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Comment by pi-e-sigma | Hacker News Reader

Nah, just google 'high power fiber cable', these are used to transfer light in laser steel cutters. As these are already in practical, industrial use we can assume that their spec is in line with what is currently realistically achievable for an optical fiber power transmission over a distance of a few meters max. For instance a SMA905 Fiber Cable can deliver meager 50 Watts and its external diameter is 5 millimeters which is laughable low compared to how much power you could push through a copper cable with the same external diameter, that is including electrical insulator.

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I don't think it's a reasonable assumption to look at industrial use cases over small distances compared to long-distance operations -- the design criteria are different.

The design criteria is exactly the same. Have an optical medium with as little attenuation as you can. If you can't have it in a medium that is only a few meters long how could you possibly achieve it in a medium of a few kilometers? Apply your logic to copper cables. Can you have a copper cable able to deliver power over a long distance but unable to deliver the same power over a shorter distance?

The analogy doesn’t hold because one of the hardest parts of this technology is building a device that can extract the optical energy as electrical energy, and dissipate whatever heat it generates. The power density of the fiber itself is easy by comparison.

The analogy holds perfectly. Remember, we are discussing a claim that optical fiber with all required cladding/protection can carry as much power as a copper wire with all insulation having the same external dimension as the fiber cable. It is simply not possible, fiber will always have less power carrying capacity and your claim that fiber power density is 'easy' has no basis in the reality.

There's absolutely no reason for the application you cited to optimize diameter and isn't approaching physical limits.

There's no reason you can't shove many optical fibers into that 5mm diameter. People don't, in short distance applications, to have simpler systems and connectors.

There's big reasons why we don't use power over fiber (the endpoints are very expensive, and the overall system efficiencies are low). None of them have to do with the highest optical power density you could hit.

> The design criteria is exactly the same. Have an optical medium with as little attenuation as you can.

If the design criteria for cables was "make a good cable", we wouldn't have thousands of different types. There are cables which have different mechanical and environmental properties, different cost constraints, prioritization of different performance characteristics, etc.

> Can you have a copper cable able to deliver power over a long distance but unable to deliver the same power over a shorter distance?

Depending on the particular application, yes. Consider some 12/2 solid Romex. It'll handle 10 watts just fine, for quite a distance, if you want. Won't work well for a 1 meter long phone charging cable, though. Different mechanical requirements.

I know things kind of derailed downthread from here, but I just have to add that your claim is either confusing or simply untrue. I visited a manufacturer of industrial laser welds [1], and they use 16 kWh lasers over reasonably small fibers. I don't know the exact dimensions, but the overall cable looked like… a reasonable cable. Somewhere like 10 to 25 mm perhaps?

[1] https://www.permanova.se/en/our-products/laser-source/

The cables look reasonably thin because they are water-cooled. That means that the power loss in the cables is so great that without active cooling the fiber would just melt. Take a look at this page https://www.photontec-berlin.com/power_delivery_fiber_cable.... which contains actual specs of the cables, for instance https://www.photontec-berlin.com/pdf/ld80.pdf or https://www.photontec-berlin.com/pdf/ld80-water.pdf

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