{"id":101754,"date":"2026-11-02T14:24:58","date_gmt":"2026-11-02T14:24:58","guid":{"rendered":"https:\/\/optics.carritech.com\/?p=101754"},"modified":"2026-11-02T17:13:42","modified_gmt":"2026-11-02T17:13:42","slug":"bidi-sfp-modules","status":"publish","type":"post","link":"https:\/\/optics.carritech.com\/pt\/bidi-sfp-modules\/","title":{"rendered":"BiDi SFP modules: every single-fibre optic from 155M to 100G"},"content":{"rendered":"

A duplex optical module assumes two fibres: one to transmit on, one to receive on. That assumption is free in a data centre and expensive almost everywhere else. In access networks, municipal ducts, rail and utility routes, mast sites and anything fed by a historic lead-in, the number of strands you have is the number somebody installed decades ago, and getting another one means a civil engineering project.<\/p>\n\n\n\n

A BiDi module solves that by putting one wavelength in each direction down a single strand<\/strong>, with a simplex LC on the front instead of a duplex pair. It doubles the capacity of fibre that is already in the ground, needs no multiplexer, and costs a fraction of trenching. The trade is that the two ends are no longer identical, and that single fact is behind most of the support calls this product class generates.<\/p>\n\n\n\n

Our guide to BiDi transceivers<\/a> covers the technology, the pairing rule and the reflection failure mode. This article is the companion to it: the complete range, 147 part numbers from 155 Mbit\/s to 100 Gigabit<\/strong>, what each speed can actually reach, and the four things to settle before you order.<\/p>\n\n\n\n

The rule that catches everybody<\/h2>\n\n\n\n

BiDi modules are sold in matched upstream and downstream halves, and two of the same half will never link.<\/strong><\/p>\n\n\n\n

A 1310\/1550 nm pair means one end transmits at 1310 and listens at 1550 while the other does exactly the reverse. Order two upstream parts and you have two modules shouting on the same wavelength and deaf to each other. There is no configuration that fixes it and no diagnostic that says so clearly — both modules report a healthy transmitter and no received power, which looks like a broken fibre.<\/p>\n\n\n\n

You can see the rule in the part numbering throughout the lists below. Every Carritech BiDi design exists as a U<\/strong> and a D<\/strong>: CT-SFP-BX-40U<\/a> pairs with CT-SFP-BX-40D<\/a>, and so on at every reach and every speed. OEM codings follow the same convention with their own letters — -D<\/strong> e -U<\/strong>, BXD<\/strong> e BXU<\/strong>, or Juniper’s descriptive 13R15<\/strong> e 15R13<\/strong>, which spell out transmit-1310-receive-1550 and the reverse. That last scheme is the clearest in the industry and the only one you can read without a data sheet.<\/p>\n\n\n\n

The practical advice is simple and we follow it on every order: buy pairs, label both ends, and record which wavelength plan a route uses somewhere other than one engineer’s memory.<\/strong> Tell us the reach and we will supply a matched pair rather than leaving you to work out which half is which.<\/p>\n\n\n\n

What each speed can actually reach<\/h2>\n\n\n\n

This table is drawn from the stocked Carritech designs rather than from a standard, because for most of this range there is no standard to draw from<\/strong> — a point the next section deals with properly.<\/p>\n\n\n\n

\n\n
Speed and package<\/th>Reaches stocked<\/th>Longest<\/th>Typical wavelength pair<\/th><\/tr><\/thead>\n
155M SFP<\/strong><\/td>20, 40, 80, 120, 160, 180 km<\/td>180 km<\/strong><\/td>1310\/1550 short, 1490\/1550 long<\/td><\/tr>\n
1.25G SFP<\/strong><\/td>3, 20, 40, 80, 120, 160, 180 km<\/td>180 km<\/strong><\/td>1310\/1550 short, 1490\/1550 long<\/td><\/tr>\n
10G SFP+<\/strong><\/td>3, 10, 20, 40, 60, 80, 100 km<\/td>100 km<\/td>1270\/1330 short, 1490\/1550 long<\/td><\/tr>\n
10G XFP<\/strong><\/td>10, 20, 40, 60, 80, 100 km<\/td>100 km<\/td>As SFP+<\/td><\/tr>\n
25G SFP28<\/strong><\/td>20, 40, 60, 80 km<\/td>80 km<\/td>1270\/1330 short, 1490\/1550 long<\/td><\/tr>\n
100G QSFP28<\/strong><\/td>10, 20, 30, 40, 80 km<\/td>80 km<\/td>1304\/1309 nm<\/strong> at 40 km<\/td><\/tr>\n<\/tbody>\n<\/table><\/figure>\n\n\n\n

Two things in that table are worth more than the rest of it.<\/p>\n\n\n\n

The slowest parts go the furthest, by a long way.<\/strong> CT-MSFP-BX-180U<\/a> is a 155 Mbit\/s BiDi module specified for 180 kilometres on a 29 dB link budget<\/strong>, drawing under a watt. That is not a typo and it is not exotic engineering: a slow signal needs far less received power per bit to decode cleanly, so the same laser buys enormously more distance. The identical logic gives the 1.25G range 180 km at CT-SFP-BX-180U<\/a>, while 10G stops at 100 and 25G at 80. If you have a very long single-strand route and a modest bandwidth requirement, the cheap slow optic is the right answer and the fast one will not reach at all.<\/strong> Our guide to 155M and 1.25G SFP modules<\/a> makes the same argument for duplex parts.<\/p>\n\n\n\n

At 100G the wavelength pair is 1304 and 1309 nm — five nanometres apart.<\/strong> Everywhere else in this range the two directions are separated by twenty to two hundred and sixty nanometres, which makes the filtering inside the module straightforward. At 100 Gigabit both directions have to sit inside the narrow window where a single-wavelength PAM4 interface works at all, so CT-QSFP28-BX-40U<\/a> separates them by five. That is a genuinely harder optical filter, and it is why 100G BiDi is a recent product rather than an old one.<\/p>\n\n\n\n

Where the standards are, and where they are not<\/h2>\n\n\n\n

This matters commercially, because it decides what "compatible" can mean.<\/p>\n\n\n\n

IEEE 802.3<\/a> defines single-strand bidirectional working only in specific places — principally the 100BASE-BX and 1000BASE-BX port types that came in through the Ethernet-in-the-first-mile work. There is no 10GBASE-BX, no 25GBASE-BX and no 100GBASE-BX.<\/strong> Everything above a gigabit in the lists below is an industry design: a manufacturer has taken a standard optical interface, split the directions across two wavelengths, and specified the result themselves.<\/p>\n\n\n\n

Two consequences follow, and both are practical rather than academic.<\/p>\n\n\n\n