被动锁模光纤激光器多孤子脉冲形成机理

基于非线性耦合薛定谔方程,在非线性偏振技术锁模的掺铒环形光纤激光器中,理论研究多孤子脉冲的形成和演化规律。研究结果表明：随着小信号增益不断增加,光纤激光器的锁模透过率函数影响多脉冲输出,多脉冲产生受色散波和脉冲分裂形成的峰值功率的限制效应和孤子能量量子化的影响,这是增益竞争和非线性偏振旋转引起的损耗之间动态平衡的最终结果。     

被动锁模光纤激光器多孤子脉冲形成机理

基于非线性耦合薛定谔方程,在非线性偏振技术锁模的掺铒环形光纤激光器中,理论研究多孤子脉冲的形成和演化规律。研究结果表明:随着小信号增益不断增加,光纤激光器的锁模透过率函数影响多脉冲输出,多脉冲产生受色散波和脉冲分裂形成的峰值功率的限制效应和孤子能量量子化的影响,这是增益竞争和非线性偏振旋转引起的损耗之间动态平衡的最终结果。     

Characteristics of pulse evolution in mode-locked thulium-doped fiber laser

We have numerically investigated the characteristics of the pulse evolution in a passively mode-locked thulium-doped fiber laser with net anomalous cavity dispersion. For the fixed resonator configuration, single-, dual-, triple-, and quadruple-pulses are generated successively by enhancing the pump power or reducing the output ratio. The characteristics of the single or multiple pulses are investigated with various cavity lengths. The separation between the two coexisting pulses changes with the pump power due to the interaction between the soliton and the dispersive waves, and then the two randomly distributed pulses could finally evolve into a soliton pair with fixed separation. According to the results of the numerical simulation, the multiple pulses are found to be generated via pulse splitting and the pulse splitting threshold decreases with the increase of the cavity length.     

Impact of pulse dynamics on timing jitter in mode-locked fiber lasers

We investigate the high-frequency timing jitter spectral density of mode-locked fiber lasers in different mode-locked regimes. Quantum-noise-limited timing jitter spectra of mode-locked-regime-switchable Yb fiber lasers are measured up to the Nyquist frequency with sub-100-as resolution. The integrated rms timing jitter of soliton, stretched-pulse, and self-similar Yb fiber lasers is measured to be 1.8, 1.1, and 2.9 fs, respectively, when integrated from 10 kHz to 40 MHz. The distinct behavior of jitter spectral density related to pulse formation mechanisms is revealed experimentally for the first time.     

Ultrashort pulse generation in diode lasers

The basic ideas and current state of the art of ultrashort pulse generation by injection lasers are reviewed. All developed techniques, including gain switching,Q-switching, and mode-locking are described and compared. A simple theoretical treatment of a diode laser which emits picosecond light pulses is discussed. Some fundamental limits of the pulse parameters are discussed. Finally, compression of chirped optical pulses by optical fibres and the soliton effect is considered.     

Comparison of pulse evolutions in low and ultra-large anomalous dispersion mode-locked fiber lasers

We have investigated and compared the pulse evolutions in low and ultra-large anomalous dispersion erbium-doped fiber lasers mode-locked by the nonlinear polarization rotation technique. Two lasers deliver the pulses that exhibit quite distinct characteristics such as pulse duration, spectral width, and spectral sidebands. Experimental observations show that the spectral width decreases from several nanometers to less than one nanometer whereas pulse duration extends about three times by changing the cavity dispersion from −0.03 to −15.50 ps². The solitons in ultra-large anomalous fiber laser show dips in the optical spectrum, which is quite distinct from that of conventional solitons observed in low anomalous regime.     

Multi-soliton complexes in mode-locked fiber lasers

We numerically investigate the formation of soliton pairs (bound states) in mode-locked fiber ring lasers in the normal dispersion domain. In the distributed mathematical model (complex cubic-quintic Ginzburg–Landau equation), we observe a discrete family of soliton pairs with equidistantly increasing peak separation. We show that stabilization of previously unstable bound states can be achieved when the finite relaxation time of the saturable absorber is taken into account. The domain of stability can be controlled by varying this relaxation time. Furthermore, we investigate the parameter domain where the region of stable bound states does not shrink to zero for vanishing absorber recovery time corresponding to a laser with an instantaneous saturable absorber. For a certain domain of the small-signal gain, we obtain a robust first level bound state with almost constant separation where the phase of the two pulses evolves independently. Moreover, their phase difference can evolve either periodically or chaotically depending on the small signal gain. Interestingly, higher level bound states exhibit a fundamentally different dynamics. They represent oscillating solutions with a phase difference alternating between zero and π.     

Tunable all-normal-dispersion Yb-doped mode-locked fiber lasers

We experimentally investigate continuous wavelength tunability in an all-normal-dispersion Yb-doped mode-locked fiber laser with an Yb-doped fiber of a fixed length. The spectral tuning over 46.3 nm, from 1024.5 to 1070.8 nm, is achieved. Spectral dynamics with different pumping levels are demonstrated. The central wavelength of the dissipative solitons shifts 8 nm towards short wavelength with pump power increasing from 168 to 456 mW.     

Intracavity self-phase modulation and pulse compression in mode-locked lasers

We show that in the presence of a bandwidth-limiting component intracavity selfphase modulation in high power mode-locked and Q-switched lasers is accompanied by interesting pulse-shaping phenomena. If the phase modulation is properly controlled the pulse duration can approach the limit given by the frequency bandwidth of the laser. For example, for a Nd: YAG laser our calculations predict an intracavity pulse compression by approximately a factor of ten over the typical pulse duration of conventional mode-locking.     

Ultrashort pulse propagation in multiple-grating fiber structures

We propose a multiple-grating fiber structure that decomposes an ultrashort broadband optical pulse simultaneously in both wavelength and time. As an initial demonstration, we used a transform-limited 1-ps Gaussian pulse centered at 1.55 mu;m as the ultrashort broadband input into a three-grating fiber structure and generated three output pulses separated in wavelength and time with good correlation between experimental results and simulations. This device structure can be used to generate a multiwavelength train of pulses for use in wavelength-division-multiplexed systems or to implement frequency-domain encoding of coherent pulses for optical code-division multiple access.     

Numerical analysis of pulse formation in solid-state lasers mode-locked with a linear external cavity

We analyse pulse formation in solid-state lasers mode-locked with a linear external cavity with translating mirror. Based on a numerical simulation the mode-locking mechanism is found to be that of an active mode-locking due to the explicitly time-dependent phase shift by the external cavity involving a slowly moving mirror and a non-zero temporal mismatch to the master cavity. Additional pulse shortening and stabilization is performed by the additive pulse mode-locking mechanism arising from self-phase modulation due to a Kerr-type non-resonant nonlinearity in the host crystal of the laser amplifier.     

Analysis of pulse dropout in harmonically mode-locked fiber lasers by use of the Lyapunov method

Using a stability analysis based on the Lyapunov method, we study pulse dropout in an actively mode-locked fiber laser. The analysis gives a limit on the maximum pulse duration and the minimum laser power that are needed for stable operation without pulse dropout. The stability of pulse trains was studied analytically and validated numerically for different pulse shapes.     

Properties of the pulse train generated by repetition-rate-doubling rational-harmonic actively mode-locked Er-doped fiber lasers

We demonstrate for the first time to our knowledge, experimentally and theoretically, that the pulse-to-pulse amplitude fluctuations that occur in pulse trains generated by actively mode-locked Er-doped fiber lasers in a repetition-rate-doubling rational-harmonic mode-locking regime are completely eliminated when the modulation frequency is properly tuned. Irregularity of the pulse position in the train was found to be the only drawback of this regime. One could reduce the irregularity to a value acceptable for applications by increasing the bandwidth of the optical filter installed in the laser cavity.     

Operating regimes and performance optimization in mode-locked fiber lasers

We derive a theoretical model to characterize the mode-locking dynamics in a single-mode fiber laser cavity with a combination of waveplates and a passive polarizer. The averaging process results in the cubic-quintic Ginzburg-Landau equation (CQGLE) where all the coefficients depend explicitly on the setting of the waveplates as well as the fiber birefringence. A comparison between full numerical simulations and the CQGLE shows a good agreement that allows for characterizing the stability and operating regimes of the laser cavity. A low-dimensional model is developed via the method of proper orthogonal decomposition (POD) to study the multi-pulsing transition of the CQGLE, and the results agree qulitatively with the CQGLE model. The theory allows one to develop guidelines for engineering and optimizing high-energy, high peak-power pulses in the laser cavity.     

Ultrahigh-repetition-rate bound-soliton harmonic passive mode-locked fiber lasers

On the basis of numerical simulation results, we put forward a way to realize harmonic passive mode locking of fiber lasers with an ultrahigh-repetition-rate pulse train. The equidistant distribution of ultrashort pulses filling the total laser cavity is due to bound-soliton mechanisms. In the case of large bound energy, such long soliton trains are very stable and have the ideal periodic structure as a soliton crystal.     

Analysis of temporal pulse development in passively mode-locked lasers

Pulse-shortening and pulse broadening effects in passively mode-locked lasers are analysed. A steady-state pulse duration limit is calculated and compared with round-trip simulations. The numerical calculations apply to a picosecond Nd: glass laser. Methods of short-pulse generation are discussed.     

Ultrashort pulse generation using fiber FM laser

Ultrashort optical pulse generation using a fiber FM laser is presented and analyzed in detail. Fiber FM laser operation is realized using a fiber ring with an internal phase modulator and an erbium-doped fiber amplifier. To compress FM laser pulses, an external dispersive single-mode fiber is employed. Furthermore, by external intensity modulation, the pulse background is removed. The background ratio of the generated ultrashort pulse is calculated and compared with the experimental results. The experimental results show an output optical pulse width of 1.77 ps and a spectral bandwidth of 0.5 THz.     

Universal soliton pattern formations in passively mode-locked fiber lasers

We investigate multiple-soliton pattern formations in a figure-of-eight passively mode-locked fiber laser. Operation in the anomalous dispersion regime with a double-clad fiber amplifier allows generation of up to several hundreds of solitons per round trip. We report the observation of remarkable soliton distributions: soliton gas, soliton liquid, soliton polycrystal, and soliton crystal, thus indicating the universality of such complexes.