Research Of Dynamic Patterns Mode-Locked Thulium-Doped Fiber Laser Based On Nonlinear Amplified Loop Mirror

At present,compared with the 1 ?m and 1.5 ?m bands,the relevant components and detection equipment in the 2 ?m fiber lasers are very expensive.Therefore,there is not much research on the 2 ?m mode-locked fiber laser.In particular,the dynamic characteristics of nonlinear amplified loop mirror(NALM)fiber laser based on different dispersions need to be explored.In addition,the single pulse energy of most of the modelocked fiber lasers is concentrated on the pJ level and needs further improvement.In this paper,the dispersion-managed Tm-doped fiber lasers are designed and analyzed,and three different types of NALM Tm-doped fiber lasers are realized.Their single pulse energy all reach nJ magnitude,and various multiple output characteristics are obtained,which are of great significance and value in both theory and practice.Its main contents include:1.Firstly,the research background and significance of this paper are analyzed,and the mode-locking principle of the 2 ?m fiber laser and the key factors to achieve the stable mode-locked pulses are also analyzed.The current research status,advantages and disadvantages of real saturable absorbers and artificial saturable absorbers at home and abroad are compared to provide a strong basis for the selection of experimental protocols based on NALM fiber lasers.Then,the basic principle of NALM is analyzed,and the gain medium,pump source and dispersion compensation component are explained in detail.At last,an experimental design scheme of Tm-doped fiber laser based on NALM modelocking is proposed.2.A fully negative dispersion mode-locked Tm-doped fiber laser based on NALM is designed and implemented.The output spectrum of the laser is not the same as the traditional negative dispersion mode-locked fiber lasers and the noise-like mode-locked pulse is achieved.The repetition frequency of the time domain waveform and spectrum is 5.32 MHz,and its size is determined by the resonator length and is accordance with the theoretical value.The pulse signal-noise ratio reaches 60 dB,and the signal-noise ratio of noise-like pulse is generally in the 40~50 dB,which shows that the stability of the noiselike mode-locked fiber laser in this paper is good.The maximum 3 dB bandwidth is 18.28 nm,which is far larger than the traditional soliton mode-locking bandwidth,and the spectrum is very smooth.This is a typical feature of noise-like mode-locking.Moreover,when the pump power is increased continuously,the center wavelength of the spectrum shifts toward the long wavelength,which means that the red shift phenomenon is obtained.In addition,the maximum single-pulse energy reaches 64.2 nJ,which is far greater than the energy of traditional negative dispersion mode-locked fiber lasers.3.Based on dispersion management,ultra-high numerical aperture fiber is added for dispersion compensation,and a NALM zero-dispersion mode-locked Tm-doped fiber laser is designed and implemented.Three different types of mode-locked pulses are obtained at different pump powers,namely Q-switched mode-locked pulses,single-wavelength modelocked pulses and dual-wavelength mode-locked pulses.The time-domain pulse diagram of the Q-switched mode-locked pulses has a typical Q-switched mode-locked pulse train.The center wavelength of the spectrum is 1987.61 nm,corresponding to a 3 dB bandwidth of 7.97 nm.In the single-wavelength mode-locked pulses,the frequency of the time domain waveform and the radio-frequency spectrum is 7.29 MHz,which is consistent with the theoretical value.The 3 dB bandwidth of the spectrum is 11.21 nm,which is larger than the conventional few nanometers.And the spectrum is a smooth Gaussian spectral distribution curve,which is a typical output characteristic near zero dispersion.In the dual-wavelength mode-locked pulses,the time-domain waveform is a pair of double pulse trains,and the spectrum appears with dual-wavelength.And the radio-frequency spectrum has a normal slight modulation.Many dynamic characteristics obtained in zero dispersion NALM fiber lasers are rarely reported.And it provides important value and reference significance for the further study of pulse dynamics characteristics of NALM dispersion management.4.In order to further study the influence of dispersion on the output pulses characteristics,continuing to add dispersion-compensated ultra-high numerical aperture fiber,and a NALM normal dispersion mode-locked Tm-doped fiber laser is implemented.The basic mode-locked pulses and various multiple-pulse sequences are obtained at different pump power.The repetition frequency of the time domain and radio-frequency spectrum is 5.24 MHz,and the signal-to-noise ratio is about 57 dB.When the pump power is increased,the center wavelength appears red shift phenomenon,and the wavelength range is 1997.67~1999.58 nm.Quasi-harmonic mode-locking is achieved when the pump power is beyond to 8.50 W.Continuing to increase the power and obtains pulse cluster,and as the power increases,the number of pulses per group of pulse cluster is also increases.In addition,when the pump power is kept constant at 9.00 W,various types of multi-pulse sequences are obtained by adjusting the polarization controller,including pulse cluster sequences,chaotic multi-pulse sequences and periodic modulated pulse sequences.In fact,adjusting the polarization controller will increase or decrease the gain loss in the cavity,which is equivalent to increasing or decreasing the pump power.Therefore,some results are similar to the adjustment of the pump power.