"approximately 200 trillion times per second" , "about 10 times the distance between the Earth and Moon."
What are you talking about? Please explain these values in proper el Reg speak.
A team of physicists claims to have created the world's sharpest laser, with a line width frequency of only 10 millihertz – opening up the possibility of improving the accuracy of optical clocks and radioastronomy experiments. Lasers are, ideally, a concentrated stream of photons at a single frequency. But in reality the light …
Here you are...all you had to do was read it.
"The light waves of the laser oscillate approximately 200 trillion times per second, and are stable for about 11 seconds before they get out of sync. By then, the wave has reached a length of approximately 3.3 million kilometres – about 10 times the distance between the Earth and Moon."
"Paragraph 1: < 10 mHz "line width frequency" - whatever a line width frequency is.
Paragraph 8: the frequency can vary by less than 0.4 aHz
El Reg is also confused. The line width frequency is essentially the variation in frequency within the laser pulse, which is correctly reported as <10 mHz (it actually says "as small as 5 mHz" in the abstract). The 4*10^-17 is not a frequency at all as El Reg suggests, it's what they call the "flicker noise floor", which is essentially the ratio of linespread to laser frequency, so ~5 mHz / 194 THz. Since it's a ratio, it has no units.
Actually, you just touched on my thought upon reading this, does the linewidth have an effect on beam spread?
I would imagine that constraining the the frequencies would increase coherence, which would limit the spread; this is reinforced by the use of the adjective "sharp".
"I would imagine that constraining the the frequencies would increase coherence"
I interpret "Stable for up to 10x the distance from Earth to Moon" as referring to the coherence length so that appears to be the case.
The irony is, of course, that a laser should be unnecessary for reading CDs & DVDs. Interference contrast ( https://en.wikipedia.org/wiki/Differential_interference_contrast_microscopy ) microscopy resolves similar phase differences with white light.
I think the author is actually talking about the monochromatic photons being coherent* in a laser beam and how the coherence has been improved. However the article fails to mention this or make too much sense without it. Needs to try harder is my rating.
* Given that by its nature a laser beam is monochromatic (all photons of the same frequency or wavelength, if you prefer), what makes a it a laser is that the photons are coherent (in step or in-phase with each other) and it is this that makes the beam more powerful than say a monochromatic sodium vapour bulb in front of a parabolic mirror.
"a laser beam is monochromatic (all photons of the same frequency or wavelength, if you prefer)"
The word linewidth itself seems to disagree. [I thought that] the original idea of the line is that small bright place on a spectrogram, which is narrower or wider, indicating how energy is spread into the nearby frequencies. If they're strictly monochromatic, the linewidth is ideally zero. Two photons with an arbitrarily small difference between their frequencies will never have the same exact trajectory after going through a prism, and that's where linewidth physically comes from. What this has to do with coherence is that if one photon and another photon are at 2 frequencies with a nonzero difference between, then they simply cannot be in step, except they can briefly appear to be in step at some intervals corresponding to the beat frequency.
disclaimer (stoopid noobie alert): once upon a time I used to skip school and read a book about lasers at the public library, but that doesn't mean I know WTF I'm talking about
"Given that by its nature a laser beam is monochromatic"
No it isn't, try actually reading the article:
"Lasers are, ideally, a concentrated stream of photons at a single frequency. But in reality the light isn't perfectly monochromatic, as components in the laser introduce flicker noise that disturbs the beam's frequency, causing it to fluctuate. Most kinds of lasers have a line width of a few kilohertz to a few megahertz."
Also, coherency has nothing whatsoever to do with power. A laser beam is not any more powerful than "a monochromatic sodium vapour bulb in front of a parabolic mirror" - if both light sources are 10W, for example, then both have exactly the same power.
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