Harmonic Mode-locked Pulses Research Articles

Ultra-high harmonic mode-locking with a micro-fiber knot resonator and Lyot filter.

We report on ultra-high harmonic mode-locking with a repetition rate of up to ∼1 THz by combining a microfiber knot resonator (MKR) and a Lyot filter. The harmonic mode-locked pulses are tunable by changing the diameter of MKR, which agrees well with the theoretical calculation. Our results indicate that the ultrafast pulse generation mechanism is due to the dissipative four-wave mixing mode-locking technique. This work provides a simple and efficient scheme to generate tunable ultrafast pulses with a high repetition rate for various applications, such as THz generation and ultrafast data communication.

Application of TiCN on passively harmonic mode-locked ultrashort pulse generation at 2 µm

TiCN could be a promising optical switch for the generation of ultrashort pulses with high repetition rates. Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using titanium carbonitride (TiCN) as a saturable absorber (SA). The linear absorption of TiCN is about 50% at 1950 nm, while the nonlinear modulation depth and saturation power are 6.76% and 1.83 kW, respectively. The fundamentally mode-locked pulses have the central wavelength of 1949.60 nm, 3-dB bandwidth of 5.20 nm, repetition rate of 27.73 MHz, and pulse width of 790 fs. The 2nd to the 16th harmonics are observed, corresponding to the repetition rates ranging from 55.44 to 444.10 MHz. To the best of our knowledge, this is the highest operating frequency in TiCN-based fiber laser. The central wavelength, 3-dB bandwidth, and pulse width are investigated as a function of harmonic order. This work would advise a new application area for TiCN.

2 µm sub-GHz harmonic mode-locked soliton generation based on a Bi2S3 saturable absorber.

Saturable absorber (SA) based harmonic mode-locking (HML) techniques at 2 µm waveband are much less reported than those at 1.5 µm waveband, the maximum repetition rate of the harmonic pulse generated by such techniques at 2 µm waveband is also much lower than those generated at 1.5 µm waveband. In this paper, the 39th harmonic with the repetition rate of 908.6 MHz is realized in a Bi2S3-based thulium-doped fiber laser. The fundamental mode-locked pulse has a central wavelength of 1954.2 nm and a 3-dB bandwidth of 5.1 nm. The repetition rate is 23.27 MHz and the pulse width is 902 fs. The characteristics of the material and harmonic mode-locked pulse are investigated. To the best of our knowledge, this is the highest and the closest resonance frequency to GHz among the reported SA-based harmonic mode-locked fiber lasers operating at 2 µm waveband.

Nonlinear optical properties of two-dimensional palladium ditelluride (PdTe2) and its application as aerosol jet printed saturable absorbers for broadband ultrafast photonics

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently gathered substantial interest in the advanced photonic field due to their strong broadband absorption, tunable bandgap, and excellent nonlinear optical (NLO) properties. Palladium ditelluride (PdTe2) is a novel TMD material with high carrier mobility, great stability, and narrow bandgap, allowing it to be used in a wide range of applications such as superconductors, phototransistors, and millimeter-wave receivers. Here, the NLO properties of PdTe2 are investigated by Z-scan technology. The highest nonlinear absorption coefficient of PdTe2 has been determined as -9.25 × 103 cm/GW (1 µm) and -0.55 × 103 cm/GW (1.5 µm), implying its admirable NLO characteristics. Subsequently, for the first time, broadband PdTe2-based saturable absorbers (SAs) are prepared by utilizing the aerosol jet printing (AJP) method. By employing this PdTe2 based SA, the generation of broadband ultrafast laser pulses from the near-to-mid-infrared regime with pulse durations in the ps to fs range (170 ps, 570 fs, and 1.59 ps in 1 μm, 1.5 μm, and 2 μm laser sources, respectively) further signify the capability of PdTe2 as an ideal NLO material for ultrafast photonics devices. Moreover, Q-switched and high-repetition-rate (0.7 GHz) based 58th harmonic mode-locked pulses in the telecommunication band are also obtained with this SA. These findings indicate that 2D-PdTe2 is a promising material for nonlinear optics and broadband ultrafast photonics applications.

Two-Dimensional Gallium Sulfide as a Novel Saturable Absorber for Broadband Ultrafast Photonics Applications.

Two-dimensional (2D) gallium sulfide (GaS) offers a plethora of exceptional electrical and optical properties, allowing it to be used in a wide range of applications, including photodetectors, hydrogen generation, and nonlinear optical devices. In this paper, ultrathin 2D GaS nanosheets are synthesized using the liquid-phase exfoliation method, and the structure, morphology, and chemical composition of the as-prepared nanosheets are extensively investigated. After depositing 2D GaS nanosheets on side polished fibers, successful saturable absorbers (SAs) are fabricated for the first time. The realized modulation depths are 10 and 5.3% at 1 and 1.5 μm, respectively, indicating the wideband saturable absorption performance of the prepared SAs. By integrating GaS-SAs into three different wavelength-based fiber laser cavities, stable mode-locked pulses are achieved, having pulse durations of 46.22 ps (1 μm), 614 fs (1.5 μm), and 1.02 ps (2 μm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 μs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband ultrafast photonics in nonlinear photonics devices.

Harmonically mode-locked Er-doped fiber laser at 1.3 GHz using a V2AlC MAX phase nanoparticle-based saturable absorber

We experimentally demonstrate a harmonically mode-locked erbium (Er)-doped fiber laser using a V2AlC-based saturable absorber (SA) at the 1.55 μm wavelength region. The prepared SA was fabricated using a side-polished fiber with V2AlC MAXphase nanoparticles via liquid-phase exfoliation (LPE). The derived saturation intensity and modulation depth of the V2AlC-based SA were approximately 63 MW/cm2 and 18%, respectively. The repetition rate was tuned from 14.98 MHz to 1.3 GHz (86th harmonic) by adjusting the pump power. The 86th harmonic mode-locked pulses at approximately 1.3 GHz repetition rate were generated with a temporal width and supermode suppression ratio of approximately 863 fs and 30 dB, respectively. These results demonstrate the application of the V2AlC MAX phase in giga-hertz frequency ultrafast fiber lasers.

Femtosecond laser direct inscribed 45° tilted fiber grating for a net-normal-dispersion mode-locked Er-doped fiber laser

The application of a femtosecond laser direct inscribed 45° tilted fiber grating (TFG) in a net-normal-dispersion mode-locked Er-doped fiber laser, as an in-fiber polarizer, has been systematically studied. The 45° TFG with a grating length of ~ 12.6 mm and a grating period of ~ 2.2 μm has a polarization dependent loss of ~ 9.8 dB and supports various types of dissipative soliton (DS) outputs. The achieved single DS mode-locked pulse centered at 1570.55 nm has a 3 dB bandwidth of 15.64 nm and a pulse duration of ~ 4.68 ps. By simply employing a section of single-mode fiber, the output pulse duration can be dechirped to ~ 250 fs. The fundamental repetition rate is 25.47 MHz and the maximum pulse output power is 24.04 mW. By appropriately adjusting the intra-cavity polarization states and increasing the pump power, the bound-state DS, as well as a harmonic mode-locked DS, can be generated. The bound-state pulse is composed of two identical DS pulses with a temporal interval of ~ 26.27 ps. The second- and third-order harmonic mode-locked DS pulses with repetition rates of 50.93 MHz and 76.40 MHz are obtained for pump powers of 298 mW and 528 mW, respectively. Our results indicate that the feasibility and versatility of the femtosecond laser inscribed 45° TFG to realize nonlinear polarization evolution mode-locking in various operation regimes. Combined with the flexibility of the femtosecond laser direct inscription technique, such a 45° TFG paves a promising route for the development of robust, compact and high-energy ultrafast all-fiber lasers.

Pulse dynamic patterns in a self-starting Mamyshev oscillator.

The pulse dynamics of a self-starting Yb-doped fiber Mamyshev oscillator without external seed pulses or additional starting arms is demonstrated experimentally. Multiple dynamic patterns of pulses, including single pulses, bound-state pulses, and harmonic mode-locking pulses, are observed at different pump powers and filter spectral separations. The generation and evolution of bound states have also been simulated by establishing the corresponding theoretical model. This is the first systematic theoretical and experimental study of the formation and evolution of bound states in Yb-doped Mamyshev oscillators. The numerical results are in excellent agreement with experiment results, providing validation of both the measurements and the numerical model.

Actively mode-locked fiber laser at 2-μm for free-space data transmission with tunable repetition rate

We experimentally demonstrate a repetition rate controllable actively mode-locked fiber laser for 2-μm free-space data transmission. The actively mode-locking can be produced by a Mach-Zehnder modulator (MZM) driven by an arbitrary waveform generator (AWG). A Lyot filter and a polarization dependent isolator (PD-ISO) are used to suppress the super-mode noise and the ambient noise. The signal-to-noise ratio (SNR) of the mode-locked pulse with repetition rate of about 690MHz can be increased from 42.64dB to 49.45dB. By inserting a high-precision variable optical delay line (VODL) in the cavity, the fundamental frequency of the cavity can be adjusted from 18.21MHz to 18.13MHz with a tuning accuracy of 73.52Hz. The RF spectra of the 330th order harmonic mode-locked pulses with continuously tunable repetition rate can be observed. In addition, the mode-locked pulse sequence can be driven by sine wave, triangular wave and square-wave, and the duration of actively mode-locked pulse can be tuned independently by changing the type of the driven signal.

Wavelength-changeable thulium-doped fiber laser based on monolayer graphene

We propose and demonstrate a wavelength changeable thulium-doped fiber laser (TDFL) based on a monolayer graphene saturable absorber (SA). With different pump powers and polarization controller states, the laser can operate in different operation modes including continuous wave, single solitons, harmonic mode-locked pulses of 2nd to 4th order, and soliton molecules. Wavelength changing over a range of 7–11 nm for different operation regimes was realized by stretching the tapered fiber and adjusting the polarization controller in the cavity. To the best of our knowledge, this is the first TDFL that is changeable in wavelength under different operation modes and that is based on a monolayer graphene SA.

2.04 µm harmonic noise-like pulses generation from a mode-locked fiber laser based on nonlinear polarization rotation

We report the generation of harmonic noise-like mode-locked pulses in a thulium-doped fiber laser. In this laser, mode-locking can be realized through the nonlinear polarization rotation (NPR) technique. The ring cavity configuration uses 5 m of a Tm-doped dual-cladding fiber, as a gain medium, pumped by a 793 nm diode laser. The cavity uses a couple of polarization controllers (PCs) and polarization-dependent isolator to form a nonlinear polarization-rotating saturable absorber. By carefully adjusting of the PCs, the laser generates noise-like mode-locked pulses at the 2.04 µm band. With the increase of the pump power, the laser generates harmonics of the mode-locking fundamental repetition frequency of 6.88 MHz, from which we can obtain the second harmonic in a controlled mode. The obtained results may be useful for enhancing the understanding of the mechanism and characteristics of noise-like pulse at the 2 µm band.

Er-doped mode-locked fiber lasers based on nonlinear polarization rotation and nonlinear multimode interference

We report an Er-doped fiber laser based on a hybrid mode-locked mechanism, which include a graded-index multimode fiber as a saturable absorber (SA) and nonlinear polarization rotation (NPR). By adopting two distinct mechanisms simultaneously, the laser can output shorter pulses with a high signal-to-noise ratio (SNR) and low noise jitter. At a pump power of 110 mW, a mode-locked pulse train with a pulse duration of 1.8 ps and a repetition rate of 49.80 MHz is generated. Compared with the implementation of the NPR mode-locked mechanism alone, the implementation of the hybrid mode-locked mechanism increased the SNR of the laser from 44 dB to 70 dB. The standard deviation of the average power fluctuation is 1.53%. Besides, a stable harmonic mode-locked pulse train can be obtained and switched by merely adjusting the pump power.

Tm-doped all-fiber structured femtosecond laser mode-locked by a novel Chem-Te saturable absorber

We succeed in developing a chemically synthesized tellurium nanocrystals (Chem-Te) thin film by liquid phase peeling technique, and demonstrate this material is an effective saturable absorber (SA) for realization of stable mode-locking in an all-fiber structured thulium-doped laser cavity. By integrating this SA into an all-fiber structured ring laser cavity, we obtained the mode-locked pulses at the repetition rate of 11.17 MHz, the achieved pulses spectral bandwidth FWHM (full width at half maximum) of 5.26 nm and the pulses duration of 890 fs with 620 mW pump power at 1560 nm, the fundamental mode-locked laser output power as high as 5.35 mW. In addition, by further increasing the pump power, the second-order and third-order harmonic mode-locked pulses train of this fiber laser were also observed. To the best of our knowledge, this is the first report on the novel Chem-Te material saturable absorption property realization in 2 μm-band fiber laser system.

Ultrafast response of harmonic modelocked THz lasers

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity’s fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

Sheet-structured bismuthene for near-infrared dual-wavelength harmonic mode-locking

Bismuthene with a similar layered lattice structure belonging to group VA is regarded as a kind of novel two-dimensional material and has excellent properties such as small indirect bandgap (less than 1 eV) and unique electronic properties, etc. Based on the large magnitude of third-order nonlinear susceptibility and high carrier motility, bismuthene can be considered as a promising material for various optoelectronics, electronics, and nonlinear optics. Compared with the mass research about the few-layer bismuthene, we focus on the characteristics and nonlinear optical properties of bismuthene nanosheets in this work. Bismuthene nanosheets present high modulation depth over 7.7%. The sheet-structured bismuthene as saturable absorbers (SAs) is a technically important issue in laser technology. Here, for the first time, it is demonstrated that bismuthene nanosheets can be served as an SA to readily generate a harmonic dual-wavelength mode-locked picosecond pulse in a highly nonlinear fiber laser. A harmonic mode-locked pulse order from 1st to 20th is obtained at the pump power from 43.2 to 201.5 mW. When the pump power is greater than 408 mW, a 52th harmonic dual-wavelength pulse (corresponding to the repetition of 208 MHz) has been obtained. This study demonstrates the bismuthene saturable absorption is an intrinsic property independent from the structural dimension. Our work attests the promise of bismuthene in optical communication, optical detecting, sensor systems, and material processing, etc.

Fundamental and harmonic mode-locked h-shaped pulse generation using a figure-of-9 thulium-doped fiber laser.

Herein, we present a fundamental and harmonic mode-locked figure-of-9 thulium-doped fiber laser using a nonlinear amplifying loop mirror. The generated fundamental mode-locked h-shaped pulse is centered at 1889 nm with an average output power reaching 282 mW and a pulse energy up to 1.23 µJ, which is the highest power and pulse energy of an h-shaped pulse. In the harmonic mode-locked regime, up to the 8th harmonic h-shaped pulse is obtained. The detailed characteristics of the h-shaped pulse are discussed. The proposed study shows that the figure-of-9 fiber laser can generate h-shaped pulses and also allows the generation of nanosecond pulses with a µJ-level pulse energy.

Harmonic Mode‐Locking: Revealing the Buildup Dynamics of Harmonic Mode‐Locking States in Ultrafast Lasers (Laser Photonics Rev. 13(9)/2019)

In article number 1800333, Xueming Liu and Meng Pang demonstrate the experimental observation of the entire buildup process of the passive harmonic mode-locking (HML) state in an ultrafast laser, which undergoes seven different ultrafast phases. Multiple HML pulses originate from the single-pulse splitting phenomenon. The acoustic resonance in the single-mode fiber stabilizes the final HML state of the laser.

Unusual Evolutions of Dissipative-Soliton-Resonance Pulses in an All-Normal Dispersion Fiber Laser

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions.

Tm3+-Doped Harmonic Dissipative Soliton Mode-Locked Fiber Laser at 1.93 $\mu$ m Based on Tungsten Disulfide in Anomalous Dispersion Regime

We report a harmonic dissipative soliton $1.93~\mu \text{m}$ thulium fiber laser based on a tungsten disulfide (WS2) saturable absorber (SA) in the anomalous dispersion regime. The multilayer WS2 nanosheets were prepared by liquid phase exfoliation method and the SA was fabricated by dropping WS2 solution onto a gold mirror. The transferred WS2 SA had a modulation depth of 2.5% and a saturation intensity of 0.82 MW/cm2. By incorporating the SA into a linear Tm3+ fiber laser cavity, harmonic mode-locked dissipative soliton laser was achieved at 1930 nm with the spectral width of 8 nm, the pulse energy of 3 nJ, the pulse width of 3.6 ns, and the repetition rate of 56.3 MHz. Based on the experimental results, it is shown that with the presence of harmonic mode-locking in $2~\mu \text{m}$ wavelength region, the multilayer WS2 serving as a SA was verified to be a good candidate for broadband high-energy mode-locking. The order of the harmonic dissipative soliton mode-locked pulses remains the same along with the increasing pump power.

Tunable high repetition rate actively mode-locked fiber laser at 2 μm

We demonstrate a tunable actively mode-locked fiber laser at 2 μm band. A segment of 4 m Tm-Ho-co-doped fiber is used as gain medium. Active mode locking pulse is realized by using intensity modulation and the signal source is high frequency sinusoidal signal. A tunable narrow bandwidth optical filter is used to narrow laser linewidth, suppress noise and achieve wavelength tuning. Stable actively mode-locked pulses with up to 2.2 GHz repetition rate is obtained, corresponding to 649 order harmonic mode-locked pulse train. The pulse width is about 200 ps. The signal-to-noise ratio of RF spectrum is 68 dB. The optical tuning range is 1907 nm~1927 nm.