Noise-like Pulses Research Articles

Harmonic Dissipative Soliton Resonance in an Er/Yb Co-Doped Fiber Laser Based On SESAM

Fundamental and harmonic mode-locking in the dissipative soliton resonance (DSR) regime is experimentally investigated in a net-normal-dispersion Er/Yb co-doped fiber laser based on the semiconductor saturable mirror (SESAM), where an equivalent narrow-band spectral filter is used for triggering multi-pulsing. By monotonically increasing the pump power with the fixed polarization controller (PC), the fundamental mode-locking operation of DSR pulses can be transformed into the harmonic mode-locking (HML) under the specific conditions. The main reason is the peak-power-clamping effect induced by the combined effects of the spectral filtering and saturable absorption of the SESAM at higher pump levels, which is applied to achieve the HML in the DSR regime for the first time. The DSR pulse broadens linearly with the pump power in a certain range, no matter whether it is in the fundamental mode-locking or HML state. By incorporating the dispersion compensation fiber (DCF) with different lengths, the pulse characteristics are discussed and compared in detail. The enhanced nonlinearity caused by increasing DCF length can lead to higher orders of HML pulses and even the generation of noise-like pulses (NLPs) with a long-enough DCF.

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 dynamics of noise-like square pulses generated in a Tm-doped figure-8 fiber laser

We report the generation of a series of noise-like square pulses (NLSPs) at 2 μm in a figure-8 Tm-doped fiber laser. The NLSP demonstrates a square shape as well in the spectral domain and possesses a resonant property in spectrum similar to that in time. The spectral resonance ranges from 10 nm to about 20 nm under an appropriate control of polarization, an indication of its wide mode-locking regime. The unique spectral dynamics exhibits a strong polarization dependence, subject to the nonlinear mode-locking mechanism. A subsequent numerical simulation shows that a uniform gain dispersion distribution accounts for the forming of the flat-topped spectra.

Improving the formation probability and stability of noise-like pulse by weakening the spectrum filtering effect.

Noise-like pulses (NLP) are extremely sought after in low-coherence tomography and supercontinuum, etc. Here, we propose an effective method to form the NLP in the all-normal-dispersion (ANDi) fiber laser by weakening the spectrum filtering for the first time. Numerical explorations are performed in detail and demonstrate that the NLP can be originated from the clustering behavior of amplified random sub-pulses led by the saturable absorber. By simulating the pulse-pattern distribution in the two-dimensional parameter space, it is further found that this kind of NLP pattern is widely distributed where are with a weak spectrum filtering. Since, the weaker the filtering, the harder the dissipative system to achieve balance, which helps to avoid the evolution from the pulse cluster to other coherent states and supports the NLP stability. To prove the feasibility experimentally, we built an ANDi fiber laser based on the nonlinear polarization rotating (NPR). The built laser can operate at the stable dissipative soliton (DS) state with a spectrum filter, which also with a 45 nm wavelength tuning performance. Replacing the filter, only NLP with a 40.2 ps pedestal and 237 fs spike can be attained. The experiments agree well with the numerical predictions. This exploration significantly broadens the design possibilities for ultrafast lasers, making them much more accessible to produce desired pulse patterns.

Bound noise-like pulse generation from Yb-doped passively mode-locked fiber lasers

We demonstrated two different types of bound noise-like pulses (NLPs) in ytterbium-doped mode-locked fiber lasers using the nonlinear polarization rotation technique. In the anomalous dispersion area, there were multiple spikes in the intensity autocorrelation (IAC) trace. In the normal dispersion area, the spike presented only in the center the of IAC trace. The results of the experiment show that the bound state NLPs may have partial coherence.

Noise-like pulse generation in an Nd-doped single-mode all-fiber mode-locked Raman laser operating at 0.93 µm.

Based on the Nd-doped single-mode fiber (SMF) as the gain medium and SMF as the Raman medium, an all-fiber mode-locked Raman laser operating at 0.93 µm waveband was demonstrated for the first time. A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress the dominant emission of Nd-doped fiber at 1.06 µm. A noise-like pulse with a pulse width of 194.70 fs, a repetition rate of 1.73 MHz and a single pulse energy of 2.03 nJ was obtained in the mode-locked Raman laser with a Stokes wavelength of 932.59 nm. Such an ultrafast all-fiber Raman laser operating at 0.93 µm has the advantages of low cost, simple structure and compactness, and can be used as an ideal light source for the two-photon microscopy.

All-Fiber Mid-Infrared Supercontinuum Generation Pumped by Ultra-Low Repetition Rate Noise-Like Pulse Mode-Locked Fiber Laser

We demonstrate broadband mid-infrared supercontinuum generation in a piece of indium fluoride fiber pumped by an ultralow repetition rate noise-like pulse mode-locked fiber laser. This figure-9 mode-locked Tm-doped fiber laser has a long laser cavity by inserting a piece of standard single mode fiber and a piece of dispersion compensation fiber, which delivers mode-locked noise-like pulse with a repetition rate of ∼190 kHz. The mode-locked noise-like pulse train has a low pulse duty cycle of 4.38 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> . With only one Tm-doped fiber amplifier, the ultra-low repetition rate noise-like pulse is amplified to ∼3 W. The pulse energy is boosted to ∼15 μJ with a peak power of ∼6.5 kW. Pumping a piece of highly nonlinear indium fluoride fiber with the amplified noise-like pulse, a flat mid-infrared supercontinuum with a covering range from ∼1500 nm to ∼4100 nm can be achieved. Due to the low operational power, no cooling installations are required to control the temperature of the supercontinuum laser system. This all-fiber mid-infrared SC laser source has a relatively simple architecture and low cost. This study demonstrates the great advantages of this type of ultra-low repetition rate noise-like pulses in mid-infrared supercontinuum generation.

Stable noise-like pulse generation from a NALM-based all-PM Tm-doped fiber laser.

An all-polarization maintaining (PM) noise-like pulse (NLP) generation from a Tm-doped fiber oscillator based on nonlinear amplifying loop mirror (NALM) that incorporated with a phase shifter and a chirped fiber Bragg grating (CFBG) is experimentally demonstrated. The 3 dB bandwidth of the output spectrum is 25 nm at the central wavelength of 1950 nm, and the maximum output average power is 13.6 mW with the repetition rate of 3.25 MHz. The noise performances of the NLP are for the first time systematically examined, and it shows an improving tendency with the increasing of the output power. At an integration frequency range from 1 kHz to 1 MHz, the minimum estimated timing jitter and the rms RIN is 139 ps and 0.58%, respectively. In addition, the long-term stable operation of the laser is verified through monitoring the output spectrum and average power.

Vector Staircase Noise-Like Pulses in an Er/Yb-Codoped Fiber Laser

We report on the experimental observation of vector staircase noise-like pulses (NLPs) produced by an Er/Yb-codoped fiber laser with a nonlinear optical loop mirror (NOLM) as the saturable absorber (SA). The staircase NLPs consist of two rectangular pulses with different energies and durations, such that the two pulse widths both increase linearly with increasing pump power with an almost constant slope while the pulse amplitude remains constant, similar to the case with Dissipative Soliton Resonance pulses. The polarization rotation state of the staircase NLPs has also been investigated along with multiple polarization dynamic patterns, including group velocity locked and polarization locked states. We have also observed harmonic mode-locked (HML) staircase NLPs by carefully adjusting the orientation of the intra-cavity polarization at fixed pump power. It was found that the HML staircase NLPs were identical to the fundamental staircase NLPs as regards the vector character and the changes in pulse duration. The experimental results are expected to enrich understanding of the vector nature of staircase NLPs and may lead to a new laser source for material processing.

Wavelength-tunable noise-like fiber laser based on PbS quantum dot saturable absorber film.

A wavelength-tunable noise-like pulse (NLP) erbium-doped fiber laser incorporating PbS quantum dot (QD) polystyrene (PS) composite film as a saturable absorber (SA) is experimentally demonstrated. The wavelength tuning is implemented via a Lyot filter consisting of a segment of polarization-maintaining fiber (PMF) and a 45° tilted fiber grating. By adjusting the polarization state of the ring cavity, the laser can deliver NLP with a continuous wavelength-tunable range from 1550.21 to 1560.64 nm. During continuous wavelength tuning, the output power varies between a range of 30.88-48.8 mW. Worthwhile noting is that the output power of 48.8 mW is the reported highest output power for wavelength-tunable NLP operation in an erbium-doped fiber laser using composite film as a saturable absorber.

Noise-Like Pulse Generated by All-Fiber Ultrafast Thulium-Doped Fiber Laser Based on Single-Wall Carbon Nanotubes

A noise-like pulse (NLP) thulium-doped fiber laser based on single-wall carbon nanotubes (SWCNTs) has been proposed and demonstrated in this paper, which the microfiber decorated with SWCNTs work as a saturable absorber to generate an ultrafast pulse. The length of the cavity is 12 m, corresponding to the fundamental frequency of 17.2 MHz. NLP with a duration of 643 fs can be obtained at 1965 nm under the signal-to-noise ratio of 56 dB. This work may assist us to better understand the dynamic characteristic of NLP with the help of materials and offer a fine laser source to the subsequent applications just like the generation of supercontinuum.

Nano-tungsten trioxide saturable absorber for L-band noise-like pulse mode-locked fiber laser

We demonstrate the generation of noise-like pulse (NLP) in L-band erbium-doped fiber laser incorporating nano-tungsten trioxide and polydimethylsiloxane (WO3/PDMS) decorated onto a tapered fiber structure. A Raman pump unit with 3.5 W pump power was employed for a core-pumping scheme of 7 m long highly doped erbium-doped fiber. At the maximum pump power, the NLP lasing wavelength was at 1602 nm with a broad spectral bandwidth of 19.05 nm. The generated pulse was characterized to have 10.38 nJ energy and 19.66 mJ/cm2 output fluence. The measured NLP spike pulse width was 178.87 fs within 56.56 ps pedestal range. This work paves way for future development of generating NLP from nano-tungsten trioxide under extreme pump powers.

Transition between noise-like pulses and Q-switching in few-mode mode-locked lasers.

Spatiotemporal mode-locked lasers have attracted extensive attention of researchers due to the complex nonlinear evolution process. Compared to single-mode mode-locked lasers, intermodal interactions greatly affect the pulses evolution in spatiotemporal mode-locked lasers. Here, we experimentally investigate the transition process between noise-like pulses and Q-switching pulses in few-mode mode-locked laser by rotating the plates, where a transition state is greatly broadened in the time domain. By means of spectral filtering, we verify that the process is the reconstruction of Q-switching between different modes to noise-like pulses. Furthermore, during the evolution of noise-like pulses, soliton collisions are detected using dispersive Fourier transform technology. Our research contributes to revealing the transient evolution process in few-mode mode-locked lasers, and enriches the study of nonlinear process.

Broadband saturable absorption in germanene for mode-locked Yb, Er, and Tm fiber lasers

Abstract Passively mode-locked lasers have been widely investigated as one of the effective methods to obtain ultrashort pulses. As an important part of passively mode-locked fiber lasers, the exploration of 2D material-based saturable absorber has become one of the hotspots in ultrafast photonics in recent years. Germanene, a novel 2D Dirac material, with ultrafast optical response and broadband optical absorption, is a promising alternative material for saturable absorber in mode-locked fiber lasers. In this paper, germanium nanosheets are prepared via liquid-phase exfoliated method, with the saturable absorption property systematically characterized in three major wavebands of the near-infrared region. The generation of ultrashort pulses based on germanene saturable absorber in fiber lasers is further realized, in a broad waveband (1000 nm) centered at 1061.1, 1559.3 and 1883.5 nm, respectively. In addition, noise-like pulses operation with central wavelength of 1558.6 nm is also obtained, and the formation of rogue waves is further demonstrated via statistical analysis. To the best of our knowledge, this is the first experimental verification of the broadband saturable absorption property of germanene-based devices, covering three major fiber laser wavelengths from 1.0 to 2.0 μm.

Noise-like pulses from all-normal dispersion ytterbium doped all-fiber oscillator with semiconductor saturable absorber

We report generation of stable train of noise like pulses (NLPs) from all-normal dispersion ytterbium doped all-fiber unidirectional ring cavity consisting of semiconductor saturable absorber (SeSA) without any bandpass filter (BPF). NLPs are characterized by their autocorrelation traces which exhibit ∼500 fs spike riding on a broad pedestal of ∼200 ps. NLP operation is also supported by its relatively broad optical spectrum along with reasonably stable longer pulse profile on oscilloscope. Numerical simulations have also been performed in detail to study the effects of cavity parameters like the cavity length, BPF bandwidth and depth of modulation (q 0) of SeSA on the evolved pulses from the cavity. The simulation results suggest that stable single clean pulse (SSCP) is generated for a certain range of cavity parameters and in the absence of BPF no SSCP is possible from the cavity if the value of q 0 is below a certain value. Under this condition either NLP or no pulse is generated in the simulation. Numerically simulated NLP in a ring cavity matches qualitatively with the experimentally obtained results for NLP. The study carried out on NLP will enrich database of pulse dynamics in modelocked oscillators and has potential application in optical coherence tomography and supercontinuum generation.

Passively mode-locked Ho-doped fiber laser with soliton rain and noise-like pulse regime

A passively Ho-doped fiber laser at 2 μm with soliton rain and noise-like pulse (NLP) operation regime has been realized by using nonlinear polarization rotation (NPR) technology. By adjusting the polarization state, the evolution phenomenon of soliton rain pulse and the NLP are obtained. The typical NLP mode-locking operation regime is achieved by finely adjusting pump power and polarization state, which delivers a corresponding pulse energy of 8.93 nJ with pulse width of 1.31 ps. The soliton rain operation regime with multiple solitons is achieved by finely adjusting pump power, which delivers a corresponding pulse energy of 22.98 nJ with pulse width of 1.21 ps. In addition, a NLP mode-locking operation regime is obtained again under a proper pump power and polarization state with a corresponding pulse energy of 34.14 nJ and pulse width of 1.18 ps. The experimental results well demonstrate the evolution of ultrashort pulse regime with the pump power and polarization state in passively mode-locked Ho-doped fiber lasers at 2 μm.

Machine learning analysis of instabilities in noise-like pulse lasers.

Neural networks have been recently shown to be highly effective in predicting time-domain properties of optical fiber instabilities based only on analyzing spectral intensity profiles. Specifically, from only spectral intensity data, a suitably trained neural network can predict temporal soliton characteristics in supercontinuum generation, as well as the presence of temporal peaks in modulation instability satisfying rogue wave criteria. Here, we extend these previous studies of machine learning prediction for single-pass fiber propagation instabilities to the more complex case of noise-like pulse dynamics in a dissipative soliton laser. Using numerical simulations of highly chaotic behaviour in a noise-like pulse laser operating around 1550 nm, we generate large ensembles of spectral and temporal data for different regimes of operation, from relatively narrowband laser spectra of 70 nm bandwidth at the -20 dB level, to broadband supercontinuum spectra spanning 200 nm at the -20 dB level and with dispersive wave and long wavelength Raman extension spanning from 1150-1700 nm. Using supervised learning techniques, a trained neural network is shown to be able to accurately correlate spectral intensity profiles with time-domain intensity peaks and to reproduce the associated temporal intensity probability distributions.

Noise-like pulse generation and amplification from soliton pulses.

The evolution of soliton pulses into noise-like pulses in a nonlinear fiber externally to the laser oscillator is demonstrated at 1.9 µm, for the first time. Soliton collapse based mechanisms induce noise-like pulses with varying properties as a function of nonlinear fiber length without requiring any laser cavity feedback. The proposed method allows the generation of noise-like pulses with a sub-300 fs spike and sub-40 ps pedestal duration. Power scaling of the noise-like pulses is demonstrated in a double-clad thulium-doped fiber amplifier with amplification up to an average power of 5.19 W, corresponding to a pulse energy of 244 nJ. This method provides an alternative route for generating fully synchronized noise-like pulses and solitons in the same system, without relying on the conventionally used mechanism of changing the intracavity nonlinearity within the laser cavity.

High-energy quasi-trapezoid noise-like pulse generation and harmonic operation in a fiber laser

We report on the observation of quasi-trapezoid noise-like pulses (NLPs) in a fiber laser with a large anomalous dispersion regime based on nonlinear polarization rotation. Trapezoidal NLPs operate at 1568 nm with a fundamental repetition rate of 946.14 kHz. The pulse width can widen from 12.54 to 32.17 ns, and the pulse energy can be increased from 8.81 to 25.84 nJ by increasing the pump power from 77.4 to 244.4 mW. The maximum intracavity energy of a trapezoidal pulse can reach ~258.4 nJ. Furthermore, stable high-order harmonics are obtained by regulating the pump power and polarization state. In the harmonic mode-locking regime, the second- to the tenth-order harmonic trapezoidal pulses can be achieved in the fiber laser.

Graded-index multimode fiber in nonlinear absorbing loop mirror as saturable absorber for noise-like pulse generation in 2 μm region

This investigation explores the potential use of graded-index multimode fiber (GIMF) in a nonlinear absorbing loop mirror (NAbLM) as an artificial saturable absorber for ultrashort pulse generation. The saturable absorption mechanism of GIMF-NAbLM was based on a combination of nonlinear phase shift and multimodal interference that was satisfied by the asymmetric positioning of the GIMF in the single-mode fiber loop mirror. The proposed saturable absorber was integrated in a mode-locked thulium-doped fiber laser with figure-of-eight cavity. A noise-like pulse of 164 fs spike pulse duration at 1950 nm central wavelength was attained in the net anomalous dispersion with a 10 m long GIMF. The GIMF-NAbLM structure demonstrates immense prospects as an effective saturable absorption device for generating stable ultrashort pulse in the 2 µm region due to its simplicity.