Mode-locked Pulses Research Articles

Hybrid mode-locked fiber laser based on nonlinear optical loop mirror and manganese phosphorus triselenide (MnPSe3) coated arc-shaped fiber

This study investigates a hybrid mode-locked fiber laser incorporating manganese phosphorus triselenide (MnPSe3) saturable absorber and nonlinear optical loop mirror (NOLM), yielding profound insights into its potential and performance. The hybrid mode-locked laser showcased an output spectrum centered at 1549 nm with an expanded 3-dB bandwidth of 12.25 nm. It generates remarkably narrow pulses, with a pulse width of 240 fs. The mode-locking operates at a repetition rate of 5.71 MHz, with a high signal-to-noise ratio (SNR) of 58 dB, and achieves an average output power of 299 mW with a pulse energy of 52.36 nJ and peak power of 218 kW. These findings underscore the potential of the hybrid mode-locking configuration utilizing MnPSe3 to enhance mode-locked pulse generation significantly compared to the NOLM configuration.

Passively mode-locked fiber lasers with broadband FeOOH saturable absorber

In this contribution, we synthesized iron oxide hydroxide (FeOOH) nanomaterials and successfully demonstrated the application of FeOOH-based saturable absorbers in ultrafast photonics at 1 μm and 1.5 μm. Various characterizations were conducted to analyze the morphological and structural features of FeOOH nanomaterials. Subsequently, a tapered fiber coated with FeOOH was integrated into the resonator as a saturable absorber, exhibiting the significant saturable absorption effect with modulation depths of 1.26 % at 1 μm and 2.61 % at 1.5 μm, respectively. In an Yb-doped fiber laser, stable noise-like mode-locking pulses were achieved with a pulse duration of 519 fs at the operating wavelength of 1037.2 nm with a full-width at half-maximum (FWHM) spectral bandwidth of 4.7 nm, yielding a maximum single pulse energy of 1.23 nJ with a high signal-to-noise ratio (SNR) of 47.8 dB. Meanwhile, the conventional soliton pulses were produced in the passively mode-locked Er-doped fiber laser, featuring a pulse duration of 1.02 ps, and an SNR of 42.5 dB at a peak wavelength of 1559.25 nm with a FWHM bandwidth of 2.8 nm. This work demonstrates the considerable nonlinear optical properties of FeOOH nanomaterials in the near-infrared range, proposing a promising saturable absorber for ultrafast photonics applications.

CW mode-locked fiber laser by improved all-polarization-maintaining figure-9 cavity at 1064 nm

In order to realize broadband continuous wave (CW) mode locking by figure-9 fiber laser at 1064 nm, we propose an improved all-polarization-maintaining figure-9 cavity structure by adding gain fiber to both the loop and the linear arm in the laser. When the two sections of gain fiber are pumped simultaneously, the CW mode locking regime can be obtained. In this work, noise-like mode-locked pulses are generated with a central wavelength of 1064 nm and a chirped fiber grating (CFBG) is used for dispersion management. The net dispersion of the cavity is about 0.01 ps2 and the output power is 234 mW. The noise-like pulses are validated by autocorrelator, the peak duration on the autocorrelation trace pedestal is about 261 fs, and the pedestal duration is about 103 ps. Numerical simulations are performed on the noise-like pulses, which agree with the experimental results. This study has inspiration for mode-locked lasers with a central wavelength of 1064 nm, and has potential application value in fields such as supercontinuum generation, fiber sensing, laser microfabrication, and laser ablation.

Wavelength-tunable spatiotemporal mode-locking in a large-mode-area Er:ZBLAN fiber laser at 2.8 µm.

We report a tunable spatiotemporally mode-locked large-mode-area Er:ZBLAN fiber laser based on the nonlinear polarization rotation technique. A diffraction grating is introduced to select the operating wavelength. Under the spectral and spatial filtering effects provided by the grating and spatial coupling respectively, stable ps-level spatiotemporally mode-locked pulses around 2.8 µm with a repetition rate of 43.4 MHz are generated. Through a careful adjustment of the grating, a broad wavelength tuning range from 2747 to 2797 nm is realized. To the best of our knowledge, this is the first wavelength-tunable spatiotemporally mode-locked fiber laser in the mid-infrared region.

Passively mode-locked Er-doped fiber laser with single and double wavelength pulses based on germanene saturable absorber

In this paper, we focus on the single crystal material germanium (Ge), which is fabricated into saturable absorbers (SAs) employing liquid phase exfoliation, and validate it with an erbium-doped fiber laser (EDFL). The Ge SA was obtained with a modulation depth of 9.8% and a saturation intensity of 11.02 MW cm−2. The single-wavelength mode-locked pulse with a minimum pulse width of 847 fs was obtained at a cavity length of 10.5 m. In addition, at a cavity length of 106 m, a dual-wavelength mode-locked phenomenon was obtained in which the central wavelengths were located at 1559.20 nm and 1561.31 nm. The experimental results show that Ge nanosheets in an EDFL provide a strong basis for the development of nonlinear optics and have a wide range of applications in the field of pulsed fiber lasers.

Internal motion within ultrafast asynchronous dual wavelength mode-locked lasers.

Realtime spectroscopy access to ultrafast fiber lasers provides new opportunities for exploring complex soliton interaction dynamics. In this study, we employ a time-stretch technique that enables real-time access to both spectral and temporal dynamics, revealing rich nonlinear processes in asynchronous dual wavelength mode-locked pulses in an ultrafast fiber laser. Due to the different group velocities of the two wavelengths, the mode-locked solitons centered at different wavelengths periodically collide with each other. We recorded the entire process of soliton establishment, stabilization, and disappearance, shedding light on the mystery of stable transmission of dual-wavelength mode-locked pulses. These processes were observed for the first time in an ultrafast fiber laser, and the experimental evidence provides important insights into the understanding of nonlinear dynamics in fiber lasers, as well as the potential for improving laser performance for application in dual-comb spectroscopy.

Pulse buildup dynamics in a self-starting Mamyshev oscillator.

The Mamyshev oscillator (MO) can generate high-performance pulses. However, due to their non-resonant cavities, they usually are not self-starting, and there is almost no effort to reveal the pulse buildup dynamics of the MO. This paper investigates the dynamic of single pulse (SP) and multi-pulse formation in a self-starting MO. It indicated that both SP self-starting and multi-pulse self-starting can be obtained by adjusting the oscillator parameters. More importantly, increasing pump power could only result in bound state pulses (BSPs) if SP self-starting was formed. With the increase of the pump power, the pulse number in BSPs would increase. However, multiple pulses could not be formed only by increasing the pump power, and the BSPs obtained here underwent SP generated from noise, amplified, and then bounded, which is different from conventional passive mode-locked fiber lasers (CPMLFLs). On the other hand, if multiple pulses were self-initiated, BSPs, pulse bunch, and harmonic mode-locked pulses (HMLPs) could be obtained by adjusting the polarization state and pump power in the cavity. Furthermore, once any of the above states are formed, if the oscillator polarization state and filter interval are unchanged, only increasing the pump power from zero, the original state can still be obtained, which is consistent with the characteristics of the CPMLFLs. These findings will provide new insights into the pulse dynamics of self-starting MO, which will be significant for studying ultrafast laser technology and nonlinear optics.

The Influence of Spectral Filtering Bandwidth and Laser Gain on the Bound-State Pulse Formation Mechanism and Evolutionary Dynamics in the All-Fiber Mamyshev Oscillator

We present a numerical investigation of the bound-state pulse formation mechanism and evolutionary dynamics based on the pump strength and spectral filtering bandwidth in the all-fiber Mamyshev oscillator. Through the numerical simulation and analysis, the different mode-locked pulses’ (such as single pulses, bound-state pulses, and chaotic multi-pulses) regime transformation conditions are quantified. The results suggest that with an increase in the pump strength, the sub-pulse energy and output coupler of the Mamyshev oscillator show an inverse proportion trend, which plays an important role in increasing the number of sub-pulses in the bound-state pulses’ state. Furthermore, optimization schemes, such as adjusting the filter bandwidth and slowing down the accumulation of nonlinear effects, are proposed to achieve a high-energy pulse output in the Mamyshev oscillator.

Investigation of low pulse repetition passively mode-locked Nd:YVO4 lasers via multi-pass periodic trajectories in a confocal cavity.

We derive the parametric equations for the geometric rays of a periodic orbit inside a confocal cavity. Based on the derived formula, we demonstrate a passively mode-locked solid-state laser with a low pulse repetition rate to obtain a pulse train traveling along zigzag multi-pass trajectories. We achieve a stable mode-locked pulse train with a pulse repetition rate of 18 MHz by designing the cavity to satisfy the dual-M trajectory. Furthermore, by precisely adjusting cavity mirrors under the same experimental setup, we can reach pulse repetition rates of 12 and 9 MHz for the mode-locked laser. It is believed that the numerical calculation and the developed experiment can provide a straightforward and convenient way to achieve a low pulse repetition rate for passively mode-locked lasers.

Emerging PbSnS2 pulse modulators for ultrafast and high repetition frequency photonic applications

Two-dimensional ternary metal sulfides receive growing concern due to the diverse physicochemical properties and broad application potential. In this study, ultrafast nonlinear optical properties of two-dimensional lead tin sulfide (PbSnS2) nanosheets are demonstrated. PbSnS2 nanosheets with the thickness less than 3 nm were fabricated by liquid-phase exfoliation, and PbSnS2 nanosheets based saturable absorber (SA) was constructed by photodeposition. The nonlinear optical absorption properties such as modulation depth (21 %) and saturation intensity (14.2 MW/cm2) of PbSnS2-SA were investigated. Dispersion management was applied to Er3+-doped fiber laser (EDFL) coupled with PbSnS2-SA. The EDFL with net dispersion −0.36 ps2 outputs 928 fs mode-locked pulses. With a cavity dispersion of −0.58 ps2, tunable 1st to 9th order harmonic mode-locked (HML) is realized by tuning the pump power only. The EDFL (net dispersion −0.98 ps2) produces the highest repetition frequency at 362.59 MHz (corresponding to 80th order HML). The results indicate that two-dimensional PbSnS2 may be widely used as ultrafast photonics material for photonics and engineering applications research.

Ge-polymer bridge waveguide for mode-locked laser pulse generation.

A Ge-polymer hybrid waveguide is sandwiched between an indium phosphide (InP) reflective gain chip and a fiber Bragg grating (FBG) to construct a laser system. The hybrid waveguide serves as a bridge between the gain chip and the fiber with tailored mode-field matching at both facets. The 50-nm amorphous Ge (α-Ge) layer shows a nonlinear absorption effect at 1550 nm. The hybrid waveguide is further verified by a femtosecond laser transmission experiment to show the pulse width compression effect. Such waveguide is then integrated inside the laser cavity as a passive saturable absorber to modulate the longitudinal modes for a pulsed output. This polymer-bridged mode-locked laser adopts an InP gain chip for compact assembly and also a FBG with a flexible length to adjust the pulse repetition rate. The mode-locked laser output around the designed 50 MHz repetition rate is demonstrated. The pulse width is measured as 147 ps, and the signal-to-noise ratio is larger than 50 dB. This work introduces a "ternary" mode-locked laser system, taking advantage of discrete photonic components bridged by a polymer-based waveguide. It also proves the feasibility of applying α-Ge films as practical and low-cost saturable absorbers in photonic devices.

The passive Q-switched and Q-switched mode-locked Nd: GYAP laser based on a novel Bi2O3 saturable absorber

With a newly fabricated Bi2O3 saturable absorber, the passive Q-switched and Q-switched mode-locked laser operations were experimentally demonstrated on a novel Nd: GYAP (Nd: Gd0.1Y0.9AlO3) disordered crystal. For the Q-switched operation, a 358 ns pulse was achieved with a short cavity laser, and a maximum output power of 0.203 W with a slope efficiency of 3.1% was realized. For the Q-switched mode-locked operation, the laser of a maximum output power of 0.264 W and a spectrum of 1.062 nm (FWHM) was also achieved on a 1.78 m Z-folded cavity. The mode-locked pulse has a calculated pulse width of sub-1000 ps and a repetition rate of 90.09 MHz. The research results will open up applications of 2D Bi2O3 saturable absorber and the Nd: GYAP crystal in the field of ultrafast lasers.

Low-noise mode-locking in a GHz repetition rate Tm3+-doped fiber laser.

We demonstrate a GHz repetition rate mode-locked Tm3+-doped fiber laser with low noise. Based on a home-made Tm3+-doped barium gallo-germanate fiber with reduced dispersion, a broad optical spectrum of mode-locking is achieved, and its amplified spontaneous emission quantum-limited timing jitter is largely suppressed. Besides, we carefully investigate the influence of the intracavity pump strength on the noise performance of the mode-locked pulses and find that manipulating the intracavity pump power can be an effective method for optimizing the timing jitter and relative intensity noise (RIN). Particularly, RIN, which originated from the relaxation oscillation, can be effectively suppressed by 33 dB at offset frequencies of >1 MHz. The integrated timing jitter and RIN are only 7.9 fs (10 kHz-10 MHz) and 0.05% (10 Hz-10 MHz), respectively.

Low-repetition-rate optical frequency comb

Reducing the pulse repetition rate of an optical frequency comb increases the pulse energy for a given average power. This enhances the efficiency of nonlinear frequency conversion and it facilitates extending the accessible wavelength range, for example, into the extreme ultraviolet (XUV). The resulting spectrally dense frequency comb can still be used for precision spectroscopy of narrow atomic or molecular transitions. In this paper, we demonstrate a low-noise infrared frequency comb with a repetition rate as low as 40 kHz using a Yb:KYW mode-locked laser, pulse picking, and subsequent amplification. The frequency comb structure is confirmed by generating a beat note with a continuous wave reference laser. A comb mode is actively stabilized to the reference laser, and the integrated rms phase noise from 20 Hz to 20 kHz is measured to be 195 mrad.

Controlled pulse generation and annihilation dynamics in ultrafast fiber lasers

In this paper, the mode-locked pulse generation and annihilation dynamics in ultrafast fiber lasers based on pump intensity modulation are investigated by using real-time Fourier transform spectral probing. The results show that the laser outputs stable mode-locked pulses when the pump modulation voltage is at a high level. As the modulation voltage jumps to a low level, the intensity of the mode-locked pulse decreases, and then undergoes a period of decaying oscillation before annihilation occurs, and after ~5 μs the soliton is reconstructed from the noise, accompanied by the generation of the <i>Q</i>-modulation instability. In the low-level phase, the annihilation process in the laser cavity occurs continuously with a period of ~55 μs. By changing the duty cycle of the modulation pump, it is possible to control the the number of times solitons continuously annihilate under low-level modulation. Further, the continuous switching process of mode-locking and soliton annihilation is related to the modulation frequency of the pump, and the increase of the modulation frequency can effectively shorten the duration of the two states, thus reducing the number of soliton annihilations. In addition, by reducing the value of the low level, the gain in the laser cavity can be reduced, resulting in a shorter period of successive soliton annihilation. The results of the study are conducive to an in-depth understanding of the formation and annihilation dynamics of solitons, and provide new perspectives for developing various operation mechanisms of ultrafast lasers.

Stable watt-level mode-locked noise-like pulse from an all-PM fiber oscillator at 2 µm

Stable watt-level mode-locked noise-like pulse from an all-PM fiber oscillator at 2 µm

MOFs-derived hierarchical NiO-Co3O4 for versatile pulses generation

Polyhedral NiO-Co3O4 materials, as a novel type of metal organic frameworks (MOFs), are considered to be attractive emerging materials in the fields of optoelectronics, energy storage systems, sensing and catalysis because of their high specific surface area, rich variable metal sites and unique morphology. Ultrafast pulse has become an important research tool in basic research and industrial application fields, including supercontinuum generation, laser material processing, laser surgery, lidar and terahertz generation. The saturable absorber (SA) is a key component that affects the laser parameters and generates ultrafast pulses, so it has caused an upsurge in the research of SA. Although MOFs and their derivatives have attracted the attention of researchers in many fields, few works have been carried out in ultrafast photonics. Here, we use the cobalt-based zeolite imidazole framework (ZIF-67-Co) as a precursor to synthesize NiO-Co3O4 polyhedron by calcination, and utilize it as an effective SA in a compact erbium-doped fiber laser (EDFL). The NiO-Co3O4 SA was measured based on home-made double-arm measurement system, which showed that its modulation depth was 5.6%. It shows that high-quality Q-switched mode-locking (QML) with a repetition rate of 5.2 MHz and a pulse width of 1.41 ps can be obtained when the pump power reaches 251.1 mW. After further improvement of the EDFL, we obtained a stable continuous wave mode-locked (CWML) pulse with a pulse duration of 1.35 ps. This is the first demonstration of a versatile pulse fiber laser based on a NiO-Co3O4 polyhedral SA. This work shows that the novel NiO-Co3O4 polyhedron is expected to serve as a highly potential nonlinear optical element in the fields of ultrafast photonics, light modulators and photodetectors.

Low repetition rate passive mode-locked semiconductor disk laser

Semiconductor disk lasers (SDLs) have advantages of high output power and good beam quality. Their flexible external cavity provides convenience for inserting additional optical element to start mode locking and produce ultra-short pulse train with duration from picosecond to femtosecond. However, the very short lifetime in a range from about a few nanoseconds to tens of nanoseconds of the carrier in semiconductor gain medium limits the decrease of pulse repetition rate, thus restricting the increase of peak power of the mode-locked laser pulse to some extent. In this work, by using the relatively shallow In<sub>0.2</sub>GaAs quantum wells, which have a relatively long carrier lifetime in the active region of gain chip, as well as the particularly designed semiconductor saturable absorption mirror (SESAM) that has a relatively small saturation flux, a passively mode-locked SDL with low repetition rate and high peak power is demonstrated. The used six-mirror cavity has a spot radius of about 200 μm on the chip and a 40 μm spot on the SESAM, and the total cavity length is about 1.92 m. The SESAM passively mode-locked SDL produces a stable pulse train with a lowest repetition rate of 78 MHz. When the temperature is 12 ℃ and the transmittance of the output coupler is <i>T</i> = 3%, an average output power value of 2.1 W and a pulse duration of 2.08 ps are achieved. The corresponding pulse peak power reaches 12.8 kW, which is about twice the reported highest peak power in an SESAM mode-locked SDL. When <i>T</i> = 2% and <i>T</i> = 5%, the obtained average output power values are 1.34 W and 1.62 W respectively, and the corresponding pulse peak power values are 8.17 kW and 9.88 kW. Based on the values reported in the literature and the results of pulse repetition rate in our experiments, the estimated lifetime of the carriers of the In<sub>0.2</sub>GaAs quantum wells in the active region of the gain used chip is 16.4 ns. This high peak power mode-locked semiconductor disk laser has important potential applications in biomedical photonics, chemistry, and nonlinear microscopy.

Organic dye-loaded reduced titanium dioxide as a broadband saturable absorber for ultrafast fiber lasers.

As a rising star among metal oxide nanomaterials, titanium dioxide (TiO2) has been widely investigated and employed in optical applications because of its excellent optical properties. In this work, we demonstrate the efficient and broadband nonlinear photonic properties of methylene blue (MB)-loaded reduced TiO2 (TiO2-x-MB) and explore the performance of a TiO2-x-MB-microfiber photonic device in broadband ultrafast photonics. Within an erbium-doped fiber laser (EDFL) system, utilizing the TiO2-x-MB-microfiber photonic device as a saturable absorber (SA), steady mode-locked pulses together with chaotic pulses were successfully achieved at the wavelength of 1.55 μm. Furthermore, by incorporating the TiO2-x-MB SA into a thulium-doped fiber laser (TDFL) system, an ultrashort single pulse and multiple pulses were obtained at 2.0 μm. These results indicate that TiO2-x-MB is an excellent nanomaterial for use in mode-locked lasers, being an alternative candidate for ultrafast fiber lasers via exploiting the chemical and physical properties of oxide nanomaterials.

Soliton Pulse Generation with MoS2 Saturable Absorber

We have successfully generated soliton mode-locked pulses in a ring Erbium-doped fiber laser (EDFL) cavity using molybdenum disulfide (MoS2) saturable absorber. The MoS2 thin film was grown onto ITO substrate through electro-deposition process using a mixture of 0.5M H2MoO4 and 0.5M Na2S2O3.5H2O as electrolyte solution. The EDFL produced stable soliton pulses operating at centre wavelength of 1557 nm and repetition rate of 1.88 MHz. The mode-locking was realized by injecting pump power within a range from 77 mW to 124 mW. It generates the maximum output power of 0.7 mW and pulse width of 3.0 ps.