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.     

Multi-soliton complexes

In this paper we introduce the concept of multi-soliton complexes (MSC). A particular example of a MSC is an incoherent soliton in a multimode fiber or in a photorefractive crystal, but there are many examples in other areas of physics. We discuss a variety of profiles of MSCs, their unusual collisional properties, the possibility of a MSC on a background and some other interesting properties of MSCs. Some of their features are also shared by single solitons, but there are many differences between the properties of the two types. (c) 2000 American Institute of Physics.     

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.     

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。     

Ultrashort pulse formation and evolution in mode-locked fiber lasers

Passive mode-locking in fiber lasers is investigated by numerical and experimental means. A non-distributed scalar model solving the nonlinear Schr?dinger equation is implemented to study the starting behavior and intra-cavity dynamics numerically. Several operation regimes at positive net-cavity dispersion are experimentally accessed and studied in different environmentally stable, linear laser configurations. In particular, pulse formation and evolution in the chirped-pulse regime at highly positive cavity dispersion is discussed. Based on the experimental results a route to highly energetic pulse solutions is shown in numerical simulations.     

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.     

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.     

Synchronization of mode-locked femtosecond lasers through a fiber link

Two mode-locked femtosecond fiber lasers, connected via a 7 km fiber link, are synchronized to an rms timing jitter of 19 fs, observed over the entire Nyquist bandwidth (half of the 93 MHz repetition frequency). This result is achieved in two steps. First, active cancellation of the fiber-transmission noise reduces timing jitter caused by path length fluctuations to a record level of 16 fs. Second, using a wide bandwidth interactivity actuator, the slave laser is synchronized to the incoming stable pulse train from the reference laser to within 10 fs. These results are confirmed by an optical cross-correlation measurement performed independently of the feedback loop operated in the microwave domain.     

Complete characterization of quantum-limited timing jitter in passively mode-locked fiber lasers

We characterize the timing jitter of passively mode-locked, femtosecond, erbium fiber lasers with unprecedented resolution, enabling the observation of quantum-origin timing jitter up to the Nyquist frequency. For a pair of nearly identical 79.4MHz dispersion-managed lasers with an output pulse energy of 450pJ, the high-frequency jitter was found to be 2.6fs [10kHz, 39.7MHz]. The results agree well with theoretical noise models over more than three decades, extending to the Nyquist frequency. It is also found that unexpected noise may occur if care is not taken in optimizing the mode-locked state.     

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.     

On the generation of high-quality Nyquist pulses in mode-locked fiber lasers

Nyquist pulses have wide applications in many areas, from electronics to optics. Mode-locked lasers are ideal platforms to generate such pulses. However, how to generate high-quality Nyquist pulses in mode-locked lasers remains elusive.We address this problem by managing different physical effects in mode-locked fiber lasers through extensive numerical simulations. We find that net dispersion, linear loss, gain and filter shaping can affect the quality of Nyquist pulses significantly. We also de...     

Intensity noise of mode-locked fiber grating external cavity semiconductor lasers

The noise of hybrid soliton pulse source (HSPS) with linearly chirped Gaussian apodized fiber Bragg grating is analyzed by couple-mode equations including spontaneous emission noise when HSPS is mode-locked. Relative intensity noise (RIN) is calculated using numerical solutions of these equations. It is shown that transform-limited pulses are generated over a wide tuning range around the fundamental mode-locking frequency with low spontaneous noise. However, high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable over a wide tuning range. Linewidth enhancement factor and spontaneous coupling factor are the most effective noise parameters and noise increases with increasing value of these parameters.     

Theoretical study on instantaneous linewidth of Fourier-domain mode-locked fiber lasers

We have numerically simulated the operation of the Fourier-domain mode-locked (FDML) fiber laser based on the wavelength reconstruction method instead of numerical solving the nonlinear Schrödinger equation. We studied the influences of the filter bandwidth and the relative time delay caused by the fiber chromatic dispersion on the instantaneous linewidth of the FDML fiber laser. The results show that the instantaneous linewidth broadens as the filter bandwidth and the relative time delay increase. When the filter has the bandwidth of 0.02 nm, the narrowest and broadest instantaneous linewidths are 0.024 and 0.042 nm, respectively. We give an understanding for the oscillation of the instantaneous linewidth of FDML. The presented result can be used to evaluate the performance achievable in the FDML fiber lasers.     

Impact of slow gain dynamics on soliton molecules in mode-locked fiber lasers

We theoretically demonstrate and experimentally confirm the major influence of gain dynamics on soliton molecules that self-assemble in mode-locked lasers. Both slow gain recovery and depletion play a pivotal role in the formation of chirped soliton molecules characterized by an increasing separation from leading to trailing pulses. These chirped molecules actually consist of many pulses and may be termed macromolecules. They are experimentally observed in a fiber laser and numerically modeled by an approach that properly includes the slow gain dynamics. Furthermore, it is shown that these processes stabilize soliton trains in fiber lasers by inhibiting internal oscillations.     

Photon-number fluctuation and correlation of bound soliton pairs in mode-locked fiber lasers

Quantum photon-number fluctuation and correlation of bound soliton pairs in mode-locked fiber lasers are studied on the basis of the complex Ginzburg-Landau equation model. We find that, depending on their phase difference, the total photon-number noise of the bound soliton pair can be larger or smaller than that of a single soliton, and the two solitons in the soliton pair have a corresponding positive or negative photon-number correlation. It is predicted for the first time to our knowledge that out-of-phase soliton pairs can exhibit less noise as a result of negative correlation.     

Frequency stabilization of mode-locked Erbium fiber lasers using pump power control

Mode-locked erbium fiber lasers can serve as frequency comb generators in the 1.5 m telecommunication band or as frequency dividers generating low noise pulse trains at microwave repetition rates from a visible or near infrared frequency standard. Such applications require suitable stabilization methods both for the lasers carrier frequency and the pulse repetition frequency. While control of the optical resonator length with the help of a piezo actuator is an obvious method, we investigate in this paper frequency stabilization of the fiber laser via pump power control. Using this method, we stabilize the pulse repetition rate of the Er:fiber-laser with respect to a hydrogen-maser with residual averaged timing errors of a few femtoseconds (for averaging times between one and one hundred seconds), an order of magnitude below the timing noise of the maser itself. PACS 42.55Wd; 42.62.Eh     

Relative intensity noise of mode-locked fiber grating external cavity semiconductor lasers

The noise of hybrid soliton pulse source (HSPS) with linearly chirped Gaussian apodized fiber Bragg grating is analyzed by couple-mode equations including spontaneous emission noise when the HSPS is mode-locked. Relative intensity noise is calculated using numerical solutions of these equations. It is shown that transform limited pulses are generated over a wide tuning range around the fundamental mode-locking frequency with low spontaneous noise. However, a high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable over a wide tuning range. It is also shown that noise is extremely sensitive to the RF and DC currents, linewidth enhancement factor, gain saturation parameter and spontaneous coupling factor.     

A theoretical and experimental study on all-normal-dispersion Yb-doped mode-locked fiber lasers

We report on a theoretical and experimental study of an all-normal-dispersion (ANDi) Yb-doped mode-locked fiber laser, in which nonlinear polarization rotation (NPR) is used to realize mode-locking without any dispersion compensation. Based on the coupled nonlinear Schrdinger (CNLS) equation, a model simulating the mode-locked process of an allnormal-dispersion ring fiber laser is developed, which shows that the achievement of stable mode-locking depends on the alignment of the polarization controller (PC) along the fast-polarization axis of the fiber, the birefringence intensity, and the net cavity dispersion. According to the theoretical analysis, stable mode-locked pulses with pulse duration 300 ps and average output power 33.9mW at repetition rate 36MHz are obtained.