Mode-locking Mechanism Research Articles

Sb_2Te_3-deposited D-shaped fiber as a saturable absorber for mode-locked Yb-doped fiber lasers

We present a study on a antimony telluride (Sb2Te3)-deposited side-polished fiber device as a saturable absorber for Yb-doped mode-locked fiber lasers. Thin layers of Sb2Te3 with variable length were deposited by a pulsed magnetron sputtering technique. We demonstrate measured characteristics of the device and show that it can be treated as a hybrid component - tunable polarizer with saturable absorption properties. The polarizing extinction ratio varies from 1.5 dB up to 17.1 dB with increasing length of the deposition. The fiber components were employed in all-normal Yb-doped fiber cavities. All devices enabled for mode-locked operation by means of hybrid mode-locking or nonlinear polarization evolution mechanism. In particular, the laser with 2 mm long Sb2Te3 absorber emitted 5.9 ps pulses with 4 mW of average output power.

Laser-diode pumped self-mode-locked praseodymium visible lasers with multi-gigahertz repetition rate.

We demonstrate efficient laser-diode pumped multi-gigahertz (GHz) self-mode-locked praseodymium (Pr3+) visible lasers with broadband spectra from green to deep red for the first time to our knowledge. With a Pr3+-doped GdLiF4 crystal, stable self-mode-locked visible pulsed lasers at the wavelengths of 522nm, 607nm, 639nm, and 720nm have been obtained with the repetition rates of 2.8GHz, 3.1GHz, 3.1GHz, and 3.0GHz, respectively. The maximum output power was 612mW with the slope efficiency of 46.9% at 639nm. The mode-locking mechanism was theoretically analyzed. The stable second-harmonic mode-locking with doubled repetition frequency was also realized based on the Fabry-Perot effect formed in the laser cavity. In addition, we find that the polarization directions were turned with lasing wavelengths. This work may provide a new way for generating efficient ultrafast pulses with high- and changeable-repetition rates in the visible range.

Emergence of resonant mode-locking via delayed feedback in quantum dot semiconductor lasers.

With conventional semiconductor lasers undergoing external optical feedback, a chaotic output is typically observed even for moderate levels of the feedback strength. In this paper we examine single mode quantum dot lasers under strong optical feedback conditions and show that an entirely new dynamical regime is found consisting of spontaneous mode-locking via a resonance between the relaxation oscillation frequency and the external cavity repetition rate. Experimental observations are supported by detailed numerical simulations of rate equations appropriate for this laser type. The phenomenon constitutes an entirely new mode-locking mechanism in semiconductor lasers.

Developing high energy dissipative soliton fiber lasers at 2 micron.

While the recent discovered new mode-locking mechanism - dissipative soliton - has successfully improved the pulse energy of 1 μm and 1.5 μm fiber lasers to tens of nanojoules, it is still hard to scale the pulse energy at 2 μm due to the anomalous dispersion of the gain fiber. After analyzing the intracavity pulse dynamics, we propose that the gain fiber should be condensed to short lengths in order to generate high energy pulse at 2 μm. Numerical simulation predicts the existence of stable 2 μm dissipative soliton solutions with pulse energy over 10 nJ, comparable to that achieved in the 1 μm and 1.5 μm regimes. Experimental operation confirms the validity of the proposal. These results will advance our understanding of mode-locked fiber lasers at different wavelengths and lay an important step in achieving high energy ultrafast laser pulses from anomalous dispersion gain media.

Unidirectional dissipative soliton operation in an all-normal-dispersion Yb-doped fiber laser without an isolator.

We demonstrate self-started unidirectional dissipative soliton operation and noise-like pulse operation in an all-normal-dispersion bidirectional Yb-doped fiber laser mode-locked by nonlinear polarization rotation. The laser works unidirectionally once mode-locking is achieved due to the cavity directional nonlinearity asymmetry along with the nonlinear polarization rotation mode-locking mechanism.

High-energy harmonic mode-locked 2 μm dissipative soliton fiber lasers

High-pulse-energy harmonic mode-locking in 2 μm Tm-doped fiber lasers (TDFLs) is realized, for the first time, by using a short piece of anomalous dispersion gain fiber and the dissipative soliton mode-locking mechanism. Appropriately designing the cavity dispersion map and adjusting the cavity gain, stable harmonic mode-locking of the dissipative soliton TDFL from the 2nd to the 4th order is achieved, with the pulsing repetition rates and pulse energy being 43.4, 65.1, 86.8 MHz, and 6.27, 4.32 and 3.29 nJ, respectively. The harmonic laser pulse has a pulse width of ~30 ps and a center wavelength of ~1929 nm with a spectral bandwidth of ~3.26 nm, giving a highly chirped laser pulse. Two types of soliton molecules are also observed in this laser system.

Graphene-deposited microfiber photonic device for ultrahigh-repetition rate pulse generation in a fiber laser.

We report on the generation of a high-repetition-rate pulse in a fiber laser using a graphene-deposited microfiber photonic device (GMPD) and a Fabry-Perot filter. Taking advantage of the unique nonlinear optical properties of the GMPD, dissipative four-wave mixing effect (DFWM) could be induced at low pump power. Based on DFWM mode-locking mechanism, the fiber laser delivers a 100 GHz repetition rate pulse train. The results indicate that the small sized GMPD offers an alternative candidate of highly nonlinear optical component to achieve high-repetition rate pulses, and also opens up possibilities for the investigation of other abundant nonlinear effects or related fields of photonics.

Synchronization of Bloch oscillations by a ring cavity.

We consider Bloch oscillations of ultracold atoms stored in a one-dimensional vertical optical lattice and simultaneously interacting with a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. We find that the feedback provided by the cavity field on the atomic motion synchronizes Bloch oscillations via a mode-locking mechanism, steering the atoms to the lowest Bloch band. It also stabilizes Bloch oscillations against noise, and even suppresses dephasing due to atom-atom interactions. Furthermore, it generates periodic bursts of light emitted into the counter-propagating cavity mode, providing a non-destructive monitor of the atomic dynamics. All these features may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations.

Pulse-Width Saturation and Kelly-Sideband Shift in a Graphene-Nanosheet Mode-Locked Fiber Laser with Weak Negative Dispersion

In ultrafast laser systems, graphene has become a popular saturable absorber to provide passive mode-locking with tunable wavelength. The authors elucidate the pulse width saturated mode-locking mechanism, focusing on the soliton compression that can be achieved by strengthening self-phase modulation in the regime of weakly negative group-delay dispersion. This mechanistic understanding allows for an optimized soliton pulse width of 500 femtoseconds or less from an erbium-doped fiber laser, for photonics and optoelectronics applications.

Hysteresis phenomenon and multistability in figure-of-eight microstructured fiber laser

We report a theoretical investigation of multi-pulse emission of a microstructured figure-of-eight fiber laser operating in passive mode-locking. The proposed laser is mode locked by the nonlinear amplifying loop mirror (NALM). We study, in this paper, the hysteresis dependence and the number of pulses in steady state as a function of both the small signal gain and the nonlinear coefficient of microstructured fiber. The numerical simulation confirms that the pulse splitting is a consequence of the energy quantization in anomalous dispersion. Moreover, our results suggest that the hysteresis phenomenon is an intrinsic feature of the mode-locked fiber lasers independently of the exact mode-locking mechanism. Finally, we identify that the nonlinear coefficient of microstructured fiber plays a key role in the formation of multi-soliton.

Using n- and p-Type Bi2Te3 Topological Insulator Nanoparticles To Enable Controlled Femtosecond Mode-Locking of Fiber Lasers

Mechanically triturated n- and p-type Bi2Te3 nanoparticles, the nanoscale topological insulators (TIs), are employed as nonlinear saturable absorbers to passively mode-lock the erbium-doped fiber lasers (EDFLs) for sub-400 fs pulse generations. A novel method is proposed to enable the control on the self-amplitude modulation (SAM) of TI by adjusting its dopant type. The dopant type of TI only shifts the Fermi level without changing its energy bandgap, that the n- and p-type Bi2Te3 nanoparticles have shown the broadband saturable absorption at 800 and 1570 nm. In addition, both the complicated pulse shortening procedure and the competition between hybrid mode-locking mechanisms in the Bi2Te3 nanoparticle mode-locked EDFL system have been elucidated. The p-type Bi2Te3 with its lower effective Fermi level results in more capacity for excited carriers than the n-type Bi2Te3, which shortens the pulse width by enlarging the SAM depth. However, the strong self-phase modulation occurs with reduced linear loss and...

Subpicosecond SESAM and nonlinear polarization evolution hybrid mode-locking ytterbium-doped fiber oscillator

We report on a high-power 960-fs subpicosecond pulse generation from a passively mode-locked ytterbium-doped large-mode-area photonic crystal fiber oscillator in the all-normal dispersion regime. Stable and self-starting operation is obtained by exploiting an efficient combination of a high modulation depth semiconductor saturable absorber mirror and the nonlinear polarization evolution mode-locking mechanism. In the single-pulse regime, the laser directly generates the output power of 3.15 W at the repetition rate of 83.7 MHz, corresponding to the pulse energy of 37 nJ. The center wavelength is 1,030 nm with the spectral bandwidth of 8 nm at the maximum output power. The output characteristics of the oscillator are discussed carefully.

Passive mode-locking under higher order effects

The response of a passive mode-locking mechanism, where gain and spectral filtering are saturated with the energy and loss saturated with the power, is examined under the presence of higher order effects. These include third-order dispersion, self-steepening, and Raman gain. The locking mechanism is maintained even with these terms; mode-locking occurs for both the anomalous and normal regimes. In the anomalous regime, these perturbations are found to affect the speed but not the structural integrity of the (locked) pulses. In fact, these pulses behave like solitons of a classical nonlinear Schrödinger equation and, as such, a soliton perturbation theory is used to verify the numerical observations. In the normal regime, the effect of the perturbations is small, in line with recent experimental observations. The results in the normal regime are verified mathematically using a Wentzel–Kramers–Brillouin-type asymptotic theory. Finally, bisolitons are found to behave as dark solitons on top of a stable background and are significantly affected by these perturbations.

Seed 레이저용 정상분산 영역에서 발진하는 간단한 구조의 이터븀 기반 모드 잠금 광섬유 레이저

In this investigation, we constructed and demonstrated a simple Yb-doped fiber laser, of which longitudinal modes are mode-locked without any additional devices to compensate the dispersion caused by optical components. Non-linear polarization rotation (NPR) was adopted for the modelocking mechanism and a polarization controller (PC) was used for a kind of spectral filters to restrict the bandwidth for mode-locking. As the result, the laser was successfully operated as mode-locked with the repetition rate of 42.2 MHz and the spectrum was broadened up to approximately 16 nm at 1033 nm center wavelength when the laser was mode-locked. In this paper, the operation of the developed Yb-doped mode-locked laser is explained with the concept of Lyot filter realized by a PC, which enables mode-locking under normal dispersion. In the industrial applications, this laser can be used as a seed laser of the high power lasers for optical manufacturing.

Widely-pulsewidth-tunable ultrashort pulse generation from a birefringent carbon nanotube mode-locked fiber laser.

We demonstrate the generation of soliton pulses covering a nearly one order-of-magnitude pulsewidth range from a simple carbon nanotube (CNT) mode-locked fiber laser with birefringence. A polarization-maintaining-fiber-pigtailed, inline polarization beam splitter and its associated birefringence is leveraged to either enable additional nonlinear polarization evolution (NPE) mode-locking effect or result in a bandwidth-tunable Lyot filter, through adjusting the intracavity polarization settings. The large pulsewidth tuning range is achieved by exploiting both the nonlinear CNT-NPE hybrid mode-locking mechanism that narrows the pulses and the linear filtering effect that broadens them. Induced vector soliton pulses with pulsewidth from 360 fs to 3 ps can be generated, and their time-bandwidth products indicate they are close to transform-limited.

Manipulating dissipative soliton ensembles in passively mode-locked fiber lasers

We review our recent experimental and theoretical results addressing the dynamics of large numbers of solitons interacting in presence of a background in passively mode-locked erbium-doped fiber lasers. We first characterize experimentally the soliton rain complex dynamics, and then we focus on ordered soliton patterns. We report that, for suitable experimental parameters, a continuous wave can impose harmonic mode locking. Two levels of modeling for a mode-locked laser subjected to the external injection of a continuous wave are developed to support the latter observation. The first one is based on a scalar master equation, while the second one takes into account the mode-locking mechanism more accurately through a vectorial approach.

Coherent supercontinuum generation using Er-doped fiber laser of hybrid mode-locking.

Supercontinuum generation is performed using femtosecond laser pulses produced by combining two distinct mode-locking mechanisms of nonlinear polarization evolution with saturable absorption on an Er-doped fiber oscillator. The generated supercontinuum yields a high level of spectral visibility of 93%, without the presence of spontaneous emission of post-amplification, being well suited for precision spectroscopy, broadband interferometry, and coherent telecommunications.

Role of cavity dispersion on soliton grouping in a fiber lasers

The effect of cavity dispersion on the dynamics of bound soliton states in a fiber laser has been studied both experimentally and numerically. The mode-locking mechanism in a laser was provided by the frequency-shifted feedback to avoid the influence of soliton attraction that could be induced by saturable absorption. It was found that phase-locked bound solitons are stable for dispersion below the "threshold" value of 0.2 ps/nm which depends on the other cavity parameters. For higher dispersion the bound states collapse resulting in the multiple weakly-interacting soliton regime, circulating randomly within the cavity.

Polarization Maintaining, Nanotube-Based Mode-Locked Lasing From Figure of Eight Fiber Laser

We propose and demonstrate an all-fiber, polarization maintaining (PM) ultrafast fiber laser using a hybrid mode-locking mechanism consisting of a nonlinear amplifying loop mirror and carbon nanotube saturable absorber (CNT-SA). By incorporating the CNT-SA, the threshold for mode-locked operation is reduced by approximately a factor of 3, from 330 to 120 mW. Moreover, the all-PM fiber design provides excellent stability and turnkey operation.

A net normal dispersion all-fiber laser using a hybrid mode-locking mechanism

We propose and demonstrate an all-fiber, dispersion-mapped, erbium-doped fiber laser with net normal dispersion generating dissipative solitons. The laser is mode-locked by a hybrid mode-locking mechanism consisting of a nonlinear amplifying loop mirror and a carbon nanotube saturable absorber. We achieve self-starting, mode-locked operation generating 2.75 nJ pulses at a fundamental repetition rate of 10.22 MHz with remarkable long term stability.