Study On Coherent Beam Combination Of Fiber Laser Based On Dithering Phase Locking Technology

Fiber lasers have been widely applied in industry, medicine and defense for theiressential characters such as high conversion efficiency, excellent beam quality, robustperformance and easy opration. However, the brightness of single fiber laser is limitedby nonlinear effect, thermal effect and so on. Coherent beam combination (CBC) ofmultiple fiber laser beams becomes one of hot topics in laser field, which can increasethe output power of fiber laser under keeping the excellient beam quality. In the presentpaper, coherent beam combination system of fiber laser based on dithering phase lockedtechnique is studied systematically and some meaningful results are achieved asfollows.1. The multi-dithering phase-locking technique is studied systematically. Thetheory of CBC of Guassian beams using multi-dithering technique is developed basedon Huygens-Fresnel principle. Then the method for selecting the cost function ispresented and the effects of optical intensity fluctuation of laser beam in the array onCBC are studied by simulation. In the experiment, the key component in phasecontrolling system, signal processor is designed and made based on field programmablegate array (FPGA) whose control bandwidth is more than10kHz and precision is betterthan λ/20. Coherent beam combination of4low power and9high power fiber lasers areimplemented in the experiment, respectively. The overall output power achieved200mW and100W. The new scheme for coherent beam combination based ondouble-PZT phase modulator is presented and the double-PZT phase modulator hasbeen made. The scheme effectively decreses complexity and cost of CBC system andexpand the wavelength of fiber laser in CBC. CBC of2μm fiber lasers is demonstratedbased on PZT phase modulators for the first time.2. The single-frequency dithering technique is presented based on time divisionmultiplexing in order to overcome the phase modulation frequency shortage inmulti-dithering technique, which is studied detailedly in theory and experiment. CBC of9fiber amplifiers using single-frequency dithering technique is demonstrated and1.08kW and1.56kW overall output power are achieved respectively. To the best of ourknowledge, this is the first report of kW level CBC of fiber lasers.3. Sine-cosine single-frequency dithering technique and single-frequency andmulti-dithering mixing phase control technique are presented and studied in theory andexperiment. The phase control speed of sine-cosine single-frequency ditheringtechnique is twice faster than single-frequency dithering technique. The experiment ofcoherent beam combination of9100-W level fiber amplifiers is implemented based onsingle frequency dithering technique for the first time.1.08kW total output power hadbeen achieved. Single-frequency and multi-dithering mixing phase control technique possesses of the advantages of single-frequency dithering technique and multi-ditheringtechnique which can be used for CBC of the larger number of fiber lasers.4. Target-in-the-loop CBC (TIL CBC) is studied detailedly. The effect ofatmosphere turbulence on phase modulation signals and the relationship betweenemitting distance and bandwidth of phase control system are detailedly analyzedtheoretically. In the experiment, TIL CBC of910W level fiber amplifiers isimplemented based on static non-cooperative target and the relavant theories aredemonstrated.5. The efficiencies of multiform array synthetic apertures are analized in theory.The calculated results show that the effect of conformal array is the best and the BPFfactors of conformal array for Gaussian and super Gausian beams are more than0.7and0.8respectively in the idear case. The scheme of beam combination of the largernumber of high power laser array is present based on above theory and the keycomponents (step-like beam combiner and water-cooled diaphragm) are designed andmade, which improves effectively the filling factor of laser beam array and cleans upthe useless light in near field.