Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser

Study on Spectrum Shifting and Pulse Splitting of Mode-Locked Fiber Lasers Based on NPR Technology

We conducted a systematic investigation into the spectral and pulse characteristics of C and L-band Nonlinear Polarization Rotation (NPR) mode-locked fiber lasers effectively employing nonlinear polarization rotation technology. In our experimental setup, we achieved a stable mode-locked state at 1560.076 nm, exhibiting a 3 dB spectral bandwidth of 9.1 nm. As the pump power increased, we observed spectral shifts accompanied by shifts in the first Kelly sideband and the generation of new Kelly sidebands. In this paper, the phenomenon of spectral deviation is elucidated through the interplay of self-phase modulation, group velocity drift, and polarization-dependent isolator (PD-ISO) filter effect, with an analysis of the formation and deviation of Kelly sidebands. Notably, spectral shift persisted even when the pump power exceeded 200 mW. However, continuous pump power escalation led to soliton splitting, resulting in the formation of new soliton beams. Based on the simultaneous generation of spectral shift and pulse splitting, our study contributes to an enhanced understanding of soliton dynamics in ultrafast fiber lasers and lays a foundation for the application of high-repetition-frequency harmonic mode-locked lasers with tunable wavelengths.

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-normal dispersion ytterbium-doped fiber laser mode locked by nonlinear multimode interference

We report a mode-locking operation using a nonlinear multimode interference (MMI)-based saturable absorber in an ytterbium-doped fiber laser under an all-normal dispersion configuration. The MMI is observed in a step-index single mode-multimode-single mode (SMS) fiber structure. The intensity and wavelength-dependent transmission of the SMS structure has been studied numerically to understand the Kerr-induced saturable absorber properties of MMI in a step-index fiber. Numerical investigations show that presence of a narrow-band wavelength-tunable spectral filter is necessary for a nonlinear MMI-based saturable absorber to be effective for mode locking. A single-pulse mode-locking operation in the laser is realized at a very low pump power of ${\sim}{56.3}\;{\rm mW}$ at which the laser generates 180 ps duration pulses at ${\sim}{9}\;{\rm MHz}$ repetition rate with measured signal-to-noise ratio of ${\sim}{50}\;{\rm dB}$. This all-fiber integrated laser operates in multi-pulse and noise-like rectangular pulse regimes at increased pump power.

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.

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.

Generation of mode-locked square-shaped and chair-like pulse based on reverse saturable absorption effect of nonlinear multimode interference

In this paper, the phenomenon of reverse saturable absorption of offset-spliced graded index multimode fibers (OS-GIMF) is revealed. And based on that, the stable square-shaped and chair-like mode-locked pulses are demonstrated with the maximum pulse energy of 0.14 µJ and 23.8 nJ respectively, while the OS-GIMF acts as a saturable absorber (SA) in fiber laser. By adjusting polarization controller (PC) and the pump power, square-shaped and chair-like pulse can be switched to each other. This multimode SA could sever as high power light source owing to its high damage threshold, compact structure and low cost.

Generation of stable clean ultrashort pulses in a simple all-fiber, all-normal dispersion ytterbium-doped mode-locked laser

We report an all-fiber, all-normal dispersion ytterbium (Yb)-doped fiber oscillator mode locked by nonlinear polarization evolution. The mode locking is realized in different pump power regimes by adjusting the polarization controllers. The average output power, pulse energy, and compressed pulse duration varies from 60 to 150 mW, 1.4 to 3.5 nJ, and 350 to 146 fs, respectively, when the pump power to the oscillator is varied from 250 to 500 mW. Particularly at 500 mW pump power, the oscillator generates highly chirped 5 ps duration pulses at 42 MHz that are compressed to a 146 fs duration with a clean temporal profile. The signal-to-noise ratio of the train of mode-locked pulses is 85 dB, indicating stable mode-locking operation. The generalized nonlinear Schrödinger equation is solved numerically to model the passive mode locking in a Yb oscillator, supporting the experimental results. In a chirped pulse Yb amplifier, the signal power is scaled to a 10 W level, and the amplified pulse is compressed to 355 fs duration.

Diverse mode of operation of an all-normal-dispersion mode-locked fiber laser employing two nonlinear loop mirrors

In this paper, we propose an all-normal-dispersion ytterbium-fiber laser with a novel ring cavity architecture having two nonlinear amplifying loop mirrors (NALM) as saturable absorbers, capable of delivering distinctly different pulses with adjustable features. By optimizing the loop lengths of the individual NALMs, the cavity can be operated to deliver Q-switched mode-locked (Q-ML) pulse bunches with adjustable repetition rates, mode-locked pulses in dissipative soliton resonance (DSR) regime or noise-like pulse (NLP) regime with tunable pulse width. The DSR pulses exhibit characteristic narrowband spectrum, while the NLPs exhibit large broadband spectrum. The operation regime of the laser can be controlled by adjusting the amplifier pump powers and the polarization controllers. To the best of the authors' knowledge, this is the first demonstration of a single mode-locked cavity where narrowband DSR pulses and broadband NLPs alongside Q-ML pulse bunches can be selectively generated by employing two NALMs.

Cavity-birefringence-dependent h-shaped pulse generation in a thulium-holmium-doped fiber laser

We report on a type of 2 μm h-shaped pulse generation in a thulium-holmium-doped fiber laser and experimentally investigate its cavity birefringence and pump power dependences. An asymmetric nonlinear optical loop mirror is employed as an artificial saturable absorber, which incorporates ∼52.7  m dispersion-shifted fiber and ∼3.8  m ultra-high numerical aperture fiber to enhance the nonlinearity. The h-shaped pulse shows both a polarization state (PS) and pump power related evolutions, even when randomly weak birefringence fibers are employed. By further incorporating different lengths of high birefringence polarization-maintaining fiber (PMF), i.e., introducing different amounts of linear cavity birefringence, much larger pulse tuning ranges can be realized. In particular, when the PMF is lengthened to ∼2.3  m through manipulating the PS, the achieved longest pulse duration of ∼318.14  ns can almost cover the whole repetition period of ∼323.96  ns, corresponding to a pulse duty circle of ∼98.2%, the largest ever reported from a fiber laser, to the best of our knowledge. We demonstrate the related characteristics in detail.