FIBER LASERS: Ultralong Raman fiber laser is virtually lossless

Scientists at Aston University (Birmingham, England) have demonstrated what they are calling the longest-ever fiber laser, with a 75-km cavity length.¹ This virtually lossless fiber span-with amplitude variations below the accuracy of their measurement technique-is the first implementation of a nonlinear system integrated in a fiber waveguide in which power losses as a function of distance are continuously compensated by propagating the signal inside an ultralong Raman fiber laser. Their demonstration is the realization of nearly lossless optical transmission-a decades-long dream for the telecommunications industry.

In the experimental setup, a length of single-mode silica-based optical fiber is pumped at both ends with lasers operating at 1360 nm (see figure). The laser cavity for the secondary pumping wave at 1455 nm is created by two Bragg-grating reflector elements with a central frequency in the vicinity of 1455 nm. When the power in the pump lasers is above the threshold at which stimulated Raman scattering overcomes the fiber attenuation at the central frequency of the gratings, the fiber span becomes an ultralong laser, generating a secondary pump due to amplified spontaneous emission (ASE). The forward and backward propagation of the secondary pump varies little along the fiber length, creating nearly constant Raman gain at the optimized signal frequency of 1550 nm, which is common for telecommunications applications.

The technique is applicable to a variety of fiber span lengths. For the 75‑km fiber span, low-signal-excursion transmission was achieved by pumping the cavity with 1322 mW from the primary pump lasers. The Bragg gratings had a reflectivity of 98% and a center wavelength of 1455 nm. Over the 75-km fiber span, a signal-power variation of 1.35 dB was observed. And experiments on a 20‑km-long fiber span pumped at 351 mW showed virtually lossless transmission of 0.001 dB (predicted variation according to the theoretical model should be lower than 0.005 dB). Using a tunable laser operating between 1520 and 1580 nm, power variation over a broad wavelength range for the 75-km span was observed to be less than 2.5 dB for any wavelength in that range.

The secret to the lossless transmission of the fiber span is a symmetric design of the cavity for the secondary pump and careful selection of the pump-laser wavelengths and power levels as they relate to the Bragg-grating center frequency and length of optical fiber. A series of mathematical calculations help derive the pump characteristics, including the threshold requirement on input pump power and the growth of radiation seeded at the secondary pump.

Practical implementation of the lossless fiber link could be important for transmission with low signal powers (with ASE noise as the main limiting factor) and for systems where fiber nonlinearity plays some positive role, such as in devices using soliton techniques. “We believe that the amplification technique based on ultralong fiber lasers could be a new enabling technology in transmission with very long amplification spans,” notes scientist Sergei Turitsyn. “Even more exciting, the created lossless fiber medium can have interesting applications in all-optical nonlinear data processing, opening methods to the design of photonic devices based on a mathematical theory of integrable nonlinear systems, with functionalities that cannot be achieved in linear optical devices.”

REFERENCE

1. J. Diego Ania-Castañón et al., Phys. Rev. Lett. 96, 023902 (January 2006).