Length dependency of modal noise and scrambling effect of fibers under incoherent light

Abstract

Fiber scrambling is important in high-precision calibration systems for radial velocity measurement for searching for exoplanets. As for laser frequency combs, the modal noise of significant laser speckles can occur due to the strong coherence of the light source, which can be effectively suppressed by vibrating the fiber. However, the fibers used for scientific target detection are coupled with polychromatic light from the celestial body. This study focuses on the fiber mode noise and length dependency under white light conditions, and proposes a new fiber scrambling method of combining different types of fiber to achieve high scrambling gain. The results show that the fiber mode noise increases with decreasing length, and that there is also significant mode noise when the fiber is less than 2m, resulting in a speckle-like pattern as the modal pattern in the near field. The combination of non-circular fibers and graded index fibers can effectively reduce mode noise and improve the scrambling gain.