Random distributed feedback fiber laser offers a new possibility, since the feedback is provided by Rayleigh scattering, which is also available at any wavelength. The wavelength tunability of random Raman fiber lasers was studied by cascaded Raman scattering, which can emit at discrete wavelength. Continuously wavelength tuning within a single Raman shift had achieved a less than 35 nm range. Recently, by combining cascaded Raman scattering and tuning of pump laser wavelength, continuously wavelength tuning of 300 nm was reported. Further wavelength extension resulted in reduced spectral purity and continuum spectrum.
Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM) clarified the mechanism which prevents the wavelength extension. They choose specialty silica fibers with zero dispersion wavelength longer than 2 μ m as the gain medium of the random distributed feedback Raman fiber laser. With an improved laser setup, they demonstrated a fiber laser which can be continuously wavelength tuned from 1 to 1.9 μ m. To the best of their knowledge, it is the widest wavelength tuning range ever reported for continuous wave fiber lasers.
The study was published in Scientific Reports
Figure. Continuously wavelength tuning from 1 to 1.9 μm. (a) Output spectra plotted for every 20 nm from 1 to 1.9 μ m. (b) Output power and inband power ratio as a function of wavelength. (Image by Zhang Lei)