Scientists from Aston University (Birmingham, England) have demonstrated what they believe to be the world’s longest laser.1 The 75-km fiber-laser cavity provides virtually lossless transmission for a communications signal at 1550 nm.
In long-distance-communication fiber links, distributed amplification can offset the attenuation that the signal would otherwise experience. Different techniques have been proposed to achieve an even distribution of the gain along the fiber span, in the hope that eventually lossless transmission would be achieved. Amplified-spontaneous-emission (ASE) noise production also needs to be controlled in any workable scheme. The Aston group has provided a solution to the problem by using the transmission fiber as a Raman laser, obtaining quasi-lossless transmission and minimizing ASE noise.
The light from two equal-power primary pump lasers emitting at around 1365 nm is launched at both ends of a silica based single-mode fiber (SMF) span. This bidirectional pumping arrangement is combined with two fiber-Bragg-grating reflectors positioned at either end of the fiber span. The central wavelength of the gratings is set to be 1455 nm, matching the primary Stokes peak of the pump laser in the fiber. The pair of gratings and the fiber span form a cavity, and if the power of the primary pumps is above the threshold necessary for stimulated Raman scattering to overcome the fiber attenuation at the central wavelength of the reflectors, the whole span becomes an ultralong laser. In this way, a dynamically stable secondary bidirectional pump at 1455 nm is generated in the cavity.
The signal beam at 1550 nm matches the Stokes wavelength for the 1455-nm pump and can be efficiently amplified with a nearly constant gain that closely matches attenuation along the fiber. The specific wavelengths of pumps and gratings used in the demonstration are not necessarily fixed for all applications, and can be modified to transmit signals in a different band.
Experimental work by the Aston group has demonstrated quasi-lossless transmission over fiber spans up to 75 km, as well as almost zero optical-signal variation over shorter distances of 25 km or less. The group has also provided a theoretical analysis that shows excellent agreement with the experiments.
“We have implemented experimentally for the first time a quasi-lossless fiber span that requires only two laser pumps,” says Juan Diego Ania-Castañón, who proposed the scheme in 2004 and led the research team. “Thanks to their extreme length, which is to the extent of our knowledge the longest achieved to date, lasers belonging to this new family exhibit very interesting properties themselves, and more important, their behavior as lossless links may have an immediate impact on different fields. This lossless fiber medium also exhibits potential multichannel capacity with a nearly flat gain response over a 36‑nm bandwidth, and can be exploited not only in optical communications, but also in all-optical nonlinear-data-processing applications, opening the possibility for the design of novel photonic devices based on a mathematical theory of integrable nonlinear systems.”
REFERENCE
1. J.D. Ania-Castañón et al., Phys. Rev. Lett.96, 023902 (2006).