Abstract: A downhole optical sensing system can include an optical fiber positioned in the well, the optical fiber including multiple cores, and one of the cores having a Brillouin scattering coefficient which is different from another one of the cores. A method of sensing strain and temperature as distributed along an optical fiber can include measuring Brillouin scattering in a core of the optical fiber disposed in a well, and measuring Brillouin scattering in another core of the optical fiber disposed in the well, the optical fiber cores being exposed to a same strain and temperature distribution in the well.

Abstract: A downhole optical sensing system can include an optical fiber positioned in the well, the optical fiber including multiple cores, and at least one well parameter being sensed in response to light being transmitted via at least one of the multiple cores in the well. The multiple cores can include a single mode core surrounded by a multiple mode core. A method of sensing at least one well parameter in a subterranean well can include transmitting light via at least one of multiple cores of an optical fiber in the well, the at least one of the multiple cores being optically coupled to a sensor in the well, and/or the at least one of the multiple cores comprising a sensor in the well, and determining the at least one well parameter based on the transmitted light.

Abstract: A downhole optical system can include an optical fiber disposed in a subterranean well, and an optical fiber amplifier which amplifies optical power in a core of the optical fiber in the well, the amplifier being optically pumped with optical power in another core of the optical fiber in the well. Another downhole optical system can include an optical fiber disposed in a subterranean well, the optical fiber comprising multiple cores, and an optical fiber amplifier comprising multiple cores optically coupled respectively to the optical fiber multiple cores in the well.