In-line Modal Couplers based on Multicore Fibers

Youngbo Shim

, Ju Il Hwang

, Sang Gwon Song

, Sang Bae Lee

and Young-Geun Han

Department of Physics, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea

Korea Institute of Science and Technology, Seoul, Korea

Keywords: Multicore Fibers, Intermodal Coupling, Fiber-optic Sensors.

Abstract: We propose an in-line modal coupler based on a multicore fiber (MCF) which can be readily fabricated by

using the adiabatic tapering method. The intermodal coupling of the in-line modal coupler apparently

generated the transmission oscillation of the center core and the multiple side core modes depending on the

waist diameter. The reduction of the waist diameter of the adiabatically tapered MCF could dramatically

change its sensitivities to strain, temperature, and ambient index. We believe that experimental results are

very useful to fabricate the in-line modal coupler based on the MCF and to improve the performance of the

fiber-optic sensors by controlling the waist diameter of the adiabatically tapered MCF.

1 INTRODUCTION

Multicore fibers (MCFs) with high core density and

low cross talk level have attracted much attention in

optical communications systems (Koshiba et al.,

2009; Li et al., 2007; Zhu et al., 2010). Recently, the

special properties of the MCF, such as small size,

well defined core separation, and good thermal

stability, have led to much interest in fiber optic

sensors (Flockhart et al., 2003; Newkirk et al., 2014).

Fiber Bragg gratings inscribed in a four-core MCF

provides a simple technique to measure the two-axis

curvature (Flockhart et al., 2003). It was reported

that the MCF-based multimode interference device

is capable of measuring temperature up to 1000

(Newkirk et al., 2014). Therefore, it is important to

consider the basic quantities and applications of the

MCF. In this paper, we discuss transmission

characteristics of an in-line modal coupler with

variations in strain, temperature, and ambient index.

The in-line modal coupler is successfully fabricated

by adiabatically tapering the MCF with seven cores.

The waist diameter of the adiabatically tapered MCF

predominantly controls the strength of the

evanescent field and the pitch size resulting in the

transmission oscillation based on the intermodal

coupling among the seven core regions. The

extinction ratio of the transmission oscillation was

gradually improved by diminishing the waist

diameter to be ~30 μm because of the enhancement

of the coupling strength among the multiple core

modes. The further reduction of the waist diameter

degraded the transmission oscillation of the in-line

modal coupler because of the sinusoidal dependence

of the normalized intensities of the center core and

multiple side core modes on the coupling coefficient

and the propagation distance. The reduction of the

waist diameter of the adiabatically tapered MCF

could dramatically change its sensitivities to strain,

temperature, and ambient index. The reduction of

the waist diameter of the adiabatically tapered MCF

could dramatically change its sensitivities to strain,

temperature, and ambient index.

2 EXPERIMENTS AND

DISCUSSION

Figures 1(a) shows the scanning electron

microscopy (SEM) image of the fabricated MCF,

respectively. The MCF has seven Ge-doped cores

surrounded by pure silica cladding. The pitch size

(

) was measured to be ~32 μm. It is important to

keep the identical size of

to suppress the crosstalk

among the multiple cores of the MCF (Zheng et al.,

2013). The core and the cladding diameters of the

MCF were measured to be 10 and 125 μm,

respectively. All seven cores in the MCF can

accommodate a single mode. Since the cladding

diameter of the MCF is exactly the same as that of

the conventional SMF, an ordinary fusion splicing

Shim, Y., Hwang, J., Song, S. and Han, Y-G.

In-line Modal Couplers based on Multicore Fibers.

DOI: 10.5220/0005777501810184

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 183-186

ISBN: 978-989-758-174-8

183

technique is readily capable of connecting the MCF

with the SMF. The micro-tapering technique was

exploited to fabricate the in-line modal coupler

based on the MCF (Yoon et al., 2012). During

softening and melting the MCF by using a computer-

controlled heater, two translation stages

simultaneously elongate the MCF resulting in the

formation of the adiabatically tapered MCF. The

temperature of the heater and the pulling speed of

the two translation stages were controlled to be

~1000

C and ~10 μm/sec, respectively. The

reduction of the core diameter and the pitch size by

tapering the MCF mainly induces the modal

coupling from the center core region to the six side

cores. It is important to investigate the effect of the

waist diameter of the adiabatically tapered MCF on

the modal coupling of the in-line modal coupler. We

fabricated the various tapered MCFs with different

diameters of 10, 20, 30, 50, 75, 125 μm. The length

of the uniform waist region of all tapered MCFs was

~12 mm. Figure 1(b) shows the images of the

tapered MCFs with different waist diameters

measured by an optical microscope.

50 ㎛

10 ㎛

20 ㎛

30 ㎛

(b)

10 μm

32 μm

(a)

Figure 1: (a) SEM image of the fabricated MCF and (b)

images of the tapered MCF measured by an optical

microscope.

To investigate the effect of the waist diameter on

the modal coupling, we spliced the core of the

conventional SMF with the center core of the MCF

and reduced the waist diameter of the MCF. We

measured the transmission spectrum based on the

modal coupling of the proposed in-line modal

coupler with various waist diameters. Figure 2

shows the transmission spectrum of the center core

mode in the in-line modal coupler. After splicing the

core of the SMF with the center core of the tapered

1520 1530 1540 1550 1560 1570 1580 1590

-30

-25

-20

-15

-10

-5

Transmission [dB]

Wavelength [nm]

Waist diameter [μm]

125 75

50 30

20 10

SMF

MCF

Figure 2: Experimental Results for the Transmission

Spectra of the Center Core in the Proposed in-Line Modal

Coupler with Various Waist Diameters.

MCF with versatile waist diameters, we measured

the output of the in-line modal coupler as seen in Fig.

2. When the waist diameter was larger than ~50 μm,

the modal coupling was not exhibited in the

transmission spectrum of the in-line modal coupling.

Since further reduction of the waist diameter

improves the coupling strength among the multiple

core modes in the in-line modal coupler, the periodic

oscillation of the transmission spectrum was

observed and the extinction ratio should be gradually

increased. When the waist diameter was ~30 μm, the

center core mode was sufficiently coupled to

multiple side core modes. The extinction ratio of the

transmission spectrum was gradually degraded by

decreasing the waist diameter because the

normalized intensity of the center core and multiple

side core modes resulting from the modal coupling

varies like

)7(cos

and

)7(sin

, respectively

(Chan et al., 2012). The oscillation periodicity of the

transmission spectrum resulting from the modal

coupling should be reduced because of the variation

of the phase depending on the coupling coefficient

and the waist diameter.

Figure 3(a) shows the peak wavelength shifts of

the tapered MCF with various waist diameters as a

function of strain. The applied strain shifted the peak

wavelengths to shorter wavelength. The reduction of

the waist diameter of the MCF successfully

enhanced the strain sensitivity of the tapered MCF.

The strain sensitivities of the tapered MCF with

different waist diameters of 30, 20, and 10 μm were

measured to be -0.5, -1.0, and -1.4 nm/mε,

PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology

184

0246810

-18

-16

-14

-12

-10

-8

-6

-4

-2

Peak wavelength shift [nm]

Strain [mε]

Waist diameter [μm]

20 30 40 50 60 70 80

0.0

0.2

0.4

0.6

0.8

1.0

Waist diameter [μm]

Peak wavelength shift [nm]

Temperature [

1.314 1.316 1.318 1.320

Waist diameter [μm]

Peak wavelength shift [nm]

Ambient index [A.U.]

(a)

(b)

(c)

Figure 3: Peak wavelength shifts of the tapered MCFs with various waist diameters as functions of strain (a), temperature

(b), and ambient index (c), respectively.

respectively. The peak wavelength of the tapered

MCF was shifted to longer wavelength as

temperature increased. The reduction of the waist

diameter of the tapered MCF degraded its

temperature sensitivity (14.2 pm/

C for 30 μm, 7.9

pm/

C for 20 μm, and 3.8 pm/

C for 10 μm) as seen

in Fig. 3(b). In Fig. 3(c), the peak wavelength of the

tapered MCF shifted to longer wavelength as the

ambient index was increased. The ambient index

sensitivity of the tapered MCF should be improved

by reducing the waist diameter of the tapered MCF.

The ambient index sensitivities of the tapered MCF

with waist diameters of 30, 20, 10 mm were

measured to be 358.6, 542.7, 809.6 nm/RIU,

respectively.

3 CONCLUSIONS

In conclusion, we discussed transmission

characteristics of an in-line modal coupler based on

the adiabatically tapered MCF with variations in

strain, temperature, and ambient index. By

controlling the waist diameters, we investigated the

sensitivity variation of the adiabatically tapered

MCF to strain, temperature, and ambient index

changes. The reduction of the waist diameter

improved the coupling strength among the multiple

core modes in the in-line modal coupler because of

the variation of the evanescent field and the pitch

size. The modal coupling of the in-line modal

coupler apparently generated the transmission

oscillation of the center core and the multiple side

core modes depending on the waist diameter. The

extinction ratio of the transmission oscillation was

gradually improved by diminishing the waist

diameter to be ~30 μm because of the enhancement

of the coupling strength among the multiple core

modes. The further reduction of the waist diameter

degraded the transmission oscillation of the in-line

modal coupler because of the sinusoidal dependence

of the normalized intensities of the center core and

multiple side core modes on the coupling coefficient

and the propagation distance. The reduction of the

waist diameter of the adiabatically tapered MCF

could dramatically change its sensitivities to strain,

temperature, and ambient index. We believe that

experimental results are very useful to fabricate the

in-line modal coupler based on the MCF and to

improve the performance of the fiber-optic sensors

by controlling the waist diameter of the adiabatically

tapered MCF.

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