Passive Mode-locking Research Articles

Linear‐cavity mode‐locked fibre laser with cylindrical vector beam generation

A cylindrical vector beam (CVB)-pulsed fibre laser with compact linear-cavity configuration has been demonstrated, using a two-mode fibre (TMF) Bragg grating (TM-FBG) for mode discrimination and a semiconductor saturable absorber mirror (SESAM) for passive mode-locking. As a result, mode-locked CVB pulses with fundamental repetition rate of 10.295 MHz have been obtained. Besides, with increased pump power harmonic, mode-locked CVB pulses have been achieved, for the first time.

Harmonic mode-locking in an external cavity tapered diode laser with saw-toothed microstructure

In the present study, we report the passive harmonic mode-locking (HML) of the external cavity tapered semiconductor laser system with saw-toothed microstructure. The stable picosecond pulse trains were observed up to the third order HML. The mechanism behind the system was analyzed and single-pulse energy of 59 pJ with a repetition rate of 412 MHz and pulse duration of 3.9 ps was realized.

Harmonic Mode‐Locking: Revealing the Buildup Dynamics of Harmonic Mode‐Locking States in Ultrafast Lasers (Laser Photonics Rev. 13(9)/2019)

In article number 1800333, Xueming Liu and Meng Pang demonstrate the experimental observation of the entire buildup process of the passive harmonic mode-locking (HML) state in an ultrafast laser, which undergoes seven different ultrafast phases. Multiple HML pulses originate from the single-pulse splitting phenomenon. The acoustic resonance in the single-mode fiber stabilizes the final HML state of the laser.

Broadband Semiconductor Mirrors with a Small Relaxation Time for Passive Mode-Locking of NIR Lasers

Two types of mirror structures with saturable absorption are under consideration: monolithic mirrors grown from semiconductor materials and mirrors with a dielectric reflector, with quantum well containing semiconductor structures transferred to the dielectric. Both types of mirrors manifest high reflectivity in the NIR range of the spectrum: the table width is about 100 nm for semiconductor reflectors and more than 200 nm for dielectric reflectors. It is shown that a maximum depth of absorption modulation from 1 to 40% is possible. The recovery time of the saturable absorber (2 ps) makes these mirrors significantly fit for using in lasers with a pulse repetition rate of 1 GHz.

Graphene mode-locked Tm,Ho-codoped crystalline garnet laser producing 70-fs pulses near 21 µm

Bilayer graphene synthesized by chemical vapor deposition is successfully applied as a saturable absorber (SA) for the passive mode-locking of a Tm,Ho:CLNGG laser at 2093nm. Near transform-limited pulses as short as 70 fs, i.e., 10 optical cycles, are produced at a 89 MHz repetition rate with 69 mW average output power. To the best of our knowledge, these are the shortest pulses ever reported from graphene-SA mode-locked Tm, or Ho-lasers in the 2 µm spectral region, including bulk and fiber lasers.

Investigation of dispersion-managed, polarization-maintaining Er-doped figure-nine ultrashort-pulse fiber laser.

Figure-nine fiber lasers can realize all-polarization-maintaining, self-started, highly stable mode-locked laser sources, and are very attractive for applications such as optical frequency combs, metrology, etc. In this work, we investigated a dispersion-managed, polarization-maintaining, Er-doped, ultrashort-pulse figure-nine fiber laser both experimentally and numerically. Stable, self-started, passive mode-locking operation was achieved in a wide net cavity dispersion region, covering the soliton, stretched pulse, and dissipative soliton mode-locking regimes. A 132 fs ultrashort pulse with spectral width of 46 nm was obtained in the stretched pulse mode-locking regime. The initial mode-locking process and dynamics inside the cavity, in addition to the fundamental characteristics of the output pulses, were examined via numerical analysis. Owing to the asymmetric configuration, the propagation behaviors were different between the two counter-propagation directions. It was found that a large breathing had already started before the passive mode-locking point in stretched pulse mode-locking operation. Intense overshoots were also observed at the beginning of passive mode-locking. Numerical results were almost in agreement with the experimental ones.

Efficient 1 µm Laser Emission of Czochralski-Grown Nd:LGSB Single Crystal.

A 5.0-at.% Nd-doped La0.64Gd0.41Sc2.95(BO3)4 (Nd:LGSB) borate laser crystal was successfully grown by the Czochralski method, for the first time to our knowledge. The spectroscopic properties of the grown crystal are discussed and 1 µm laser emission, under end-pumping with a fiber-coupled diode laser at 807 nm, is reported. A c-cut Nd:LGSB medium yielded 1.35 W continuous-wave output power at 0.63 overall optical-to-optical efficiency, with respect to the absorbed pump power, together with the high 0.68 slope efficiency. With an a-cut Nd:LGSB sample, 0.81 W output power at 0.52 optical-to-optical efficiency was obtained. The laser emission performances under quasi-continuous wave pumping are presented as well, for both c-cut and a-cut crystals. Passive Q-switching was investigated with a semiconductor saturable absorber mirror (SESAM). Laser pulses with 2.2 µJ energy and 32.8 ns durations were recorded from a-cut Nd:LGSB. The average output power reached 0.36 W at 1.55 W absorbed pump power. Passive mode-locking with SESAM was achieved in a long Z-type resonator. Ultrashort pulses with 0.19 W average power, 1.63 nJ energy, and 1.43 ps pulse duration, at 118 MHz repetition rate, are demonstrated for the a-cut Nd:LGSB medium.

Numerical investigation of GHz repetition rate fundamentally mode-locked all-fiber lasers.

GHz repetition rate fundamentally mode-locked lasers have attracted great interest for a variety of scientific and practical applications. A passively mode-locked laser in all-fiber format has the advantages of high stability, maintenance-free operation, super compactness, and reliability. In this paper, we present numerical investigation on passive mode-locking of all-fiber lasers operating at repetition rates of 1-20 GHz. Our calculations show that the reflectivity of the output coupler, the small signal gain of the doped fiber, the total net cavity dispersion, and the modulation depth of the saturable absorber are the key parameters for producing stable fundamentally mode-locked pulses at GHz repetition rates in very short all-fiber linear cavities. The instabilities of GHz repetition rate fundamentally mode-locked all-fiber lasers with different parameters were calculated and analyzed. Compared to a regular MHz repetition rate mode-locked all-fiber laser, the pump power range for the mode-locking of a GHz repetition rate all-fiber laser is much larger due to the several orders of magnitude lower accumulated nonlinearity in the fiber cavity. The presented numerical study provides valuable guidance for the design and development of highly stable mode-locked all-fiber lasers operating at GHz repetition rates.

Fe3O4 nanoparticles as the saturable absorber for a mode-locked fiber laser at 1558 nm

We demonstrated a mode-locked erbium doped fiber laser (EDFL) operating at 1558 nm by exploiting ferroferric-oxide nanoparticles (FONPs) as the saturable absorber (SA) material. FONPs, with an average diameter of ~20 nm, were prepared by a thermal decomposition method, then mixed with sodium carboxymethylcelluose to form an FONP film. The FONP SA was fabricated through a sandwiched FONP film between two fiber connectors. By inserting the FONPs SA into an EDFL cavity pumped by a 980 nm laser diode, stable passive mode-locking was achieved with a threshold pump power of ~120 mW. The obtained mode-locked laser had a central wavelength of ~1558 nm, a spectral bandwidth of ~0.8 nm, a pulse width of ~4.35 ns, and a repetition rate of ~37.32 MHz. Furthermore, a maximum average power of ~17 mW was obtained for a pump power of 340 mW. To the best of our knowledge, this is the first time that it has been demonstrated that FONPs can be used for constructing mode-locked fiber lasers.

Lutetium (III) oxide film as passive mode locker device for erbium-doped fibre laser cavity

We reported a newly fabricated saturable absorber (SA) namely Lutetium oxide (Lu2O3) thin film for generating a stable and compact mode-locked Erbium-doped fibre laser (EDFL) operating at 1564 nm. The film SA was synthesized by embedding Lu2O3 into polyvinyl alcohol (PVA) host film. By incorporating the film SA along with 200 m long single mode fibre (SMF) into an EDFL cavity, the mode-locked laser with 0.97 MHz repetition rate stably emerged at an increasing pump power, ranging within 145-187mW. The pulse width was measured to be 2.12 ps. The highest pulse energy and peak power were measured as 7.64 nJ and 3.61 kW respectively and were obtained at 187 mW pump power. The fundamental frequency was examined to have a signal to noise ratio (SNR) of 61 dB. These laser performances suggest that the Lu2O3 film SA is feasible for becoming an alternative passive SA in the 1.55μm region.

Single and dual-wavelength noise-like pulses with different shapes in a double-clad Er/Yb fiber laser.

We report an experimental study of passive harmonic mode-locking in an all-fiber switchable dual-wavelength Er/Yb double-clad laser. The proposed scheme supports single- and dual-wavelength operation of mode-locked pulses with rectangular, h-like and trapezoidal shapes in a noise-like pulse regime. Single-wavelength emissions at λ1 = 1545.1 and λ2 = 1563.6 nm were obtained for pump power values of 9.42 and 6.31 W, achieving pulse durations of up to 18 and 11.8 ns, respectively. At an intermediary pump power of 7.5 W, dual-wavelength emission is obtained and pulses of around 3.59 ns are generated. Additionally, the transition dynamics until 4th-order harmonic mode-locking is also observed. Different laser operation regimes of fundamental and different orders of harmonic mode-locking, with rectangular, h-shaped or trapezoidal shaped pulses are obtained with the same laser configuration with simple and well-defined plates and pump power adjustments.

Passively Q-switched and mode-locked ytterbium fiber laser with Bi2S3 nanowire

In this paper, we report a passively Q-switched and mode-locked ytterbium fiber laser employing Bi2S3 nanowire as a saturable absorber (SA). The Q-switching and mode-locking can be achieved in the same cavity at the same pump power through carefully rotating the intra-cavity polarization controller. In the Q-switching experiment, the maximum output pulse energy is 157.2 nJ with a repetition rate of 22.26 kHz at pump power of 116.7 mW, while for passive mode-locking, the pulse train has a repetition rate of 3.94 MHz, pulse width of 782 ps and SNR ~49 dB. The laser wavelength of Q-switching and mode-locking are both centered at ~1038 nm, with 3 dB optical spectrum width of 0.3 nm and 0.2 nm, respectively. We show that Bi2S3 nanowire can be an efficient SA for an all-normal dispersion ultrafast ytterbium fiber laser.

Study of dielectric coatings for broadband operation of surface-emitting semiconductor lasers

We investigate antireflection (AR) coating materials for two different operation lasing regimes requiring broad spectral bandwidth. We characterize high-power continuous-wave (CW) wavelength-tunable vertical-external-cavity surface-emitting semiconductor lasers and their passive mode-locking capabilities when using semiconductor saturable absorber mirrors. One laser gain design was investigated with different single dielectric layers as AR coatings. The dielectric coating materials used were SiO2, Al2O3, Ta2O5, and TiO2. The AR coating was designed to reduce pump reflection and increase the confinement factor of the microcavity. Average power of 4.6 W in CW and a total wavelength tuning range of 42 nm has been observed with the SiO2-coated structure. The shortest pulse of 708 fs was also observed for the SiO2-coated structure, with a corresponding CW wavelength tuning range of 36 nm.

Spectral compression in ring similariton fiber laser

We present a theoretical model of a ring similariton fiber laser employing nonlinear spectrum compression. A pair of tunnel-coupled optical fibers is used for passive mode-locking. The laser characteristics are simulated for a wide range of laser operation regimes and configuration parameters. The optimized laser system design enables generation of pulses with the maximal efficiency and spectral density providing the lowest losses of energy inside the cavity.

Sub-40-fs high-power Yb:CALYO laser pumped by single-mode fiber laser

We report on the study of single-mode fiber-laser-pumped mode-locked Yb:CALYO lasers via using a passive saturable absorber and Kerr-lens mode-locking technique, respectively. Up to 3.1-W average power with 103-fs pulse duration is obtained from the passive mode-locking, and down to 36-fs pulse duration with more than 2-W average power is achieved by the pure Kerr-lens mode-locking, which is to the best of our knowledge, the highest average power from a reported sub-40-fs Yb-based solid-state oscillator.

Repetition rate transitions and timing stability improvement in monolithic multi-section semiconductor lasers

We study numerically passive mode-locking(PML) dynamics in a 4mm long multi-section semiconductor laser with interdigitated absorber placement. Using the simulation tool FreeTWM, we identify repetition rate (RR) transitions from fundamental to higher harmonic PML by gain current control. The calculated timing jitter (TJ) varies from 17fs to 5000fs with the lowest TJ observed at the third harmonic RR. The simulations are confirmed by experimental investigations of a multi-section quantum-dot laser having the same geometry as in the simulations. RR transitions from 10GHz to 30GHz and TJ between 3fs and 1000fs are observed

PMMA Sandwiched Bi2Te3 Layer as a Saturable Absorber in Mode-Locked Fiber Laser

In this paper, we fabricated a PMMA sandwiched Bi2Te3 self-assembly layer as a saturable absorber device, which was used as a passive mode locker for ultrafast pulse generation at the telecommunication band. Nanosheets of Bi2Te3 as a bulk topological insulator were successfully synthesized through a solvothermal treatment and self-assemble method to form a thin film at a water-air interface. In order to transfer the Bi2Te3 self-assembly layer to the optical fiber end, we design a construction of two PMMA layers sandwiched self-assembly layer. By incorporating this saturable absorber into an erbium-doped fiber laser, femtosecond mode-locking operation was experimentally demonstrated. The output pulse width is about 505 fs. Our results indicate that PMMA sandwiched topological insulator layer structure could be an improvement technology in traditional PMMA transfer method and could be used as a long-term stable saturable absorber for the passively mode locking lasers.

Passive harmonic mode-locking in an erbium-doped fibre laser

We report an all-fibre erbium laser operating in the regime of passive harmonic mode-locking based on the effect of nonlinear polarisation rotation. The generation of the 67th harmonic with a pulse repetition rate of 5.62 GHz and a duration of 1.8 ps is demonstrated. It is shown that as the harmonic number increases, both the frequency modulation parameters and the pulse duration change.

Emission Regimes of 1.06 μm Spectral Bandwidth Two-Sectional Lasers with Quantum Dot Based Active Layer

We 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.

Theoretical and Experimental Investigation of Unidirectionality in an Integrated Semiconductor Ring Mode-Locked Laser With Two Saturable Absorbers

The lack of integrated optical isolators in standard integrated semiconductor photonics platforms leaves integrated lasers vulnerable to back reflections. A hypothesis is that by making a ring laser operate unidirectionally, external reflections will couple back into the non-lasing direction, and the sensitivity toward reflections will be reduced. In this paper, unidirectionality is pursued by placing two saturable absorbers asymmetrically in a ring mode-locked laser. In the case of passive mode-locking, and for internal reflections below −75 dB, simulations, based on the slowly varying envelope approximation, show extinction ratios of up to 37 dB between the two propagation directions. For larger reflections, the extinction ratio degrades linearly with reflection magnitude. The extinction ratio is shown to increase when the reverse bias of the saturable absorbers is modulated actively with a phase difference. Our simulations show that unidirectional operation reduces the amplitude noise in the frequency range from 1 MHz to 10 GHz from around 0.022% to 0.004% when compared to bidirectional operation, whereas the timing jitter of the pulse train increases from around 172 to 412 fs. Experiments show that the extinction ratio of a realization of this mode-locked laser design is limited to around 3 dB. Simulations suggest that this is due to internal butt-joint reflections in the laser cavity.