Modal Noise Mitigation and Scrambling Performance of a Compound Scrambler

Abstract Fiber scrambling is integral to high-precision calibration systems for radial velocity (RV) measurements, crucial in the quest for exoplanet discovery. Coherent light sources, like laser frequency combs, often induce laser speckles, a form of modal noise, which can result in spectra calibration errors. The starlight collected in astronomical observations is often polychromatic light. This study investigates the speckle-like mode noise generated under incoherent light, identifying a clear correlation between fiber length and the intensity of this noise. Remarkably, experiments using LED illumination showed that fibers shorter than 2 m exhibit significant speckle-like noise, manifesting as distinctive ripple-like structures, a newly identified phenomenon referred to as modal patterns. Mitigation techniques, including squeezing and vibrating methods, can reduce contrast and suppress modal patterns effectively. To further enhance scrambling performance, a novel fiber scrambling approach, the compound scrambler, is proposed. This design integrates laser polishing, thermal heating, and fiber tapering techniques to enhance near-field uniformity. Utilizing a combination of non-circular and graded index fibers, the compound scrambler achieves notable improvements in scrambling gain (SG). A refined nondestructive fabrication method, leveraging the superior scrambling effect of non-circular fibers and the efficiency of adiabatic taper structures, achieves a high SG of 1064. These findings contribute to advancing high-precision spectrograph design, offering practical solutions to enhance RV measurement accuracy. The compound scrambler's integrity and modular design promise stability, longevity, and scalability, holding immense potential for astronomical instrumentation.