Phase locking of two fiber lasers is demonstrated experimentally by the use of a self-imaging resonator with a spatial filter. The high-contrast interference strips of the coherent beam profile are observed. The coherent output power of the fiber array exceeds 12W and the efficiency of coherent power combination is 88% with pump power of 60W. The whole system operates quite stably and, for the spatial filter, no thermal effects have been observed, which means that we can increase the coherent output power further by this method.
©2006 Optical Society of America
Recently, the output power of a single fiber laser has been improved rapidly and has exceeded kilowatt magnitude [1,2]. However, the ultimate output from a single fiber laser is limited by the nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). Beam combination is an effective geometry which can improve output power with excellent beam quality. At present, many researchers have brought up various techniques of beam combination, including the Master-Oscillator-Power-Amplifier arrangement (MOPA) [3,4], the self-organization mechanism in a multicore fiber laser array [5,6], the all-fiber coherent beam combining technique [7–11], the self-Fourier (S-F) resonators [12,13], etc.. The MOPA system, making use of active phase correction, involves complicated interferometric detection and phase modulation of each fiber laser of the array. The self-organization of a multicore fiber array and the all-fiber coherent beam combining can’t actually avoid the ultimate power limitation of a single fiber and the fabrication of a multicore fiber involves very complicated processings. Because the principle of the S-F resonator is the same as Talbot cavity, it involves complicated and rapidly evolving field amplitudes and wavefronts and can’t operate in single-mode. For various operation mediums such as Nd:YVO4 and Nd:YAG, beams coherent addition of laser arrays has been demonstrated by the use of self-image resonator [14,15]. Fiber laser arrays with self-imaging resonator have been realized under low power condition [16,17]. In this method, phase correction of fiber lasers is passive, which is realized by means of self-adjusting process of the resonance frequencies of fiber lasers array to adapt to changes in the optical path lengths. Using this method, a number of fiber lasers with different lengths are coupled into a common self-imaging resonator with a spatial filter for phase locking. Generally speaking, the self-adjusting process does not easily occur, but performs best in laser systems with broad gain bandwidth, long and unequal lengths, and low-Q resonator. Fortunately, the fiber lasers array occupies these favored conditions. Therefore, this method allows a large number of longitude modes for mode-selecting and does not have to use single-mode fiber lasers. Furthermore, this method does not need polarization controlling of each fiber laser beam, because a polarization eigen state can always be fund in the two elements system regardless of relative orientation.
In this paper, we demonstrate experimentally phase locking of two fiber lasers via a self-imaging resonator with a spatial filter for mode selecting. The patterns of the beam profile at the output mirror exhibits the high-contrast interference strips. Even if the individual laser optic length changes, these strips are still in the state of relative stability. By using the time-independent steady-state rate equations, we have studied the distinction of the slope efficiency between two fiber lasers and obtain the optimum length for the Yb-doped double-cladding fiber. For pump power of 60W, the coherent output power of 12.3W is obtained and corresponding coherent power combination efficiency is as high as 88%. This is up to now the highest output power with the same method. The experiment shows that this method is a potential approach to high power beam combination.
2. Experiment setup
By using the self-imaging resonator with a spatial filter, the phase-locking of two fiber lasers with different fiber lengths has been demonstrated under low power level [16,17]. In Ref [16,17], the stable interference strips and the modulation of longitudinal modes, which caused by the length difference, are observed. The researchers have found that the self-imaging resonator with a spatial filter appears a self-adjusting process and explained the self-adjusting process in reason by means of selection of common resonances in a compound resonator. The experimental setup, shown in Fig. 1, is the same with that in Ref. [16,17]. The self-imaging resonator consists of two flat dichroic input mirrors (M1 and M2) with high reflectivity of >99.8% for 1080nm~1150nm and high transmission of ~95% for 975nm, a convergent lens (L3) with focal length f=500mm as Fourier transform lens in this system, and a flat semitransparent output mirror (M3) with 50% transmission at 975nm. It is demonstrated experimentally that M1 and M2 can tolerate high pump power of ~280W . Two 975nm diode lasers are used as pump source in the experiment of phase locking. Two Yb-doped double-clad D-shape fibers with a core diameter of 16μm and a inner clad diameter of 400/450μm for the shorter/longer axis are used as active media. Both the two fibers belong to the same pre-form. The fiber lengths are 18.5m and 13.2m, respectively. The nominal numerical aperture is 0.16 for the core and 0.37 for the inner cladding. The small-signal absorption coefficient is 1.05 dB/m at 975nm and the slope efficiency is ~50% as free-running lasers with the fiber length of 20m. Both CW output and pulse output exceed 100W using this fiber [18,19]. M1 and M2 are attached to the input end of fibers, and the other ends of fibers are perpendicularly cleaved with 4% Fresnel reflectivity. Two plano-convex lenses (L1 and L2) with 6.28mm diameter are set on the output end of fibers as the collimators. The beams from fiber ends are expanded to a diameter of 2.56mm. The collimated beams are placed symmetrically about the resonator optical axis on the front focal planes of Fourier transform lens L3, and their center-to-center spacing is 6.3mm. The semitransparent output mirror M3 is set on the back focal panes of L3.
3. Results and discussion
The beam profiles at the front focal plane of L3 and at the output plane of the system are related to each other through a Fourier transform. Thus a beam profile symmetrically about the optical axis reproduces itself after a round-trip in the resonator. Two counter-propagating waves are coupled. We examine the beam profile of the output at the output mirror (M3) by means of a CHOU4810 CCD camera and a laser beam analyzer (LBA-PC300; soft version 3.23; Spricon Inc.). Figure 2(a) shows the beam profile at the back focal plane of the lens (L3) under the circumstance that the fiber laser array is in free-running without a spatial filter. Just as Liping Liu’s description, the beam profile exhibits low-contrast interference stripes that are constantly moving without irregular pace and direction. The movement of the stripes exacerbates and the contrast degrades when the individual fibers are subject to mechanical perturbation. These phenomena are resulted from the mode competition. In order to stabilize the phase of the fiber laser array, a spatial filter is placed on the output mirror (M3). Considering high power conditions, we adopt two high melting point platinum (melting point: 1769°C) wires with 20μm diameter as the spatial filter. The platinum wires are placed parallel spaced 90μm which is the distance between the two first intensity minimum of the predicted mode patterns at the output mirror (M3). Ideally, the spatial filter is placed at the position matching with the in-phase mode patterns, which results in low loss for the in-phase mode and high loss for out-of-phase mode. Figure 2(b) shows the beam profile of the in-phase mode. The beam profile of the in-phase mode exhibits high-contrast interference strips. The center strip width is 91 μm which is well in agreement with the calculated result. With the spatial filter in place, the phase locking is stable even when the optical lengths are deliberately changed and mechanical perturbation is added. For example, no slight strips movement is observed when the temperature of one of the fibers is raised 10°C, or when one of the fibers is bent slightly. Obviously, the fiber laser array with self-imaging resonator takes on a self-adjusting process to adapt to the optical length changes, because the fiber laser array with self-imaging resonator possesses three important properties: broad gain bandwidth, long and unequal lengths, and low-Q value. The broad gain bandwidth and long lengths provide a large number of closely spaced longitudinal modes within the gain bandwidth. The low-Q values of resonator broaden the resonance lines to allow those near the common resonance to overlap .
The output power versus pump power relation of the laser array in the in-phase mode and the individual lasers are shown in Fig. 3. The slope efficiencies are 36%, and 23% for Laser1 and Laser2, respectively, and 26% for the phase locking of the fiber laser array in the in-phase mode. All of the three slope efficiencies are lower than that of the previous reports, which results from higher cavity loss and larger coupling loss by using the high reflectivity output mirror (M3). Therefore, reducing properly the reflectivity of the output mirror (M3) not only improves the slope efficiencies of the individual laser and the phase-locking fiber laser array, but also enhances the output power of the fiber laser array in the in-phase mode. The slope efficiency of Laser1 is 13% higher than that of Laser2, which results from the different fiber lengths. When pump power is a constant, the output power of a fiber laser is a function of the fiber length. In Fig. 4, we numerically simulate the curves of the output power versus fiber length with pump power of 30W, 50W, and 70W by means of the time-independent steady-state rate equations [20,21]. With the length of fiber increasing, the output power increases rapidly and reaches a maximum value, and then falls smoothly. Note, however, that the output power varies slowly near the optimum length so that similar output powers (less than 0.05% difference of the maximum value) are obtained in the range of 18–24m. The length of Laser1 (18.5m) is just in the optimum range. However, the Laser2 length of 13.2m is much shorter than the optimum length, which leads that the pump light isn’t absorbed completely by the Yb ions in the core and loses. We also note that the optimum range does not vary with the different pump power. Therefore, we can improve the slope efficiency of each fiber laser by choosing proper length. This also benefits the slope efficiency and output of the whole laser array.
When the total pump power rises to 60W, the coherent output power of the phase locking fiber laser array, 12.3W, is obtained, and the coherent power combination is 88%. When the whole system operates under the high power condition, the physical properties of the spatial filter do not change such as glowing, breaking, melting and so on, which means that the filter may tolerate the high power. Therefore, reducing properly the reflectivity of the output mirror (M3), increasing the number of fibers, enhancing the pump power, selecting the optimum length fibers, or optimizing the construct of the resonator, we can improve the coherent output power greatly, which will be done in the future work.
We have demonstrated experimentally the phase locking of two fiber lasers by means of a self-imaging resonator and a special spatial filter. The pattern of the coherent beam profile exhibits the steady interference strips. The phase locking is due to a self-adjusting process of the fiber laser array with the long lengths, broad gain bandwidth, and low-Q value. When pump power rise to 60W, the coherent output power of 12.3W is obtained and the coherent power combination is 88%. There is no doubt that the coherent output power can be increased greatly by the same method if we optimize the parameters of the resonator and the fiber laser array.
The authors gratefully acknowledge Jinyan Li, Shiyu Li and other colleagues in Fiberhome Telecommunication Tech Co. Ltd for their great endeavor in designing and fabricating the fiber. This work was supported by the 863 Key Program Foundation of China (NO. 2005AA828030) and by the Knowledge Innovation Program of the Chinese Academy of Science.
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