# Is there a maximum power rating for fiber optics cables?

I'm at an internship and I've been tasked with projecting the image with a projector in a direction different than where it would normally go.

I am looking into fiber optics, but it doesn't seem like there's too much info on this topic.

I was just wondering if there's a maximum power rating for fiber optic cables (like the "image conduits") that I would have to worry about if pounding 5+ watts of light through the fiber and expect a decent beam (after external optics) to be projected out the other side.

• Fiber optics can't do that. How about a mirror? Cheap, easy and, most importantly, it will actually work. – CuriousOne Jul 1 '16 at 22:28
• @CuriousOne - I was going to say it was impossible, but with a coherent fibre bundle careful input/output f numbers and not too much FRD it isn't "entirely" impossible - just wildly impractical ! – Martin Beckett Jul 2 '16 at 4:17
• @MartinBeckett: I was thinking about matching a regular projector to a fiber bundle... I think that's far outside of the "usefulness" envelope. With custom optics it's a different matter, of course, but compared to a mirror the results will still be far inferior. – CuriousOne Jul 2 '16 at 5:53
• Point the projector in a direction different from where it normally points? No, seriously! If that's not a reasonable answer, then the problem you are trying to solve has constraints that you haven't told us about. What are the constraints? – Solomon Slow Jun 17 '17 at 3:30

As the curious one and Martin Beckett points out in the comments, the idea of trying to transmit an image in a fiber is wildly impractical and there are probably simpler solutions.

However, to answer the title question, yes most fibers have important power limits. For example, we avoid putting more than 300mW into the single-mode fibers we use in my lab. This might seem small, but keep in mind that the mode-field diameter of such a fiber is quite small, so that 300mW has been focused to a diameter of about 5 $$\mu$$m. There are two main limitations to this - damaging the fiber itself at the air to glass interface and damaging a connector or termination component. For example, many fibers are terminated in a ceramic ferrule, and the fiber is held in the ferrule using epoxy. If the power is too high, the laser light can vaporize the epoxy causing an epoxy reside to coat the fiber, thus increasing scattering and heating, creating a small positive feedback loop and probably causing major damage to the fiber.

We have some higher power fibers which don't have the ceramic cladding or have an "end-cap" design where the air-to-glass transition happens at a much larger mode-field diameter before the mode is shrunk to the single-mode size. But eventually these types of issues would still potentially melt part of the fiber connector. Multi-mode fibers will have much higher limits because the fiber cores are much larger. The fiber core itself can handle much higher powers which allows applications like that mentioned by Martin, but to handle such high powers extreme care must be taken. I think for example the powers Martin is referring to are coming from fiber lasers, so the light is actually generated inside the fiber and is not so subject to the issues I'm mentioning.

In the end you have to ask the manufacturer what powers they recommend / have tested for a given fiber. For example, thorlabs has a nice guide for this:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3882

They are commonly used for 10KW laser cutting - so I wouldn't worry about that.

I would worry about exactly what optics you were thinking of