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I'm naive in this field, but I will need to use optic fibers for optogenetics application. I already know that my question will sound stupid for the most of you, please forgive me for that.

My problem is the following: we are going to buy a laser for our optogenetic application and we decided to buy a pigtailed laser diode. At this point I was wondering which could be the best fiber to couple to the laser. The company suggested us to couple a single mode fiber (0.07 NA, 35 microns core) but, since we will use multimode fiber (50 microns core, 0.22 NA and 200 microns core, 0.22 NA) in our experiment, I'm scared that this actually will not work. The distance the light has to travel is very short, so the actual problem would be the data loss at the interface.

What would you suggest?

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  • \$\begingroup\$ What kind of data? How is it modulated? What data rate or subcarrier frequency? What wavelength? \$\endgroup\$ – The Photon Feb 19 at 15:28
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Launching from single mode fiber to multimode fiber is fairly easy. You're routing a small pipe into a big pipe. You should not lose much optical power at this interface.

However, you will excite a restricted set of modes in the multimode fiber, and this could cause the behavior to be different from what you'd normally expect. You could find the multimode fiber's effective bandwidth is much better than usual with the restricted mode launch. Or you could find that a single pulse launched into the fiber is split into two pulses at the output. Neither of these effects is likely to be very significant if the fiber length is short (a few meters or less) and you're using on-off keying (OOK) or amplitude modulation.

The fact your single mode fiber has 35 um core diameter implies you're using a very long optical wavelength (compared to the 650-1600 nm normally used for optical communication). This will likely mean the 50-um fiber, while multimode, will really only support a few (10 or fewer?) modes at your wavelength. This can also cause some hard-to-predict effects, so you might be best off using the 200-um option if you want to launch from a single mode fiber.

Unless there's something you're not telling us, you should be fine using a laser with a single mode pigtail fiber.

If you're using some complex modulation scheme, or expecting to extract data from the phase of the optical signal or something, then you might want to reconsider (but I'd be surprised you're using multimode fiber at all in these scenarios).

Final note: Be careful to use the same polish type on the ends of the two fibers you're mating. If the single mode pigtail comes with an angled facet (APC connector type) you'll need to mate it to a multimode fiber with APC polish. This will likely be a special-ordered connector, because APC is not normally needed for multimode applications.

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