Hacker News Reader

Comment by rlt | Hacker News Reader

Why couldn’t intersatellite laser links have similar capacity to fiber? In principle, if not practice?

Is it because ISLs will require more powerful lasers because the light is focused through a lens instead of guided through a fiber?

View on HN

>> Why couldn’t intersatellite laser links have similar capacity to fiber? In principle, if not practice?

Because laser energies, signal paths, traveling through free space must literally share that space with each other. It is effectively one wire. An underground cable can have hundreds, thousands, even millions of wires lying beside each other without interference.

It is comparable to the difference between a wifi connection and bundle of ethernet cables. One can scale and one cannot.

I'll start by saying my understanding of the laser connection on the satellites is virtually nil.

Could you just add more beams on each satellite?

No. The beams would overlap, which is fine of they are on different frequencies, but eventually you run out of frequencies to use. Fiber confines the beam to inside the fiber. No overlap between one fiber and another means you can run multiple down the same pipe.

Why would they overlap?

Because a beam in an open medium always diverges due to wavefront errors and diffraction. A very good laser system might possibly have a divergence of only 1 microradians. For reference a typical laser pointer has a divergence angle of 1 to 2 milliradians. With the 2000km or so between satellites for Starlink the laser "spot" on the receiving satellite with the very good laser will be about 2 meter. Not enough to receive multiple beams on one satellite without overlapping.

Thanks for explaining. Would it work if they put receivers on a long pole, spaced 3m apart? Maybe they could mount them on the edge of the solar panel array, which is ~30m long [0].

[0] https://lilibots.blogspot.com/2020/04/starlink-satellite-dim...

The link estimates the surface of the panels to be 30m2. Since the panels are 3.1m wide the length would be 9.6 meters. But even at 30 meters it would be very very hard to engineer a system that can keep a laser beam aligned on a moving target with sub microradian precision. I'd guess Starlink laser are more divergent to make hitting the target easier. Keeps the exit pupil a manageable size too (narrower beams need bigger optics).

Ok, but I was asking about a single space laser vs a single fiber, not the theoretical limits of many of them.

Space is pretty big, lasers are pretty narrowly focused, and not many people have lasers in space yet.

High end fibre routers use multi-spectral bandwith, effectively allowing for multiple data streams to be transmitted on one fibre cable, and re-broken out at the other end. This significantly increases bandwidth.

I doubt the laser links support this, and I wonder if any could, when in a non-encapsulated environment. EG, spectral pollution from sunlight/etc.

probably less (literal) bandwidth, lasers are expensive

That and because the atmosphere is not clear (this is why 1Gbps free space laser network bridges have very limited range), the links from satellite to earth in any rf band will be very small in capacity as compared to a singlemode fiber cable. Space to space lasers, sure... Much clearer.

Starlink ground links are RF

That's what I said

oops my bad

Reply on news.ycombinator.com