When I send two separate laser beams directly into each other one of the beams goes completely out. These are inexpensive 5mv red diode lasers. What’s happening inside the laser to make it “turn off”? When the beams are not opposing the “turned off” laser returns to its normal function. Note: The use of a a beam splitter allows me to see effects. Update here are two videos I posted showing my experiment results. https://www.youtube.com/watch?v=n66R_OVeiGc https://www.youtube.com/watch?v=bsJ5sMncu_g

  • $\begingroup$ This is interesting, do you have any photographs or a video showing the effect? $\endgroup$
    – Philip
    Commented Jan 25, 2020 at 16:22
  • $\begingroup$ Also can you be more clear what you mean by effects? And can you describe (perhaps with a diagram) your experimental setup in more detail? $\endgroup$
    – Liam Clink
    Commented Jan 25, 2020 at 16:23
  • $\begingroup$ @Philip For some reason my phone won’t allow me to up load a photo, not sure why. The two lasers are pointing directly at each other. A beam splitter is placed between them at 45degrees. When the beams are directed into each other one beam goes completely out. $\endgroup$
    – Lambda
    Commented Jan 25, 2020 at 16:38
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    $\begingroup$ Generating lots of excess carriers could well short out one diode. $\endgroup$
    – Jon Custer
    Commented Jan 25, 2020 at 18:07
  • 1
    $\begingroup$ Could be either depending on the device and the power supply (e.g. current limited). $\endgroup$
    – Jon Custer
    Commented Jan 25, 2020 at 18:13

2 Answers 2


I do not believe interference is the reason you don't see the beam from laser 2. I believe laser 1 is somehow turning off laser 2.

There are spots and side fringes on both cards that do not change when laser 2 is turned on or off. They are entirely from laser 1. Laser 1 is shining into laser 2, reflecting back out, bouncing off the beamsplitter and lighting up the left card. You can see this particularly at the end of the second video. So the dim spot on the left card is not output from laser 2.

Laser 2 always turns abruptly on or off. This happens when light from laser 1 is blocked, or even dimmed by smoke. If this was interference, you would expect laser 2 to fade out as smoke dimmed laser 1.

Plainly the situation is not symmetric. Laser 2 does not shut off laser 1. Perhaps laser 2 is not precisely aimed into laser 1. Perhaps laser 1 is brighter than laser 2. Perhaps it is something else.

I can speculate about a mechanism where one laser would turn another off, but I could easily be wrong.

A laser is an active medium in a cavity. An energy source keeps exciting atoms in the medium. The atoms emit light when they decay back to a lower energy state.

When an atom is excited, it will spontaneously decay if left alone. But if another photon of the right frequency hits it, it can be stimulated to decay and emit a photon. When this happens, the emitted photon has the same direction and phase as the first photon. This is what makes laser light coherent.

Without a cavity, an active medium is just a light. Not enough photons stimulate atoms before escaping.

The cavity is two mirrors facing each other so that light on the right path repeats the path over and over. One mirror is partially reflecting so that some light can get out of the cavity.

When a laser starts up, light is emitted in all directions. Much of it escapes. But some happens to be in just the right direction to reflect back and forth. This light stays in the medium and is much more likely to stimulate atoms to emit light. The stimulated light is also in just the right direction to stay in the cavity. This light is amplified, while other light is not. In a very short time, almost all the light in the cavity is traveling in the right direction. Some gets through the partially reflecting mirror, and this is the beam you see. This gives us the word laser: Light Amplification by the Stimulated Emission of Radiation.

Excited atoms emit a narrow range of wavelengths. Note that the wavelength of the light must match the cavity length just right. The round trip distance in the cavity must be an integer number of wavelengths for constructive interference. In a gas laser with a very long cavity, this isn't a problem. Suppose a frequency where 1,000,000,000 wavelengths makes a round trip isn't quite perfect. A slightly higher frequency where 1,000,000,001 wavelengths is a round trip might be better. There may be several possibilities within the range emitted by the medium.

Laser diodes have much shorter cavities. They may still have multiple possible wavelengths (or modes). But there will not be as many. Frequencies where 100 or 101 wavelengths fit are more widely separated.

If you shine a second laser into the cavity, it most likely will not be exactly lined up with the cavity mirrors. Two independent diode lasers will have different cavity lengths. They are not likely to match exactly the same frequencies. So it will not reflect back and forth many times. It might reflect a few times at best, but it will not contribute to a laser beam.

There are losses in a laser cavity. Some light is absorbed by the medium. Some through the mirror. Some is spontaneously emitted in the wrong direction. For a laser beam to form, amplification must exceed loss.

Light from a second laser is another source of loss. Excited atoms are stimulated to emit in the wrong direction. This might be enough to make loss exceed amplification, and stop the laser.

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    $\begingroup$ After additional testing I think your right. Here are some results I have discovered: If one laser is removed and replace by a mirror I am unable the turn off the laser with its own light. Not sure why but seems to be the case. In the two laser configuration if I slowly decrease the beam of the “dominant” laser by rotating polarizing film the dominant beam fades as the other increases. $\endgroup$
    – Lambda
    Commented Jan 26, 2020 at 18:11

This is really a comment:

Lasing is a quantum mechanical effect. This means that the quantum mechanical wave functions have to take into account the boundary conditions of the whole set up.

See this MIT video where it shows that when there is in total blackout interference of two beams the photons go back to the laser that gave them substance.

I suspect if your two lasers are of the same frequency hitting the atoms giving the stimulated emmision the lasing crystal of one of them gets total "black out " interference in stimulating an emission, depending on the distance between them and the phases.

Edit after comments:

In this link the question is raised whether independent light beams interfere, and a lot of links are given that answer in the affirmative with experiments done from the beginning of laser research. Also the effect is used commercially:

Photonic structures such as gratings can be rapidly fabricated by laser interference lithography, where multiple laser beams are overlapped in a photosensitive material. The spatial intensity distribution of the interfering beams is translated into a physical structure of the photoresist.

So independent coherent beams do interfere. My comment above tried to look at it at the quantum mechanical level, where individual photons are taken into account as well as energy conservation.

It seems that the question did not catch the attention of experts in optics following this site.

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    $\begingroup$ Given the linewidth of a typical cheap laser diode, any destructive interference between two separate lasers would last less than a microsecond. $\endgroup$
    – The Photon
    Commented Jan 25, 2020 at 17:37
  • $\begingroup$ @ThePhoton I m not talking of interference of the two beams. If you watch the video I gave a link for, you will see I am talking of the system, including the two lasing media. $\endgroup$
    – anna v
    Commented Jan 25, 2020 at 17:48
  • $\begingroup$ In that case how does your answer relate to OP's question, which was about light from one laser entering a second laser's aperture and causing the second laser to turn off? $\endgroup$
    – The Photon
    Commented Jan 25, 2020 at 17:51
  • $\begingroup$ Since I posted I have more info: Not sure this happens with all lasers. The lasers I have used are focusable three wires (red, yellow and black)TTL 35khz lasers. I have them connected so they stay on. Red and yellow together to one side of the battery and black to the other. Sorry for this late info. Did consider it relevant until now. $\endgroup$
    – Lambda
    Commented Jan 25, 2020 at 17:54
  • $\begingroup$ Just tried it with a simple two wire laser diode and it still turns off when another laser is directed into it. So the question is what’s causing this to happen. It seems that only a laser of the same or close to the same frequency works. A blue laser will not turn off a red. $\endgroup$
    – Lambda
    Commented Jan 25, 2020 at 18:02

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