Capacitive sensor

I am trying to build a sensor to detect the optical fiber inside soil. Can we use capacitive sensor to detect the optical fiber inside soil as there is a difference between the dielectric constant of soil with water and the optical fiber.

Your suggestion will be really helpful.

You are correct but you need to consider the differences in mass between the fiber and the soil as well as the ratio between them. The deeper the fiber is buried the harder it becomes. I as assuming this is a fiber cable buried in the ground. The jacket may be better for sensing. For starters you might want to set up some experiments to see what the differences are so you can start with enough knowledge to design the sensor. You will probably come up with the same result it is not possible with today’s technology at a reasonable price. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!

Are the fibres held in a steel armoured cable ? In which case detecting the steel would be the way forward

Sure there'll be a difference in dielectric constant for soil + fibre vs. just soil, but is this difference big enough? Compared to different types of soil? Compared to wet and dry soil?

I'm afraid that without a metal jacket around that cable you won't be able to find it, and even than you'll only be able to find it if it's pretty close to the surface (have a look at metal detectors and how deep they reach).