optical circulator as a bidirectional one fiber solution


There is a lack of bidirectional one fiber (BIDI) options for 40G and 100G optics. Usually BIDI is implemented using two CWDM wavelengths, one for tx and one for rx. However there is also a lack of CWDM and DWDM options for 40G and 100G.

Would it be possible to use an optical circulator like this one (customized to 1310 nm)?


Combined with a traditional two fiber 1310 nm 10 km 40G QSFP module like this: https://www.fs.com/de/en/products/24422.html

The link distance would be 5 km.

The optical circulator separates tx and rx by the direction the light travels in. It would work even though both directions use the same wavelength. There will likely be some reflection but hopefully attenuated enough that it is regarded as background noise.

Has anyone done this? Any reason it would not work?



The main issue you're going to run into (especially trying to plug anything into a DWDM shelf) is 40G and 100G transceivers usually emit 4 lanes of traffic instead of a single lane like 10 and 1G optics do.

I'd imagine that's why there are so few solutions that don't involve things like OTN.

Something that is broadly the same as a coherent 100G QPSK single wavelength optical module, but in two different frequencies, and a passive CWDM mux/demux prism at each end might work. The limitation would be availability of optics for a modern 100G MSA that are both coherent and Tx/Rx at two different THz frequencies.

Or with some box and vendor equipment in between, such as:


Good news about almost all optics, their Rx window is pretty wide. Meaning a 1550nm optic will activate the receiver on a 1560nm optic just fine (and probably anything in the 1500nm band). Careful use of specialized single strand DWDM muxes (FS.com) can yield great bidi-like results with increased channel count.


For 1 and 10Gbps OOK modulation yes, but not for something like a ITU DWDM grid channelized or tunable coherent optic. In which the (QPSK, 8PSK, 16QAM) signal has a specific THz width and frequency not unlike a radio operating in a very, very narrow waveguide.

You would still need to frequency shift TX and RX. They are travelling opposite directions on the same piece of glass; as the traffic rate increases the likelihood of collisions increases and you’ll start to get errors. The collision would either cancel the ‘bit’ or act like OBI and get erroneous bits and checksum errors or missing chunks from the constellation. There are BIDI 100G transceivers for Multi-mode available today based on the Foxconn optics, I’d imagine we’ll see BIDI for the O and C bands in the next 18 months.

What about 100Ghz ITU spacing on the tx, are the rx optics broad enough to take the off-band input?


Non-coherent receivers usually seem to even when paired with DWDM grid transceivers. You're paying for the tightly controlled laser on those, not so much the receiver.

Coherent of course will demod whatever you tune them to (and pretty well reject other stuff) but are therefore subject to their LO tuning range.

Figure your typical OOK optical receiver is roughly like a single-stage AM radio receiver (crystal radio or similar, just followed by amplification to logic level) without any front-end filtering on it. The demux in front of it, where present, is the front-end filter. It'll capture and downconvert to RF basically anything it sees.

If we were talking 10G, adjacent channels, add a TFFL filter it Should work. 100G isn’t just on-off at a high clock rate, it’s also modulated around the center frequency, I don’t think it’d work even with a wideband receiver.

There are some DCO 100G coherent optics on the market, but I think this thread is more about why there’s not much in the way of 40/100g transmission speed, but it really is about how 10G was one of the last walls where OOK was a thing. Once you went 40G/100G you went to parallel signals, either in CDWDM form or parallel signals on parallel fibers (eg: those MPT/MPO cables we all get to enjoy).

If you talk to the optics folks you can get customized tx/rx optics, and in theory you could get an optimized 100G-LR4 or 100G-ER4-Lite optic, but even those the reach is quite limited to around 25Km while in the 10G space you can get up to 80-120km with a filter and a bunch of pluggable fixed channel (or tunable) optics.

You can get decent 100G OEO solutions for a fair price per channel but you’re still looking at $$ compared to just building a 40x10G system due to the cost differences.

Without more details like your link budget, etc.. it’s hard to suggest options other than get at least 2 fibers :slight_smile:

I know some vendors are working on some creative 1 fiber solutions due to the way the dark fiber is taxed in Europe, so you should be talking to the usual suspects..

- Jared

Hi Baldur,

Would it be possible to use an optical circulator like this one
(customized to 1310 nm)?

Has anyone done this? Any reason it would not work?

While it often does work, reflectance is a bitch.

I'd advise you to order some attenuators to plug on RX ports in case the
ghost signal is too high.

Of course, you should use APC connectors all along the optical path.

Also bear in mind that circulators have a narrow spectrum range (likely
1280-1340 in your case, good for 100G-LR4/ER4, but not for 100G-CWDM4)
and are rarely polarization neutral, meaning a DP-QPSK coherent optic
cannot be used.

Best regards,