Measuring small differences in the phase of light

[pyroartist]

Recently I read something in NASA Tech Briefs about an instrument that could measure phase differences as small as .001 degree in light. I would like to create an instrument that could do this for every pixel in a picture, even if it was only a few hundred pixels (such as a 20x30 pixel array). How is this measurement made?
Electronically or is it mostly optical devices?

 

[davenn]

Recently I read something in NASA Tech Briefs about an instrument that could measure phase differences as small as .001 degree in light. I would like to create an instrument that could do this for every pixel in a picture, even if it was only a few hundred pixels (such as a 20x30 pixel array).

And you budget for this project is how many $100's of thousands ?

 

[eq1]

That's a pretty amazing resolution. I wonder at what wavelengths and through what medium it was achieved. It pretty has to be some kind of interferometer.
https://en.wikipedia.org/wiki/Interferometry

 

[tech99]

Recently I read something in NASA Tech Briefs about an instrument that could measure phase differences as small as .001 degree in light. I would like to create an instrument that could do this for every pixel in a picture, even if it was only a few hundred pixels (such as a 20x30 pixel array). How is this measurement made?
Electronically or is it mostly optical devices?

How will you keep the picture still enough! It sounds as if normal atomic movement will upset such a sensitive reading.

 

[Baluncore]

How is this measurement made?
Electronically or is it mostly optical devices?

The sum nodes in an interferometer are very wide, while the difference nulls are very narrow. To get such fine phase information requires the use of the nulls. By adjusting an optical path length, to get close to total extinction in an interferometer, it is possible to detect very small path length changes by the resulting brightness variation with an optoelectronic sensor.

To do that with an image would require a stationary target object with monochromatic laser illumination. Each pixel would require the equivalent of a servo-controlled optical wedge to adjust the path length to zero phase-shift, prior to beginning the observation.

White light interferometry does not have the same resolution as monochromatic null measurements.
https://en.wikipedia.org/wiki/Photoelasticity

It would help to have a reference to the original article, and also to know why such an imaging device is needed.

 

[sophiecentaur]

I'd like to know what application the OP would envisage which would need or could use that sort of information - apart from " 'cos they can".
There would presumably be more information in the Tech Note.

 

[pyroartist]

Thanks to everyone for the thoughts about how to do this. I have been busy and just read them now. I also found the article I mentioned. Thought I had thrown it out. Here is the quick summary: Daniel Shaddock of Liquid Instruments of Australia worked for JPL on a project called LISA (Laser Interferometer Space Antenna).
The phasemeter in this device can measure the difference between two incoming light beams down to a millionth of a wavelength(!)
They are now building a compact instrument called Moku:Lab which has multiple instruments in it including a phasemeter (it doesn't say if it is as sensitive as the LISA one).
More: https://spinoff.nasa.gov/Spinoff2018/ip_3.html
I would still like to know how they did this. No details in this article.

 

[Tom.G]

Details here:
https://trs.jpl.nasa.gov/bitstream/handle/2014/40283/06-2898.pdf?sequence=1

The above and 170 total hits were found with:
https://www.google.com/search?&q=lisa+phasemeter+site:nasa.gov

Cheers,
Tom

 

[CWatters]

What sort of picture? What's it illuminated with? Phase relative to?

Your regular LED screen pixel is big compared to the wave length of light and doesn't have _a_ phase.

 

[pyroartist]

What sort of picture? What's it illuminated with? Phase relative to?

Your regular LED screen pixel is big compared to the wave length of light and doesn't have _a_ phase.

I am hoping to come up with a method to detect and display air turbulence passively at a distance of at least 300 meters. Illumination would be sunlight.
It seems like the small phase differences caused by the turbulence (air refraction) will not be detectable because there is not a stable signal to compare them with.

 

[russ_watters]

I am hoping to come up with a method to detect and display air turbulence passively at a distance of at least 300 meters. Illumination would be sunlight.

This is already a thing: Google for telescope adaptive optics laser guide star.

 

[sophiecentaur]

Illumination would be sunlight.

I'm not sure what you have in mind but this sort of thing would normally require a highly coherent light source and the Sun is not one of them.

 

[pyroartist]

I'm not sure what you have in mind but this sort of thing would normally require a highly coherent light source and the Sun is not one of them.

Yes, that is what I was afraid of. The reference beam must be stable and coherent. The telescope system only works at night and uses a "guide" star and I am working only in the day. All ideas on how to view turbulence in the sky appreciated. LIDAR works but powerful pulsed lasers (>50mJ.) are still very expensive. Someday.

 

[Tom.G]

Visually, stars twinkle at night. Is that because of their point source characteristic and we are seeing the moving interference fringes produced by the turbulent atmosphere? If that is the case, a sufficiently small point source at a distance should produce what you are after. Try a Google search for 'schlieren'. What remains is generating the point source, like the artificial guide stars.

 

[pyroartist]

Yes, that is the problem. In the daytime sky there are usually no point sources other than the sun which is nearly vertical above. I want to look out horizontally. I have studied the Schlieren methods and none will work looking horizontally above the horizon.

 

[anorlunda]

Doppler radar is a traditional solution, but it works at distances, scales, and weather conditions that may not meet your needs.

One can imagine a shorter wavelength version of Doppler radar, but success is not just a question of the electronics, but also of the properties of the atmosphere. If such a thing exists, I would expect it first in the military; for example the ability to detect a sniper exhaling at a range of 1 km.

 

[sophiecentaur]

I am working only in the day.

It's a shame that you can't work at night because there are stars all over the sky. Would it not be possible to set up an autonomous measurement system which could run at night, even if you couldn't be there? It would have to involve an Observatory building which could be shut in inclement weather but that need not cost more than a few Thousand Dollars. (Fibreglass ones for amateur use are pretty common). Alternatively, it might be possible to approach your local Astro Society and get them involved in running an experiment. Amateur Astronomers are usually very interested in 'legit' Science experiments and the equipment for imaging could even be borrowed.
The state of the atmosphere affects astronomers a lot; it's what limits what we can get to see from Earth, of course. But the 'seeing' which describes the effect of the atmosphere at any time can be measured by looking at the image of a star and how much it departs from the ideal Airey Disc shape.

Did you consider making use of the vast amount of data (star images, via terrestrial telescopes ) that is available to the public? Also, there would be records of the weather in many places at the times the images were obtained. You might find that you may not need to do an actual experiment but just use existing data. Could be a very cheap option.

 

[Baluncore]

The coherence of a laser is lost after only a few hundred metres in the atmosphere. That is why evacuated tubes must be used for the several kilometre optical paths of long baseline interferometers such as LIGO. Longer baseline gravity wave detectors will need to be in the vacuum of space.

Amateur optical communications over a few kilometres show that lasers do not offer any advantage over LEDs as the light source. Lasers are not used for record breaking optical links, between mountain peaks. A heliograph is used during the day, while at night a high power LED array with fresnel lens is used.

Any mapping of atmospheric disturbance will need to be done with a known target. The image of that target could be gathered using digital cameras fitted to one or more telescopes at the observatory site, where FFT convolution processing of the images will reveal the atmospheric distortion.

The target could be an array of tracking heliographs, or spherical reflectors on a hillside, illuminated by sunlight. Or it could be a laser painting a grid of points on a building or screen of some sort. High altitude aircraft and the International Space Station are visible during the day, as are the moon and Jupiter. But in those cases you need to know where to look. Prediction and tracking is available for all those targets.

The artificial stars used to dynamically align big telescopes rely on a layer in the upper atmosphere that has a high sodium ion concentration as the screen. That will not work during the day when sunlight illuminates the Na layer.