Dear people of laser science, my understanding of fiber optic gyro is: a single laser beam runs into a cube splitter, then the 2 beams are driven into the opposing ends of a fiber (coil). Then in the fiber they encounter a relative phase shift in case the coil rotates. Finally their interference causes a measurable intensity change that correlates to the speed of rotation.

My "proposal" is to drive the light into one end only of fiber (after the splitter), then on the opposing end couple up the fiber with a phase conjugation mirror.

The beam travels back along the fiber and runs again through the splitter ending up in the detector. Would it improve the system or nothing good comes of it? My theory is that the "time reflected" (phase conjugation mirror) counter-beam may eliminate some error and may extend the length of coil for greater resolution. ?? doubled up phase shift ??


If I undertsand your scenario correctly the two beams overlapping at the detector would be 1) a beam that has travelled through the length of the fibre, and back; and 2) a beam that hasn't travelled through the fibre at all.

There's a couple of problems with this:

  • gyroscope fibres are often very long (kilometres), but the coherence length of lasers is not that long (typically up to tens of centimetres, but coherence lengths of 100m or so are possible), so you wouldn't get a useful interference signal
  • when sending the same beam back, your beam would be phase-shifted in one direction on its way out to the mirror attached to the end of the fibre, and then shifted back when travelling back in the opposite direction (by the same amount if the angular velocity doesn't change)
  • $\begingroup$ oops, i see (the beam that did not pass the cable) $\endgroup$ – Leventete Jan 10 '20 at 23:32
  • $\begingroup$ oops, i see (the beam that did not pass the cable). that part i dodged poor me. back to the standard layout FOGs they advertise with upto 5km coil length. how it goes through the coherence problem you mentioned? $\endgroup$ – Leventete Jan 10 '20 at 23:49
  • $\begingroup$ i've read up around coherence trouble (thanx to come up with that). looks like "fiber lasers" can generate highly coherence laser light, up to 100km. (ok, nothing for free). then i have another question about use of phase conjugation mirror. let suppose i use a "cheap" laser source with short coherence length. i split it, drive it into the coil's ends. then i break the middle length of coil, push the messed up beam through the "time mirror" and let the reflection travel the next half of coil. (as an amplifying station). so 1 "time mirror" for each direction. on the ends back to good? $\endgroup$ – Leventete Jan 11 '20 at 13:29
  • $\begingroup$ In the standard FOGs the coherence length isn't really a problem as long as the difference in path length between the two arms (which you use to detect the rotation) is smaller than the coherence length. $\endgroup$ – Holzner Jan 13 '20 at 10:46
  • $\begingroup$ oh, is it like there is a coil for the cw running light and separate coil for the ccw running light? not the same fiber for both? my gosh, me such newborn $\endgroup$ – Leventete Jan 13 '20 at 22:27

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