But, yes, a laser link indeed is desirable. Sure, we can still contact Voyager with radio telescopes, but even from the Mars rovers, notice how it takes so long to get from Mars to grainy B&W picture back on Earth?
Sending back live video feeds and more full colour images sets the data rate bar much, much higher. Getting this much data back quickly is limited by the frequency of the radio waves/light. Laser light has an over 1,000 times shorter wavelength than Ka band radio telescopes can manage (that's what NASA uses now to talk to the Mars probes), which increases the potential amount of data that can be sent in a given timeframe by essentially that amount.
In addition, because laser light is focused so narrowly, it wastes much less energy than a radio antenna which must spray a good portion of space with radio waves in order to hit Earth. Imagine focusing your mag-light in the dark... the narrower the focus, the brighter the beam gets, because more energy is packed into less space. The challenge though, is that you have to aim much more precisely at Earth to compensate for that more focused beam.
Here's a great overview of JPL's long-term vision:
Well... compared to the speed of light, Earth and Jupiter aren't moving too fast relative to each other. However, even with a lot of doppler shifting (which would be negligible in this situation), things would be okay. All photodetectors have a frequency response *range* (looks like a hump on a graph). So a 650 nm detector might respond decently to 600 nm and 700 nm light, just not quite as strongly, assuming the detector has a very narrow band.
I don't think the problem doesn't lie with diode laser modulation... the data rate for a free space link will never reach rates we see in a single-frequency fiber optic link, and it's easy enough to switch a diode laser on and off for rates up to 10Gbit/s, or whatever the upper limit of the free space goal would be, w/o phase shifting techniques.
The more pressing issue, I think, comes with lack of peak power associated with continuous wave-type lasers (like diodes). NASA is also researching work on novel resonator designs to reduce diode-pumped Q-switched laser regeneration times, something which holds promise over a diode laser due to the high peak power from a pulse which Q-switching can give. Currently, however, such lasers cannot be pulsed quickly enough to achieve the desired data rates.
A high peak power Q-switch dumps all of the photons from a pulse in a very short burst in time (say, less than 1 nanosecond), which allows for higher confidence in determining exactly where in a dataframe that pulse belongs. Essentially, this allows fewer photons to need to be received on Earth in order to resolve the signal. So while the overall pulse frequency can't be as high, the laser can be lower-power with a reduced data error rate.
1. The laser here consumes on the order of 10 Watts of power or less. A radio antenna broadcasts energy into a wide swath of space (imagine a cone spreading out from a radio dish into the sky, now imagine Earth as a tiny point in the sky... lots of radio signal going nowhere near that star), while the laser's cone is much much narrower. So, assuming you can point the thing accurately at Earth, you can get away with using much less energy, because all of the energy is aimed right where you want it (at Earth), rather than into the void of space (like 99% of your radio antenna signal).
2. Yes. The visible-light absorption is a concern. The signal doesn't "lose power" as much as the energy spreads out in space, albeit slowly, (and some light is further absorbed in the atmosphere). Assuming you have a nice ~50cm beam coming from your spacecraft orbiting Mars, the beam width diverges to about the size of Earth when it reaches Earth. So the problem now is shifted to amplifying a weak signal. However, since the laser wavelength is several orders of magnitude shorter than your radio wave, there's a potential to increase the overall bandwidth of your connection, even after factoring in bandwidth reduction due to the need for stronger signal amplification. The need for such a large receiving antenna will stem from the fact that only tens or hundreds of photons will be arriving to represent one bit of data. In addition the Sun now becomes a gigantic source of visible-wavelength noise, which means the antenna will need an extremely narrow field of view to be pointed exactly at your distant planet in order to keep random light from the sun from bouncing into the antenna and drowning out the signal.
So the main reason is, given the challenges, there's still a potential for an order of magnitude increase in the amount of data you can send back from deep space. No more minutes for downloading a single black and white image... think live video feeds from Mars, and you'll begin to see why the simple increase in potential data bandwidth is enough to keep NASA on this project.
Slashdot Mirror
JPL's been working on it too for a while now... and with similar datarates, and a ground acquisition plan to boot.
http://lasers.jpl.nasa.gov/PAGES/pubs.html#ocd
But, yes, a laser link indeed is desirable. Sure, we can still contact Voyager with radio telescopes, but even from the Mars rovers, notice how it takes so long to get from Mars to grainy B&W picture back on Earth?
Sending back live video feeds and more full colour images sets the data rate bar much, much higher. Getting this much data back quickly is limited by the frequency of the radio waves/light. Laser light has an over 1,000 times shorter wavelength than Ka band radio telescopes can manage (that's what NASA uses now to talk to the Mars probes), which increases the potential amount of data that can be sent in a given timeframe by essentially that amount.
In addition, because laser light is focused so narrowly, it wastes much less energy than a radio antenna which must spray a good portion of space with radio waves in order to hit Earth. Imagine focusing your mag-light in the dark... the narrower the focus, the brighter the beam gets, because more energy is packed into less space. The challenge though, is that you have to aim much more precisely at Earth to compensate for that more focused beam.
Here's a great overview of JPL's long-term vision:
http://lasers.jpl.nasa.gov/PAPERS/REVIEW/overview.pdf
[Doppler shifts]
Well... compared to the speed of light, Earth and Jupiter aren't moving too fast relative to each other. However, even with a lot of doppler shifting (which would be negligible in this situation), things would be okay. All photodetectors have a frequency response *range* (looks like a hump on a graph). So a 650 nm detector might respond decently to 600 nm and 700 nm light, just not quite as strongly, assuming the detector has a very narrow band.
I don't think the problem doesn't lie with diode laser modulation... the data rate for a free space link will never reach rates we see in a single-frequency fiber optic link, and it's easy enough to switch a diode laser on and off for rates up to 10Gbit/s, or whatever the upper limit of the free space goal would be, w/o phase shifting techniques.
The more pressing issue, I think, comes with lack of peak power associated with continuous wave-type lasers (like diodes). NASA is also researching work on novel resonator designs to reduce diode-pumped Q-switched laser regeneration times, something which holds promise over a diode laser due to the high peak power from a pulse which Q-switching can give. Currently, however, such lasers cannot be pulsed quickly enough to achieve the desired data rates.
A high peak power Q-switch dumps all of the photons from a pulse in a very short burst in time (say, less than 1 nanosecond), which allows for higher confidence in determining exactly where in a dataframe that pulse belongs. Essentially, this allows fewer photons to need to be received on Earth in order to resolve the signal. So while the overall pulse frequency can't be as high, the laser can be lower-power with a reduced data error rate.
Notes to remember about efficiency:
1. The laser here consumes on the order of 10 Watts of power or less. A radio antenna broadcasts energy into a wide swath of space (imagine a cone spreading out from a radio dish into the sky, now imagine Earth as a tiny point in the sky... lots of radio signal going nowhere near that star), while the laser's cone is much much narrower. So, assuming you can point the thing accurately at Earth, you can get away with using much less energy, because all of the energy is aimed right where you want it (at Earth), rather than into the void of space (like 99% of your radio antenna signal).
2. Yes. The visible-light absorption is a concern. The signal doesn't "lose power" as much as the energy spreads out in space, albeit slowly, (and some light is further absorbed in the atmosphere). Assuming you have a nice ~50cm beam coming from your spacecraft orbiting Mars, the beam width diverges to about the size of Earth when it reaches Earth. So the problem now is shifted to amplifying a weak signal. However, since the laser wavelength is several orders of magnitude shorter than your radio wave, there's a potential to increase the overall bandwidth of your connection, even after factoring in bandwidth reduction due to the need for stronger signal amplification. The need for such a large receiving antenna will stem from the fact that only tens or hundreds of photons will be arriving to represent one bit of data. In addition the Sun now becomes a gigantic source of visible-wavelength noise, which means the antenna will need an extremely narrow field of view to be pointed exactly at your distant planet in order to keep random light from the sun from bouncing into the antenna and drowning out the signal.
So the main reason is, given the challenges, there's still a potential for an order of magnitude increase in the amount of data you can send back from deep space. No more minutes for downloading a single black and white image... think live video feeds from Mars, and you'll begin to see why the simple increase in potential data bandwidth is enough to keep NASA on this project.