Optical Fiber Technology 18 (2012) 220–225

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Optical Fiber Technology

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Multiple defect core photonic crystal fiber with high birefringence inducedby squeezed lattice with elliptical holes in soft glass

Ireneusz Kujawa a, Ryszard Buczynski a,b,⇑, Tadeusz Martynkien c, Marek Sadowski c, Dariusz Pysz a,Ryszard Stepien a, Andrew Waddie d, Mohammad R. Taghizadeh d

a Glass Laboratory, Institute of Electronic Materials Technology, Wolczynska 133, 01-919, Warsaw, Polandb Faculty of Physics, University of Warsaw, Pasteura 7, 02-093 Warsaw, Polandc Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Polandd School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 December 2011Revised 8 March 2012Available online 26 June 2012

Keywords:Photonic crystal fibersBirefringent fibersFiber sensorsSoft glass

1068-5200/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.yofte.2012.04.004

⇑ Corresponding author at: Faculty of Physics, Univ02-093 Warsaw, Poland. Fax: +48 22 5546822.

E-mail address: rbuczyns@igf.fuw.edu.pl (R. Buczy

We present a dual mode, large core highly birefringent photonic crystal fiber with a photonic claddingcomposed of elliptical holes ordered in a rectangular lattice. The fiber is made of borosilicate glass andhas a regular set of elliptical holes with an aspect ratio of 1.27 and a filling factor near 0.5. The group bire-fringence (G) and effective mode area were measured at 1550 nm for the fundamental mode and werefound to equal 2 � 10�4 and 20 lm2 respectively. We discuss the influence of structural parametersincluding the ellipticity of the air holes and the aspect ratio of the rectangular lattice on the birefringenceand on the fundamental and second modes of the fiber.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

The development of highly birefringent fibers remains one ofthe most promising applications of photonic crystal fibers (PCFs).It is well known that photonic crystal fibers (PCFs) can exhibitmuch higher birefringence values than their conventional counterparts such as bow tie and panda type optical fibers. Since the bire-fringence in PCFs results from the asymmetric distribution of therefractive index in the fiber cross-section, not form stress inducedphenomena, they are highly sensitive to temperature variations [1]and, therefore, good candidates for various mechanical sensorssuch as strain, stress or pressure [2].

There are several methods proposed to break symmetry andachieve the asymmetric distribution of the field in PCFs includingelliptical cores [3], small air holes in the core [4], varied sizes of cir-cular holes in the cladding [5,6] and adding large air holes outsidethe cladding [7]. A lot of interest has also been focused on PCFswith elliptical holes in a hexagonal or rectangular lattice [8,9]. Thissolution offers extremely high birefringence, however the practicaldevelopment of these types of structure suffers from a number oftechnological difficulties [10]. A successful demonstration of apolymer PCF with elliptical holes made from polymethyl methac-rylate (PMMA) using an extrusion method was reported by Issa

ll rights reserved.

ersity of Warsaw, Pasteura 7,

nski).

et al. [11]. This polymer PCF exhibited a birefringence on the orderof 10�4 at 850 nm. Recently two groups reported the successfuldevelopment of PCFs with elliptical holes using soft [12] and silicaglasses [13].

In most of work to date, the major focus has been on the reali-zation of the maximum birefringence that it is possible to obtain inthe photonic structures. As a result, the proposed structures possesa very small core and high coupling losses are expected. On theother hand, large mode fibers offer a likely route to single modeperformance and high birefringence.

Several groups have reported the use of step-index two-modehighly birefringent fibers as a good candidate for multi-parametersensors for the simultaneous measurement of strain and tempera-ture [14], the very sensitive measurement of strain or temperatureindividually [15,16] or the measurement of the acousto-optic fre-quency shift [17]. The two mode highly birefringent fiber can workas an interferometer using two spatial modes as the two interfer-ometer arms – in this configuration the group delay is similar,while the difference in the phase delays is large [18]. The use ofa photonic crystal fiber allows the development of two-mode fibersfor a wide wavelength range [19]. Two mode photonic crystal fi-bers have been successfully used as inteferometric strain sensorsat wavelengths from 650 nm to 1300 nm [20], as an interferomet-ric torsion sensor [21], a tunable acousto-optic filter from 700 to1700 nm [22] and as a temperature sensor [23].

In this paper we present a large core fiber design with ellipticalholes on a rectangular lattice in the cladding. In order to increase

I. Kujawa et al. / Optical Fiber Technology 18 (2012) 220–225 221

the mode area, the core is created by omitting the 4 central holes inthe structures.

2. Influence of hole ellipticity on fiber properties

The total birefringence of the squeezed lattice fibers is a combi-nation of the birefringence induced by the rectangular lattice andthe birefringence induced by the ellipticity of the air holes in thecladding.

To study the influence of hole ellipticity, we have simulated atest structure with a rectangular lattice and various hole shapeswith a constant minor axis diameter of 0.88 lm and major axislengths between 0.88 lm and 1.2 lm (Fig. 1). We assume thatthe photonic crystal fiber is composed of four rings of air holes or-dered in a rectangular lattice with lattice constants of Kx = 2.6 lmand Ky = 1.6 lm. The core of the fiber is formed by omitting thefour central holes in the structure. In the simulations we considerthe fundamental as well as second guided modes.

The simulations were performed using the finite element meth-od in Comsol 3.4 [24]. The effective refractive indices of the guidedmodes were calculated taking into account the material dispersionof the glass. Uniaxial perfectly matched layer (UPML) boundaryconditions were used to determine the confinement loss of the se-lected guided modes in the fibers. These simulations show that the

Fig. 1. The scheme of core and first two rings in photonic cladding in the photoniccrystal fiber with rectangular lattice and various shapes of air-holes from circularwith the diameter of 0.88 lm (dotted line) to elliptical with the diameter of majorand minor axis of 1.2 lm and 0.88 lm, respectively (solid line).

×1×10-4

Fig. 2. Calculated phase (B) and group (G) birefringence for fundamental LP01 and seconration between fast and slow axis of elliptical holes. Rectangular lattice has lattice cons

considered structures are capable of effectively guiding up to 2modes. Based on the calculated refractive index properties, thephase birefringence B, defined as the difference between the prop-agation constants bx and by of the two orthogonally polarized com-ponents (LPx

01 and LPy01 for the fundamental mode/LPx

11 and LPy11

for the second mode), is calculated according to the formula:

B ¼ nx � ny ¼k

2pðbx � byÞ ð1Þ

Whilst the group birefringence G is defined as:

G ¼ B� kdBdk

ð2Þ

We observe that the ellipticity of the air holes has consequencesfor the birefringence ratio between the first and second modes. Forcircular holes (e = 1), the birefringence of the second mode LP11 ishigher than that of the fundamental mode LP01 – independent ofthe hole diameter. In addition, the observed change in the birefrin-gence with increasing ellipticity of the air holes is different for thefundamental and second modes. The phase birefringence of the

e=1

e=1.15

e=1.2

e=1.25

e=1.3

e=1.35

0-4

d mode LP11 of the PCF structures with various shape of air-holes: e denotes aspecttant of Kx = 1.25 lm and Ky = 2.25 lm.

Fig. 3. Phase birefringence vs. ellipticity of air-holes for fundamental LP11 (a) andhigher mode LP11 (b) for a wavelength of 1200 nm.

wavelength [µm]

Loss

[dB/

m]

e=1.2

e=1.25

e=1.3 LP01x

LP11x

LP01y

LP11y

wavelength [µm]

Fig. 4. Calculated confinement loses of guided fundamental (thin lines) and second (thick lines) modes in PCFs with different aspect ratio of elliptical holes e = 1.2, 1.25 and1.3. Material loses are not taken into account.

222 I. Kujawa et al. / Optical Fiber Technology 18 (2012) 220–225

fundamental mode (LP01) increases with ellipticity, (Fig. 2a)whereas the phase birefringence of the second mode (LP11) de-creases with ellipticity (Fig. 2b).

Based on those simulations we can determine a target ellipticityof the air-holes, where the birefringence of both the fundamentaland second modes are equal for a particular wavelength of illumi-nation (Fig. 3). This property has important consequences for thecharacterization of the fiber and the determination of the measure-ment mode, since the confinement losses of the LP11 modes are oneorder of magnitude higher than for the fundamental mode (Fig. 4).

3. Development of large core birefringent PCF with elliptical air-holes

A borosilicate glass (NC-21A) is used for the PCF development.This multi-component glass, synthesized in-house at ITME, hasan oxide composition by weight of 55.0% SiO2, 1.0% Al2O3, 26.0%B2O3, 3.0% Li2O, 9.5% Na2O, 5.5% K2O and 0.8% As2O3. This glass iswell suited for the development of complex fiber structures withthe stack and draw technology due to its very good rheologicalproperties [12,25,26]. The main physical properties of NC21A are:refractive index nD = 1.533, density q = 2.50 g/cm3, coefficient ofthermal expansion a20-300 = 82 � 10�7 K�1, glass transition temper-ature Tg = 500 �C and softening point DTM = 530 �C. The transmis-sion of NC-21A glass is limited to the range 380–2700 nm with arelatively high attenuation of 4 dB/m.

Fig. 5. Subpreform of large core birefringent PCF with rectangular air-holes – general view

For the preform assembly, we used rectangular cross-sectionglass capillaries (Fig. 5c) with an axis aspect ratio of 0.33 and linearfilling factors of fx = 0.90 and fy = 0.75 ordered in a rectangular lat-tice. The rectangular capillaries are drawn by a fiber drawing towerto a size of 2 by 4 mm. As a preform for these capillaries, a rectangu-lar tube built from two L-shaped glass profiles fused in a furnace at atemperature slightly above the glass softening point was used. It iswell known that the lattice geometry used at the PCF preform stageinfluences the shape of the air-holes formed during the fiber draw-ing process (Fig. 5). Choosing a rectangular lattice at the preformstage therefore results in elliptical air-holes during drawing of thesubpreform rods. The core of the fiber is formed with four rectangu-lar rods and surrounded by four concentric rings of elliptical rods.

During sub-preform and fiber drawing, we use a low-speeddrawing process to ensure a homogenous heat distribution in thesubpreform and a relatively low pulling temperature of 730 �C topreserve the ellipticity of the air holes. The drawing process, per-formed on a fiber drawing tower, uses a feeding speed of 1.5 mm/min and a pulling speed of 0.6 m/min. Accurate control and adjust-ment of these drawing parameters is essential to obtaining ellipticalholes, since the subpreform air-holes tend to become circular. Forour final fiber we chose a rectangular cross-section which allowseasy identification of the main axis and simplifies the orientationand avoids twisting of the fiber during measurement. The fabri-cated fibers have lattice constants of Kx = 2.6 lm and Ky = 1.6 lmfor main axes X and Y respectively (Fig. 6). The size of the ellipticalholes varies slightly depending on the location in the structure

(a), core area (b) and rectangular-like microcapillary – the preform component (c).

Fig. 6. SEM photograph of PCF with elliptical air-holes in the cladding (a) rectangular cross-section of the fiber, (b) photonic cladding, and (c) rectangular core.

I. Kujawa et al. / Optical Fiber Technology 18 (2012) 220–225 223

(Fig. 6c). This is due to too high a preform temperature during fiberdrawing corresponding to too low a glass viscosity. Under theseconditions, small differences in the inner dimensions of the preformcapillaries result in pressure differences and, consequently, an in-crease in the diameter of some of the holes. Most of the enlargedholes are located in the outside of the ring structure which corre-lates well with the temperature gradient in the structural preformduring drawing. These imperfections don’t significantly degradethe fiber birefringence, attenuation or modal properties. The distor-tion of the rectangular lattice results form some irregularities ofrectangular-like capillaries obtained during individual capillarydrawing process (Fig. 5c) These imperfections resulted in latticeshifts during sub-preform drawing process (Fig. 5b) and it couldn’tbe further corrected during final fiber drawing process. The holeshave minor and major axes of dx = 1.12 lm and dy = 0.88 lm,respectively, yielding linear filling factors of fx = 0.43 and fy = 0.55(Fig. 6). The average measured ellipticity of the air holes is aboutg = 1.275. The core of the fiber is rectangular with dimensions4.1 � 6.5 lm.

4. Characterization and modeling of developed fibers

We used the standard spectral interferometric method withcrossed polarizers to determine the group birefringence. The mea-surement setup is presented in Fig. 7.

Fig. 7. Group birefringence

Fig. 8. Registered interferograms for fundamental (

Polarized supercontinuum light is launched into the sample fi-ber. The first polarizer is aligned such that both polarization modesare equally excited. At the output of our sample, an analyser is ori-ented at 90� with respect to the input polarizer. The output signalis registered using a spectrum analyser and monitored with a CCDcamera. The CCD camera allows the verification of the near fielddistribution of the fiber output and ensures proper light couplinginto the core of the PCF. The spectrum analyser records the modu-lation of the intensity as a function of wavelength which resultsfrom the interference between the polarized components of thepropagating mode. Maximum intensity occurs when

dD/dk

Dk ¼ �2p ð3Þ

where D/ is the phase shift corresponding to successive fringes inthe output spectrum represented by their maxima and Dk is the dis-tance between successive fringes.

By changing the input coupling conditions, we can selectivelyexcite the fundamental and second modes. The selected mode isverified with a CCD camera simultaneously imaging the output fa-cet of the measured fiber during the spectrum measurements(Fig. 7). The registered interferograms are presented in Fig. 8 (fiberlength L = 0.5 m). The decrease of output signal we observe above1.4 lm is related to the water absorption peak in the fiber (phe-nomena observed for both modes), but the further decrease of

measurement set-up.

a) and higher mode (b). Fiber length L = 0.5 m.

wavelength [µm]

phas

e B

and

grou

p G

bire

fring

ence

wavelength [µm]

phas

e B

and

gro

up G

bire

fring

ence

(a) (b)Fig. 9. Experimental (points) and numerical simulation (lines) results of phase and group modal birefringence vs. wavelength for developed fiber based SEM photos (a) andbased on estimated idealized fiber structures (b). Both the fundamental mode (black) and higher order mode (red) results are shown. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

224 I. Kujawa et al. / Optical Fiber Technology 18 (2012) 220–225

intensity above 1.5 lm is related to the increasing losses for longerwavelengths above the sensitivity of the spectrometer.

Based on the interferogams we calculated the group birefrin-gence for the sample fiber (Fig. 9) as:

jGj ¼ k2

DkLð4Þ

where k is the average wavelength between two successive fringesand L is the length of the measured fiber. We have obtained a groupbirefringence magnitude of G = 2 � 10�4 at 1550 nm. The sign of Gcannot be directly determined in our experiment; however, themodeling results show a negative sign for G.

Simultaneously we have calculated the phase and group bire-fringence based on the SEM micrographs (Fig. 6a) using Eqs. (1)and (2). The calculated values presented in Fig. 9a are in very goodagreement with the measured group and phase birefringence val-ues. To estimate an influence of imperfection in fiber developmenton the output characteristics we have calculated the group and

Fig. 10. A SEM photo based (solid ellipses) and idealized rectangular lattice (emptyellipses) structures of the developed fiber.

phase birefringence of a perfect structure based on the calculatedaverage values of the hole dimensions and lattice pitch (Fig. 9b).The developed and idealized structures based on the drawn fiberare shown in Fig. 10. The experimental results match perfectlythe numerical simulations for the drawn fiber based SEM photos(a) and those based on the estimated idealized fiber structures.This implies that imperfections introduced during the fiber manu-facturing process have a minimal effect on the fiber birefringenceand shows a good tolerance to fabrication errors of the designed fi-ber structure.

According to the numerical simulations of the actual fiber, theconfinement losses of the higher modes are about one order ofmagnitude greater than those observed for the fundamental mode(Fig. 11). This observation is confirmed by the very low signal forthe second mode above 1.5 lm. In practice even short lengths ofthe sample fiber can be treated as single mode above 1.5 lm. Thefundamental mode has an effective mode area of 20 lm2. This rel-atively large mode area ensures efficient coupling with standardsingle mode fiber.

Fig. 11. Calculated confinement losses vs. wavelength for fundamental (thin lines)and higher order mode (thick lines).

I. Kujawa et al. / Optical Fiber Technology 18 (2012) 220–225 225

5. Conclusion

We have reported the development of a highly birefringent PCFwith elliptical air holes and squeezed lattice cladding with a largemode area of 20 lm2. The large core is created by multiple defectsof the photonic structures and allows the guiding of two propagat-ing modes. The birefringence of both modes has been measured andshown to be in good agreement with numerical simulations. Thephase birefringence of the fundamental mode is on the order of10�4, due to the low air filling factor, and the group birefringenceG = 2 � 10�4 at 1550 nm. These PCFs are well suited to applicationsin optical fiber directional transverse strain sensors where theirrectangular cross-section allows the avoidance of twisting duringmounting and provides straightforward orientation of the polariza-tion axes with respect to the direction of applied force.

Acknowledgments

This work was supported in part by Polish Ministry of Science ofScience and Higher Education Research Grant NN515244737 andby an internal scientific grant of ITME.

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