Generation of Different Mode-Locked States in Nonlinear Multimodal Interference-Based Fiber Lasers

A femtosecond mode-lock all-fiber laser based on the nonlinear multimode interference (NL-MMI) effect is demonstrated. The saturable absorber (SA) in this laser is a graded-index multimode fiber-step index multimode fibergraded-index multimode fiber (GIMF-SIMF-GIMF) structure. The laser can work in mode-lock or soliton mode-lock two states. Through finely turning the polarization controller (PC), the laser will change the operation state. When the laser is running in the mode-locked state, the pump threshold is 51.4 mW. The center wavelength of the mode-lock pulse is 1559.71 nm with a spectral bandwidth of 10.73 nm, 354 fs duration and the fundamental repetition rate of 10.8 MHz. When the laser working in the soliton mode-lock state, the pump threshold is 54.1 mW. The center wavelength of the soliton pulse is 1561.85 nm, and the spectral bandwidth is 4.01 nm. The lasers have environmental stability. After a slight disturbance, the laser can stabilize itself to the mode-locked state.

A mode-locked all-fiber laser based on the nonlinear multimode interference (NL-MMI) effect of the graded-index multimode fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step-index multimode fiber (SIMF) connected by a GIMF. Compared with the theoretically proposed SMF–GIMF–SMF structure, the introduction of the SIMF eliminates the restriction on the length of the GIMF and provides a more flexible method of making an SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulsewidth of ∼446 fs, the bandwidth of 4.48 nm, and the fundamental repetition rate of 11.73 MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in the large-mode-area double-clad fiber laser make it highly attractive in ultrafast photonics.

2.11 GHz all-fiber harmonic mode-locked laser based on highly nonlinear hybrid multimode interference saturable absorber

We report the experimental observation of high-energy square pulses in all-normal-dispersion passively mode-locked Yb-doped fiber laser under dissipative soliton resonance (DSR) condition using a nonlinear polarization rotation technique. The generated square pulses are mode locked with fundamental cavity repetition rate of 1.86 MHz. The maximum output average power and the corresponding pulse energy are 62.1 mW and 34 nJ, respectively. The output square pulses are wave-breaking-free and can be tuned in pulsewidth from hundreds of picoseconds to several nanoseconds by changing the pump power. The multistability of DSR in all-normal-dispersion cavity has been first demonstrated in experiment, which is in agreement with theoretical prediction. The observed experimental results will enhance the understanding of fundamental physics of DSR phenomenon.

The dissipative soliton resonance (DSR) pulses are demonstrated experimentally in an all polarization-maintaining (PM) thulium-doped mode-locked fiber laser. The mode-locked operation is achieved by using of the nonlinear amplifying loop mirror mechanism (NALM). Each loop of the apparatus includes an independently controlled amplifier and a section of gain fiber. The experiment is carried out in central wavelength of 1969.8 nm and 3.5 ns as the maximum output pulse width. Single pulse energy increases from 5 to 10 nJ with the enhancement of amplifier power A 2 , when amplifier power A 1 is set to 2 W. In addition, the DSR pulse properties are experimentally verified by changing the length of PM-1950 fiber spliced into the NALM loop. We have achieved environmentally stable mode-locked pulses at 3, 2.1, and 1.62 MHz corresponding to the maximum pulse energies of 6.8, 20 and 29.1 nJ. The output pulse is stable and robust at different states.

Ultrafast fibre lasers have been developing at an accelerated pace during the past two decades, as a result from both technological and conceptual advances. This chapter emphasizes on the latter. It presents the key enabling concepts that have allowed the development of innovative cavity architectures supporting original pulse dynamics, including energetic short pulses in fibre laser cavities using standard telecom-grade components. Popular techniques of passive mode-locking for fibre lasers are introduced, based on either saturable absorber materials or non-linear interferences. The concept of a soliton laser is presented with its energy scaling and dispersive wave radiation features. Going beyond, the dissipative soliton paradigm is introduced, explaining the stable propagation of high-energy chirped-pulses in laser cavities endowed with normal dispersion, as well as the pattern formation of soliton molecules. The dissipative soliton paradigm encompasses a diversity of self-consistent ultrafast cavity dynamics such as the self-similar evolution followed by filtering, multicolour Raman dissipative solitons, dissipative soliton resonance, and even incoherent noise-like pulse emission. Finally, an outlook is provided, mentioning hot emerging topics such as smart lasers and spatiotemporal lasers.

A detailed investigation of the nonlinear multimodal interference in a short graded-index multimode optical fiber is presented. The analysis is performed for a specific device geometry, where the light is coupled in and out of the multimode fiber via single-mode fibers. The same device geometry was recently used to obtain ultra-low-loss coupling between two single-mode optical fibers with very different mode-field diameters. Our results indicate the potential application of this simple geometry for nonlinear devices, such as in nonlinear switching, optical signal processing, or as saturable absorbers in mode-locked fiber lasers. Saturable absorption in this all-fiber configuration is discussed and it is shown that it provides attractive properties that can potentially be used in high pulse energy mode-locked fiber lasers.

International audience

A 102 kHz repetition rate of dissipative soliton mode-locked pulses was experimentally achieved in this work. The oscillator was based on normal dispersion, with the Nb2CTx/ZnO composite acting as the saturable absorber, which was fabricated using the delamination and hydrothermal methods. This output was a dissipative soliton at a fundamental repetition rate of 40.6 MHz and a pulse width of 8.4 ps. By amplifying the seed pulses through a ytterbium-doped fiber amplifier, an average optical output power of 104 mW was achieved. These amplified mode-locked pulses were then passed through an acousto-optic modulator with a pulse picker module to lower the repetition rate. After the pulse-picking process, a repetition rate of 102 kHz was achieved with a pulse duration of 11.4 ps. At the same time, it increases the pulse energy of the mode-locked pulses from 0.1 nJ (at the oscillator) to 22.5 nJ (after the pulse picker) and the peak power from 4.8 W to 1.9 kW. This experimental configuration demonstrates the scalability of pulse energy without compromising the dispersion and cavity length of the oscillator, resulting in no significant broadening of the pulse duration.

The experiment reports a kind of L-band dissipative soliton resonance (DSR) nanosecond Er-doped fiber laser. The passive mode-locked L-band DSR pulse is achieved in a figure-eight structure by using the nonlinear-amplifying-loop-mirror (NALM). The repetition rate of pulse is 468 kHz. At the maximum pump power of 398 mW, the output power is 25 mW with a pulse energy of 45 nJ. By adjustment of polarization state in cavity, the output pulse can be switched among 4 different DSR states with central wavelength of 1572, 1585, 1590 and 1605 nm discretely, presenting different peak power and pulse width. We can determine that the adjustment of polarization state changes the loss spectra of cavity and the 4 DSR states operate in different balance points. The tunable DSR pulse duration is from 120 to 150 ns by increasing the central wavelength.

We report on the investigation of large energy dissipative soliton resonance (DSR) pulse generation in a simple linear cavity Tm3+-doped fiber laser (TDFL), by using semiconductor saturable absorption mirror as saturable absorber. This DSR TDFL is implemented by utilizing ultra-high numerical aperture 4 (UHNA4) fiber for dispersion compensation. With further dispersion management, the DSR TDFL can generate an average output power of 242 mW with corresponding pulse energy of ~61 nJ. Moreover, the soliton molecules mode-locked operation has also been demonstrated in this TDFL. Our results can provide an easily accessible way to develop large energy TDFLs.

This work presents a comparative review of two classes of advanced, ultrafast, fiber lasers: dissipative soliton resonance (DSR) oscillators and Mamyshev oscillators. These two classes have received significant attention in recent years and have arguably the highest potential among new ultrafast fiber oscillators, which motivated this work. Working principles are carefully described, and the mechanisms used to mitigate or exploit nonlinearity are highlighted and discussed. An analysis of existing laser systems based on the two classes is performed, with focus on pulse duration, energy, and peak power. Examples of both classes based on ytterbium-, erbium-, and thulium-doped fibers are presented. It is found that the DSR laser generally achieves higher pulse energies than the Mamyshev oscillator, but the latter results in shorter pulse durations. Finally, it is concluded that Mamyshev oscillators perform better in terms of peak power and stability since they do not rely on suppressing nonlinearity. Suggestions for further improvements are made.

We demonstrated an ultrafast Yb-doped fiber laser with a single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure based saturable absorber. The GIMF was placed in the groove of an in-line fiber polarization controller to adjust its birefringence, enabling the SMF-GIMF-SMF structure to realize efficient saturable absorption based on nonlinear multimode interference without strict length restriction. By adjusting two intra-cavity polarization controllers, stable dissipation solitons and noise-like pulses were achieved in the 1030 nm waveband with pulse durations of 10.67 ps and 276 fs, respectively. We also realized Q-switched mode-locked pulses in the same fiber laser cavity. By the dispersive Fourier transform method, the real-time spectral evolution in the buildup process of the Q-switched mode-locked state was captured, which showed that the continuous-wave in this laser could gradually evolved into the stable Q-switched mode-locked pulses through unstable self-pulsation, relaxation oscillation and rogue Q-switching stage. To the best of our knowledge, our work reveals the buildup dynamics of the Q-switched mode-locked operation in a fiber laser for the first time. And we also studied the real-time spectral evolution of the stable Q-switched mode-locked pulses, which exhibited periodic breathing property.

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe2, a new two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe2-based SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is 0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked pulses in an Er-doped fiber laser has been obtained by using PtSe2 as a saturable absorber.

Special cavities like Figure-8 and Figure-9 are exploited in lasers to enable self-starting passive mode-locking using nonlinear amplifying loop mirrors (NALMs) or nonlinear optical loop mirrors (NOLMs). Their significance lies in enhanced nonlinearity and intracavity feedback, enabling stable, self-sustained mode-locked pulses suitable for ultrafast fiber lasers. In this paper, we propose the design of femtosecond pulse width passively mode-locked Holmium-doped fiber laser (HDFL) operating at 2090 nm and 12.5 MHz repetition rate based on Figure-9 (F9) cavity. The F9 cavity is implemented utilizing three different reflectors, including saturable absorber (SA), simple mirror (SM), and fiber loop mirror (FLM). The performance of the proposed laser is compared for different reflectors considering characteristics of slope efficiency (SE), pulse width, optical signal to noise ratio (OSNR), peak power, and pulse energy. SA, SM, and FLM configurations yield mode-locked pulses with SEs of 35.6%, 8%, and 8.8%, pulse widths of 357.2 fs, 294 fs, and 231 fs, OSNRs of 36.4 dB, 46 dB, and 50 dB, peak powers of 13.53 kW, 6.12 kW, and 9 kW, and pulse energies of 4.83 nJ, 2 nJ, and 2.1 nJ, respectively. The analysis reveals that the FLM-based reflector achieves the shortest pulse width and highest OSNR, while the SA-based reflector delivers the highest peak power and pulse energy, highlighting trade-offs between pulse quality and energy performance in the proposed laser design.