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39

The photons move at the speed of light in a straight line from the laser to the moon and back. The spot on the moon can move faster than light. There is no law against that. The spot is not a physical object, just an image. When you turn your wrist nothing happens to the photons which are already on the way to the moon - they continue on the same trajectory. ...


38

How interesting. Presumably the "glow-in-the-dark" effect comes from the decay of a meta-stable excited state. It gets charged by sufficiently energetic photons, and decays slowly because some selection rule prevents a direct transition without an external influence. If this is the case, we can guess that the laser is exciting the meta-stable state to a ...


25

The ball is probably glowing because it has strontium aluminate in, which produces light by phosphoresence. It's a characteristic of phosphorescence that the light emission is quite long lived. This happens because when you shine light onto a phosphor the light promotes it into an excited state that subsequently decays by interactions with the solid lattice ...


18

Yes easily. Fiber-fed 5-10kW Nd-Yag lasers are commonly used for cutting metal in machine shops. Fibers are so transparent, especially when designed for a single wavelength laser, that the power loss and so heating in the fiber is very small. It's generally less than an optically fed laser where dirt accumulates on the lenses. Many systems have a thin ...


14

This paper seems relevant to your question. If I'm reading the abstract correctly, the answers to your questions are: Q: It seems that if the coherence length of a laser is big enough, it is possible to observe a (moving) interference picture by combining them. Is it true? A: Yes Q: How fast should photo-detectors be for observing of the ...


13

There are two thermodynamic aspects to laser cooling that are worth mentioning. The first, as others have noted, has to do with the frequency of the light that is absorbed and emitted. In Doppler cooling, the laser is tuned slightly below the frequency that the laser wants to absorb. An atom moving toward the laser sees that light shifted slightly up in ...


13

Dear Thomas, the diameter of the beams of these HeNe lasers is between 0.5 and 1 millimeter, so the power 1 mW is coming to $10^{-6}$ squared meters or so. The ratio of power and area is $10^{-3}/10^{-6} = 10^{+3}$ Watts per squared meter. On the other hand, when a 3W LED is watched from the distance 0.1 meters, the power of 3 W is divided to $4\pi R^2 = ...


11

There is a limit to how small you can focus an ideal single-mode laser beam. The product of the divergence half-angle $\Theta$ and the radius $w_0$ of the beam at its waist (narrowest point) is constant for any given beam. (This quantity is called the beam parameter product, and is related to the $M^2$ beam quality measure you may have heard of.) For an ...


11

I would guess that in this case the word faster means that more data per second can be transferred. This is because light has a much higher frequency than microwave so it can be modulated at a much higher frequency. Microwave frequencies are in the range 1 GHz to 100 GHz while light is around 600 THz, so light can in principle transfer 1,000 to 100,000 times ...


10

Yes. 100W CO2 laser is doable at home, and some in fact did that. 100W one will BURN really well. No other types of power lasers are doable at home. (well, probably there is also killing 200 DVD-RW drives and collimating them all - I am actually doing that, I have 45 RW drives ;-) ) The only problems is that you still need few rare things like IR mirrors ...


9

Not all lasers are inefficient. Infrared diodes are quite efficient. This paper shows 64% electrical to optical conversion efficiency: http://www.jdsu.com/ProductLiterature/paper_hipower_910_980_laser_diodes.pdf An argon gas laser on the other hand is quite inefficient.


9

It is possible to do diffractive optics really cheap - either with photography (with a non-digital camera) or a widely available DTP polygraphic technique. What you need is to: Make a Fourier transform of an image you want to have (it should be relatively simple). Make an image which is white when the real part of the FT is positive, and black - when it is ...


9

As previous answers have stated, the wavelength (or frequency) and intensity of the beam are important, as well as the type and amount of impurities in the air. The beam must be of a wavelength that is visible to humans, and fog or dust scatters the light very strongly so that you can see it. However, even in pure, clean air, you will be able to see a laser ...


8

The usual way linear polarisation is measured is by shining polarised light onto a polarising filter, rotating that filter and then using Malus' law to fit the data to a $I_0 cos^2(\theta_{beam} - \theta_{polariser})$ shape. By finding the angular position of the intensity peak we can infer the angle of polarisation of the incoming beam. Now, assume we ...


8

Detectors as fast as 50 GHz can be easily bought (if you have the money :P). This means that if the difference in the frequency of the lasers is smaller then 50 GHz or wavelength difference is smaller than 60 pm then you can detect the beating using these fast detectors. This wavelength difference can be achieved (sorry but I am not in the mood of finding a ...


8

Some laser rangefinding uses a retroreflector, which will bounce the laser light back in the direction it came regardless of orientation. Otherwise, lasers operate at a very specific frequency, so the signal/noise ratio only needs to be strong enough to be detectable at that frequency. If you shine a normal laser pointer on a wall, even if the wall is ...


8

Human color vision is based on four types of receptors in the retina: rods, and three types of cones. Their response to different wavelengths is shown in this graph: . It shows clearly how certain wavelenghts, mostly around the yellow-green portion of the spectrum, are absorbed more strongly, and by more types of cells, than the rest. So it is normal ...


8

The word "stimulated" means that the emission of the photon is "encouraged" by the existence of photons in the same state as the state where the new photon may be added. The "same state" is one that has the same frequency, the same polarization, and the same direction of motion. Such a one-photon state may be described by the wave vector and the polarization ...


8

If you look at reflectances of common materials used to make mirrors with (for example, the topmost graph found on this wiki page), you'll see that not 100% of the light is reflected, especially at the shorter wavelengths. I'm still looking for a somewhat better source for similar curves for household mirrors, but I know that the idea is roughly the same ...


7

Laser light is spatially and temporally coherent. The stimulated emission is mainly responsible for the temporal coherence. So the answer is yes, you can create an electromagnetic beam that is spatially but not temporally coherent by placing a pinhole close to the source, and then another pinhole in the far field of the first pinhole. This beam will not ...


7

If we have the same popular press in mind, the official name of this device - informally known as anti-laser - is a "coherent perfect absorber". See http://en.wikipedia.org/wiki/Coherent_perfect_absorber It was proposed in early 2010 by A. Douglas Stone and collaborators: http://arxiv.org/abs/1003.4968 ...


7

Typically, a laser will damage an optical surface in one of two ways. The first is just what you would expect: the laser heats the material up until something bad happens. The second is also pretty simple, but less common because (AFAIK) it is really only a problem with very short pulses (on the order of femtoseconds). In this case a small but rapidly ...


7

As far as I know you have to worry about how much energy being deposited per surface area. And the area of the "hotspot" of the laser can be very small. So the deposited energy it enough to kill cells on your retina. Actually there is a whole article on Wikipedia about it.


7

The most widespread type of laser cooling is "Doppler cooling". The laser light, coming from the sides, is adjusted to a frequency that is slightly below the natural frequency of the atom. So the atom will only absorb the light if it is moving against the laser beam. Consequently, the velocity will drop. When it absorbs the photon, the energy goes to its ...


7

so only a tiny fraction of beam energy is reflected back to the device. This tiny fraction is enough. With respect to ambient light: One can modulate the laser beam, and filter the the voltage of the receiving photodiode for this modulation frequency and phase. Another precaution is to have a light filter in front of the receiving photodiode which ...


7

As Rococo already pointed out, the no-cloning theorem doesn't forbid cloning of all specific states. It just states that you cannot make copies of arbitrary (general) states. Let me (briefly) reiterate the core of the theorem: To clone a state you need a linear operator C that maps a state $|a\rangle|0\rangle$ to $|a\rangle|a\rangle$. This is not possible ...


7

This is wrong. A wide region (width $\gg\lambda$) of many, evenly distributed in-phase spherical wave emitters, like the Huygens equivalent of a laser beam's cross section, behaves like a phased antenna array, with the result that destructive interference between the emitters cancels radiation deviating significantly from the beam and reinforces radiation ...


7

I'd like to add to Ruslan's, Gregsan's and Oscar Lazo's answers, particularly Oscar's. All these answers are perfectly valid. The multiple bounces in a laser raise the probability of a given photon's stimulating another in a stimulated emission event AND shape the output spectrum. But why is there a spectrum to shape? And how does the cavity shape the ...


7

Due to Heisenberg uncertainty principle $\Delta x\Delta p\gtrsim \frac{\hbar}2$, one can't really make a quantum have zero momentum in any direction. So you can't say that photons go in the same direction - this is just a simplified description of laser operation. In reality, the thinner the beam, the higher the divergence. Compare e.g. a DPSS laser (e.g. ...


6

can You read German? here http://de.wikipedia.org/wiki/Laserschneiden all Your questions are answered. The English pendant is much shorter and does not deal Your problems. In case You can't read that, some short excerpt: Main problem with copper and Aluminium are high reflectivity at 10 µm, the high thermal conductivity and and no "assistance" from ...



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