Single and dual-wavelength noise-like pulses with different shapes in a double-clad Er/Yb fiber laser

Frequency-domain-resolved investigation of unitary h-shaped pulses

We experimentally investigate the generation of h-shaped pulse in an all-polarization-maintaining (PM) and all-normal-dispersion (ANDi) mode-locked fiber laser. The generated pulse is demonstrated to be a unitary pulse, instead of a noise-like pulse (NLP). Furthermore, by employing an external filtering system, the obtained h-shaped pulse can be resolved into rectangular-shaped pulses, chair-like pulses, and Gaussian pulses. The authentic AC traces with a double-scale structure of unitary h-shaped pulses and chair-like pulses are observed on the autocorrelator. The chirp of h-shaped pulses is also proved similar to that of DSR pulses. To the best of our knowledge, this is the first time that the existence of unitary h-shaped pulse generation has been confirmed. Moreover, our experimental results reveal the close relationship of formation mechanisms of dissipative soliton resonance (DSR) pulses, h-shaped pulses, and chair-like pulses, which helps to unify the essences of such "DSR-like" pulses.

All-fiber bidirectional mode-locked laser to generate cylindrical vector beam asynchronous noise-like pulses

We proposed and demonstrated a bidirectional mode-locked fiber laser to generate cylindrical vector beam (CVB) asynchronous pulses based on a graded index multimode fiber. A homemade fused taper two-mode fiber optical coupler (TMF-OC) is employed as a mode converter. The central wavelength for clockwise (CW) pulses can be tuned from 1030.32 nm to 1041.04 nm due to the filtering effect based on multimode interference, that of counterclockwise (CCW) pulses is from 1030.81 nm to 1039.28 nm. When the central wavelengths are 1033.22 nm and 1032.71 nm for CW direction and CCW direction respectively, CVB asynchronous noise-like pulses with a repetition rate difference of ∼436.9 Hz can be obtained. The purity of CVB in CW direction and CCW direction is 95.7% and 93.4% respectively. This bidirectional mode-locked fiber laser with CVB output can be better applied to laser gyroscopes, asynchronous sampling, and dual-comb technique, and impel the interdisciplinary studies in the future.

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.

Evolutions of Q-switched mode-locked square noise-like pulse with different cavity lengths

A $Q$-switched mode-locked square noise-like pulse (QMLSNLP) is generated in a nonlinear polarization rotation passively mode-locked fiber laser. When the pump power changes from 154 mW to 414 mW, the frequency of the $Q$-switched envelope varies from 21.7 kHz to 38.9 kHz, while the duration of the $Q$-switched envelope decreases from $5.1\,\,{\unicode{x00B5}\rm s}$ to $3.2{\unicode{x00B5}\rm s}$, correspondingly. In the meantime, QMLSNLP keeps the fundamental repetition rate constant, and the pulse duration increases from 3.4 ns to 6.7 ns. By inserting different lengths of single-mode fiber into the ring cavity, the evolutions of QMLSNLP are measured and analyzed. According to the cavity parameters and experimental results, impacts of the cavity length on QMLSNLP are investigated in detail.

Observation of square- and h-shaped pulse from a mode-locked erbium-doped fiber laser

We demonstrate the generation of a type of square-shaped pulse in a passively mode-locked erbium-doped fiber laser based on the nonlinear optical loop mirror technique. Through adjusting the pump power and polarization state, square-shaped pulses are generated. Furthermore, we investigate the pulse profile in relation to the optical spectrum. By filtering out short-wavelength spectrum components gradually, pulse shaping is achieved, and the top of the square-shaped pulse becomes flat. Subsequently, by filtering out long-wavelength spectrum components, a type of h-shaped pulse is obtained and the formation reason is also investigated.

Generation, characterization, and experimental analysis of noise-like pulse envelopes with complex shapes

We present the generation of 41 noise-like pulse (NLP) envelopes with complex shapes using a passively mode-locked, erbium-doped figure-eight fiber laser (EDFEFL). The tuning of each of the complex forms was carried out by varying the polarization state within the laser cavity, using a quarter-wave retarder (QWR2) inside a nonlinear optical loop mirror (NOLM), which is part of the EDFEFL. The position of the retarder plate was identified and recorded for each of the complex shapes. The temporal and spectral characterization was done using the position of the WQR2 wave plate as an independent variable. We present a single-shot analysis of the dynamics for the temporal amplitude, the full width at half-maximum (FWHM), and the root-mean-square (RMS) width for each of the 360 cycles measured for the 41 complex envelopes. We also perform an analysis for the case in which the pulse is completely divided into subpackets. We analyze the corresponding spectral profile for each of the complex forms generated. Finally, we evaluate the performance of the NOLM theoretical model with our experimental results. The wavelength of the NLPs is 1560 nm; the period of the cavity fiber laser is 1.1 ms; and the temporal FWHM is within the range of nanoseconds.

Fe3O4 nanoparticle-enabled mode-locking in an erbium-doped fiber laser

In this paper, we have proposed and demonstrated the generation of passively mode-locked pulses and dissipative soliton resonance in an erbium-doped fiber laser based on Fe₃O₄ nanoparticles as saturable absorbers. We obtained self-starting mode-locked pulses with fundamental repetition frequency of 7.69 MHz and center wavelength of 1561 nm. The output of a pulsed laser has spectral width of 0.69 nm and pulse duration of 14 ns with rectangular pulse profile at the pump power of 190 mW. As far as we know, this is the firsttimethatFe₃O₄ nanoparticles have been developed as low-dimensional materials for passive mode-locking with rectangular pulse. Our experiments have confirmed that Fe₃O₄ has a wide prospect as a nonlinear photonics device for ultrafast fiber laser applications.

Versatile multi-soliton patterns of noise-like pulses in a passively mode-locked fiber laser

We experimentally report the dynamics of multi-soliton patterns noise-like pulses (NLPs) in a passively mode-locked fiber laser, which the pulse duration can be linearly tuned from 8.21 ns to 128.23 ns by 2.936 ns / 10 mW. Benefiting from the drastically strengthened nonlinear effects in the cavity and the high gain amplification in the unidirectional ring (UR), the transformation from rectangular-shaped NLP to Gaussian-shaped NLP is experimentally achieved. Versatile multi-soliton patterns are observed in NLP regime for the first time, namely, single-scale soliton clusters, high-order harmonic mode-locking, and localized chaotic multiple pulses. In particular, the spectrum evolution with pump power and spectrum stability in 2 hours are also monitored. The obtained results demonstrate the rectangular-shaped NLP can fully transform into Gaussian-shaped NLP, and the multi-soliton patterns can exist in the NLP regime, which contributes to further understanding the nature and mechanism of the NLP in a passively mode-locked fiber laser.

Fundamental and harmonic mode-locked h-shaped pulse generation using a figure-of-9 thulium-doped fiber laser

Herein, we present a fundamental and harmonic mode-locked figure-of-9 thulium-doped fiber laser using a nonlinear amplifying loop mirror. The generated fundamental mode-locked h-shaped pulse is centered at 1889 nm with an average output power reaching 282 mW and a pulse energy up to 1.23 µJ, which is the highest power and pulse energy of an h-shaped pulse. In the harmonic mode-locked regime, up to the 8th harmonic h-shaped pulse is obtained. The detailed characteristics of the h-shaped pulse are discussed. The proposed study shows that the figure-of-9 fiber laser can generate h-shaped pulses and also allows the generation of nanosecond pulses with a µJ-level pulse energy.

Gain-driven spectral-temporal noise-like pulse dynamics in a passively mode-locked fiber laser

We study numerically complex noise-like pulse dynamics in a passively mode-locked erbium-doped fiber laser. Wavelength-dependent gain dynamics is modeled as a combination of a three-level and a four-level system, which approximate the gain behavior in the 1530-nm and 1560-nm regions, respectively. The typical deformation of the erbium gain spectrum as it saturates is properly reproduced by this approach. Several puzzling noise-like pulse dynamics that were recently observed experimentally are qualitatively reproduced numerically, in particular slow quasi-periodic energy variations and the emergence and walkoff of wavelength-shifted radiation components. These results clearly reveal that gain dynamics is deeply involved in the onset of such complex temporal and spectral instabilities in these sources.

High-energy mode-locked holmium-doped fiber laser operating in noise-like pulse regime

An all-fiber mode-locked holmium-doped fiber laser operating in the noise-like pulse regime is reported. Employing a nonlinear amplifying loop mirror as the artificial saturable absorber, the laser generates pulses with energy up to 280 nJ, which is, to the best of our knowledge, the highest pulse energy ever achieved from mode-locked holmium-doped fiber lasers. We also discover a peculiar mode-locking operation mode using the laser. The peculiar mode falls into the category of a noise-like pulse, while it possesses a number of unique features (strong spikes in the optical spectrum, high temporal coherence, and chair-like pulse profile) that are not observed in previous research works regarding noise-like pulse lasers.