Mode-locked State Research Articles

Q-switched and mode-locked Er-doped fiber laser using PtSe2 as a saturable absorber

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe2, a new two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe2-based SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is 0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked pulses in an Er-doped fiber laser has been obtained by using PtSe2 as a saturable absorber.

Decaying evolution dynamics of double-pulse mode-locking

Taking advantage of the dispersive Fourier transformation technique, the decaying evolution processes of double-pulse mode-locking in a single-walled carbon-nanotube-based Er-doped fiber laser are observed in detail for the first time to our knowledge. The decaying dynamics of the double-pulse mode-locking state is analyzed in the spectral and temporal domains. We reveal that the two pulses in one cluster disappear either simultaneously or one by one during the decaying processes of double-pulse mode-locking states. In addition, the spectral evolution patterns of the special double-pulse states (i.e., bound states) are extremely distinct at different decline rates of the pump power.

Self Q-switched mode-locking in compact red Pr3+-doped ZBLAN fiber laser

The self Q-switching mode-locking effect in compact red fiber laser has been investigated. The Q-switched mode-locking state can keep operating with a pump power ranging from 120.5 mW to 564.8 mW. The maximum average output power of this self Q-switched mode-locking laser is 22.5 mW, and the central wavelength of the output optical spectrum is 635.9 nm with a 3 dB bandwidth of 2.4 nm, as well as the fundamental repetition rate of mode-locking pulse trains being 25.8 MHz. This work can provide an attractive method for Q-switched mode-locking pulse generation in visible fiber lasers.

Graphene-Assisted All-Fiber Optical-Controllable Laser

All-optical controlling provides the burgeoning, precise, and compact access in photonics and optoelectronics with tunable performance. Through tuning physical parameters of optical-controlled devices, such as effective index, bandwidth and dispersion, the output characteristics of free-running lasers can be manipulated for widespread applications in spectroscopy and laser biology. Among which the flexible optical-thermal tunability of graphene introduces an advantageous avenue for the enhancement of photosensitivity of optical-controlled devices to efficiently regulate laser properties. Here, we fabricate an optical-controllable fiber filter based on a graphene-assisted micro-fiber Bragg grating, whose wavelength can be linearly tuned by controlling pump light, and a tuning step of 3.1 pm is achieved. By cooperating it with a fiber laser operating at a Q-switched mode-locked state, the lasing wavelength is all-optically controlled with a linear tuning sensitivity of 23.5 pm/mW. Considering the optical controlling performances, such a wavelength-tunable fiber laser may find potential applications in all-optical computing, all-optical logic gating, and generation of microwave source.

Polarization Rotation Dynamics in Harmonically Mode-Locked Vector Soliton Fiber Lasers

Polarization rotation dynamics for group velocity locked vector solitons in higher order harmonically mode-locked soliton fiber lasers are studied for the first time, in a thulium/holmium fiber laser. In a linear ultrafast fiber laser with net anomalous dispersion, harmonically mode-locked states are generated continuously by increasing the pump power for specific net cavity birefringence settings. The polarization evolution frequency remains the same for all harmonically mode-locked pulse trains, converging toward a singular solution close to the maximum pulse intensity modulation depth for each respective harmonically mode-locked state. With a polarization resolved output, an intensity modulation of the ultrafast pulse train with varying pulse binning is recorded, corresponding to the polarization rotation of the vector solitons. In addition, a polarization rotation vector soliton state with two different sets of RF sidebands symmetric to the repetition rate is observed, which leads to a more complex modulated pulse train. The experimental results of the polarization evolution also matche well with the presented theoretical modeling. Similar relative intensity noise <0.4% for all harmonically mode-locked states is measured. This method to modulate the intensity of an ultrafast pulse train and tune its pulse spacing by integer fractions of the initial round trip time shows a strong potential for applications in optical communications, polarization de multiplexing schemes, and sensing.

Passively Mode-Locked Fiber Lasers Based on Nonlinearity at 2-μm Band

2- μ m fiber lasers are of wide potential applications in the fields of radar, sensing, and free-space communications. We introduced a simple approach to generate passively mode-locked pulse in thulium-doped fiber lasers based on nonlinear polarization rotation (NPR) in our previous work. The high repetition rate of 1.78 GHz is obtained by both NPR and semiconductor saturable absorption mirror. By using a polarization-maintaining fiber in the experiment, a birefringence Lyot filter as a fiber comb filter can be constructed, and then the tuning range of 94 nm can be obtained for the mode-locked laser. In this paper, we also demonstrated our recent research work, a square-wave mode-locked fiber laser at 2- μ m band based on nonlinear amplifying loop mirror. The square-wave noiselike pulse (NLP) at longer wavelength of 2- μ m band can be observed. With the increase of the pump power, pulse width can be increased from 0.75 to 1.3 ns. The polarization vector characteristics of the square-wave NLP were analyzed experimentally. In addition, with the increase of the pump power, a kind of square-wave mode-locked pulse with low-intensity multipulse bunches can be observed, and these multipulse bunches can be operated at the multipulse mode-locked state. The side-mode suppression ratio decreased from 50 to 30 dB.

Dual-pulses and harmonic patterns of a square-wave soliton in passively mode-locked fiber laser

We demonstrate a square-wave soliton pulse passively mode-locked fiber laser. The mode-locked pulses are achieved by using a nonlinear amplifying loop mirror. Single-pulse operation at a fundamental repetition rate of 3.2 MHz is obtained. The optical spectrum presents the soliton feature of several sidebands. The pulse duration expands with increasing pump power, but the amplitude hardly varies. Pulse breaking occurs and a stable dual-pulse is obtained with a fixed interval of 48 ns. Harmonic mode-locked states can be achieved when the total pump power is higher than 740 mW. The harmonic pulses can also operate in both single-pulse and dual-pulse states.

Observation of soliton molecules in a spatiotemporal mode-locked multimode fiber laser.

We report on the first experimental observation, to the best of our knowledge, of soliton molecules in a spatiotemporal mode-locked multimode fiber (MMF) laser. By adjusting the waveplates inside the cavity, not only the spatiotemporal mode-locking state with a stable single pulse but also soliton molecules are observed. Various soliton molecules, including soliton pairs, soliton triplets, and soliton quartets with different pulse separations, are achieved. Transition of different operation states with pump power is given. The results would be beneficial for further understanding of the nonlinear dynamics in spatiotemporal mode-locked MMF lasers.

Spectral periodicity in soliton explosions on a broadband mode-locked Yb fiber laser using time-stretch spectroscopy.

Experimentally, we demonstrate, to the best of our knowledge, the first observation of periodic spectrum changing via soliton explosion in a passively mode-locked fiber laser by a nonlinear polarization evolution. Using time stretch to capture 7220 consecutive single-shot spectra over a 100 μs time window in real time, the soliton explosions appear in a transition between two different mode-locking states. Simultaneous measurements of spectrum and pulse energy at three different output points in the laser cavity show that the soliton explosion's dynamics are related to residual dispersion. This study improves the understanding of pulse formation and evolution in the unstable mode-locking regime of lasers.

1.6 μm dissipative soliton fiber laser mode-locked by cesium lead halide perovskite quantum dots.

We demonstrate a stable, picosecond fiber laser mode-locked by cesium lead halide perovskite quantum dots (CsPbBr3-QDs). The saturable absorber is produced by depositing the CsPbBr3-QDs nanocrystals onto the endface of a fiber ferrule through light pressure. A balanced two-detector measurement shows that it has a modulation depth of 2.5% and a saturation power of 17.29 MW/cm2. After incorporating the fabricated device into an Er3+-doped fiber ring cavity with a net normal dispersion of 0.238 ps2, we obtain stable dissipative soliton with a pulse duration of 14.4 ps and a center wavelength at 1600 nm together with an edge-to-dege bandwidth of 4.5 nm. The linear chirped phase can be compensated by 25 m single mode fiber, resulting into a compressed pulse duration of 1.046 ps. This experimental works proves that such CsPbBr3-QDs materials are effective choice for ultrafast laser operating with devious mode-locking states.

Optical properties and applications for MoS2-Sb2Te3-MoS2heterostructure materials

Two-dimensional (2D) materials with potential applications in photonic and optoelectronic devices have attracted increasing attention due to their unique structures and captivating properties. However, generation of stable high-energy ultrashort pulses requires further boosting of these materials’ optical properties, such as higher damage threshold and larger modulation depth. Here we investigate a new type of heterostructure material with uniformity by employing the magnetron sputtering technique. Heterostructure materials are synthesized with van der Waals heterostructures consisting of MoS2 and Sb2Te3. The bandgap, carrier mobility, and carrier concentration of the MoS2-Sb2Te3-MoS2 heterostructure materials are calculated theoretically. By using these materials as saturable absorbers (SAs), applications in fiber lasers with Q-switching and mode-locking states are demonstrated experimentally. The modulation depth and damage threshold of SAs are measured to be 64.17% and 14.13 J/cm2, respectively. Both theoretical and experimental results indicate that MoS2-Sb2Te3-MoS2 heterostructure materials have large modulation depth, and can resist high power during the generation of ultrashort pulses. The MoS2-Sb2Te3-MoS2 heterostructure materials have the advantages of low cost, high reliability, and suitability for mass production, and provide a promising solution for the development of 2D-material-based devices with desirable electronic and optoelectronic properties.

Black phosphorus: broadband nonlinear optical absorption and application

Black phosphorus (BP), 2D layered material with layered dependent direct bandgap (0.3 eV (bulk), 2.0 eV (single layer)) that has gained renewed attention, has been demonstrated as an extremely appropriate optical material for broadband optical applications from infrared to mid-infrared wavebands. Herein, by coupling multi-layer BP films with microfiber, we fabricated a nonlinear optical device with long light–matter interaction distance and enhanced damage threshold. Through taking full advantage of its fine nonlinear optical absorption property, we obtained stable mode-locking (51 ps) and Q-switched mode-locking states in Yb-doped or Er-doped (403.7 fs) all-fiber lasers and the single-longitudinal-mode operation (53 kHz) in an Er-doped fiber laser with enhanced power tolerance, using the same nonlinear optical device. Our results showed that BP could be a favorable nonlinear optical material for developing BP-enabled wave-guiding photonic devices, and revealed new insight into BP for high optical power unexplored optical devices.

Generation of cylindrical vector beams in a mode-locked fiber laser using a mode-selective coupler

We experimentally obtain cylindrical vector beams (CVBs) in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber (SMF) and a two-mode fiber (TMF) is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.

Microresonator-based high-resolution gas spectroscopy.

We report the first demonstration of a microresonator-based tunable mode-locked frequency comb source. We achieve a mode-hop-free tuning range of 16GHz by simultaneously tuning both the pump laser and the cavity resonance while keeping the system in a multi-soliton mode-locked state. The optical spectrum spans 2520-4125 cm-1 (2.425-3.970μm) pumping at 3508 cm-1 (2.850μm) in a silicon microresonator with a comb line spacing of 4.23 cm-1 (127GHz). Our scanning technique can be used to increase the effective resolution of the microresonator-based comb spectroscopy. As a proof-of-principle demonstration, we record the absorption spectrum of the rovibrational transitions of the υ3 and υ2+(υ4+υ5)+0 bands of acetylene. We measure absorption features as narrow as 0.21 cm-1 (6.4GHz) full width at half-maximum at a frequency sampling step of 80MHz.

Self-Tuning Mode-Locked Fiber Lasers Based on Prior Collection of Polarization Settings

We present a method to discriminate stable mode-locked states of an all fiber erbium-doped laser based on nonlinear polarization rotation mechanism. The method has a balance between the accuracy and rapidness of the pulses discrimination. Mode-locked states are distinguished and confirmed by an amplitude discriminator, which reflects the repetition rate information of the pulses. An electronic polarization controller is used to precisely adjust the intracavity polarization states. The driving voltages of the electronic polarization controller at those mode-locked states are recorded as populations, and it is benefit to reduce the build-up time for mode locking. The populations are also recorded at operation temperature covering from 20 °C to 50 °C, which exhibits the repeatability and stability of the mode-locked states. Rapid self-tuning mode-locked fiber lasers could be achieved by the repeatability of the polarization settings under the stable temperature environment.

Mode-locking behavior of Izhikevich neurons under periodic external forcing.

Many neurons in the auditory system of the brain must encode periodic signals. These neurons under periodic stimulation display rich dynamical states including mode locking and chaotic responses. Periodic stimuli such as sinusoidal waves and amplitude modulated sounds can lead to various forms of n:m mode-locked states, in which a neuron fires n action potentials per m cycles of the stimulus. Here, we study mode-locking in the Izhikevich neurons, a reduced model of the Hodgkin-Huxley neurons. The Izhikevich model is much simpler in terms of the dimension of the coupled nonlinear differential equations compared with other existing models, but excellent for generating the complex spiking patterns observed in real neurons. We obtained the regions of existence of the various mode-locked states on the frequency-amplitude plane, called Arnold tongues, for the Izhikevich neurons. Arnold tongue analysis provides useful insight into the organization of mode-locking behavior of neurons under periodic forcing. We find these tongues for both class-1 and class-2 excitable neurons in both deterministic and noisy regimes.

Analysis of Bound-Soliton States in a Dual-Wavelength Mode-Locked Fiber Laser Based on Bi2Se 3

Wavelength switchable and dual-wavelength soliton operations are demonstrated in an erbium-doped fiber laser with three pieces of TI Bi2Se3 polyvinyl alcohol (Bi2Se3-PVA) film. Wavelength switchable operation from 1532 to 1557 nm could be achieved just by adjusting the pump power. With the appropriate settings of PC and pump power, a novel dual-wavelength soliton operation was observed. The dual-wavelength pulses operate at different mode-locked states. One is the single pulse mode-locking operation at 1532 nm, and the other one is the bound-soliton states at 1557 nm. However, the dual-wavelength soliton operation cannot be stable for a long time. The numerical results demonstrate that the separations of bound pulses have small variation and that the phase difference of bound pulses appear jumping change of $\pi $ .

Widely tunable 2 μm continuous-wave and mode-locked fiber laser.

We propose and experimentally demonstrate a widely tunable mode-locked thulium-doped fiber laser. The laser can operate at both continuous-wave and mode-locked states at different pump power levels. A classic nonlinear polarization rotation structure is employed to obtain the passive mode-locked laser. A birefringence Lyot filter as a fiber comb filter is used to expand the tuning range. Thanks to the filtering component, the tuning range of the continuous-wave laser can reach 127nm (1823-1950nm). The tuning ranges of the single-wavelength and dual-wavelength mode-locked lasers are 94nm (1835-1929nm) and 87nm (1831-1918nm), respectively, with a 3.085MHz repetition rate and 75ps pulse width.

Passively mode-locked InAs quantum dot lasers on a silicon substrate by Pd-GaAs wafer bonding

We demonstrate an electrically pumped InAs quantum dot (QD) two-section passively mode-locked laser (MLL) on a silicon substrate by low temperature (250 °C) Pd-GaAs wafer bonding technology. The saturable absorber of the QD-MLL is electrically isolated by a 15-μm wide dry-etching gap which resulted in ∼30 kΩ resistance from the gain regions of the MLL. At room temperature, the laser operates in the O-band (1.3 μm) telecommunication wavelength regime with a threshold current of 94 mA and laser bar cavity and absorber lengths of 6 mm and 300 μm, respectively. The optimum mode-locked conditions are observed under injection current and reverse bias voltage of 124 mA and −7 V, which generates pulses at a repetition rate of 7.3 GHz, an optical bandwidth of 0.97 nm, and a nearly transform limited pulse width of 2 ps (sech2 pulse profile). These results enable QD-MLLs to be integrated with silicon photonic integrated circuits, such as optical time division multiplexing and optical clocks.

Optofluidic tunable mode-locked fiber laser using a long-period grating integrated microfluidic chip.

An optofluidic tunable mode-locked fiber laser using a microfluidic chip integrated with long-period grating (LPG) is presented. The microfluidic chip enables ultrafine adjustment of the liquid's refractive index and, thus, LPG's spectrum via tuning the mixing ratio of the microfluidic flows. With such an optofluidic spectrum-tunable filter, the central wavelength of the mode-locked laser can be tuned continuously, while the mode-locking state is steadily maintained. The mode-locked pulses are measured with a pulse duration of 0.9 ps and repetition rate of 12.14 MHz, respectively. Moreover, bound solitons with variable soliton separations are experimentally demonstrated.