Generation of flat supercontinuum in specialty optical fibers
Supercontinuum (SC) laser sources exhibiting excellent wide bandwidth and good beam quality have attracted much attention because of their potential applications including spectroscopy, optical coherence tomography, chemical detection, biomedicine, as well as homeland security, etc. SC is generally obtained by pumping the highly nonlinear media by ultra-short pulses. When the ultra-short pulse is transmitted in the nonlinear medium, the spectrum of the pulse will produce spectral broadening under the combination of nonlinear effects and dispersion, thus forming a SC spectrum. Studying the generation of broadband flat supercontinuum is not only academically significant, but also very important in practical applications. Especially, the flatness of the spectrum is of great interest, because it can improve the accuracy of measurement in practical applications.
Schematic diagram of supercontinuum generation
This paper reviews the state of art for generating and improving SC spectral width and flatness in specialty optical fibers. The specialty optical fibers involved include photonic crystal fibers, soft glass fibers as well as germanium doped fibers. Especially, Fluoride fibers (especially ZBLAN fiber) have lower transmission loss and higher damage threshold in the mid-infrared wavelength range, and using it as a nonlinear medium can extend the long wavelength limit of SC to the mid-infrared. The 2~5 μm mid-infrared wave band contains several relatively transparent and low-loss windows in the atmosphere, and the characteristic lines of many important molecules are also distributed in this wave band. Therefore, the flat SC source covering mid-infrared wave is very important in practical applications such as atmospheric remote sensing, infrared imaging, and biological detection. In practical applications, the broadband flat SC sources can not only improve the accuracy of measurement, but also reduce the difficulty of power balance. In addition to the review, this paper proposes the future research direction of flat SC sources.
The research group of Huanhuan Liu from Shanghai University is dedicated to studying the effects of initial pulse center wavelength, peak power, initial pulse width, chirp, as well as nonlinear effects in nonlinear fibers on SC flatness. In order to make supercontinuum better applied to various fields, Liu Huanhuan's research team focused on exploring solutions to improve spectral flatness. The key technology is using chirped pulse amplification and dispersion compensation to achieve large energy narrow pulse output, and then using the optimized narrow pulse to pump fluoride fiber to generate SC. Through continuous improvement, at present, they have realized SC generation with a good spectral flatness with fluctuation of less than 1.29 dB and the obtained SC source has potential application value in the optimization of deep biological imaging in short-wave infrared region.
Liu H H, Yu Y, Song W, Jiang Q, Pang F F. Recent development of flat supercontinuum generation in specialty optical fibers. Opto-Electron Adv 2, 180020 (2019).