Bidirectional Oscillating-Amplifying Integrated Fiber Lasers (B-OAIFL) combine the advantages of Oscillating-Amplifying Integrated Fiber Lasers (OAIFL) and bidirectional output fiber lasers, including high efficiency, simple control logic, good anti-back reflection, low cost, and small volume. In this work, we introduced the (2+1)×1 Side Pump Signal Combiner (SPSC) between the Output Coupler Fiber Bragg Grating (OC-FBG) and the amplifying sections to address the temporal instability of the laser at low power levels under the counter pumping. Two groups of wavelength-stabilized 976 nm Laser Diodes (LDs) were directly coupled into the oscillating section to increase the laser power within the cavity, and then, a 60 W-level time-stabilized threshold signed as photodetector-detected no-pulse characteristics was achieved. We compared the output characteristics with two different fiber-coiling structures in the oscillating section (STR1, STR2). STR1 was coiled in a single-track-shaped fiber groove with a diameter ranging from 8.0 cm to 10.7 cm, while the coiling diameters of the two output ports were different. For STR2, an '8'-shaped coiling was adopted with gradually varying diameters from 8.0 cm to 9.5 cm and the same coiling diameter of 8.0 cm at both ports. The Transverse Mode Instability (TMI) threshold under the unidirectional pumping at end A and end B were 2271 W and 2530 W, respectively, with Structure one. With Structure two, the thresholds at end A and end B were 2735 W and 2508 W, respectively. Based on STR2 under the bidirectional pumping with a total pump power of 6997 W, we finally achieved a laser output of 2655 W at the end A and 2491 W at the end B, with a total power of 5146 W and a good beam quality of M2 to approximately 1.32 at end A and M2 to approximately 1.45 at end B, respectively. The Stimulated Raman Scattering (SRS) suppression ratio exceeded 40 dB for both end A and end B.
Oscillating-amplifying integrated fiber laser (OAIFL) can take into account the advantages of fiber laser oscillators and amplifiers, including well anti-back reflected light ability, high conversion efficiency, good nonlinear suppression capability and compact structure. In this work, based on the steady-state rate equations of the fiber laser, combined with the special boundary conditions of OAIFL, the rate equation model of the OAIFL is established. Based on this model, the effects of the pump configuration, the fiber length ratio of the oscillating section and the amplifying section, and the reflectivity of the output coupler fiber Bragg grating on the output characteristics of the OAIFL are theoretically studied. In addition, experiments are carried out to verify the reliability of the pump control logic of the laser under different pump configurations. The results show that OAIFL can maintain a safe working state when using the pump control sequence that inevitably causes laser burnout in conventional fiber laser amplifiers.
A novel Bidirectional output Oscillating-Amplifying Integrated Fiber Laser (B-OAIFL) which combines the advantages of the bidirectional output fiber laser oscillator, and the Oscillating-Amplifying Integrated Fiber Laser (OAIFL) is proposed and studied theoretically. The influences of the reflectivity of Fiber Bragg Grating (FBG), the length ratio, the absorption coefficient and other related parameters on the characteristics of the output laser were studied theoretically. The reflectivity of FBGs and the length of active fiber in the oscillating section show a negative correlation with the output power and efficiency. The 915 nm-pumping and the center wavelength of FBGs in the range from 1070 nm to 1080 nm show a better performance on the suppression of ASE. And the center wavelength of FBGs shows little effect on the output power and efficiency.
With the development of high power fiber oscillators, it is urgent to fabricate fiber Bragg gratings (FBGs) on large core fibers. Here, a pair of FBGs in fibers with core diameter of 30 μm are fabricated based on femtosecond laser phase mask scanning method. The reflectivity of high-reflectivity FBG (HR-FBG) is more than 99% with the central wavelength of 1080 nm and the 3dB bandwidth of 3.6 nm. The reflectivity of output coupler FBG (OC-FBG) is about 10% with the central wavelength of 1079.8 nm and the 3dB bandwidth of 2 nm. Furthermore, an all-fiber oscillator is built based on the fs-written FBGs, and the maximum output power of 6.4 kW is realized with the optical-optical conversion efficiency of 74%. The temperatures of HR-FBG and LR-FBG are 90 and 49℃, respectively. This work demonstrates that the large core diameter FBG written by femtosecond laser has excellent performance, which is of great significance for the development of high power fiber oscillators.
The spindle-shaped fiber is a novel fiber with a gradual longitudinal structure. The small size section and large size section have the advantages of controlling the number of modes and reducing the laser power density respectively. Based on the steady-state rate equations and coupled mode theory of fiber lasers, this paper conducts theoretical research and comparative analysis on SRS and mode evolution in fiber lasers based on conventional fibers and spindle-shaped fibers, respectively. The results show that, compared with conventional fibers with the similar core diameter, the spindle-shaped fiber can effectively balance the mitigation of SRS and TMI in fiber lasers, and has great potential for achieving higher power laser with better beam quality.
In this work, we reported a 2 kW monolithic master oscillator-power amplifier (MOPA) configuration based on a novel constant-cladding tapered-core (CCTC) Yb-doped fiber. The CCTC fiber has a constant cladding diameter of ~400 μm and a varying core diameter along the fiber, with a ~24 μm at both ends and ~31 μm in the middle. This special fiber design can not only effectively suppress the stimulated Raman scattering, but also smooth the thermal load in the fiber. The output performance of this fiber was carefully investigated in a bidirectional pump MOPA configuration with respective co-pump and counter-pump scheme, especially on the aspects of the SRS and TMI. As a result, the TMI threshold is measured at ~870 W and ~1980 W in the co-pump and counter-pump scheme, respectively. The maximum output power of 2023 W is achieved with no sign of SRS in the counter-pump scheme. Before the TMI threshold, the beam quality (M2 factor) remains ~1.4 without any mode distortion, and the M2 factor is measured to be ~1.65 at the maximum output power. These results indicate that the CCTC fiber has great potential to simultaneously mitigate the SRS and TMI effect in the high power operation.
Tapered fiber is expected to effectively suppress nonlinear effects while ensuring excellent beam quality, and may achieve higher power output by breaking through the power limit of ordinary uniform size fiber. Base on the rate equations and the proposed three types of tapered fibers, a nonlinear theoretical model including spectrum, power and stimulated Raman scattering (SRS) is established. Using this model, the SRS characteristics of the three tapered fiber amplifiers are numerically simulated in the condition of keeping same total absorption coefficient of different fibers guaranteed. The simulation results show that the type I fiber has the worst suppression effect on SRS, and the convex type III fiber has the best suppression effect on SRS. When using the backward pumping method, the convex type II fiber can reach almost consistent suppression effect of the convex type III fiber. Theoretical studies show that the use of tapered ytterbium-doped fiber to build fiber amplifiers has a certain suppression effect on SRS in fiber amplifiers, and provides a reference for the selection of fibers in high-power fiber amplifiers.
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