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.
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?"