Please email your questions to babbitt@physics.montana.edu
 
 

Here are the questions and answers.
 
 

Question 1

What's the difference between the laser used in the holo class and the

laser used for cutting? What different properties turns a laser into a

cutting tool?

Answer 1

The lasers in the holo labs have continuous outputs and put out an average of 1-10 milliwatts (a milliwatt is 1/1000 of a Watt, a 100W light bulb puts out about a Watt of light). Laser used for cutting are usually pulsed to momentarily increase their energy per unit time. They also have an average power of 10-100 Watts. By concentrating the energy in a short period of time (i.e. pulses), the material being cut doesn't have time during the pulse to remove the energy (which is usually transformed from light into heat) by heat conduction. The rapid heating of the small spot on the material at the time of the pulse usually causes ablation, essentially the rapid removal of the top of the surface the pulse hits. The pulsing also allows time for the ablated material to fly away before the next pulse, so that the ablated material in the air doesn't absorb the laser pulse and rob energy from getting to the surface to be cut.

Another difference is that cutting lasers are usually 10.6 micron (10,600 nanometers) wavelength, far into the infrared. This wavelength is readily absorbed. Also, it is easier/cheaper to make a high power laser at this wavelength. But, you can also make cutting lasers with visible wavelengths (400-700 nanometers).

Follow-up question

What role do wavelengths play with all this?

Answer

For each material, different wavelengths of light are absorbed differently. Also, the reflectivity of the material varies with wavelength. So if you know the material you are going to cut, getting a laser at the right wavelength could be useful. But, for general cutting, 10.6 micron lasers work well on most materials.

Question 2

Let's say there are 2 red light lasers of which both have the exact

same wave lengths. The only difference is one of the lasers amplitude is

bigger than the other. What will happen?

Answer 2

A single laser with a coherent (uniform) wavefront causes a uniform illumination of whatever it illuminates (let's say film). If a second laser that is weaker (or if you split the same laser in two unequal beams, one much weaker than the other) is interfered with the first laser, at points they will interfere constructively or destructively. But, since the second beam is weak, rather than doubling or canceling the first laser, it will only slightly enhance or decrease the first laser. So the spatial modulation (grating) produced by the interference would be on top of a uniform illumination (low contrast).
 
 

Question 3

In a spatially coherent wave, we have several waves matching each

other uniformly right? Since there are in-phase, can they be represented

as one huge wave?

Answer 3

I'm not sure I understand the question. If the coherent waves are all hitting one spot, they produce a huge wave at that spot. If the waves are spread out along a broad plane, each point on the plane sees a fraction of all the waves and not all of them. Those that hit a single spot do add to a bigger wave.

An example is a big laser light beam the uniform illuminates a few inch circle. If you use a lens to concentrate it to a smaller spot, they that spot sees the summation of a lot more waves.

Follow-up question

A laser beam is actually made up of several coherent light waves right? So can the waves in this beam be represented as one big wave?

Answer

If the waves are 1) at the same wavelength, 2) in-phase, and 3) spatially overlapping, they add together to one "big" wave. By big, I mean a big amplitude, and thus a high intensity. The lasing action of the laser depends on stimulated emission of light. Stimulated emission has the property that a light wave entering the lasing medium generates more light waves of the same wavelength, phase, and direction as the entering wave. This wave adds to the entering wave, thus the output is just an amplified version of the entering wave. If the lasing medium is put between two mirrors, the amplified wave is passed again and again though the laser, making the strong laser light beam. Eventually, the lasing medium saturates (can only amplify the light so much), which limits the output power of the laser.
 
 
 
 

Questions 4

Is it possible to shoot lasers like what we see in the Star Wars

movies? And can lightsabers be produced?

Answer 4

Star Wars had light sabers and the ray from the death star.

The light saber

A strong laser will appear as a beam in a dusty room, so producing the beam is possible. However, the beam would continue to travel until it slowly diminished to zero from all the scattering, not end abruptly as in the end of a light saber. In a dustfree environment (like space) you won't see the beams.

The sword like nature of the light saber (i.e. that they hit into each other) is not very believable. Light does not interact with light. The fact that two light beams can pass through each other makes possible much of the microwave and optical communication of today. Only if there is something (atoms or molecules) at the point the light beams cross would be interact. That interact until extraordinary circumstance may cause the beams to deflect from one another. But, I don't think the force of the deflection could possible be carried back along the lightbeam and cause you to feel the force in the sword handle. They has to be a lot for to the light saber than just light to make it a remote possibility to work.

As for the death star

The death start shot out four beams that combined into one big beams. One explanation is that at the point at which the four beams crossed is a laser material that puts out the big beam. The four beams are the pumps (supply the energy) for the laser material. The death star could shot out the laser material just in time to the crossing point, the laser could lase momentarily, then blow up after a short while due to the thermal strain. This may sound far fetched, but it is not too different from the x-ray lasers the Defense Department had designed in the 1980's. The idea was to have an x-ray laser shot up into space. The laser was surrounded by a nuclear bomb. The radiation from the bomb pumped the xray laser. The laser would put out a burst of directed, coherent x-ray beam. This was made possible because the radiation from the bomb travels at the speed of light, much faster than the shock wave from the blast. So the laser had time to lase, before it was destroyed by the blast.
 
 

Follow-up question

What other types of laser material are there (instead of the lens)?

And what do you mean by "So the laser had time to lase, before it was

destroyed by the blast"? Does that mean the laser itself gets blow up

to?

Answer

A lens is not a laser material. A lens does not amplify the light wave, it concentrates it. Consider a collimated beam of light entering a lens. If you measure the power going into a lens and the power at the focus of the lens, they will be the same (actually, the focus spot will be a little less than the input due to reflective losses in the lens). But, the intensity of the light will be much greater at the focal spot than at the input of the lens. The intensity is defined as the power per unit area. The power is roughly the same entering the lens as at the focal spot. But, the light is concentrated by the lens into a much smaller spot at the focus. Thus, the intensity is much greater (i.e. power divided by big collimated laser spot is much less than the same power divided by a tiny focussed laser spot)

Lastly, since light can be thought of as the movement of molecules, does

it have any mass at all?