Requirements on double-cladding Yb-doped fiber for power scaling of diffraction-limited fiber amplifiers.

Requirements on the double-cladding Yb-doped fiber (DCYF) for power scaling of diffraction-limited fiber amplifiers are studied in this paper. By considering the limitations of thermal lens and transverse mode instability separately, it is found that the power scaling limit can be enlarged to more-than 100 kW and 80 kW, respectively, with the proper selection of pump and signal wavelengths. It is also found that the requirements on DCYF strongly depend on the wavelength and brightness of pump light. It is predicted that smaller-than 20-µm core diameter is required to achieve the 20-kW output power, as long as the 976-nm pump brightness can be high enough. The requirements on the inner-cladding diameter and cladding-to-core ratio of DCYF are also investigated.

Demonstration of hundred-watt-level near-diffraction-limited monolithic fiber laser near 980 nm with step-index double-cladding Yb-doped fiber.

In this paper, a hundred-watt-level near-diffraction-limited step-index Yb-doped fiber (YDF) laser near 980 nm is demonstrated firstly, to the best of our knowledge. By using the 11.7-W 979-nm single-mode seed light, the in-band amplified spontaneous emission (ASE) is well suppressed and the maximum output power of 101.5 W with the beam quality (M2 factor) of 1.285 was obtained. This work does not only propose an effective method for the suppression of in-band ASE, but also provides a cost-effective solution of hundred-Watt-level near-diffraction-limited fiber lasers near 980 nm.

Demonstration of kilowatt monolithic Yb-doped fiber laser operation near 980 nm.

In this Letter, to the best of our knowledge, the first kilowatt monolithic Yb-doped fiber laser operating near 980 nm is demonstrated. The fiber laser is achieved with the help of an all-fiber amplifier fabricated with a 105/250-µm core/cladding-diameter double-cladding Yb-doped fiber. With 11-W seed light, about 1.11-kW output power was obtained with 65.3% slope efficiency. The center wavelength was around 978 nm with a 10-dB bandwidth of about 0.6 nm. About 34-dB suppression of 1030-nm amplified spontaneous emission was realized, and the output beam quality (M2 factor) was about 16.2 at maximum output power. Better beam quality can be expected by optimizing the seed light and active fiber. This work can provide significant guidance for the study and designation of high-power fiber lasers operating near 980 nm and other three-level fiber sources.

Study on thermal-lens induced mode coupling in step-index large mode area fiber lasers.

In this paper, the thermal-lens induced mode coupling in step-index large mode area fiber laser is systematically investigated. The pertinent mode coupling coefficient is studied firstly, to the best of our knowledge. It is demonstrated that the mode coupling can be induced by the thermal-lens induced waveguide changing along the active fiber. It is found that the mode coupling can be enhanced mainly by the thermally-induced mode distortion and refractive index variation, both of which will become severe with the large thermal load. The impacts of fiber configuration parameters on the mode coupling are discussed. It is found that in the straight fiber, the mode coupling in a larger-core fiber can be weakened when the thermal load is low, but it will become stronger when thermal-lens effect is severe enough. However, in the bent fiber, enlarging core size, reducing core numerical aperture (NA), or decreasing bend radius will all aggravate the mode coupling. Especially when NA is excessively reduced, the mode coupling will be dramatically raised even with a small thermal load. The pertinent study is significant for understanding the mode coupling phenomenon in high-power fiber lasers.

Analytical study on the stimulated Raman scattering threshold in distributed-pumped fiber amplifiers.

In this paper, the stimulated Raman scattering (SRS) threshold of the distributed-pumped fiber amplifier consisting of multiple sub-amplifiers is analytically studied based on the rate-equation model. The SRS threshold formula is presented where the predictions agree well with the numerical results. With the help of the formula, the effect of the pumping scheme on the SRS suppression of the distributed-pumped fiber amplifier is systematically investigated. It is found that with a given number of pump injection ports, the SRS suppression can be optimized by making only some of sub-amplifiers counter-pumped with the others bi-directional-pumped. It is also suggested that the first sub-amplifier should be adopted to be counter-pumped for suppressing SRS if only one sub-amplifier can be single-directional-pumped. The variation of SRS threshold with the number of sub-amplifiers is also discussed in detail. We believe that these results can provide significant guidance on understanding and designing distributed-pumped fiber lasers and amplifiers.

Study on the adiabaticity criterion of the thermally-guided very-large-mode-area fiber.

The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adiabaticity criterion can be met as long as it is small enough. Then, the effects of the configuration parameters of TG VLMA fiber on the up-limitation of pump absorption for the adiabaticity criterion are investigated. It is found that for the straight TG VLMA fiber, reducing the initial refraction index and inner-cladding diameter and utilizing the bi-directional pumping scheme are beneficial to the adiabatic propagation of fundamental mode. The bent TG VLMA fiber is also studied. It is found that the bent fiber is much more difficult to meet the adiabaticity criterion than the straight one. The results show that even with the 100-cm bend radius, the pump absorption should be smaller than 1 dB/m to meet the adiabaticity criterion. It is suggested that enlarging the core-to-cladding ratio can be helpful for loosening the adiabaticity criterion of bent TG VLMA fiber. These pertinent results can provide significant guidance for understanding and designing the TG VLMA fiber and pertinent lasers and amplifiers.

Study on the power scalability of all-fiber superfluorescent source operating near 980 nm.

The high-power all-fiber superfluorescent source operating near 980 nm is studied experimentally and numerically. In experiment, an all-fiber superfluorescent source operating near 980 nm is fabricated with the distributed side-coupled cladding-pumping (DSCCP) Yb-doped fiber (YDF). By optimizing the active fiber and angle-cleaving of the output port, a recorded 17.1-W output power and 14.6% slope efficiency of 980-nm ASE are obtained. No parasitic laser oscillation is observed at the maximum output power. The power scalability of the source is also numerically investigated. A simple but effective method is present to numerically determine the threshold of parasitic laser oscillation. It is found that the output power can be scaled up to 50 W and 100 W with the optical feedback of each output port suppressed to 1.2 × 10-6 and 7 × 10-7, respectively. It is also revealed that coupling coefficient should be larger than 6 to realize more than 50% slope efficiency. These results provide significant guidance for understanding and designing the high-power superfluorescent Yb-doped source (SYFS) operating near 980 nm and other sorts of three-level fiber sources.

Fabrication of chirped and tilted fiber Bragg gratings and suppression of stimulated Raman scattering in fiber amplifiers.

Stimulated Raman scattering (SRS) is one of the main limits for fiber lasers further power scaling. We report on the suppression of the stimulated Raman scattering in fiber laser amplifier using chirped and tilted fiber Bragg gratings (CTFBGs) for the first time. In this paper, we design and fabricate a CTFBG used to suppress the SRS in 1090 nm fiber laser output, and establish a system to test the effect of suppression. A maximum suppression ratio nearly 25 dB is achieved. Experimental results demonstrate that CTFBGs can increase the Raman threshold and promote the slope efficiency of the whole system, which is significant for further power scaling in high power oscillators and amplifiers.

Thermal-induced transverse-mode evolution in thermally guiding index-antiguided-core fiber.

The transverse-mode evolution in thermally guided (TG) index-antiguided-core (IAGc) fibers is numerically studied in this paper. With the finite-element method and thermal conduction equations, the fundamental mode evolution with a thermal load is investigated, and four evolution stages (i.e., the cladding-confined, quasi-cladding-confined, quasi-core-confined, and core-confined) are revealed. Thermal load thresholds corresponding to these stages are presented. Furthermore, the field evolutions of high-order modes also are investigated; the filling factors of these modes are discussed as well. The pertinent results can provide significant reference for designing TG IAGc fibers and understanding the thermal effect on the transverse mode of fibers involving the index-antiguided core.

Comparison of fiber lasers based on distributed side-coupled cladding-pumped fibers and double-cladding fibers.

We compare both analytically and numerically the distributed side-coupled cladding-pumped (DSCCP) fiber lasers and double cladding fiber (DCF) lasers. We show that, through optimization of the coupling and absorbing coefficients, the optical-to-optical efficiency of DSCCP fiber lasers can be made as high as that of DCF lasers. At the same time, DSCCP fiber lasers are better than the DCF lasers in terms of thermal management.

High-power all-fiberized superfluorescent source with distributed side-coupled cladding-pumped fiber.

An all-fiberized broadband superfluorescent source based on a distributed side-coupled cladding-pumped Yb-doped fiber is demonstrated. A recorded 102 W combined output power is obtained with the angle-cleaved output ends. The bandwidth is larger than 10 nm. The power scalability of the superfluorescent source is analyzed numerically. It is found that, besides the optical feedback, the active fiber length is another key. The result shows that, by optimizing the active fiber length, it is promising to realize a 600 W superfluorescent source with this scheme.

Experimental study on the all-fiberized continuous-wave ytterbium-doped laser operating near 980 nm.

All-fiberized continuous-wave Yb-doped fiber lasers operating near 980 nm are fabricated, and 1.73 W, 980 nm lasing is obtained. Moreover, the output properties of the 980 nm fiber laser are studied by experiment. It is demonstrated, for the first time to the best of our knowledge, that the output power curve versus the active fiber length experiences double-peak values, which are caused by the red shift of the lasing wavelength induced by the longitudinal-mode competition. It is also demonstrated that the pump threshold increases exponentially with the active fiber length. The relationship between the pump threshold and the optimum active fiber length is examined.

Continuous wave, dual-wavelength-pumped supercontinuum generation in an all-fiber device.

We propose a continuous wave dual-wavelength-pumped scheme for visible supercontinuum (SC) generation. The scheme is numerically studied in this paper. In the scheme, the dual-wavelength pump source is produced through a four-wave mixing process in a photonic crystal fiber. SC generation is numerically investigated by solving the generalized nonlinear Schrödinger equation. The results verify that the visible SC can be generated by the scheme, which implies that the scheme is promising for generating visible SC with high spectral power densities.

Dynamical model for self-organized fiber laser arrays.

A dynamics model of self-organized fiber laser arrays is presented in this paper. The model does not only break the limitation of the standard slowly varying wave approximation, but also be built on the basis of Maxwell-Bloch equations which make this model more suitable to study the dynamics (especially phase dynamics) of fiber laser arrays. In this paper, this model is applied to analyze fiber laser array of interferometric configuration. The results agree well with the reported experimental results. It is also revealed that the coupling strength of 2-fiber laser array of interferometric configuration have a negligible effect on the phase-locked state of the array.

Self-organization of arrays of two mutually-injected fiber lasers: theoretical investigation.

The array of two mutually-injected fiber lasers is theoretically studied. It is found that the self-organization mechanism of this array is virtually the longitudinal-mode competition in the compound laser cavity. Two phase-locked states of this array are predicted. The performance of this array is also investigated, and the theoretical result agrees well with the experimental observation. Based on the theoretical analysis, some advices for building this array are also given.

Effect of polarization controlling on coherent beam combining of two-fiber laser arrays of interferometric configuration.

The effect of polarization controlling on coherent beam combining of two-fiber laser arrays of interferometric configuration is researched. Three kinds of arrays, built on the basis of Michelson and Mach-Zehnder interferometers, are investigated experimentally. It is found that polarization controlling is not necessary for coherent beam combining of the Michelson interferometric array but necessary for that of Mach-Zehnder interferometric arrays. These results reveal the important role of polarization in the self-organization process of interferometric laser arrays.