This is adapted from an answer to a question at the Physics Stack Exchange site. The questioner asked:

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? How fast should photo-detectors be for observing of the interference of beams from two of the “best available” lasers?

This is a question about the itnerference of light waves, which is traditionally demonstrated via the famous “double slit” experiment, where a single laser is sent through a barrier with two narrow slits cut in it. The light waves passing through the two slits overlap with each other, and produce a characteristic pattern of bright and dark spots, like this image from MIT showing three different lasers going through a double-slit, producing patterns with different spacing:

i-3d6f0640a91be3edae44102c931da12f-double-slit-pattern.jpg

As I said, this is traditionally done with a single laser and two slits. In principle, you ought to be able to do it with two completely independent lasers of the same wavelength, and see an interference pattern that way. In practice, though, this is very hard to do, as the phase difference between two independent lasers will jump around randomly at a very high rate. At any given instant, the two lasers will interfere with each other to make a nice double-slit pattern, but the positions of the bright and dark spots will shift around randomly, so fast that your eye can’t follow them.

In principle, though, you ought to be able to “see” this sort of pattern using an electronic detector system. The question from Stack Exchange is, essentially, what would you need to see this sort of thing? (At least, that’s how I interpreted the question…)

The key number for this is time scale for the fluctuations of the phase. The pattern between the lasers would be steady for a time on the order of the “coherence time” of the lasers. A good rule of thumb for finding the coherence time for a fairly ordinary laser is that the coherence length of the laser is roughly equal to the length of the cavity, and the coherence time is the coherence length divided by the speed of light. A typical gas laser of the sort you would readily find on the shelves of your local physics department stockroom has a cavity that is roughly one foot long, which means a coherence time of the time required for light to travel one foot, which is one nanosecond (one of the rare cases where American units are more convenient than SI units).

So, you would want to be able to pick up the pattern in less than a nanosecond, which means your detector needs to be able to handle count rates of at least one gigahertz. That’s not too difficult to manage– you can get photodiodes with a bandwidth of 50-60GHz pretty easily. In order to see a spatial pattern, though, you really want a linear array of these at the very least, and a CCD camera would be even better. You also need to be able to pick up multiple photons in that span, in order to be able to see the pattern, so you can clearly resolve the difference between bright and dark fringes– I would say you probably want at least 100 photons/pixel in the bright fringes to be able to get decent fringe contrast. This is doable with fairly basic lasers– a few milliwatts in the red range of wavelengths is about 1015 photons per second, or about 106 photons per nanosecond, so you can spread that out over a few thousand pixels and still be safe. And, of course, you need a low level of “dark counts” in your detector, so it can readily tell the difference between a bright spot with 100 photons and a dark spot with none.

That’s a pretty challenging set of detector requirements. You want, essentially, a CCD detector with single-photon sensitivity and a bandwidth of 100 GHz. I don’t think you’re going to find that just lying around. Even the fast CCD detectors people use for real-time monitoring of BEC experiments and the like have a frame readout time of a millisecond or so (at least, that’s the time between frames in the movies they usually show, and I assume that if they could go faster, they would), and those aren’t single-photon sensitive. Single-photon detectors tend to be things like single avalanche photodiodes, which are easily damaged by photon count rates of a few tens of kilohertz. You might be able to construct a detector with these specs, but it would be an extremely difficult problem, and not something that would be worth doing just to see a spatial interference pattern between two independent lasers.

If you can bump the coherence time of the lasers up by a factor of 1000 (which is not a trivial undertaking) then it becomes a little easier to do, but it’s still very difficult– you’re looking for a CCD with a frame readout time of a microsecond or so, able to handle photon counts in the megahertz range. Which is still a very hard problem. Another factor of 100-1000 would put you solidly in the region where you could reasonably expect to find a detector that would do the job, but then you’re talking coherence times of a tenth of a second or so, and that requires some really exceptional laser design– we’re talking the sort of lasers that they use for optical clock experiments.

This is an interesting problem, though. You will often hear the statement “A photon interferes only with itself” thrown around (I’ve seen it attributed to both Wigner and Dirac, but I’m not sure which said it first), which leaves open the possibility that photons from two completely independent sources might not be able to interfere with each other. Because of that, people have looked closely at this process, and in fact, the existence of a spatial interference pattern between two independent lasers was verified way back in 1967 by Pfleegor and Mandel. They even did it with single photons.

The Pfleegor and Mandel experiment is pretty clever, as you would expect of an experiment doing this with 1960′s technology. It’s kind of involved, though, so explaining their trick will wait for another post.

Comments

  1. #1 anders
    November 15, 2010

    How about if I happen to find two iodide stabilized HeNe lasers in the stockroom?

  2. #2 Anonymous Coward
    November 15, 2010

    Your typical stockroom iodide-stabilized HeNe still has a MHz linewidth, so you’d be in Prof. Orzel’s “If you can bump the coherence time of the lasers up by a factor of 1000″ regime.

  3. #3 Peter Davis
    November 15, 2010

    Is this anything like the dissonance or beating between two barely out of tune piccolos? Or is coherence time a more fundamental sort of randomness in the laser’s phase?

  4. #4 Wilson
    November 15, 2010

    the time required for light to travel one foot, which is one nanosecond (one of the rare cases where American units are more convenient than SI units).

    Exactly one nanosecond, or very, very close? Would 30.5cm (12.007874″) be close enough? 30cm (11.811″)?

    Not nitpicking, just curious.

  5. #5 anders
    November 15, 2010

    Wilson: by definition, light in vacuum travels 29.9792458 cm in a nanosecond. That would be about 0.98357 feet.

    So, if iodide-stabilization is only going to get MHz level stability, how about injection locking? (i.e. feeding some small part of laser1 output into the cavity of laser2?)

  6. #6 Mary
    November 15, 2010

    Injection locking would work for sure, but it’s sort of cheating, since they are no longer two independent lasers.

    In fact, it’s possible to phase-lock lasers electronically as well, but again, once you couple them together like that, the problem is too easy — seeing their interference fringes is no more difficult than the single laser case.

    The challenge here is how to see fringes when the lasers aren’t coupled…

    An interesting related question: if you have a laser gain medium that can’t deal with powers of more than 100mW but you want a 1 watt output, is there any way to combine the beams of ten of the 100 mW lasers coherently to get your 1 Watt? This is actually its own subfield of research, though the first time I saw I talk about it all I could think about were the green rays of the Death Star, which join together and shoot outwards to destroy planets… Anyway, this page has an intro to “coherent beam combining” if you scroll down…

  7. #7 Manu
    November 15, 2010

    The detection problem is approached from a totally wrong angle in this post. There is no need for ultrafast CCDs with GHz bandwidth as there is no need to time-resolve the interference pattern. Just the exposure time has to be short enough to be within the coherence time. After a short exposure you can take your time to read out the CCD.

    And if the shutter of the camera is not fast enough, you can always switch the laser beams. With acousto-optic modulator you can switch a laser beam in tens of nanoseconds, which would be short enough to measure the interference of two research grade lasers.

    So pretty much any well equipped cold atoms lab should be able to measure this.

  8. #8 Andrew
    November 15, 2010

    How about amplified pulsed femtosecond lasers? The coherence time of a femtosecond oscillator is (substantially) longer than the pulse duration, and single amplified pulses are visible to the naked eye.

    You’d have to get the timing of the two pulses right, of course. You can probably use a piezo to tune the cavity length of one of your femtosecond oscillators so the repetition rates of the two lasers are precisely the same, and then a delay line for fine temporal overlap. You’d want the laser colors the same too, so your interference pattern wouldn’t drift too much over one pulse duration.

  9. #9 perry
    November 15, 2010

    I think it was Dirac, but the point is that photons are excitations of a mode, and it is the modes that interfere.

  10. #10 ROBERTO OLIVEIRA DA SILVA
    November 16, 2010

    It’s very interesting!
    I really liked these pictures, I show my colleagues in the university course.

    Roberto Oliveira
    student- UFRPE

    Brazil

  11. #11 Robert S
    November 16, 2010

    Couldn’t you also use a time gated ICCD? I forget the exact timing available, but seem to remember that it was on the order of light feet. So a few nanoseconds. So, increasing the coherence length to a couple meters ( a mere factor of 10) might be enough to see effects

    Never mind, I looked at a couple sources and there are gated ICCD cameras that operate down in the the hundreds of ps exposure time. Laser tubes a foot or so long should show coherence over that time if I understand the concepts.

  12. #12 lane ranger
    November 17, 2010

    A little-known feature of the Pfleegor and Mandel system is that their interference is not dependent on the state of the laser field. The laser does not need to have an intrinsic (even randomly varying) phase, and the interference can be explained as detection based. The laser beams don’t interfere, the detections do!
    Whilst we might not like it, it is difficult to argue against.

  13. #13 Damien
    January 8, 2011

    Here is the way I’ve always seen it, tell me if you think I am wrong.

    You can very well say with Dirac that the photon interferes only with itself. The thing is: the photon is not shot out of laser 1 or laser 2. The photon is an excitation of a mode in the entire space, or at least in the entire smallest cavity containing both laser devices (typically: the laboratory).

    Note that with their perfectly defined frequencies, photons have a perfectly undefined position, they are in the whole of the smallest opaque box in which they were created.

    When you see the light shooting out of a laser device, it is not the photons that are moving, what happens is that the excitations of the various modes of the cavity interfere destructively ahead of the beam and constructively inside the beam.

    The lasers’ cavities are not closed (that’s why you can see the laser light outside of the lasing medium). The word cavity is convenient to talk about them, though.

    The photon is in the lasing medium of laser 1, in the lasing medium of laser 2, and in both beams outside of these media. The creation of the photon is due to the processes taking place in both laser media.

    Maybe that’s what perry meant by “the point is that photons are excitations of a mode, and it is the modes that interfere.”

    Well, let me know.