Passive Mode-locking Research Articles

Multi-pulse dynamic patterns in a mode-locked erbium-doped fiber laser based on an Sb2S3-PVA saturable absorber

Multi-pulse dynamic patterns have been experimentally documented in a passively mode-locked (PML) erbium-doped fiber (EDF) laser using an Sb2S3-PVA saturable absorber (SA). The fundamental mode-locking operation, with a repetition rate of ∼3.22 MHz, a pulse width of ∼2.5 ps, a signal-to-noise ratio (SNR) of ∼50 dB and a peak power over 200 W, was achieved under a pump power from 280 to 360 mW with appropriate polarization states introduced by the polarization controllers (PCs). By rotating the orientation of the intra-cavity PCs carefully and slowly at a pump power of 350 mW, it was found that a multi-pulse bunch was transformed gradually from a single-pulse to a twelve-pulse bunch, with several intermediate transition states of multi-pulse bunches being observed. In addition, other characteristic modes including disordered multi-pulses and soliton rains have been experimentally observed by meticulously adjusting the polarization states of PCs at a pump power of 350 mW. Our systematic study clearly demonstrates that Sb2S3 has potential as an effective SA for generating different operation states of multi-pulses in PML anomalous-dispersion EDF lasers.

Synchronized triple-wavelength fiber lasers at 1, 1.55, and 1.9 µm.

Synchronized lasers working at different wavelengths are of great significance for numerous applications, such as high-energy femtosecond pulse emission, Raman microscopy, and precise timing distribution. Here, we report synchronized triple-wavelength fiber lasers working at 1, 1.55, and 1.9 µm, respectively, by combining the coupling and injection configurations. The laser system consists of three fiber resonators gained by ytterbium-doped fiber, erbium-doped fiber, and thulium-doped fiber, respectively. Ultrafast optical pulses formed in these resonators are obtained by passive mode-locking with the use of a carbon-nanotube saturable absorber. A maximum cavity mismatch of ∼1.4 mm is reached by the synchronized triple-wavelength fiber lasers in the synchronization regime by finely tuning the variable optical delay lines incorporated in the fiber cavities. In addition, we investigate the synchronization characteristics of a non-polarization-maintaining fiber laser in an injection configuration. Our results provide a new, to the best of our knowledge, perspective on multi-color synchronized ultrafast lasers with broad spectral coverage, high compactness, and a tunable repetition rate.

Long- and Short-Term Stability of All Polarization-Maintaining Thulium Doped Passively Mode-Locked Fiber Lasers with Emission Wavelengths at 1.95 μm and 2.07 μm

In this work, we compare the operation of a passively modelocked polarization-maintaining emission in two thulium-doped fiber lasers pumped at 1561 nm, with emission at wavelengths of 1.951 μm in one case and 2.07 μm in the other. We obtained a sequence of light pulses at 15.6 MHz, whose temporal width was 81 ps at 1.95 μm, and a sequence of light pulses at 13.1 MHz, whose temporal width was 94 ps at 2.07 μm. Finally, we also measured the long-term stability of this setup during a 24-h operation, as well as the short-term stability in a simulated harsh environment. The results confirm the superior performance of fiber laser systems with a fully polarization-maintaining design.

Low-Noise Femtosecond SESAM Modelocked Diode-Pumped Cr:ZnS Oscillator

Cr-doped ZnS and ZnSe are excellent gain mediums for high power and broadband ultrashort pulse generation in the 2 – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{m}$ </tex-math></inline-formula> wavelength range. SESAM modelocked Cr:ZnS oscillators have the advantage of reliable, self-starting passive modelocking. We present a diode-pumped, SESAM modelocked Cr:ZnS oscillator delivering ultrashort pulses of 189 fs at 550 mW average output power with a repetition rate of 435 MHz with low relative intensity noise (RIN) and timing jitter. We measured an integrated RIN of 0.05% within a frequency span of [10 Hz, 5 MHz] dominated by the 1560-nm pump diode, and a very low integrated timing jitter of 10.9 fs [2 kHz, 10 MHz]. This type of laser source benefits not only from very low noise but also from reduced complexity and cost due to direct diode pumping, which is suitable for many applications such as spectroscopy, ranging, and frequency conversion.

Nb2AlC as saturable absorber for a passively mode-locked operation of Tm:YLF laser

The Nb2AlC material has attracted more and more attention in optoelectronics due to its excellent optical properties. Herein, the Nb2AlC-based saturated absorber (SA) was fabricated by ultrasonic thermal dissolution method and spin coating means, and a passively mode-locked (PML) Tm:YLF laser was perfectly demonstrated with the SA. With an incident pump power of 21.63W, a 0.379-W average output power and a 200-ps pulse width were achieved at 1872.1nm, corresponding to a per pulse energy of 3.15nJ and a peak power of 15.75W. In addition, the beam quality factors were measured to be 1.04 and 1.05 in the horizontal direction and vertical direction.

The master equation for passive modelocking

Passive modelocking (PML) of lasers is pivotal in modern science and industry. Here, solving a half-century-long challenge, we present the first master equation (ME) describing PML on all time scales, from Q-switched to fundamental and harmonic modelocking, valid for both slow and fast saturable absorption, short and long cavities. The proposed ME should become the workhorse for analytical and numerical studies of ultrafast lasers in both the photonics engineering and laser physics communities.

Passive mode-locked cesium diode-pumped alkali laser.

A first, to the best of our knowledge, demonstration of passive mode-locking in diode-pumped alkali laser (DPAL) is reported in this paper. An intracavity cesium vapor cell, buffered by atmospheric pressure methane, is used to passively mode lock a continuously pumped cesium DPAL with a static gain medium. A train of short pulses with duration less than 460 ps was observed using a 2.0-GHz bandwidth detector that limited the real time duration measurements. The calculated minimum duration for these pulses is 57 ps.

Dispersion-Managed Tm-Ho Co-Doped Ultrashort Pulse Fiber Laser Using Single-Walled Carbon Nanotube and Spectral Filter

In this paper, we have demonstrated a dispersion-managed, high-power, Tm-Ho co-doped ultrashort pulse fiber laser using a single walled carbon nanotube (SWNT) dispersed in polyimide film. An in-line type spectral filter was developed to control the output pulse spectra. Two SWNT films with different modulation depths were examined as a mode-locker. Normal dispersion fiber was used in the fiber laser oscillator, and dependence on net cavity dispersion was investigated. Passive mode-locking was achieved in a wide dispersion range, from −0.319 to +0.101 ps2. Stable soliton mode-locking operation and dissipative soliton mode-locking operations were observed. The pumping efficiency was ~3 times higher than that in a Tm-doped fiber laser with a similar configuration. The developed fiber laser showed self-start and stable operations, and this laser is useful for practical applications.

Ultrafast laser spectroscopy uncovers mechanisms of light energy conversion in photosynthesis and sustainable energy materials

The invention of the laser in 1960 gave us the ruby laser, which generally produced chaotic pulses of light. Six years later, in 1966, a concept called passive mode-locking applied to neodymium-glass lasers produced reasonably well-behaving picosecond pulses. This triggered an intense activity, with respect to developing improved laser pulse sources, measurement techniques, and application to chemistry, physics, and biology. Initially, only ∼10 –ps-long pulses at a few wavelengths were available. Nevertheless, insight into the function of complex biological systems, like photosynthetic proteins, and molecules of chemical interest was gained in very early studies. Today, both duration and color of ultrashort pulses can be tuned to almost any value. This has of course opened up possibilities to study almost any atomic, molecular, or solid-state system and any dynamic process. This review focuses on the use of laser spectroscopy to investigate light energy conversion mechanisms in both natural photosynthesis and a topical selection of novel materials for solar energy conversion. More specifically, in photosynthesis we will review light harvesting and primary electron transfer; materials for solar energy conversion that we discuss include sensitized semiconductors (dye sensitized solar cells), polymer:fullerene and polymer:polymer bulk heterojunctions (organic solar cells), organometal halide perovskites, as well as molecular and hybrid systems for production of solar fuel and valuable chemicals. All these scientific areas, and in particular photosynthesis and the solar cell materials, have been extensively studied with ultrafast spectroscopy, resulting in a vast literature; a comprehensive review of the individual materials is, therefore, not feasible, and we will limit our discussion to work that we think has been of particular importance for understanding the function of the respective systems.

Study on the mechanically exfoliated graphene saturable absorption dependence on the number of layers and its influence on passive mode-locking erbium-doped fiber laser dynamics

We experimentally investigated the saturable absorption influence of graphene layers with natural stacking order in an erbium-doped fiber laser passive mode-locking. Mechanically exfoliated graphene saturable absorber (MEGSA) samples, ranging from 1 to 6 layers, were fabricated preserving their natural ABA stacking order and precisely characterized by 2D band profile from Raman spectroscopy. By incorporating the samples as saturable absorbers (SA) in the fiber laser, mode-locking performances with pulse duration from 670–780 fs and bandwidth from 3.8–4.6 nm could be generated. Also, we identified a transition in the mode-locking activation mechanism from non-self-starting, for monolayer and bilayer graphene, to self-starting, for trilayer and few-layer graphene, which is a strong indicative of fast-to-slow saturable absorption response dependence on the number of graphene layers.

A passively mode-locked of Tm:YAP laser with a zeolitic imidazolate frameworks-8 (ZIF-8) saturable absorber

Metal-organic framework materials have shown outstanding potential and applications in many fundamental fields, while the zeolitic imidazolate frameworks-8 (ZIF-8) material has received little attention. In this paper, ZIF-8 particles have been used as a saturable absorber (SA), which favors the Tm:YAP solid-state laser (SSL) working well in a passively mode-locked（PML）mode. Under the PML mode, an average output power of 912 mW is obtained from the laser with a pulse width of 737 ps at 97.98 MHz. Also, the as-calculated per pulse energy and the peak power are 9.3 nJ and 12.6 W, respectively.

Graphene saturable absorber mirror for passive mode-locking of mid-infrared QCLs

Passive mode-locking in quantum cascade lasers (QCLs) remains one of the huge challenges because of the fast relaxation time of the excited carriers which is typically in the range of sub-picoseconds. The use of conventional techniques such as the semiconductor saturable absorber mirror is inefficient because the spatial hole burning effect dominates the carrier dynamics. To overcome this effect, longitudinal transition structures with relaxation time around $$50~\mathrm {ps}$$ were proposed. However, mode-locking is assured with an external modulation at a cavity roundtrip frequency. In this paper, we demonstrate that a single-layer graphene used as a saturable absorber permits to generate stable pulses in such structures. The graphene is integrated with a highly reflective mirror to increase the internal electric field and achieve the saturation intensity. The dynamic of the QCL is modeled with Maxwell-Bloch equations while Maxwell-Ampere equation is used for the graphene layer by considering a nonlinear conductivity. This system of equations is solved using the one-dimensional Finite-Difference Time-Domain ( FDTD) method. To model the single-layer graphene of a $$0.33~\mathrm {nm}$$ thickness, a specific sub-cell is implemented based on Maloney method. Simulation results of a $${\mkern 1mu} 6.2~\mu {\text{m}}$$ QCL with a diagonal radiative transition show a generation of isolated pulses with a peak electric field of $$80~\mathrm {MV.m^{-1}}$$ and a duration of $$51~\mathrm {fs}$$ . The mode-locking remains stable for QCLs with a vertical transition having a relaxation time below $$5~\mathrm {ps}$$ .

Quantum size-dependent luminescence and nonlinear optical properties of silicon quantum dots/SiO2 multilayer

The luminescence and nonlinear optical properties of silicon quantum dots (Si QDs)/SiO2 multilayers with different QDs diameters (2.5, 3 and 4 nm) were investigated by utilizing continue wave (325 nm) and picosecond laser (tp = 25 ps, λ = 1064 nm), respectively. It was found that both the linear and nonlinear optical properties of Si QDs/SiO2 multilayers strongly depended on the QDs size. The photoluminescence intensity could be enhanced by ∼32-fold, and about 17 times increase in the saturable absorption (SA) coefficient was achieved, which could be ascribed to the controllable density of interface-states as well as their coupling effect with the Si QDs core states. Moreover, a transformation from reverse saturable absorption (RSA) characteristic through two-step absorption process into the SA behavior due to the interface-state filling effect was observed when increasing the incident laser intensity. We proposed that the ultrafast free carries trapping process by the interface states would greatly influence the self-defocusing effect. In addition, the free carries absorption and SA effect should be considered in accounting for the variation of nonlinear parameters as changing the pump intensity. Our results suggest that the widely tunable SA feature through controlling the Si QDs size provides great potential to develop high-efficient Si QDs-based nonlinear optical devices such as passive mode-locking or Q-switching laser.

New polymethine dyes for liquid and polymer passive Q-switches of neodymium lasers

Spectral and nonlinear optical properties of the new tricarbocyanine dyes, derivatives of the benzo[cd]indole, which absorb efficiently in the region 1.05–1.08 µm, have been investigated. Based on these compounds, the liquid (ethanol solutions) and polymer (in a form of triplexes, where dye-doped polyurethane films are placed between two optical glass substrates) passive Q-switches for the neodymium lasers have been developed. The modulation depth of 100% at the generation of the ultrashort pulse (USP) trains has been obtained. The high-stable regime of passive mode-locking has been realized; it is characterized with the 100% probability appearance of a single pulse in an axial period with average durability 25–30 ps and energy 1–2 mJ, depending on the dye structure. Advantages of the new Q-switches over the corresponding analogues, based on organic dyes, nanotubes, graphene, inorganic semiconductors and mirrors (SESAM), LiF:F2ˉ and Cr4+:YAG, have been demonstrated.

Passive mode-locking and terahertz frequency comb generation in resonant-tunneling-diode oscillator

Optical frequency combs in the terahertz frequency range are long-awaited frequency standards for spectroscopy of molecules and high-speed wireless communications. However, a terahertz frequency comb based on a low-cost, energy-efficient, and room-temperature-operating device remains unavailable especially in the frequency range of 0.1 to 3 THz. In this paper, we show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb. The standard deviation of the spacing between the comb lines, i.e., the repetition frequency, is reduced to less than 420 mHz by applying external bias modulation. A simulation model successfully reproduces the mode-locking behavior by including the nonlinear capacitance of RTD and multiple optical feedback. Since the mode-locked RTD oscillator is a simple semiconductor device that operates at room temperature and covers the frequency range of 0.1 to 2 THz (potentially up to 3 THz), it can be used as a frequency standard for future terahertz sensing and wireless communications.

Nonlinear chalcogenide optical fiber couplers.

We demonstrate chalcogenide optical fiber couplers with a power-dependent coupling coefficient. The couplers are designed and fabricated using an As2Se3 fiber and characterized at a wavelength of 1938 nm, leading to a critical power of 126 W, the lowest ever reported for any optical fiber coupler. These nonlinear couplers enable all-optical switching and will be useful for passive mode-locking over a wide wavelength range from the telecommunication band to the mid-infrared.

Passive modelocking MIR quantum cascade laser incorporating self-induced transparency

Passive modelocking MIR quantum cascade laser incorporating self-induced transparency

Threshold reduction of GHz-repetition-rate passive mode-locking by tapering the gain fiber.

Passively mode-locked fiber lasers with GHz repetition rates have recently attracted significant attention in frontier research areas, including frequency-comb spectroscopy, coherent optical communication, photonic radar, micromachining, etc. In general, the threshold of passive mode-locking increases with the fundamental repetition rate, which is inversely proportional to the cavity length, and this sets a limit on the scalability of the fundamental repetition rate. To overcome this issue, here we propose to reduce the threshold of continuous-wave mode-locking (CWML) by precisely tapering the gain fiber, which can enhance the power density incident on the semiconductor saturable absorber mirror. Assisted by the analysis of guiding property, an experimental scheme is established for tapering standard Yb-doped fibers (125 µm cladding diameter), and tapered Yb-doped fibers with different waist diameters can be fabricated. Using a tapered Yb-doped gain fiber with waist cladding diameter of 90 µm, we are able to achieve CWML with a fundamental repetition rate of 3.3 GHz, and reduce its mode-locking threshold by 31%. More importantly, the optical spectrum of the CWML is found to be broadened with the waist diameter reduction of the gain fiber, which is beneficial for generating shorter transform-limited pulses. The efforts made in this work can provide a promising route to realize stable high-repetition-rate mode-locked fiber lasers with moderate levels of pump power.

(INVITED)Fiber-based polarization dependent devices and their application for passive mode-locking

Fiber-based polarization dependent devices (FPDDs), such as optical polarizer, polarization beam splitter are of significant importance in a variety of applications, especially in optical fiber communication and fiber laser systems. Over the past decades, a significant progress has been made in the development of compact and high-efficiency polarization dependent devices based on fibers. In this review, we mainly summarize and review the recent progress of kinds of FPDDs based on fiber grating, polished fiber, photonic crystal fiber and microfiber. And some common and representative applications of these FPDDs are presented and illustrated. The development of FPDDs and the progress of related application technology will create new research approaches and applications.

Mode-locked erbium-doped fiber laser based on a mechanically exfoliated ReS2 saturable absorber onto D-shaped optical fiber

In this work, we report a femtosecond mode-locking Erbium-doped fiber laser using mechanically exfoliated rhenium disulfide (ReS2) deposited onto the polished surface of a D-shaped optical fiber. By performing the polarization and saturable absorption measurements, the sample exhibited a polarization extinction ratio of 10 dB (90%) and nonlinear transmittance variation of 3.40%. When incorporated into the cavity as a saturable absorber (SA), the passive mode-locking performance of 220 fs was achieved. This is the best mode-locking performance ever reported in literature achieved with all-fiber based ReS2 SA. By using density functional theory (DFT) calculations, we obtained the electronic states and the optical absorption spectrum at 1550 nm attributed by defects in the ReS2 structures, which is consistent with its linear and nonlinear optical absorption in the laser mode-locking mechanism.