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I read this paper (the last sentence of Section 3.1). It is stated that in fiber optic interferometric sensors, the interference pattern cannot be detected if visible light is used because most fiber optics are transparent for longer wavelengths. From my understanding, the interference pattern is more apparent when coherent and monochromatic light source is used.

I have shone a 650 nm laser into a plastic fiber optic and the image below is the output of the fiber.

enter image description here

As you can see, there is still some speckle pattern. So, here comes my question: "Is visible light appropriate as the light source for fiber optic interferometric sensors or do we have to use infrared light?"

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It is not necessary to use infrared light in a fiberoptic interferometric sensor. Any coherent light that is transmitted by the fiber well enough to return light with a reasonably measurable intensity can be used. Speckle is not a serious issue, because interferometry can fairly easily cope with stationary speckle.

The difference between visible and infrared transparency of an optical fiber only becomes important when the length of the light path in the fiber is extremely long, resulting in large absorption losses due to less-than-excellent transparency.

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  • $\begingroup$ Thank you for your answer. It really helped! However, I have a follow up question: Does it still apply to intrinsic fiber optic sensors, in particular, acutely bent fiber optic in which Whispering Gallery Mode (WGM) is induced? I am wondering if the visible light from a laser can maintain its coherent property despite transmitting through a bent fiber optic. $\endgroup$ – Xenon Oct 30 '20 at 7:19
  • $\begingroup$ @Xenon I think the only issue is possible large loss at bends if the delta-n between core and cladding at visible wavelengths is small. Remember that fiber optics depend on "total internal reflection" to keep the light in the core. $\endgroup$ – Carl Witthoft Oct 30 '20 at 12:03
  • $\begingroup$ @CarlWitthoft thanks for the answer. I am just wondering if a coherent source would lose its coherence upon traversing from different media since the WGM eventually couples back with the core mode at the end of the bending region. $\endgroup$ – Xenon Nov 2 '20 at 8:14
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    $\begingroup$ Coherence can be a rather complicated and subtle thing. The simple answer is "No, the light does not lose its coherence in traversing from one medium to another". However if there is scattering at the interface it will effectively lose its spatial coherence. If the interface is moving as the light traverses it, the light can effectively lose its temporal coherence. However in both cases, if the information is not lost concerning the scatterer or the motion, then in principle & in practice the coherence can be "restored"; that's why I say "effective" coherence. $\endgroup$ – S. McGrew Nov 2 '20 at 15:55

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