Search

Artificial saturable absorbers (SA) are nonlinear optical devices widely employed in mode‐locked lasers. Among the artificial SAs are nonlinear polarization evolution (NPE) and nonlinear amplifying loop mirrors that can work in either figure‐of‐eight (Fo8) or figure‐of‐nine (Fo9) laser cavities. The NPE technique is highly sensitive to environmental perturbations. Both Fo8 and Fo9 laser cavities exhibit a higher environmental stability than the NPE technique. Here the recent advances of the Fo9 laser cavity, with a focus on the pulse formation mechanisms and the role of different cavity parameters that can enable ultrafast mode‐locking analyses are discussed. Besides, the recent development of using the Fo9 laser cavity to generate high‐energy rectangular laser pulses through either dissipative soliton resonance or noise‐like pulse regimes is also reviewed. In conclusion, the current issues and challenges of pulse formation through the Fo9 laser cavity are highlighted and recommendations for future research directions are proposed. This review is expected to provide a deeper insight into the Fo9 laser cavity as the next generation of artificial SA with interesting cavity structures, laser features, and output performances. [ABSTRACT FROM AUTHOR]

Soliton "molecules", i.e., bound states of two or several solitons, represent a fundamental concept, which manifests itself in various nonlinear systems. They are dynamically similar to chemical molecules, attracting great interest to fundamental studies and offering potential applications (such as multilevel encoding of optical data). Here, the study demonstrates a novel dichromatic soliton‐molecule compounds (DSMC) built as a hybrid bound state of multiple bound soliton pulses carried by two wavelengths in a fiber laser. The DSMCs are maintained by two different binding mechanisms, viz., the self‐phase modulation (SPM) interaction between temporal solitons at the same wavelength, mediated by their tails, and the cross‐phase modulation (XPM) interaction between solitons at different wavelengths. They also exhibit unique temporal and spectral vibration profiles. Both static DSMCs and ones with robust internal vibrations are generated experimentally in the fiber laser, and numerically as solutions of the corresponding dissipative nonlinear model. The findings reported here expand the concept of soliton molecules and further promote their similarity to chemical molecules. [ABSTRACT FROM AUTHOR]

We numerically investigate the influence of higher-order dispersion on the dynamics of quartic solitons in a passively mode-locked fiber laser. The simulation results show that the output pulse characteristics of the quartic soliton fiber laser are not only affected by the intra-cavity net dispersion but also related to the position of the single-mode fiber (SMF). For different intra-cavity net higher-order dispersion values, the quartic soliton undergoes multiple alternations between the stable single soliton state and pulsating state in the fiber laser, and the number and intensity of Kelly sidebands in the output pulse spectrum are significantly different. For different positions of the SMF, the dynamics of the quartic soliton are also different. In addition, we also observe two types of novel pulsation-creeping behavior and time shift. These findings provide new insights into the output characteristics of quartic solitons in fiber lasers. [ABSTRACT FROM AUTHOR]

Dispersive Fourier transform (DFT) has emerged as a powerful technique, enabling the transformation of spectral information from an optical pulse into a temporal waveform. This advancement facilitates the implementation of absorption spectroscopy using a single-pixel photodetector and a pulsed laser, particularly effective when operating on wavelengths near the absorption lines of the gas under study. This paper introduces a DFT-spectrometer employing a mode-locked tunable fiber laser with the central wavelength of 1531.6 nm. We demonstrate fast acquisition NH 3 absorption spectroscopy with a 0.2 nm spectral resolution, achieved through the utilization of the HITRAN database for estimating ammonia concentrations. Alongside the successful demonstration of NH 3 absorption spectroscopy, we explore practical limiting factors influencing the system's performance. Furthermore, we discuss potential avenues for enhancing sensitivity and spectral resolution, aiming to enable more robust and accurate gas sensing applications. [ABSTRACT FROM AUTHOR]

The wavelength and interval tunable second-order bound solitons are generated in a nonlinear polarization rotation mode-locked fiber laser based on a Lyot filter in this paper. The Lyot filter structure is composed of two polarization controllers, two in-line polarizers and polarization maintaining fiber. The transmission curves of Lyot filter have the characteristic of periodic modulation, so it has spectral filtering effect. The change of two polarization controllers of Lyot filter can cause a shift of the transmission curve and the variation in the modulation depth. The wavelength tunable second-order bound solitons can be obtained due to the shift of transmission curve. A second-order bound solitons with a maximum wavelength tuning range of 18.56 nm can be obtained in the cavity. In addition, the second-order bound solitons with different pulse intervals can be obtained due to the variation in the modulation depth. But the second-order bound solitons will be turned into complex higher-order bound solitons through increasing the pump power. It can be recovered to second-order bound solitons by adjusting the polarization controllers. The numerical simulation further verifies the phenomenon on second-order bound solitons, which is consistent with experimental results.

Highly stable pulsed fiber lasers are key optical components in optical communication, optical sensing, and precision micromachining systems due to the high beam quality, high peak power, and compact configurations. However, the available optical modulators in the fiber laser suffer from the operation bandwidth limitations and poor long-term physicochemical stability. Here, we have investigated the broadband nonlinear optical absorption behavior of the chromium iodide (CrI3) film, which exhibits broadband saturable absorption towards the mid-infrared regime and excellent long-term stability. The conventional soliton fiber laser operating at telecom wavelength has been obtained from an Er3+-doped fiber laser (EDFL) utilizing CrI3 film with a signal-to-noise ratio (SNR) of 92.4 dB and a pulse width of 492 fs. In addition, a passively Q-switched operation around 2.8 μm has also been obtained from an Er3+-doped ZBLAN fiber laser (EDZFL) modulated by the CrI3 film with a SNR of 46.8 dB and a pulse width of 766 ns. The demonstration shows that the CrI3 film exhibits robust broadband optical modulation, and may make inroads for developing highly stable ultrafast optoelectronic devices.

Pulse lasers with an output wavelength of 976 nm play an important role in scientific research and industrial fields. Using a nonlinear optical loop mirror (NOLM), we build a 976 nm mode-locked fiber laser with a figure-9 resonant cavity. By adding a 30 m passive fiber to the loop mirror, the noise-like pulses (NLPs) with a repetition frequency of 4.79 MHz, a maximal average output power of 13.35 mW and corresponding pulse energy of 2.79 nJ, are achieved. Furthermore, the variation of the pulse characteristics at different pump powers is investigated. The pulse energy and pulse width both increase with increasing pump power, but the coherent peak width in the autocorrelation curve decreases with increasing pump power. The laser provides a simple and costeffective solution for the generation of 976 nm noise-like pulses, which has potential application value in the generation of supercontinuum spectrum and the study of noise-like pulse dynamics. [ABSTRACT FROM AUTHOR]

Real‐time investigation in mode‐locked fiber lasers (MLFLs) is important to understand the ultrafast dynamics before, during, and after the formation of stable laser pulses. However, the experimental measurement of these dynamics is restricted by the resolution of conventional oscilloscopes. The development of a time‐stretched dispersive Fourier transform (TS‐DFT) technique provides the shot‐to‐shot measurement of mapping the spectral information of an ultrashort optical pulse into time‐stretched waveform in the MLFL. Here, the recent progress and development of various fascinating dynamics in the MLFL, including the study of birth, evolution, and extinction process in MLFL, different types of MLFL, and complex motion dynamics in MLFL, have been reviewed. The issues and challenges encountered in this research area are discussed and several recommendations are suggested to overcome these problems. The integration of the TS‐DFT technique is expected to provide deeper insight into the real‐time investigation of various dynamics in the MLFL by displaying fascinating phenomenon and revealing unexplored trajectories. [ABSTRACT FROM AUTHOR]

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 CO 2 -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. [ABSTRACT FROM AUTHOR]

In this manuscript, experimental studies on the instability and hysteresis in an all-normal dispersion (ANDi) fiber laser have been presented. The laser was mode-locked by using a semiconductor saturable absorber mirror and a chirped fiber Bragg grating in linear cavity configuration and under the stable conditions it delivered stationary dissipative soliton pulses with characteristic rectangular-shaped steep-edged spectrum. With increasing the pump power, the laser transits to a non-stationary state with a near trapezoidal-shaped spectrum with significant temporal instabilities. The hysteresis associated with the state transition and variations in spectral characteristics has been studied including dispersive Fourier transform based analysis. Pump power induced state transition in an ANDi linear cavity with a physical saturable absorber without the influence of any physical polarization controller or apparent limitation due to spectral filtering is the key observation presented in this paper. [ABSTRACT FROM AUTHOR]

From the perspective of the differential phase delay experienced by the two counterpropagating optical fields, the self-starting of the mode-locked fiber laser with a non-linear amplifying loop mirror (NALM) is theoretically studied. Although it is generally believed that NALM shows a saturable absorption effect on both continuous wave (CW) light and pulses, we find a counter-intuitive fact that cross-phase modulation (XPM) leads to opposite signs of differential non-linear phase shifts (NPSs) in these two cases, resulting in inverse saturable absorption (ISA) during the pulse formation process. The ISA is not helpful for the self-starting of laser mode-locking and can be alleviated by introducing a non-reciprocal phase shifter into the fiber loop. These results are helpful for optimizing the design of NALM and lowering the self-starting threshold of the high-repetition-rate mode-locked fiber laser. [ABSTRACT FROM AUTHOR]

Dispersive Fourier transform (DFT) has emerged as a powerful technique, enabling the transformation of spectral information from an optical pulse into a temporal waveform. This advancement facilitates the implementation of absorption spectroscopy using a single-pixel photodetector and a pulsed laser, particularly effective when operating on wavelengths near the absorption lines of the gas under study. This paper introduces a DFT-spectrometer employing a mode-locked tunable fiber laser with the central wavelength of 1531.6 nm. We demonstrate fast acquisition NH3 absorption spectroscopy with a 0.2 nm spectral resolution, achieved through the utilization of the HITRAN database for estimating ammonia concentrations. Alongside the successful demonstration of NH3 absorption spectroscopy, we explore practical limiting factors influencing the system’s performance. Furthermore, we discuss potential avenues for enhancing sensitivity and spectral resolution, aiming to enable more robust and accurate gas sensing applications.

Dispersion management plays an important role in improving the output performance of a mode-locked fiber laser. Therefore, dispersion management is carried out by introducing the grating pair in our experiment. Through adjusting the distance between the grating pair, mode-locked pulses corresponding to different dispersion regimes can be realized, which typically range from soliton state in the anomalous dispersion regime to the dissipative soliton format in the normal dispersion regime. Furthermore, tunable spectrum distribution can be achieved by adjusting two intra-cavity polarization controllers. The proposed dispersion management method complements mode-locking techniques based on nonlinear multimode interference (NL-MMI). The laser can operate with self-start mode locking stably and is useful for practice applications. [ABSTRACT FROM AUTHOR]

We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 x 10-11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime. [ABSTRACT FROM AUTHOR]

As a recent addition to the emerging 2D bismuth oxychalcogenides (Bi2O2X, where X = S, Se, and Te), atomically‐thin bismuth oxytellurium (Bi2O2Te) exhibits unique thermoelectric and photoelectric properties. In this study, high‐quality Bi2O2Te nanosheets (NSs)‐based saturable absorber (SA) with a modulation depth of 8.2% is developed using the liquid phase exfoliation (LPE) method and is successfully applied to a passively mode‐locked Er‐doped fiber laser (EDFL) for the first time (with reference to the available literature). Due to the excellent nonlinear saturable absorption property of Bi2O2Te NSs, various switchable and stable mode‐locking states can be realized in the same EDFL, including fundamental frequency conventional soliton (CS), soliton molecules, and harmonic mode‐locking (HML) states. Among them, the CS pulse at 7.48 MHz (fundamental frequency) with a high signal‐to‐noise ratio (SNR) of 67.25 dB is obtained. The 2nd, 3rd, and 4th‐order soliton molecules are observed with a soliton pulse separation of ≈6.2 ps. Moreover, for HML, a maximum repetition rate of 1.78 GHz (i.e., corresponding to the 238th‐order harmonic) is obtained. The results reveal that Bi2O2Te NSs demonstrate excellent nonlinear optical modulation properties and can serve as a fundamental basis for promotion of academic research and innovation of engineering applications involving ultrafast photonics. [ABSTRACT FROM AUTHOR]

• Stable noise-like pulses are obtained in a linear-cavity Tm fiber mode-locked laser using the SMF-FMF-SMF structure as a saturable absorber (SA). • The mode-locked Tm fiber laser based on FMF SA delivered a maximum power of 116 mW with a pulse energy of 24.73 nJ at the pump power of 2.2 W. • Noise-like mode-locking Tm fiber lasers are constructed and studied with FMF-based SA and GIMF-based SA, respectively. The experimental results reveal that the mode-locking performances of the FMF-based SA is significantly better than that of GIMF-based SA, especially in terms of the improvement of output power. As a real saturable absorber (SA) based on the nonlinear multimode interference (NL-MMI) effect, the devices based on the few-mode fiber possess numerous exciting characteristics due to their all-fiber structure, straightforward manufacturing process. In this paper, by employing a SA with the SMF-FMF-SMF structure in a linear cavity, we demonstrate a stable mode-locked Tm-doped fiber laser. At the pump power of 1 W, a single noise-like pulse (NLP) with a pedestal pulse duration of 28 ps and coherent peak width of 0.77 ps is generated at a central wavelength of 1940 nm with a 3 dB bandwidth of 10.51 nm. The stable noise-like pulse can be sustained from the lasing pump threshold up to the maximum pump power of 2.2 W without experiencing pulse splitting. The experiment results in a peak average output power of 116 mW, along with a corresponding maximum pulse energy of 24.73 nJ at a repetition frequency of 4.69 MHz. The high stability of our fiber laser is confirmed by the signal-to-Noise Ratio (SNR) of 63 dB, and its enduring stability is additionally validated over an 8-hour period. The experimental findings suggest that the SMF-FMF-SMF structure has significant potential in the simple and robust all-fiber mode-locked lasers operating in the noise-like pulse regime. [ABSTRACT FROM AUTHOR]

Extra-cavity synchronous intensity modulation is a straightforward and effective approach to suppress the full-bandwidth relative intensity noise (RIN) for noise-like pulse (NLP) fiber lasers. In this work, the picosecond NLPs are produced by a homemade Er/Yb co-doped mode-locked fiber laser and subsequent optimized by the synchronous intensity modulation device. The photodetector (PD) and electro-optical modulator (EOM) allow the real-time measurement and modulation for each optical pulse. A detailed description of key parameters for components is presented. The impact of DC bias voltage and RF signal amplitude on resulting RIN is experimentally investigated. We demonstrate a noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. Numerical investigations suggest that this method is potential to suppress RIN to below 1% under ideal conditions. The key factor in RIN reduction is correct operating region and high photoelectric-conversion linearity. The synchronous intensity modulation is an effective method to obtain low-RIN NLP laser sources, valuable for some practical applications. • Full-bandwidth relative intensity noise suppression based on synchronous intensity modulation. • The real-time measurement and modulation for each optical pulse. • A noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. • Correct operating region and high photoelectric-conversion linearity benefit to RIN reduction. [ABSTRACT FROM AUTHOR]

• We have demonstrated the realization of the CSs with narrow spectral bandwidth in an Er-doped linear-cavity fiber laser. • This is the first report of the GIMF-based SA in the narrow bandwidth soliton generation. • Compared with previous reports, our work provides a compact and low-cost alternative for the narrow bandwidth solitons. We have demonstrated the realization of the nearly transform-limited conventional solitons (CSs) with narrow spectral bandwidth in an Er-doped linear-cavity fiber laser, which is mode-locked by the graded index multimode fiber (GIMF)-based saturable absorber (SA). To our knowledge, this is the first report of the GIMF-based SA in the narrow bandwidth soliton generation. Simultaneously, the wavelength tunability of 0.48 nm and controllable Kelly sidebands for the narrow bandwidth solitons can be realized by the polarization controller (PC). Thereby, the minimum spectral bandwidth of the CSs reaches to be 0.1 nm, accompanied by the pulse duration of 20.8 ps and pulse energy of 1.07 nJ. Our work provides a compact and low-cost alternative for the narrow bandwidth solitons and offers a promising platform in the fields such as spectroscopy, quantum optics and other fields. [ABSTRACT FROM AUTHOR]

A ∼ 1.5 μ m amplified spontaneous emissions spectral bandwidth of 60 nm and gain coefficient of 1.52 dB/cm were obtained from a 5-cm-long Er: YAG crystal-derived silica fiber (ECDSF), which was fabricated using a CO 2 laser-heated drawing technique. An all-fiber-integrated passively femtosecond harmonic mode-locked fiber laser (fs-HMLFL) was constructed, in which a 5-cm-long ECDSF was used as a gain fiber and a long-period grating acted as a polarizer to meet nonlinear polarization rotation mechanism. To the best of our knowledge, this is the first demonstration of a fs-HMLFL in similar ECDSFs. Up to 751.7500 MHz repetition rate lasing pulse could be achieved with a pulse width of 629.75 fs, corresponding to the 21st-order harmonics of fundamental frequency. Moreover, its output spectral bandwidth, polarization degree, and super-mode suppression ratio are 8.37 nm, 98.7%, and 35.56 dB respectively. In addition, by adjusting the intracavity polarization state, a broad spectrum fundamental frequency mode-locked state was observed, exhibiting a spectral bandwidth of 11.11 nm and a pulse duration of 412.60 fs. These results suggest that this fs-HMLFL based on broadband and high-gain ECDSFs are promising for astronomical frequency combs and ultra-large capacity optical fiber communication systems. Especially, the utilization of centimeter-level ECDSF shows good performance in terms of spectral bandwidth, gain, and nonlinearity and provides an idea for the miniaturization and integration of ultrafast pulse lasers, which lays the foundation for applications in optical communications, precision spectroscopy, and micro-nano scale material processing. • A high performance ECDSF were obtained using a CO 2 laser-heated drawing technique. • Based on a 5-cm-long ECDSF and PD-HLPG, a passively fs-HMLF was constructed. • This is the first demonstration of a harmonic fs-HMLFL in similar ECDSFs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rigorous mode-locking performance comparison between MoS 2 and MoSe 2 is demonstrated. • Results indicate that MoS 2 has shorter excited state carrier lifetimes than MoSe 2 in visible region. • The MoS 2 and MoSe 2 show excellent performance in achieving femtosecond fiber laser. Recently, MoS 2 and MoSe 2 have attracted significant attention in ultrafast photonics owing to their excellent nonlinear optical properties. The mode-locking performance of them were also investigated intensively. However, so far, there is still no answer to who has better mode-locking performance between MoS 2 and MoSe 2. In this work, we prepared MoS 2 and MoSe 2 nanosheets and the transient absorption spectra indicate that both have excited-state carrier lifetimes of picosecond level in the visible region, and MoS 2 has a shorter recovery time than MoSe 2. The mode-locked Er-doped fiber lasers (EDFL) based on MoS 2 and MoSe 2 were carried out in the same ring laser cavity, which allows for an accurate comparison of their mode-locked performance. The mode-locked EDFL based on MoS 2 saturable absorber (SA) shows a pulse width of 470 fs, which is much shorter than that based on MoSe 2 SA (639 fs). This suggests that MoS 2 has better mode-locking performance than MoSe 2 from the viewpoint of pulse width. [ABSTRACT FROM AUTHOR]

• Multi-pulse emission analyzed under the perspective of single-shot measurements. • Mode-locking operation based on the nonlinear polarization rotation effect. • Noise-like pulse emission with broad-bandwidth supercontinuum spectrum. • Multi-emission in the form of bunches of pulses. • Noise-like pulse emission with complex pulse patterns. Single-shot characterization of three categories of multi-pulse emissions in a passively mode-locked fiber laser based on nonlinear polarization rotation is experimentally reported. The mechanism to control the intracavity polarization state relies on a polarizer beam splitter cube together with a half-wave and a quarter-wave retarders. With a precise adjustment of the retarders, the fiber laser can reach different mode locking regimes involving noise-like pulses (NLP) and solitons. These regimes include (i) NLP emission exhibiting broad-bandwidth supercontinuum spectrum, (ii) multiple soliton emission in the form of soliton bunches with rectangular-shaped envelope, and (iii) NLP emission with complex pulse patterns, such as the one obtained from the breakup of a rectangular NLP, or the harmonic mode locking regime exhibiting multiple, uniformly spaced NLP wave packets. The experimental results presented in this work provide useful information for a better understanding of different types of multi-pulse emission from the same fiber laser system, complementing the studies focused on the nature and physical mechanisms underlying NLP emission. [ABSTRACT FROM AUTHOR]

We report the evolution of single-pulse and multi-pulse in a mode-locked all polarization-maintaining fiber laser based on a nonlinear optical loop mirror (NOLM). The main parameters of the laser were determined by numerically simulating the relationship between the number of pulses and the basic parameters of each element. Experiments demonstrate generation of 1 to 6 pulses per cavity round trip. There is an unstable regime between the stable N and N + 1 pulses states when gradually increasing the pump power, while the intervals of the unstable regime shrink or even disappear when the direction of pump power variation is reversed. Our experimental results are different from those previously reported where the intervals of the unstable pulse regime are almost unchanged regardless of the pump power adjustment direction. We also describe how to obtain fundamental mode-locking from the multi-pulse state. The laser operated in the single-pulse regime shows excellent stability.

We report on a self-starting mode-locked all-polarization-maintaining fiber laser using a nonlinear amplifying loop mirror. It delivers a 7.81 MHz train of pulses with an average power of 213 mW, corresponding to pulse energy higher than 27 nJ. The output chirped pulses can be recompressed down to ∼114fs with a grating pair compressor. The nonlinear phase shifts accumulated in the long fiber cavity can be used to compensate for the mismatched third order dispersion and increase the compressed pulse quality. The laser shows excellent stability and environmental robustness during the test.

The influences on the tuning range of a wavelength-tunable L-band mode-locked fiber lasers incorporating a long-period fiber grating (LPFG) are investigated. Through controlling both resonance wavelength and bandwidth of the LPFG, the net gain of the laser can be designed appropriately, hence leading to an optimized tuning range of the L-band mode-locked lasers. A wide spectral tuning range of 23.98 nm is obtained when using the LPFG with 21.60 nm bandwidth. Benefiting from the narrow band optical attenuation characteristic of LPFG, the lowest power variation of the laser is merely 4.15 µW/nm. Moreover, theoretical gain analysis corresponds to the experimental observation well.

Photonics-based microwave generation brings the advantages of photonic oscillators, such as high stability, wide bandwidth, and low loss, to the microwave domain. In this paper, the generation of tunable microwave signals was investigated both theoretically and experimentally based on an all-polarization-maintaining 1-GHz mode-locked fiber laser. Based on beating between two highly chirped optical pulse trains with a relative time delay at the photodetector, tunable microwave signals could be obtained. The numerical simulations show that 40 GHz or higher microwave signals could be obtained by tuning the time delay and dispersion. To experimentally validate the theoretical model, the generation of tunable microwave signals from 2–4 GHz was demonstrated. Due to the utilization of polarization-maintaining devices, the optical output has a high degree of linear polarization of more than 99%, which verifies the enhanced system stability. These demonstrations are imperative for solidifying the advancements of recent years and could promote the utilization of photonics-based microwave generation in microwave photonics. [ABSTRACT FROM AUTHOR]

Numerous approaches have been developed to generate optical vortex beams carrying orbital angular momentum (OAM) over the past decades, but the direct intracavity generation of such beams with practical output powers in the femtosecond regime still remains a challenge. Here we propose and experimentally demonstrate the efficient generation of high-peak-power femtosecond optical vortex pulses from a Mamyshev oscillator (MO) based on few-mode polarization-maintaining (PM) ytterbium-doped fibers (YDFs). By employing an appropriate intracavity transverse spatial mode selection technique, ultrafast pulses carrying OAM with selectable topological charge of l = ±1 are successfully generated with an average output power of ∼5.72 W at ∼24.35 MHz repetition rate, corresponding to a single pulse energy of ∼235 nJ. The chirped pulses can be compressed to ∼76 fs outside the cavity, leading to a pulse peak power of ∼2.2 MW. To the best of our knowledge, this is by far the highest pulse energy and peak power for optical vortex pulses ever generated directly from a fiber oscillator. This unprecedented level of performance should be of great interest for a variety of applications including materials processing and imaging.

We suggest and demonstrate a scheme to generate dissipative solitons and self-similar pulses in a fiber laser mode-locked by nonlinear polarization rotation (NPR). The scheme is based on inserting a bandwidth tunable Mach-Zehnder interferometer (MZI) filter in the cavity. The MZI filter is fabricated by cascading two 3 dB optical couplers (OCs). The propagating light is split into two arms in the first OC and is combined again in the second OC. The free spectral range of the MZI, and the corresponding bandwidth of the MZI filter, depend on the optical path difference between these two arms. By applying stretching on one of the arms, both dissipative solitons and self-similar pulses have been obtained both numerically and experimentally. Furthermore, upon exploiting the dependence of the peak wavelength of the MZI filter on the amount of stretching, dissipative solitons with tunable wavelength have also been obtained.

We experimentally demonstrate a novel 2 μm soliton laser using a bidirectional nonlinear polarization evolution mode-locked Tm3+-doped fiber oscillator. The mode-locking operations between the laser signals propagating in clockwise and counterclockwise directions can be achieved through the intracavity polarization state adjustments independently, and the maximum achieved output powers are 332.5 and 367.8 mW with transfer efficiencies of 17.2 % and 18.3 %, respectively. Hysteresis in the output powers and spectra transformation between the two laser signals propagating in counter directions are observed and investigations are conducted in detail. Benefiting from the robust birefringence filtering effect, the central wavelength of this soliton laser can be continuously tuned from 1951.51 to 2015.89 nm. Stable high-order harmonic mode-locking (HML) operations are observed, and up to 633.02 MHz repetition rate with a central wavelength of 1992.55 nm is achieved. The monitored intensity spectra stability and average output power fluctuations exhibit excellent performance of this proposed 2 μm soliton fiber laser. The phase noise levels in two counter directions propagating light signals are characterized at different average output powers. Our experiment provides a potential technique for the generation of 2 μm solitons fiber laser that can be widely applied in ultrafast time-resolved measurements, mid-infrared frequency metrology, molecular spectroscopy, etc.

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.

Ultrafast fiber lasers, due to their short pulse duration, excellent beam quality, and high brightness, are extensively used in precision processing, biomedicine, nonlinear optics, and spectroscopy. However, great challenges still exist in improving the optical conversion efficiency in glass‐based gain media because of the high non‐radiative transition probability. Here, we demonstrate an oxyfluoride glass–ceramic (GC) fiber containing NaYF4:Yb3+ nanocrystal that enables enhanced 1064‐nm continuous‐wave laser output with an optical signal‐to‐noise ratio of 60 dB. Compared with the as‐prepared glass fiber, the optical conversion efficiency of GC fiber is improved from 24.2% to 30.0%. The improvement of laser action is mainly caused by the preferential incorporation of Yb3+ into the NaYF4 nanocrystal with low phonon energy. Using this well‐developed GC fiber, we successfully built a passively mode‐locked pulsed fiber laser that deliveries laser pulses with a pulse duration of 8.1 ps and a repetition frequency of 56.92 MHz. These results highlight that the GC strategy may provide a roadmap for the development of ultrafast fiber laser and the application of GC fibers in various optoelectronic fields. [ABSTRACT FROM AUTHOR]

A wavelength-tunable passive mode-locked linear-cavity Yb-doped fiber (YDF) laser has been demonstrated. The mode-locked operation was realized by using a semiconductor saturation absorption mirror as a cavity end-reflector. The laser wavelength was tuned by moving the horizontally adjustable platforms to bend a segment of the graded-index multimode fibers (GIMFs) in the cavity. The central wavelength of the output pulse is continuously adjustable in our experiment and the tunable range is from 1031.99 to 1039.32 nm. The output pulses have a 3 dB bandwidth of 1.77 nm and the fundamental repetition rate of 15.33 MHz. The average output power is 15.18 mW, the corresponding pulse energy is 0.99 nJ. The experimental results show that GIMF can be used as an efficient and simple way to tune the wavelength output in the mode-locked linear-cavity YDF laser. [ABSTRACT FROM AUTHOR]

Recently high-order modes (HOMs) lasers have been extensively investigated due to their potential applications in mode-division multiplexing. In this paper, we present two schemes of generating HOMs from the mode-locked fiber lasers (MLFLs) in the dissipative soliton resonance (DSR) regime. Watt-level HOM outputs are implemented through intra-cavity mode conversion. 3 W average-power HOMs with an efficiency slope of 25% can be obtained based on an MLFL in the DSR regime, which is achieved by incorporating a long-period fiber grating (LPFG) and a dual-resonant acoustically induced fiber grating (AIFG), respectively. Their different spectrum responses enable flexible mode conversion in the MLFLs. Both fiber mode converters are exploited to show their robust capability of efficient mode manipulation. The MLFL with an LPFG inserted in the cavity can achieve wide-bandwidth intra-cavity optical vortex beams (OVBs) near the dispersion turning around point because of the pulses of the fundamental mode and high-order vortex eigenmodes oscillating in the cavity with the same group velocity to form spatiotemporal mode locking. The MLFL based on a dual-resonant AIFG can perform the function of fast switching (∼0.3 ms) in LP01, LP11a, and LP11b modes with a high modal purity of 96%. These different modes with high-energy pulses can be flexibly switched with programmable radio frequency modulation. Furthermore, a quarter-wave plate and a polarizer are employed at the output of fiber laser to realize the controllability of the mode field, which is possible to generate a controllable mode field of OVBs based on the first-order Poincaré sphere. This control method can be integrated with the MLFLs to extend the flexibility of high-power HOMs generation.

• Enhanced nonlinear optical properties were observed in Mo 4/3 B 2 T 2 /Co 3 O 4 vdW heterostructure. • The enhancement is due to the carrier migration in the heterostructure. • Stable conventional soliton and harmonic solitons output are experimentally demonstrated by using Mo 4/3 B 2 T 2 /Co 3 O 4 heterostructure SA. • Multi harmonic solitons are realized successfully with Mo 4/3 B 2 T 2 /Co 3 O 4 vdW SA and the highest repetition rate is to be 189.72 MHz. VdW heterostructure have garnered significant interest in ultrafast laser generation because of their attractive saturable absorption properties, holding promise for applications in optics, medicine, and optoelectronics. In this study, the saturable absorption of Mo 4/3 B 2 T 2 /Co 3 O 4 vdW is investigated for the first time. Mo 4/3 B 2 T 2 /Co 3 O 4 vdW SA is successfully applied to passively mode-locked Er-doped fiber lasers. Due to the excellent saturable absorption properties of Mo 4/3 B 2 T 2 /Co 3 O 4 vdW SA, stable mode-locked fiber lasers can be realized in the same cavity, including fundamentally conventional soliton, bound soliton, and harmonic soliton. Among them, the conventional soliton operates at a repetition rate of 11.16 MHz with a high SNR of 56.26 dB. Bound soliton is observed with a soliton pulse separation of 5.26 ps. Furthermore, 3rd-, 5th-, 8th-, 10th-, 12th-, 15th-, and 17th-order harmonic solitons are observed, corresponding to repetition rates of 33.48 MHz, 55.8 MHz, 89.28 MHz, 111.6 MHz, 133.92 MHz, 167.4 MHz and 189.72 MHz, respectively. For vdW heterostructure, the saturable absorption properties are strongly modulated by the interlayer coupling, which may be introduced by the Fermi level recombination and photogenerated carrier migration between the heterostructure. We established a model to understand the saturable absorption properties of the Mo 4/3 B 2 T 2 /Co 3 O 4 , which suggests the enhancements of the saturable absorption are attributed to the carrier migration between Mo 4/3 B 2 T 2 and Co 3 O 4. The results indicate that Mo 4/3 B 2 T 2 /Co 3 O 4 vdW heterostructure can be used as an excellent nonlinear optical material, which can be used as SA in ultrafast pulse generation. [ABSTRACT FROM AUTHOR]

• A model based on the rate equation plus the GNLSE was established, simulating the mode-locking process of the Figure-9 laser. • Simulation results provide effective quantitative guidance for the self-starting generation of high-quality pulses. • The self-starting single-pulse Figure-9 Yb-doped fiber oscillators with different bandwidth filter widths were developed.. The self-starting single-pulse mode-locked mechanisms in an all-normal-dispersion (ANDi) Figure-9 fiber laser are portrayed via an integrated simulation approach, which combines rate equations with the General Nonlinear Schrödinger Equation (GNLSE). The simulation results indicate that the mode-locking performances and pulse characteristics are significantly influenced by the splitting ratio, linear phase shift, gain, and asymmetric placement of gain fiber positions within the loop directly, providing effective quantified guidance for self-starting high-quality pulse generation. An experimental oscillation based on a nonlinear amplifying loop mirror (NALM) was established, operating under different widths of bandpass filters. Combining simulation for parameter optimization, the introduced linear phase shift and the coupling ratio were adjusted by changing the rotation angle of the combination wave plate to determine the optimal mode-locked operating point. A spectral bandwidth of 10.4 nm and a pulse duration of 17.5 ps, delivering 0.67 nJ of pulse energy, have been achieved in a modified mode-locked Yb-doped fiber laser. [ABSTRACT FROM AUTHOR]

• Breathing soliton pairs with different breathing ratios, pulse separations and breathing phase synchronization are observed. • For the first time, the pump hysteresis for the breathing ratio of both single soliton and soliton pair are obtained. • The transient dynamics of the transition between breather pairs and stable single soliton are unveiled. The emergence of breathing solitons, as well as the breathing soliton molecules undergoing periodic evolution in peak power and spectral width has attracted growing research attentions in dissipative systems. Beyond the intense pulse interactions within tightly-bound states with picoseconds interval, the long-range interplay between pulses separated by nanosecond-scale starts to be investigated in recent years, thus filling the long-standing academic gap between the bound solitons and independent multiple pulses. In this work, we report on the real-time observations of breathing dissipative soliton pairs with nanoseconds' separations in a fiber laser operating under the threshold of stationary mode locking. Compared with conventional soliton counterpart, here the breathing soliton pair exhibit even more fascinating inter-pulse nonlinear dynamics due to the intriguing self-excited periodic evolutions. The shot-to-shot measurements of pulse intensity profiles, associated with dispersive Fourier transform clearly uncover not only the different breathing phase synchronization and pulse separations among different pulses but also the hysteresis behaviors of the breathing ratio by continuously varying the laser parameters. Moreover, we observed various transient stages in this unstable lasing regime, including the transition between stable single soliton and breathing soliton pair, as well as their long-range breather interaction. These results reveal the breathing multi-pulse state and the instantaneous non-equilibrium dynamics involving the breather pairs, enriching the framework of breathing solitons and opening a novel possibility to explore the internal motions of breathing soliton complex in nonlinear systems. [ABSTRACT FROM AUTHOR]

A novel feature of noise-like pulses (NLPs) from a dual pump module based figure-9 cavity has been presented in this manuscript. The laser is mode-locked by taking advantage of a non-linear amplifying loop mirror (NALM) and the cavity contains a LOOP and an ARM, each section comprises an Yb-doped fiber pumped by an independently controllable pump module. In stable conditions, the laser delivers nanosecond rectangular pulses at a fundamental repetition rate of 1.96 MHz. The NLPs are recognized from their double-scaled auto-correlation trace. Akin to typical NLPs, the pulse duration increases with the pump power of the pump modules placed at the LOOP, whereas, it is noticed that the pulse width tends to reduce with the power of the pump module placed outside the LOOP, which is an unconventional behavior for NLPs. Alongside the pulse duration, the peak power also behaves oppositely with the pump power of individual modules. In the best operating condition, the maximum average power is 766 mW corresponding to 136 W peak power and pulse energy of 390 nJ. This study offers a possible direction to explore the dynamics of NLPs in laser cavities with two independent pump modules. [ABSTRACT FROM AUTHOR]

• Cerium oxide (CeO2) composite is spin-coated on a microfiber as saturable absorber. • Incorporation of CeO2 saturable absorber in a ring-cavity thulium-doped fiber laser. • Mode-locked fiber laser generates high pulse energy at 1973 nm emission wavelength. • Fiber laser's long-term stability indicates no signs of performance degradation. This work presents the use of cerium oxide (CeO 2) as saturable absorber (SA) in the mode-locked pulse fiber laser generation. The CeO 2 materials were prepared through sonication which then incorporated into a polydimethylsiloxane (PDMS) polymer to be coated on a tapered fiber. The CeO 2 /PDMS-SA exhibited saturation intensity of 44 MW/cm2 and modulation depth of 3.4%, confirming its nonlinear intensity-dependent transmission characteristics. Integrated into a thulium-doped fiber laser cavity, the CeO 2 /PDMS-SA enabled mode-locking at a pump power of 420 mW, leading to a fundamental mode-locking operation up to 1484 mW within the net anomalous dispersion regime. The laser emitted pulses at wavelength of 1973 nm with signal-to-noise ratio of 54 dB, repetition rate of 9.04 MHz and pulse width of 1.49 ps. The CeO 2 /PDMS-SA mode-locked fiber laser demonstrated stability of an average 3 dB bandwidth of 2.89 ± 0.08 nm, average central wavelength of 1973.05 ± 0.14 nm and average pulse width of 1.5 ± 0.01 ps for the duration of 120 min. Notably, the CeO 2 /PDMS-SA demonstrated a high pulse energy of 10.62 nJ and average output power of 96 mW. This investigation suggests their viability for use in future studies. The results also demonstrated the potential of CeO 2 for use in various photonics applications in the "eye-safe" region. [ABSTRACT FROM AUTHOR]

This work demonstrates the employment of tungsten trioxide/polydimethylsiloxane nanocomposite saturable absorber (WO3/PDMS-SA) in realizing mode-locked conventional soliton (CS) and noise-like pulse (NLP) laser generation in net anomalous dispersion. The switching formation from CS regime of 970.0 fs pulse duration to NLP regime of 182.0 fs coherent spike with 65.3 ps pedestal was achieved by varying its pump power. The pulse laser exhibited good stability of 50.76 and 49.82 dB signal-to-noise ratio at 9.09 MHz fundamental repetition rate and trivial variation during stability test for CS and NLP regime, respectively. This work expresses the feasibility of WO3/PDMS-SA in attaining various types of mode-locked pulse phenomena using a fixed cavity configuration conceivably beneficial for compact dual-purpose laser systems.

From the perspective of the differential phase delay experienced by the two counterpropagating optical fields, the self-starting of the mode-locked fiber laser with a non-linear amplifying loop mirror (NALM) is theoretically studied. Although it is generally believed that NALM shows a saturable absorption effect on both continuous wave (CW) light and pulses, we find a counter-intuitive fact that cross-phase modulation (XPM) leads to opposite signs of differential non-linear phase shifts (NPSs) in these two cases, resulting in inverse saturable absorption (ISA) during the pulse formation process. The ISA is not helpful for the self-starting of laser mode-locking and can be alleviated by introducing a non-reciprocal phase shifter into the fiber loop. These results are helpful for optimizing the design of NALM and lowering the self-starting threshold of the high-repetition-rate mode-locked fiber laser.

Fiber lasers have long remained relevant for various applications worldwide in many industries. This paper presents a mode-locked ytterbium-doped fiber laser (YDFL) using our home-made topological insulator Bi2Se3 nanosheets (TI Bi2Se3) as the saturable absorber. The fabricated TI Bi2Se3 is transported to the end of the fiber ferrule using an optical deposition process, which is a key ingredient for initiating a pulsed fiber laser. With a pump power of 211.1 mW, the captured repetition rate and pulse width are 8.3 MHz and 6.2 ns, respectively. The length of the setup configuration is approximately 20 m, which corresponds to an output power measurement of 12.4 mW with a calculated pulse energy of 1.5 nJ. There are no significant Kelly sidebands, but the strong stability of the pulsed laser is defined by a high signal-to-noise ratio (SNR) of around 60.35 dB. [ABSTRACT FROM AUTHOR]

Fiber-based polarization dependent devices (FPDDs), such as optical polarizer, polarization beam splitter are of significant importance in a variety of applications, especially in optical fiber communication and fiber laser systems. Over the past decades, a significant progress has been made in the development of compact and high-efficiency polarization dependent devices based on fibers. In this review, we mainly summarize and review the recent progress of kinds of FPDDs based on fiber grating, polished fiber, photonic crystal fiber and microfiber. And some common and representative applications of these FPDDs are presented and illustrated. The development of FPDDs and the progress of related application technology will create new research approaches and applications.

Numerous approaches have been developed to generate optical vortex beams carrying orbital angular momentum (OAM) over the past decades, but the direct intracavity generation of such beams with practical output powers in the femtosecond regime still remains a challenge. Here we propose and experimentally demonstrate the efficient generation of high-peak-power femtosecond optical vortex pulses from a Mamyshev oscillator (MO) based on few-mode polarization-maintaining (PM) ytterbium-doped fibers (YDFs). By employing an appropriate intracavity transverse spatial mode selection technique, ultrafast pulses carrying OAM with selectable topological charge of l = ±1 are successfully generated with an average output power of ∼5.72 W at ∼24.35 MHz repetition rate, corresponding to a single pulse energy of ∼235 nJ. The chirped pulses can be compressed to ∼76 fs outside the cavity, leading to a pulse peak power of ∼2.2 MW. To the best of our knowledge, this is by far the highest pulse energy and peak power for optical vortex pulses ever generated directly from a fiber oscillator. This unprecedented level of performance should be of great interest for a variety of applications including materials processing and imaging. [ABSTRACT FROM AUTHOR]

In this paper, we experimentally demonstrate two types of dissipative soliton resonant (DSR) and noise-like pulse (NLP) in a mode-locked fiber laser using the nonlinear optical loop mirror (NOLM). By appropriately adjusting the polarization states, the switchable generation of DSR and NLP can be achieved from one mode-locked fiber laser. By adjusting the pump power, the pulse width of DSR increases gradually from 2.45 to 13.35 ns with a constant peak intensity, while the NLP just has a little increase, even splitting into two narrower pulses at higher pump power. Two types of DSR and NLP have the same pulse periods of 1.29 μs, corresponding to the cavity length of the fiber laser. The obtained results display the evolution process of DSR pulse and NLP in mode-locked fiber laser and have some application in optical sensing, spectral reflectometry, micro-machining, and other relative domains. [ABSTRACT FROM AUTHOR]

Mode-locked lasers with low noise performance, such as low timing jitter and relative intensity noise are essential for high-precision applications. In this letter, we demonstrated a 122-MHz all-polarization-maintaining mode-locked Er-doped fiber laser based on a nonlinear amplifying loop mirror. A hybrid component integrating a wavelength division multiplexer and a phase shifter is applied to increase the repetition rate. The relative intensity noise is 0.004% and 0.76% rms (integrated from 1 MHz to 100 Hz) from two output ports. The timing jitter characteristic is measured by the delayed optical heterodyne method. The timing jitter is ~1.9 fs rms (integrated from 1 MHz to 10 Hz) of both two output ports. The origin of the timing jitter of this laser is analyzed numerically based on the well-established theoretical model using the laser’s repetition rate, cavity dispersion, etc. [ABSTRACT FROM AUTHOR]

We demonstrate sub-femtosecond timing jitter of ultrashort pulses from a dispersion managed Yb-fiber laser mode-locked by a SESAM. Timing jitters of the mode-locked Yb-laser under different dispersion conditions (when the intra-cavity dispersions are −0.005 ps2, +0.001 ps2, +0.006 ps2) are characterized by the single-crystal balanced optical cross-correlator. In our experiment, an ultra-low timing jitter of 48 attoseconds is achieved when the jitter spectral density is integrated from 3 kHz to 1 MHz offset frequency. We think that the obtained timing jitter level is the lowest for fiber lasers mode-locked by real saturable absorbers. This ultra-low timing-jitter, stably mode-locked fiber laser source is expected to be used for high precision applications outside of the laboratory. [ABSTRACT FROM AUTHOR]