Fiber Bragg gratings fabricated in fibers with different geometries by femtosecond laser written through the coating and their applications in strain sensing and fiber laser.

Applications of the type-I fiber Bragg gratings (FBGs) written through the coating (TTC) in strain sensing and tunable distributed Bragg reflector (DBR) fiber lasers were demonstrated. We reported the principle of selecting the distance between the fiber and the phase mask when writing type-I TTC FBGs. Type-I TTC FBGs written in commercially available acrylate-coated fibers with various geometries and their strain responses were demonstrated. Results showed that the strain sensitivity of FBGs increases as the core-diameter decreases, probably due to the waveguide effect. In addition, a continuously tunable DBR fiber laser based on TTC FBGs was achieved with a wavelength tuning range of 19.934 nm around 1080 nm, by applying a strain of 0-21265.8 µÉ› to the laser resonant cavity. The wavelength tuning range was limited by the splice point between the gain fiber and the passive fiber for transmitting pump and signal lasers. When the pump power was 100 mW, the relative intensity noises were -97.334 dB/Hz at the relaxation oscillation peak of 880 kHz and -128 dB/Hz at frequencies greater than 3 MHz. The results open a potential scheme to design and implement continuously tunable fiber lasers and fiber laser sensors for strain sensing with a higher resolution.

Linewidth compression of a single longitudinal mode ytterbium-doped fiber laser based on femtosecond laser fabricated fiber Bragg gratings.

We demonstrate a single longitudinal mode distributed Bragg reflection (DBR) fiber laser by directly fabricating fiber Bragg gratings (FBGs) on an ytterbium-doped fiber (YDF) using a femtosecond laser. A simple optical self-injection feedback method was used to effectively compress the linewidth and reduce relative intensity noise (RIN) of a single longitudinal mode DBR fiber laser. Further, we investigated the effect of self-injection feedback cavity length and reflectivity on linewidth compression and determined that the linewidth tends to decrease with the increase of the external cavity photon lifetime. By a self-injection feedback, the laser linewidth was compressed from 31.8 kHz to 1.4 kHz. Meanwhile, the relaxation oscillation peak from -103.2d B/H z at 1.51 MHz was suppressed to -122.3d B/H z at 0.16 MHz. This low-noise narrow linewidth single longitudinal mode fiber laser is expected to be a promising candidate for applications such as active detection of neutral atmosphere and distributed fiber sensing.

Ultra-high-temperature sensing using fiber grating sensor and demodulation method based on support vector regression optimized by a genetic algorithm.

We propose an ultra-high-temperature sensing method using a fiber Bragg grating (FBG) and demodulation technique based on support vector regression optimized by a genetic algorithm (GA-SVR). A type-I FBG inscribed by a femtosecond laser in a silica fiber was packaged with a tube and used as a temperature sensor. The external ambient temperature was retrieved from the transient FBG wavelength and its increase rate in reaching thermal equilibrium of the FBG with the external environment using GA-SVR. The temperature sensing in the range of 400 to 1000 °C was realized with an accuracy of 4.8 °C. The highest sensing temperature exceeded the FBG resisting temperature of 700 °C. The demodulation time was decreased to approximately 15 s, only 3.14% of the FBG sensor time constant. The proposed method could realize the external ambient temperature determination before the FBG temperature reached the thermal equilibrium state, which enables to attain a demodulation time shorter than the time constant of the FBG sensor and a sensing temperature higher than the FBG resisting temperature. This method could be potentially applied in temperature inspection of combustion and other fields.

Optical properties and applications of molybdenum disulfide/SiO2 saturable absorber fabricated by sol-gel technique.

We investigate a new type of molybdenum disulfide (MoS2)-doped sol-gel glass saturable absorber (SA) fabricated by sol-gel technique. The reagents used for the sol-gel glass contain Tetraethyl orthosilicate (TEOS), ethanol, water, and hydrochloric acid. Different from the traditional ways of fabricating SAs, the MoS2 in our method is encapsulated by inorganic sol-gel glass instead of polymer compound with low laser damage resistance, which greatly increases the optical damage threshold of MoS2 SA. The MoS2-doped sol-gel glass as an SA is experimentally demonstrated in a passively mode-locked ytterbium-doped fiber laser (YDFL). Stable mode-locked pulse trains are successfully generated in the normal dispersion regime with a pulse width of 13.8 ps and the average output power of 34.6 mW. The fluctuation of the central wavelength and spectral bandwidth is as low as 0.9% in one week, which indicates that the mode-locking state has good environmental stability. To the best of our knowledge, it is the first example of sol-gel glass SA for ultrafast pulses generated in YDFL, which potentially gives a new approach to improve optical damage threshold and long-term working stability for broadband absorbers.

Er-doped Q-switched fiber laser with a black phosphorus/polymethyl methacrylate saturable absorber.

A stable Q-switched Er-doped fiber (EDF) laser is successfully obtained by using a black phosphorus (BP)/polymethyl methacrylate (PMMA) film as the saturable absorber (SA). To avoid the oxidization of nanomaterials, the BP nanoparticles are fabricated via a liquid-phase exfoliation method and then embedded into a PMMA film that possesses excellent optical transparency in the selected spectrum range. The modulation depth (MD) of the BP/PMMA film SA is 14.3% and the saturable intensity (Isat) is 6.9 MW/cm2. By inserting the BP/PMMA film into the EDF laser cavity, we achieve the stable passive Q-switching operation over the wavelength range from 1561.21 nm to 1564.16 nm. The repetition rate increases from 10.348 kHz to 30.098 kHz, and the pulse duration decreases from 25.01 µs to 2.98 µs by altering the pump power from 9 mW to 90 mW. The maximum single pulse energy is 283.91 nJ. To the best of our knowledge, 283.91 nJ is the largest single pulse energy among the Q-switched fiber lasers with BP as the Q-switcher at the 1.5 µm wavelength region. The experimental results evidently show that the BP/PMMA film SA can work as a promising Q-switcher for large pulse energy fiber lasers.

Molybdenum Carbide Buried in D-Shaped Fibers as a Novel Saturable Absorber Device for Ultrafast Photonics Applications.

Study of nonlinear laser-matter interactions in 2D materials has promoted development of photonics applications. As a typical MXene material, molybdenum carbide (Mo2C) has attracted much attention because of its graphene-like structure. Here, a type of D-shaped fiber (DF)-buried Mo2C saturable absorber (SA) fabricated by magnetron-sputtering deposition (MSD) and sol-gel technique is reported. The Mo2C material was buried between the bottom DF and the upper amorphous silica fabricated by sol-gel technology. Therefore, the DF-based SA effectively solves the problem of material shedding and aging, thus improving the stability and damage threshold of the fiber laser. Application of the SA in erbium-doped fiber laser and stable passive Q-switched operation with a maximum pulse energy of 430.47 nJ is realized. By adjusting the polarization state and pump power, high-power mode-locked pulses are generated with a pulse duration and output power of 199 fs and 54.13 mW, respectively. Further, bound-state soliton pulses are obtained with a pulse width of 312 fs and soliton interval of 1.26 ps for the first time based on MXene materials. Moreover, by application of the SA in ytterbium-doped fiber lasers, a stable dissipative soliton mode-locked pulse is obtained with a pulse width of 23 ps. These results indicate that the DF-based buried Mo2C as a novel SA provides a reliable method for all-fiber and multifunctional high-power ultrafast laser.

Passively Mode-Locked Fiber Laser with WS2/SiO2 Saturable Absorber Fabricated by Sol-Gel Technique.

High-performance ultrafast fiber lasers require saturable absorbers (SAs) of high optical damage threshold and high operation stability. Here, the optical properties and application of the WS2/SiO2 SA prepared by the sol-gel method are reported. SiO2 prepared by sol-gel technique has similar properties to fiber in ultrafast fiber lasers, such as mechanical strength, refractive index, optical transmission, and absorption. For the SA device by the sol-gel method combined with WS2 material, not only will the additional scattering loss not be introduced, but also, the damage threshold of the SA device can be effectively increased. Furthermore, SA material is wrapped by SiO2, which insulates the influence of the external environment. Based on the first preparation of the WS2/SiO2 glass SA, stable soliton pulses are obtained in ytterbium-doped fiber lasers (YDFLs) with a pulse width of 58 ps, an average output power of 56.8 mW, and a repetition rate of 19.03 MHz. In addition, a stable mode-locked operation with a pulse width of 325 fs and an output power of 39.6 mW is also achieved in an erbium-doped fiber laser (EDFL). These results demonstrate that the WS2/SiO2 glass prepared by the sol-gel method can significantly increase laser output power and shorten pulse width in the fiber laser, which provides a new opportunity for the traditional preparation method of the SA device.

Mode-Locked Er-Doped Fiber Laser by Using MoS2/SiO2 Saturable Absorber.

The two-dimensional (2D) layered material MoS2 has attracted numerous attentions for electronics and optoelectronics applications. In this work, a novel type of MoS2-doped sol-gel glass composite material is prepared. The nonlinear optical properties of prepared MoS2/SiO2 composite material are measured with modulation depth (ΔT) of 3.5% and saturable intensity (Isat) of 20.15 MW/cm2. The optical damage threshold is 3.46 J/cm2. Using the MoS2/SiO2 composite material as saturable absorber (SA), a passive mode-locked Er-doped fiber (EDF) laser is realized. Stable conventional soliton mode-locking pulses are successfully generated with a pulse width of 780 fs at the pump power of 90 mW. In the pump power range of 100-600 mW, another stable mode-locking operation is obtained. The pulse width is 1.21 ps and the maximum output power is 5.11 mW. The results indicate that MoS2/SiO2 composite materials could offer a new way for optical applications.

Optical Nonlinearity of ZrS2 and Applications in Fiber Laser.

Group VIB transition metal dichalcogenides (TMDs) have been successfully demonstrated as saturable absorbers (SAs) for pulsed fiber lasers. For the group comprising IVB TMDs, applications in this field remain unexplored. In this work, ZrS2-based SA is prepared by depositing a ZrS2 nanostructured film onto the side surface of a D-shaped fiber. The nonlinear optical properties of the prepared SA are investigated, which had a modulation depth of 3.3% and a saturable intensity of 13.26 MW/cm². In a pump power range of 144⁻479 mW, the Er-doped fiber (EDF) laser with ZrS2 can operate in the dual-wavelength Q-switching state. The pulse duration declined from 10.0 µs down to 2.3 µs. The single pulse energy reached 53.0 nJ. The usage of ZrS2 as a SA for pulse generation in fiber lasers is presented for the first time. Compared to the experimental results of dual-wavelength Q-switched fiber lasers with two-dimensional (2D) materials, our laser performance was better. Our work indicates that the group comprising IVB TMD ZrS2 has bright prospects for nonlinear optical applications.