High-power And Coherent Mid-infrared Supercontinuum Sources Based On Dispersion-engineered Fluorotellurite Fibers

2-5 ?m mid-infrared supercontinuum light sources have attracted much attention due to their numerous applications in frequency metrology,molecular spectroscopy,biomedicine,remote sensing and military.Up to now,mid-infrared supercontinuum light sources with high output power(>20 W)are mainly generated in fluoride glass fiber.However,since fluoride glass fibers have poor chemical stability,thermal stability and mechanical properties,those affect the application of fluoride glass fiber in high-power mid-infrared supercontinuum light sources.For this purpose,our research group has explored a new type of highly stable fluorotellurite glass fiber and developed a mid-infrared supercontinuum light source with an output power of more than 20 W by using the fluorotellurite glass fiber as a nonlinear medium.Whereas,the above light source is not all-fibered structure,the energy proportion of the long wavelength region of the spectrum is low,and the coherence of the light source needs to be further studied.During the doctoral years,the author has worked on how to realize all-fiber high-power mid-infrared supercontinuum light sources,how to improve the energy proportion of the long wavelength region of the spectrum and the coherence of light sources.The author achieved a series of research results as follows:(1)By using the improved low temperature fusion technology realized the low loss fusion between fluorotellurite glass fiber and sillca fiber,developed a 25.8 W allfiber mid-infrared supercontinuum light source.In the experiment,a 56 cm fluorotellurite glass fiber with a core diameter of 11 ?m is used as the nonlinear medium and a high-power 1.93-2.5 ?m all-fiber supercontinuum light source as the pump source,the fluorotellurite glass fiber is connected to the output silica fiber of the pump source by direct fusion splicing,the insertion loss is reduced to ?1.65 d B by optimizing the position of the heating zone and the connection point of the heterogeneous fiber.For a pump power of ?42.6 W,we obtained a ?25.8 W all-fiber supercontinuum light source with a spectral range from 0.93 to 3.99 ?m.The corresponding optical-to-optical conversion efficiency is about 60.6%.(2)Designed and fabricated the fluorotellurite glass fiber with double zero dispersion wavelength and the red-shifted dispersion wave with the wavelength of about 4 ?m is generated by using it as the nonlinear medium.In the experiment,a1.98 ?m femtosecond pulse laser as the pump source and the fluorotellurite glass fiber as nonlinear medium,the second zero dispersion wavelength was moved to 3.551 ?m by optimizing the fiber core diameter.For a pump power of ?1 W,the Raman soliton generated in the fiber moved to the second zero dispersion wavelength of the fiber,and at the same time,a red-shifted dispersive wave at 4 ?m is generated as soliton self-frequencyshift cancellation occured.This provides a way to realize 4 ?m allfibered coherent light sources.(3)Designed and fabricated the dispersion-gradient fluotellurite glass fiber and observed the enhancement of red-shifted dispersion wave,the energy proportion of the long wavelength region of the supercontinuum is increased to 85%.In the experiment,a 1.98 ?m femtosecond fiber laser as the pump source,a 3 m long dispersion-gradient fluotellurite glass fiber as the nonlinear medium and it initially has two zero dispersion wavelengths,the fiber core diameter was gradually reduced with the extension of fiber length and the dispersion of fiber gradually changed to allnormal dispersion.With the increase of pump power,the red-shifted dispersion wave moved to the long-wavelength due to the cascaded Raman amplification effect and its intensity increases sharply;when the pump power is 500 m W,obtained a supercontinuum light source with a wavelength range of 1.08-3.065 ?m and the energy proportion of the long wavelength region(2.5-3.065 ?m)is about 85%.(4)Using the dispersion-engineered fluorotellurite glass fiber as nonlinear medium,we developed a 10 GHz octave-spanning optical frequency comb.In the experiment,a fluorotellurite glass fiber with the zero dispersion wavelength of 1.431?m was used as nonlinear medium and the pulse laser of 1.55 ?m with repetition rate of 10 GHz generated by the directly modulated micro-cavity laser was amplified as the pump light,developed the optical frequency comb with a spanning from 900 to2400 nm,the beat-notes at different wavelengths were measured,with a linewidth of about 910 Hz and a signal-noise-ratio of about 44 d B.