Why is Laser Light Different?

Thanks SB,

Clarity and simplicity for my simple mind!

sd,

I understand your desire to be simplistic, but you've definitely gone over the edge into error! Any later attempts to understand laser light will be incomprehensible if armed with your explanation.

No offence.

Did I tell you that it is all in the family ?

You have the smallest baby as GASER (gamma ray) with maximium power and the family tree moves on to the XASER (X ray), MASER (Microwave) through to RASER (Radio Waves)

Now ERASER though except in primary classes and movies.

I'm bias to U V lasers. Vaser any one. How about a millisecond tan but not two milliseconds.

But I'm still confused on how a Phaser works. Must be the unobtainium Dilithium crystals.

Regular light is like noise, laser light is like the sound of a tuning fork.

First, to highlight why you should go further - here are some possible misconceptions you might arrive at from reading sb's "simplified" explanation?

"After scatter/diffraction, no destructive interference will take place"
. incomprehensible, or wrong, or both
. the most significant difference between laser light and "ordinary" light is quite simply that diffraction effects do not usually average out

"and anyway the particle scattering them will be vaporised in an instant"
. lasers are not necessarily particularly powerful, but
. yes they can focus as well as is possible for their frequency (wavelength) of light

What is correct is how laser light is produced - amplification of incoming light, and comments about the existence of coherence - but...

Now to look at coherence:
Let us first consider the focusing of light from everyday objects - usually, it will produce an image. If we block all the light except that starting out from a very small area, we will have an image of a point. The image will be larger than the original, because the wavelength of the light is not zero (its size is determined by interference effects - see Airy disc for some more detail). A beam that focuses to the minimum possible size is called "spatially coherent", and there are multiple different measures for the quality of this spatial coherence. Laser light is often spatially coherent as is theoretically possible; NB that you will not usually see the structure of the Airy disc if you focus all the light from a laser - this is suppressed by the gradual way that the intensity of the beam decays at its edges. . . So, lasers can be focused down to the smallest size that is practical for the wavelength.
Now let us think about colour, (or wavelength or frequency). Normal (even what appears to the eye as single-colour) light is built from a range of wavelengths. As nothing is ever perfect, this is even true of laser light. A measure of colour purity used for lasers is known as "temporal coherence"; paradoxically, this is usually measured as a length! This is based on the observation that if you split the beam into two paths and recombine them after they have travelled different distances you will sometimes still be able to see interference patterns. The difference in the path lengths for which interference remains visible is known as the "coherence length". For ordinary white light, this distance is the order of a micron (a millionth of a meter). For typical sodium street lighting it is a few mm. Mercury vapour lamps (still not lasers) can give half a metre. Conditioned lasers can give coherence lengths of millions of km. So you can see from this that, contrary to how you might have read sb's explanation, you are far more likely to get interference of any sort from a laser than from normal sources - and this would of course include destructive interference.

So, what does all this mean in practical terms: practical 'ordinary' lamps can give very high power, but this will not come from a single point - so we cannot focus this power into a very small area; all the power in a laser beam can be focused down to a diffraction-limited spot, so a powerful laser can be very good for cutting (by burning through a very small width). Another application of spatial coherence is that it allows us to create the most a tightly-defined parallel beam; this can be useful for surveying, or for free-space optical communications (to get the maximum practical power to a distant receiver.

The applications of temporal coherence are in general more subtle, so I'll come to those later. However, you can see the combined effects of temporal and spatial coherence in the speckle pattern that appears whenever you shine a laser onto a matte surface - though even here the spatial coherence is of more importance - a coherence length similar to the depth of the coat of paint (usually much less than a mm) will usually be adequate.

So, where is extreme temporal coherence important?
Coherence lengths of a few metres are necessary to get the best from room-temperature spectroscopy; longer coherence lengths can be exploited at lower temperatures; but possibly the largest benefit of lasers for this application is that they can be made tunable.
The interference between beams that have travelled different distances is useful in accurate determination of distance - the beneficial coherence length can be up to twice the distance to the object being measured [Note: even if the measurement error is many (even hundreds/thousands of) wavelengths, this can contribute to our ability to detect the returning beam].

I think that's more than enough for now...