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High Power Laser Science and Engineering, (2018), Vol. 6, e56, 9 pages.

licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.

doi:10.1017/hpl.2018.50

Loss mechanism of all-ﬁber cascaded side

pumping combiner

Chengmin Lei

, Zilun Chen

1,2,3

, Yanran Gu

, Hu Xiao

1,2,3

, and Jing Hou

1,2,3

College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China

Hunan Provincial Key Laboratory of High Energy Laser Technology, National University of Defense Technology,

Changsha 410073, China

Hunan Provincial Collaborative Innovation Center of High Power Fiber Laser, National University of Defense Technology, Changsha

410073, China

(Received 10 July 2018; revised 31 August 2018; accepted 26 September 2018)

Abstract

Compared with end pumping ﬁber combiner, one of the advantages for side pumping combiner is the unlimited pumping

points, which means multi-point or cascaded side pumping can be realized. However, the loss mechanism of the cascaded

structure is rarely discussed. In this paper, we present the numerical and experimental investigation about the loss

mechanism of a two-stage-cascaded side pumping combiner based on tapered-fused technique. The inﬂuence of loss

mechanism on the coupling efﬁciency and thermal load of the ﬁber coating is analyzed according to simulations and

experiments with different tapering ratios for the ﬁrst stage. Based on the analysis, a cascaded component with total

pump coupling efﬁciency of 96.4% handling a pump power of 1088 W is achieved by employing 1018 nm ﬁber laser as

the pump source. Future work to further improve the performance of a cascaded side pumping combiner is discussed and

prospected.

Keywords: ﬁber combiner; ﬁber laser; side pumping

1. Introduction

Fiber lasers have seen progressive development and been

widely applied in industrial ﬁeld, defense technology and

medical science over the past several decades. The investi-

gation of all-ﬁber components for the highly integrated high

power ﬁber laser and ampliﬁer systems has been increased

in recent years. One of the most key components is a

pump/signal combiner. The side pumping combiner, in

which the pump light is coupled into the double-clad ﬁber

through the ﬁber side, is of great interest in these two

decades. The attractive characteristics of side pumping com-

biner, such as uninterrupted signal ﬁber core and unlimited

pump points, which an end pumping combiner (such as

tapered-fused bundle (TFB) technique

[1]

) cannot achieve,

facilitate their potential applications in ﬁber laser and ampli-

ﬁers. Among the reported side pumping couplers, those in

3D structure such as adhered micro-prism

[2]

, V-groove

[2, 3]

embedded mirror

[4]

and diffraction grating

[5, 6]

, restricted

the further improvement of the stability and compactness of

Correspondence to: Z. Chen, College of Advanced Interdisciplinary

Studies, National University of Defense Technology, Changsha 410073,

China. Email: zilun2003@163.com

the system, as well as the pump power handling capability

(only W level to 10 W level) and pump coupling efﬁciency

(not higher than 90%). The all-ﬁber structure has seen more

advantages. A tapered-fused method, which is based on

direct fusion of one or several tapered pump ﬁbers to the

surface of a double-clad signal ﬁber, seems to be a more

potential method because of its high coupling efﬁciency

and the capability of handling high pump power up to kW

class

[7–11]

In order to further increase the output power from the

ﬁber laser, higher pumping power coupled into double-clad

signal ﬁber is required. Since the pumping points in a side

pumping scheme are not limited, multi-point or cascaded

side pumping can be realized. Tan et al. reported the

cascaded combiner

[12]

, consisting of ﬁve combiners by using

tapered-fused technique, and constructed a bi-directionally

pumped ﬁber laser oscillator with 780 W output power when

injecting 1096 W pump power. The laser was pumped via

intra-cavity ﬁber-cascaded combiners and extra-cavity ﬁber-

cascaded combiners. This work showed the advantage of

side pumping technique that it enhances the pump power

through cascaded structure as well as the potential of multi-

point distributed pumping. However, compared with TFB

2 C. Lei et al.

end pumping combiner which can easily achieve (N +

1) × 1 structure (N can be 7 or more) with high coupling

efﬁciency for every pump ﬁber

[13–15]

, for a tapered-fused

side pumping combiner, the increase of pump ﬁbers and high

pump coupling efﬁciency requirement of the components

cannot be achieved simultaneously as illustrated in some

experimental work. The increased number of pump ﬁbers

at one pump point was proven to result in a decrease of

pump coupling efﬁciency in Ref. [7]. This is why a (2 +

1) × 1 structure is more advisable for high pump coupling

efﬁciency requirement, while using cascaded scheme by

inserting more pump ﬁbers at several positions along the

signal ﬁber will also reduce the coupling efﬁciency and

insert more signal loss and beam quality degradation

[12]

Since in a cascaded scheme, different side combiners will

inﬂuence each other on pump coupling efﬁciency and pump

power loss, it is of great importance to ﬁgure out the pump

power loss mechanism in a cascaded scheme and ﬁnd out

a balance between the increase of pump arms and high

coupling efﬁciency. Though this effect has been pointed out

phenomenally, the causes and physical mechanism of extra

pump power loss for side pumping combiners in cascaded

structure were rarely investigated and analyzed thoroughly.

In this paper, we report simulations and experiments

about the pump power loss mechanism of two-cascaded side

pumping combiner based on tapered-fused technique for the

ﬁrst time. First, the optical design and numerical modeling

of two-cascaded combiner are depicted. Then, simulation

results for different parameters of the two combiners are

shown in Section 2. These simulation results can be used

as an instruction to the parameter selection and optimization

during the fabrication of the components. Then, the experi-

ment of the two-cascaded (1+1)×1 combiners with different

parameters is demonstrated. Last, a cascaded component is

used for handling a pump power of 1088 W when employing

1018 nm ﬁber laser as the pump source, with total pump

coupling efﬁciency of 96.4%.

2. Theoretical analysis

2.1. Numerical model

The numerical simulation is carried out by beam propagation

method (BPM), which is a method for solving the Helmholtz

equation for optical ﬁbers

[16]

. The schematic diagram of the

numerical model for the two-cascaded combiner is shown

in Figure 1, which is composed by two (1 + 1) × 1 side

pumping combiners. The core/cladding diameter of the

double-clad signal ﬁber is 20 and 400 µm, respectively,

with a cladding numerical aperture (NA) of 0.46. The

core/cladding diameter of two pump ﬁbers is 220/242 µm

with NA = 0.22. L

taper

= 9 mm and L

waist

= 1 mm are

the lengths of the transition region and the taper waist. The

diameter of the taper waist is deﬁned as D

waist

. As for the

Figure 1. The longitudinal scheme of the cascaded combiner.

geometrical shape of the taper in the longitudinal direction,

for simplicity, a linear shape is assumed in the simulations

instead of the parabolic shape. Since the simulation of side

pumping combiner with the consideration of fused depth

may be more reliable as discussed in our previous work

[10]

similarly, in this numerical model, we assume that the fused

depth increases gradually along a length of δL

taper

and

reaches the maximum at the taper waist (this maximum depth

is deﬁned as fused depth in the calculation). The distance

between the coating edge and the taper waist end of the

pump ﬁber is assumed to be 5 mm and the coating length for

every combiner to be 5 mm. Fused depth for both stages is

5 µm. Detailed information about the parameter setting and

numerical model description of noncascaded side pumping

combiner can be found in Ref. [10]. By using BPM, the

integral power in different waveguides can be monitored

along the longitudinal direction. The total power is set to

1. The pump wavelength is set to 976 nm.

The analysis of the loss mechanism for a noncascaded

side pumping combiner by tapered-fused method has been

proposed and well-discussed recently. The simulations about

the inﬂuence of taper ratio, taper length, pump light input NA

and number of pump ports on the pump efﬁciency and loss

mechanism were performed by using the ray tracing method

in Ref. [7]. Furthermore, we ﬁrst calculated the evolution of

the leakage power into the coating of the signal ﬁber (LPC)

along the coating length, which is the most critical pump

power loss of the side pumping combiner. However, in terms

of a cascaded structure as shown in Figure 1, when the pump

light (inserted at Pump ﬁber 1) that has already coupled into

the inner cladding of the signal ﬁber through the coupling

region of Stage 1 propagates to Stage 2, it may introduce

extra power loss due to the irregular waveguide shape in its

coupling region, giving rise to a reduction of total coupling

efﬁciency. Hence, it is of high importance to explore how

the pump light which is inserted from the pump arm at the

former-stage combiner propagates in the cascaded scheme,

especially in the latter stage. In our numerical simulation,

we mainly calculate the coupling efﬁciency at Stage 1 (η

)

and Stage 2 (η

), LPC for Stage 1 and for Stage 2 as well as

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