SnSe2 Nanosheets for Subpicosecond Harmonic Mode-Locked Pulse Generation.

Tin diselenide (SnSe2 ) nanosheets as novel 2D layered materials have excellent optical properties with many promising application prospects, such as photoelectric detectors, nonlinear optics, infrared photoelectric devices, and ultrafast photonics. Among them, ultrafast photonics has attracted much attention due to its enormous advantages; for instance, extremely fast pulse, strong peak power, and narrow bandwidth. In this work, SnSe2 nanosheets are fabricated by using solvothermal treatment, and the characteristics of SnSe2 are systemically investigated. In addition, the solution of SnSe2 nanosheets is successfully prepared as a fiber-based saturable absorber by utilizing the evanescent field effect, which can bear a high pump power. 31st-order subpicosecond harmonic mode locking is generated in an Er-doped fiber laser, corresponding to the maximum repetition rate of 257.3 MHz and pulse duration of 887 fs. The results show that SnSe2 can be used as an excellent nonlinear photonic device in many fields, such as frequency comb, lasers, photodetectors, etc.

Low-Temperature PECVD Growth of Germanium for Mode-Locking of Er-Doped Fiber Laser.

A low-temperature plasma-enhanced chemical vapor deposition grown germanium (Ge) thin-film is employed as a nonlinear saturable absorber (SA). This Ge SA can passively mode-lock the erbium-doped fiber laser (EDFL) for soliton generation at a central wavelength of 1600 nm. The lift-off and transfer of the Ge film synthesized upon the SiO2/Si substrate are performed by buffered oxide etching and direct imprinting. The Ge film with a thickness of 200 nm exhibits its Raman peak at 297 cm-1, which both the nanocrystalline and polycrystalline Ge phases contribute to. In addition, the Ge thin-film is somewhat oxidized but still provides two primary crystal phases at the (111) and (311) orientations with corresponding diffraction ring radii of 0.317 and 0.173 nm, respectively. The nanocrystalline structure at (111) orientation with a corresponding d-spacing of 0.319 nm is also observed. The linear and nonlinear transmittances of the Ge thin-film are measured to show its self-amplitude modulation coefficient of 0.016. This is better than nano-scale charcoal and carbon-black SA particles for initiating the mode-locking at the first stage. After the Ge-based saturable absorber into the L-band EDFL system without using any polarized components, the narrowest pulsewidth and broadest linewidth of the soliton pulse are determined as 654.4 fs and 4.2 nm, respectively, with a corresponding time-bandwidth product of 0.32 under high pumping conditions.

Efficient Saturable Absorber Based on Ferromagnetic Insulator Cr2Ge2Te6 in Er-Doped Mode-Locked Fiber Laser.

A ferromagnetic insulator Cr2Ge2Te6 as a saturable absorber in an Er-doped fiber laser (EDFL) was demonstrated. In this work, a CGT-PVA composite film was successfully fabricated using the liquid-phase exfoliation method and employed in an EDFL. The modulation depth and saturation intensity of the SA are 4.26% and 89.40 MW/cm2, respectively. Stable pulses with a minimum pulse width of 978.5 fs when the repetition rate was 3.25 MHz were recorded experimentally. Furthermore, stable solitons still need to be obtained when the pulse energy in the cavity is as high as 11.6 nJ. The results fully suggest that CGT has outstanding nonlinear absorption properties, which may have broad potential applications in ultrafast photons.

Simultaneous second harmonic generation of multiple wavelength laser outputs for medical sensing.

Multiple wavelength light sources in the medical spectral window region are useful for various medical sensing applications in tissue by distinguishing the absorption and scattering coefficients optically. We propose a simultaneous second harmonic generation of multiple wavelength fiber laser output using parallel channels of periodically-poled lithium niobate (PPLN) waveguides. High intensity dual wavelength lasing output is experimentally realized with two tunable fiber Bragg gratings of 1,547.20 nm and 1,554.48 nm for the efficient conversion to the half wavelengths, 773.60 nm and 777.24 nm, by using two parallel PPLN channels. Compared with a conventional dual second harmonic generation (SHG) configuration based on two different input wavelengths from each independent light source, this method has a relatively higher efficiency to align the input light beam into the adjacent parallel PPLN channels simultaneously. The use of fiber lasers offers several advantages since they are relatively inexpensive, provide high power in excess of tens of watts, are widely tunable, and can produce pulses from milliseconds to femtoseconds.

Versatile Mode-Locked Operations in an Er-Doped Fiber Laser with a Film-Type Indium Tin Oxide Saturable Absorber.

We demonstrate the generation of versatile mode-locked operations in an Er-doped fiber laser with an indium tin oxide (ITO) saturable absorber (SA). As an epsilon-near-zero material, ITO has been only used to fashion a mode-locked fiber laser as an ITO nanoparticle-polyvinyl alcohol SA. However, this type of SA cannot work at high power or ensure that the SA materials can be transmitted by the light. Thus, we covered the end face of a fiber with a uniform ITO film using the radio frequency magnetron sputtering technology to fabricate a novel ITO SA. Using this new type of SA, single-wavelength pulses, dual-wavelength pulses, and triple-wavelength multi-pulses were achieved easily. The pulse durations of these mode-locked operations were 1.67, 6.91, and 1 ns, respectively. At the dual-wavelength mode-locked state, the fiber laser could achieve an output power of 2.91 mW and a pulse energy of 1.48 nJ. This study reveals that such a proposed film-type ITO SA has excellent nonlinear absorption properties, which can promote the application of ITO film for ultrafast photonics.

Nonlinear Optical Properties of Zirconium Diselenide and Its Ultra-Fast Modulator Application.

Recently, two-dimensional (2D) materials have been widely studied by researchers due to their exceptional 2D structure and excellent optical characteristics. As one of the typically-layered 2D transition metal dichalcogenide (TMD) semiconductors from group IVB with a bandgap value of 0.9-1.2 eV (bulk to monolayer), the characteristics of zirconium diselenide (ZrSe2) have already been extensively investigated in many fields. However, the nonlinear absorption properties of ZrSe2 in ultra-fast lasers have not been previously demonstrated. In this work, we measured various parameters in order to investigate the characteristics of the nonlinear saturable absorption of ZrSe2. A ZrSe2-polyvinyl alcohol (PVA) film was successfully prepared, which was employed as a saturable absorber (SA) to demonstrate, for the first time, an erbium (Er)-doped passive mode-locking fiber laser with a ring cavity. The saturation intensity of the ZrSe2-PVA film-type SA is 12.72 MW/cm2, while its modulation depth is 2.3%. The stable soliton state with a maximum output power of 11.37 mW and a narrowest monopulse duration of 12.5 ps at a repetition frequency of 21.22 MHz was detected. The experimental results conclusively proved that ZrSe2, with its suitable bandgap value and excellent nonlinear absorption properties, as well as its high damage threshold, should have extensive potential applications within the field of ultra-fast pulse lasers.