Ultralong Lasers Cavity Length Limits Explored
Juan Diego Ania-Castañón of Instituto de Óptica (CSIC), Spain
[This is an invited article based on a recent work by the author and his collaborators from UK and Russia. -- 2Physics.com]
Author: Juan Diego Ania Castañón
Affiliation: Instituto de Óptica, CSIC, Spain
Since their inception, lasers have been considered simply as sources of light. However, ultralong lasers implemented in optical fiber can also be seen as unique transmission media opening the way to a new outlook on information transmission and secure communications.
In our recent paper in Physical Review Letters  we present a study of the physical mechanisms that restrict the achievable cavity length in a fiber laser cavity, achieving in the process what is to date the longest laser ever built, reaching 270 km.
P. Harper, S. Turitsyn, D. Churkin, A.E. El-Taher (Photonics Research Group, Aston University, UK)
Ultralong lasers, first proposed in 2004  and experimentally demonstrated in 2006 , have been shown to induce virtual transparency in optical fiber, offering quasi-lossless transmission conditions which are ideal for the implementation of soliton-based systems and signal processing. Departing from previous application-oriented studies, we set out on this occasion to explore the fundamental limits of laser operation. This endeavor has resulted in the discovery of interesting new physical regimes of operation, different from those observed in traditional lasers.
S. Kablukov, E.V. Podivilov and S. Babin (Institute of Automation of Electrometry, Russia)
A typical ultra-long Raman fiber laser consists of one or more reels of optical fiber, which act as both the active medium and the potential transmission medium, one or more pump sources that inject radiation into the cavity in order to induce lasing, and a set of fiber Bragg grating reflectors (tuned to the Stokes wavelength) which delimit the cavity. These pump sources are themselves usually standard short-length Raman fiber lasers. The Raman frequency shift in optical fiber is of ~ 13 THz, which translates into roughly 100 nm at the usual infrared wavelengths used for telecommunication.
Taking advantage of the reduced attenuation offered by standard optical fiber in the telecommunication spectral window, we were able to strech cavity length up to 270 km while still retaining a resolvable cavity mode structure, confirming the formation of an ultralong standing electromagnetic wave. Our initial pump sources at 1450 nm were used to induce lasing in the cavity at the corresponding Stokes wavelength of 1550 nm.
Of course, such an enormous resonant cavity presents a similarly extraordinary number of longitudinal cavity modes . Indeed, the observed spectral separation between modes clearly follows the classical formula ∆ν =c/2nL, where n is the refractive index of the fiber core, c the speed of light and L the cavity length, which for a typical grating bandwidth of 100 GHz, brings the number of modes to the hundreds of millions. These modes are broadened and eventually washed out as they interact with each other through intensity-dependent, turbulent-like four-wave mixing processes in the fibre, meaning that in order for the mode-structure to be resolvable at such extended cavity lengths, Stokes wave intensity must be kept low.
Perhaps even more interestingly, at such long cavity lengths there is an additional physical effect that contributes to the washing out of the cavity modes: Rayleigh backscattering. The random backreflection of Stokes photons by Silica molecules along the optical fiber forms a family of overlapping cavities of randomly varying length. Our calculations show that for a system such as ours, the amount of radiation reflected in these random scatterings and the amount of radiation reflected at the ultra-long laser cavity gratings themselves become comparable when the length of fiber is of the order of 250 km, in agreement with the observed experimental limit for mode resolution.
Ultralong lasers already present unique applications in a variety of areas ranging from the implementation of effectively lossless broadband transmission links, the posibility of new classical means for secure key distribution and the design of highly efficient supercontinuum sources, but most importantly, they represent a new and rich field of study that combines diverse areas of Physics such as nonlinear science, the theory of disordered systems or wave turbulence. This richness allows us to anticipate that new applications and technologies of ultralong lasers will continue to emerge in the future.
 S.K. Turitsyn, J.D. Ania-Castañón, S.A. Babin, V. Karalekas, P. Harper, D. Churkin, S.I. Kablukov, A.E. El-Taher, E. V. Podivilov, and V. K. Mezentsev, “270 km Ultralong Raman Fiber Laser”, Phys. Rev. Lett. 103 133901 (2009). Abstract.
 J.D. Ania-Castañón, “Quasi-lossless transmission using second-order Raman amplification and fibre Bragg gratings”, Optics Express, 12(19), 4372-4377 (2004). Abstract.
 Juan Diego Ania-Castañón, Tim J. Ellingham, R. Ibbotson, X. Chen, L. Zhang, and Sergei K. Turitsyn, “Ultralong Raman Fiber Lasers as Virtually Lossless Optical Media”, Phys. Rev. Lett. 96 23902 (2006). Abstract.
 S. A. Babin, V. Karalekas, P. Harper, E. V. Podivilov, V. K. Mezentsev, J. D. Ania-Castañón, and S. K. Turitsyn, “Experimental demonstration of mode structure in ultralong Raman fiber lasers”, Opt. Lett. 32(9) 1135-1137 (2007). Abstract.