Trenching fibre costs between a few tens and a few hundreds of pounds per metre depending on what is above it. The optics that light it cost a few hundred pounds. That ratio is the entire argument for coloured optics: if you can put eight, or forty, or eighty 10G channels down one pair of fibres instead of one, you have multiplied the value of the expensive asset using the cheap one.
10G CWDM and DWDM SFP+ modules are how most organisations do that without buying a transport platform. They drop into ordinary SFP+ switch and router ports, they look like normal 10G interfaces to the host, and all the wavelength-division work happens in a passive multiplexer in the rack. No transponders, no shelf, no line cards.
This guide covers the two grids and what they are for, the reach and receiver differences between 40 km and 80 km variants, fixed versus tunable channels, how coloured optics work in someone else’s DWDM system, and the channel-planning mistakes that strand fibre.
CWDM and DWDM, in one section
Both put multiple wavelengths on one fibre pair. The difference is how closely spaced they are, and everything else follows from that.
| CWDM | DWDM | |
|---|---|---|
| ITU grid | ITU-T G.694.2 | ITU-T G.694.1 |
| Channel spacing | 20 nm | 100 GHz (~0.8 nm) or 50 GHz (~0.4 nm) |
| Channels available | 18 across 1271–1611 nm | 40 at 100 GHz, 80 at 50 GHz across the C-band |
| Laser | Uncooled DFB or EML | Temperature-stabilised, thermoelectrically cooled |
| Wavelength stability needed | Loose — the channel is 13 nm wide | Tight — drift of a fraction of a nanometre matters |
| Module cost | Lower | Higher |
| Amplifiable | Only the channels inside the EDFA window | Yes — the whole C-band |
| Typical use | 8–18 channels, metro and access, up to ~80 km | High channel counts, long haul, anything amplified |
The short decision rule: if you need fewer than about sixteen channels and no optical amplification, CWDM is cheaper and simpler. If you need more channels, or you need amplifiers, or you are feeding an existing DWDM line system, it has to be DWDM. The wider comparison, including the passive components either side of the module, is in CWDM vs DWDM.
The CWDM grid, and the channels nobody uses
G.694.2 defines eighteen CWDM wavelengths at 20 nm spacing from 1271 nm to 1611 nm. In practice the range splits into two halves that behave quite differently on real fibre.
- 1271–1451 nm. The lower channels. On older ITU-T G.652 fibre these sit on or near the water absorption peak around 1383 nm, where attenuation rises sharply. On modern low-water-peak fibre (G.652.C and G.652.D) they are usable; on 1990s cable plant some of them are not. Check what fibre you actually have before planning a channel there.
- 1471–1611 nm. The upper eight channels. Low loss on essentially any single-mode fibre, and the reason most eight-channel CWDM systems use exactly these. This is the band Carritech’s base CWDM designs cover.
One more CWDM limitation to plan around: only the top few CWDM channels fall inside the gain window of a standard erbium-doped fibre amplifier. If your design might ever need amplification, CWDM closes that door for most of its channels. That is not a reason to avoid CWDM — it is a reason to be honest about the ceiling before you commit the mux.
The DWDM C-band grid
G.694.1 anchors the DWDM grid to 193.1 THz and spaces channels at 100 GHz, 50 GHz or tighter. At 100 GHz spacing the C-band gives forty channels, and that forty-channel plan from 1530.33 nm to 1561.42 nm is what the overwhelming majority of fixed 10G DWDM SFP+ modules are built for. Carritech stocks the complete forty-channel plan in two different OEM codings, listed in full further down.
Two things about DWDM modules that CWDM modules do not have to worry about:
- The laser is temperature-stabilised. A 100 GHz channel is about 0.8 nm wide and the module has to stay inside it across its whole operating range, so there is a thermoelectric cooler and a control loop in there. That is part of why a DWDM module costs more and why its power draw is less trivial than a grey optic’s.
- The channel is in the part number, and it is not field-changeable. A fixed DWDM SFP+ is one wavelength for life. Order the wrong channel and you have bought a module you cannot use, which is the single most common DWDM procurement error and the reason the tunable option below exists.
40 km or 80 km: it is the receiver, not the laser
Both Carritech base designs at each grid use an EML transmitter. The difference between the 40 km and 80 km parts is mostly on the receive side, and it is worth understanding because it determines what you can and cannot get away with.
| 40 km variant | 80 km variant | |
|---|---|---|
| CWDM | CT-SFP+CWXX-40 — EML and PIN | CT-SFP+CWXX-80 — cooled EML and APD |
| DWDM | CT-SFP+DWXX-40 — cooled DFB-EML and PIN, 100 GHz grid | CT-SFP+DWXX-80 — EML and APD, 50 GHz grid |
| Receiver | PIN photodiode | Avalanche photodiode |
| Power | < 1.5 W | < 1.5 W |
| Standards | IEEE 802.3ae, SFF-8431, SFF-8472 | IEEE 802.3ae, SFF-8431/8432, SFF-8472 |
An avalanche photodiode is several decibels more sensitive than a PIN diode, which is what buys the extra forty kilometres. It also has a narrower dynamic range at the top end, so an 80 km module on a 10 km span can be overloaded — put an attenuator in or buy the right part. The underlying signalling is plain IEEE 802.3ae 10 Gigabit Ethernet in both cases, with the module memory map and diagnostics defined in the SFF specifications.
Beyond about 80 km on G.652 fibre at 10G, chromatic dispersion starts to be a design factor in its own right rather than something the module absorbs, and the honest answer is usually an amplified design or a coherent one rather than a longer-reach pluggable. Work it properly with the optical link budget rather than the datasheet kilometre.
Fixed or tunable?
A tunable DWDM SFP+ can be set to any channel on the C-band grid, usually over the module’s management interface or by the host platform. One part number covers the whole plan.
The arithmetic is straightforward. A forty-channel fixed plan means forty part numbers, and sparing it properly means holding stock of each one. One tunable part number spares the entire plan, and it means a channel re-plan does not require a procurement cycle. Against that, a tunable module costs more per unit, draws more power, and has to be configured — an unconfigured tunable module in a port is a link that does not come up for a reason nobody can see on the fibre.
Carritech’s tunable design is CT-SFP+TDWXX-80, with OEM-coded equivalents including DWDM-SFP10G-C-S= and DWDM-SFP10G-E-I=, 10G-SFPP-ZRD-T, SFP-10G-ZDWT-L, SFPP-10G-CT50-ZR, SFPP-10GE-DWDM-IT, JL250A. The full case, including where tunability stops paying for itself, is in why tunable optical transceivers can reduce complexity in DWDM networks, and the tunable 10G DWDM range is here.
Black link: your optics in someone else’s DWDM system
A question that comes up constantly: can you put a coloured SFP+ straight into a switch and feed it into a DWDM line system that somebody else owns, or that came from a different vendor to the switch?
Usually yes, and there is a standard for exactly this arrangement. ITU-T G.698.2 specifies amplified multichannel DWDM applications with single-channel optical interfaces — the "black link" model, where the transmitter and receiver are specified at the points where they meet the line system, and what happens inside the line system is the line system’s problem. It is what makes third-party coloured pluggables into managed DWDM infrastructure a design rather than a gamble.
What you need from the line system owner to make it work: the channel allocation, the permitted launch power range at the input, the expected power presented at the output, and whether the system expects 100 GHz or 50 GHz spacing. With those four numbers the module choice is arithmetic. Without them it is guesswork, and the usual failure is a module launching too hot into an amplified span and degrading every other channel on the fibre.
What 10G coloured optics also carry
It is easy to forget that a 10G SFP+ cage is not only an Ethernet port. The same wavelength plan, and in most cases the same modules, carry:
- OTU2 and OTU2e transport frames, with the ITU-T G.709 wrapper and its RS(255,239) forward error correction — which is why transport people expect more reach from the same optics than Ethernet people do. The FEC is doing real work; the mechanism is in our guide to forward error correction.
- SONET and SDH at STM-64 and OC-192, on legacy transport that is still very much in service.
- 10G Fibre Channel on some storage interconnects — see Fibre Channel transceivers for how the storage speed names map onto the Ethernet ones.
Check the specific part before assuming a given module is rate-flexible, but the point stands: coloured 10G pluggables are the connective tissue between the data centre and the transport network, which is why they turn up in data centre interconnect designs long after the data centre itself has moved to 100G and beyond.
Diagnostics, and the one thing to monitor
Every module here reports digital diagnostics: temperature, supply voltage, laser bias current, and transmit and receive optical power. On a WDM link received optical power per channel is the number that matters, and it is worth trending rather than merely alarming on.
The reason is that a WDM link degrades in ways a grey link does not. A mux port can be dirty while every other port is clean. An amplifier can tilt its gain across the band so that the channels at one end of the plan quietly lose margin. A patch panel reconfiguration can add half a decibel that nobody records. None of that shows up as an error until it shows up as an outage, and the only early warning is the received power trend. Our guide to spotting a failing fibre link before it drops covers how to use it, and fibre optic connector cleaning covers the cheapest decibels available anywhere — which in a WDM rack full of LC adapters is a larger number than most people expect.
Five mistakes that strand fibre
- Buying 100 GHz modules for a 50 GHz plan, or the reverse. A 100 GHz module will sit on a 50 GHz grid channel if that channel coincides with its wavelength, and will not otherwise. Confirm the spacing before the part number.
- Planning CWDM channels your fibre cannot carry. The lower CWDM wavelengths on older cable plant can lose far more than the design assumed. Test the span, or stay in the 1471–1611 nm band.
- Ignoring the mux insertion loss. A multiplexer and demultiplexer pair can easily take several decibels out of the budget before the fibre does anything at all, and an add-drop node takes more. It belongs in the budget calculation, not in the margin.
- Using an 80 km module on a short span. APD receivers saturate. Attenuate or buy the 40 km part.
- Treating the channel plan as a document rather than a record. Wavelength allocation has to be tracked somewhere authoritative, because the cost of getting it wrong is a module you cannot use and, worse, a channel collision that takes down a working service.
The 10G CWDM and DWDM SFP+ modules Carritech stocks
A hundred and sixteen tested modules across the two grids, plus the tunable range, every one coded for the platform it is going into.
CWDM: the eight-channel upper band
Cisco-coded across 1470 to 1610 nm: CWDM-SFP10G-1470, CWDM-SFP10G-1490, CWDM-SFP10G-1510, CWDM-SFP10G-1530, CWDM-SFP10G-1550, CWDM-SFP10G-1570, CWDM-SFP10G-1590 and CWDM-SFP10G-1610.
CWDM: the full eighteen-channel span
Ubiquiti-coded parts extend down into the lower band: UACC-OM-SFP10-1270, UACC-OM-SFP10-1290, UACC-OM-SFP10-1310, UACC-OM-SFP10-1330, UACC-OM-SFP10-1450, UACC-OM-SFP10-1470, UACC-OM-SFP10-1490, UACC-OM-SFP10-1510, UACC-OM-SFP10-1530, UACC-OM-SFP10-1550, UACC-OM-SFP10-1570 and UACC-OM-SFP10-1590. Nokia 3FE65831AB and 3FE65832AB, and the Nortel-heritage AA1403165-E6, complete the CWDM range. The whole 10G CWDM SFP+ range is here.
DWDM: the forty-channel 100 GHz plan
Both common OEM codings across the complete C-band plan, so you can match whatever the far end already has. Find your wavelength in the table and the part numbers are alongside it.
Plus the HPE and H3C parts JG990A, JG991A and JG992A, the multi-channel DWDM-SFP10G-C and SFP-10G-DWDM, the Juniper-style channel range SFP-10G-W17 to SFP-10G-W61, SFP-10G-DZ-T, SFPP-10G-DT-ZRC2 and the Finisar FTLX6872MxC. The full 10G DWDM SFP+ range is here.
The Carritech base parts
Five designs sit behind the whole range. CT-SFP+CWXX-40 covers the CWDM grid from 1470 to 1610 nm to 40 km with an EML transmitter and PIN receiver; CT-SFP+CWXX-80 does the same wavelengths to 80 km with a cooled EML and an APD. CT-SFP+DWXX-40 covers the 100 GHz C-band plan to 40 km; CT-SFP+DWXX-80 covers the 50 GHz plan to 80 km. CT-SFP+TDWXX-80 is the tunable 80 km design. All five draw under 1.5 W and report full digital diagnostics.
Five designs, well over a hundred part numbers, and the difference is the channel plus the vendor identification string. For a multi-vendor estate that is exactly the problem the Carritech Opticode coding box and the approach in building a smarter transceiver stock strategy are for.
Where to go next
If you are weighing coloured optics against simply lighting another pair, start with the link budget and CWDM vs DWDM. If the span is short and the problem is fibre count rather than distance, BiDi transceivers solve it with one fibre instead of two and no multiplexer at all — the 10G BiDi SFP+ range covers it. If the requirement is really 100G or more over a metro span, 400ZR vs OpenZR+ and data centre interconnect are the right starting points.
Getting the right channel first time
Carritech Optics holds the 10G SFP+ and XFP range in UK and EU stock — 10G CWDM, 10G DWDM, tunable DWDM and the grey 10G duplex SFP+ modules, plus the wider 10G transceiver range and patch cords.
Every module is coded for the target platform and measured against its specification before despatch — see the 22 checks a module passes and our transceiver testing process, which includes the wavelength and optical power verification that matters most on a WDM link — and carries a lifetime warranty. On procurement, compatible optical transceivers vs OEM and do third-party optical transceivers void your warranty? answer the questions that always come up.
Send us the channel plan, the span length and the platform at each end and we will tell you which parts fit and where the budget is tight. Our compatibility checker returns the tested Carritech equivalent of an OEM part number in seconds, upload a whole channel list and we will price it, or request a quote.
