Mode-locked Pulse Duration Research Articles

Oxygen vacancies based BiOBr nanoflowers saturable absorber and its application in Q-switched mode-locked infrared laser

In this work, BiOBr nanoflowers with oxygen vacancies as saturable absorber are prepared using hydrothermal method. Passively Q-switched mode-locked infrared laser is achieved. The output power reaches the maximum 312 mW. The minimum Q-switched envelope is 242 ns with the maximum repetition rate of 180.5 kHz. Meanwhile, the mode-locked pulse duration within the Q-switched envelope is approximately 315 ps with repetition rate of 232 MHz. The results of X-ray diffraction (XRD), Electron paramagnetic resonance (EPR), UV–Vis–NIR diffuse reflectance spectra, and X-ray photoelectron spectroscopy (XPS) demonstrate the oxygen vacancies introduce intermediate energy levels in the energy band of BiOBr, which enhances the optical absorption in the near-infrared region. Therefore, BiOBr saturable absorber exhibits good saturable absorption characteristics in the near-infrared region and is a promising modulation element in Q-switched mode-locked infrared lasers.

17-GHz waveguide lasers modulated by a heterostructure layered material

In this work, a thin-film MoS2/WS2/MoS2/WS2 heterostructure is employed as a saturable absorber (SA) material for efficient ultrafast pulsed laser operation, achieving a 17.54 GHz Q-switched mode-locked Nd:GGG waveguide laser fabricated by femtosecond laser direct-writing (FsLDW). The mode-locked pulse duration is measured to be as short as 31 ps. The maximum laser slope efficiency and the average output power are determined to be 29.27% and 310 mW, respectively. Such a high-performance compact pulsed laser exhibits promising applications of crystalline waveguide structures and layered heterostructures in ultrafast integrated photonics. This work represents the very first experimental demonstration of a pulsed laser based on FsLDW Nd:GGG waveguides and the very first demonstration of using a MoS2/WS2/MoS2/WS2 heterostructure as an efficient SA.

Low threshold soliton and a noise-like pulse conversion in an all-polarization-maintaining figure-eight cavity.

We experimentally report a low threshold soliton and a noise-like mode-locked fiber laser using an all-polarization-maintaining figure-eight cavity. We built a bidirectional pump structure without a phase shifter at the beginning of the experiment. The resonator has a high mode-locking threshold of 620 mW. Afterwards, we used a phase shifter in the resonator, and the laser can self-start in a conventional soliton (CS) mode-locked state when pump1 reaches the threshold of only 70 mW. The CS pulse with a duration of 863.8 fs can be observed at 1560 nm. When the two pump powers increase to 350 mW and 50 mW, the conventional soliton can convert to noise-like pulses. The central wavelength and pulse duration of noise-like mode-locked pulse are 1560.4 nm and 417.9 fs, respectively. The laser can realize conversion between ultrafast pulses and high-energy pulses, and have a low threshold that can be used for nonlinear frequency conversion, supercontinuum generation, sensing, etc.

Frequency comb with 100 GHz spacing generated by an asymmetric MQW passively mode-locked laser.

A L-band two-section AlGaInAs/InP asymmetric multiple quantum well (QW) passively mode-locked laser has been used to generate a frequency comb with a 100 GHz spacing at a central wavelength of 1610 nm. The comb contains 10 optical lines within a -3dB bandwidth of 8.05 nm and 34 optical lines within a -20dB bandwidth of 30 nm. The mode-locked pulse duration was 440 fs. To the best of our knowledge, this is the shortest pulse duration from any directly electrically pumped QW semiconductor mode-locked laser.

Generation of 131 fs mode-locked pulses from 2.8 μm Er:ZBLAN fiber laser

We demonstrated a femtosecond mode-locked Er:ZrF4-BaF2-LaF3-AlF3-NaF (Er:ZBLAN) fiber laser at 2.8 μm based on the nonlinear polarization rotation technique. The laser generated an average output power of 317 mW with a repetition rate of 107 MHz and pulse duration as short as 131 fs. To the best of our knowledge, this is the shortest pulse generated directly from a mid-infrared mode-locked Er:ZBLAN fiber laser to date. Numerical simulation and experimental results confirm that reducing the gain fiber length is an effective way to shorten the mode-locked pulse duration in the Er:ZBLAN fiber laser. The work takes an important step towards sub-100-fs mid-infrared pulse generation from mode-locked Er:ZBLAN fiber lasers.

Self-Q-switching and passively Q-switched mode-locking of dual-wavelength Nd:YSAG laser

In this paper, self-Q-switched (SQS) and passively Q-switched mode-locked (QML) dual-wavelength Nd:YSAG lasers are reported, that works at 1058.97 and 1061.49 nm, simultaneously. Till now, the performance on SQS, and passively QML operation by using black phosphorus (BP) in Nd:YSAG crystal lasers have not been reported as per the author’s knowledge. The Nd:YSAG laser SQS operation was realized by designing a simple plano-plano resonator cavity. The maximum output power of 0.471 W at absorbed pump power 2.64 W and the pulse duration was 885 ns. The Nd:YSAG laser QML operation was realized by designing a Z-type resonator cavity. The duration of mode-locked pulse inside the Q-switched envelop was estimated to be 235 ps with 96.5 MHz repetition rate at the absorbed pump power of 4.08 W in QML with black phosphorus saturable absorber (BP-SA).

Nonlinear absorption properties of indium selenide and its application for demonstrating pulsed Er-doped fiber laser

Two-dimensional materials are now receiving continuously increasing attention due to their exceptional optical properties. Recently, it has been proved that indium selenide (InSe) is also an excellent photoelectric material, with wide applications in the field of photodetecters and so on. In our work, layered InSe was prepared through the liquid-phase exfoliation method, and the nonlinear saturable absorption properties of InSe were also experimentally investigated. In addition, InSe-polyvinyl alcohol film was successfully prepared and employed as an saturable absorber for demonstrating an Er-doped passive Q-switched mode-locked fiber laser for the first time. The largest Q-switched pulse energy was as high as 112.97 nJ and the narrowest mode-locked pulse duration was 2.96 ns at a repetition rate of 1.74 MHz. Our results fully indicate that InSe also has excellent nonlinear absorption properties in comparison with other commonly used 2D materials. Thus, the applications of InSe can be deeply investigated to promote the development of ultra-fast optics in the near future.

65 GHz Q-switched mode-locked waveguide lasers based on two-dimensional materials as saturable absorbers

Two-dimensional (2D) materials have generated great interest in the past few years opening up a new dimension in the development of optoelectronics and photonics. In this paper, we demonstrate 6.5 GHz fundamentally Q-switched mode-locked lasers with high performances in the femtosecond laser-written waveguide platform by applying graphene, MoS2 and Bi2Se3 as saturable absorbers (SAs). The minimum mode-locked pulse duration was measured to be as short as 26 ps in the case of Bi2Se3 SA. The maximum slope efficiency reached 53% in the case of MoS2 SA. This is the first demonstration of Q-switched mode-locked waveguide lasers based on MoS2 and Bi2Se3 in the waveguide platform. These high-performance Q-switched mode-locked waveguide lasers based on 2D materials pave the way for practical applications of compact ultrafast photonics.

Effects of nanomaterial saturable absorption on gain-guide soliton in a positive group-dispersion fiber laser: Simulations and experiments

In recent years, several kinds of nanomaterials have been discovered, and successfully used as saturable absorbers (SAs) for passively mode-locked fiber lasers. However, it is found that most of nanomaterials-based SAs cannot stably generate gain-guide solitons in positive group-dispersion fiber lasers, which is urgently expected to fully understand the inherent reasons. In this paper, we numerically and experimentally investigate the effects of nanomaterial saturable absorption (e.g. modulation depth and saturation optical power) on gain-guide soliton in positive group-dispersion Er3+-doped fiber laser (PGD-EDFL). By numerically solving the Ginzburg–Landau equation, the evolutions of both the mode-locked optical spectrum and pulse duration as a function of modulation depth and saturation optical power are analyzed, respectively. In experiment, we firstly prepare five nanomaterial SAs with the similar insertion loss, which have the different modulation depth from 1.80% to 23.36%, and the different saturation optical power from 8.8 to 536 W. We then perform the experimental comparison by incorporating the five SAs in a same PGD-EDFL cavity, respectively. The experimental results are in good agreement with the numerical ones. Our result reveals that: (1) a low modulation depth cannot support the formation of gain-guide soliton, (2) as the modulation depth increases, the spectral bandwidth of gain-guide soliton increases, the pulse duration decreases and the pulse chirp becomes large, (3) the saturation optical power has the weak influences on the gain-guide soliton performances.

Phase-Modulation-Mode-Locked Continuous-Wave MgO:PPLN Optical Parametric Oscillator

We report phase-modulation mode-locking of a continuous-wave (cw) singly-resonant optical parametric oscillator (OPO) in the near- and mid-infrared based on MgO-doped periodically poled LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (MgO:PPLN) and pumped by an Yb-fiber laser at 1064 nm. By investigating the spectral output and pulse formation characteristics of the OPO, we observe a clear transition from an unstable spectrum in cw operation to a stable and smooth profile under phase modulation, confirming mode-locked operation. Pulse generation and spectral characteristics under different operating conditions are studied, including the effects of the location of the intracavity phase modulator and the number of oscillating modes on mode-locked output pulse duration.

Passively Q-switched and mode-locked dual-wavelength Nd:GGG laser with Cr4+:YAG as a saturable absorber

By using neodymium-doped gadolinium gallium garnet (Nd:GGG) as a laser medium, a simultaneously passively Q-switched and mode-locked (QML) dual-wavelength laser with Cr4+:YAG as a saturable absorber is presented. The laser simultaneously oscillated at 1061nm and 1063nm, corresponding to a frequency difference of 0.53THz. QML pulses with nearly 100% modulation depth were observed. The mode-locked pulse duration underneath the Q-switched envelope was estimated to be about 908ps. The experimental results indicated that the dual-wavelength QML Nd:GGG laser can be an excellent candidate for the generation of THz waves.

Mode-locked, continuous-wave, singly resonant optical parametric oscillator

We report an actively mode-locked continuous-wave (cw) optical parametric oscillator in singly resonant oscillator (SRO) configuration, generating stable 230 ps pulses at 80 MHz repetition rate. The idler-resonant cw SRO, configured in standing-wave cavity, is based on MgO:sPPLT as the nonlinear gain material and pumped at 532 nm. Mode-locking is achieved by direct deployment of an intracavity phase modulator close to one of the SRO cavity end mirrors. The effects of modulation depth and modulation frequency on mode-locked pulse duration and repetition rate are investigated for both idler and signal pulses. Mode-locking is further confirmed by enhancement of single-pass second-harmonic generation of SRO output in the crystal of β-BaB(2)O(4).

Possibility for mode-locked operation of a femtosecond UV storage ring free-electron laser using a low-loss Fabry–Perot resonator

A storage ring free-electron laser (SR-FEL) is inherently a self-mode-locked optical system. The gain broadening due to electron energy spread affects the small signal gain in order to determine the output coupling. Here, the dependence of the small signal gain, the optimum output coupling, and pulse duration on both electron energy spread and loss of a Fabry–Perot resonator in UV SR-FEL were investigated. It was shown that the output coupling strongly affects the mode-locked pulse duration and the present picosecond pulses can be shortened to femtosecond ones using a proper low-loss resonator.

Subpicosecond pulse generation from a 156 μm mode-locked VECSEL

Near-transform-limited subpicosecond pulses at 1.56 μm were generated from an optically pumped InP-based vertical-external-cavity surface-emitting laser (VECSEL) passively mode-locked at 2 GHz repetition rate with a fast InGaAsNSb/GaAs semiconductor saturable absorber mirror (SESAM). The SESAM microcavity resonance was adjusted via a selective etching of phase layers specifically designed to control the magnitude of both the modulation depth and the intracavity group delay dispersion of the SESAM. Using the same VECSEL chip, we observed that the mode-locked pulse duration could be reduced from several picoseconds to less than 1 ps with a detuned resonant SESAM.

Fabrication and characterization of double-wall carbon nanotube absorber for passive mode-locked Nd:GdVO4 laser

Using the vertical evaporation technique we fabricated saturable absorbers by transferring the double-wall carbon nanotubes (DWCNT) onto a hydrophilic quartz substrate. The fast recovery time and the saturation intensity of the absorber were measured to be 228 fs and 130 μJ/cm2, respectively, at 1060 nm. The modulation depth of the absorber was about 3.7%. Passive mode-locked Nd:GdVO4 laser was demonstrated. The continuous wave mode-locked pulses pulse duration is 5.6 ps and the largest average output power is 1.2 W at the pump power of 9.5 W. To the best of our knowledge, this is the first demonstration of high power continuous wave mode locking laser with DWCNT absorber.

Passively Q-switched mode-locking in a diode-pumped Nd:LuVO4 laser with V:YAG

A passively Q-switched and mode-locked Nd:LuVO4 laser with V:YAG at 1.34 μm was successfully demonstrated. Comparisons between c-cut and a-cut Nd:LuVO4 lasers were experimentally made. The maximum average output power of 170 mW, the highest Q-switched pulse energy of 4.5 μJ were obtained in c-cut Nd:LuVO4 laser. The duration of mode-locked pulse was estimated to be less than 540 ps with repetition rate of 110 MHz.

Laser ceramic materials for subpicosecond solid-state lasers using Nd^3+-doped mixed scandium garnets

We have successfully developed and demonstrated broadband emission Nd-doped mixed scandium garnets based on laser ceramic technology. The inhomogeneous broadening of Nd(3+) fluorescence lines results in a bandwidth above 5 nm that is significantly broader than that for Nd:YAG and enables subpicosecond mode-locked pulse durations. We have also found the emission cross section of 7.8 × 10(-20) cm(2) to be adequate for efficient energy extraction and thermal conductivity of 4.7 W/mK from these new Nd-doped laser ceramics. The new laser ceramics are good candidates for laser host material in a diode-pumped subpicosecond laser system with high efficiency and high repetition rate.

LD end pumped, actively mode locked and passively Q-switched Nd:YAP laser at 1341 nm

An end pumped Nd:YAP laser at 1341 nm is actively mode locked and passively Q-switched. Pumping was done with a pulsed high power laser diode with maximum power 425 W. V 3+:YAG with 61% initial transmission served as saturable absorber, and an acousto-optic modulator is used for active mode locking. The output pulse train with 69 ns duration has a total energy of 3.2 mJ with ±4% shot-to-shot fluctuation. The peak output energy of a single mode locked pulse is 0.25 mJ. The pulse duration of a single mode locked pulse is less than 800 ps. The output laser beam is nearly diffraction limited with 1.6 mm diameter, and beam propagation factor M 2 about 1.3.

Diode-pumped passively mode-locked 1342 nm Nd:YVO_4 laser with an AlGaInAs quantum-well saturable absorber

We demonstrate what we believe to be the first use of AlGaInAs quantum wells (QWs) as a saturable absorber for a diode-pumped passively mode locked Nd:YVO(4) laser at 1342 nm. The QWs are grown on a Fe-doped InP substrate that is transparent at lasing wavelength. At an incident pump power of 13.5 W an average output power of 1.05 W with a continuous mode-locked pulse duration of 26.4 ps at a repetition rate of 152 MHz was generated.

Analytical and computational study of self-induced transparency mode locking in quantum cascade lasers

The possibility of using the self-induced transparency effect to achieve laser mode locking has been discussed since the late 1960s but has never been observed. In prior work, we proposed that quantum cascade lasers are the ideal tool to realize self-induced transparency mode locking due to their rapid gain recovery times and relatively long coherence times, and because it is possible to interleave gain and absorbing periods. Here, we present designs of quantum cascade lasers that satisfy the requirements for self-induced transparency mode locking at both 8 and $12\text{ }\ensuremath{\mu}\text{m}$, indicating that it is possible to satisfy these requirements over a wide wavelength range. The coupled Maxwell-Bloch equations that define the dynamics in quantum cascade lasers that have both gain and absorbing periods have been solved both analytically and computationally. Analytical mode-locked solutions have previously been found under the conditions that there is no frequency detuning, the absorbing periods have a dipole moment twice that of the gain periods, the input pulse is a $\ensuremath{\pi}$ pulse in the gain medium, and the gain recovery times in the gain and absorbing periods are much longer than the coherence time ${T}_{2}$ and are short compared to the round-trip time. It was shown that the mode-locked pulse durations are on the order of ${T}_{2}$, which is typically about 100 fs in quantum cascade lasers. In this work, these analytical results are reviewed and extended to include the effects of partial inversion in the gain and absorbing periods and of frequency detuning. An energy theorem in the limit of long coherence times is derived. The Maxwell-Bloch equations have been solved computationally to determine the robustness of the mode-locked solutions when frequency detuning is present, the dipole moment of the absorbing periods differs from twice that of the gain periods, the gain relaxation time is on the order of 1--10 ps, as is typically obtained in quantum cascade lasers, and the initial pulse is not a $\ensuremath{\pi}$ pulse in the gain medium. We find that mode-locked solutions exist over a broad parameter range. We have also investigated the evolution of initial pulses that are initially much broader than the final mode-locked pulses. As long as the initial pulse duration is on the order of ${T}_{1}$ or shorter and has enough energy to create a $\ensuremath{\pi}$ pulse in the medium, a mode-locked pulse with a duration on the order of ${T}_{2}$ will ultimately form.