With the fascinating properties corresponding to excessive beam high quality and handy thermal administration, high-power fiber lasers have been extensively utilized in a variety of purposes, together with materials processing, medical remedy, scientific researches and plenty of different fields [1], [2], [3], [4], [5]. Nonetheless, the facility scaling functionality of a steady wave monolithic fiber laser is restricted by non-linear impact, thermal impact, transverse mode instability and so forth [6], [7], [8], [9]. As for breaking by the bodily limitations of a single fiber laser, the coherent beam combining (CBC) approach is a promising approach scheme [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Laser array increasing is likely one of the important analysis instructions of the CBC approach, which may enhance the general output energy successfully. Within the earlier analysis, the variety of sub-apertures of CBC system has exceeded the dimensions of 1000 beamlets [22].
As for reaching environment friendly coherent combining and bettering the output energy, CBC system requires the part distinction amongst completely different beamlets stably to be about zero. Owing to the part noise brought on by environmental fluctuation and thermal impact, the combining effectivity of CBC system is decreased. Therefore, energetic phase-locking strategies employed to exactly compensate the part noise is extremely wanted. To repeat with the problem, varied energetic phase-locking strategies have been demonstrated. With out lack of generality, the energetic phase-locking strategy of CBC could be thought-about as a suggestions phase-locking loop, and the prevailing energetic phase-locking approach could be divided into two classes in line with the suggestions construction. One is the exterior phase-locking approach [23], [24], [25], [26], [27]the suggestions indicators are measured by detecting the output beamlets using exterior optical components. As for implementing this technique, large-aperture lenses and mirrors are used to focus and pattern the mixed beam array, which is able to elevate sensible difficulties when the CBC system is expended to a big array.
In distinction to exterior phase-locking approach, the interior phase-locking approach may detect and compensate part distinction earlier than the fiber lasers being emitted into free area. The interior phase-locking approach is principally primarily based on spatial configuration and all-fiber construction. As for the spatial configuration technique [28], [29], [30], [31]splitter mirrors are positioned behind the collimators, most energy is transmitted to free area to type output laser array, whereas the remainder energy of beamlets are targeted to detect the suggestions indicators. As for the all-fiber construction [32], [33], [34]the suggestions indicators are detected by the interference of the Fresnel reflection lights on the collimators’ glass-air interface. Section-locking could be achieved earlier than being emitted into free area with out utilization of spatial optical components, the all-fiber construction is extra compact. In 2013, B. J. David et al. proposed the idea of inner phase-locking optical phased array and realized the energetic inner phase-locking of three laser beams for the primary time [30]. In 2016, L. E. Roberts et al. used uneven fiber couplers to realize phase-locking of two 15 W laser amplifiers [33]. Just lately, our analysis group proposed a distributed inner phase-locking approach, and CBC of two fiber lasers was realized primarily based on the stochastic parallel gradient descent (SPGD) algorithm [34]. With the benefit of reaching phase-locking earlier than lasers being emitted into free area, the all-fiber construction will present reference for the appliance of coherent beam combining system, corresponding to optical vortex lattices [29]cylindrical vector beams [35]free-space optical communications [36] and so forth.
At current, the researches of inner phase-locking of all-fiber laser array are nonetheless within the stage of approach verification, and face many difficulties in energy scaling and array increasing. Firstly, the fiber lasers transmit ahead and backward twice between the coupler and the collimator, which is able to end result within the attainable π-ambiguity among the many laser beams, and additional, end result within the power inconcentration within the far-field. Secondly, the utmost variety of amplifiers reported is not more than 3 channels, therefore, a bigger phase-locking laser array primarily based on all-fiber construction is must demonstrated experimentally. Thirdly, the facility scaling functionality of optical fiber amplifiers must be additional studied. To repeat with the challenges, this paper constructed a CBC system of seven channels to confirm the interior phase-locking approach primarily based on all-fiber construction.
On this paper, a 7-channel fiber laser array is constructed to confirm the all-fiber inner phase-locking approach experimentally. When the system is in closed loop, the relative phases of the output beamlets are locked effectively. The beamlets constructively intervene within the far-field of ∼ 95 % fringe distinction and ∼λ/21 part deviation. The affect of part error, energy inconsistency and tilt error to the combining effectivity are analyzed. With the benefits of compact construction, easy-expanding and no want of the exterior optics, the phase-locking technique primarily based on all-fiber construction has the potential to be employed within the high-power huge laser array system.
