All-fiber 7 × 1 signal combiner with high beam
quality for high-power fiber lasers
Hang Zhou (周 航), Zilun Chen (陈子伦)*, Xuanfeng Zhou (周旋风),
Jing Hou (侯 静), and Jinbao Chen (陈金宝)
College of Optoelectronic Science and Engineering, National University of Defense Technology,
Changsha 410073, China
*Corresponding author: zilun2003@hotmail.com
Received January 11, 2015; accepted April 23, 2015; posted online May 28, 2015
All-fiber signal combiner is a key component for augmenting the fiber laser power. Presently the reported 7 × 1
signal combiners are all have output fibers with core diameters larger than 100 μm. In order to improve the beam
quality of the combiner, a fiber with smaller core of 50 μm diameter is chose to be the output fiber. An all-fiber
7 × 1 signal combiner is fabricated with measured power transmission efficiency around 99% for each port. The
beam quality is improved and the measured M
2
are around 6 which are matched well with the theoretically
calculated results.
OCIS codes: 140.3510, 140.3298, 060.2310, 060.2340, 230.2285.
doi: 10.3788/COL201513.061406.
Fiber lasers which have the output power extended into
kilowatt (kW) regime have applied in many fields due
to their advanced features such as high beam quality,
compactness, and thermal management
[1–6]
. However, the
output power of fiber laser from a single fiber is limited by
strong nonlinear effects. In order to achieve higher power
output, incoherent beam combining through signal com-
biner has been found to be a significantly easy way
[7–9]
.
Nowadays, the applications of combined fiber lasers have
drawn increasingly more attention on the beam quality of
the combiner in addition to the output power.
In 2011, Noordegraaf
[10]
reported an all-fiber 7 × 1 signal
combiner which supported up to 2.5 kW combined output
power and only a minor temperature increase was
observed. At an intermediate power level of 600 W the
beam parameter product (BPP) was measured to be
2.22 mm·mrad, corresponding to an M
2
value of 6.5.
Yan
[11]
et al. achieved a 4 kW continuous-wave (cw) fiber
laser with an all-fiber 4 × 1 signal combiner, and the cou-
pling efficiency was nearly 98%. In Ref. [
12], Shamir et al.
reported a 3 kW level incoherently combined laser with a
measured power transmission efficiency of 98% using an
all-fiber Y-branch combiner, and the M
2
factor of ≤ 3.5
was measured at a 600 W output level.
However, although fiber signal combiners supporting
kW laser output have been reported, the output fiber cores
are almost larger than 100 μm and the beam quality can-
not be improved. In Ref. [
11] the output fiber had a core of
50 μm, but the manufacture details were not given clearly
and the beam quality was not measured. The influence of
adopting smaller-core fiber as output fiber on the beam
quality of combiner has not been studied and reported
(to our best knowledge). Theoretica lly using fiber of
smaller core as the output fiber can improve the beam
quality of the output optical field effectively. In this work,
we investigate the beam quality of a 7×1 signal combiner
with smaller output fiber. First, we theoretically analyze
the power transmission efficiency and the beam quality of
signal combiner. Then experiments are conducted to fab-
ricate 7 × 1 signal combiners based on the theoretical
results. The beam quality of the fabricated 7 × 1 signal
combiner is measured and the beam quality factors are
6.0 and 6.3 which are better than that of signal combiner
with larger-core output fiber. Moreover with the power
transmission efficiency is also measured to be as high as
around 99% for each port.
In order to fabricate fiber combiners of high transmis-
sion efficiency, two basic principles should be followed:
adiabatic tapering and brightness conservation. Adiabatic
tapering is using a long enough taper length to ensure the
taper angle is small enough. Based on adiabatic tapering,
the optical field propagating in the tapered fiber bundle
will not change dramatically. Thus the transmission effi-
ciency in the taper region will be high. Through theoretical
analysis, a suitable taper length of the fiber bundle is
calculated to ensure a high transmission efficiency of
the combiner. The taper length of the fabricated combiner
is equal to the calculated taper length .
Brightness conservation is the other important principle
in the design of fiber combiner. If the incident optical
field is uniform, the integrated brightness (IB) can be
expressed as
IB ¼ AR
Z
Ω
ϕ
0
dΩ ∝ A
Z
ϕ
0
sinðφÞdφ ¼ A½1 − cosðϕÞ; (1)
where A is the optical field area, R is the radiation inten-
sity contour, and Φ and φ is the receiving angle of fiber.
The numeri cal aperture (NA) of fiber is proportional to
sinðΦÞ, and sinðΦÞ ≈ Φ,so
IB ∝ A½1 − cosðϕÞ ≈
A
2
NA
2
: (2)
COL 13(6), 061406(2015) CHINESE OPTICS LETTERS June 10, 2015
1671-7694/2015/061406(4) 061406-1 © 2015 Chinese Optics Letters