Gaining chirped pulse amplification with fiber only

Lynn Savage, lynn.savage@photonics.com

Optical parametric amplification is a common method for improving laser-based telecommunications by boosting signal strength across long lengths of fiber optics. If you chirp the signal pulses – stretching and recompressing each pulse – you get amplified broadband pulses that have high gain, increased conversion efficiency and several other benefits. What you don’t have, however, is a simple system, because the technique, dubbed optical parametric chirped pulse amplification, requires the incorporation of bulky nonlinear crystals several millimeters long that further demand careful alignment at all times.

A schematic shows the experimental setup of a fiber-based optical parametric chirped pulse amplifier. FOPA = fiber optical parametric amplifier; RF = radio frequency; TL = tunable laser; PM = phase modulator; EDFA = erbium-doped fiber amplifier; PC = polarization controller; HNLF = highly nonlinear fiber; CFBG = chirped fiber Bragg grating; PBS = polarization beamsplitter. Reused with permission of the Optical Society of America.

Now, researchers led by Arnaud Mussot of Université de Lille 1 have streamlined the amplification process by making it an all-fiber system. The results are promising not only for improving telecommunications systems but also, perhaps, for advancing studies of the interactions between laser light and matter.

Fiber-based optical parametric chirped pulse amplification systems were proposed several years ago, but, according to Mussot, the original group did not have the ability at the time to develop the photonic crystal fiber needed to replace the nonlinear crystal typically used to create chirp. His group, however, did.

Mussot and his colleagues at Université de Mons in Belgium, at Commissariat à l’Energie Atomique (CEA) in Le Bop and at the University of Alcalá in Alcalá de Henares, Spain, integrated a linearly chirped fiber Bragg grating into a fiber optical parametric amplifier. They created pulses using a tunable continuous-wave laser as a seed source and a microwave source to modulate the beam.

The 15-cm-long chirped fiber Bragg grating stretched the picosecond-scale pulses of 1550-nm light by 24.6 dB and then compressed the pulses to their initial length without significant distortion.

The group reported its technique in the June 1, 2010, issue of Optics Letters.

According to Mussot, the team currently is adapting the technique to work with femtosecond-scale pulses at about 1 µm, which would be useful for laser-matter experiments. Working in this wavelength range requires the development of novel photonic crystal fibers and the use of an Yb-doped mode-locked fiber laser as the seed signal. If successful, the gain in such a system is expected to be greater than 60 dB.

The technique’s potential is not all that excites Mussot, however. The collaborative effort between his university and CEA is paying major dividends.

“It was a strong collaboration between these two institutes, [and it] is still running,” he said.