Passive Mode-locking Research Articles

Composite two-dimensional material GO-MoS 2 -based passively mode-locked Erbium-doped fiber laser

In this paper, for improving the performance and stability of MoS2 saturable absorber, graphene oxide (GO) as colloidal surfactant is used to exfoliate MoS2 bulk material for obtaining few-layer GO-MoS2 nano-flakes. Further research on few-layer GO-MoS2 saturable absorber to mode-lock erbium-doped fiber laser (EDFL) is then conducted. In the experiment, a stable mode-locked pulsed laser is achieved with a center wavelength of 1558 nm, a repetition rate of 7.86 MHz and a pulse width of 1.9 ps. When the pump power reaches 60.5 mW, the output power is 0.48 mW and the pulse peak power is calculated to be 32.1 W. This work shows that the new composite 2D material prepared by this method is beneficial to maintain the stability of few-layer MoS2 and increase the damage threshold of the MoS2 saturable absorber for passive mode-locking.

Recent Progress in Broadband Carbon Nanotube Saturable Absorbers for Ultrafast Bulk Solid-State Lasers

Carbon nanotubes (CNTs) are one of the most investigated nanomaterials for a variety of applications due to their unique electronic and optical characteristics. Over the last two decades, CNTs have been widely investigated to develop efficient passive mode-locking devices applicable for diverse ultrafast laser systems, because they exhibit high nonlinearity, photo-excited ultrafast carrier recovery, broadband applicability and relatively easy processes for device fabrication. In this overview article, recent progress of CNT-based saturable absorbers applicable for mode-locking ultrafast bulk solid-state lasers operating in a broad spectral range between 800 nm and 2100 nm will be reviewed.

Femtosecond Tm–Ho co-doped fiber laser using a bulk-structured Bi2Se3 topological insulator**Project supported by the National Research Foundation of Korea (NRF) Grant Funded by the Korea Government (MSIT), Republic of Korea (Grant No. NRF-2018R1A2B6001641), Ministry of Science and ICT (MSIT) under the Information Technology Research Center (ITRC) Support Program (Grant No. IITP-2018-2015-0-00385), and supervised by the Institute for Information and Communications

We experimentally demonstrate a femtosecond mode-locked thulium–holmium (Tm–Ho) co-doped fiber laser incorporating a saturable absorber (SA) based on a bulk-structured bismuth selenide (Bi2Se3) topological insulator (TI). The SA was prepared by depositing a mechanically exfoliated Bi2Se3 TI layer onto a side-polished optical fiber platform. Unlike high-quality nano-structured Bi2Se3 TI-based SA, bulk-structured Bi2Se3 with non-negligible oxidation was used as a saturable absorption material for this experimental demonstration due to its easy fabrication process. The saturation power and modulation depth of the prepared SA were measured to be ∼28.6 W and ∼13.4%, respectively. By incorporating the prepared SA into a Tm–Ho co-doped fiber ring cavity, stable soliton pulses with a temporal width of ∼853 fs could be generated at 1912.12 nm. The 3-dB bandwidth of the mode-locked pulse was measured to be ∼4.87 nm. This experimental demonstration reaffirms that Bi2Se3 is a superb base material for mid-infrared passive mode-locking even under oxidation.

Passive mode-locking in erbium-doped fibre laser based on BN-GO saturable absorber

ABSTRACTA C-band mode-locked fibre laser incorporating a boron nitride-doped graphene oxide (BN-GO)-based saturable absorber (SA) is proposed and demonstrated. The SA is fabricated by depositing multiple layers of synthesized BN-GO nanoparticles onto the polished surface of a side-polished fibre, which is then inserted into an erbium-doped fibre laser cavity to generate the desired pulsed output. The strong nonlinear optical response and light absorption of the BN-GO nanoparticles induces the generation of a highly stable mode-locked pulse at 1567.32 nm with visible Kelly’s sidebands. The pulses have a measured repetition rate of 13.56 MHz and a pulse width of 1.18 ps at the maximum pump power of 280.5 mW. The pulses have a frequency signal-to-noise ratio of ∼53 dB, indicating a highly stable output. The proposed laser would find significant telecommunications applications, particularly for dense wavelength division multiplexing systems.

Narrow core standard single mode fiber for supercontinuum generation from graphene-based mode-locked pulses

In this work, a supercontinuum (SC) source is proposed and demonstrated using a highly-doped, 2 m long zirconia-erbium doped fiber (Zr-EDF) with a dopant concentration of 3800 ppm/wt and a narrow core 100 m long single-mode-fiber (SMF). A graphene based saturable absorber (SA) is used as a passive mode-locker, generating pulses with an average output power, pulse energy and peak power of ∼0.9 mW, ∼69.8 pJ and ∼83 W respectively and a repetition rate of 12.9 MHz. This is then amplified using a 140 mW amplifier, giving an output power, pulse energy and peak power of ∼75.0 mW, 5.8 nJ and ∼6.76 kW respectively and injected into a 100 m long SMF, to generate an SC output which spans from 1450 to more than 1700 nm, with a pulse width at its full-width-at-half-maximum (FWHM) of 120 fs, 7 times lower than the 840 fs FWHM width of the pulses from the fiber laser. The narrow core SMF performs comparably to longer lengths of standard SMF-28 fibers.

Saturable absorption by carbon nanotubes on silica microtoroids

Saturable absorption is a key technology for shaping the waveform of light such as in passive mode-locking. The combination of high-Q optical microcavities with a saturable absorber allows stable lasing and soliton formation. This work describes saturable absorption by carbon nanotubes (CNTs) on silica microtoroids. CNTs, which are saturable absorbers capable of a fast response time and broadband absorption, were grown on silica microtoroids by chemical vapor deposition (CVD). Raman spectroscopy revealed that the CNTs are in good quality (G/D ratio ∼ 7) and about 1.0 nm in diameter, thus confirming that a sample for use in the telecommunication band can be prepared by CVD. A counter-propagating pump-probe experiment enabled us to investigate the characteristics of CNTs as saturable absorbers while suppressing thermo-optic bistability in a microcavity system. The results revealed a saturable absorption coefficient of 0.042 cm−1, a saturable intensity of 25.9 MW/cm2, and a modulation depth of 28%. This is the first step toward the demonstration of the robust mode-locking in a silica microtoroid consisting of CNTs.

Mapping the dynamical regimes of a SESAM mode-locked VECSEL with a long cavity using time series analysis

The different dynamical regions of an optically-pumped SESAM mode-locked, long-cavity VECSEL system with a fundamental pulse repetition frequency of ~200 MHz are investigated. The output power, captured as 250 μs long time series using a sampling rate of 200 GSa/s, for each operating condition of the system, is analyzed to determine the dynamical state. A wavelength range of 985-995 nm and optical pump powers of 10 W-16.3 W is studied. The system produces high quality fundamental passive mode-locking (FML) over an extensive part of the parameter space, but the different dynamical regions outside of FML are the primary focus of this study. We report five types of output: CW emission, FML, mode-locking of a few modes, double pulsing, and, semi-stable 4th harmonic mode-locking. The high sampling rate of the oscilloscope, combined with the long duration of the time series analyzed, enables insight into how the structure and substructure of pulses vary systematically over thousands of round trips of the laser cavity. Higher average output power is obtained in regions characterized by semi-stable 4th harmonic mode-locking than observed for FML, raising whether such average powers might be achieved for FML. The observed dynamic transitions from fundamental mode-locking provide insights into instability challenges in developing a stable, widely tunable, low repetition rate, turn-key system; and to inform future modelling of the system.

Picosecond Pulse Generation and Pulse Train Stability of A Monolithic Passively Mode-Locked Semiconductor Quantum-Well Laser at 1070 nm

We experimentally study the pulse generation and the pulse train timing and amplitude stability of a monolithic passively mode-locked multisection quantum-well semiconductor laser. The laser emits a stable optical pulse train at a wavelength of 1070 nm and at a fundamental repetition rate of 20 GHz. Timing jitter and amplitude jitter analysis allows identifying distinct laser operating regimes of stable picosecond optical pulses. We obtain an ultra-low pulse-to-pulse timing jitter of 22 fs and find emission regimes which are unaffected by amplitude jitter. The lowest pulse width amounts to 2.6 ps, and the highest pulse peak power is estimated to be 556 mW at a low timing jitter of 170 fs and a negligible relative amplitude jitter of 0.05.

Ultrawideband Millimeter-Wave Oscillators Based on Two Coupled Gyro-TWTs With Helical Waveguide

We are considering the possibility of ultrawideband millimeter-wave generation in microwave oscillators consisting of two coupled helical-waveguide gyro-TWTs, one of which operating as an amplifier whereas the other acting as a saturable absorber or as a nonlinear converter. In the first case, the regime of passive mode-locking is realized when the radiation represents a periodical train of ultrashort electromagnetic pulses. The spectral domain corresponds to the formation of a microwave “frequency comb.” On the other hand, if the second gyro-TWT acts as a nonlinear converter with a fast variation of the phase characteristic, it leads to the generation of chaotic signals with quasi-continuous spectrum and a bandwidth of up to ~10% which is defined by the bandwidth of the gyro-TWTs.

Effects of Nanomaterial Saturable Absorption on Passively Mode-Locked Fiber Lasers in an Anomalous Dispersion Regime: Simulations and Experiments

In recent years, several kinds of nanomaterials have been successfully used for passive mode-locking, but it is not fully understand how the mode-locking performance is influenced by the different characteristics of these saturable absorbers (SAs). In this paper, we numerically and experimentally investigate the effects of nanomaterial saturable absorption (e.g., modulation depth and saturation intensity) on a passively mode-locked fiber laser in an anomalous dispersion regime. First, by numerically solving the Ginzburg–Landau equation, we analyze the evolution of the output performances (spectral bandwidth, pulse duration, and peak power) of passively mode-locked Er $^{3+}$ -doped fiber laser as the SA's modulation depth or saturation intensity. Then, we fabricate four nanomaterial-based SAs, which have the different modulation depth from 1.8% to 19.1%, the different saturation intensity from 11 to 180 MW/cm $^{2}$ , and the similar insertion loss of $\sim$ 3 dB. Finally, we perform the experimental comparison of passively mode-locked Er $^{3+}$ -doped fiber laser using the four nanomaterial-based SAs, respectively. Our results reveal that: 1) as the modulation depth increases, the mode-locked spectral bandwidth becomes wide and the pulse duration becomes short; and 2) the SA's saturation intensity has little influence on the output performance. The experimental results are in good agreement with the numerical simulations. This work could provide a useful guideline for choosing proper nanomaterial-based SA for different practical applications.

High-Power Sub-300-Femtosecond Quantum Dot Semiconductor Disk Lasers

Self-assembled quantum dots (QDs) as active media for ultrafast semiconductor disk laser offer large gain bandwidths, fast gain dynamics, and high temperature stability. We report on the shortest pulses and the highest pulse peak power from an optically pumped vertical external cavity surface emitting laser (VECSEL) based on QDs and optimized for passive modelocking at 1035 nm using a semiconductor saturable absorber mirror. We demonstrate 216-fs pulses with an average output power of 269 mW at a pulse repetition rate of 2.77 GHz and 396 W peak power. At a lower pulse repetition rate of 1.67 GHz, we achieve 193-fs pulses with 112 mW of average output power. We remark a higher optical-to-optical pump efficiency compared to our previous QW VECSELs in the sub-300-fs regime. This is further confirmed by a comparative analysis of the saturation recovery which reveals longer carrier lifetimes for the QD compared to QW VECSELs.

Electrostatic Functionalization and Passivation of Water-Exfoliated Few-Layer Black Phosphorus by Poly Dimethyldiallyl Ammonium Chloride and Its Ultrafast Laser Application.

Few-layer black phosphorus (BP) which exhibits excellent optical and electronic properties, has great potential applications in nanodevices. However, BP inevitably suffers from the rapid degradation in ambient air because of the high reactivity of P atoms with oxygen and water, which greatly hinders its wide applications. Herein, we demonstrate the electrostatic functionalization as an effective way to simultaneously enhance the stability and dispersity of aqueous phase exfoliated few-layer BP. The poly dimethyldiallyl ammonium chloride (PDDA) is selected to spontaneously and uniformly adsorb on the surface of few-layer BP via electrostatic interaction. The positive charge-center of the N atom of PDDA, which passivates the lone-pair electrons of P, plays a critical role in stabilizing the BP. Meanwhile, the PDDA could serve as hydrophilic ligands to improve the dispersity of exfoliated BP in water. The thinner PDDA-BP nanosheets can stabilize in both air and water even after 15 days of exposure. Finally, the uniform PDDA-BP-polymer film was used as a saturable absorber to realize passive mode-locking operations in a fiber laser, delivering a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides a new way to obtain highly stable few-layer BP, which shows great promise in ultrafast optics application.

Low threshold linear cavity mode-locked fiber laser using microfiber-based carbon nanotube saturable absorber

In this work, we demonstrate a linear cavity mode-locked erbium-doped fiber laser in C-band wavelength region. The passive mode-locking is achieved using a microfiber-based carbon nanotube saturable absorber. The carbon nanotube saturable absorber has low saturation fluence of 0.98 µJ/cm2. Together with the linear cavity architecture, the fiber laser starts to produce soliton pulses at low pump power of 22.6 mW. The proposed fiber laser generates fundamental soliton pulses with a center wavelength, pulse width, and repetition rate of 1557.1 nm, 820 fs, and 5.41 MHz, respectively. This mode-locked laser scheme presents a viable option in the development of low threshold ultrashort pulse system for deployment as a seed laser.

Precise fiber length measurement using harmonic detection of phase-locked cavity modes

In this paper, precise length measurements of optical fibers are performed by employing harmonic detection of the pulse-train frequency of a passively mode-locked fiber laser. This frequency is proportional to the length of the laser cavity in which the measured fiber is installed. Our proposed technique enables length measurement of long fibers from 1 to 40 km with precision from 0.4 to 8 mm and short fibers of few meters with precision as low as 26 µm. Such superior precision is achieved not only by the selection of higher harmonics of up to 1410, but also by the careful control of the wavelength at which the passive mode-locking occur, because of the broadband nature of the used gain medium.

Topological Insulators for Mode-locking of 2-μm Fiber Lasers

We performed density functional theory calculations for the topological insulators (TIs) of Bi2Se3 and CoSb3 (skutterudite) in a comparison with Bi2Te3 and In2Co4 Sb12 (filled skutterudite) to explore whether the former TIs can serve as saturable-absorption materials for laser mode-locking at the wavelengths of 2 μm and beyond. The calculated electronic-band structures and optical-absorption spectra indicate the potential of these TIs in terms of midinfrared saturable-absorption applications. Especially, in consideration of the film and bulk forms of Bi2Se3 and Bi2Te3, we investigated the electronic and/or optical properties of the metallic surface states and the semiconducting interior states, respectively. They exhibited an excellent broadband operability regardless of the state, and particularly, we noted a linearly dispersive relation around the Fermi levels of all of the electrons of the Bi2Se3 surface in both the conduction and valence bands, whereas this is not evident for Bi2Te3. Assuming the importance of linear dispersion for a fast electronic relaxation, and considering the decent oxidation resistance of the Bi2Se3 surface, we suggest that Bi2Se3 is superior to Bi2Te3 for midinfrared mode-locking. A review of the experimental studies regarding TI-based 2-μm saturable absorbers that have been performed so far is presented, and the discussion and rationalization of the possible universality of TIs regarding mid- to far-infrared passive mode lockers results in the suggestion of a new CoSb3-TI-based saturable absorber.

非线性放大环形镜被动锁模光纤激光器重复频率精确锁定研究

非线性放大环形镜被动锁模光纤激光器重复频率精确锁定研究

Режимы излучения двухсекционных лазеров спектрального диапазона 1.06 μm с активной областью на основе квантовых точек

AbstractWe have investigated two-sectional semiconductor lasers with an active region comprising five layers of InGaAs quantum dots, emitting in the spectral range near 1.06 μm. Regimes of passive mode-locking, passive Q-switching, and mode-locking with pulse modulated amplitude are realized. The transition conditions between generation regimes are investigated. The frequency tuning range with current increase in the Q-switched regime exceeds more than 4 times. The duration of the mode-locked pulses was 2 ps at the pulse repetition rate of 44.3 GHz.

Watt-level 10-gigahertz solid-state laser enabled by self-defocusing nonlinearities in an aperiodically poled crystal

Femtosecond modelocked lasers with multi-gigahertz pulse repetition rates are attractive sources for all applications that require individually resolvable frequency comb lines or high sampling rates. However, the modelocked laser architectures demonstrated so far have several issues, including the need for single-mode pump lasers, limited output power, Q-switching instabilities and challenging cavity geometries. Here, we introduce a technique that solves these issues. In a two-dimensionally patterned quasi-phase-matching (QPM) device, we create a large, low-loss self-defocusing nonlinearity, which simultaneously provides SESAM-assisted soliton modelocking in the normal dispersion regime and suppresses Q-switching induced damage. We demonstrate femtosecond passive modelocking at 10-GHz pulse repetition rates from a simple straight laser cavity, directly pumped by a low-cost highly spatially multimode pump diode. The 10.6-GHz Yb:CaGdAlO4 (Yb:CALGO) laser delivers 166-fs pulses at 1.2 W of average output power. This enables a new class of femtosecond modelocked diode-pumped solid-state lasers with repetition rates at 10 GHz and beyond.

Unveiling multi-scale laser dynamics through time-stretch and time-lens spectroscopies

Spectro-temporal studies on the nonlinear physics of complex laser dynamics are essential in approaching its ultimate performance as well as understanding interdisciplinary problems. Unfortunately, it has long been limited by the insufficient spectro-temporal resolving power of conventional temporal and spectral analyzers, particularly when an indefinite optical signal ensemble contains polychromatic mixtures of continuous-wave (CW) and short pulse. In this work, we propose a real-time optical spectro-temporal analyzer (ROSTA) with three synchronized processing channels (i.e., multi-core) for single-shot studies on laser dynamics. It simultaneously provides temporal resolutions of ~70 ps in the time domain and 10’s ns (or 10’s MHz frame rate) in the spectral domain, as well as a high spectral resolution for multiscale optical inputs, i.e., ranging from CW to fs pulses. Its non-trivial record length of up to 6.4 ms enables continuous observations of non-repetitive optical events over an extensive time period ― equivalent to a propagation distance of ~1900 km. To showcase its practical applications, ROSTA is applied to visualize the onset of passive mode-locking of a fiber laser, and interesting phenomena, i.e., evolution from quasi-CW noise burst to strong shock, transition from fluctuation to mode-locking, and coexistence of CW and mode-locked pulses, have been spectro-temporally observed in a single-shot manner for the first time. It is anticipated that ROSTA will be a powerful technology for spectro-temporal optical diagnosis in different areas involving polychromatic transients.

Influence of gain fiber on dissipative soliton pairs in passively mode-locked fiber laser based on BP as a saturable absorber

We investigate the influence of gain fiber on dissipative soliton pairs in passively mode-locked (PML) fiber laser based on black phosphorus (BP) as a saturable absorber. Numerical simulations show that we can generate the dissipative soliton pairs in PML fiber laser when the gain fiber parameters (gain saturation energy and gain bandwidth) are in an appropriate dynamic range, and the dissipative soliton pairs become unstable once the range is exceeded. Then we analyze the dynamic evolution of the dissipative soliton pairs and the influence of gain fiber on the pulse separation, peak power, and single-pulse energy of the dissipative solitons pairs.