Mode-locked Laser Research Articles

Direct generation of ultrafast visible vortex beam in passively mode-locked Pr:YLF laser using a spot-defect mirror

We demonstrate the direct generation of SESAM (Semiconductor Saturable Absorption Mirror)-based Pr:YLF mode-locked vortex lasers in the visible spectral region, employing a spot-defect cavity mirror. A first-order Laguerre-Gaussian (LG01) mode-locked pulse has been achieved, delivering an output power of 67.4 mW accompanied by a pulse width of 60 ps. Furthermore, through precise adjustments to the radius of the spot-defect, a second-order Laguerre-Gaussian (LG02) mode-locked pulse has been achieved, yielding 57.3 mW of output power with a slightly extended pulse width of 92 ps. This work offers a more suitable method for generating higher-order ultrafast visible vortex laser and may open up potential prospect in metal materials processing and super-resolution imaging.

Relative intensity noise reduction for noise-like pulses based on synchronous intensity modulation

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.

All-polarization-maintaining passively mode-locked Ho-doped fiber laser oscillator and amplifier based on a nonlinear optical loop mirror

Our study reveals results from an all-polarization-maintaining Ho-doped fiber laser that is passively mode-locked, consisting of both an oscillator and an amplifier. A stable, self-starting, noise-like mode-locked pulse is achieved in the Ho-doped fiber laser oscillator based on a nonlinear optical loop mirror at a wavelength of 2.05 μm. The mode-locked oscillator achieves a peak output power of 20.2 mW, generating pulses with a duration of 284 ps and exhibiting a coherence peak at 690 fs. Moreover, with the use of Ho-doped fiber amplifier, the mode-locked pulse can reach a peak output of 1.05 W while maintaining a power stability of 1.43 % over the course of an hour. The highest pulse energy reaches approximately 333.6 nJ, which equates to a peak power of 1.18 kW. The polarization extinction ratio is approximately 24.9 dB.

Background-free correlation spectroscopy using an infrared mode-locked laser.

The recent advances in infrared laser technology are expanding the capabilities and applications of vibrational spectroscopy. A promising approach utilizing broadband infrared mode-locked lasers is background-free (BF) absorption spectroscopy. This method captures the free-induction decay (FID) of excited molecules while suppressing the background light. It is unique in that the signal strength increases with input optical power but eventually struggles with detector noise when targeting fewer molecules. In this paper, we present a novel method of multiplexed background-free spectroscopy using a spectral mask whose transmittance has a strong correlation with the absorption spectrum of a target molecule. We successfully demonstrate an order of magnitude increase in the sensitivity due to multiplexing as well as a high molecular contrast due to the spectral correlation. The presented results indicate the promising potential of the method for sensitive and selective detection of trace molecules.

Characterization of noise spectra in low-jitter GHz mode-locked fs-laser-inscribed waveguide lasers

We demonstrate low-noise, continuous-wave mode-locking of a diode-pumped Yb:KLuW waveguide laser operating at fundamental repetition rates in the GHz range. Utilizing a femtosecond-laser-inscribed Yb:KLuW channel waveguide laser with a tunable cavity length of a few centimeters, we successfully generate femtosecond pulses near 1030 nm, employing either single-walled carbon nanotubes or semiconductor saturable absorbers. Although timing noise is one of the most critical factors in the implementation of high-repetition-rate laser sources for various applications, investigations into timing noise characteristics in waveguide lasers remain limited. This study fills this gap by presenting timing jitter measurements for GHz mode-locked waveguide lasers with diverse cavity parameters. To quantitatively analyze noise spectral characteristics, including relaxation oscillation processes induced by intensity noise features, we introduce a theoretical modeling approach. The present investigation directly enables the provision of strategies for further suppression of timing jitter and designing high-performance, low-noise mode-locked lasers.

Bandwidth-induced saturation in multimode fiber-based absorbers.

Multimode fiber-based saturable absorbers enable mode-locking in lasers, generating ultrafast pulses and providing an exceptional platform for investigating nonlinear phenomena. Previous analyses in the continuous-wave (CW) limit showed that saturable absorption can be obtained due to nonlinear interactions between transverse modes. We find experimentally that saturable absorption can be achieved, thanks to the interplay of single-mode fiber nonlinearity and the wavelength-dependent linear transmission of the multimode fiber, even with negligible intermodal nonlinearities. We further show that even when intermodal nonlinearities are significant, the CW analysis may not be sufficient for long multimode fibers. Understanding the underlying mechanisms of multimode fiber-based saturable absorbers opens new possibilities for developing programmable devices for ultrafast control.

Dual-wavelength erbium-doped mode-locked fiber laser based on CoPS3 saturable absorber

An erbium-doped mode-locked fiber laser based on CoPS3 saturable absorber was successfully constructed and ultrashort pulsed light was obtained. The CoPS3 nanosheets solution was prepared using the liquid-phase exfoliation technique, and then drop-coated onto a tapered fiber as a saturable absorber. The fiber laser based on CoPS3 saturable absorber could output conventional solitons at the central wavelengths of 1557.46 nm or 1532.12 nm, respectively, as well as simultaneously output conventional solitons at dual wavelengths of 1531.56 nm and 1557.32 nm, with the ability to switch between the three operational states. The achieved minimum pulse width was 1.59 ps. In addition to conventional solitons, the laser demonstrated the capability to generate bound-state solitons. This study highlights the effectiveness of CoPS3 as a reliable optical modulator for fiber lasers and illustrates its potential for applications in ultrafast optics.

Discovery of dissipative microwave photonic soliton molecules in dual-bandpass optoelectronic oscillator

Optoelectronic oscillators (OEOs), which have attracted extensive studies in the past decades, are high quality-factor optoelectronic feedback loops for generating various ultra-pure microwave signals. In essence, OEOs are also dissipative nonlinear systems with multiple timescale characteristics and abundant nonlinearities, which open the possibilities for exploring localized dissipative solitary waves. In this paper, we demonstrate a new-class temporal dissipative soliton, i.e., dissipative microwave photonic soliton molecule (DMPSM), in a dual-bandpass OEO. Both the numerical simulation and experiment are conducted to reveal the physical mechanism of DMPSM generation and to evaluate the characteristics of the generated DMPSM sequences. Unlike optical soliton molecules in mode-locked lasers, the formation of DMPSMs arises from the combined action of multiple timescale coupling, nonlinear bistability, and time-delayed feedback in the OEO cavity, where the soliton interval and number in a DMPSM can be well-controlled through varying the multiple timescale variables in the OEO cavity, and the repetition frequency of the DMPSMs can be tuned through changing that of the initially injected perturbation signal. Meanwhile, the generated DMPSM sequence performs with high stability and excellent coherence, which shows enormous application potentials in pulse radar detection, dense microwave comb generation, and neuromorphology.

638-nm laser-diode pumped alexandrite femtosecond laser passively mode-locked by single-walled carbon nanotubes.

A passively mode-locked alexandrite laser was developed with a single-walled carbon nanotubes (SWCNTs) saturable absorber (SA), which was pumped by a 638 nm red laser. After using a pair of prisms for dispersion compensation, the narrowest pulse width of 70 fs was achieved at a repetition rate of 100 MHz. The mode-locked laser had a signal-to-noise ratio greater than 55 dB and a beam quality factor of less than 1.13. A high average output power of 386 mW was achieved with a slope efficiency of 12.4%. It is the first time a passively mode-locked alexandrite femtosecond laser pumped by a 638 nm red laser has been developed. Moreover, it is also the first time that SWCNTs are used as SAs to achieve a passively mode-locked laser in the visible light range.

Wavelength tunable noise-like pulses in a hybrid mode-locked erbium-doped fiber laser

The study demonstrates a new type of laser that generates wavelength-tunable h-shaped noise-like pulses (NLPs). We incorporated the nonlinear polarization rotation (NPR) mechanism into a figure-eight erbium-doped hybrid mode-locked fiber laser based on the nonlinear amplifier loop mirror (NALM). Under an intra-cavity net dispersion of −5.535 ps2, the system produced NLPs with a maximum pulse duration of 24.54 ns, a maximum single energy of 19.96 nJ and a maximum output average power of 14.46 mW when the pump power was set to 543 mW. By adjusting the pump power and fine-tuning the polarization controller, the laser can output NLPs with tunable wavelengths ranging from 1560 to 1601 nm. The flexibility of wavelength-tunable techniques has potential applications in laser detection, high-energy physics experiments, and other fields, enriching the research framework of noise-like applications.

Transient behaviors in a spectrum-tailored all-PM NALM mode-locked fiber laser.

We demonstrate an all-polarization-maintaining (PM) dispersion-managed mode-locked ytterbium-doped fiber oscillator based on a nonlinear amplifying loop mirror (NALM). Experimentally, three mode-locking regimes with distinct tailored spectra are achieved with carefully designed cavity setup and adjusted pump power. Using the dispersive Fourier transformation (DFT) technique, the real-time transition dynamics among these regimes are observed as the pump power decreases. It is revealed that the intracavity gain does not decrease continuously during the relaxation period of the gain fiber. Instead, it decreases first and then increases, ultimately leading to the overdriving of NALM. This is attributed to the increase in the population inversion rate of gain fiber under the remaining pump. The dynamic evolution of intracavity gain and non-monotonic transmission properties of NALM collectively induce chaotic pulsation during the transformation. This work not only provides a new perspective for the design and development of novel spectrum-tailored laser sources, but also deepens the understanding of the transient dynamics of ultrafast fiber lasers.

Mode-locking and wavelength-tuning of a NPR fiber laser based on optical speckle.

Passively mode-locked fiber lasers based on nonlinear polarization rotation (NPR) have been widely used due to their ability to produce short pulses with high peak power. Nevertheless, environmental perturbations can influence the mode-locked state, making it a challenge for the practical implementation. Therefore, researchers are searching for assessment criteria to quickly assist and maintain mode-locking of NPR fiber lasers. Speckle patterns containing spectral information can be generated when the laser transmits through a scattering medium, which can serve as indicators of the mode-locked state. The mode-locked regions are confined to the area close to the minimum texture contrast of speckle patterns. Based on these characteristics, we manually simulate the automatic mode-locking (AML). In addition, we utilize convolutional neural networks (CNNs) to recognize speckle patterns of wavelength tunable lasers and determine the center wavelength.

Indium selenide for mode-locked pulse generation in a Mid-infrared Er:ZBLAN fiber laser

Two-dimensional materials are increasingly recognized for their distinctive nonlinear optical properties. Indium selenide (InSe), a notable optoelectronic material, demonstrates unique advantages in terms of nonlinear absorption behavior, making it a promising candidate in mid-infrared mode-locked lasers. By preparing a InSe saturable absorber (SA) and applying it in a Er: ZBLAN fiber laser, we demonstrate a stable mode-locked mid-infrared fiber laser at 2.8 μm. The obtained mode-locked pulses have a repetition frequency and pulse duration of 29.875 MHz and 13.6 ps, respectively. The obtained maximum value of pulse energy is 4.3 nJ. The signal-to-noise ratio (SNR) is 55 dB, indicating the high stability of the laser system. Our experimental results validate that InSe is a promising SA in the near 3 μm band. This work paves the way for further investigations and applications of InSe-based devices in nonlinear optics and mode-locked laser systems.

Multiple soliton operation in Ge2Sb2Te5 saturable absorber based fiber lasers

With the rapid development of fiber lasers, the invention of novel ultrafast modulators based on the new materials is critical for furthering basic research and practical applications of mode-locked fiber lasers. Ge2Sb2Te5 is a kind of phase-change and thermoelectric material. The fields of optical fiber temperature sensors, optical switches, and phase-change memories have all made extensive use of Ge2Sb2Te5. Nevertheless, the performance of Ge2Sb2Te5 is currently unproven in nonlinear optics and ultrafast laser applications. In this study, Ge2Sb2Te5-based saturable absorbers (SAs), were successfully prepared by depositing nanosheets on a tapered optical fiber. Conventional solitons, second-order harmonic mode-locked, and double-, triple-, quadruple-, and quintuple-pulse phenomena at different net dispersions are observed. A conventional soliton was obtained at 166 mW-987 mW with a pulse interval of 102 ns and a signal-to-noise ratio of 59 dB. At 440 mW of pump power, second-order harmonic mode-locked with a 3 dB bandwidth of 3.29 nm was obtained. The experimental results show that Ge2Sb2Te5 has a broad research prospect in designing ultrafast photonic devices in the future by virtue of its low cost, narrow bandgap, high damage threshold and high stability.

270 MHz passively mode-locked Er-doped fiber laser using single walled carbon nanotube polymer film.

We demonstrated a short-cavity mode-locked erbium-doped fiber laser based on single walled carbon nanotube polymer composite film saturable absorber with a maximum fundamental repetition rate of 270.5 MHz. To the best of our knowledge, this is the highest fundamental repetition rate among mode-locked erbium-doped ring fiber lasers based on nanomaterial polymer composite films. Samples with non-saturable loss range from 13.2% to 60% and modulation depth from 1.63% to 7.08% are prepared by controlling carbon nanotube concentration and film thickness. We systematically investigate the impacts of saturable absorbers on mode-locking repetition rate. Our results show that when the laser repetition rate continuously increases from 15.45 MHz to 270.5 MHz, samples with higher non-saturable loss will push forward the mode-locking threshold and eventually lose mode-locking. In addition, the increased modulation depth will enhance the pulse shaping ability and result in a shorter pulse. Our results may help for high repetition rate mode-locked fiber laser design and improve the laser characteristics. The demonstrated laser may support applications such as frequency comb, etc.

Observation of topological frequency combs.

On-chip generation of optical frequency combs using nonlinear ring resonators has enabled numerous applications of combs that were otherwise limited to mode-locked lasers. Nevertheless, on-chip frequency combs have relied predominantly on single-ring resonators. In this study, we experimentally demonstrate the generation of a novel class of frequency combs, the topological frequency combs, in a two-dimensional lattice of hundreds of ring resonators that hosts fabrication-robust topological edge states with linear dispersion. By pumping these edge states, we demonstrate the generation of a nested frequency comb that shows oscillation of multiple edge state resonances across ≈40 longitudinal modes and is spatially confined at the lattice edge. Our results provide an opportunity to explore the interplay between topological physics and nonlinear frequency comb generation in a commercially available nanophotonic platform.

Theoretical analysis of passively mode-locked semiconductor ring lasers.

In this paper, the theoretical analysis of the passive mode-locked semiconductor ring lasers (PML-SRLs) is investigated based on a travelling wave model. It is found that both the optical confinement factor and the injection current make great contributions to the operation regime and the performance of PML-SRLs. All operation regimes of PML-SRLs are governed by the transient gain-loss balance. Such balance is closely associated with the relationship among the stimulated rate in the semiconductor optical amplifier (SOA), the carrier lifetime in the saturated absorber (SA), and the roundtrip time of the ring resonator. Furthermore, our investigation indicates that the mode-locked state of such PML-SRLs is independent of the passive waveguide, but the performance degrades with the increased waveguide loss or the shortened waveguide length, once the material bandwidth is wide enough. Another discovery is that it is possible to achieve the high energy pulse in the PML-SRL with shortening the passive length and narrowing the gain spectrum meanwhile. Overall, such investigations should benefit designing the required PML-SRLs and achieving the high performance.

Tunable mode-locked holmium-doped fiber laser based on GIMF-SIMF-GIMF structure at 2.09 µm

In this study, a structure of graded-index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) was used as a saturable absorber (SA) to generate mode-locking in a holmium-doped fiber laser (HDFL). The HDFL was pumped with a compact in-house thulium-doped fiber laser (TDFL) operating at 1943 nm with a maximum output power of 2.1 W. Mode-locked pulses with a pulse duration of 1.61 ps were successfully generated at the center wavelength of 2091.1 nm. The pulse had a repetition rate of 12.79 MHz with a bit of fluctuation of 2.09 %, indicating stable pulses. The mode-locked pulses were also tunable with a range from 2086.5 nm to 2091.1 nm by stretching the GIMF part of the SA. This first report demonstrated mode-locking using nonlinear multimode interference (NL-MMI) using GIMF-SIMF-GIMF structure as a saturable absorber in HDFL operating near 2.1 µm wavelength.

439 MHz, 94 fs, low-threshold mode-locked all fiber ring laser

Pulsed fiber lasers with high repetition rates play a crucial role in applications such as high-efficiency processing and bio-optical imaging. However, many ring fiber mode-locked lasers primarily focus on shortening the cavity length to achieve high repetition rate pulse output. Attention is often directed towards the gain fiber and cavity structure, with less consideration given to the challenge of increasing mode-locking threshold power. In this study, we present a high-repetition-rate annular all-fiber laser system with dispersion management. Through strategic adjustment of the intracavity dispersion away from the near-zero dispersion region and careful control of the intracavity gain strength, we have successfully lowered the mode-locking threshold power. This approach can generate pulses with a repetition rate of 439.71 MHz and a mode-locking threshold power of approximately 600 mW. Following pulse compression, the achieved pulse width is 94.8 fs. Its time-bandwidth product is 0.368. To our knowledge, this represents the highest fundamental repetition rate currently attained using a ring all-fiber resonator, accompanied by the lowest mode-locking threshold. Our work introduces a ring fiber laser system characterized by a low mode-locking threshold and a high repetition rate, offering a valuable method for achieving higher fundamental repetition rate pulses.

Molten fluoride salt-assisted synthesis of titanium carbide (Ti2C) MXene and its application for 2 µm mode-locking in a thulium-doped fiber laser

Titanium carbide (Ti2C), a new two-dimensional material named MXenes, has attracted interest due to its potential applications in numerous fields. Of the many unique characteristics of Ti2C MXene, its nonlinear properties are attractive for optoelectronic applications, specifically for ultrafast laser generation. In this work, a Ti2C MXene was fabricated by etching a MAX phase precursor titanium aluminum carbide (Ti2AlC) using a mixture of lithium fluoride and hydrochloric acid, eliminating the risk of using the harmful hydrofluoric acid. The Ti2C MXene was prepared in solution form and then dropped onto a reduced core diameter of tapered fiber before being used as a saturable absorber (SA). The SA device was inserted into a thulium-doped fiber laser to generate stable mode-locked pulses at a center wavelength of 1951 nm with a pulse width of 1.67 ps. The mode-locked laser was highly stable when tested over time, with peak optical power fluctuations of as little as 0.005 dB measured. The results show that the Ti2C MXene exhibit outstanding performance for ultrafast laser generation.