Jianqiu Cao

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.

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.

Power scalability of a single-stage Yb-doped superfluorescent fiber source

A high-power all-fiber broadband superfluorescent source is fabricated by using a distributed side-coupled cladding-pumped Yb-doped fiber. It is found experimentally that besides the optical feedback, the active fiber parameters have a significant effect on the power scaling. By optimizing the fiber length, a record amplified spontaneous emission output of 186.3 W is achieved, and the slope efficiency with respect to launched pump power is 78%. The bandwidths are larger than 20 nm. Furthermore, the power scalability of the superfluorescent source is analyzed numerically. The parasitic laser oscillation can be suppressed further with a large core to cladding ratio.

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.

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.

Investigation on Power Scalability of Diffraction-Limited Yb-Doped Fiber Lasers

The power scalability of diffraction-limited Yb-doped fiber lasers is investigated with a new model which takes into consideration the spectrums of absorption and emission cross sections of active ions. It is found that the maximum output power of a fiber laser is not only dependent on the pump wavelength, but also depends on the core size and cladding-to-core area ratio of active fiber. To increase the pump wavelength is not sufficient to improve the maximum output power, where at the same time the large core, small cladding, high pump brightness and dopant concentration are simultaneously required. Further investigations show that to maximize the output power, the best choice of pump wavelength is highly dependent on parameters such as the pump brightness, dopant concentration, core size and cladding-to-core area ratio. It is found that the pump wavelength with the largest absorption cross section should be the best choice when the core size is small. Besides, the upper limitation of pump brightness for elevating the output power of fiber lasers is given.

Theoretical Study on Phase Locking of Arrays of Mutually-Injected Fiber Lasers

Phase locking of arrays of mutually-injected fiber lasers is investigated theoretically. The phase-locked states of this array are analytically discussed. It is revealed that phase locking of this array is affected by the transfer characteristic of the coupler used for mutual injection. Arrays fabricated with two kinds of couplers (i.e., 1 × 3 coupler and the compound coupler built with two 1 × 2 couplers) are studied. It is found that the compound coupler is more suitable for phase locking of the array. The relationship between the phase-locked states and the number of elementary lasers of the array is also studied.

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.

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.

Experimental Study on All-Fiberized Continuous-Wave Yb-Doped Fiber Amplifier Operating Near 980 nm

The all-fiberized continuous-wave 980-nm amplifier with an Yb-doped double-cladding fiber (YDCF) is experimentally investigated first, to the best of our knowledge. It is revealed that besides the core-to-cladding ratio of YDCF, the filling of the seed light in the active core is also of great importance for the 980-nm signal amplification. By improving the seed light filling in the active core, the 6.8-W 980-nm radiance is obtained from the 60/130 fiber amplifier and the slope efficiency can reach to 32%. Besides, it is also revealed that not only the 1064-nm, but also the 985-nm amplified spontaneous emission will be induced in the YDCF amplifier operating near 980 nm.