Q-switched Mode-locking Research Articles

1.4 W Passively Q-Switched Mode-Locked Tm:CALGO Laser with a MoS2 Saturable Absorber

The passively Q-switched mode-locked (QML) operation of a Tm:CaGdAlO4 (Tm:CALGO) bulk laser pumped by a wavelength-tunable Ti:sapphire oscillator using molybdenum disulfide (MoS2) as a saturable absorber was demonstrated. By using an output coupler with 3% transmittance, Q-switched mode-locked operation can be achieved with 3.56 W absorbed pump power. At a pump power of 6.77 W, laser pulses with the maximum average power of 1.44 W were obtained, corresponding to a slope efficiency of 21.3%. The laser delivered pulses centered at 1927 nm with a repetition frequency of 131.6 MHz. The experimental results confirm the promising application of the MoS2 in high-power Q-switched mode-locked solid-state lasers at 2 µm.

Dynamic gain driven mode-locking in GHz fiber laser

Ultrafast lasers have become powerful tools in various fields, and increasing their fundamental repetition rates to the gigahertz (GHz) level holds great potential for frontier scientific and industrial applications. Among various schemes, passive mode-locking in ultrashort-cavity fiber laser is promising for generating GHz ultrashort pulses (typically solitons), for its simplicity and robustness. However, its pulse energy is far lower than the critical value of the existing theory, leading to open questions on the mode-locking mechanism of GHz fiber lasers. Here, we study the passive mode-locking in GHz fiber lasers by exploring dynamic gain depletion and recovery (GDR) effect, and establish a theoretical model for comprehensively understanding its low-threshold mode-locking mechanism with multi-GHz fundamental repetition rates. Specifically, the GDR effect yields an effective interaction force and thereby binds multi-GHz solitons to form a counterpart of soliton crystals. It is found that the resulting collective behavior of the solitons effectively reduces the saturation energy of the gain fiber and permits orders of magnitude lower pulse energy for continuous-wave mode-locking (CWML). A new concept of quasi-single soliton defined in a strongly correlated length is also proposed to gain insight into the dynamics of soliton assembling, which enables the crossover from the present mode-locking theory to the existing one. Specifically, two distinguishing dynamics of Q-switched mode-locking that respectively exhibit rectangular- and Gaussian-shape envelopes are theoretically indicated and experimentally verified in the mode-locked GHz fiber laser through the measurements using both the standard real-time oscilloscope and emerging time-lens magnification. Based on the proposed criterion of CWML, we finally implement a GDR-mediated mode-locked fiber laser with an unprecedentedly high fundamental repetition rate of up to 21 GHz and a signal-to-noise ratio of 85.9 dB.

High-power idler-resonant intracavity KTA OPO driven by a dual-loss-modulated Q-switched mode-locked laser with AOM and Sb2Te3 nanosheets

By using Sb2Te3 nanosheets as saturable absorbers (SA) and an acousto-optic modulator (AOM), a laser-diode (LD) end-pumped idler-resonant KTiOAsO4 (KTA)-based intracavity optical parametric oscillator (IOPO) pumped by a dual-loss-modulated Q-switched mode-locked (QML) laser has been realized. The experimental results show that the pulse widths of the Q-switched envelope and the mode-locking pulse numbers underneath the Q-switched envelope decrease as the pump power increases. When the pump power reaches a certain value, only one mode-locking pulse underneath a Q-switched envelope exists, resulting in the generation of the subnanosecond mode-locking pulses of OPO with the repetition rate of AOM. The minimum mode-locking pulse durations of the signal and idler waves were measured to be 545 and 936 ps at an AOM frequency of 1 kHz and a diode pump power of 22.45 W, corresponding to the maximum peak powers of 648 and 185 kW, respectively. Furthermore, a set of coupled rate equations for the dual-loss-modulated QML laser-pumped intracavity idler-resonant OPO was formulated according to the Gauss distribution of intracavity photon density. The numerical simulations of these equations agree with the experimental results. These results collectively suggest the potential application of Sb2Te3 as a promising nanomaterial in the realm of optoelectronics.

Voltage-controlled nonlinear optical properties in gold nanofilms via electrothermal effect

Dynamic control of the optical properties of gold nanostructures is crucial for advancing photonics technologies spanning optical signal processing, on-chip light sources and optical computing. Despite recent advances in tunable plasmons in gold nanostructures, most studies are limited to the linear or static regime, leaving the dynamic manipulation of nonlinear optical properties unexplored. This study demonstrates the voltage-controlled Kerr nonlinear optical response of gold nanofilms via the electrothermal effect. By applying relatively low voltages (~10 V), the nonlinear absorption coefficient and refractive index are reduced by 40.4% and 33.1%, respectively, due to the increased damping coefficient of gold nanofilm. Furthermore, a voltage-controlled all-fiber gold nanofilm saturable absorber is fabricated and used in mode-locked fiber lasers, enabling reversible wavelength-tuning and operation regimes switching (e.g., mode-locking—Q-switched mode-locking). These findings advance the understanding of electrically controlled nonlinear optical responses in gold nanofilms and offer a flexible approach for controlling fiber laser operations.

Observation of 2 µm multiple annular structured vortex pulsed beams by cavity-mode tailoring.

In the past few years, annular structured beams have been extensively studied due to their unique "doughnut" structure and characteristics such as phase and polarization vortices. Especially in the 2 µm wavelength range, they have shown promising applications in fields such as novel laser communication, optical processing, and quantum information processing. In this Letter, we observed basis vector patterns with orthogonality and completeness by finely cavity-mode tailoring with end-mirror space position in a Tm:CaYAlO4 laser. Multiple annular structured beams including azimuthally, linearly, and radially polarized beams (APB, LPB, and RPB) operated at a Q-switched mode-locking (QML) state with a typical output power of ∼18 mW around 1962 nm. Further numerical simulation proved that the multiple annular structured beams are the coherent superposition of different Hermitian Gaussian modes. Using a self-made M-Z interferometer, we have demonstrated that the obtained multiple annular beams have a vortex phase with orbital angular momentum (OAM) of l = ±1. To the best of our knowledge, this is the first observation of vector and scalar annular vortex beams in the 2 µm solid-state laser.

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Nonlinear optical response of strain-mediated gallium arsenide microwire in the near-infrared region

Abstract Gallium arsenide (GaAs) semiconductor wires have emerged as potent candidates for nonlinear optical devices, necessitating bandgap engineering for an expanded operational wavelength range. We report the successful growth of strain-mediated GaAs microwires (MWs) with an average diameter of 1.1 μm. The axial tensile strain in these wires, as measured by X-ray diffraction and Raman scattering, ranges from 1.61 % to 1.95 % and from 1.44 % to 2.03 %, respectively. This strain condition significantly reduces the bandgap of GaAs MWs compared to bulk GaAs, enabling a response wavelength extension up to 1.1 μm. Open aperture Z-scan measurements reveal a nonlinear absorption coefficient of −15.9 cm/MW and a third-order magnetic susceptibility of −2.8 × 10−8 esu at 800 nm for these MWs. I-scan measurements further show that the GaAs saturable absorber has a modulation depth of 7.9 % and a nonsaturation loss of 3.3 % at 1050 nm. In laser applications, GaAs MWs have been effectively used as saturable absorbers for achieving Q-switched and dual-wavelength synchronous mode-locking operations in Yb-bulk lasers. These results not only offer new insights into the use of large diameter semiconductor wires but also expand the potential for applications requiring bandgap tuning.

Germanene: demonstration of the conversion from mode-locked to Q-switched mode-locked in Er-doped fiber laser

Some two-dimensional layered mono-elemental materials have been reported as saturable absorbers (SAs) for the generation of various soliton phenomena, and many excellent results have been achieved. In our experiment, we made thin films of germanene-polyvinyl alcohol (Ge-PVA) and applied them as SA in Er-doped fiber lasers, investigated Ge’s characteristics, and demonstrated the conversion from mode-locked to Q-switched mode-locked (QML). The conventional mode-locked operation with a repetition rate of 9.63 MHz and a central wavelength of 1559.7 nm was stably triggered when the pump power exceeded the threshold of 40 mW. QML pulse with a central wavelength of 1530.26 nm and the maximum pulse energy of 86 nJ can be obtained by changing the pump power and polarization state of the light in the cavity. This work reveals the excellent optical properties of Ge SA in ultrafast fiber lasers and provide a new approach for the generation of QML pulses.

MOFs-derived hierarchical NiO-Co3O4 for versatile pulses generation

Polyhedral NiO-Co3O4 materials, as a novel type of metal organic frameworks (MOFs), are considered to be attractive emerging materials in the fields of optoelectronics, energy storage systems, sensing and catalysis because of their high specific surface area, rich variable metal sites and unique morphology. Ultrafast pulse has become an important research tool in basic research and industrial application fields, including supercontinuum generation, laser material processing, laser surgery, lidar and terahertz generation. The saturable absorber (SA) is a key component that affects the laser parameters and generates ultrafast pulses, so it has caused an upsurge in the research of SA. Although MOFs and their derivatives have attracted the attention of researchers in many fields, few works have been carried out in ultrafast photonics. Here, we use the cobalt-based zeolite imidazole framework (ZIF-67-Co) as a precursor to synthesize NiO-Co3O4 polyhedron by calcination, and utilize it as an effective SA in a compact erbium-doped fiber laser (EDFL). The NiO-Co3O4 SA was measured based on home-made double-arm measurement system, which showed that its modulation depth was 5.6%. It shows that high-quality Q-switched mode-locking (QML) with a repetition rate of 5.2 MHz and a pulse width of 1.41 ps can be obtained when the pump power reaches 251.1 mW. After further improvement of the EDFL, we obtained a stable continuous wave mode-locked (CWML) pulse with a pulse duration of 1.35 ps. This is the first demonstration of a versatile pulse fiber laser based on a NiO-Co3O4 polyhedral SA. This work shows that the novel NiO-Co3O4 polyhedron is expected to serve as a highly potential nonlinear optical element in the fields of ultrafast photonics, light modulators and photodetectors.

Q‐switched mode‐locked Nd:GGG laser with MoS2–SnSe2 heterojunction nanosheets

AbstractIn this paper, we used the liquid phase stripping method to manufacture MoS2–SnSe2 heterojunction saturable absorber (SA). The scanning electron microscope, the atomic force microscope, and the Raman spectrometer were used to characterize the MoS2–SnSe2 heterojunction SA. The Q‐switched mode‐locked (QSML) Nd:GGG laser with MoS2–SnSe2 heterojunction SA was presented. The maximum output power of the QSML Nd:GGG laser was 236.4 mW with a repetition rate of 83.3 MHz and pulse width of 447 ps. In short, the MoS2–SnSe2 heterojunction nanosheets present excellent saturable absorption characteristics.

Buildup of multiple spatiotemporal nonlinear dynamics in an all-fiber multimode laser

Ultrafast lasers based on multimode fibers have attracted extensive attention owing to the large mode-field area and nonlinear tolerance. The high spatial degree of freedom of multimode fibers is significant for spatiotemporal pulses locked both in transverse and longitudinal modes, where the energy of output pulses can be remarkably improved. Herein, the 1.5-μm all-fiber spatiotemporal mode-locked laser was realized based on carbon nanotubes as a saturable absorber. Moreover, by tuning the polarization controller and the pump power carefully, the output wavelengths can be ranged from 1529 to 1565 nm based on the multimode interference filter. In addition, Q-switched mode-locking and spatiotemporal mode-locked dual combs were also observed by further adjusting the polarization controller. Such a kind of an all-fiber multimode laser offers a crucial insight into the spatiotemporal nonlinear dynamics, which is of great significance in scientific research and practical applications.

Dual-loss-modulated QML erbium-doped fiber laser with microfiber and NPR technique

We demonstrated a dual-loss-modulated Q-switched mode-locked (QML) erbium-doped fiber laser based on the nonlinear polarization rotation (NPR) technique with a microfiber. The microfiber served as both a highly nonlinear medium and a saturable absorber, which together with the NPR technique enables stable QML operation under significant pump power fluctuations. We evaluated the nonlinear properties of microfibers with different waist diameters. A maximum QML envelope energy of 467.15 nJ is achieved, corresponding to an average single-pulse energy of 4.06 nJ beneath the Q-switched envelope, which is 2.35 times higher than the maximum pulse energy attained with dissipative soliton mode-locking under similar pump power. In addition, the QML laser was verified by numerical simulation based on the coupled Ginzburg-Landau equation. The microfiber's customizable size enables significant variations in non-linear and saturable absorption characteristics, creating the potential for QML pulse regulation. This research provides novel insights for designing QML lasers.

High Peak Power Ultrashort Pulsed Passively Q-Switched Mode-Locked Bragg Laser Diode

We demonstrate a Bragg waveguide laser diode with an unbiased saturable absorber exhibiting passive Q-switched mode-locking with two AlInGaAs quantum wells. The widths of the observed passively mode-locked pulses ranged from 250 fs to 1640 fs under a Q-switching envelope. The frequency response showed the fundamental peak and smallest linewidths, for Q-switched operation, to range from 0.35 GHz to 1.15 GHz and 1.4 MHz to 9.1 MHz respectively. At 25 °C the devices demonstrated the highest peak power of ~10.1 W @ ~791 nm and highest peak to average power ratio of ~2800.

Passively Q-switched mode-locking Er-doped fiber laser with tunable wavelength and pulsating soliton

A passively Q-switched mode-locking erbium-doped fiber laser with tunable wavelength and pulsating soliton is demonstrated. The polarization-maintaining erbium-doped fiber, working as the gain medium, combines with nonlinear polarization rotation to form a Lyot filter in the cavity. A tapered fiber is placed between two polarization controllers to cause a significant insertion loss in the optical path, which makes it easy to generate Q-switched mode-locking (QML) pulses. The wavelength tunable QML laser is implemented simply and easily, and the tunable wavelength ranged from 1528.05 nm to 1569.55 nm, as large as 41.5 nm. In addition, a new type of QML pulse involving pulsating soliton has been discovered in the laser, which was called Q-switched harmonic mode-locking pulsating soliton (QHML-PS). We find that the harmonic order and repetition frequency increases with the rising of the pump power. Our work has tremendous significance for expanding the research and application of emerging QML fiber lasers.

Operation of a passively mode‐locked Tm:YAG laser with tantalum aluminum carbide as a saturable absorber

AbstractIn this paper, a compact diode‐pumped continuous wave (CW) and passively Q‐switched mode‐locked (QML) Tm:YAG lasers operation on the 3F4–3H6 transition is realized by using tantalum aluminum carbide (Ta4AlC3) phase ceramic (MAX) material as a saturable absorber (SA). The first stable QML operation of the mid‐infrared Tm:YAG laser at 2012.38 nm is demonstrated with a Ta4AlC3‐SA. A stable QML Tm:YAG laser was achieved when the absorption pumping power reached 14 W. Under QML mode, Tm:YAG laser generated a maximum average output power of 560 mW at 2012.38 nm with an optical‐to‐optical conversion efficiency of 4%, corresponding to a shortest pulse duration of 541 ps and a high repetition rate of 168 MHz. The experiment shows that Ta4AlC3‐SA is an attractive candidate for diode‐pumped CW and QML mid‐infrared all‐solid‐state lasers emitting around 2.0 μm.

ASE recirculation effects in pulsed frequency shifted feedback lasers at large frequency shifts.

An analysis of the different emission regimes (continuous wave, Q-switched, and different forms of modelocking) of a C-band Er:fiber frequency shifted feedback laser at large frequency shifts is presented. We clarify the role of amplified spontaneous emission (ASE) recirculation in the origin of various spectral and dynamical properties of this type of laser. Specifically, we show that Q-switched pulses are supported by a noisy, quasiperiodic ASE recirculation pattern that univocally identifies the pulses within the sequence, and that these Q-switched pulses are chirped as a consequence of the frequency shift. A specific pattern of ASE recirculation, in the form of a periodic stream of pulses, is identified in resonant cavities, namely, those where the free spectral range and the shifting frequency are commensurable. The phenomenology associated with this pattern is explained through the moving comb model of ASE recirculation. Modelocked emission is induced from both integer and fractional resonant conditions. It is shown that ASE recirculation coexists with modelocked pulses, originates a secondary peak in the optical spectrum, and also drives Q-switched modelocking near resonant conditions. Harmonic modelocking with variable harmonic index is also observed in non-resonant cavities.

All-fiber low threshold tunable multimode Q-switched mode locked fiber laser using a few mode Er-doped fiber

We report an all-fiber low threshold (70 mW) multimode Q-switched mode-locked (QML) fiber laser at 1.5 m. Nonlinear polarization rotation and a few mode Er-doped fiber (FM-EDF) were both adopted to achieve a low threshold QML operation initially, and the later experimental results showed and verified that the FM-EDF was the key factor of the QML pulses generation. A multimode QML state at 70 mW pump power was obtained with an 18 μs Q-switched pulse width and a 7.41 kHz repetition rate. The repetition rate and pulse width of the mode-locked pulses within the Q-switched envelope were 16.9 MHz and ∼910 ps respectively. The multimode QML fiber laser can be turned from 1561.8 nm to 1575.1 nm owing to the multimode interference and the fiber birefringence filtering effect. The QML pulses with dual-wavelength and three-wavelength were also observed by manipulating the polarization controllers and increasing the pump power. The experimental results showed that multimode QML pulses can be achieved with a short FM-EDF at a lower pump power in an all-fiber multimode cavity. The generated low threshold and tunable QML multimode fiber laser has potential application in measuring and mode-division multiplexed system.

High-efficiency mode-locked erbium-doped ZBLAN fiber laser around 2.8 µm by directly depositing Bi2S3 particles onto a cavity mirror.

Mid-infrared (MIR) pulsed lasers near a 3µm waveband show great potential for the high absorption of water molecules and many important gas molecules. A passively Q-switched mode-locked (QSML) E r 3+-doped fluoride fiber laser with a low laser threshold and high slope efficiency around a 2.8µm waveband is reported. The improvement is achieved by depositing bismuth sulfide (B i 2 S 3) particles onto the cavity mirror directly as a saturable absorber and using the cleaved end of the fluoride fiber as output directly. -QSML pulsesbegin to appear with the pump power of 280mW. The repetition rate of the QSML pulses reaches a maximum of 33.59kHz with the pump power of 540mW. When the pump power is further increased, the output of the fiber laser switches from the QSML to the continuous-wave mode-locked operation with the repetition rate of 28.64MHz and the slope efficiency of 12.2%. The results indicate that B i 2 S 3 is a promising modulator for the pulsed lasers near a 3µm waveband, which paves the way for further development of various applications in MIR wavebands, including material processing, MIR frequency combs, and modern healthcare.

2 µm cylindrical vector beam generation from a c-cut Tm:CaYAlO4 crystal resonator.

Different from the traditional ideal column symmetry cavities, we directly generated the cylindrical vector pulsed beams in the folded six-mirror cavity by employing a c-cut Tm:CaYAlO4 (Tm:CYA) crystal and SESAM. By adjusting the distance between the curved cavity mirror (M4) and the SESAM, both the radially polarized beam and azimuthally polarized beam are generated around 1962 nm and the two vectorial modes can be freely switched in the resonator. Further increased the pump power to 7 W, the stable radially polarized Q-switched mode-locked (QML) cylindrical vector beams were also obtained with an output power of 55 mW, the sub-pulse repetition rate of 120.42 MHz, pulse duration of ∼0.5 ns and the beam quality factor M2 of ∼2.9. To our knowledge, this is the first report of radially and azimuthally polarized beams in the 2 µm wavelength solid-state resonator.

Passively Q-switched mode-locked thulium-doped fiber laser using nonlinear polarization rotation technique

A Q-switched mode-locked (QML) thulium-doped fiber laser based on nonlinear polarization rotation (NPR) and negative group velocity dispersion (GVD) is demonstrated. This regime consists in the generation of a mode-locked (ML) pulse train under a Q-switched (QS) envelope. In this regard, by properly adjusting the polarization controllers and increasing the pump power in a range from 1 to 3 W, QML regime is obtained. With a maximum pump power of 3 W, a QS pulse duration of 1.46 μs and a pulse energy of 5.96 μJ are obtained. The maximum frequencies reached are 26 kHz and 2.63 MHz, corresponding to the Q-switched and mode-locked pulses, respectively. This method for generating a QML at 2 μm has the advantages of being eye-safe and robust, providing an ideal dual-frequency comb (DFC) generation method with high potential for various applications.