| Fiber lasers have become the preferable laser sources in many application fields because of their compactness,high laser conversion efficiency,and high beam quality.In recent years,ultrafast lasers with wavelengths ranging from 1800 nm to 2100 nm are significant in the applications of biomedical surgery,environmental science,and laser industrial processing,greatly driving the research and development of 2 μm ultrafast fiber lasers.Among them,ultrafast thulium(Tm)-doped fiber laser with an emission wavelength of>2000 nm and pulse repetition rate of～100 MHz or>100 MHz not only possesses the fundamental characteristics of conventional ultrafast laser(short pulse duration,high peak power,and broad laser spectrum)but also has the advantages of high pulse repetition rate,avoiding the absorption of water vapor,and ease of power scaling.Such ultrafast fiber laser is beneficial for synchronously pumping ultrafast mid-infrared optical parametric oscillators,improving the accuracy of gas detection and boosting the ultrafast laser processing efficiency.Therefore,investigations on the technologies of the>2000-nm high repetition rate ultrafast laser generation based on Tm-doped fiber and nonlinear pulse compression can enrich the functions of 2 μm laser and extend its application fields.This paper initially investigates the evolution mechanisms and laws of the ultrafast laser pulse transmitting inside the fiber and preliminarily determines the key parameters of designing an ultrafast fiber laser.The gain management is carried out for the Tm-doped fiber,and the>2000 nm high repetition rate ultrafast Tm-doped fiber laser oscillator is realized.Then,the fiber master oscillator power amplification(MOPA)technology and the fiber chirped pulse amplification(CPA)technology are respectively employed to scale up the ultrafast laser pulse energy.Finally,based on the normal dispersion fiber,the 2-μm high repetition rate nonlinear pulse compression technology is also investigated.The main research contents of this paper are as follows:1.The theoretical model of the ultrafast laser pulse transmitting inside the fiber is built,and the influences of different optical effects on the characteristics of femtosecond pulse are numerically investigated.Simulation results show that as the ultrafast laser pulse transmits inside the anomalous dispersion fiber,the induced self-phase modulation,pulse self-steepening,and stimulated Raman scattering can broaden the laser spectrum,and makes the ultrafast laser pulse evolve into the breathing pulse,and even degenerate into multi-pulses.In addition,the simulation results further reveal that the spectral sidebands exist as an independent pulse in the time domain and transmit synchronously with the main pulse.While in the normal dispersion fiber,ultrafast laser pulse prefers to maintain the single pulse transmission.2.Considering the problem of low gain of Tm-doped fiber in the wavelength region of>2000 nm,the reabsorption effect of Tm-doped fiber and the wavelength-dependent transmittance modulation of the nonlinear polarization rotation(NPR)effect are taken to redshift the net gain of Tm-doped fiber to>2000-nm wavelength region.The theoretical model of the NPR-based mode-locking fiber oscillator is built,which determines the key optical parameters for stable mode-locking operation.The high repetition rate mode-locking Tm-doped fiber oscillator is experimentally verified,which outputs the conventional soliton pulse with a center wavelength of 2007 nm,a pulse repetition rate of～96 MHz,and a pulse duration of 470 fs,matching well with the numerical simulation results.Further optimizing the pump laser power and the polarization state of the intracavity pulse makes the Tm-doped fiber oscillator go into the harmonic mode-locking regime,which can operate at the center wavelengths of 1962 nm,1982 nm,and 2005 nm,respectively.The maximum pulse repetition ratio is>1 GHz,the average output power reaches>300 mW,and the pulse duration is<500 fs for each wavelength harmonic mode-locking operation.3.For breaking the limitation of the gain bandwidth of Tm-doped fiber in>2000 nm wavelength region,the soliton self-frequency shift(SSFS)is stimulated inside the Tm-doped all polarization-maintaining fiber MOPA laser amplifier to further extend the laser wavelength.After amplifying the～100-MHz fundamental seed laser pulse,the 4th-order Raman solitons with the center wavelengths of 2017 nm,2115 nm,2243 nm,and 2346 nm are successfully excited.With the further introduction of an optical filter,a near Fourier transform limited laser pulse at 2346 nm with a spectral bandwidth of 25.7 nm is separated,corresponding to the pulse duration of 229 fs and an average power of 250 mW.The amplification ability of the MOPA laser amplifier for different wavelength GHz ultrafast laser pulses is also investigated.The average powers of the GHz ultrafast laser pulses at 1962 nm,1982 nm,and 2005 nm are respectively amplified to～4 W,and the SSFS effect is also observed.Based on the general nonlinear Schrodinger equation(GNLSE),the MOPA laser amplifier is further analyzed,demonstrating the occurrence of pulse splitting for the amplified GHz ultrafast laser pulse.4.In order to overcome the problem of pulse splitting in the MOPA laser amplifier,a Tmdoped all polarization-maintaining fiber CPA laser amplifier is designed to scale up the pulse energy of the high repetition rate pulsed laser.Theoretical simulation and experiment demonstrate that it is feasible to separate the Kelly sidebands and main soliton pulse by introducing a spatial filter inside the grating-based pulse stretcher,and the Kelly sidebands are experimentally proved to exist as an independent pulse in the time domain.The power amplification ability of the CPA laser amplifier for different chirped main soliton pulses is studied.The 460-fs main soliton pulses are amplified to 10.35 W and 9.1 W after anomalous dispersion and normal dispersion broadening respectively.After compressing the amplified pulse,the average output powers are>7 W,and the shortest pulse duration is 346 fs.The spectral intensity of the main soliton pulse is further pre-modulated to suppress the spectrum-narrowing effect of the laser amplifier,making the laser pulse shortened to 208 fs.5.Aiming at the problem that ultrafast fiber laser oscillators and amplifiers are difficult to generate the high repetition rate few-cycle laser pulses,the nonlinear pulse compression technology is investigated based on the normal dispersion fiber,which is characterized with a large normal dispersion and a moderate nonlinear coefficient.The 470-fs,101.4-MHz,2.14-W,2-μm driving laser pulses are compressed to 70 fs after transmitting through the one-stage nonlinear pulse compressor.As the laser pulses further go through the cascaded stage of the nonlinear pulse compressor,the pulse duration is further shortened to 28.3 fs,corresponding to the 4.3 optical cycles,a pulse compression ratio of 16,and an average output power of>1 W.With further optimizing the fiber length,the all-fiber cascaded nonlinear pulse compressor is successfully realized,which delivers the ultrafast laser pulse with a pulse duration of 35.1 fs.The corresponding pulse compression ratio is 13.7,and the average output power is 1.28 W.Based on the GNLSE,the evolution of the ultrafast laser pulse inside the all-fiber nonlinear pulse compressor is further analyzed.The simulation results are well consistent with the measured results. |
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