Harmonic Mode-locking Research Articles

Low-noise 2-GHz figure-9 fiber laser based on passive harmonic mode-locking.

We have proposed and demonstrated the generation of a high-repetition-rate ultrashort pulse with long-term stability and low noise based on a harmonic mode-locked (HML) figure-9 fiber laser. Different HML orders from the 2nd to the 13th are generated by adjusting the pump power, net dispersion, and wave plate angles. A 2-GHz HML pulse is obtained with a 155-MHz fundamental repetition rate in a Yb-doped fiber laser, and the corresponding supermode suppression level is as high as 50 dB. The average power of the output pulse is nearly 200 mW, and the compressed pulse duration is 535 fs. To the best of our knowledge, 2 GHz and 196 mW represent, respectively, the highest repetition rate and output power in a figure-9 fiber laser. This work offers a potential pathway to achieve high-repetition-rate ultrashort pulses in the GHz range with high power and low noise levels.

Passively mode-locking fiber laser based on Cr2Si2Te6 saturable absorber

This paper reports on the generation of third-order harmonic mode-locking, double-pulse phenomena, and conventional solitons in an erbium-doped fiber laser using Cr2Si2Te6 as a saturable absorber (SA). The double-balance probing method was employed to examine the nonlinear characteristics of Cr2Si2Te6-SA. Its modulation depth is 2.35 %, and its saturation intensity is 29.74 MW/cm2. We discovered conventional soliton functioning with a center wavelength of 1561.8 nm at a pump power of 34.76 mW. It has a repetition frequency of 6.76 MHz and a signal-to-noise ratio of 45 dB. As the pump power increased, the traditional soliton was able to maintain its existence in the range of 34.76 mW to 80.96 mW. The maximum output power reaches 1.1 mW when the pump power is 80.96 mW, and the maximum single-pulse energy of the conventional soliton is 0.16 nJ. Furthermore, we observed third-order harmonic mode-locking and double-pulse phenomena at pump outputs of 53.65 mW and 71.16 mW. The experimental findings demonstrate the excellent nonlinear effect of the SA based on Cr2Si2Te6. In ultrashort-pulse fiber lasers, Cr2Si2Te6 nanosheets can be employed as an efficient saturable absorption material.

Continuous harmonic mode-locked pulsed ultrafast fiber laser based on PtS2 saturable absorber

Saturable absorbers (SAs) play an important role in mode-locked fiber lasers. The search for SAs with excellent modulation properties has become a hot topic in the field of ultrafast photonics. PtS2 is a ideal material for SA due to its suitable band gap and Easily peelable layer structure. We prepared PtS2-SA by liquid-phase exfoliation (LPE) and tapered fiber. Subsequently, we measured the modulation depth of the prepared SA using the two-arm detection method as 6.4%, and the saturation intensity is 4.5 MW/cm2. By varying the input power in the annular cavity, a conventional solitons (CS) with a center wavelength is 1530.3 nm and a repetition frequency of 6.1 MHz was obtained. Continuing to increase the power, a stable second-fourth orders harmonic mode-locked (HML) was obtained. Our work shows that PtS2-SA has excellent modulation in mode-locked fiber lasers. It also proves that PtS2 has good saturable absorption properties and improves the knowledge of research workers about the performance of PtS2. There is hope in future research work to further exploit the output properties of PtS2.

Pulse instabilities in harmonic active mode-locking: a time-delayed approach.

We propose a time-delayed model for the study of active mode-locking that is valid for large values of the round trip gain and losses. It allows us to access the typical regimes encountered in semiconductor lasers and to perform an extended bifurcation analysis. Close to the harmonic resonances and to the lasing threshold, we recover the Hermite-Gauss solutions. However, the presence of the linewidth enhancement factor induces complex regimes in which even the fundamental solution becomes unstable. Finally, we discover a global bifurcation scenario in which a single pulse can jump, over a slow time scale, between the different minima of the modulation potential.

Copper Functionalized SnSe Nanoflakes Enabling Nonlinear Optical Features for Ultrafast Photonics.

This study enhances the ultrafast photonics application of tin selenide (SnSe) nanoflakes via copper (Cu) functionalization to overcome challenges such as low conductivity and weak near-infrared (NIR) absorption. Cu functionalization enhances concentration, induces strain, and reduces the bandgap through Sn substitution and Sn vacancy filling with Cu ions. Demonstrated by density functional theory calculations and experimental analyses, Cu-functionalized SnSe exhibits improved NIR optical absorption and superior third-order nonlinear optical properties. Z-scan measurements and femtosecond transient absorption spectroscopy reveal better performance of Cu-functionalized SnSe in terms of nonlinear optical properties and shorter carrier relaxation times compared to pristine SnSe. Furthermore, saturable absorbers based on both SnSe types, when integrated into an erbium-doped fiber laser, show that Cu functionalization leads to a decrease in pulse duration to 798 fs and an increase in 3dB spectral bandwidth to 3.44nm. Additionally, it enables stable harmonic mode-locking of bound-state solitons. This work suggests a new direction for improving wide bandgap 2D materials by highlighting the enhanced nonlinear optical propertiesand potential of Cu-functionalized SnSe in ultrafast photonics.

Passively harmonic mode-locked GHz U-band femtosecond pulse generation based on soliton self-frequency shift.

We demonstrate a GHz U-band fiber laser harnessing soliton self-frequency shift (SSFS). The seed source is a passively harmonic mode-locked (HML) fiber laser based on carbon nanotubes (CNTs) polymer film. By adjusting the pump power and polarization controller (PC) appropriately, the repetition rate can be tuned up to 1.104 GHz with a side mode suppression ratio (SMSR) of 42.5 dB. The seed source with different repetition rates can be frequency shifted from C-band to U-band in a piece of dispersion-shifted fiber (DSF) after amplification. We selected three repetition rates (457.8 MHz, 700.1 MHz, and 1104.1 MHz) and systematically studied the impact of different repetition rates on SSFS. The broad tuning range of wavelengths covering the entire U-band can be obtained by adjusting the pump power of the main amplifier. To the best of our knowledge, the demonstrated laser is the first U-band femtosecond fiber laser based on passively HML with a repetition rate over 1 GHz.

Passive harmonic mode-locking in Tm–Ho doped fiber laser with MoS2–ZnO saturable absorber deposited on the arc-shaped fiber

This work reports a harmonic mode-locked Tm–Ho doped fiber laser (THDFL) using newly fabricated MoS2–ZnO as a saturable absorber (SA). It generates harmonic mode-locking (HML) operation with a center wavelength of 1911.4 nm and a 3-dB bandwidth of 3.02 nm. The fundamental repetition rate and pulse width were 14.1 MHz and 2.05 ps, respectively. Based on the radio frequency (RF) spectrum, the observed signal-to-noise (SNR) of the fundamental HML pulse was 67.22 dB. Increasing the pump power with fine-tuning of the polarisation controller (PC), higher order HML pulses were also observed using the MoS2–ZnO SA. The highest stable HML frequency was 300 MHz, observed at the 21st HML. The central wavelength and 3-dB bandwidth of the HML pulse were 1915.2 nm and 2.29 nm, respectively. The 21st HML pulses demonstrated a SNR of 40.09 dB. No supermode noise was observed in the RF spectrum, indicating that the 21st HML pulses were relatively stable. With industry demand, this high repetition rate HML with MoS2–ZnO SA opens new avenues for comprehension and manipulation of electronic phenomena, paving the way for future innovations in ultrafast lasers.

Passively harmonic dissipative soliton generation in normal dispersion erbium-doped fiber laser using SMS fiber as artificial saturable absorber

Passively harmonic mode-locking has been experimentally demonstrated in an erbium-doped fiber laser with large normal dispersion using single-multi-single mode structure as artificial saturable absorber. By increasing the pump power under the same polarization setting, the mode-locking operation can switch from fundamental mode-locked to 5th order harmonic mode-locked. Highest repetition rate of 4.26 MHz (5th order harmonic) is observed, with pulse width and pulse energy ascertained at 290 fs and 3.0 nJ, respectively. Excellent signal-to-noise ratio (SNR) of above 50 dB is observed for all harmonic orders. The findings validated that SMS structure can be used to generate stable and switchable high order of harmonic mode-locked. The low-cost SMS fiber for harmonic mode-locked generation technique could lay the groundwork for future sustainable industrial growth.

High signal-to-noise ratio harmonic mode-locking Mamyshev oscillator at 1550 nm

Harmonic mode-locking is the effective method to improve ultrafast pulse repetition rate for the current research. However, this way of increasing the repetition rate based on pulse splitting has inherent instability characteristics, manifested as low signal-to-noise ratio and loss of mode-locking. In this paper, Mamyshev regenerative oscillation mode-locking technique is used to effectively improve the signal-to-noise ratio of pulses by compressing the nonlinear chirp in the cavity under the condition of near-zero dispersion. In the experiment, we successfully achieved a harmonic mode-locked pulse of up to 16 orders, corresponding to a repetition rate of 78.56 MHz, and the signal-to-noise ratio of the highest harmonic order reached 70 dB. Our research not only solves the inherent defect of low repetition rate of Mamyshev oscillator, but also effectively improves the signal-to-noise ratio of harmonic mode-locked pulse. This mode-locked structure based on Mamyshev oscillator provides reference and operability for improving the stability of ultrafast fiber lasers.

Generation of a switchable and tunable rectangular pulse and multi-pulse with different peak powers at the L-band

A novel, to our knowledge, L-band erbium-doped fiber laser, utilizing a nonlinear optical loop mirror (NOLM) as a mode-locker, is presented in this study. Through precise adjustments of the polarization controllers (PCs), the laser achieves the generation of rectangular pulses with distinct single wavelengths, λ1=1593nm and λ2=1571nm, as well as dual-wavelength operation. The laser’s operational mode can extend further to include harmonic mode-locking (HML). Furthermore, the investigation reveals the emergence of trapezoidal pulses and low-peak-power rectangular pulses within proximity of the conventional rectangular pulses. Notably, the evolutions of these low-peak-power pulses with the pump power also adhere to the peak power clamping (PPC) effect. Remarkably, the relative positioning of these pulses remains consistent across varying pump power levels or harmonic orders. Intriguingly, the evolution of the trapezoidal pulse with respect to pump power stands in stark contrast to that of the h-shaped pulse.

Double harmonic mode-locking in soliton fiber ring laser acquired through the resonant optoacoustic coupling

Passive harmonic mode-locking of a soliton fiber laser locked to optoacoustic resonance (OAR) in the cavity fiber ensures high-frequency laser operation, high pulse stability, and low timing jitter. However, the pulse repetition rate (PRR) of such lasers is limited to ∼1 GHz for standard fibers due to the available acoustic modes. Here, we address these limitations by demonstrating a soliton fiber laser built from standard fiber components and subjected to double harmonic mode-locking (DHML). As an example, the laser adjusted to operate at the 15th harmonic of its cavity matching the OAR at ∼199 MHz could be driven to operate at a high harmonic of this particular OAR frequency, thus reaching ∼12 GHz. This breakthrough is made possible through controllable optoacoustic interactions in a short, 50 cm segment of unjacketed cavity fiber. We propose that the precise alignment of the laser cavity harmonic and fiber acoustic modes leads to a long-lived narrow-band acoustic vibration. This vibration sets the pace for the pulses circulating in the cavity by suppressing modes that do not conform to the Vernier principle. The surviving modes, equally spaced by the OAR frequency, in cooperation with the gain depletion and recovery mechanism, facilitate the formation of stable high-frequency pulse sequences, enabling DHML. In this process, the OAR rather than the laser cavity defines the elementary step for laser PRR tuning. Throughout the entire PRR tuning range, the soliton fiber laser exhibits enhanced stability, demonstrating supermode suppression levels better than ∼40 dB and picosecond pulse timing jitter.

Passively harmonic mode-locked erbium-doped fiber laser with a gold nanofilm saturable absorber

We demonstrate a 1.5 GHz harmonic mode-locked erbium-doped fiber laser by incorporating gold nanofilm as a saturable absorber (SA). The high-quality gold nanofilm SA fabricated by the physical vapor deposition method possesses a high modulation depth of 12.9% and a low saturation intensity of 1.69 MW/cm2 at 1.56 µm, facilitating the generation of harmonic mode-locking. The fundamental mode-locked operation was obtained at 1564.7 nm, with a pulse duration of 586 fs and a repetition rate of 34.235 MHz. At the pump power of 610 mW, 44th-order harmonic mode-locking with a repetition rate of 1.506 GHz was achieved, which is the highest yet reported in mode-locked fiber lasers using gold nanomaterials as SAs. Moreover, the gold nanofilm-based harmonic mode-locked fiber laser shows relatively high signal-to-noise ratios, high output power, and good stability. These results highlight the advantage of the gold nanofilm-based SA in realizing high repetition rate laser sources.

Harmonic Mode-Locking From Erbium-Doped Fiber Self-Starting Mamyshev Oscillator

Harmonic Mode-Locking From Erbium-Doped Fiber Self-Starting Mamyshev Oscillator

Raman-assisted Kerr platicon and perfect soliton crystal formation in the cavity

In this paper, we investigate the soliton evolution dynamics influenced by the stimulated Raman scattering effect. The strong Raman gain ensures the breathing platicon and stable platicon formation at the cavity with normal quartic dispersion. The switching waves benefitted from the power transformation between pump mode and Raman gain spectrum could modulate the temporal waveform at the breathing platicon existence region. The oscillated characteristics of the temporal waveform mainly rely on the central frequency of the Raman gain spectrum. Meanwhile, at the anomalous quadratic dispersion regime, the Raman effects could capture the perfect soliton crystal formation which provides one novel method for the implementation of the harmonic mode-locking process. Our findings provide a viable way to demonstrate the cavity soliton dynamics and soliton-based applications.

Harmonic mode-locking based on Tm, Ho:Ca(Gd,Lu)AlO4 laser near 2.1 μm wavelength region

In this paper, a laser diode (LD) pumped passive mode-locking (ML) Tm, Ho:Ca(Gd, Lu)AlO4 laser based on a semiconductor saturable absorber mirror (SESAM) is reported. With the introduced group delay dispersion (GDD) of −4000 fs2, a stable pulse train with the fundamental repetition rate of 105 MHz is achieved. By increasing the pump power, a stable harmonic mode-locking (HML) pulse with the repetition rate of 211.0 MHz and a 4th-order HML pulse train with the repetition rate of 420 MHz are obtained. By adjusting the introduced GDD to −2600 fs2, a stable 2nd-order HML pulse with repetition rate of 190.8 MHz at 2078.5 nm is also obtained. To the best of our knowledge, this is the first report on the HML Tm, Ho:Ca(Gd, Lu)AlO4 lasers and also the first report on LD pumped mode-locking Tm, Ho:Ca(Gd, Lu)AlO4 laser.

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

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

Revealing of long-range soliton interaction induced time and phase dynamics in a mode-locked fiber laser.

By means of the dispersive temporal interferometry technique, we carried out a real-time observation of the time separation and relative phase evolutions of two pulses toward harmonic mode-locking. During the separation stage of the buildup process, the time separation increases, while the relative phase decreases synchronously, and the largest change rates are 0.247 fs/r and -0.017 rad/r, respectively. Meanwhile, the two rates show a linear relation with the former 17.4 times larger than the latter. Moreover, a residual phase change rate of -3.89 × 10-4 rad/r is observed in a steady non-uniform dual-soliton state, while such phase change is absent in a uniform four-soliton state. This study unveils the soliton interaction dynamics in lasers, which not only help to reduce timing jitter in multi-soliton mode-locking, but also bring insight to a temporal tweezing of femtosecond pulse.

Enhanced nonlinear effect for generation of both bound state solitons and harmonic mode-locking with high signal-to-noise ratio based on crossed bandpass transmittance filters

Enhanced nonlinear effect for generation of both bound state solitons and harmonic mode-locking with high signal-to-noise ratio based on crossed bandpass transmittance filters

A synthetic pulse interaction for modelocking at ~100 GHz

Harmonic modelocking allows high repetition rates but has limited controllability and scalability. We propose a synthetic pulse-to-pulse interaction, acting like a saturable absorber, but imparting loss as a function of temporal delay, as a route to ultrahigh frequency fiber lasers.

976-nm pulse-type switchable all-fiber mode-locked laser based upon nonlinear multimode interference

A 976-nm pulse-type switchable mode-locked fiber laser is experimentally demonstrated, for the first time to our knowledge, with a single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure-based saturable absorber (SA). The GIMF is bent into a given state to strengthen its effect on establishing mode-locking. By adjusting the intra-cavity polarization controller, the switching between dissipative solitons (DSs) and noise-like pulses (NLPs) can be achieved. Meanwhile, we also realize a second harmonic mode-locking (HML) in the same laser cavity. The laser provides a solution to realize 976 nm mode-locking operation with a simple and low-cost configuration and can serve as a potential tool for various applications.