Lasers in Space Twinkle too!
I mentioned in my last blog “Why do stars twinkle?” that I am an experimental quantum physicist, which means I’m a scientist who does quantum physics experiments with lasers. Quantum physics is the “science of the tiny” like how do atoms move and interact with each other. I work for a research lab at a university, and conduct experiments that explore how the universe works at this fundamental level of atoms. I also do experiments with individual particles of light called photons. I use lasers to do scientific tests, and for real-world demonstrates like secure global communication using quantum satellites.
Lasers are used EVERYWHERE in our modern world, from grocery store bar code scanners to medical imaging devices and tools, and even transmitting the internet through fibre optic cable! The space industry has been using lasers for communication and sensing since the 1960s. We use lasers to image the Earth from space to monitor climate change, in radar detection, and for communicating to satellites or for satellite-to-satellite links.
Turbulence in our atmosphere affects laser beams just like it does starlight, causing the laser light to ‘twinkle’ (i.e. speckle pattern). It’s important for scientists to understand how turbulence affects a laser to study if quantum physics experiments will work with satellites in space.
There’s lots of ways to generate turbulence in the lab for these tests. One of the simplest ways is to send the laser through a box of circulating hot air. One limitation with this box design is that it’s difficult to precisely control, reproduce or change the strength of turbulence. Since we are doing very sensitive quantum physics experiments, precision is essential so this method is not ideal!
We chose instead to study the effects of turbulence on a laser in the lab using a special device called a Spatial Light Modulator (SLM). A SLM is a small liquid crystal (LC) screen, similar to your phone screen or computer monitor. I can create turbulence on the laser light by changing the SLM settings. Here’s a picture of the SLM in my experiment at the university lab:
The SLM controller talks to the LC screen to interpret the black-and-white image (i.e. phase map) into electrical signals for the SLM. These signals change the voltage across the LC cells (pixels), which makes turbulence on the light. You can think of a phase map like a map of elevation but instead of mapping terrain height, it is a map of phase (black = 0 to white = pi radians). Just as more lines close together on a terrain map represents rapid changes in height, more lines close together on a phase map means stronger turbulence.
Here are some examples of how different phase maps generate different levels of turbulence on the light. The black and white images are the phase maps with the corresponding false-color images of the laser beam above them (dark red = brightest, dark blue = less bright).
Combining quantum physics with space has real-world applications, like allowing two distant parties to communicate securely from anywhere in the world. In my next blog, I will introduce some new quantum space experiments that are happening soon to do just that, including Canada’s first quantum satellite mission called the Quantum EncrYption and Science Satellite (QEYSSat)!