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.     

孤子激光器及其发展

激光和孤子研究的进展,导致一种新型的、输出脉宽及形状可控的超短光脉冲激光器——孤子激光器的诞生。孤子激光器在光通讯、光计算、超高速现象的研究、超高速光电子器件的研制等方面有其独特的应用。它也使我们有可能对孤子效应本身进行深入研究。本文将叙述在单模光纤中形成孤子的机制,孤子激光及喇曼孤子激光原理,孤子及孤子激光研究的进展,包括孤子激光的稳定性、孤子自频移以及孤子间的相互作用力等。最后介绍一些已经出现的或将来可能出现的应用,并指出这是一个正在迅速发展的研究领域。     

Non-Gibbsian stochastic light-mode dynamics of passive mode locking

We study a stochastic light-mode system with non-Gibbsian steady state statistics, unravelling global nonequilibrium phase transition properties. It relates to the onset of passive mode-locking in the general case of lasers with arbitrary dispersion and Kerr nonlinearity that includes the nonsolitonic regime. The solution is facilitated by a special stationarity criterion imposed by the system gain balance. We show that the mode-locking phase transition is generic, and give exact expressions for the pulse power and its stability map. We find that at the boundary of the mode-locking stability the pulse power is exactly one half of the total intracavity power, and that the parameter region for the most resistant pulses against noise destabilization is not at the soliton condition.     

Frequency doubling of femtosecond erbium-fiber soliton lasers in periodically poled lithium niobate

We report efficient frequency doubling of passively mode-locked femtosecond erbium-fiber lasers. Quasi-phase-matched second-harmonic generation in periodically poled lithium niobate is used to generate 8.1 mW of 190-fs (FWHM), 90-pJ pulses at 777 nm with a conversion efficiency greater than can be obtained with existing birefringently phase-matched nonlinear materials. A dispersion-compensation-free soliton oscillator generating transform-limited 230-fs (FWHM) pulses at 1554 nm is used as a pump laser.     

Application of the coherent photon seeding technique to synchronously pumped and hybridly mode-locked dye lasers

The mode-locking performances of synchronously pumped and hybridly mode-locked dye lasers self-stabilized by coherent photon seeding (CPS) are investigated experimentally. The variation of the optimum values of the feedback parameters and the pulse quality of the stabilized laser with the cavity detuning is studied over a wide range of this parameter. The results presented for the case of pure synchronous pumping complement those previously reported for the generation of transform-limited pulses of variable length. The first experiment on the application of CPS to a conventional dual-jet hybridly mode-locked laser confirms the predictions of the numerical modelling.     

Soliton Rectangular Pulses and Bound States in a Dissipative System Modeled by the Variable-Coefficients Complex Cubic-Quintic Ginzburg-Landau Equation

The complex cubic-quintic Ginzburg-Landau equation(CQGLE) is a universal model for describing a dissipative system,especially fiber laser.The analytic one-soliton solution of the variable-coefficients CQGLE is calculated by a modified Hirota method.Then,phenomena of soliton pulses splitting and stable bound states of two solitons are investigated.Moreover,rectangular dissipative soliton pulses of the variable-coefficients CQGLE are realized and controlled effectively in the theoretical research for the first time,which breaks through energy limitation of soliton pulses and is expected to provide theoretical basis for preparation of high-energy soliton pulses in 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.     

Direct generation of a 10 GHz 816 fs pulse train from an erbium-fiber soliton laser with asynchronous phase modulation

By employing the technique of asynchronous mode locking, we have successfully demonstrated direct generation of stable 10 GHz 816 fs pulse trains with a supermode-suppression ratio >70 dB from a hybrid mode-locked Er-fiber laser. When the modulation frequency deviates from the cavity harmonic frequency by 15-40 kHz, stable femtosecond soliton pulses are formed. Our results demonstrate that asynchronous mode locking can act as an effective mechanism for achieving a shorter pulse width and for stabilizing high-repetition-rate pulse trains in soliton fiber lasers.     

Polarized soliton pulses generation using nonlinear micro ring resonators for multi- and long distance links

We propose a new concept of quantum soliton pulses generation using a soliton pulse in the micro ring resonators. Firstly, the chaotic soliton pulses are generated and circulated within the integrated micro ring resonators. Secondly, the specific second harmonic pulses are selected by using the appropriate ring parameters. The superposition of the second harmonic pulses within the micro ring devices introduces the randomly polarized photons within the micro ring device. The entangled photon visibility of the polarized photon is seen after passing the polarization control devices and projecting on the detectors. The optimum entangled photon visibility is obtained. The advantage of such a system is that the quantum repeater unit can be redundant for long distance quantum communication link, whereas the use of the system for multi-entangled photon sources and links is also available. The system degradation via the entangled photon states timing walk-off is also discussed.     

Artificial spectral filtering in dissipative soliton fiber lasers with invisible bandpass filters

We numerically study the artificial spectral-filtering effect in dissipative soliton fiber lasers without intracavity spectral filters. It is found that in dissipative soliton lasers with real saturable absorbers (SAs), the dynamic spectral filtering of the real SAs serves as an artificial spectral filter and contributes to the pulse shaping. While in the dissi- pative soliton lasers with artificial SAs, such as nonlinear polarization rotation, the spectral filtering introduced by the intracavity polarization-dependent components acts as an artificial spectral filter and shapes the pulses to obtain mode-locking. An investigation of the artificial spectral-filtering effect reveals the operating mechanisms of the dissipative soliton fiber lasers without visible bandpass filters.     

Widely-wavelength-tunable few-cycle optical pulse generation from an all-fiber erbium-doped laser system

A scheme for the construction of fiber laser systems for the generation of tunable ultrashort optical pulses is proposed. The scheme is based on the self-Raman shift of the soliton frequency in dispersion-decreasing fibers with the subsequent spectral broadening owing to the supercontinuum generation in a short highly nonlinear fiber and the compression in the corresponding fiber compressor. An all-fiber laser system for the generation of ultrashort laser pulses in the wavelength range 1.6–2.0 μm is experimentally demonstrated. In particular, the shortest pulses with a duration of 24 fs are generated at wavelengths of 1.8–1.9 μm, which corresponds to less than four optical cycles.     

Self-similar evolution of parabolic pulses in a laser

Self-similar propagation of ultrashort, parabolic pulses in a laser resonator is observed theoretically and experimentally. This constitutes a new type of pulse shaping in mode-locked lasers: in contrast to the well-known static (solitonlike) and breathing (dispersion-managed soliton) pulse evolutions, asymptotic solutions to the nonlinear wave equation that governs pulse propagation in most of the laser cavity are observed. Stable self-similar pulses exist with energies much greater than can be tolerated in solitonlike pulse shaping, and this has implications for practical lasers.     

Modulation instability induced by periodic power variation in soliton fiber ring lasers

Modulation instability with subsideband generation induced by periodic power variation in soliton fiber ring lasers is reported. We found that different wavelength shifts of subsideband generation are related to different periodic power variation. The period of power variation and wavelength shifts of subsideband can be changed by altering the linear cavity phase delay. It is also found that the periodic power variation is caused by the interaction between the nonuniform polarization state of the circulating light and the polarizer in the laser cavity.     

Relativistic electromagnetic solitons in the electron-ion plasma

We present the results of the investigation about the ion motion influence on relativistic soliton structure, and show that in the case of moving multinode solitons the effect of the ion dynamics results in the limiting of its amplitude. The constraint on the maximum amplitude corresponds to either the ion motion breaking in the low-node-number case, or to the electron trajectory self-intersection in the case of high-node-number solitons. The soliton breaking leads to the generation of fast ions, and provides a novel mechanism for the ion acceleration in the plasma irradiated by the high-intensity laser pulses.     

Propagation of short soliton pulses through a parabolic index fiber with dispersion decreasing along length

A parabolic index dispersion decreasing fiber (DDF) has been designed and optimized to produce high capacity soliton communication system. Variation of different fiber parameters such as core radius, effective core area and GVD factor along the 25 km of DDF length has been carried out to optimize a best possible DDF which can sustain the propagation of fundamental soliton. The variation of non-linearity with length along with the conventional power and GVD factor variation has been included in the generalized non-linear Schrodinger equation (NLSE). This NLSE has been solved numerically by split step Fourier method for shorter pulse propagation, incorporating the term for third order dispersion and intrapulse Raman scattering. Stable soliton pulses in transmission system have been achieved by our simulation, when a correction factor due to Raman induced soliton mean frequency shift is incorporated to the GVD profile predicted by the fundamental soliton condition. The interaction of neighboring soliton pulse pair through the proposed fiber has also been studied.     

正色散激光器中的增益导引孤子

采用非线性薛定谔方程模拟了全正色散介质激光器中实现脉冲锁模输出的动态过程,理论表明,不同于工作于净色散为负的孤子锁模激光器以及腔内具有色散延迟线净色散为正的色散管理孤子和自相似脉冲锁模激光器,在纯正色散介质构成的锁模激光器中,增益饱和和增益窄化对形成稳定的锁模脉冲起重要的作用,在两者的作用下脉冲形成增益导引孤子.采用不同的锁模方式在纯正色散光纤锁模激光器中得到了稳定的增益导引孤子锁模,实验结果与理论模拟一致. 关键词： 锁模 正色散 增益导引孤子     

Observation of soliton bound states in a graphene mode locked erbium-doped fiber laser

We report on the experimental observation of bound states of solitons in a graphene mode locked erbium-doped fiber laser. By using graphene as a saturable absorber, we have obtained stable, single and bound soliton pulses, the latter with a full width at half maximum (FWHM) of 1.49 ps and a fixed pulse separation of 2.46 ps. Our results once again show that bound soliton pulses is an intrinsic feature of fiber lasers independent of the exact mode locking mechanism.     

Soliton strings and interactions in mode-locked lasers

Soliton strings in mode-locked lasers are obtained using a variant of the nonlinear Schrödinger equation, appropriately modified to model power (intensity) and energy saturation. This equation goes beyond the well-known master equation often used to model these systems. It admits mode-locking and soliton strings in both the constant dispersion and dispersion-managed systems in the (net) anomalous and normal regimes; the master equation is contained as a limiting case. Analysis of soliton interactions show that soliton strings can form when pulses are a certain distance apart relative to their width. Anti-symmetric bi-soliton states are also obtained. Initial states mode-lock to these states under evolution. In the anomalous regime individual soliton pulses are well approximated by the solutions of the unperturbed nonlinear Schrödinger equation, while in the normal regime the pulses are much wider and strongly chirped.     

Dark pulse generation in a dispersion-managed fiber laser

We report an experimental observation of dark pulse generation in a dispersion-managed erbium-doped fiber laser with net anomalous cavity group-velocity dispersion. It is found that apart from the bright soliton pulses, dark pulses with spectral sidebands could be obtained in the laser under appropriate operating conditions. The generation of dark pulses may be attributed to soliton shaping in the cavity.     

Dark solitons in mode-locked lasers

Dark soliton formation in mode-locked lasers is investigated by means of a power-energy saturation model that incorporates gain and filtering saturated with energy, and loss saturated with power. It is found that general initial conditions evolve (mode-lock) into dark solitons under appropriate requirements also met in experimental observations. The resulting pulses are essentially dark solitons of the unperturbed nonlinear Schr?dinger equation. Notably, the same framework also describes bright pulses in anomalous and normally dispersive lasers.