Supposing that the power imbalance of the input
beams in the second-stage combination cannot be
compensated, a small portion of the optical powers
will be lost, and the optical power of the output
beam can be expressed by:
PBC-2 DPC
P
2
3
sub
out sub 3
3
sub
10 10
10
1
1
2
1
10 10
EL IL
IL
P
P
P P P
P
P
After calculation, it can be noted that P'
out
is equal to
be 0.57 mW.
The average optical power of the output beam
detected in the experiment is 0.60 mW when loop
closed, and greater than the theoretical optical power
of the output beam under the assumption that the
power imbalance of the input beams in the second-
stage combination cannot be compensated, which
indicates that the fiber-based CPBC with cascaded
PL and PT controls can break through the limitation
in the fiber-based CPBC with PL control that the
power imbalance will restrict the combining
efficiency, and can combine three laser beams to one
linearly polarized beam efficiently.
On the other hand, in the fiber-based CPBC of
three beams with PT control, two DPCs as well as
eight control signals are required. It is obvious that
the closed loop will be achieved after more than 3.3
ms (the experimental results of fiber-based CPBC of
two input beams) of SPGD optimization in the
experiment of fiber-based CPBC of three laser
beams with PT control. It can be noted that,
compared with the fiber-based CPBC with PT
control, the fiber-based CPBC with cascaded PL and
PT controls needs less control signals, resulting in a
lower complexity and faster convergence rate.
4 CONCLUSION
In conclusion, we have reported on the research
progress of the fiber-based CPBC in IOE, CAS. Two
control strategies, PL control and PT control, are
proposed and experimentally validated. It can be
noted that the CPBC with PL control and CPBC
with PT control both can combine individual beams
efficiently. Nevertheless, both of the two fiber-based
CPBC schemes have some limitations that will
restrict the enhancement of the combining
performance. In the CPBC with PL control, the
power imbalance of the input beams will inevitably
degrade the combining efficiency of the CPBC and
cannot be compensated. In the CPBC with PT
control, a major drawback is the increased
complexity and decreased convergence rate due to
the required multiple control signals. To break
through the limitations in previous fiber-based
CPBC schemes, the CPBC with cascaded PL and PT
controls is proposed. It is the synthesis of the PL
control and PT control, and can combine the
advantages of the PL control and the PT control
together. We believe that the proposed fiber-based
CPBC in this paper has great potential in coherent
FSO communications employing the multi-aperture
receiver with phased array.
This work is supported by the National Natural
Science Foundation of China under grant No.
61675205 and the CAS “Light of West China”
program.
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