Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXene's potential in laser technology is not explored. It is demonstrated here that Ti3 CN, one of MXene compounds, can serve as an excellent mode-locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti3 CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode-locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q-switched lasers using Ti3 CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial-based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers.

By coupling few-layer Molybdenum Disulfide (MoS2) with fiber-taper evanescent light field, a new type of MoS2 based nonlinear optical modulating element had been successfully fabricated as a two-dimensional layered saturable absorber with strong light-matter interaction. This MoS2-taper-fiber device is not only capable of passively mode-locking an all-normal-dispersion ytterbium-doped fiber laser and enduring high power laser excitation (up to 1 W), but also functions as a polarization sensitive optical modulating component (that is, different polarized light can induce different nonlinear optical response). Thanks to the combined advantages from the strong nonlinear optical response in MoS2 together with the sufficiently-long-range interaction between light and MoS2, this device allows for the generation of high power stable dissipative solitons at 1042.6 nm with pulse duration of 656 ps and a repetition rate of 6.74 MHz at a pump power of 210 mW. Our work may also constitute the first example of MoS2-enabled wave-guiding photonic device, and potential y give some new insights into two-dimensional layered materials related photonics.

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Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction

We report on the generation of a femtosecond pulse in a fiber ring laser by using a polyvinyl alcohol (PVA)-based molybdenum disulfide (MoS2) saturable absorber (SA). With a saturable optical intensity of 34  MW/cm2 and a modulation depth of ∼4.3%, the PVA-based MoS2 SA had been employed with an erbium-doped fiber ring laser as a mode locker. The mode-locking operation could be achieved at a low pump threshold of 22 mW. A ∼710  fs pulse centered at 1569.5 nm wavelength with a repetition rate of 12.09 MHz had been achieved with proper cavity dispersion. With the variation of net cavity dispersion, output pulses with durations from 0.71 to 1.46 ps were obtained. The achievement of a femtosecond pulse at 1.55 μm waveband demonstrates the broadband saturable absorption of MoS2, and also indicates that the filmy PVA-based MoS2 SA is indeed a good candidate for an ultrafast saturable absorption device.

Femtosecond pulse erbium-doped fiber laser by a few-layer MoS 2 saturable absorber

We propose and demonstrate 1, 1.5, and 2 μm passively Q-switched fiber lasers by exploiting a few-layer Molybdenum sulfide (MoS2) polymer composite as broadband saturable absorber (SA), respectively. The few-layer MoS2 nanosheets are prepared by the liquid-phase exfoliation method, and are composited with polyvinyl alcohol (PVA). The PVA-MoS2 film is sandwiched between two fiber ferrules to form the fiber-compatible SA. The few-layer MoS2 not only shows good transparency from ultraviolet to mid-infrared spectral region, but also possesses the nonlinear saturable absorption. The modulation depth and saturation optical intensity of the PVA-MoS2 film are measured to be 1.6% and 13 MW/cm2 at 1566 nm by the balanced twin-detector technique, respectively. By further inserting the filmy PVA-MoS2 SA into the cavities of Yb-, Er- and Tm-doped fiber lasers, we achieve stable Q-switching operations at 1.06, 1.56, and 2.03 μm, respectively. The output characteristics of the Q-switched pulses at the three wavelengths have been investigated, respectively. The MoS2-based Q-switching enables the large pulse energy of ∼1 μJ with a pulse width of 1.76 μs. This is, to the best of our knowledge, the first demonstration of MoS2-based Q-switched fiber lasers in a wide wavelength range (from 1 to 2 μm). Our results experimentally confirm that the new-type 2-D material, few-layer MoS2, is a promising broadband SA to Q-switch fiber lasers covering all major wavelengths from near- to mid-infrared region.

1-, 1.5-, and 2-μm Fiber Lasers Q-Switched by a Broadband Few-Layer MoS 2 Saturable Absorber

We reported on the generation of femtosecond pulse in a fiber ring laser by using a polyvinyl alcohol (PVA)-based topological insulator (TI), Bi2Se3 saturable absorber (SA). The PVA-TI composite has a low saturable optical intensity of 12 MW/cm2 and a modulation depth of ~3.9%. By incorporating the fabricated PVA-TISA into a fiber laser, mode-locking operation could be achieved at a low pump threshold of 25 mW. After an optimization of the cavity parameters, optical pulse with ~660 fs centered at 1557.5 nm wavelength had been generated. The experimental results demonstrate that the PVA could be an excellent host material for fabricating high-performance TISA, and also indicate that the filmy PVA-TISA is indeed a good candidate for ultrafast saturable absorption device.

Femtosecond pulse generation from a topological insulator mode-locked fiber laser

Dual-wavelength passively harmonic mode locking (HML) operation is demonstrated in an erbium-doped fiber laser with a microfiber-based topological insulator (TI) saturable absorber. It was found that the dual-wavelength pulse-trains possess different HML orders due to the different cavity nonlinear effects experienced by the two wavelengths. By properly adjusting the cavity parameters, dual-wavelength HML pulses with repetition rates of 388 and 239 MHz were achieved. Moreover, wavelength switchable operation of dual-wavelength HML pulses was also obtained. The experimental results reveal that the microfiber-based TI could indeed be employed as a high-performance dual-function photonic device with the saturable absorption and high nonlinear effects for the applications in fields of ultrafast and nonlinear photonics.

Dual-Wavelength Harmonically Mode-Locked Fiber Laser With Topological Insulator Saturable Absorber

We reported on the generation of dual-wavelength rectangular pulses in a Yb-doped fiber laser (YDFL) by using a microfiber-based graphene saturable absorber (GSA). The duration of dual-wavelength rectangular pulse could be varied from 1.41 ns to 4.23 ns with the increasing pump power. With a tunable bandpass filter, it was found that the characteristics of the rectangular pulses centered at 1061.8 nm and 1068.8 nm are similar to each other. Moreover, the dual-wavelength switchable operation was also realized by properly rotating the polarization controllers (PCs). The demonstration of the dual-wavelength rectangular pulses from a YDFL would open some applications for fields such as spectroscopy, biomedicine and sensing research.

Dual-wavelength rectangular pulse Yb-doped fiber laser using a microfiber-based graphene saturable absorber.

We report on the generation of a high-energy noiselike rectangular pulse in a mode-locked figure-eight fiber laser. The noiselike pulse appeared to have a rectangular shape on the oscilloscope. The pulse duration increased with increasing pump power, while the peak amplitude remained constant, which is very similar to the pulse evolution of dissipative soliton resonance. However, the pulse type is confirmed as a noiselike pulse using an autocorrelator. With the maximum pump power of 350 mW, the 135 nJ noiselike rectangular pulse with 76 ns duration was achieved. The results provide a new guideline for clarifying an alternative formation mechanism of the high-energy rectangular pulses in fiber lasers.

https://iopscience.iop.org/article/10.7567/APEX.7.042701/pdf

High-energy noiselike rectangular pulse in a passively mode-locked figure-eight fiber laser

We report on the trapping of noise-like pulse in a figure-eight fiber laser mode locked by nonlinear amplifier loop mirror (NALM). After achievement of noise-like vector pulse, it was found that the wavelength shift of the two resolved polarization components responsible for the pulse trapping was very sensitive to the cavity birefringence. By properly rotating the polarization controllers (PCs), the wavelength shift could be up to 4.8 nm, which is much larger than that of conventional soliton trapping. The observed results would shed some light on the fundamental physics of noise-like pulse as well as its vector features in fiber lasers.

Nian Zhao

Papers

Femtosecond pulse generation from a topological insulator mode-locked fiber laser

Dual-Wavelength Harmonically Mode-Locked Fiber Laser With Topological Insulator Saturable Absorber

Dual-wavelength rectangular pulse Yb-doped fiber laser using a microfiber-based graphene saturable absorber.

High-energy noiselike rectangular pulse in a passively mode-locked figure-eight fiber laser

Noise-like pulse trapping in a figure-eight fiber laser