Mode-locked Soliton Research Articles

Two-Dimensional Black Arsenic Phosphorus for Ultrafast Photonics in Near- and Mid-Infrared Regimes.

Black arsenic phosphorus (b-AsP), as one kind of novel two-dimensional (2D) materials, bridges the band gap between black phosphorus and graphene. Thanks to its great advantages, including high carrier mobility, excellent in-plane anisotropy, and broad tunability band gap, b-AsP has aroused great interest in fields of photonics and photoelectronics. In this paper, ultrathin 2D b-AsP nanomaterials were fabricated by the liquid-phase exfoliation method, and their strong broadband linear and nonlinear absorptions were characterized by ultraviolet-visible-infrared and Z-scan technology. The experimental determination of the nonlinear absorption coefficient and low saturation intensity of b-AsP were -0.23 cm/GW and 3.336 GW/cm2, respectively. Based on density functional theory, the partial charge density and band structure at the conduction band minimum and valence band maximum were calculated, which further proves the excellent optical properties of 2D b-AsP. By first using 2D b-AsP as a novel saturable absorber in both erbium-doped and thulium-doped fiber lasers, mode-locked soliton pulses can stably operate at 1.5 and 2 μm. The laser pulses generated by 2D b-AsP possess higher stability to resist self-splitting than those generated by other 2D material-based mode-lockers. These experimental results highlight that 2D b-AsP has great application potential as a novel optical material in ultrafast photonics from near- to mid-infrared regimes.

Symbiotic quadratic soliton mode-locked non-degenerate optical parametric oscillators.

We analytically and numerically unveil the existence condition of symbiotic solitons in doubly resonant non-degenerate optical parametric oscillators. Resonant signal and idler with terahertz comb bandwidth and femtosecond pulse duration in the mid-infrared are attainable through this symbiotic soliton mode-locking technique. The group velocity mismatches between the three interacting waves are the dominant cause of the symbiotic soliton perturbation, and their effects are numerically investigated in detail. The principle can be applied to commonly used mid-infrared material platforms, making it a competitive ultrashort pulse and broadband comb source architecture.

Environmentally Stable 1.56-μm Femtosecond Laser System With Discontinuous Selectable Repetition Rate

We demonstrate here an environmentally stable and compact Er-doped fiber laser system capable of delivering sub-100-fs pulse duration with discontinuous selectable repetition rate from 10 to 70 MHz. This laser source employs a SESAM harmonic mode-locked soliton laser to facilitate tunable repetition rate. A single-mode-fiber (SMF) amplifier and a double-cladding-fiber (DCF) amplifier (both with double-pass configuration) bridged by a three-stage YVO4-based divider are used to manage the dispersion map and boost the seed pulses. With the help of ×8 replicas, the seed pulses are simultaneously amplified and compressed with 90% recombining efficiency, generating 4.60-nJ pulse energy at 10 MHz and 6.13-nJ pulse energy at 70 MHz. The main advantage of the proposed laser system is that when pulse repetition rate is switched on-demand, the pulse duration can be kept almost unchanged only by simply adjusting the pump powers of the fiber amplifiers, instead of changing any optical device (the spacing between gratings or the length of fibers).

Versatile mode-locked patterns in a fiber laser using silica-coated gold nanorods as saturable absorber

We demonstrate the generation of versatile mode-locked patterns in fiber laser with the saturable absorber (SA) based on gold nanorods (GNRs). The GNRs were synthesized by the seed-mediated growth method, and then were encased with silica in order to improve their photo-thermal stability. The silica-encased GNRs (GNRs@SiO2) SA presents a modulation depth of 3.9% and nonsaturable loss of 29.7% through a balanced twin detector measurement technique. Using the SA with high nonlinearity and excellent saturable absorption in the fiber laser cavity, versatile mode-locked patterns such as fundamental mode-locked soliton, harmonic soliton, dual-wavelength bound soliton, and soliton bundle can be achieved. The experimental results indicate that the GNRs@SiO2 could be a promising nonlinear optical material for potential applications in the field of ultrafast photonics.

Tin monoselenide based saturable absorbers for the generation of ultrashort pulses

The IV-VI semiconducting compounds, a family of two-dimensional material with attractive optical and electrical properties, have received extensive attention in recent years. Here, we demonstrate that tin monoselenide (SnSe) can be utilized as a saturable absorber (SA) for erbium-doped fiber laser (EDFL). By sandwiching the film of polyvinyl alcohol (PVA) and SnSe between two fiber connectors and depositing the SnSe nanoparticles onto the side-polished fiber (SPF), two types of SnSe based SAs are fabricated. The nonlinear optical properties of these two SAs are investigated, the modulation depth of the SnSe-SPF and SnSe-PVA SAs are 19.5% and 16.8%, respectively. Stable soliton mode-locking operations with center wavelength of 1556.6 (1558.2) nm and pulse width of less than 2 ps are obtained in the EDFL based on SnSe-SPF (SnSe-PVA) SA. Furthermore, the experimental results confirm that the damage threshold of the SnSe-SPF SA is much higher than that of the SnSe-PVA SA. Numerous photonics devices may benefit from the nonlinear saturable absorption properties of SnSe, such as near-infrared pulsed lasers, modulators, and so on.

Generation of Flat-Top Q-Switched Envelope Dissipative Soliton Resonance Pulse in Passively Mode-Locked Fiber Laser

We present demonstration of an erbium-doped fiber laser based on the nonlinear amplifying loop mirror mechanism, with a stable phenomenon of flat-top Q-switched mode-locked (FT-QML) dissipative soliton resonance (DSR) pulse, for the first time. In the experiment, we can observe both the flat-top Q-switched envelope and square-wave pulse by the peak power clamping effect inside the cavity. The characteristics of both continuous DSR pulse and flat-top Q-switched mode-locked square-wave are analyzed comprehensively. With the maximum pump power of 363 mW, single pulse energy in the FT-QML envelope can be up to 14 nJ as three times high as the one of continuous DSR. Essentially, the temporal width of the Q-switched mode-locked envelope can reach 4.6 μs with flat-top duration of 3.5 μs. The amplitude fluctuation of the sub-pulse inside the flat-top is stable at about 0.12% by using the formula of RMSE. Therefore, the laser can provide flat-top burst mode pulse sequence, and the pulse width can be controlled by tuning the pump power.

“Periodic” soliton explosions in a dual-wavelength mode-locked Yb-doped fiber laser

We report the “periodic” soliton explosions induced by intracavity soliton collisions in a dual-wavelength mode-locked Yb-doped fiber laser. Owing to the different group velocities of the two wavelengths, the mode-locked solitons centered at different wavelengths would periodically collide with each other. By using the dispersive Fourier transformation technique, it was found that each collision would induce soliton explosions, but none of them would be identical. Therefore, this phenomenon was termed as “periodic” soliton explosions. In addition, the dissipative rogue waves were detected in the dual-wavelength mode-locked state. The experimental results would be fruitful to the communities interested in soliton dynamics and dual-comb lasers.

Mulitiscale spatiotemporal structures in mode-locked fiber lasers

Using an Er-doped fiber laser as a test bed, here we for the first time experimentally demonstrate the simultaneous effect of the fast scale (round-trip time) and slow scale (thousands round-trip time) instabilities on the emergence of breathers similar to the Akhmediev breathers, Peregrine solitons, and partially mode-locked chaotic solitons. The anomalous statistics of the laser output power justifies the connection of the observed spatiotemporal structures with bright and dark rogue waves. Apart from the interest in laser physics for revealing mechanisms of the multiscale dynamics, the obtained results can be of fundamental interest for studying spatiotemporal patterns induced by the interplay of the mechanisms mentioned above in various distributed systems.

Soliton Mode-Locked Large-Mode-Area Tm-Doped Fiber Oscillator

We report a passively mode-locked fiber oscillator based on thulium-doped large-mode-area (LMA) fiber. Reliably mode-locking soliton operation can be readily achieved after the suppression of the higher-order modes by using mode field adapters in combination with coiling fiber method. The laser delivers 600-fs level pulses at $2~\mu \text{m}$ with an average power of up to 243 mW and a repetition rate of ~53.7 MHz. The output corresponds to a maximum pulse energy of 4.5 nJ, >5 times larger than that of a similar homemade mode-locked Tm laser with the LMA fibers replaced by single mode fibers, coinciding with theoretical prediction. The successful demonstration of our soliton mode-locked LMA fiber laser at $2~\mu \text{m}$ offers another strategy for high performance ultrafast fiber sources at mid-infrared region.

Generation of 1.8-W Average Power From a Diode-Pumped Femtosecond Rotary Nd:Glass Disk Laser

We demonstrate 1.8 W of average output power with 350 fs of pulse duration from a diode-pumped mode-locked rotary Nd:glass disk laser. In this laser with rotating Nd:glass disk structure, we use a Nd:glass disk with a diameter of 50 mm, which disperses heat to a wide area, to extend the upper limit of pumping power of Nd:glass for high output average power. We achieve passively soliton mode-locking for obtaining femtosecond pulses based on a semiconductor saturable absorber mirror. To our best knowledge, this is the highest average power reported from a diode-pumped femtosecond Nd:glass laser oscillator. The results are significant for power scaling of femtosecond Nd:glass lasers.

Stabilization of passive harmonic mode locking in a fiber ring laser

We propose the model of a harmonically mode-locked soliton fiber ring laser based on the nonlinear polarization rotation taking into account the gain depletion and recovery effects. It is shown that a specific timing jitter could arise in such lasers, since the pulses in the cavity are not strongly identical. To suppress the jitter and stabilize the harmonic mode-locking operation, a method using a small frequency shift followed by the laser radiation filtering is described. The performed numerical simulation shows that the proposed method is able to provide extremely stable harmonic mode locking in a soliton fiber ring laser.

Real-time transition dynamics between multi-pulsing states in a mode-locked fiber laser

We experimentally studied the transition dynamics between consecutive multi-pulsing states, specifically the build-up and annihilation of soliton pulses between a double pulsing and a three-pulse state, utilizing the dispersive Fourier transform technique. The birth of an additional pulse in a mode-locked soliton fiber laser in a multi-pulsing regime arises from a dispersive wave-induced narrow-band pulse that experiences strong intensity fluctuations, while the other soliton pulses maintain their shapes. During the decaying process to a double pulsing state, it is observed that all the pulses undergo a unique breathing behavior before settling into a steady state.

Single- and dual-wavelength switchable mode-locked dissipative soliton Yb-doped fiber laser based on graphene/WS2 nanocomposites modelocker and polarization controller

We report a single- and dual-wavelength switchable mode-locked dissipative soliton in an ytterbium-doped fiber laser based on graphene/WS2 nanocomposites saturable absorber. The graphene/WS2 nanocomposites film possesses efficient saturable absorption characteristics with large modulation depth (45.1%), which results in the stable single-wavelength mode-locking operation with pulse duration of 450 ps. By carefully rotating the polarization controller, the mode-locking operation can be switched to a dual-wavelength at both 1066.4 nm and 1069.2 nm due to the modulation of the intracavity birefringence. Our work extends the studies for 2D nanomaterials and supplies potential applications in the multi-wavelength fiber laser.

Experimental study on buildup dynamics of a harmonic mode-locking soliton fiber laser.

In this paper, we experimentally investigate the onset dynamics of harmonic mode-locking (HML) in a short-cavity all-polarization-maintaining fiber laser using time-stretch spectroscopy. We observe a transient multi-pulse state evolving into a stable HML state. Moreover, a bunch of metastable short-lived mode-locking states are recorded before the laser entered the HML state. In these transient states, sudden changes including the formation and destruction of single broadband pulses are observed.

860 femtoseconds mode-locked fiber laser by Gallium co-doped erbium fiber (Ga-EDF)

We proposed and demonstrated a high power mode-locked fiber laser using a new type of gain medium which is called as Erbium Gallium co-doped fiber (Ga-EDF). The mode-locking mechanism is enabled by a graphene-based saturable absorber, which is fabricated by slotting a single layer graphene (SLG) thin film in between two fiber ferrules connected through an adaptor. The Ga-EDF has an absorption rate of 25 dB/m at 980 nm. With a 2-m-long Ga-EDF utilized as the primary gain medium in the laser system, the proposed laser can generate mode-locked solitons, with the central wavelength of 1560 nm, a 3 dB bandwidth of 3.20 nm and an average output power of 18.23 mW. The generated pulse yields a repetition rate of 12.25 MHz with pulse duration and pulse energy of 860 fs and 1.49 nJ respectively. For comparison purpose, the experiment is repeated by replacing the Ga-EDF in the laser cavity with the same length of 2-m-long conventional erbium-doped fiber (EDF), and similar measurement of the mode-locked output performance is undertaken. The result obtained shows in comparison an improvement of the mode-locked Ga-EDF laser output performance to that of the mode-locked EDF laser in terms of the 3 dB bandwidth, pulse width and signal-to-noise ratio. The proposed work is the first time, to the knowledge of the authors, that the application of Ga-EDF as an active gain medium in the development of mode-locked fiber laser incorporating graphene which is a thin layer film as a saturable absorber is explored.

Pulse train stability in passively Mode-locked Erbium-doped fiber laser with a nonlinear polarization rotation

<span>A nonlinear polarization rotation technique-based mode-locked erbium-doped fiber laser has been experimentally demonstrated using a 0.75 m long highly concentrated erbium-doped fiber as the gain medium. With unintentional polarization dependent loss induced by twisting single-mode fiber in an intracavity polarization controller, the optical output of the laser shifts from continuous wavelength to modelocked soliton pulse. A stable and cleaner pulse with a repetition rate of 15.32 MHz laser wavelengths and a pulse energy of 0.496 nJ with optical signal-to noise ratio of more than 33 dB are successfully obtained using a simple ring cavity.</span>

Revealing the Transition Dynamics from Q Switching to Mode Locking in a Soliton Laser

Q switching (QS) and mode locking (ML) are the two main techniques enabling generation of ultrashort pulses. Here, we report the first observation of pulse evolution and dynamics in the QS-ML transition stage, where the ML soliton formation evolves from the QS pulses instead of relaxation oscillations (or quasi-continuous-wave oscillations) reported in previous studies. We discover a new way of soliton buildup in an ultrafast laser, passing through four stages: initial spontaneous noise, QS, beating dynamics, and ML. We reveal that multiple subnanosecond pulses coexist within the laser cavity during the QS, with one dominant pulse transforming into a soliton when reaching the ML stage. We propose a theoretical model to simulate the spectrotemporal beating dynamics (a critical process of QS-ML transition) and the Kelly sidebands of the as-formed solitons. Numerical results show that beating dynamics is induced by the interference between a dominant pulse and multiple subordinate pulses with varying temporal delays, in agreement with experimental observations. Our results allow a better understanding of soliton formation in ultrafast lasers, which have widespread applications in science and technology.

All fiber mode-locked thulium-doped fiber laser using a novel femtosecond-laser-inscribed 45°-plane-by-plane-tilted fiber grating

Using the novel 45°-plane-by-plane-tilted fiber grating inscribed by a femtosecond laser as an in-line fiber polarizer, we demonstrate an all-fiber passively mode-locked thulium-doped fiber laser. We characterize the grating for polarization-dependent loss, indicating high contrast and broad operating wavelength. The 45°-tilted fiber grating together with polarization controller act as an artificial saturable absorber suitable for mode-locking through nonlinear polarization rotation. By properly pre-adjusting the polarization controller, stable mode-locking is achieved for a pump power in the range of 300 mW–400 mW at 1565 nm. Owing to the net anomalous dispersion, a stable mode-locked soliton pulse centered at 1870 nm with a pulse width of 862 fs is reported at a fundamental repetition rate of 22.34 MHz. A long-term stability test reveals that the laser is quite stable for 24 h under laboratory conditions, confirming the capabilities of the 45°-tilted fiber grating for stable laser operation.

The use of polarization-imbalanced NOLM to improve the quality of the spectrum compression

In the present work, we investigated the polarization-imbalanced non-linear optical loop mirror (NOLM) for compressing the soliton spectrum as well as removing the sidebands. In the passive mode-locked soliton fiber laser, the spectra of solitons are usually accompanied with dispersive waves that can form spectral sidebands. The sidebands are the result of a constructive interference between the soliton and dispersive waves that take place at frequencies in which the phase difference is 2π. These sidebands may contain a significant part of the energy of the output spectrum, it is important first clean the spectrum by removing the sidebands and then compress the spectrum to have an optimal soliton profile. Our NOLM consists of a nearly symmetrical coupler with a 51/49 coupling ratio, and a fiber loop which consists of two 50-m pieces of SMF-28 fibers twisted in opposite directions. The maximum transmission of the NOLM at high power was 34%. At the NOLM output we reached a spectral compression of more than 50% for different values of input power. Although the sidebands around 1555 nm and 1570 nm were totally cleaned, nearly 5% of the sidebands were remaining around 1560 nm and 1567 nm. In order to get stronger spectral compression, the signal at the NOLM output was first amplified then sent into 1 km of SMF-28. For minimum input power the full width at half maximum (FWHM) bandwidth of the spectrum decreased from 5.5 nm to 0.67 nm. For higher input power the FWHM decreased from 4.23 nm to 0.59 nm.

Generation of high-energy and low-repetition rate hybrid pulses from a passively mode-locked Tm-Ho co-doped laser

This study presents a passively mode-locked Tm-Ho co-doped fiber laser at around 1840 nm with a low-repetition rate and high pulse energy. The laser delivers pulses at 93 kHz and produces a pulse energy of 18.5 nJ at 0.55 W pump power when the cavity length is 2.21 km. When the pump power is increased, high order of repetition rate outputs are observed. By a comparison of the spectra at various pump power levels indicates that the laser can be operated at hybrid multi-wavelength soliton mode-locking, depending on the pumping intensity. With the increase in pump power, single-wavelength soliton mode-locking and simultaneous of single- and dual-wavelength soliton mode locking are also observed. Besides, the system produces a laser at 1417 nm, which is induced by two-step absorption and the consequent stimulated emission. The low–repetition rate and high–pulse energy fiber laser at around 1840 nm has potential applications in sensing, processing, and surgery research.