Mode-locked Laser Pulses Research Articles

Dual-Wavelength Self-Similar Pulse Fiber Laser Based On Topological Insulator

Multi wavelength ultrashort pulse is of great significance in the scientific field. The dual-wavelength self-similar (SS) pulse mode-locked fiber laser with topological insulator (TI) is studied in this paper. The study on evolution of dual-wavelength SS pulse is based on Ginzburg-Landau equation (GLE) by split-step Fourier transforms (SSFT). The results have a high theoretical significance and value in practice.

Pulse-amplitude equalization in the monotonic region of the modulator of a rational-harmonically mode-locked fiber ring laser

We investigate characteristics of the modulator and classify the modulation regions to study pulse-amplitude equalization (PAE) of a rational-harmonically mode-locked fiber ring laser pulses. PAE was accomplished experimentally in a polarization-maintaining laser resonator simply by optimizing the modulator driving power without any additional devices, and stable pulse-amplitude-equalized pulses were obtained in the monotonic region of the modulator at repetition rates of 20 GH, 30 GH, and 40 GH.

Vanadium disulfide for ultrafast photonic application

The investigation of two-dimensional (2D) nonlinear optical materials offers a promising way to construct the high-performance optical devices in fundamental and industrial applications because of their rich distinct optoelectronic properties. Herein, by utilizing the liquid exfoliation method, vanadium disulfide (VS2) nanosheets are prepared and the thickness is measured to be 3.16 nm. In addition, we have fabricated the VS2-based optical device and the nonlinear optical property is characterized with modulation depth of 23.97%. By using VS2 as saturable absorber, a high stable passively mode-locking Er-doped fiber laser is obtained with pulse duration of 169 fs and the largest average output power of 70.5 mW. The slope efficiency is up to 7.9%. In comparison to recent results of mode-locking fiber lasers with 2D materials, the VS2-based fiber laser demonstrates better performance. To the best of our knowledge, this is the first example of using VS2 for generating femtosecond mode-locked laser pulse. Our experimental results not only reveal VS2 ultrafast photonics application, but also advance the high-performance applications for information science and nonlinear optics.

Dark pulse mode-locked fibre laser with zirconia-based erbium-doped fibre (Zr-EDF) and Black phosphorus saturable absorber

A dark pulse multi-wavelength mode-locked fibre laser was demonstrated via the enhanced zirconia-yttria-aluminium co-doped silica fibre (Zr-EDF) as a gain medium with the utilisation of black phosphorus as saturable absorber (BPSA). A self-started dark pulse was accomplished by increasing the pump power and carefully rotating a polarisation controller (PC). Five lasing lines of multi-wavelength were generated with small constant channel spacing of 0.6 nm. The measured fundamental repetition rate was 1 MHz with a pulse duration of 3.46 ps. At output power 9.89 mW, the calculated pulse energy was 9.89 nJ, whereas the optical signal to noise ratio (OSNR) was as high as 74.8 dB, which confirmed the stability of the dark pulse train.

Spectral dynamics on saturable absorber in mode-locking with time stretch spectroscopy

A mode-locked laser that can produce a broadband spectrum and ultrashort pulse has been applied for many applications in an extensive range of scientific fields. To obtain stable mode-locking during a long time alignment-free, a semiconductor saturable absorber is one of the most suitable devices. Dynamics from noise to a stable mode-locking state in the spectral-domain are known as complex and a non-repetitive phenomenon with the time scale from nanoseconds to milliseconds. Thus, a conventional spectrometer, which is composed of a grating and line sensor, cannot capture the spectral behavior from noise to stable mode-locking. As a powerful spectral measurement technique, a time-stretch dispersive Fourier transformation (TS-DFT) has been recently used to enable a successive single-shot spectral measurement over a couple of milliseconds time span. Here, we experimentally demonstrate real-time spectral evolution of femtosecond pulse build-up in a homemade passive mode-locked Yb fiber laser with a semiconductor saturable absorber mirror using TS-DFT. Capturing 700 consecutive spectra (~ 17 µs time window) in real-time using the time-stretch technique, we are able to resolve the transient dynamics that lead to stable mode-locking. Before setting stable mode-locking, an oscillating or shifting fringe pattern in the consecutive spectra was detected. This signature proves the existence of multiple pulses (including a soliton molecule) which is temporally separated with a different relative phase. The dynamics on multiple pulses is originated from a fast relaxation time of the saturable absorption effect. This study provides novel insights into understanding the pulse behavior during the birth of an ultrafast mode-locked laser pulse and the stable single-pulse operation which is highly stabilized.

Investigation of tungsten trioxide as a saturable absorber for mode-locked generation

Low-dimensional materials as saturable absorbers (SA) for pulsed laser applications have gain wide interest due to high nonlinearity and strong light-matter interaction. Various materials have been investigated such as graphene, transition metal oxides (TMO), transition metal dichalcogenides, and topological insulators. Tungsten trioxide (WO3); a cheap and nontoxic type of TMO has not yet been investigated for mode-locked pulsed fiber laser (MLFL) generation despite extensive employment as applications in other fields. WO3 is very attractive with spectral absorption extending in the broad near infra-red region, good mechanical strength, high resistance to photo corrosion, and is one of the few oxides that has high thermal and chemical stability. A tungsten trioxide (WO3) based saturable absorber (SA) fabricated via deposition of WO3 composite on tapered fiber was demonstrated. The WO3 weight percentage was varied from 0.005 to 1.235 wt% to investigate the effect on nonlinear saturable parameters and on mode-locked lasing performance. It was determined that the nonlinear saturable parameters were not dependent on weight percentage within its working range; 0.025 to 1.103 wt%. The generation self-started with 35–70 mW pump power, where the pulse durations and spectral bandwidths fall within the range of 810–940 fs and 8–12 nm, respectively. This experimental investigation provides insight on WO3 as a saturable absorber for mode-locked pulse laser generation. The utilization of this nanomaterial is aimed to cut down the cost the existing SA technology while providing enhanced durability and shelf-life.

Temporal Imaging for Ultrafast Spectral-Temporal Optical Signal Processing and Characterization

Temporal imaging is one of the most promising all-optical signal processing techniques that enable single-shot ultrafast signal characterization in both spectral and temporal domains. However, conventional temporal imaging systems have inherent constraints such as limited record length, narrow spectral observation range and complex system configuration, which prevent their potential to be fully unleashed. In this article, we review the recent efforts in developing advanced temporal imaging systems with much enhanced performance. Their powerful applications are demonstrated by real-time single-shot characterization of Kerr soliton generation in microresonator, noise-like pulses in mode-locked fiber lasers, broadband frequency combs and the dynamic waveform in ultrafast optical tomography. In addition, potential future developments of this technique in several important directions are discussed.

Harmonic mode-locking of asynchronous dual-wavelength pulses in mode-locked all-fiber lasers

We report the observation of various harmonic mode-locking (HML) of a dual-wavelength all-fiber laser mode-locked with different repetition rates, and show the stability of the repetition rate difference. Interestingly, a state of second-order HML at both wavelengths is achieved. The repetition rate difference is recorded, resulting in a standard deviation of 9.3 Hz, though the repetition rates of both wavelengths fluctuate ∼860 Hz. It enriches the nonlinear dynamics of mode-locked lasers, and has potential applications related to the dual-comb based on free-running lasers, provided that the stability of HML is further improved.

Indium Tin Oxide Coated D-Shape Fiber as a Saturable Absorber for Generating a Dark Pulse Mode-Locked Laser**Supported by the Ministry of Education of Malaysia (Grant No. LR001A-2016A) and University of Malaya (Grant No. RP039C-18AFR).

A dark pulse mode-locked laser is experimentally demonstrated using the indium tin oxide (ITO) coated D-shape fiber as a saturable absorber (SA). Using the polishing wheel technique, a D-shape single mode fiber was fabricated. A 60-nm-thick layer of ITO was deposited over the D-shape fiber using the electron beam deposition method. The SA has a saturation intensity of 40.32 MW/cm2 and a modulation depth of 3.5%. A stable dark pulse mode-locked laser was observed at a central wavelength of 1559.4 nm with repetition rate 0.98 MHz, pulse width 370 ns and signal-to-noise ratio 61 dB.

Q-Switched and Mode-Locked Nd/Cr:YAG Ceramic Pulse Laser

A mode-locked and Q-switched short pulse laser using the Nd3+/Cr3+:YAG ceramic has been constructed with a SESAM and Cr4+:YAG crystal optical switch based on excite state absorption (ESA). Laser oscillations of the pulse laser were observed experimentally. The Nd/Cr:YAG ceramic laser has a high conversion efficiency from white light (such as lamp light or solar light) to the laser. The Nd/Cr:YAG ceramic has a higher laser gain than the Nd:YAG laser for the same pumping power. The laser oscillation can be obtained very easily. A single-mode-locked laser pulse with fast modulation on the order of 100 ps was obtained in some pump power regimes when using the Cr4+:YAG crystal. The obtained pulse duration of the short pulse was a few hundred ps. A maximum peak power of 60 kW was obtained when using a SESAM. The same level of peak power (60 kW) was also obtained when using the Cr4+:YAG crystal.

High-power mid-infrared picosecond pulse bunch generation through difference frequency generation

We report a high average power mid-infrared picosecond (ps) pulse bunch output by means of direct difference frequency generation (DFG) in periodically poled magnesium-doped lithium niobate between a linearly polarized ps pulse bunch Yb fiber laser and a synchronized Er fiber laser. The ps pulse bunch Yb fiber laser was composed of an all polarization maintained “figure of eight” structured mode-locked Yb fiber laser as the seed, a pulse multiplier, and two stages of Yb fiber amplifiers. The mode-locked Yb fiber laser has an output ps laser pulse at 1030 nm with a repetition rate of 16.32 MHz. The pulses were then transformed to the pulse bunches through the pulse multiplier. Within each bunch, there were 16 equally spaced pulses with pulse widths of 8.5 ps and time intervals around 300 ps. The Er fiber laser had a gain switched seed laser diode working at 1550 nm with a pulse width around 5.1 ns, which was synchronized to the mode-locked Yb fiber laser pulse bunch, and two stages of Er fiber amplifiers. Under the average power of 27.8 W of Yb fiber laser, we obtained an average output power of 3.1 W at 3.07 µm. This is, to the best of our knowledge, the highest average power mid-infrared ps pulse bunch obtained via DFG directly.

VSe2 nanosheets for ultrafast fiber lasers

In this work, we investigate VSe2 for generating femtosecond and large energy mode-locked laser pulses for the first time.

Poly(3-hexylthiophene-2,5-diyl) regioregular (P3HT) thin film as saturable absorber for passively Q-switched and mode-locked Erbium-doped fiber laser

Abstract Q-switched and mode-locked Erbium-doped fiber lasers (EDFLs) were demonstrated using poly(3-hexylthiophene-2,5-diyl) regioregular (P3HT) organic semiconductor in the form of thin film as a saturable absorber (SA). The P3HT thin film is cut into small pieces and sandwiched between two ferrules in EDFL cavity to generate a stable Q-switching and mode-locking pulse train. By increasing the 980 nm pump power from 63 mW to 125 mW, the repetition rate of the EDFL increases from 62.97 kHz to 78.63 kHz while the pulse width decreases from 5.52 µs to 3.79 μs. The maximum pulse energy of 15.05 nJ is generated with repetition rate and pulse width of 78.63 kHz and 3.79 μs, respectively. Meanwhile, self-starting mode-locked EDFL arises as the pump power raises from 125 mW to 187 mW. As the pump power increases to the maximum, the repetition rate is almost constant of 1.838 MHz and pulse width of 2.62 ps are observed. The mode-locked EDFL operated at 1563 nm wavelength. This experiment shows that P3HT SA can be used to generate stable Q-switched and mode-locked pulsed lasers.

Splitting and Motions of Dissipative Soliton Resonance Pulses in Mode-Locked Fiber Lasers

We numerically investigated the splitting and motions of dissipative soliton resonance (DSR) pulses in an all-normal-dispersion Yb-doped fiber laser mode-locked by a nonlinear optical loop mirror. At certain values of the system parameters, the initial single Gaussian pulse can evolve into an unstable DSR pulse with several dark solitons inside. After collisions of dark solitons, a big intensity dip occurs at the center of the DSR pulse, leading to the splitting of the DSR pulse. After splitting, the peak-to-peak separation between two DSR pulses increases at first and finally reaches a fixed value of 193 ps, indicating a steady-state multipulse operation. The relative motions of separated DSR pulse originate from the phase shift.

Graphdiyne for Ultrashort Pulse Generation in an Erbium-Doped Hybrid Mode-Locked Fiber Laser

An erbium-doped hybrid passively mode-locked fiber laser based on few-layer graphdiyne (GDY) saturable absorber (SA) has been investigated for the first time. Hybrid mode-locked fiber laser is composed of nonlinear polarization rotation (NPR) technology and GDY-SA. The central wavelength, pulse width and repetition rate of the output pulse are 1530.7 nm, 690.2 fs and 14.7 MHz, respectively. Compared with the passively mode-locked pulse laser with GDY-SA or NPR technology alone, the output pulse width of hybrid passively mode-locked fiber laser is reduced more than 50 fs. It is demonstrated that the performance of GDY can be potentially applied in ultrafast laser.

Fiber Optic Raman Distributed Temperature Sensor Based on an Ultrashort Pulse Mode-Locked Fiber Laser

The testing of an all-fiber erbium ultrashort pulsed laser in a distributed fiber temperature sensor as a source of probing pulses has been performed. Among the prospects of such an approach are an improved signal–noise ratio in the receiving system and a better spatial resolution of the temperature sensor. The experiments have revealed the factors that limit the effective length of the fiber temperature sensor, such as a high pulse repetition frequency and intrinsic laser noises. As a result of the performed work, the distributed fiber optical temperature sensor with a near-room-temperature resolution of ~1.5 K, an effective length of ~3 m, and a spatial resolution of ~10 cm has been developed.

In2Se3 nanosheets with broadband saturable absorption used for near-infrared femtosecond laser mode locking

Indium selenide (In2Se3) has attracted tremendous attention due to its favorable electronic features, broad tunable bandgap, high stability and other attractive properties. However, its further applications for nonlinear optics have not yet been fully explored. In this work, we demonstrate that few-layer α-In2Se3 nanosheets exhibit strong saturable absorption properties over a wide wavelength range covering 800, 1064 and 1550 nm. The few-layer α-In2Se3 nanosheets used for this experiment are fabricated via a simple ultrasonic exfoliation in liquid. Stable ultrafast mode-locking laser pulses are obtained from both ytterbium-doped and erbium-doped fiber laser systems operating at 1064 and 1550 nm, respectively. A pulse duration as short as 215 fs was achieved in the Er-doped fiber laser system. Stable output pulses over 6 h of operation were obtained in both laser systems. The pulse energy and peak power of the laser output pulses were increased by reducing the In2Se3 thickness. These results indicate that α-In2Se3 nanosheets with low layer numbers are promising candidates for broad ultrafast photonics devices, such as optical switchers, Q-switchers and mode lockers.

All-in-One Fiber Laser Based on a Liquid Crystal Transducer

We report a new method of generating continuous-wave (cw) radiation, Q-Switched as well as mode-locked laser pulses from a single linear fiber laser cavity. The three operating modes can be selected by the electric signal that is applied to an intracavity liquid crystal (LC) transducer cell. In addition, this is the first demonstration of using an LC modulator as an active mode-locking device in a fiber laser oscillator to date. In the proof-of-principle experiments presented here, the Q-Switched laser pulses with a duration of 1.3 $ {\mu }\text{s}$ , a maximum energy of 2.7 $ {\mu }\text{J}$ , and a peak power of 1.93 W were generated at low repetition rates ranging from 0.2 to 6 kHz. In mode-locked operation, pulse trains with repetition rates as high as 944.5 kHz and a shorter duration of 65 ns were achieved. Finally, a slight modification of the LC cell allows for a direct comparison between amplitude modulation (AM) and frequency modulation (FM) mode-locking.

All-Fiber Supercontinuum Source Pumped by Noise-Like Pulse Mode Locked Laser

We demonstrate flat supercontinuum generation in a piece of highly nonlinear fiber pumped with a stable noise-like pulse mode-locked fiber laser. The compact all-fiber laser realizes mode locking based on a nonlinear amplifying loop mirror and emits noise-like mode-locked pulse with high peak power and high average power. The output pulse has a 3-dB bandwidth of ~35.8 nm with the central wavelength of ~1581 nm. Pumping a 102-m highly nonlinear fiber with the noise-like pulse mode-locked fiber laser directly, a flat and weak residual-pump supercontinuum spanning from ~1370 to ~2200 nm is generated. The spectrum within 1865 to 2200 nm contains more than 45% of the whole energy of the supercontinuum.

Switchable and tunable multi-wavelength emissions in pulsed ytterbium fiber lasers with black phosphorus saturable absorbers and polarization-maintaining fiber Bragg gratings

In this paper, we present a systematic study of ytterbium-doped pulsed fiber lasers, both Q-switched and mode-locked, that emitted in switchable and tunable multi-wavelength states. The pulsed lasers were initiated by black phosphorus saturable absorbers (BP-SAs) prepared by an optically driven process. One relatively thicker BP-SA with a modulation depth of 16.8% and a saturable intensity of 13.09 MW/cm2 was used for Q switching, and one thinner BP-SA with a modulation depth of 6.7% and a saturable intensity of 7.82 MW/cm2 was used for mode locking. The switchable and tunable wavelength-emitting status was achieved by a Bragg grating written in polarization maintaining fibers (PM-FBG). The fabricated fiber Bragg grating had two reflection peaks. By adjusting an in-cavity polarization controller, the wavelength-emitting status of the pulsed lasers was flexibly switched in three discrete states: individual wavelength of 1063.8 and 1064.1 nm, respectively; and simultaneously emitting both wavelengths with tuning capability. Stable Q-switched and mode-locked laser pulses were observed with all three wavelength-emitting states. Furthermore, by applying stresses at the fast or slow axis of the utilized PM-FBG, the wavelength spacing could be tuned from 0.02 to 0.52 nm in the dual-wavelength emission state.