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Scientific RepoRts | 7: 2199 | DOI:10.1038/s41598-017-02118-z
www.nature.com/scientificreports
Realization of large energy
proportion in the central lobe by
coherent beam combination based
on conformal projection system
Dong Zhi, Zhixin Zhang, Yanxing Ma, Xiaolin Wang, Zilun Chen, Wuming Wu, Pu Zhou & Lei Si
In this paper, we experimentally validate a tiled-aperture conformal projection system with the largest
array lling factor and element beam truncation factor to the best of our knowledge. The conformal
projection system, which is made up of a hexagonal adaptive ber-optics collimator (AFOC) array with
the proximate ideal intensity distributions, is fabricated and the performance of output beam is tested
and evaluated properly and carefully. Both of the active phase-locking control and precise tip-tilt control
are implemented successfully in the CBC of the hexagonal seven-beam-array. Experimental results
show a large energy proportion (47%, which increases by over 10% comparing with the previously
demonstrated largest value) in the central lobe is achieved and the residual phase error is lower than
λ/27. When the AFOC array performs, the precise tilt control makes the combining beams overlap well
and the average normalized metric value is improved from 0.336 without control to 0.947 with both
of active piston and tip-tilt phase controls while the fringe contrast increases from 19% to more than
91% correspondingly. This work presents a promising structure for the achievement of large energy
proportion in the central lobe in high power ber laser CBC systems.
ere has been an ongoing eort to achieve a high power ber laser with high brightness via coherent beam com-
bination (CBC) technique
1, 2
. CBC of ber laser array, which is a promising approach to overcome the power scal-
ing limitations of the single ber laser and to achieve high-brightness laser with good beam quality, can be used in
beam projection area and free space laser communication system with enormous potential. CBC systems mainly
contain two subsets, which are characterized by the output formatting, tiled-aperture and lled-aperture imple-
mentations
1–3
. Comparing with lled-aperture format, CBC based on tiled-aperture-stitching method has the
advantage free of power limitation induced by single optical component (diraction optical element, self-imaging
waveguide, polarization beam combiner, et al.) and the disadvantage of energy wastage in the side-lobes in the
far-eld pattern
4–7
. e side-lobes are the direct results of lower energy proportion in the central lobe, which is an
important character concerned in most CBC applications. For the tiled-aperture combination scheme, the energy
proportion of side-lobes is determined by the assembling of the beam array, which is dicult to select and to
implement
8, 9
. is assembly requires a trade-o between an under-lled aperture which suers from signicant
side-lobes in the far-eld, and an over-lled aperture which suers from clipping power loss in the near eld
9
.
Lots of tiled-aperture-based CBC experiments focusing on the active phasing algorithms have been carried out
and demonstrated well with high fringe contrast and control accuracy
8–13
. However, due to the less-than-unity
lling factor of beam array and the small truncation factor of single beam element, the energy proportion in
the central lobe has been small
11–15
. Up to now, the largest energy proportion in the central lobe experimen-
tally achieved is almost 60%, which is coincidence with the theoretical calculation value, by J. Anderegg in 2006
based on tiled-aperture CBC architecture utilizing bulk free space optical elements
16, 17
. In order to decrease the
complexity, researchers use the integrative ber collimator to carry out CBC experiments
18–22
. At the same time,
the lling factor of the integrated collimator array is usually below 70% and the main-lobe energy proportion is
usually below 30%
19–22
. To improve the array lling factor, researchers utilize the microlens array to arrange the
array beams
23–26
. Using this method, J. Bourderionnet and his co-workers improved the energy proportion in the
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, 410073,
China. Correspondence and requests for materials should be addressed to Y.M. (email: xm_wisdom@163.com) or
P.Z. (email: zhoupu203@163.com) or L.S. (email: w_zt@163.com)
Received: 25 January 2017
Accepted: 5 April 2017
Published: xx xx xxxx
OPEN