Scientists from Germany and the U.S. have built the world’s fastest single-shot laser camera – 1,000x faster than its predecessors at capturing extremely short-lived events. They used the camera to provide the most precise view yet of how a hydrocarbon flame produces soot, which can teach us about how this important climate pollutant is produced in kitchen stoves, car engines, and wildfires.
The device’s technique is called laser-sheet compressed ultrafast photography (LS-CUP). It “can resolve a plane of a three-dimensional object like a flame or spray or any turbid media and can “resolve physical or chemical processes” in space and time, Yogeshwar Nath Mishra, a coauthor of the work’s paper, published in Light: Science & Applications, told The Hindu by email.
Dr. Mishra is a researcher with the University of Gothenburg, Sweden, the NASA Jet Propulsion Laboratory in California, and at the Caltech Optical Imaging Laboratory. He hails from Azamgarh.
Experimental evidence
The researchers’ LS-CUP device can capture images at 12.5 billion frames per second (fps). To compare, the standard frame rate for films and TV shows is 24 fps. They used it to capture the “emission, soot temperature, primary nanoparticle size, soot aggregate size, and the number of monomers” of polycyclic aromatic hydrocarbons – molecules that go on to form soot.
They wrote in their paper that they found “strong experimental evidence in support of the theory and modelling of soot inception and growth mechanism in flames”. Such evidence is required to validate models used to predict how much soot forms in different industrial processes. Soot changes rainfall patterns and melts glaciers faster.
Their device can also be used to photograph shockwaves in nuclear reactors, combustion of fine sprays, and an enigmatic process called sonoluminescence (sometimes, when excited by sound, bubbles in a liquid implode and release light at a temperature of ~10,000 K), all of which involve processes that happen in a few nanoseconds.
Four types of radiation
LS-CUP has three components: “We have combined laser sheet imaging with compressed sensing on a standard streak camera system,” in Dr. Mishra’s words.
The laser sheet is a sheet of laser light that illuminates a stable kerosene flame. The sheet is emitted as a pulse, with a width of 15 nanoseconds, at the flame to cause it to emit four types of electromagnetic radiation:
- Laser-induced fluorescence (an atom absorbs laser light and reemits it),
- Laser-induced incandescence (light from very hot soot particles),
- Elastic scattering (very small soot particles scattered off the laser light particles), and
- Luminosity (light from hot soot particles and chemical reactions of the particles being combusted)
The first three depend on the properties of the laser.
A beam-splitter splits the radiation signals to two separate measurement devices, to simultaneously study the evolution of different types of radiation. The streak camera, compressed sensing (which entails advanced signal-processing techniques), and some prior knowledge of the time intervals at which each type of radiation is emitted are used to reconstruct and interpret the signals.
In a test, the group was able to reconstruct 200 frames per shot. Dr. Mishra explained that the laser sheet helps achieve wide-field imaging as well as the faster imaging speed.
Pros and con
The camera is founded on CUP, an older technology using which other scientists have achieved a frame rate of 70 trillion fps, but only for line-of-sight imaging. LS-CUP on the other hand is capable of planar imaging. “Planar imaging means imaging a 2D plane of a 3D object,” Dr. Mishra explained. “This helps us resolve all the details of dynamics happening in that plane. We can selectively choose a plane where most activities are going on.”
LS-CUP also uses a nanosecond laser to excite the flame just enough to emit some signals for the camera, whereas CUP requires femtosecond lasers, which are more sophisticated and expensive (1 femtosecond is one-millionth of a nanosecond). However, Dr. Mishra said the LS-CUP device itself “can be cost-intensive for some laboratories”.
How much? Per Dr. Mishra, more than ₹1.5 crore for their setup.
