Fiber laser reaches record length

Marie Freebody, mariefreebody@physics.org

In a bid to push the boundaries of laser science, a team of scientists from the UK, Spain and Russia has built the longest-ever fiber laser cavity. The group’s aim was to find out just how long it could make a fiber laser cavity in which lasing with a resolvable longitudinal mode structure could still be observed. And the answer is pretty long: 270 km, to be exact.

Such ultralong fiber lasers could be used in future high-speed optical communications systems. Rather than being a source of coherent light, ultralong fiber lasers are an ideal transmission medium for carrying information and providing secure communication.

The record-breaking fiber laser exhibits a resolvable mode structure with a width approximately 120Hz and peak separation of around 380Hz in the radio-frequency spectrum, said Sergei K. Turitsyn, a researcher from Aston University.

“Lasers with such a long cavity and so large a number of modes have never before been studied,” Turitsyn said. “We hope that our results will open up a new research field closely linked with a range of other areas of physics, such as nonlinear science, theory of disordered systems and wave turbulence.”

Despite extraordinary advances in laser physics, only recently have the fundamental limits of laser cavity length become an area of exploration. Turitsyn and colleagues achieved their record-approaching theoretical limits set by relevant physical effects; first, backreflection of light resulting from fiber medium inhomogeneities.

As reported in the Sept. 25, 2009, issue of Physical Review Letters, the technique combines several optical technologies, including the fundamental nonlinear effect of stimulated Raman scattering.

This schematic depicts an ultralong Raman fiber laser design and optical power distribution. Pumping at 1450 nm generates laser radiation at 1550 nm. Reprinted with permission from Turitsyn et al, Physical Review Letters, 103, 133901 (2009). ©American Physical Society 2009.

First, distributed Raman fiber amplification provides a gain medium for signal transmission in standard telecommunication optical fiber over distances of 100 km or more. Second, the laser cavity is formed by fiber Bragg gratings, which act as reflectors for 1.5-µm wavelength light. And, finally, both the pumping and signal waves propagate close to the window of transparency of silica, minimizing fiber losses.

The team believes that new applications and technologies will continue to emerge from studying the physics of ultralong fiber lasers.

“Our results indicate that the physical mechanisms underlying the operation of such lasers involve nontrivial, nonlinear interactions of the resonator modes and are quite different from those in other types of lasers,” Turitsyn concluded. “We have revealed interesting connections between the new field of ultralong fiber lasers and many areas of fundamental science, and we plan to study the physics involved in more detail.”