Physicists created the world’s sharpest laser

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line width

Physicists created the world’s sharpest laser, with a line width frequency of only 10 millihertz. This precision is useful for various applications such as optical atomic clocks, precision spectroscopy, radio astronomy.

Lasers concentrated with the stream of photons at a single frequency. But, in reality light isn’t perfectly monochromatic. The components in the laser introduce flicker noise that disturbs beam’s frequency, causing it to fluctuate. Most kinds of lasers have a line width of a few kilohertz to a few megahertz.

Researchers from Physikalisch-Technische Bundesanstalt and other university researchers have found a way to narrow the line width for a laser with a wavelength of 1.5 micrometers, establishing a new world record. The project took nearly a decade to complete and works by reducing the thermal fluctuations of the laser resonator.

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laser resonator

A Fabry-Pérot silicon resonator made out of two highly reflecting mirrors that ensure laser light wave reflecting from the mirrors is kept at a fixed frequency. The mirrors fixed at 21cm apart from each other to ensure stability and line width of the laser.

The light waves oscillate approximately 200 trillion times per second, it only gets out of sync after 11 seconds. By then, the wave has reached a length of approximately 3.3 million kilometers about 10 times the distance between the Earth and Moon.

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The scientists had isolated the resonator nearly perfectly from all environmental influences which might change its length. For this reason, researchers cooled the silicon down to a temperature of -150°C (-238°F) to reduce the random Brownian motion of the material from thermal energy.

The fluctuations of resonator length are in the range of 10 attometers (10-18), no more than a ten-millionth of the diameter of a hydrogen atom. It means that the frequency of the laser can vary by less than 4×10-17Hz. But, such a tiny improvement is worth pursuing since it can greatly boost the precision of several experiments that rely on laser measurements.

However, the researchers are eyeing up the possibilities of reducing the line width further, to less than 1 millihertz. A long stability period will be key to taking measurements in radio astronomy, where the laser travels for longer distances.

More information: [Physical Review Letters]

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