Proc. SPIE. 11128, Infrared Remote Sensing and Instrumentation XXVII
KEYWORDS: Optical fibers, Fiber amplifiers, Optical amplifiers, Modulation, Signal attenuation, Remote sensing, L band, Wavelength division multiplexing, Single mode fibers, Time metrology, Dense wavelength division multiplexing, S band
Optical fibers are becoming commonly used beside data transmissions for dissemination of ultra-precise and stable quantities or alternatively as distributed sensors of for example acoustic and mechanic vibrations, seismic waves, temperature etc. There have been developed methods for these transfers and their stabilization, allowing thus to achieve excellent performances. Such performance is bound with utilization of single physical medium for both ways of propagation. These methods are attractive both for very high-performance applications and as a secure alternative complementary to radio and satellite-based transfer methods. From economical point of view, sharing fibers with regular data traffic is an advantage, especially for longer distances and large infrastructures. Unfortunately, the most often used wavelengths are located almost in the middle of telecommunication band. Due to continuous data traffic growth and utilization of flexible spectral allocation, the collision in wavelength plan will occur more and more often. In this paper we overview alternative wavelengths suitable for these transfers, we also propose suitable methods for all-optical reach extension, by all-optical amplification. Shared line design allowing transfer of ultra-stable quantities in three different spectral bands is proposed and such design is evaluated.
The reach of any all-optical transmission is limited by attenuation of transmission path and other factors as signal to noise ratio, and it can be extended by all-optical amplification. Bidirectional single fibre transmission introduces an issue of bidirectional symmetrical amplifiers in order not to lose advantage of path symmetry. In case of time transfer, quasibidirectional amplification might be acceptable when supported by specific arrangements, e.g. as much as possible equal arrangement for disjoint segments of the path. Time transfer with best available accuracy or optical frequency transfers require single path optical amplifiers that are further considered. In this constitution, unfortunately, reflections together with Rayleigh back-scattering will create feedback. In case feedback is strong enough and discrete amplifier operates in high gain regime (about 20dB), the whole system will start to oscillate. It saturates the gain of amplifiers and also can generate errors, when lasing in a transmission band. In the article, we review possible all optical amplification methods including those allowing to use untraditional transmission bands (outside C band).
Long distance precise frequency and accurate time transfer methods based on optical fiber links have evolved rapidly in recent years, demonstrating excellent performance. They are attractive both for very high-performance applications and as a secure alternative complement to radio- and satellite-based methods. In this paper, we present development of infrastructure for such transmission containing 700+km of transmission lines, with planned cross border optical frequency connectivity. According to our knowledge, this will be the third such line globally. The infrastructure also shares fibers with existing data transmissions, both amplitude and phase modulated, which poses high demands on mutual isolation and insensitivity to cross talks.