Single-Mode Multicore Fibers With Integrated Bragg Filters

Fiber Bragg gratings (FBGs) in single-mode fibers, used as narrowband filters, have reached a high maturity level, especially for applications in telecommunications and sensing. In spectroscopic applications, e.g., in Raman spectroscopy, FBG filters can be used to separate laser probing light from generated sensor light which simplifies Raman probes. For this purpose, light from a sample has to be collected with high efficiency into a fiber. The light collection efficiency in single-mode fibers is, however, strongly limited due to the small fiber core area. Therefore, we have investigated the possibility to use single-mode multicore fibers to increase the light collection efficiency while maintaining Bragg filter performance. An optical density filtering effect of 1.5, which is equivalent to a transmission of -15 dB, has been achieved with a FBG in a 61-core fiber. Simulations have been performed in order to clarify the influence of the fiber geometry and refractive index distribution on grating performance. The application of chirped gratings in the multicore fibers improves filter performance in comparison to homogeneous gratings.