Fiber-based laser and amplifier systems provide high output power, excellent beam quality and easy handling by utilizing a setup consisting of fiber components like signal-pump combiners and cladding-light strippers. Large-mode-area fibers are used in these systems to improve the performance while maintaining single-mode operation. Compared to pure single-mode fibers, these fibers always guide higher order modes, which can have an impact on the system’s performance. An instrument was developed based on the S2-method, which is able to perform high-speed, in-line measurements of the modal composition and polarization extinction ratio. This device is used to monitor the fiber component manufacturing process using large-mode-area fibers in real-time, enabling us to improve critical process steps and, thereby, optimize the manufactured components, namely signal-pump combiners and fiber endcaps.