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BRIEF PAPER

Fabrication of Polarization-Maintaining Photonic Crystal Fiber
Coupler with Air Hole State Control Using CO2 Laser
Irradiation TechniqueHirohisa YOKOTA†a) , Member, Yusuke ITO† , Hiroki KAWASHIRI† , Hideyuki KIUE†† , Hideo TOBITA†† ,Yoh IMAI† , Nonmembers, and Yutaka SASAKI† , Fellow

SUMMARY
Polarization-maintaining photonic crystal fiber couplers(PM-PCFCs) were fabricated using a CO2 laser irradiation technique. We
could control the states of air holes in the tapered region of couplers byadjusting the laser power density in the fusion and the elongation processes. It was demonstrated that the air hole remaining PM-PCFC exhibited polarization-splitting characteristics and that the air hole collapsed
PM-PCFC had polarization insensitive coupling characteristics.
key words: photonic crystal fiber, polarization-maintaining fiber, opticalfiber coupler, polarization dependent coupling characteristics, CO2 laser
irradiation technique1.

Introduction

Photonic crystal fibers (PCFs) that have air holes around thecenters of their cross sections exhibit various interesting features not seen in conventional optical fibers by designing thesizes and the arrangements of the air holes. Examples of thefeatures are endlessly single-mode, arbitral dispersion controllability, extremely high/low nonlinearity, high birefringence, and so on. At first, PCFs have been applied to functional fibers in optical fiber systems because of their uniquefeatures [1]. Recently, applications of PCFs as transmission
lines have been investigated since low loss PCFs had beendeveloped [2]. An ultra-wideband WDM transmission wasreported using the feature of endlessly single-mode [3]. An
ultra-high bit rate transmission using optimally dispersion
controlled single polarization PCF was also proposed [4].
A polarization-maintaining PCF (PM-PCF) that is realized by an air hole arrangement in two-fold rotational
symmetrically can exhibit higher modal birefringence compared to conventional polarization-maintaining fibers suchas a PANDA fiber, an elliptical jacket fiber, and a bow-tie
fiber [5]. Because of its high birefringence, PM-PCFs areexpected to be the applications for optical transmission systems and optical sensing systems with polarization control.
To construct PM-PCF transmission and sensing systems using PM-PCF features, optical fiber devices fabricated withPM-PCFs are preferred. An optical fiber coupler is one ofManuscript received January 26, 2012.
Manuscript revised May 1, 2012.
†
The authors are with the Graduate School of Science and Engineering, Ibaraki University, Hitachi-shi, 316-8511 Japan.
††The authors are with Tobitarika Glass Seisakusho, Ltd.,
Hitachi-shi, 319-1233 Japan.
a) E-mail: hirohisa@mx.ibaraki.ac.jp
DOI: 10.1587/transele.E95.C.1689

the basic components in optical fiber systems; therefore a realization of PM-PCF coupler (PM-PCFC) is desired to construct PM-PCF systems.
In this paper, we report fabrications of PM-PCFCs using the CO2 laser irradiation technique. The state of airholes in the PM-PCFC taper can be controlled by appropriate adjustment of laser power density in the fusion and theelongation processes. Coupling characteristics dependingon the state of air holes in the coupler taper are experimentally clarified.
2.

PM-PCFC Fabrications with Air Hole State Control

In fabrications of PCF couplers (PCFCs) including PMPCFCs, states of air holes in the PCFC tapers should becontrolled because the states of air holes in the PCFC tapers aﬀect light propagations in the tapers i.e. the characteristics of the PCFCs depend on the states of air holes. Although fused optical fiber couplers are fabricated using gas
burners or electric heaters in general, we used CO2 laser
irradiation technique for PM-PCFC fabrication. Since thistechnique can precisely control the energy to heat fiber, anaccurate control of fiber temperature is possible in coupler
fabrication. Therefore, it is suitable for PM-PCFC fabrications with air hole state control. It was reported that the stateof air holes could be controlled in the elongation process of
the PCFC fabrication using CO2 laser irradiation techniquein which the laser power density was appropriately adjusted
as the PCFs were elongated and that wavelength characteristics of PCFCs depended on the states of air holes [6]. The
technique also has advantages that are contamination-freeand short fabrication time.
Figure 1 shows a PM-PCFC fabrication setup using
CO2 laser irradiation. The laser beam was irradiated to thesides of PM-PCFs through two cylindrical lenses. Using
these lenses, the laser beam was focused in radial direction
to fibers and defocused in longitudinal direction to fibers.
The PM-PCFs were fixed to the mechanical stages with stepping motors. Tension members were used to apply tension
to fibers stably, and also have a function to rotate fibers inthe setting; therefore we can easily set the polarization axesof PM-PCFs parallel in the fiber setting to suppress polarization crosstalk in coupler outputs. The polarization axesof the PM-PCFs were set parallel, observing facets of twistc 2012 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. ELECTRON., VOL.E95–C, NO.10 OCTOBER 2012

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Fig. 1Fig. 2

Cross section of the PM-PCF for coupler fabrication.

Fig. 3

Irradiated laser power density in PM-PCFC fabrications.

PM-PCFC fabrication setup using CO2 laser irradiation.

free PM-PCFs using a microscope. In the elongation process, the power density of the irradiated laser was appropriately increased to make fiber temperature constant because
the temperature of laser irradiated fiber is decided by theirradiated laser power density and the fiber diameter. By
the laser power density adjustment, the fiber break can beavoided, and the state of air holes in the tapered region canbe controlled whether air holes were remaining or collapsed.
3.

Experimental Results and Discussions

The cross section of the PM-PCF used for coupler fabrication is shown in Fig. 2. It has 6 air hole rings. The cladding
diameter is 123.2 µm, and the air hole pitch is 4.3 − 5.5 µm.
The diameters of large and small air holes are 4.9 µm and2.7 − 3.4 µm, respectively. In the cross section, we defined
the x-axis as the direction to major axis of the elliptical areasurrounded by large air holes. The y-axis was defined as theorthogonal direction to the x-axis.
We used a pulsed CO2 laser with 14.285 kHz repetition
frequency for PM-PCFC fabrication. The maximum valueof the averaged laser output power was 25 W. The diameterof the irradiated laser beam on fibers through two cylindricallenses was ∼ 14 mm in longitudinal direction to fibers. Theincrement of the laser power density was brought by the increment of the average laser power output and by decreasing
the diameter of the laser beam in radial direction to fibers.
Figure 3 shows the irradiated laser power density for time in
the PM-PCFC fabrications. The CO2 laser beam was irradiated for 100 seconds to fuse PM-PCFs before elongation.
After fiber fusion, the PM-PCFs were elongated for 300 seconds with 60 µm/s elongation speed while CO2 laser beam
was irradiated to the PM-PCFs. In fabrication of PM-PCFC
with air hole collapsed taper, the gradient of increment in
temporal laser power density was made steeper comparedto the air hole remaining PM-PCFC fabrication, so that thefiber temperature was made higher to obtain lower glass viscosity for air hole collapsing in elongation process.
Figure 4 shows photographs of the fabricated air holeremaining PM-PCFC taper side view. It is found that the airholes are remaining in the whole of the tapered region. The
fiber diameter of the taper waist is ∼ 38% of the fiber diameter before elongation. The taper length that is defined as theFig. 4view.

Photographs of fabricated air hole remaining PM-PCFC taper side

distance between the points of 10% diameter reduction was∼ 25 mm. Figure 5 shows a photograph of the fabricated airhole remaining PM-PCFC cross section at taper waist. It isfound that the air holes are remaining and the polarization
axes are almost set parallel. Figure 6 shows photographs of
the fabricated air hole collapsed PM-PCFC taper side view.
The air holes are collapsed at the taper transition region. The
fiber diameter of the taper waist is ∼ 14% of the fiber diameter before elongation. The taper length was ∼ 22 mm, andBRIEF PAPER

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Fig. 5 Photograph of fabricated air hole remaining PM-PCFC crosssection at taper waist.

is expected by optimizing the shape of coupler taper suchas a minimum taper diameter and a taper length. In the airhole collapsed PM-PCFC, the coupling characteristics werepolarization insensitive, where the polarizations of the transmitted lights at the output ports were maintained. The extinction ratios at straight and cross ports were 54% and 46%
for both x- and y-polarizations. The polarization crosstalk ateach port was ∼ −30 dB. It is considered that the characteristics were caused by the air hole collapsed PM-PCFC taper
without birefringence.
The excess losses of fabricated PM-PCFCs were ∼6 dB. This value is too high to use them in practical systems. It is considered that the loss is induced by the cutoﬀof fundamental mode in PCF taper [7]. We expect that optimally designing of the air hole arrangement and the taper
profile is eﬀective for the excess loss reduction.
4.

Conclusions

In this paper, we reported the fabrications of PM-PCFCs using CO2 laser irradiation technique. The states of air holes in
PM-PCFC tapers could be successfully controlled by appropriately adjusting the laser power density. The fabricated airhole remaining PM-PCFC exhibited polarization-splitting
characteristics. On the other hand, the air hole collapsed
PM-PCFC had polarization insensitive coupling characteristics, where the polarizations of the transmitted lights at theoutput ports were maintained.
Acknowledgement
Fig. 6view.

Photographs of fabricated air hole collapsed PM-PCFC taper side

Table 1 Extinction ratios in fabricated PM-PCFCs.
Air holes
Polarization
Straight
Cross
x83%
17%
Remainingy19%81%
Collapsedx, y54%46%

the length of the air hole collapsed region was ∼ 24 mm.
As shown in these results, we could successfully control thestate of air holes in the tapered region of the PM-PCFCs byadjustment of laser power density.
Coupling characteristics of fabricated PM-PCFCs at
1550 nm-wavelength were measured. They are summarizedin Table 1. In the air hole remaining PM-PCFC, the extinction ratios at straight and cross ports for x-polarization were83% and 17%, respectively. On the other hand, those for ypolarization were 19% and 81%. The polarization crosstalkat each port was up to −30 dB. It is found that the air holeremaining PM-PCFC exhibited polarization-splitting characteristics. It is considered that the characteristics werebrought by the birefringence in the tapered region with airholes. Although the obtained characteristics are not suﬃcient in practical use, the improvement of the characteristics

This work was supported by Grants-in-Aid for Scientific Research (21560347) of Japan Society for the Promotion ofScience (JSPS).
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