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Isn't it possible to use a laser as an "antenna" that transmits data? This would save quite a lot of mass since you don't need a large dish.

The only problem I can think of is how would you modulate the signal to encode bits? But lasers can modulate their signal somehow, can't they? I'm pretty sure that you could use amplitude modulation, or at the very least, on/off key.

EDIT: Another problem is that lasers won't be able to receive any signal, so a dish is necessary for the probe to receive signals with a reasonable gain.

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  • $\begingroup$ Laser data transmission has been used (not in space) since the first laser printer back at PARC in the 70s, if not before: a telescope pairing was used to beam data to the printer in another building across the street. $\endgroup$ – Nathan Tuggy Jul 15 '15 at 19:43
  • $\begingroup$ @NathanTuggy Can you please explain what a telescoping pairing is? If it's a laser beaming light into another laser, I don't see how it's received. I thought lasers can only emit light, not receive it (unlike a normal antenna which can do both). $\endgroup$ – DrZ214 Jul 20 '15 at 22:01
  • $\begingroup$ A laser beaming through one telescope, across the gap, and into another telescope and an optical receiver. (I believe it was simplex transmission, i.e. unidirectional, but beam splitters and the like can make half-duplex or even full-duplex transmission practical by switching between allowing light from the local laser to go through the telescope or light from the distant laser to come through instead.) $\endgroup$ – Nathan Tuggy Jul 20 '15 at 22:04
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We're working on it.

You do need a little telescope on the spacecraft to serve as the antenna. You can have two-way laser communications. It may in fact be essential to achieve the required pointing accuracy. There are many modulation schemes -- that's not an issue. Pulse position modulation is a front runner.

The upside is much higher data rates with the same power. E.g. two orders of magnitude. The downside is that you need much more accurate pointing. You need large telescopes on the ground to play the role of the large DSN antennas, but in the optical range. You are much more vulnerable to weather conditions at the Earth sites.

Due to the high pointing accuracy, you will likely want to have a backup radio communication system to debug problems pointing the laser accurately enough.

If you like, you can read the book on the subject.

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  • $\begingroup$ Added the book to the meta list. Please feel free to edit in other awesome books you know either on Meta or on GitHub. $\endgroup$ – Deer Hunter Jul 15 '15 at 8:26
  • $\begingroup$ Why does it have to be an optical laser? Can't we use microwaves, or any frequency you want? $\endgroup$ – DrZ214 Jul 15 '15 at 18:59
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    $\begingroup$ Lasers are easier than masers, and provide higher data rates. As for other wavelengths in general, you want the highest frequency for the highest data rate, and you want Earth's atmosphere to be transparent to it. As you can see from the linked graphic, that would be visible light. $\endgroup$ – Mark Adler Jul 15 '15 at 21:12
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You modulate the signal for a comms laser the same way you do for radio antenna (effectively) so this isn't the issue.

The problems are:

  • Powerful lasers are heavy
  • They require a fair bit of power
  • They are not very robust, compared to a dish antenna, which can take a fair amount of damage from fast moving particles and just suffer incremental loss of signal strength
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  • $\begingroup$ I'm not sure I can agree with your 2nd point. Lasers must have an even higher gain than dishes (or any other antenna for that matter). If so, that would imply less power needed for a given signal strength. $\endgroup$ – DrZ214 Jul 14 '15 at 21:52
  • $\begingroup$ Nope - that's just not the case. Look at it from the perspective of power received by the receiver (as that's a nice useful measure) - lasers require far more power to achieve the same signal level (or more accurately signal to noise ratio against background noise) $\endgroup$ – Rory Alsop Jul 14 '15 at 22:19
  • $\begingroup$ Can you provide a cite comparing lasers with radio dishes on a per watt basis? I'm having a hard time believing that coherent laser light, all emitted in one direction (as opposed to broadcast from an antenna), somehow has less gain than a dish. $\endgroup$ – DrZ214 Jul 14 '15 at 22:32
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    $\begingroup$ @DrZ214 The laser emitter cannot function as a receiver, but neither can a radio transmitter. The optics can be used for both, analogous to the radio 'dish.' $\endgroup$ – 2012rcampion Jul 15 '15 at 3:46
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    $\begingroup$ @DrZ214 high power lasers are actually horribly inefficient. You'd have to read up on the full process, it won't fit in this box, but the coherent light that comes out is only a portion of the energy you put in. Inside a tube there's plenty of light that isn't coherent and has to be filtered out, and the charge pump itself isn't exactly efficient either. All this lost energy is why you need to have some serious cooling in place to run a serious laser setup. Diode lasers are better but not nearly so powerful yet afaik. $\endgroup$ – Kaithar Aug 7 '15 at 19:46

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