Diameter Control Of Micro/nanofiber And Its Application In Mode-locked Laser

Micro/nanofiber has a variety of excellent optical characteristics and has been widely used in recent years.At present,the precise preparation of micro/nanofiber is the key to its further development and large-scale application.Based on the previous researches on the preparation of micro-nano fibers,this paper proposes a convenient and scalable micro/nanofiber(MNF)fabrication technology based on the principle of glass fiber and the evolution of the mode during the fiber stretching process,which has the characteristics of highly accurate diameter control.When a certain range of light wavelengths(for example,400 nm-1000 nm)are coupled into a 1550 nm single-mode fiber(SMF-28e),if the fiber is tapered under the adiabatic condition,as the fiber gradually tapers,the original fiber mode is gradually coupled into the MNF mode.When the fiber continues to be thinner,each higher-order mode existing in the MNF will be cut off in turn,which will be reflected as a sudden change in the spectral transmittance.In the end,only the fundamental mode exists in the MNF.Based on this,we have built a fiber taper system to achieve precise diameter control.Unlike earlier work,we used a white light source and a marine spectrometer to monitor the transmittance of each wavelength during the fiber drawing process.Since the cut-off of a specific mode at a specific wavelength will correspond to a particular cut-off diameter,we can use the sudden decrease in transmission intensity caused by the cut-off of the TE01 mode as an indicator signal to stop the taper process.In the experiment,different wavelengths are selected in the wavelength range of 400 nm-1000 nm to obtain different indication signals,so that an MNF with a desired diameter can be fabricated in a specific diameter range.The diameter control range of 360 nm to 680nm has been achieved in experiments,and the accuracy and precision of diameter control have reached 2nm and 3nm,respectively.If the cutoff of other higher order modes is selected as the signal,this method can be used to extend the diameter control range to a few microns.On the other hand,this paper describes the second-order and third-order dispersion characteristics of MNFs at 2μm,and introduces the experimental setup and mode-locking mechanism of a 2μm mode-locked fiber laser.Because we can precisely control the diameter of MNF,it can be used as a dispersion adjustment element in mode-locked fiber lasers.By measuring and calculating the dispersion of MNFs,an MNF with a diameter of 1μm and a uniform length of 10 cm is selected as the positive dispersion element.In this experiment,the net dispersion in the cavity was adjusted by changing the length of the common fiber,and various pulse spectra were output.Based on the output spectrum,the MNF’s ability to compensate for second-and third-order dispersion was experimentally verified,and a 50 nm wide spectrum and a 195-fs pulse width dissipative soliton pulse were finally achieved.In particular,due to the MNF provides third-order dispersion compensation for the laser cavity,an almost flat and symmetrical output spectrum is obtained experimentally.Compared with the previous research of dissipative soliton fiber lasers,we are more effective for dispersion management in mode locking,and also make the whole device more fiber-optic and flexible.Looking to the future,measures such as adopting a hybrid mode-locking scheme,adding a band-pass filter,or designing a shorter laser cavity will further allow 2μm dissipative soliton to have higher pulse energy,wider spectrum,and shorter duration.