Net Anomalous Dispersion Research Articles

Induced dark solitary pulse in an anomalous dispersion cavity fiber laser.

We report on the formation of induced dark solitary pulses in a net anomalous dispersion cavity fiber laser. In a weak birefringence cavity fiber laser simultaneous laser oscillation along the two orthogonal polarization directions of the cavity could be achieved. Under suitable conditions bright cavity solitons could be formed along one polarization direction while CW emission occurs along the orthogonal polarization direction. In a previous paper we have shown that under incoherent polarization coupling a bright soliton always induces a broad dark pulse on the CW beam. In the paper we further show that under coherent polarization coupling a bright soliton could further induce either a weak bright or a dark solitary pulse on the bottom of the broad dark pulse. Numerical simulations have also well reproduced the experimental observations, and further show whether a weak dark or bright solitary pulse is induced is determined by the presence or absence of a phase jump in the induced pulse.

Generation of dual-wavelength square pulse in a figure-eight erbium-doped fiber laser with ultra-large net-anomalous dispersion.

A type of wave-breaking-free mode-locked dual-wavelength square pulse was experimentally observed in a figure-eight erbium-doped fiber laser with ultra-large net-anomalous dispersion. A 2.7km long single-mode fiber (SMF) was incorporated as a nonlinear optical loop mirror (NOLM) and provided largely nonlinear phase accumulation and anomalous dispersion, which enhanced the four-wave-mixing effect to improve the stability of the dual-wavelength operation. In the NOLM, the long SMF with small birefringence supported the Sagnac interference as a filter to manage the dual-wavelength lasing. The dual-wavelength operation was made switchable by adjusting the intra-cavity polarization loss and phase delay corresponding to two square pulses. When the pump power was increased, the duration of the square pulse increased continuously while the peak pulse power gradually decreased. This square-type pulse can potentially be utilized for signal transmission and sensing.

95 nJ dispersion-mapped amplifier similariton fiber laser at 86 MHz repetition rate with linear cavity configuration

A high-energy and low repetition rate dispersion-mapped amplifier similariton oscillator with a large net intracavity anomalous dispersion and a linear cavity configuration is demonstrated experimentally at 1 μm. The numerical results confirm that self-similar evolution is accomplished in the gain fiber, and both the parabolic- and Gauss-shaped pulses can be emitted at different ports of the cavity, respectively. The maximum output power of 820 mW at a repetition rate of 8.6 MHz under a pump power of 12.76 W, corresponding to a pulse energy as high as of 95 nJ has been obtained.

Numerical study on the minimum modulation depth of a saturable absorber for stable fiber laser mode locking

We conducted a numerical study to investigate the minimum modulation depth of a saturable absorber that is essentially required for stable optical pulses to be generated from a passively mode-locked fiber laser cavity. More specifically, an extended nonlinear Schrodinger equation was numerically solved in order to analyze the impact of the cavity group velocity dispersion (GVD), cavity χ(3) nonlinearity, and saturation gain of an active fiber on the minimum modulation depth that is required. The net cavity GVD is shown to significantly influence the required minimum modulation depth level of a saturable absorber. A cavity with little net anomalous dispersion was found to be readily mode locked through the use of a saturable absorber with a very small amount of modulation depth. Furthermore, the reason that a fiber laser cavity with a zero cavity GVD becomes unstable and needs a much higher modulation depth was investigated, and such a condition was found to be associated with the fiber χ(3) nonlinearity. The minimum modulation depth was also shown to vary according to the saturation gain level of an active fiber.

Dark pulse generation in fiber lasers incorporating carbon nanotubes

We demonstrate the generation of dark pulses from carbon nanotube (CNT) incorporated erbium-doped fiber ring lasers with net anomalous dispersion. A side-polished fiber coated with CNT layer by optically-driven deposition method is embedded into the laser in order to enhance the birefringence and nonlinearity of the laser cavity. The dual-wavelength domain-wall dark pulses are obtained from the developed CNT-incorporated fiber laser at a relatively low pump threshold of 50.6 mW. Dark pulses repeated at the fifth-order harmonic of the fundamental cavity frequency are observed by adjusting the intra-cavity polarization state.

On the formation of noise-like pulses in fiber ring cavity configurations

We give an overview of the current status of fiber-based noise-like pulse (NLP) research conducted over the past decade, together with presenting the newly conducted, systematic study on their temporal, spectral, and coherence characteristics in nonlinear polarization rotation (NPR)-based erbium-doped fiber ring cavity configurations. Firstly, our study includes experimental investigations on the characteristic features of NLPs both in the net anomalous dispersion regime and in the net normal dispersion regime, in comparison with coherent optical pulses that can alternatively be obtained from the same cavity configurations, i.e., with the conventional and dissipative solitons. Secondly, our study includes numerical simulations on the formation of NLPs, utilizing a simplified, scalar-field model based on the characteristic transfer function of the NPR mechanism in conjunction with the split-step Fourier algorithm, which offer a great help in exploring the interrelationship between the NLP formation and various cavity parameters, and eventually present good agreement with the experimental results. We stress that if the cavity operates with excessively high gain, i.e., higher than the levels just required for generating coherent mode-locked pulses, i.e., conventional solitons and dissipative solitons, it may trigger NLPs, depending on the characteristic transfer function of the NPR mechanism induced in the cavity. In particular, the NPR transfer function is characterized by the critical saturation power and the linear loss ratio. Finally, we also report on the applications of the fiber-based NLP sources, including supercontinuum generation in a master-oscillator power amplifier configuration seeded by a fiber-based NLP source, as one typical example. We expect that the NLP-related research area will continue to expand, and that NLP-based sources will also find more applications in the future.

Systematical analysis of mode-locked fiber lasers using single-walled carbon nanotube saturable absorbers

The output characteristics of the Er-doped mode-locked fiber laser using a single-walled carbon nanotube saturable absorber are investigated theoretically with a nonlinear Schrödinger equation and a saturable absorption equation using realistic parameters. Stable self-starting mode-locking pulses are achieved under net normal, net zero, and net anomalous cavity group velocity dispersion (GVD) respectively. A spectrum with a flat top is obtained from the net normal cavity GVD laser while a spectrum with Kelly side-bands is obtained from the net anomalous cavity GVD laser. The characteristics of the pulse duration changing with cavity GVD and modulation depth of the single-walled carbon nanotubes are discussed. The characteristics of the mode-locking pulses from net normal, net zero, and net anomalous cavity GVD mode-locked fiber lasers are compared. These systematical results are useful for designing mode-locked fiber lasers with saturable absorbers made by different kinds of carbon nano-materials.

2 µm bright–dark pulses in Tm-doped fiber ring laser with net anomalous dispersion

We demonstrate bright–dark pulses near 2 µm in a Tm-doped fiber laser (TDFL) for the first time. Bright–dark pulses could be achieved when the laser works in the passively mode-locking regime and the net group velocity dispersion is quite anomalous. A polarization controller is employed to manipulate the polarization states of the laser oscillation modes. The dark and bright pulses are confirmed to be two orthogonal linear polarization components that are coupled incoherently in the laser cavity. Experimental results also show that the dark and bright pulses have different spectral distributions.

Dissipative soliton resonance in a passively mode-locked figure-eight fiber laser

The generation of mode-locked rectangular pulses operating in dissipative soliton resonance (DSR) region is demonstrated in an erbium-doped figure-eight fiber laser with net anomalous dispersion. The duration of the wave-breaking-free rectangular pulse broadens with the increase of pump power. At a maximum pump power of 341 mW, the pulse energy can be up to 3.25 nJ with a repetition rate of 3.54 MHz. Particularly, the spectrum of rectangular pulse operating in DSR exhibits conventional soliton sidebands. The observed results show that the formation of pulse operating in DSR region is independent of mode-locking techniques, which may be helpful for further understanding the DSR phenomenon.

Numerical investigation of wavelength tuning in a graphene-based mode-locked fiber laser

We numerically demonstrate a wavelength-tunable graphene-based mode-locked fiber laser with net anomalous cavity group-velocity dispersion. It is shown that by utilizing fiber birefringence, the central wavelength of mode-locked pulses could be continuously tuned over the range from 1553 to 1566 nm. The numerical results are in agreement with the experimental observations in the literature. The mechanism of wavelength tuning may be attributed to an artificial fiber birefringence-induced filter in the cavity.

Bright–Dark Pulse Pair in a Figure-Eight Dispersion-Managed Passively Mode-Locked Fiber Laser

We report on the generation of bright–dark pulse pair in a figure-eight dispersion-managed (DM) passively mode-locked fiber laser with net anomalous cavity group velocity dispersion (GVD). The nonlinear amplifying loop mirror (NALM) is employed in this configuration to allow for passively mode-locked operation. Through carefully adjusting the polarization controllers (PCs), the bright–dark pulse pair could be easily achieved. Furthermore, it is found experimentally that the pulse pair is formed due to the cross coupling between the bright and dark pulses, which are located at two different wavebands.

Generation and Propagation of Bound-State Pulses in a Passively Mode-Locked Figure-Eight Laser

We have investigated the generation and propagation of robust bound-state pulses emitted from a passively mode-locked figure-eight laser with net-anomalous dispersion. Two pulses with the pulse duration of 1.3 ps and the separation of 2.2 ps exhibit a strongly modulated spectral profile. The experimental observations show that the pulse duration and the separation increase approximately linearly along the extracavity single-mode fiber (SMF). The pulse duration and separation are broadened by a factor of 7 and 5.7, respectively, after the pulse pair propagates through the standard SMF of 202 m. The theoretical results well agree with the experimental observations.

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.

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.

Stability analysis of pulsed cascaded Raman lasers in dispersion-managed systems

We investigate synchronous first and second order pulsed Raman lasers that can achieve frequency spacing of up to 1000 cm−1 for CARS microscopy applications. In particular, we focus on analytical and numerical analysis of pulsed stability derived for Raman lasers by using dispersion-managed telecom fibers and pumping at near 1530 nm telecom wavelengths. We show the evolution of the first and second order Stokes signals at the output for different peak pump power and the net anomalous dispersion combinations. We determine the stability condition for dispersion-managed synchronous Raman lasers up to second order. The results show that the stable second order Raman Stokes pulses with 0.02 W to 0.1 W peak power and 1 ps to 2.1 ps pulse width can be achieved in proposed dispersion-managed systems.

Mode-locked ytterbium fiber lasers.

A compact fiber laser is demonstrated with use of a Gires-Tournois compensator and a short length (2-4 cm-long) of highly doped ytterbium (Yb) fiber providing net anomalous group-velocity dispersion. With use of a novel semiconductor saturable absorber mirror based on GaInNAs structure, self-started 1.5-ps-pulse mode-locked operation was obtained at 1023 nm with a repetition rate of 95 MHz. A mode-locked Yb-doped fiber laser was developed without the use of any dispersion compensation technique. Overall group-velocity dispersion was minimized by using a short length of highly doped Yb fiber in a compact amplifying loop cavity. Self-started mode-locked operation was obtained in 980-1030-nm wavelength range with a fundamental repetition rate of 140 MHz.

Energy enhancement of dispersion managed soliton transmission system using mostly normal dispersion fiber

The dispersion managed soliton transmission system using mostly normal dispersion fiber is investigated. It is shown that, with the same net anomalous dispersion, the optimum energy enhancement is larger for the system using mostly normal dispersion fiber than the system using mostly anomalous dispersion fiber. The allowed transmission distance for the system using mostly normal dispersion fiber is longer than those using mostly anomalous dispersion fiber.