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I'm evaluating the suitability of optical SFP modules as a low cost continuous wave source. Is it possible to set optical SFP modules into a CW mode? Can this be done with off-the-shelf racks or is custom code required?

Based on this answer: Using SFP modules from a microcontroller it seems there is a high-pass filter blocking DC/CW operation. However, the reference link is broken so I don't know if this is generally true for the interface or if it is specific to one implementation.

user339443
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  • fixed that link in the reference answer for you – Marcus Müller May 09 '23 at 10:39
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    Ideally there is one out there for you which can do it. Many of the commercial ones are AC-coupled which would require you to open and solder a bypass at a minimum. – winny May 09 '23 at 10:41
  • yep, considering the interface is differential, and laser diodes aren't, and that all optical comms standards assume a base clock that's always transmitted to keep the receiversynchronized, the assumption that any reasonable SFP transmitter would be AC coupled seems at the very least reasonable. – Marcus Müller May 09 '23 at 10:42
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    What do you mean "off-the-shelf rack"? "Rack" is something where I put servers, switches and the like, and an SFP transceiver is a thing that you plug in to a network card or switch, so there's quite a big logical distance between "racks" and "SFP transceivers" – Marcus Müller May 09 '23 at 10:44
  • @MarcusMüller Rack was probably not the correct term. I guess I've seen rack mounted switches with a large number of ports. Was hoping there was something along those lines that would allow me to set modules into a CW mode. – user339443 May 09 '23 at 10:48
  • You can build or buy SFP breakout boards, like [this](https://osmocom.org/projects/misc-hardware/wiki/Sfp-experimenter) one. But standard networking equipment will never try to transmit a constant amplitude, so, everything that goes in a server rack is useless. Again, as alluded to, it's unlikely any SFP module is designed for DC. – Marcus Müller May 09 '23 at 10:52
  • by the way, I'm assuming "CW" is referring to the laser being constantly on, and not to the laser being constantly intensity-modulated with a sine wave. – Marcus Müller May 09 '23 at 10:53
  • What's wrong with using a laser diode? –  May 09 '23 at 12:56
  • @winny, AC coupling is required by the SFP MSA. However that's only for the data signal. The laser bias is controlled internally. It's conceivable that there's an SFP out there that will produce a CW output at about the same as its normal average output power when there's no data signal present. – The Photon May 09 '23 at 15:53

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