Photonics Question - Laser and Total Internal Reflection

  1. Hello, this is my first post to this site. I had this idea a few nights ago, and I've been trying to wrap my head around it. Before I go ordering supplies to run some tests, I thought it would be worthwhile to gain some insight from you guys. Anyways, as you probably know, total internal reflection occurs at certain angles when light hits a medium boundary. Light bounces off this medium boundary with total energy being conserved, making this a perfect mirror. I know we utilize this with lasers in fiber optics to transmit information, but my question is this: What would happen if a laser was shown into a fiber optic that was bent to make a doughnut or torroid shape. Would the collimated beam keep bouncing along the inside of the fiber optic medium? What would happen after the laser input was removed? Would the system be self sustaining for a period of time before aberrations and losses took over? You would think this would have been done before for the pure simplicity of the idea. However I have looked all of the internet and nobody has done this. Ideas? Thanks!
  2. jcsd
  3. Drakkith

    Staff: Mentor

    Fiber optic cables absorb light as it travels through. This places a limit on how far a signal can be transmitted before it needs to be amplified. In fact, all mediums do this.
  4. Ah yes, I forgot about attenuation. This still doesn't answer the core of my question however. I'm sure the laser would be able to complete the loop many many times before being absorbed by the optically clear material.
  5. Drakkith

    Staff: Mentor

    Sure, but what does that accomplish? The light is fully attenuated in such a short amount of time, a fraction of a second, that I don't see any useful application. And this certainly wouldn't classify as "self sustaining".
  6. Ring down spectroscopy uses a similar principle to determine gas concentration to ppm accuracy. A high finesse cavity can have thousands of round trips before extinction.
  7. Drakkith

    Staff: Mentor

    Ohh, interesting. From wiki:

    Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseous samples which absorb light at specific wavelengths, and in turn to determine mole fractions down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS)
  8. Bobbywhy

    Bobbywhy 1,864
    Gold Member

    Here’s a company (among several) that sells several different mirrors they claim are 100% reflective. For example, the mirror part number #MHR2X75 is claimed to be 100% reflective at 1.06 microns. Flat/Flat Model 700 Series #M100Rror 100% Reflective@1.06 Micron Flat/Flat Model 700 Series

    Here’s another source of mirrors with wide-band reflectivity above 99.999%:

    Lastly, here is a mirror that reflects 100% of the incident radiation, providing the angle of incidence is 35 degrees! Now, one might construct an array of these mirrors so that the angles of incidence and reflection all equal 35 degrees, and send in a specific wavelength pulse. Then you will have “stored” your light pulse indefinitely!
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