Passive Mode-locked Fiber Laser Research Articles

Easy wavelength-tunable passive mode-locked fiber laser through a Lyot filter with a SESAM

In this work, we present an easy wavelength-tunable passive mode-locked fiber laser with a semiconductor saturable absorber mirror as a mode-locker. The cavity includes a Lyot filter as a wide wavelength-tunable filter that can be tuned by simply increasing the retardance of a variable wave retarder. The Lyot filter consists of the variable wave retarder and highly birefringent fiber inserted between two linear polarizers oriented at the same angle. The highly birefringent fiber has 8.196 cm length and linear birefringence ∆n=4.805x10−4 which provides a 61 nm period. Continuous mode-locking was experimentally achieved from 1561.9 to 1546.1 nm by simply increasing the retardance from 0.0778π to 0.4778π rad delivering sub-ps pulses width at 10.57 MHz repetition rate.

Generation of Bright–Dark Pulse Pairs in the Er-Doped Mode-Locked Fiber Laser Based on Doped Fiber Saturable Absorber

This study reports new types of passive mode-locked Er-doped fiber laser (EDFL) based on a segment of doped fiber saturable absorber (DFSA) with Tm/Ho-doped fiber (THDF), Yb-doped fiber (YDF), and Er-doped fiber (EDF). By employing THDF-SA, a bright pulse sequence with a fundamental repetition rate of 17.86 MHz was obtained. In addition, various mode-locked output states, including dark pulses, dark–bright pulse pairs, bright–dark pulse pairs, and second-harmonic pulses, were obtained through polarization modulation and gain modulation, and the orthogonality of dark–bright pulses in both polarization directions was verified. Furthermore, using EDF-SA and YDF-SA, dark pulses and dark–bright pulses were obtained. A comparison of the three experiments revealed that THDF-SA effectively reduces the mode-locked threshold and improves the average output power. Compared with bright pulses, dark pulses offer several advantages such as resisting noise, increasing propagation speed, and suppressing nonlinear scattering (such as pulse-intrinsic Raman scattering); thus, the EDFL can find broad application in long-distance transmission, precision measurement, and other fields.

21 kW, 166 fs pulses from passive mode-locked ytterbium-doped fiber laser employing Bi2O2Te nanosheets as saturable absorber in 1 μm

21 kW, 166 fs pulses from passive mode-locked ytterbium-doped fiber laser employing Bi2O2Te nanosheets as saturable absorber in 1 μm

Enhanced Nonradiative Charge Recombination in Microfiber-Based Bismuthene.

After the first report of a graphene-based passive mode-locking ultrafast fiber laser, two-dimensional materials as efficient saturable absorbers offer a new horizon in ultrafast fiber laser. However, the interactions on atomic scale between these two-dimensional materials and fiber and the fiber effect on the carrier dynamics have not been realized. To figure out the exact role of fiber and the carrier dynamics affected by the fiber substrate related to ultrafast photonics, bismuthene, a newly reported 2D quantum material used in a passive mode-locking fiber laser, deposited on α-quartz has been investigated. We surprisingly found that the α-quartz substrate can strongly accelerate the nonradiative electron-hole recombination of bismuthene in theory, and the transient absorption spectra of bismuthene on normal glass and α-quartz further verify the substrate effect on carrier dynamics of bismuthene. The discovery provides new thinking about substrate effect to regulate the performance of ultrafast mode-locking fiber lasers as well as ultrafast photonics.

Switchable Single- to Multiwavelength Conventional Soliton and Bound-State Soliton Generated from a NbTe2 Saturable Absorber-Based Passive Mode-Locked Erbium-Doped Fiber Laser.

As a member of transition metal dichalcogenides (TMDs), NbTe2 has a work function of 5.32 eV and a band gap of 0 eV at the Fermi level, which enables it to possess broadband absorption characteristics and has huge potential in optoelectronic devices. In this work, a combination of liquid phase exfoliation (LPE) and optical deposition methods (ODMs) were used to fabricate a NbTe2 saturable absorber (SA). Based on the NbTe2 SA, a ring passive mode-locked erbium-doped fiber laser (PML-EDFL) was constructed by adding NbTe2 SA into the laser cavity. A switchable single- to multiwavelength (dual/triple/quadruple) conventional soliton (CS) and a bound-state soliton (BS) were observed for the first time. The results reveal that NbTe2 SA has excellent saturable absorption characteristics (modulation depth of 2.6%, saturation intensity of 177.4 MW/cm2, and unsaturated loss of 63.8%) and can suppress mode competition and stabilize multiwavelength oscillation. This study expands the applications of NbTe2 nanosheets in ultrafast optoelectronics. The proposed switchable PML-EDFL has extensive applications in high-capacity all-optical communication, high-sensitivity optical fiber sensing, high-precision spectral measurements, and high-energy-efficiency photon neural networks.

Dynamic characteristics for 2 μm-self-similar pulse thulium-doped fiber laser

A passive mode-locked thulium-doped fiber laser is a generator for 2 μm ultra-short laser pulses. In addition, mode-locked self-similar pulses also exhibit rich nonlinear dynamic characteristics in thulium-doped fiber lasers. Therefore, this article studies the dynamic characteristics of self-similar pulses for the 2 μm passive mode-locked thulium-doped fiber laser. The research results also indicate that as the gain coefficient increases, the peak power of self-similar pulses increases, pulse width widens, and the linearity of chirp increases.

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.

Tunable Three-Wavelength Fiber Laser and Transient Switching between Three-Wavelength Soliton and Q-Switched Mode-Locked States

We report a passive mode-locked fiber laser that can realize single-wavelength tuning and multi-wavelength spacing tuning simultaneously. The tuning range is from 1528 nm–1560 nm, and up to three bands of soliton states can be output at the same time. These results are confirmed by a nonlinear Schrödinger equation model based on the split-step Fourier method. In addition, we reveal a way to transform the multi-wavelength soliton state into the Q-switched mode-locked state, which is period doubling. These results will promote the development of optical communication, optical sensing and multi-signal pulse emission.

Pulse energy exchange of vector solitons in a fiber laser

The energy exchange between orthogonal polarization components is crucial for the build-up of vector solitons (VSs). Unlike previous observations of energy exchange in the frequency domain, our experiments analyzed pulse energy flows in the time domain. We provide evidence to demonstrate the influence of the four-wave mixing (FWM) and cross-phase modulation (XPM) effect on VSs build-up in passive mode-locked fiber lasers through a perspective of pulse energy exchange. The results indicate that the energy exchange of PRVS caused by FWM and XPM is stronger than that of PLVS. The liner energy exchange caused by the birefringence of fiber and PC influences the period of energy exchange. After stabilization, the intra-cavity energy evolution period is one roundtrip for PLVS while serval roundtrips for PRVS. In the future, we can achieve PLVS by adjusting the linear energy exchange through cavity birefringence, thereby meeting the industrial demand for stable and uniform pulse trains.

Passively mode-locked fiber lasers dynamic behavior by multimode fiber polarization controller

Passively mode locked fiber lasers (PML-FLs) can produce a variety of phenomena due to the nonlinear effects. With the introduction of the nonlinear polarization rotation (NPR) and a multimode fiber polarization controller (MMF-PC), we observed nonlinear effects in our erbium-doped mode-locked fiber laser, such as the evolution of the soliton bunch to dissipates soliton resonance (DSR) pulses. In the evolution of the soliton bunch to DSR pulses, the soliton square bunch and the multi-longitudinal mode oscillation also appeared in our experiment with the increasing the pump power. To illustrate the importance of MMF-PC, a control experiment is conducted by replacing the MMF-PC with a single mode fiber PC (SMF-PC). The experimental results indicate that the MMF-PC are crucial for the evolution of soliton pairs to DSR pulses and the generation of soliton pairs. The NPR mode-locked structure is used to achieve dual-wavelength filtering, its strength-dependent non-uniform loss can effectively suppress the mode competition in the gain fibers. The MMF-PC in the mode-locked structure adds a polarization burned hole effect inside the resonator. We achieved dual-wavelength mode locking by adjusting the PC to control the birefringence and loss in the resonator.

The soft actor–critic algorithm for automatic mode-locked fiber lasers

With the development of artificial intelligence, deep reinforcement learning (DRL) has been applied for fiber laser. In this paper, the intelligent passively mode-locked fiber laser (PMLFL) with the Soft Actor–Critic (SAC) algorithm is reported. The SAC algorithm is a DRL algorithm with random policy, which combines the Actor–Critic framework and the maximum entropy. The agent learns the logic of mode-locking by outputting actions and inputting states of the laser. Due to the maximum entropy model, more exploration is encouraged, which means that multiple policies can be learned to maximize the reward, and the robustness is enhanced accordingly. The results show that the logic learned by the agent is similar to that of human. In 80 random initial state of polarization mode-locked tests, 37 explorations are needed on average, and the frequency of achieving mode-locked state exceeds 0.8 within 60 explorations. Further, the laser system can be monitored or controlled remotely, which expands the application scenarios.

Tunable Harmonics Generation from Low Average Power Mode-Locked Er-Fiber Laser Using Periodic Poled Nonlinear Crystals

Abstract: Sufficient second, third, and fourth harmonic generation of mode-locked Er-doped fiber laser (ML - EDFL) was experimentally demonstrated. This was achieved by using periodically poled KTiOPO4 (PPKTP) and PP LiNbO3 (PPLN) PPLN nonlinear crystals. Harmonic generation in both crystals depended on the direction of polarization. The highest conversion efficiency was obtained by using a half-wave plate to rotate the polarization before the crystals. When using the PPKTP nonlinear crystal, conversion efficiencies of 4.88%, 0.02%, and 0.002% were obtained for second, third, and fourth harmonics generation at wavelengths of 980, 520, and 390 nm, respectively. For the PPLN nonlinear crystal, temperature and polarization direction were optimized for each harmonic generation wavelength. As a result, conversion efficiencies of 8.6%, 0.1%, and 0.007% were obtained for second, third, and fourth harmonic generation at the same respective wavelengths. Tunable wavelength ranges and their SHGs, as well as the multi-wavelength output and their corresponding SHG wavelengths, were also reported. Keywords: Er-fiber laser, Passive mode-locked fiber laser, Nonlinear polarization rotation, PPKTP, PPLN, SHG, THG, FHG. PACS: Fiber lasers, 42.55.Wd, Mode locking, 42.60.Fc.

Molybdenum gallium carbide as saturable absorbers in 1.5-μm passive mode-locked fibre laser

A saturable absorber (SA) is one of the important components in a passive mode-locked fibre laser configuration to attain picosecond- to femtosecond-competent ultrashort pulses for fulfilling the imperatives of industries, particularly engineering and life sciences. The substantiation of a newfound SA in the generation of mode-locked pulses utilising molybdenum gallium carbide (Mo2Ga2C)—a MAX phase nanomaterial—was authenticated in this work. A Mo2Ga2C SA thin film was fabricated via a simple solution with polyvinyl alcohol (PVA) as the hosting medium. Linear absorption, modulation depth, non-saturable absorption, and saturation intensity of the 17.141-μm Mo2Ga2C SA were found to be 8.29%, 19.82%, 9.21% and 2.85 kW/cm2, respectively. The fabricated Mo2Ga2C SA was then implemented into an erbium-doped fibre laser (EDFL) system to realise the eminently stable mode-locked fibre laser at a centre wavelength of 1559.2 nm and rudimentary signal-to-noise ratio (SNR) of 54.8 dB. The pulse repetition rate and pulse width were measured to be 968.72 kHz and 2.028 ps successively. The maximum pulse energy and peak power are 10.219 nJ and 24.12 mW. In corroboration with previous reports, the implementation of Mo2Ga2C as a mode-locker is propitious in validating the impressively stable and ultrafast nonlinear fibre laser system for further multidisciplinary applications, for instance, medical instrumentation, sensing and range finding, communication, and military due to its excellent optical, physical, thermal, and mechanical properties.

Compact and robust dual-color linearly polarized illumination source for three-photon fluorescence imaging

The miniaturized femtosecond laser in near infrared-II region is the core equipment of three-photon microscopy. In this paper, we design a compact and robust illumination source that emits dual-color linearly polarized light for three-photon microscopy. Based on an all-polarization-maintaining passive mode-locked fiber laser, we shift the center wavelength of the pulses to the 1.7[Formula: see text][Formula: see text]m band utilizing cascade Raman effect, thereby generate dual-wavelength pulses. To enhance clarity, the two wavelengths are separated through the graded-index multimode fiber. Then we obtain the dual-pulse sequences with 1639.4[Formula: see text]nm and 1683.7[Formula: see text]nm wavelengths, 920[Formula: see text]fs pulse duration, and 23.75[Formula: see text]MHz pulse repetition rate. The average power of the signal is 53.64[Formula: see text]mW, corresponding to a single pulse energy of 2.25[Formula: see text]nJ. This illumination source can be further amplified and compressed for three-photon fluorescence imaging, especially dual-color three-photon fluorescence imaging, making it an ideal option for biomedical applications.

Femtosecond Fiber Laser Based on BiSbTeSe2 Quaternary Material Saturable Absorber

Topological insulator materials, including Bi2Te3, Sb2Te3, Sb2Te3, and Bi2Se3, have attracted some attention due to their narrow band gaps, high carrier mobility, wide spectral absorption ranges and other characteristics. We report a new multi-compound topological insulator material BiSbTeSe2 that, compared with the traditional topological insulator composed of two elements, can integrate the physical advantages of each element, helpful to build an experimental platform with rich physical properties. The nonlinear optical characteristics of the quaternary material BiSbTeSe2 is obtained in the erbium-doped fiber laser. Using the BiSbTeSe2 as a saturable absorber material, the passive Q-switched and mode-locked fiber lasers are achieved. The pulse duration and signal-to-noise ratio (SNR) of the Q-switched fiber laser are 854 ns and 70 dB, respectively. Meanwhile, the pulse duration and SNR of the mode-locked fiber laser are 259 fs and 87.75 dB, respectively. This work proves that the BiSbTeSe2 has a considerable application prospect as a saturable absorber in fiber lasers, and provides a new reference for selection of high-performance saturable absorber materials.

Impact of Intracavity Power Variations toward Ultrashort Pulse Generation

This study demonstrates a passive mode-locked erbium-doped fiber laser with a graphene nanoplatelet-saturable absorber (GNP-SA) that generates ultrashort pulses within femtosecond pulse duration. The GNP-SA is fabricated via a direct transfer approach by mechanically exfoliated graphene on a fiber ferrule. Its characteristics include 0.8% modulation depth, 8.7 MW/cm2 saturation fluence, and 36.8% absorbance. The quality of ultrashort pulses is studied with a variation of intracavity circulating powers that is controlled through an optical coupler. By changing the light intensity in the cavity, the optical amplification property in the erbium-doped fiber is also impacted. The increment of the output coupling ratio increases the population inversion in the active gain medium, which leads to the change of lasing wavelength from 1558 to 1532 nm. Using a 50% output coupling ratio, the fiber laser generates 960 fs pulse duration, 11.08 MHz repetition rate, and 6.05 mW output power. This study contributes to the understanding of oscillating light behavior while changing its intracavity power that affects the optical amplification properties.

Nonlinear photoresponse of PdSe2 nanosheets for soliton operations in passive mode-locked Er-doped fiber lasers

In this study, we investigated the nonlinear absorption properties of palladium diselenide (PdSe2). We employed PdSe2 with a saturable intensity of 23.40 MW/cm2 and a modulation depth of 7.39 % as a saturable absorber (SA), which was successfully exploited to attain a stable mode-locked pulse within an Er-doped fiber laser (EDFL). Centered at 1557 nm, the EDFL produced stable pulses when the pulse width was 1.31 ps and the fundamental frequency rate was 12.56 MHz. Single, double, and triple pulses were observed by changing the polarization state of the EDFL. The third, fourth, sixth, tenth and eleventh harmonic mode-locked pulses were obtained by adjusting the pump power as well as the polarization state of the laser cavity. This proved that PdSe2 was an outstanding SA in the research of ultrafast optical modulators due to its excellent ultra-fast modulation properties.

Long- and Short-Term Stability of All Polarization-Maintaining Thulium Doped Passively Mode-Locked Fiber Lasers with Emission Wavelengths at 1.95 μm and 2.07 μm

In this work, we compare the operation of a passively modelocked polarization-maintaining emission in two thulium-doped fiber lasers pumped at 1561 nm, with emission at wavelengths of 1.951 μm in one case and 2.07 μm in the other. We obtained a sequence of light pulses at 15.6 MHz, whose temporal width was 81 ps at 1.95 μm, and a sequence of light pulses at 13.1 MHz, whose temporal width was 94 ps at 2.07 μm. Finally, we also measured the long-term stability of this setup during a 24-h operation, as well as the short-term stability in a simulated harsh environment. The results confirm the superior performance of fiber laser systems with a fully polarization-maintaining design.

CoS nanosheets for generation of vector soliton and bound solitons in nonlinear optical fiber system

Transition metal chalcogenides (TMCs) with two-dimensional (2D) structures are of great interest for their extraordinary electrical, optical, magnetic, and mechanical properties. In this work, a novel cobalt sulfide (CoS) nanosheets (NSs) based saturable absorber (SA) with a modulation depth of 6.4 % is fabricated through liquid phase exfoliation (LPE) method, and successfully applied to a passive mode-locked erbium-doped fiber laser (PML-EDFL) for the first time as far as we know. By adjusting the pump power and optimizing the orientation of the intra-cavity polarization controller, a stable conventional solitons (CS) pulse with a duration of 905 fs and signal-to-noise ratio (SNR) of 58.5 dB at fundamental repetition frequency of 4.03 MHz, a pair of bound solitons with time interval of 4.9 ps between two 1.8 ps pulses are generated in proposed PML-EDFL. Meanwhile, the vector characteristics of generated soliton pulse are investigated, and polarization-locked vector solitons are observed. The results reveal that the CoS NSs have superior polarization insensitive nonlinear optical modulation properties and will find extensive application potential in the ultrafast photonics.

Study on the mechanically exfoliated graphene saturable absorption dependence on the number of layers and its influence on passive mode-locking erbium-doped fiber laser dynamics

We experimentally investigated the saturable absorption influence of graphene layers with natural stacking order in an erbium-doped fiber laser passive mode-locking. Mechanically exfoliated graphene saturable absorber (MEGSA) samples, ranging from 1 to 6 layers, were fabricated preserving their natural ABA stacking order and precisely characterized by 2D band profile from Raman spectroscopy. By incorporating the samples as saturable absorbers (SA) in the fiber laser, mode-locking performances with pulse duration from 670–780 fs and bandwidth from 3.8–4.6 nm could be generated. Also, we identified a transition in the mode-locking activation mechanism from non-self-starting, for monolayer and bilayer graphene, to self-starting, for trilayer and few-layer graphene, which is a strong indicative of fast-to-slow saturable absorption response dependence on the number of graphene layers.