Mode-locked State Research Articles

Ultra-short wavelength of 1.7 μ m ultrafast Tm-doped fiber laser

Thulium-doped fiber lasers (TDFLs) operating at 1.7 μm have immense potential for multiphoton microscopy and imaging applications. However, further research and development are needed, particularly in the ultra-short wavelength range below 1720 nm. Herein, we demonstrate the mode-locked TDFL at 1.7 μm, utilizing a microfiber InSb saturable absorber. By employing intracavity filters, optimizing the gain fiber length, and incorporating a broadband evanescent field absorber, we achieve high-performance ultra-short wavelength solitons in the TDFL. The solitons exhibit a central wavelength of 1707 nm, a pulse width of 895 fs, a repetition rate of 23.36 MHz, and a single pulse energy of 1.7 nJ. Moreover, our experiments demonstrate the generation of diverse soliton states, including high-order harmonic and bound solitons. We also explore the dynamics of mode-locking through experimental investigations. Our findings establish a solid foundation for the research and control of diverse 1.7 μm mode-locked operation state, advancing the field of ultrafast fiber lasers in this wavelength band.

Multipulse operation in Mamyshev oscillator: influence of external seed source

We study the influence of an external seed source on the startup of a fiber Mamyshev oscillator (MO). Depending on the seed source parameters, while keeping MO parameters fixed, we observe diverse mode-locked operations from the MO, generating stable single, double, triple and even up to a group of seven pulses. The probability of the occurrence of such mode-locked operations is studied. There can be different operating regimes in a particular multipulse operation. We find that there is an optimum spectral width of the seed source for which the probability of generating mode-locked stable train of single pulses from a fiber MO is maximum. Further, we find that the power of the seed source has less influence on the mode-locking state of the fiber MO; however, the probability of single or double pulse operation is relatively higher than that of multipulse operation at higher power of the seed source. These experimental observations will enrich the database of multipulse patterns and help in understanding pulse dynamics in the fiber MO. Suitably chosen seed source parameters can generate the desired pulse pattern. Such tailored outputs could have prospective applications in laser micromachining and high bit rate data transfer in optical networks.

Single-shot dynamics of dual-comb generation in a polarization-multiplexing fiber laser

Dual optical frequency combs have been a recurrent case of study over the last decade due to their wide use in a variety of metrology applications. Utilizing a single cavity laser to generate a dual comb reduces system complexity and facilitates suppression of common noise. However, a dual-comb regime in single cavity lasers tends to be more unstable and difficult to achieve. Therefore, having a better understanding about the way they are generated could improve and automate their generation and control. In this paper, we investigate the build-up dynamics and collision of dual comb in a polarization-multiplexing ring-cavity fiber laser using DFT (Dispersive Fourier Transform) method. We observe a bunch of meta-stable short-lived mode-locking states before the laser entered the dual-comb mode-locking state. The energy level of this short-lived initial pulses determines its evolution. If it decreases too much, the pulse will eventually collapse while if it stays above certain level, it will be successfully generated. The results presented in this paper increase the understanding of dual-comb generation inside a single cavity laser and may contribute in future attempts to increase the stabilization of this regime.

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.

Observation of RIN reduction via spectral broadening in an NPR-based stretched pulse fiber laser.

We show that an optimum mode-locking state with low relative intensity noise (RIN) can be identified by continuous broadening of an optical spectrum in a stretched-pulse fiber laser based on nonlinear polarization rotation (NPR). Under the premise of keeping the overall spectral shape unchanged, either gradually increasing the pump power or unidirectionally adjusting the polarization controller (PC) can effectively reduce RIN as the optical spectral bandwidth broadens. The optimized intensity noise performance of the laser can be attributed to the increased pulse energy and reduced intra-cavity net dispersion. Moreover, the integrated RIN will further decrease as the maximum 3-dB bandwidth extends. In our experiment, the detected minimum integrated rms RIN is below 0.003% (from 100 Hz to 100 kHz). Our experimental results find that the absolute spectral width is not a necessary key condition for obtaining low RIN mode-locked laser, whereas it may help understand and design versatile low-noise ultrafast laser sources.

The impact of saturable absorber recovery time on hybrid mode-locked fiber lasers with near zero net dispersion

The hybrid mode-locking scheme holds great potential in obtaining high power ultrashort laser source since it overcomes low damage threshold, self-starting difficulty in single real saturable absorber (SA) or artificial SA mode-locking scheme. Here we numerically studied the effect of SA’s recovery time on the stability of laser output in an all polarization-maintaining (PM) Erbium-doped fiber laser with hybrid mode-locking scheme, operating at the near zero net dispersion. The mapping relationship between fiber fused angles of nonlinear polarization rotation (NPR) section and different mode-locking states is demonstrated. The proportion of stable single pulse operating points increased from 7 % to more than 77 %, as the recovery time of the SA was reduced from 10 ps to 0.1 ps. The results indicate that the laser output states with hybrid mode-locking scheme could be highly affected by the SA recovery time, when the cavity net dispersion is in near zero regime.

Wavelength tunable actively mode-locked Tm3+ doped fiber laser with GHz repetition rate

Using a phase modulator to synchronize the longitudinal modes in an all-fiber cavity, we demonstrate a wavelength tunable (1955–2045 nm) diode pumped actively mode-locked Thulium doped fiber laser with tunable repetition rate up to 10 GHz with the shortest measured pulse width of 42 ps and average output power of 5 mW. We have performed a detailed analysis of the stable mode-locking regimes, including the self mode-locking condition that challenges the harmonic mode-locked operation. We demonstrate a supermode suppression ratio of > 50 dB across the entire wavelength band of operation while maintaining a pulse width and timing jitter of < 50 ps and < 1 ps, respectively.

Reproduction of mode-locked pulses by spectrotemporal domain-informed deep learning.

The accurate reproduction of unique pulse states in a mode-locked fiber laser is an important scientific issue and has wide applications in the laser industry. We present what we believe to be a novel method for automatically and precisely reproducing targeted soliton states in a mode-locked fiber laser by spectrotemporal domain-informed deep learning. Targeted solitons are experimentally reproduced via a superior matching process with a spectrotemporal mean square error (MSE) of 3.99 × 10-5. The outstanding feature of our reproduction algorithm is that the pulse information in both the spectral and temporal domains is jointly adopted for reconstructing targeted soliton states from white noise, rather than establishing arbitrary mode-locked pulse states, as described in previous studies. Additionally, a single-layer perceptron model is proposed to retrieve the phase distribution of a mode-locked pulse, validating the physical completeness of our reproduction approach. Our approach advances ultrafast laser technology, enabling the precise control of pulse dynamics in applications such as optical communication and nonlinear optics.

Revealing the buildup of dynamic mode-switchable frequency-shifted feedback laser based on photon–phonon interaction

Photon-phonon interaction (PPI) is an intriguing physical phenomenon in which the dynamic photon-phonon energy transfer allows the manipulation of acousto-optic properties in the media. Recently, there has been significant interest of PPI as it accompanies the conservations of energy and momentum processes. Mode-locking is a process in which temporal and spatial resonant modes establish stable synchronization through non-linear interactions. Here, we propose and reveal the buildup of a dynamic mode-switchable frequency-shifted feedback laser in which frequency shift and mode conversion are achieved based on PPI. Moreover, dynamic switching between LP11a/b mode and stable donut mode output is achieved and precisely controlled by PPI. Finally, by using the time-stretch dispersive Fourier transform (TS-DFT) detective system, we observe the transient dynamics established in frequency-shifted feedback (FSF) fiber laser, such as the direct evolution from continuous wave (CW) state to mode-locked state, CW state to multipulse state and subsequently to stable mode-locked state. All these findings would unveil the buildup of mode-locking mechanisms of dynamic mode-switchable FSF fiber lasers based on PPI with more degrees of freedom and unlock new possibilities in ultrafast laser-based technology, optical communication and light control.

Exploring the spatial hole burning effect on the mode-locking characteristics of self-mode-locked Nd:YVO4 lasers

The spatial hole burning (SHB) effect on the mode-locking characteristics of Nd:YVO4 self-mode-locked (SML) lasers are investigated. By varying the cavity length, pump power, and crystal temperature, we manipulate the SHB effect on the longitudinal mode distributions and the temporal waveforms of the SML lasers. The experimental results indicate that, at a pump power of 3395 mW and a cavity length of 23 mm, the SHB effect is minimal, the temporal waveform remains stable and the longitudinal mode spacing equals the fundamental longitudinal mode interval (FLMI). As increasing the cavity length to 75 mm, the experimental longitudinal mode spacing becomes 1 to 5 times the FLMI and the temporal waveforms are then multi-pulse mode-locked state, indicating an aggravation in the SHB effect of the SML laser. With the cavity length of 23 mm, as the pump power increases from 3.4 W to 9.7 W, the SHB effect on the mode-locking characteristics gradually also aggravates. The laser gradually changes from the fundamental mode-locked state to the spaced mode-locked state. By adjusting the crystal temperature from 25 °C to 35 °C, the aggravated SHB effect caused by the increase in pump power is counteracted.

Transient dynamics in mode-locked all-PM Er-doped fiber laser with NALM

In this study, we utilized the Time-Stretch Dispersive Fourier Transform (TS-DFT) method to explore rapid transient states in both time and frequency domains of an all-polarization-maintaining dispersion-managed figure-eight mode-locked laser with Nonlinear Amplifying Loop Mirror (NALM). We detail the different stages of build-up dynamics at varying pump powers, encompassing the Q-switching (QS) stage, transient single pulse formation, pulsation beating patterns, bound states, and the final transition to a single pulse state. Our findings reveal that the mode-locked bound state regime emerges from a single pulse stage following QS, different from previously reported directly resulting from QS pulses or relaxation oscillations. In a novel contribution to the field, we have experimentally demonstrated the transient single pulse stage of build-up dynamics and compared it with the fundamental mode-locked single pulse.

Characterization of the ultrafast fiber laser mode-locked by nonlinear multimode interference

A passively mode-locked fiber laser with switchable output states is demonstrated by using an all-fiber saturable absorber (SA) based on a hybrid single mode fiber-graded index multimode fiber-single mode fiber-graded index multimode fiber-single mode fiber (SMSMS) incorporated into a ring cavity. We investigate the vectorial characteristics of the output soliton in order to analyze the influence of polarization dependence of the SMSMS structure on solitons generation. Due to the appearance of vector solitons, it is proved that nonlinear multimode interference (NL-MMI) plays a dominant role in solitons generation but not nonlinear polarization rotation (NPR). Meanwhile, abundant mode-locked states are achieved in the laser, such as fundamental soliton, higher-order harmonics, bound twin-pulse solitons, and quasi-harmonic bound states. This configuration provides a useful platform for investigating ultrafast soliton dynamics.

Soliton pulse pairs at multiple colours in normal dispersion microresonators

AbstractSoliton microcombs are helping to advance the miniaturization of a range of comb systems. These combs mode lock through the formation of short temporal pulses in anomalous dispersion resonators. Here, a new microcomb is demonstrated that mode locks through the formation of pulse pairs in coupled normal dispersion resonators. Unlike conventional microcombs, pulses in this system cannot exist alone, and instead phase lock in pairs wherein pulses in each pair feature different optical spectra. The pairwise mode-locking modality extends to multiple pulse pairs and beyond two rings, and it greatly constrains mode-locking states. Two- (bipartite) and three-ring (tripartite) states containing many pulse pairs are demonstrated, including crystal states. Pulse pairs can also form at recurring spectral windows. We obtained the results using an ultra-low-loss Si3N4 platform that has not previously produced bright solitons on account of its inherent normal dispersion. The ability to generate multicolour pulse pairs over multiple rings is an important new feature for microcombs. It can extend the concept of all-optical soliton buffers and memories to multiple storage rings that multiplex pulses with respect to soliton colour and that are spatially addressable. The results also suggest a new platform for the study of topological photonics and quantum combs.

Influence of saturable absorber parameters on the hybrid mode-locking performance of fiber lasers

In this paper, we numerically study the influence of saturable absorber parameters, namely, modulation depth, recovery time, and saturation energy, on the performance of an ultrafast fiber laser that is mode-locked by a hybrid scheme of a saturation absorber (SA) and a nonlinear polarization rotation (NPR). Maps of mode-locked states related to intracavity wave-plates are created to evaluate the operation of hybrid mode-locking under different modulation depths of the SA. Along with the improvement of modulation depth of the SA, the mode-locked pulse results from a combination effect of the SA and NPR. The numerical analysis reveals that the SA parameters can significantly impact the pulse profile, peak power, and operation state. Moreover, they can also impact the selection of NPR mode-locked points, resulting in variation in stable single pulse output. A higher modulation depth can suppress the unstable multiple pulses’ operation and convert them into harmonic mode-locked pulses. The obtained results indicate that the selection of appropriate SA parameters can effectively improve the output characteristics of hybrid mode-locked fiber lasers.

Multiple Bound State Soliton Pulses in the All Polarization Maintaining Fiber Laser.

The bound state soliton pulse, a novel mode-locked output state of fiber lasers, has been studied extensively to gain a better understanding of soliton interactions and to explain the mechanism behind the generation of mode-locked pulses. In this particular research, we utilized a self-made saturable absorber (SA) consisting of single-walled carbon nanotubes (SWCNT) in a fully polarization maintaining (PM) erbium-doped fiber optical path. Through this setup, we observed various bound state pulse phenomena, including the double bound state with different phase differences, the bound state formed by two double pulse bound states, the multi-pulse bound state, etc. The abundant bound soliton pulse states demonstrated the excellent nonlinear absorption characteristics of the SA as well as the excellent optical properties of the all-PM fiber laser. It contributed to exploring the relationship between sub pulses and mode-locked pulses in the future. Additionally, due to the strong interaction between bound state solitons and the inherent stability of the PM optical path, there was potential for utilizing this setup as a seed source to enhance the stability of high-power fiber lasers.

Versatile wavelength tunable noise-like pulse generation from an erbium doped fiber laser using an intra-cavity Michelson interferometer

We report, versatile wavelength tunable mode locked noise-like pulses (NLP) from an erbium doped fiber ring laser, using a novel intra-cavity Michelson interferometer (MI) as a tunable filter. Our results demonstrate independent control of the central wavelength and the bandwidth using fine and coarse controls in one arm of the MI. The central wavelength is continuously tunable across the entire C-band (1530 nm to 1566 nm), while the bandwidth can be varied from 3 nm to 19.5 nm. The NLPs have a repetition rate of 1.307 MHz with pulse energies of ∼1 nJ. Additionally, harmonic mode-locked states and NLP pairs can also be produced using the same laser. On account of the adaptable output spectrum, we believe, such lasers will be useful for a variety of applications including sensing, spectroscopy and optical coherence tomography. The MI-based fiber laser can also serve as an ideal platform for gaining insight and exploring the rich dynamics of NLPs.

1.7 μm all-fiber figure-9 mode-locked laser based on a fiber Bragg grating

Abstract Fiber lasers operating at 1.7 μm have very important applications in biomedicine, optical imaging, laser welding, optical communication and other fields because of their rich spectral characteristics in the near-infrared band. We designed and experimentally implemented a 1.7 μm all-fiber figure-9 (F9) mode-locked laser, with a fiber Bragg grating (FBG) acting as both the mirror and the spectrum filter. The all-fiber F9 design made the laser work in the mode-locking state more efficiently. We obtained mode-locked pulses with a central wavelength of 1724.76 nm and a repetition rate of 14.39 MHz when the pump power was 1.1 W, and the pulse width was about 54 ps. Limited by the bandwidth of the FBG, the 3 dB bandwidth of the mode-locked spectrum was about 0.18 nm. The output power was 52 mW at a pump power of 2.5 W. The multi-pulse dynamics were studied by adjusting the pump power and the polarization controllers, and pulse trains of up to six pulses in a group were achieved. The 1.7 μm narrow-bandwidth all-fiber F9 mode-locked laser is simple in structure and easy to build, with potential application as a seed source in high-energy ultrashort pulse lasers.

All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating

As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical long-period grating (PMF-HLPG) was designed and fabricated as a polarizer using the CO2-laser direct-write technique to realize the NPR effect. A homemade fiber Bragg grating (FBG) was also introduced into the laser system to enable a narrow-bandwidth lasing output and wavelength tunability. Based on the PMF-HLPG and FBG mentioned above, an all-fiber mode-locked laser with a spectra bandwidth of 0.15 nm was constructed to generate stable short pulses with a fundamental repetition rate of 12.7122 MHz and a pulse duration of 30.08 ps. In particular, its signal-to-noise ratio is up to 84.5 dB, showing the high stability of the laser. Further, the operating wavelength of the laser can be tuned from 1559.65 nm to 1560.29 nm via heating the FBG while maintaining its mode-locked state with stability. The results indicate that the PMF-HLPG could be used as a polarizer to meet the NPR mechanism for ultrashort pulse laser applications in optical communication, optical sensing, and biomedical imaging.

Evolution between bright and dark pulses in a MoxW1-xTe2 based fiber laser.

We proposed an erbium-doped fiber laser mode-locked with a MoxW1-xTe2-based nonlinear optical modulator for the first time to our best knowledge. This fiber laser can deliver bright pulses, bright-dark pulse pairs, dark pulses, bright-dark-bright pulses, and dark-dark-bright pulses. The modulation depth and saturation intensity of the MoxW1-xTe2-based saturable absorber were about 7.8% and 8.6 MW/cm2, respectively. When 10% of the laser in the cavity was output, conventional soliton pulses with central wavelength of 1560.1 nm can be obtained in the cavity. When 70% of the laser was output, dual-wavelength domain-wall dark pulses appeared in the laser cavity. This experiment revealed that an appropriate increase in the ratio of output energy can improve the chance of dark pulses in fiber lasers. The mode-locking states in this fiber laser can evolve with each other between bright pulses, bright-dark pulse pairs and dark pulses by adjusting the polarization controller. The results indicated that the MoxW1-xTe2 can be used to make modulators for generating dark pulses. Furthermore, our work will be of great help to improve the chance of the generation of dark pulse in fiber lasers.

Longitudinal to transversal conversion of mode-locked states in an empty optical resonator.

A longitudinal mode-locked state can be converted to a transverse mode-locked state by exploiting the spectral and spatial filtering of an empty optical resonator. Carrier and amplitude modulation sidebands were simultaneously transmitted by the conversion resonator, yielding phase-locked superpositions of up to five transverse modes. Equivalently, an amplitude-modulated beam was converted into a beam that periodically moved across the transverse plane. Precise control over the spatial beam shape during oscillation was gained by independently altering the set of transverse modes and their respective powers, which demonstrated an increased level of control in the generation of transverse mode-locked states.