSAs and ROSAs
AWG (arrayed waveguide grating, silica PLC)	phased array of waveguides disperses wavelengths to separate outputs	2–3 dB	compact for many channels; temperature-sensitive unless athermalised; used in some LR4 and in DWDM equipment
Free-space grating / prism	bulk optics	2–4 dB	rare in pluggables
Silicon-photonics mux	ring resonators or echelle gratings on-chip	1–3 dB	integrated with modulators in SiPh FR4 designs; may need thermal tuning
BiDi filter	single 45° dichroic plate: passes one band to the PD, reflects the laser's band into the fibre	0.5–1 dB	isolation 30–40 dB between Tx and Rx bands

Internal mux + demux loss (2–5 dB round trip) is part of the reason WDM modules launch more power per lane than parallel ones and why their DDM Tx per lane looks lower than the laser's actual output (Typical values).

Why LR4 needs cooling and CWDM4 does not

An uncooled DFB drifts about 0.1 nm/°C — 7 nm over a 0–70 °C case range.

Grid	Spacing	Filter passband	Uncooled drift fits?
CWDM (CWDM4, FR4)	20 nm	±6.5 nm	yes — no TEC, lower power, cheaper
LAN-WDM (LR4, ER4)	800 GHz ≈ 4.5 nm	±1 nm or less	no — each laser sits on a TEC at a fixed temperature (or a wide-temperature-range laser design is used)
DWDM	100 GHz ≈ 0.8 nm, 50 GHz ≈ 0.4 nm	±0.1 nm	no — TEC plus a wavelength locker

This is why 100G LR4 modules draw 3.5–4.5 W where CWDM4 draws 2–3 W, and why LR4 TOSAs are box packages rather than TO-cans (Optical packaging).

Holding a DWDM wavelength

 laser on TEC ──► 1–5 % tap ──► etalon (Fabry–Pérot filter) ──► PD1
                            └──────────────────────────────► PD2 (reference)
 ratio PD1/PD2 → MCU → adjust TEC set point (fine) or laser current → wavelength locked

TEC + thermistor — a Peltier element under the laser submount holds it at a set point (25–45 °C) to ±0.1 °C; the MCU runs the loop. DDM may expose laser temperature and TEC current (Temperature & voltage).
Wavelength locker — an etalon whose transmission ripples with the ITU grid period; comparing the tapped power through it with a reference tells the controller which side of the channel the laser is on. Accuracy ±1–2.5 GHz over life.
Tunable lasers add a coarse mechanism: sampled-grating DBR sections, a DFB array with a combiner and thermal tuning, or an external cavity (ECL); packaged as ITLA / micro-ITLA / nano-ITLA in coherent modules, or as compact tunable TOSAs in SFP+/XFP. Channel selection and status go through SFF-8690 / CMIS page 04h–12h; tuning takes seconds and the Tx is squelched meanwhile (Tunable transceivers).

BiDi and PON optics

A BOSA puts a laser and a photodiode behind one fibre port. A 45° WDM filter reflects the laser's wavelength into the fibre and passes the incoming wavelength to the photodiode; isolation between the two bands (30–40 dB) and the filter's edge steepness decide how much of the module's own light leaks into its receiver. PON adds burst-mode electronics on the ONU (laser on only during its time slot) and on the OLT (a receiver that resets its threshold between bursts from different ONUs) — the reason an OLT's DDM Rx is an average of many senders (PON transceivers).

BiDi modules come in paired variants (-U/-D, or by wavelength) because the filter in each is tuned the opposite way; two of the same kind never link (Physical mismatches).

Polarisation and reflections

Internal filters and AWGs are slightly polarisation-dependent (PDL 0.1–0.3 dB); fibre polarisation wanders, so the received power may wobble by a few tenths of a dB with no fault. Every internal surface also reflects a little; isolators after each laser keep this from reaching the die (Optical packaging).

Failure modes specific to internal WDM

Failure	Signature	Notes
TEC failure on LR4/DWDM	laser temperature drifts, TEC current at limit, one lane's Rx at the far end falls as it slides off the filter	module temperature may look normal
Filter delamination / misalignment	one lane low in both Tx (into fibre) and far-end Rx, wavelength-dependent	mechanical shock, thermal cycling
Wavelength locker drift	DWDM channel off-grid, filtered by the mux → low Rx, errors	DDM Tx unchanged — the light is there, at the wrong wavelength
BiDi filter leakage	poor sensitivity, errors on short links (own Tx swamps the Rx)	worse with high launch power and reflections
Ageing of one laser in a 4-lane WDM module	one lane's bias up, then power down; the other three fine	Per-lane diagnostics

In CodingBox

The identity bytes describe the WDM design: wavelength (1310 for LR4/CWDM4 as the nominal centre, or the exact channel for DWDM), the compliance or application codes (LR4 vs CWDM4 vs FR4) and, for tunable modules, the channel grid and current channel on the tunable pages. Per-lane DDM on the DDM screen shows each internal laser separately; laser temperature and TEC current appear where the module reports them.