A good source of fibre

Abstract

An Erbium-doped ultrashort pulse fibre laser that matches or surpasses the best solid-state sources is presented in work from Shanghai Jiao Tong University. Based on commercially available components, its creators believe this design could lead to compact, cost-effective, highly stable and reliable laser sources for a vast range of demanding ultrafast laser applications. The experimental setup in the lab at Shanghai Jiao Tong University. The laser is based on commercially available components to provide a more accessible source Ultrashort pulse lasers are important for numerous applications, including micro-machining, surgery, femto-chemistry, terahertz generation, and medical imaging. Some applications, like high-speed fibre communication systems, frequency metrology, photonic analogue-to-digital converters and optical coherence tomography (OCT), also require laser sources that have broadband spectrum and high repetition rate. Fibre lasers, in which the laser amplification medium is a rare-earth doped gain fibre, present a potential alternative to more bulky, expensive and complex solid-state ultra-fast lasers, but only if they can be made to match the performance characteristics – pulse duration, spectrum width, timing jitter and repetition rate – of the best solid-state sources. In their current Electronics Letters paper, the team from Shanghai Jiao Tong University present an Erbium-doped fibre laser that has all these characteristics, combined with simplicity, and is based on commercial components without using any complex fibre structures or materials. The 1550 nm laser has demonstrated 34.3 fs pulses at a fundamental repetition frequency of up to 200 MHz with average output power of 38.5 mW. Its time jitter integrated from 1 kHz to 10 MHz is 24 fs and it generates a broad and flat spectrum with a full width at half magnitude of 148 nm. The main challenge for the Shanghai Jiao Tong researchers was delivering ultrashort pulses whilst achieving high repetition rates. In fibre lasers, gain and loss are fixed by the fibre characteristics in the laser cavity and pulse shaping is regulated by the mutual interactions between chromatic dispersion and optical nonlinear effect. The output characteristics of stretched-pulse fibre lasers are determined primarily by the net group velocity dispersion (GVD), of the whole cavity and, to a lesser extent, by the lengths of the fibres composing the cavity. With regard to optical nonlinear effect, if the accumulated nonlinear phase shift is too small, mode-locking action of laser, based on nonlinear polarisation rotation, can't be implemented effectively. On the other hand, excessive nonlinear phase shift can saturate the mode-locking behaviour and lead to optical wave breaking or multiple pulse operation. In this work, these two factors have been controlled through careful selection, combination and arrangement of three different commercial fibres to form the laser cavity. The resulting cavity has a net GVD near zero and optimally controlled nonlinearity. The polarisation state in the cavity is optimised via fine tuning of the waveplates and pump power, and a prism pair is used outside the cavity to provide external pulse compression (dechirping) of the broadband laser. “It has a lot of potential for applications in wavelength-division multiplexing-based optical sampling to achieve hundreds of parallel channels, and in long haul optical communications networks, where a hundred different closely spaced channels of data are simultaneously transmitted in an optical fibre, with each channel sent by a slightly different laser wavelength,” said team member Prof. Weiwen Zou. The pulses are shorted from 49.1 fs to 34.3 fs by dechirping the output from the laser cavity using an external prism pair “This laser already satisfies many of the requirements for ultrafast optical science. It could increase the performance and range of applications, including femtosecond laser frequency comb generation for optical ultra-precise distance measurements, ultrahigh-resolution molecular spectroscopy, large dynamic range and high-speed fibre Bragg grating strain sensor interrogation, and high-resolution OCT, which have been extensively demonstrated in recent years.” The team say that the laser could be used almost immediately in high-speed and high-resolution photonic analogue-to-digital conversion systems that require hundreds of parallel channels, where electronic analogue-to-digital converters are severely limited; as well as ultra-low phase noise generation of radiofrequency and microwave signals for applications of coherent radar imaging systems, and scientific facility synchronisation.