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What is Near-Space?
Space begins 100 km above the ground. Beyond 20 km (the Armstrong Limit), we are in the upper atmosphere or "near-space". To get to this region, one must cross the jet-stream where there is very fast (about 100 kph) and very cold (about -50 C) wind. In near-space, it is also essentially vacuum. The environmental conditions are very untypical of what you find in a lab or even in a ground-based field test making near-space an ideal region to test devices that are meant to go into orbit.
Prepping for Near-Space Flight
Our first flight into near-space was in 2012 with the team of Christoph Wildfeuer. That experience provided us with the data and experience necessary to perform more balloon flights. With this experience behind us, we began planning to demonstrate a down conversion source in near-space. The aim was to ensure that the down conversion source would operate as well in near-space as on the ground.
We prepared two units of the down conversion experiment (code-named CS2 and CS3), and subjected them to vibration, thermal cycling and vacuum testing in the labs. Then we hand-carried them to Sursee, Switzerland where Zhongkan and Cliff carried out on-site tests to prove that the trip did not damage the units. What can go wrong in travel? Well, this is all new to us at the moment, so we had to be concerned about the random knocks it would take during flight turbulence and even the effects of the X-Ray machines at airport security! The BBO crystals used in SPEQS contain Barium atoms that have a huge scattering cross-section for X-rays (see the X-ray images below) so at first there was some concern that the X-Rays can cause damage to the BBO material.
Randomly placed optical elements inside a test SPEQS unit. Most of the elements are aluminium optical holders. There is one BBO crystal with an aluminium cap. X-rays are a tool we can use when we need to analyse what has gone wrong inside a sealed SPEQS unit.
Can you identify the components in this X-ray picture using the photo on the left? The red circle shows the location of the 6 mm BBO crystal - note how opaque it is compared to 3 mm of aluminium. The green circles have small glass components (e.g. dichroic mirrors) attached to aluminium caps.
Flight and Recovery
The Singapore-Swiss team launched the 1.6 kg weather balloon on the morning of the 29th of September. There was some concern that the cloud cover might cause icing on the balloon during ascent, adding mass, and preventing the balloon from reaching an altitude where it could burst. Such a scenario would cause the balloon to drift for days and make recovery difficult, if not impossible. To account for this, Christoph added more helium to the balloon causing it to ascend more quickly, but the additional gas also caused the balloon to burst at 35.5 km (compared to 40+km in our 2012 test). The balloon flight path is seen below.
The balloon package carried a simple camera and a view from the balloon is included below. The package made a safe landing at Bazenheit and the team retrieved it successfully and you can also see the experimental package after retrieval below. All data was safely retrieved.
Results and Future Plans
The results were quite good - the SPEQS experiment worked flawlessly in the entire flight. Instead of repeating the results here, we suggest you get a copy of our open access paper at Scientific Reports.
Very briefly, some interesting environmental observations we made were:
a. The balloon burst is quite violent. Our accelerometer suggested that it was over 20 grms.
b. The package landing was not very gentle although it was on a grassy field. The deceleration was over 20 grms and even higher than the balloon burst.
c. Above the jetstream, the temperature of the atmosphere is quite comfortable!
All observations are reported in detail in the Scientific Reports article.
The balloon test has been very promising, and paves the way for entangled photon technology based on bulk SPDC to be deployed on low-resource platforms. We are now hoping to get a similar experiment onto a CubeSat, and to launch more complex devices on weather balloons in the coming years.
Media Links to Balloon Flight (from local Swiss media and the Sursee Radio Club)
Google Sites
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