Single-frequency fiber lasers based on metal thiophosphites with sub-kHz linewidths.

Metal thiophosphites (CrPX4) (X = S and Se), emerging layered metal chalcogenophosphate materials, have found extensive utility in the fields of advanced electronic devices, magnetic technologies, and electrochemical systems. However, their applications in single-frequency fiber lasers have been relatively limited. Here, liquid phase exfoliation technology is used to exfoliate the CrPX4 into few-layered samples. Subsequently, we explored the application of CrPX4-based saturable absorbers (CrPX4-SA) by precisely depositing CrPX4 nanosheets onto the surface of D-shaped fibers. This led to the successful creation of two stable single-frequency fiber lasers (SFFLs) based on CrPS4 and CrPSe4 with center wavelengths of 1550.068 nm and 1550.092 nm, respectively. The linewidth of CrPSe4 was tested and determined to be roughly 375 Hz, which is the narrowest ever value attained for an SFFL based on its structure, and its signal-to-noise ratio (SNR) was 74.17 dB. The maximum output power of each oscillator was 0.796 mW and 0.963 mW, and their corresponding power fluctuations were 2.5% and 1.4%, respectively.

2D Semi-Metallic Hafnium Ditelluride: A Novel Nonlinear Optical Material for Ultrafast and Ultranarrow Photonics Applications.

2D semi-metallic hafnium ditelluride material is used in several applications such as solar steam generation, gas sensing, and catalysis owing to its strong near-infrared absorbance, high sensitivity, and distinctive electronic structure. The zero-bandgap characteristics, along with the thermal and dynamic stability of 2D-HfTe2, make it a desirable choice for developing long-wavelength-range photonics devices. Herein, the HfTe2 -nanosheets are prepared using the liquid-phase exfoliation method, and their superior nonlinear optical properties are demonstrated by the obtained modulation depth of 11.9% (800 nm) and 6.35% (1560 nm), respectively. In addition, the observed transition from saturable to reverse saturable absorption indicates adaptability of the prepared material in nonlinear optics. By utilizing a side polished fiber-based HfTe2 -saturable absorber (SA) inside an Er-doped fiber laser cavity, a mode-locked laser with 724 fs pulse width and 56.63 dB signal-to-noise ratio (SNR) is realized for the first time. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser development with improved efficiency and reduced thermal impact. Finally, employing this HfTe2 -SA, a highly stable single-frequency fiber laser (SNR ≈ 74.56 dB; linewidth ≈ 1.268 kHz) is generated for the first time, indicating its promising ultranarrow photonic application.

VTe2: Broadband Saturable Absorber for Passively Q-Switched Lasers in the Near- and Mid-Infrared Regions.

In this study, we demonstrate the fabrication of a novel 2D transition metal dichalcogenide, VTe2, into a saturable absorber (SA) by using the liquid phase exfoliation method. Furthermore, the first-principles calculations were conducted to elucidate the electronic band structures and absorption spectrum. The nonlinear optical absorption properties of VTe2 at 1.0, 2.0, and 3.0 µm were measured using open-aperture Z-scan and P-scan methods, which showed saturation intensities and modulation depths of 95.57 GW/cm2 and 9.24%, 3.11 GW/cm2 and 7.26%, and 15.8 MW/cm2 and 17.1%, respectively. Furthermore, in the realm of practical implementation, the achievement of stable passively Q-switched (PQS) lasers employing SA composed of few-layered VTe2 nanosheets has manifested itself with broadband operating wavelengths from 1.0 to ∼3.0 µm. Specifically, PQS laser operations from near-infrared to mid-infrared with pulse durations of 195 and 563 ns for 1.0 and 2.0 µm solid-state lasers, respectively, and 749 ns for an Er3+-doped fluoride fiber laser at 3.0 µm were obtained. Our experimental results demonstrate that VTe2 is a potential broadband SA device for achieving PQS lasers. To the best of our knowledge, this is the first demonstration of using VTe2 as an SA in PQS lasers in the near- and mid-infrared regions, which highlights the potential of VTe2 for future research and applications in optoelectronic devices.

MXene Core-Shell Nanosheets: Facile Synthesis, Optical Properties, and Versatile Photonics Applications.

In recent years, the transition metal carbonitrides(MXenes) have been widely applied to photoelectric field, and better performance of these applications was achieved via MXene complex structures. In our work, we proposed a MXene core-shell nanosheet composed of a Ti2C (MXene) phase and gold nanoparticles, and applied it to mode-locked and single-frequency fiber laser applications. The optoelectronic results suggested that the performances of these two applications were both improved when MXene core-shell nanosheets were applied. As a result, we obtained a mode-locking operation with 670 fs pulses, and the threshold pump power reached to as low as 20 mW. Besides, a single-frequency laser with the narrowest linewidth of ~1 kHz is also demonstrated experimentally. Our research work proved that MXene core-shell nanosheets could be used as saturable absorbers (SAs) to promote versatile photonic applications.

Sub-kilohertz linewidth fiber laser by using Bragg grating filters.

A single-frequency (SF) fiber laser is widely applied in many fields. A superior ultra-narrow-bandwidth fiber filter, which consists of an in-series commercial common fiber Bragg grating (FBG) filters with a partial overlap in their reflection spectrum, is presented in this paper. The bandwidth of the filter with a center reflection wavelength of 1549.83 nm reaches 0.024 nm at 3 dB reflection, a remarkable reduction of approximately 300% by combination usage narrower than a single FBG used bandwidth. We also demonstrate a method to further narrowing of the filter bandwidth by increasing the usage times of filters, and the bandwidth can reach 0.0025 nm at 3 dB reflection. A long linear cavity single-frequency fiber laser with an entire cavity of 30 cm is implemented with the linewidth of 941 Hz at 3 dB, which is about 210% narrower than the previous record of SF fiber laser based on FBG filters in a linear cavity. This research may open a new door toward ultra-narrow-bandwidth filters and SF fiber lasers.

Zero-Dimensional MXene-Based Optical Devices for Ultrafast and Ultranarrow Photonics Applications.

In recent years, MXene has become a hotspot because of its good conductivity, strong broadband absorption, and tunable band gap. In this contribution, 0D MXene Ti3C2Tx quantum dots are synthesized by a liquid exfoliation method and a wideband nonlinear optical response from 800 to 1550 nm is studied, which have a larger nonlinear absorption coefficient ß of -(11.24 ± 0.14) × 10-2 cm GW-1. The carrier dynamic processes of 0D MXene are explored with ultrahigh time resolution nondegenerate transient absorption (TA) spectroscopy, which indicates that the TA signal reaches its maximum in 1.28 ps. Furthermore, 0D MXene is used to generate ultrashort pulses in erbium or ytterbium-doped fiber laser cavity. High signal-to-noise (72 dB) femtosecond lasers with pulse durations as short as 170 fs with spectrum bandwidth of 14.8 nm are obtained. Finally, an ultranarrow fiber laser based on 0D MXene is also investigated and has a full width at half maximum of only 5 kHz, and the power fluctuation is less than 0.75% of the average power. The experimental works prove that 0D MXene is an excellent SA and has a promising application in ultrafast and ultranarrow photonics.