How to communicate between planets?

[Haorong Wu]

TL;DR Summary

How to communicate between planets while the laser would diverge when propagating?

Hello, I am recently studying communication in curved spacetime.

For example, if i use a laser in 1000nm and the initial beam waist is 1m. Then the rayleigh range is $3 \times 10^6m$. The nearest distance between Earth and Mars is about 5500km. So the beam waist at the Mars will be around $2 \times 10^4 m$.

It seems it would yield impossibility to transfer information by using the orbital angular momentum of photons because the light would diverge too large to be detected.

Could it be compensated somehow? For example, i could increase the radius of the laser to 500m, and then Mars would be within the Rayleigh range.

 

[Vanadium 50]

Why not just use a radio?

 

[Haorong Wu]

Why not just use a radio?

Lol. My research group is specialized in the oam of photons. My supervisor would like me to study how the quantum information encoded in oam evolves in curved spacetime.

 

[Delta2]

Summary:: How to communicate between planets while the laser would diverge when propagating?

The nearest distance between Earth and Mars is about 5500km

I know this is sort of irrelevant to the content of the OP but in order to be as accurate as possible the nearest distance between Earth and Mars is probably about 5.500.000 km.

Summary:: How to communicate between planets while the laser would diverge when propagating?

i could increase the radius of the laser to 500m

A laser with beam diameter of 1km, is way too big for our current technology isn't it?

 

[russ_watters]

I know this is sort of irrelevant to the content of the OP but in order to be as accurate as possible the nearest distance between Earth and Mars is probably about 5.500.000 km.

55 million.

 

[Delta2]

55 million.

Interesting hehe, OP was wrong with about a factor of 10 relative to my answer and i was wrong also with a factor of 10 relative to the real answer. Yes i guess if it was only 5.5M km we would be able to see Mars as a smallish moon in the sky.

 

[Nugatory]

How to communicate between planets while the laser would diverge when propagating?

What are you assuming is the minimum intensity required for detection?

We can receive radio communications from unmanned spacecraft on Mars, and a directional radio signal is not as well collimated as a laser.

 

[Haorong Wu]

@Delta2 , sorry about that error. I forgot the factor $10^4$, lol. Well, I am just suppose a possible solution, because I do not know how to overcome the difficulty.

@nasu , it has been shown that photons can carry orbital angular momentum, or oam. For example, a Laguerre -Gaussian mode will pocess such oam. It provides another space to encode information. Well, my supervisor is interested in how the Laguerre -Gaussian mode light would propagate in a curved spacetime. There is some recent paper about this subject. But it did not study the LG modes. Instead, it gives a prediction for Hermite-Gaussian modes.

@Nugatory , yes, I suppose since the laser would be diverge so largely, the intensity in a unit area would be quite low for detecting.

Well, I am not sure, should I just study this subject without considering the lack of realization?

 

[jbriggs444]

propagate in a curved spacetime.

What has curved spacetime to do with Earth and Mars? Surely the Newtonian approximation is good enough?

 

[Haorong Wu]

@jbriggs444 , I am sorry I used an inappropriate example. In fact, we are studying to send a LG-mode light passing near the sun to the opposite point in the Earth's orbit. The transverse mode will be perturbed near the sun.

 

[Charles Link]

In reading the OP, it appears to contain some unrealistic scenarios. Perhaps it is worthwhile presenting a couple basic concepts: The basis of the Rayleigh length is diffraction. The aperture size affects the beam divergence.
In a simple approach, using $m \lambda=b \sin{\theta}$ for the zeros of the diffraction pattern, with $m \neq 0$, basically $\Delta \theta=\lambda/b$. This means at a distance $s$, the beam width is $w=s \Delta \theta=s \lambda/b$. (Doing a non-Gaussian analysis here, where $b$ can be thought of as the waist). Setting $w=2b$, we get $s \approx 2b^2/\lambda$. This I believe is the idea behind Rayleigh's formula with the $\pi$. (The diffraction formula is really for far-field=these are simply estimates).
Meanwhile, one needs to be practical with what is commercially available. Perhaps there are ways to make $b$ very large, but I think the divergence of a typical off-the-shelf HeNe is $\Delta \theta \approx 2$ mrad.
See https://www.thorlabs.com/tutorials.cfm?tabID=9F3258E0-FB6B-46D4-944D-B31A84EF1DAA
See https://www.quora.com/What-is-reali...e adaptive optics,(aperture) diameter you use.

 

[sophiecentaur]

A laser with beam diameter of 1km, is way too big for our current technology isn't it?

Beamwidth is much bigger than that for all practical radio comms links so why would this not be a similar problem with optical comms? The wonderful thing about the Inverse Square law is that doubling the link distance only worsens the Carrier to Noise ratio by 6dB. Powers of two will rapidly take you to huge link distances (the Viking space probes, for instance).

I am not too sure of the basic difference between the main factors involved in THz and GHz links but isn't the CNR a similar factor for both? I'm speaking from the standpoint of RF Engineering so you may be able to put me right here.

 

[berkeman]

Beamwidth is much bigger than that for all practical radio comms links so why would this not be a similar problem with optical comms?

Maybe the difference in the RX concentrator function between optics (mirrors) versus RF reflectors? Both would be parabolic, but the tolerances for surface smoothness would be very different, no?

 

[Delta2]

Beamwidth is much bigger than that for all practical radio comms links so why would this not be a similar problem with optical comms? The wonderful thing about the Inverse Square law is that doubling the link distance only worsens the Carrier to Noise ratio by 6dB. Powers of two will rapidly take you to huge link distances (the Viking space probes, for instance).

I am not too sure of the basic difference between the main factors involved in THz and GHz links but isn't the CNR a similar factor for both? I'm speaking from the standpoint of RF Engineering so you may be able to put me right here.

I am not an RF engineer actually I am a mathematician with interest in electromagnetism. I am not very familiar with the ray leigh range and the beam width, i just don't think we have lasers with beam width 1km have we?

 

[tech99]

I am not an expert but I did look into this topic when the technique was publicly launched some years ago, with statements that it would revolutionise communcation. As far as I recall, the idea of angular momentum for communication is a bit like circular polarisation but allows many different communication channels to be used. However, it requires the receiver to be within the Radiation Near Zone, or Fresnel Region, of the transmitting antenna. The Radiation Near Zone is usually defined by the Rayleigh Distance, which is Diameter^2/(2 x lambda). As we move away from the TX antenna the number of possible channels falls, so it is very limted benefit even at the Rayleigh Distance.
As far as I can see, the idea is akin to having controlled near-in hot spots of antenna radiation, so we need to be close to the TX antenna.

 

[davenn]

i just don't think we have lasers with beam width 1km have we?

I think you misunderstand
The laser beam doesn't start at 1km ... the 1km is referring to the divergence ( spreading out of the beam with distance)

 

[Delta2]

I think you misunderstand
The laser beam doesn't start at 1km ... the 1km is referring to the divergence ( spreading out of the beam with distance)

I better get myself out of this thread, there are many things I don't understand, i shouldn't have posted at first place...

 

[Algr]

What has curved spacetime to do with Earth and Mars? Surely the Newtonian approximation is good enough?

I was wondering this too. It's not like the expansion of the universe or an event horizon is going to be a factor.

 

[nasu]

I think you misunderstand
The laser beam doesn't start at 1km ... the 1km is referring to the divergence ( spreading out of the beam with distance)

The OP mentions INITIAL beam waist of 1m and then he asks if he can increase the radius to 500 m in order to increase the Rayleigh range. The Rayleigh range depends on the initial beam waist, doesn't it?

 

[sophiecentaur]

I am a mathematician with interest in electromagnetism.

I just don't think we have lasers with beam width 1km have we?

I suspected that the Engineering aspects of this subject were not being considered in the OP. That doesn't invalidate the question, though; many useful advances have been made when apparently fruitless ideas have been applied successfully in practice.

A laser beam can be made into any width you want, with suitable optics; just consider how a hologram is made by illuminating a large object with one laser.

 

[tech99]

Could it be compensated somehow? For example, i could increase the radius of the laser to 500m, and then Mars would be within the Rayleigh range.

You would have difficulty making an optical aperture with a diameter of 500m and a surface error of a quarter of the wavelength (250nm). If it were based on the surface of Earth, what about atmospheric turbulence?

 

[sophiecentaur]

Summary:: How to communicate between planets while the laser would diverge when propagating?

Could it be compensated somehow?

This has to be a non-starter, I think, if you need to control the position of the received beam to within 500m. The direction of the beam would need to be controlled constantly with a pointing accuracy of 500/5.5E7 radians. The loop delay would be several minutes (can't remember how much but that does't matter for my argument. Transmitter and receiver would have to be in orbit, just to reduce atmospheric distortion (I imagine that's already been considered) but the orbits of the two satellites will be perturbed on the way round and you would need almost GPS style positioning accuracy.
But my comments only apply if you cannot handle a realistic pointing error and compensate for a lot of spillage. There is no way that you'd be dealing with 'lab conditions'. Moreover, the link would, presumably need to work at much greater distances than 55 million km over the two planetary orbits.

I think what's needed is for a robust figure for the optical requirements of this system. It's got to come down to almost 'typical' link calculations - just with some different figures.

 

[Aonghus Lynch]

Hello there!

I think I can help with this.

Not only does optical/laser communication with Mars seem to be possible and potentially very useful, but Nasa is already developing it!. There seems to be a big experiment coming up in 2022.

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https://www.inverse.com/science/hum...ill-use-lasers-to-communicate-with-earth-nasa
"Laser beam-based communication will be put to the test in the year 2022, when NASA launches its Psyche mission, which will travel to study a metal asteroid orbiting the Sun between Mars and Jupiter.
The orbiter will carry a test laser-communication terminal onboard, designed to transmit data and images to an observatory at Southern California's Palomar Mountain."

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https://www.forbes.com/sites/jamiec...ts-broadband-moment-and-live-video-from-mars/
“The primary importance of laser communications is increasing the rates and volumes of scientific data returned,” said Biswas. “For example, streaming back HD video can easily be enabled by laser comms.”

 

[tech99]

I think laser communication is a practical propositiion but not using photon orbital momentum, as this requires impracticable radiating structures to place Mars within the radiation near field of the radiating aperture.

 

[Haorong Wu]

Thanks, everyone. I have learn much from your posts.

I am sorry I caused some confusions because I used some inappropriate statements.

I should not say communication. Rather, I should say quantum information. Also, instead of saying communication between Earth and mars, I should say near the sun, or near a black hole.

Again, I am sorry about it.

In a recent paper, it seems that a light passing a rotating black hole may picks up some orbital angular momentum. So, I think I maybe start with a black hole. Maybe that would tell me how the information transfers between gravitational field and EM field. That seems interesting.

Thanks, guys.

 

[sophiecentaur]

Not only does optical/laser communication with Mars seem to be possible and potentially very useful, but Nasa is already developing it!.

They will be treating it as an Engineering Project, of course and that will involve much the same considerations as an RF system but with some terrific advantages. It seems that most of the problems with terrestrial 'free space' optical links go away then you do away with the atmosphere. I can imagine a network of additional comms satellites with up and down links plus 'out-links' to the rest of the galaxy. Positively an Arthur C Clarke world.

 

[wrobel]

How to communicate between planets?

Pigeon post?