Is it possible to build two mirrors in vacuum such that a light pulse (single photon) gets refleted many times in order to increase the distance it travels? For example like that?

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Say I want the photon to travel 1 km. The two mirrors have a length of 1m and a distance of 10 cm. Hence I need the pulse to bounce back and forth approximately 10,000 times.

Is it possible to build something like that?

Another question is: What happens to the pulse? Will it disperse? How will the incident pulse differ from the outgoing pulse?

And at least: Is there any advantage over an optical fiber with a length of 1 km? Or is it even worse (concerning losses and dispersion)?


2 Answers 2


Yes, this is exactly the principle used in an etalon.

A notable example of this is the LIGO experiment to detect gravitational waves. LIGO uses arms that are 4 km long, but the mirrors at each end bounce the light 150 times (75 round trips) so the optical length of the arms is 600 km. It's this large optical path length that enables the tiny effects of gravitational waves to be detected.

  • $\begingroup$ Thanks for your response. But my question was focused more on the particular numbers I gave and the question for the difference between an optical fiber and two mirrors to increase the path length. And how will the device affect the pulse? $\endgroup$
    – thyme
    Aug 7, 2014 at 12:56

For every reflection in the mirror you will loose some light. In my knowledge the best polished mirror has ~ 99% reflecting ability. But lets stick to 90%. A common mirror has less than 10% reflectivity. After 10000 reflections the intensity of the out going light is $(0.9)^{10000}$ of the incoming light. On the other hand, optical fiber uses total internal reflection. So there is no loss of light. You don't need a optical fiber 1km long. If you know the radius of the fiber and you know the critical angle, with a similar method you can find out the length of the fiber you need.

  • $\begingroup$ 1. Even 0.999^10000 is close to zero (0.000045); 2. One major difference between mirrors in vacuum and the optical fiber is that the speed of light in the fiber is much lower than in vacuum and depends on the material used. $\endgroup$
    – johnfound
    Aug 7, 2014 at 16:13
  • $\begingroup$ In 1km the speed hardly matters. In vacuum it takes about 0.33*10^(-5) sec. In optical fiver it takes around 0.5*10^(-5) sec. $\endgroup$ Aug 7, 2014 at 16:20
  • $\begingroup$ Well, it depends on why this path is needed. For example if the OP wants to measure the speed of light in vacuum, using optical fiber will not work at all. If he wants to measure something else, but the speed of light is part of the process, using optical fiber ca ruin the precision, because of not clearly known speed of light. $\endgroup$
    – johnfound
    Aug 7, 2014 at 16:27

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