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# destructive interference?

#### Maelstrom

##### New member
Have you guys ever created complete destructive intereference by shining two lasers together? IE shining two beams through a prism or whatever you sue to combine beams and have nothing come out( or it be very dim)? This is the only example of destructive intereference with light I can seem to find, or that was ever presented to me...

#### climbak

##### New member
To have interference the light needs to be coherent. Light from different sources is mutually incoherent. This means that two sources of light wont interfere with each other. There may be a way to make seperate sources coherent, but I'm not sure off the top of my head and it's pretty safe to say that in everday type things you won't encounter this. The double slit interference is just one example. Diffraction gratings and holograms are other examples. Here is a picture of an optical vortex, which is basically a fancy way of saying a hole in the middle, using a forked grating:

#### Maelstrom

##### New member
THanks. But say you were able to create two sources of light; same speed, frequency, and wavelength( making them coherent, correct?), would it be possible then? In the physics class Im in at school, we only touched on complete constuctive and destructive intereference, so is there partial interference? FInally( last question) why is it so much easier to have destructive interference with sound than light?

#### climbak

##### New member
Having two seperate, but identical light sources does not make them mutually coherent. There may be a way to force two sources into coherence by passing through slits or some other means, but I am not sure exactly how it would be done. I suppose if you were able to create a purely single frequency source then it wouldn't be nearly as hard, but even lasers usually have a fairly high spread in frequency; diodes more so than gas for example. As for the partial interference, the answer is yes. Take the output from the double slit experiment. The only places where fully constructive or destructive interference will happen is right on the minima and maxima. Everywhere else the interference is just partial. My guess to why it's easier to have interference with sound is that sound travels through a medium where light needs no mediumand the tolerance levels when trying to match frequencies are much more leniant with sound. I am sure there are other reasons as well, but thats just my guess.

#### Maelstrom

##### New member
Thanks!
Are you a physicist or aiming to be one? You always seem to be very knowledgable and chime in or know answers to these kind of topics....

#### NovoRei

##### New member
So, imagine you join two beams dephased by 180 degrees.

The energy is lost? Could you separate the beams?

#### climbak

##### New member
Maelstrom said:
Thanks!
Are you a physicist or aiming to be one? You always seem to be very knowledgable and chime in or know answers to these kind of topics....
yes actually. I have a year left before I get my BS in Physics. After that I dont know yet.

NovaRei:
I dont think that two seperate beams would cancel eath other out. Even though they may be 180 degrees out of phase, that doesnt mean they are coherent. Their electric fields would add up and basically just double the intensity. For example, with the laser system I am working on right now we are counting on the fact that seperate laser sources will be mutually incoherent and not interfere with each other when writing holograms. Even a beam from the same source will lose coherence after a certain length.

#### flogged

##### New member
Maelstrom said:
Have you guys ever created complete destructive intereference by shining two lasers together? IE shining two beams through a prism or whatever you sue to combine beams and have nothing come out( or it be very dim)? This is the only example of destructive intereference with light I can seem to find, or that was ever presented to me...

Climbak is right - two laser beams from different sources, even if the same frequency, will not interfere with each other.

I'm not certain what the reasons for this are. At least with diode lasers there are minute variances between the frequency of two given diodes, depending on temperature and manufacturing. Basically the two sources need to somehow be correlated with each other at some point in the past before they will interfere.

Usually when making holograms a single laser is used, and the beam is split and then recombined to interfere at the object to create the hologram. I posted your question to <alt.lasers> years ago. Seem to recall I got a response from someone saying that '4th order' interference (whatever that means) has been observed from two separate gas lasers, though it's not easy to do.

I know that at radio frequencies (still EM radiation) interference techniques are used all the time, for example in phased arrays and superhet receivers.

I like this little animation on interference:

Interference is one of the things that fascinates me about lasers - you don't have to look hard to find it. It makes the wave nature of light obvious. For example my avatar is made by simply reflecting a laser beam off a frosted incandescent light bulb. The projected spot is an interference pattern modulated by the surface of the bulb. Laser speckle is also interference - the space in the room is filled with the interference pattern, it's why you can't focus on it. I'm nearsighted, and it's easier for me to see speckle when I take my glasses off - everything gets a little fuzzy, but the speckle remains sharp.

L

#### likewhat

##### Guest
As said above the lasers would have to have the same frequency and in addition have some definite phase correlation in order to "cancel" the light.

Even if you use actively stabilized lasers the instantaneous linewidth of which can be one the order a few Hz that means that after a second the phase would have changed relative to another laser of the same frequency. Even the world record most highly stabilized lasers would only perfectly destructively interfere for a second before they would constructively interfere.

That is not to say that when you overlap 2 different lasers you dont get interference. You most certainly do, in fact you can overlap 2 lasers of different frequencies and create a new source of coherent light at the difference of the two frequencies. This is one of the ways that Terahertz radiation sources are created.

The thing you have to understand is that since laser radiation is of such a high frequency the difference in the frequency of two lasers if not actively stabilized will be GHz and constantly drifting. That means that the interference pattern will be turning the light on and off many times per nanosecond so you will just see and average and it will appear to be light the whole time. If you overlap two lasers on a very fast photo detector and connect it to an oscilloscope you will see a wave appear that will be the difference of the 2 frequencies of the lasers. In fact if you can control the frequencies of your lasers you can use this method with some electronics to control the frequency of one laser relative to another (it is called rf offset locking).

So the answer is yes, but not in the way you were thinking of due to the lack of a definite phase relationship between the lasers and the difference in their frequencies.

#### pseudonomen137

##### New member
NovoRei said:
So, imagine you join two beams dephased by 180 degrees.

The energy is lost? Could you separate the beams?

Nope, its not impossible for the energy to be lost (or so I'm told ). In the case of destructive intereference about to happen, you'll find instead of the laser continuing along its logical path, it'll instead get reflected so you don't end up with any loopholes to the laws of physics.

If you're interested in that type of stuff, you should try making your own simple interferometer. Not a lab-grade measuring device, but if you pick up some surplus optics (some front/first surface mirrors and either some 50/50 beamsplitter prisms of 50/50 half-silvered mirrors [those helping hands alligator clip things make simple cheap holders for aligning this stuff]) you can make a simple Michelson or Mach-Zedner interferometer with a laser and a few tens of dollars (if even that). Its pretty cool once you get it setup!

#### LikeitBright

##### New member
Maelstrom said:
Have you guys ever created  complete destructive intereference by shining two lasers together? IE shining two beams through a prism or whatever you sue to combine beams and have nothing come out( or it be very dim)? This is the only example of destructive intereference with light I can seem to find, or that was ever presented to me...
The individual lasers won't be correctly phased so you probably won't see any interference.  Usually one source is optically split and recombined to demonstrate interference. Interference is just a matter of redistributing the optical energy. Bright fringes become brighter than the contribution from either source while the dark areas of destructive interference are 'nulls'. And yes, it is a bit weird.

#### brigman

##### New member
Have you guys ever created complete destructive intereference by shining two lasers together? IE shining two beams through a prism or whatever you sue to combine beams and have nothing come out( or it be very dim)? This is the only example of destructive intereference with light I can seem to find, or that was ever presented to me...

A laser diode is highly coherent quantum system and it has been observed that only photons or particles from the same quantum source will display the type of interference you describe (non-classical interference). See this link for more information.

#### lasersbee

##### Well-known member
Hey brigman...

As a new Member.... you should perhaps READ the date of the Post
That Thread has been DEAD for over one and a half years....

Jerry

#### f22warzone

##### New member
Thanks for bringing it up tho i have never seen this post and its very cool and informative a bit old but still cool : )

#### brigman

##### New member
Hey brigman...

As a new Member.... you should perhaps READ the date of the Post
That Thread has been DEAD for over one and a half years....

Jerry

Hey Jerry, you will probably spend a lot of your time wondering why your relationships lack that certain something: it's just you.

I hope you work it out mate.

BTW: @Admins kick me, ban me or whatever, in what world would I care?

#### HIMNL9

##### New member
Hey Jerry, you will probably spend a lot of your time wondering why your relationships lack that certain something: it's just you.

I hope you work it out mate.

BTW: @Admins kick me, ban me or whatever, in what world would I care?

Ehm, sorry if i say you this, but, don't you think that you're overreacting a bit ?

After all, Jerry just pointed you to the fact that the thread was dead, and from a long time .....

In these cases, is usually better to start a new thread, asking for the same thing, and maybe also linking the old one as example, instead to resurrect the old one, you don't think ?

(and, by the way, this forum usually don't ban peoples just if they resurrect old threads for error, as instead other forums do ..... but asking the admins to do it, in this way, just cause you don't like a reply ..... you don't think that is just a bit too unfair ? ..... not bashing you for that, just asking .....)