Pulse Bunch Research Articles

Supercontinuum generation by nonlinear polarization rotation mode-locked fiber laser with pulse type switchable

We report on the generation of a supercontinuum (SC) by nonlinear polarization-rotating (NPR) mode-locked fiber laser, which includes a pulsed switchable NPR mode-locked fiber laser, an Er-doped fiber amplifier, and high nonlinear fiber (HNLF). In the experiment, by adjusting the polarization controller (PC) angle and pump power, we obtained three different pulses. They are traditional soliton bunch pulses, the coexistence of soliton bunch pulses and square pulses, and multi-longitudinal mode noise like square pulses. All three pulses can generate SC with spectrum width be about of 1000 nm. We fixed the pump power in the laser and observed the influence of amplifier power on the spectrum width and output power of SC. The SC width of soliton bunch pulse reaches 950.9 nm, the SC of soliton bunch pulse and square pulse coexisting pulse is 1009.7 nm, and the SC spectrum width of multi-longitudinal mode noise like square pulse is 887.1 nm. Our results demonstrate the generation of SC with three types of switchable pulses, opening up possibilities for various application requirements.

Pulse buildup dynamics in a self-starting Mamyshev oscillator.

The Mamyshev oscillator (MO) can generate high-performance pulses. However, due to their non-resonant cavities, they usually are not self-starting, and there is almost no effort to reveal the pulse buildup dynamics of the MO. This paper investigates the dynamic of single pulse (SP) and multi-pulse formation in a self-starting MO. It indicated that both SP self-starting and multi-pulse self-starting can be obtained by adjusting the oscillator parameters. More importantly, increasing pump power could only result in bound state pulses (BSPs) if SP self-starting was formed. With the increase of the pump power, the pulse number in BSPs would increase. However, multiple pulses could not be formed only by increasing the pump power, and the BSPs obtained here underwent SP generated from noise, amplified, and then bounded, which is different from conventional passive mode-locked fiber lasers (CPMLFLs). On the other hand, if multiple pulses were self-initiated, BSPs, pulse bunch, and harmonic mode-locked pulses (HMLPs) could be obtained by adjusting the polarization state and pump power in the cavity. Furthermore, once any of the above states are formed, if the oscillator polarization state and filter interval are unchanged, only increasing the pump power from zero, the original state can still be obtained, which is consistent with the characteristics of the CPMLFLs. These findings will provide new insights into the pulse dynamics of self-starting MO, which will be significant for studying ultrafast laser technology and nonlinear optics.

Revealing the pulse dynamics in a Mamyshev oscillator: from seed signal to oscillator pulse.

The Mamyshev oscillator (MO) is a promising platform to generate high-peak-power pulse with environmentally stable operation. However, rare efforts have been dedicated to unveil the dynamics from seed signal to oscillator pulse, particularly for the multi-pulse operation. Herein, we investigate the buildup dynamics of the oscillator pulse from the seed signal in a fiber MO. It is revealed that the gain competition among the successively injected seed pulses leads to higher pump power that is required to ignite the MO, hence resulting in the higher optical gain that supports buildup of multiple oscillator pulses. The multiple oscillator pulses are identified to be evolved from the multiple seed pulses. Moreover, the dispersive Fourier transform (DFT) technique is used to reveals the real-time spectral dynamics during the starting process. As a proof-of-concept demonstration, a highly intensity-modulated pulse bunch was employed as the seed signal to reduce the gain competition effect and avoid the multi-pulse starting operation. The experimental results are verified by numerical simulations. These findings would give new insights into the pulse dynamics in MO, which will be meaningful to the communities interested in ultrafast laser technologies and nonlinear optics.

Mode-locked Yb-doped fiber laser with graded-index multimode fiber as saturable absorber with gain management

We experimentally demonstrate a novel, linear-cavity mode-locked Yb-doped fiber laser based on the graded-index multimode fiber (GIMF) as saturable absorber, and propose the use of varying gain levels by doubling gain fiber length to mitigate the technical difficulty of multimode fiber as a saturable absorber in the length accuracy requirements. It provides a more flexible solution to achieve a highly stable mode-locked fiber laser based on GIMF. By analogy to dispersion management, we refer to this scheme as gain management. By adopting the two mechanisms of flexible GIMF length and gain management simultaneously, a stable mode-locked pulse train with the repetition rate of 10.96 MHz and pulse duration of ∼83.4 ps at 1064.07 nm was generated. Stable pulse bunches with pulse numbers up to 12 in a single bunch were obtained when increasing the pump power. The proposed gain management method opens a promising way for practical highly stable mode-locked technology based on nonlinear multimode interference mechanism.

Tunable all fiber multi-wavelength mode-locked laser with a large dynamic range using polarization controller coiled SMF-GIMF-SMF structure as both saturable absorber and comb filter

We have demonstrated a tunable, multi-wavelength mode-locked fiber laser with a large dynamic range based on polarization controller (PC) coiled graded index multimode fiber (GIMF) as a saturable absorber (SA) and comb filter for the first time. The SA device that consists of a section of GIMF coiled into a polarization controller and two sections of single mode fibers aim to mitigate the technical difficulty of multimode fiber (MMF) in the length accuracy requirements and tune multi-wavelength operation in an Yb-doped fiber laser. Nonlinear multimode interference and transverse light field evolution in multimode fiber are simulated. Experimentally, we achieved the stable continuous-wave mode-locked (CWML), pulse bunches, and high-harmonic mode-locked (HML) operation. Furthermore, the PC-based GIMF structure can also be used as a filter to tune the laser line numbers. Continuously tunable multi-wavelength mode-locked operation with a large dynamic range was demonstrated by varying the polarization orientation of MMF. The range of six-wavelength spectrum is up to 28 nm. Our results indicate that the PC coiled GIMF could serve as an SA and a filter, which makes it beneficial to realize tunable, multi-wavelength mode-locked laser with a large dynamic range.

Polarization attractors driven by vector soliton rain.

Soliton rain is a bunch of small soliton pulses slowly drifting near the main pulse having the period of a round trip. For Er-doped fiber laser mode-locked by carbon nanotubes, for the first time, we demonstrate both experimentally and theoretically a new type of polarization attractors controllable by vector soliton rain. With adjusting the pump power, vector soliton rain takes the form of pulses with rotating states of polarization which enable transforming slowly evolving trajectories on the Poincaré sphere from the double-scroll spiral to the circle. The obtained results on controlling complex multisoliton dynamics can be of interest in laser physics and engineering with potential applications in spectroscopy, metrology, and biomedical diagnostics.

Harmonic mode-locked noise-like pulses under a Q-switched envelope in an erbium doped all-fiber ring laser.

We report for the first time, to the best of our knowledge, harmonic mode-locked noise-like pulses under a Q-switched envelope in an all-fiber erbium doped ring laser cavity, mode locked using the nonlinear polarization rotation (NPR) technique. For a cavity with a fundamental repetition rate of 1.33 MHz, stable mode-locked noise-like pulses, with few nanoseconds durations, single pulse energies around 30-40 nJ, and Q-switched repetition rates up to 31 kHz, were produced and characterized from the fundamental to the eighth harmonic. The formation and evolution of Q-switched harmonic mode-locked noise-like pulses were preceded by the formation of Q-switched mode-locked noise-like pulse bunches. This is also the first report on noise-like bunches under a Q-switched envelope, to our knowledge. Our studies also provide further insights into the interplay of different physical mechanisms involved in the production of such ultrashort pulses. Such sources should prove especially useful for efficient supercontinuum generation and laser micromachining.

Spectral-temporal patterned supercontinuum generation and pulse compression with tunable sub-terahertz repetition rate

We report on the generation of a spectral-temporal patterned supercontinuum (SC) that is a train of ultrashort pulse bunches with an ultra-broadband spectrum changing from pulse to pulse in a controllable manner. The SC is attained in a highly nonlinear silica fiber pumped by a specially designed mode-locked fiber laser that can generate homogeneous soliton molecules (SMs) consisting of similar high-amplitude dissipative solitons and heterogeneous SMs consisting of interleaved high- and low-amplitude pulses. The temporal distance between the spectrally broadened pulses can be adjusted in a wide range from 5 to 170 ps by tuning the laser. The SC spans from 1350 to beyond 1700 nm. It is formed by Raman-shifted solitons in the long-wavelength region and dispersive waves in the short-wavelength region, whose wavelength shift regularly varies from pulse to pulse depending on the amplitudes of the pump pulses. By selecting different parts of the SC spectrum it is possible to obtain different patterned optical pulse trains. It was shown that the pulse duration in the SMs can be decreased below 100 fs due to soliton-effect compression directly in the highly-nonlinear fiber.

Spectral filtering effect-induced temporal rogue waves in a Tm-doped fiber laser.

We have experimentally and theoretically investigated optical rogue waves (ORWs) in a net negative dispersion Tm-doped fiber laser with a long cavity, adopting nonlinear polarization evolution as a mode-locker as well as a spectral filter. We obtained a state with numerous pulses bunched in a burst accompanied by perturbation within the burst, in which the spectrum was partially perturbed. After statistical analysis, we found that ORWs have existed in this bunching state. By adjusting the intra-cavity polarization controllers, the perturbed pulse bunching turned into a chaotic pulse bunching state, which gave rise to giant pulses with ultra-high amplitudes, and the giant pulses were a precursor of a broad-spectrum noise-like pulse. The probability of occurrence of ORWs was increased in the chaotic state, which is caused by multi-pulse instability induced by the spectral filtering effect. Simulation results confirm the experimental results and demonstrate that the spectral filter bandwidth (SFB) is directly related to the probability of the emergence of ORWs. When increasing the SFB across the range of multi-pulse instability at a fixed pump power, the frequency with which ORWs appear increases.

Modulated comb-like pulse distribution induced by intracavity Sagnac filtering in a fiber laser

We demonstrate experimentally a modulated comb-like pulse distribution generated from a mode-locked fiber laser with an intracavity Sagnac interferometer. The modulated comb-like pulse bunch profile is composed of a sequence of symmetrically distributed pulses with a fixed temporal separation. The modulated bunch profiles can be dynamically adjusted by manipulating the pump power and the polarization settings of the fiber laser. The pulse separation in the bunch is determined by the optical delay line of the Sagnac interferometer. We develop a theory to model our experimental results and calculate the transient generation process of the comb-like pulse bunch. The theoretical results are in accordance with the experimental results. For the first time the generation of the modulated comb-like pulse distribution is proved by theoretical and experimental analyses to be a result of intracavity Sagnac filtering. These results will help in realizing adjustable modulated pulse bunch generation in noise-driven dissipative systems for potential applications in laser measuring and information processing.

Experimental design of radiation reaction by 1 PW laser pulse and linear accelerator electron bunch

Radiation reaction (RR) is the frictional effect of a relativistically energetic and radiating electron. This situation can be created when the energetic electron collides with an ultra-intense laser pulse. We propose a RR experiment by making use of the collision of a 600 MeV electron beam from the linear accelerator and an ultra-intense laser pulse focused to 1022W/cm2 at ELI-NP. Its purpose is to distinguish RR in the quantum-dominant (semi-classical) regime when χ≈0.4 in comparison with the classical RR model, where, χ∝(electronenergy)×laserintensity. The numerical studies demonstrate that scattered electrons with the energy between 80–200 MeV and emitted photons above 300 MeV are needed to clarify the difference between these RR models. The layout of the experiment is proposed to enable the measurements of electron energy spectra and photon polarization by following the numerical results. The parameter N indicating the number of absorbed laser photons by an electron is introduced to express the nonlinearity of RR. The parameter set of (χ,N) resolves the physical regimes of RR and demonstrates a roadmap of the past and future of the RR experiments. The ELI-NP parameter set can provide a plot in the higher-N region as N>5×105 with the almost fixing χ parameter by comparing with the past researches.

Multiple dissipative solitons with equal and invariant temporal spacing in Yb-doped all-normal-dispersion fiber lasers

A new category of multiple dissipative solitons (DSs) with equal and invariant temporal pulses spacing was observed in a Yb-doped passively mode-locked all-normal-dispersion fiber laser based on nonlinear polarization rotation (NPR). It is observed that the number of DSs in a pulse bunch per roundtrip can be adjusted continuously by the pump power while the temporal spacing between each pulse remains invariant and is independent of the cavity‘s length. A theoretical model corresponding to the experimental configuration established to simulate the pulse generation based on coupled nonlinear Schrödinger equations. It is found that the experimental phenomenon can be well predicted by the theoretical model. It is expected that the multiple DSs with equal-temporal spacing may find applications in sensing or optical communications and also provide fundamental evidence in soliton dynamics and nonlinear systems.

PCA-Based Matrix CFAR Detection for Radar Target.

In radar target detection, constant false alarm rate (CFAR), which stands for the adaptive threshold adjustment with variation of clutter to maintain the constant probability of false alarm during the detection, plays an important role. Matrix CFAR detection performed on the manifold of Hermitian positive-definite (HPD) covariance matrices is an efficient detection method that is based on information geometry. However, the HPD covariance matrix, which is constructed by a small bunch of pulses, describes the correlations among received data and suffers from severe information redundancy that limits the improvement of detection performance. This paper proposes a Principal Component Analysis (PCA) based matrix CFAR detection method for dealing with the point target detection problems in clutter. The proposed method can not only reduce dimensionality of HPD covariance matrix, but also reduce the redundant information and enhance the distinguishability between target and clutter. We first apply PCA to the cell under test, and construct a transformation matrix to map higher-dimensional matrix space to a lower-dimensional matrix space. Subsequently, the corresponding detection statistics and detection decision on matrix manifold are derived. Meanwhile, the corresponding signal-to-clutter ratio (SCR) is improved. Finally, the simulation experiment and real sea clutter data experiment show that the proposed method can achieve a better detection performance.

Wavelength tunable picosecond pulse bunch laser around 3 µm through PPMgLN –based DFG with temperature insensitive high power scaling

We report a bunch-mode-operated, wavelength-tunable, temperature-insensitive high power, mid-infrared (MIR) laser based on difference frequency generation (DFG) in a periodically poled Magneoisum-oxide doped Lithium Niobate (PPMgLN) chip. Two synchronized fiber lasers each with master oscillator power amplifier (MOPA) structures are utilized as pump and signal sources for the DFG. The pump laser is seeded by a mode-locked Yb fiber laser around 1030 nm with fundamental repetition rate of 16.32 MHz and pulse duration of 8.5 ps. Each pulse of the pump seed is multiplied by a factor of 16, forming the pulse bunch, and then amplified to 27.8 W. A super-luminescent light emitting diode (SLED) around 1580 nm is triggered by the first pulses of the pump bunch and adopted as the signal seed, which is amplified to 4 W afterwards. The pulse duration and spectral bandwidth of the signal are ~ 5 ns and 50 nm, respectively, covering the total duration of the pump bunch in the time domain and signal wavelength tuning range in the spectral domain, respectively, enabling wavelength-tuning DFG with temperature-insensitive power scaling. High power pulse bunch MIR laser can always be realized within the crystal temperature range from 25 to 95 °C with corresponding idler wavelength from 3.03 μm to 2.84 μm. The highest idler power of 4.6 W is achieved at 3.0 μm with pump-to-idler conversion efficiency of ~ 16.5%. It is believed that such a bunch-mode-operated, mid-infrared ps laser source with high output power within the whole temperature-wavelength tuning range are of great value to the evaluation of wavelength dependent laser bio-ablation.

High-power mid-infrared picosecond pulse bunch generation through difference frequency generation

We report a high average power mid-infrared picosecond (ps) pulse bunch output by means of direct difference frequency generation (DFG) in periodically poled magnesium-doped lithium niobate between a linearly polarized ps pulse bunch Yb fiber laser and a synchronized Er fiber laser. The ps pulse bunch Yb fiber laser was composed of an all polarization maintained “figure of eight” structured mode-locked Yb fiber laser as the seed, a pulse multiplier, and two stages of Yb fiber amplifiers. The mode-locked Yb fiber laser has an output ps laser pulse at 1030 nm with a repetition rate of 16.32 MHz. The pulses were then transformed to the pulse bunches through the pulse multiplier. Within each bunch, there were 16 equally spaced pulses with pulse widths of 8.5 ps and time intervals around 300 ps. The Er fiber laser had a gain switched seed laser diode working at 1550 nm with a pulse width around 5.1 ns, which was synchronized to the mode-locked Yb fiber laser pulse bunch, and two stages of Er fiber amplifiers. Under the average power of 27.8 W of Yb fiber laser, we obtained an average output power of 3.1 W at 3.07 µm. This is, to the best of our knowledge, the highest average power mid-infrared ps pulse bunch obtained via DFG directly.

Spectral Norm Based Mean Matrix Estimation and Its Application to Radar Target CFAR Detection

The estimation of mean matrix for a set of Hermitian positive definite (HPD) matrices can be considered based on a geometric framework. This kind of estimator is closely related to geometric measures, which can be established to measure the dissimilarity between two matrices on the matrix manifold. In this paper, the matrix spectral norm is exploited to derive a new geometric measure on the matrix manifold consisting of all HPD matrices. Correspondingly, the mean matrix associated with the new geometric measure on the matrix manifold is considered via two effective solution techniques. More specifically, the first method transforms the mean matrix estimation into a semidefinite programming problem solved by the interior point method; the second method exploits joint approximate diagonalization to diagonalize the given covariance matrices and then recasts the mean matrix estimation into an optimization problem with linear inequality constraints. Furthermore, two matrix detectors employing the proposed mean matrix estimators are designed for target detection using pulse Doppler radar with a small bunch of pulses. At the analysis stage, we show that the proposed matrix detector can guarantee the bounded constant false alarm rate property in terms of the clutter covariance matrix. Finally, numerical experiments based on simulated clutter data and real sea clutter data are performed to verify the effectiveness and robustness of the proposed matrix detectors.

Tailoring of multi-pulse dynamics in mode-locked laser via optoacoustic manipulation of quasi-continuous-wave background

Various nonequilibrium multi-pulse states can emerge in a mode-locked laser through interactions between the quasi-continuous-wave background (qCWB) and pulses inside the laser cavity. While they have been regarded as unpredictable and hardly controllable due to the noise-like nature of qCWB, we here demonstrate that the qCWB landscape can be manipulated via optoacoustically mediated pulse-to-qCWB interactions, which alters the behaviors of multi-pulse dynamics. In this process, impulsive qCWB modulations are created at well-defined temporal locations, which act as the point emitters and attractive potentials for drifting pulse bunches and soliton rains. Hence, we can transport a single pulse bunch from a certain temporal position to another, and also make soliton rains created and collided exclusively at specific temporal locations. Our study opens up possibilities to control the nonequilibrium multi-pulse phenomena precisely in the time domain, which would offer a practical means of advanced optical information processing.

Initiation of Kerosene Combustion in a Supersonic Air Flow by a Bunch of Gas-Dynamic Pulses

In this paper, we propose a method to initiate combustion of kerosene in a supersonic air flow in part of a combustion chamber with a constant cross section and experimentally confirm the possibility of its implementation. An intensive combustion mode is implemented using a bunch of pulses with variable energy and is maintained when the exposure is turned off after the combustion is arranged in the diverging part of the combustion chamber.

New matrix constant false alarm rate detectors for radar target detection

Matrix constant false alarm rate (M-CFAR) detector based on information geometry provides a novel scheme to the detection of target in sea clutter environment for pulse Doppler radar with small bunch of pulses. The detection performance of the existing M-CFAR detectors is greatly influenced by the geometric measures on the matrix manifold. Different geometrical measures lead to different detection performance, some of which are not satisfactory. Motivated by this, the authors utilise Root-Euclidean, Power-Euclidean, Cholesky-Euclidean and new Riemannian measure to design several kinds of detection schemes, which improve the detection performance. Finally, simulation results verify that the designed detectors achieve a better detection performance with a relatively lower computational complexity.

Large-energy mode-locked ytterbium-doped linear-cavity fiber laser based on chemical vapor deposition-Bi2Se3 as a saturable absorber.

We reported on the generation of pulse bunch and large-energy dark pulses in a mode-locked ytterbium-doped linear-cavity fiber laser based on Bi2Se3 as a saturable absorber (SA). Bi2Se3 nanosheets were successfully synthesized by the chemical vapor deposition (CVD) method and transferred to the end facet of a fiber connector for the proposed SA. Its saturation intensity and modulation depth were measured to be 52 MW/cm2 and 14.5%, respectively. By inserting the Bi2Se3-based SA into the Yb-doped all-fiber linear cavity, stable pulse bunches were observed. In addition, dark soliton operation with a maximum average output power of 32.6 mW and a pulse energy of 61.8 nJ were also achieved. To the best of our knowledge, this is the first demonstration of a dark soliton within a linear cavity with much larger pulse energy than previous works. Our study fully indicated that CVD-Bi2Se3 could be an excellent SA for achieving large-energy pulse operations.