artykuł

MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 64, 1, (2012), 22-25
www.ptcer.pl/mccm
Birefringence of glass-air photonic crystal ¿bre
induced by elliptical microholes of cladding
IRENEUSZ KUJAWA1*, RYSZARD BUCZYēSKI1, 2, DARIUSZ PYSZ1, TADEUSZ MARTYNKIEN3, HUGO THIENPONT4
Institute of Electronic Materials Technology, Glass Laboratory, WólczyĔska 133, 01-919 Warsaw, Poland
University of Warsaw, Faculty of Physics, Pasteura 7, 02-093 Warsaw, Poland
3
Wroclaw University of Technology, Institute of Physics, WybrzeĪe WyspiaĔskiego 27, 50-370 Wroclaw, Poland
4
Vrije Universiteit Brussel, Department of Applied Physics and Photonics, Pleinlaan 2, 1050 Brussels, Belgium
*e-mail: [email protected]
1
2
Abstract
In this paper, we report on the fabrication of the birefringent photonic crystal ¿bre with a photonic cladding composed of elliptical holes
ordered in a rectangular lattice. Choice of such con¿guration allows obtaining birefringence in photonic crystal ¿bres. In this case two-fold
rotational symmetry is achieved and the polarized orthogonal modes (HE11x and HE11y) are not degenerated. We discuss the inÀuence of
structural parameters including the ellipticity of the air holes and the aspect ratio of the rectangular lattice on the birefringence and on the
modal properties of this glass-air microstructural ¿bre.
Keywords: Photonic crystal ¿bres, Microstructured ¿bres, Birefringence, Multicomponent glass
DWÓJàOMNOĝû W SZKLANO-POWIETRZNYM ĝWIATàOWODZIE FOTONICZNYM WYWOàANA
ELIPTYCZNYMI MIKROOTWORAMI PàASZCZA
W artykule przedstawiono uzyskane wáókno dwójáomne o anizotropii optycznej wywoáanej eliptycznym ksztaátem elementów páaszcza
fotonicznego i prostokątnym ksztaátem przekroju. WyraĨnie okreĞlona dwuosiowoĞü prowadzi do duĪych róĪnic efektywnych wspóáczynników zaáamania dla dwóch podstawowych i ortogonalnie spolaryzowanych modów HE11x i HE11y. DziĊki temu mody te nie są zdegenerowane. ZaleĪnie od parametrów strukturalnych wáókna moĪliwe jest uzyskanie znacznych wartoĞci dwójáomnoĞci. W artykule przedstawiono zarys technologii wytwarzania szklanych wáókien tego typu, przedyskutowano wpáyw parametrów strukturalnych na wartoĞü dwójáomnoĞci i wáasnoĞci modowe oraz wyniki symulacji i pomiarów dla uzyskanego wáókna.
Sáowa kluczowe: Ğwiatáowody fotoniczne, Ğwiatáowody mikrostrukturalne, dwójáomnoĞü, szkáa wieloskáadnikowe
1. Introduction
The high birefringence in the photonic crystal ¿bres
(PCFs) is a result of large effective index anisotropy possible to achieve only in the two-fold rotational symmetry
photonic structure [1]. A typical PCF which has three-fold
rotational symmetry (m = 3) with an ideal hexagonal lattice
and circular holes is not birefringent [2]. For real samples
of such ¿bres, degeneracy of fundamental mode breaks
down due to fabrication imperfections and low-birefringence
always occurs. However, a photonic crystal ¿bre with global
anisotropy can achieve extremely high birefringence of the
order of 10-2 when rectangular lattice and elliptical-like air
holes are applied [3]. Such PCFs can ¿nd applications in
the optical ¿bre directional transverse strain sensors with the
bene¿t that their rectangular cross-section allows avoiding
twist during mounting and provides straightforward orientation of the polarization axis with respect to the direction of
applied force, and in many others applications for example
in sensing and in the telecommunication [4, 5]. Unfortunately
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elliptical holes with controlled dimensions are indeed much
more dif¿cult to obtain than circular holes.
In this paper we deal with our latest achievements in
the fabrication of such rectangular lattice. First, we brieÀy
describe the fabrication process. Second, we deal with the
measurement set-up and with our experimental results.
Finally, we present the results our simulations.
2. Fabrication
The lattice geometry used at the PCF preform stage
inÀuences the shape of the air holes formed during the ¿bre
drawing process [6, 7]. One typically observes the tendency
to form air holes with a shape that matches the lattice geometry. This effect can be controlled for structures with a high
air-¿lling factor. Choosing a rectangular lattice at the preform
stage therefore results in elliptical air holes during drawing
of the subpreform (Fig. 1).
However, during ¿bre drawing, the subpreform air holes
tend to become circular, which renders controlling the ellipti-
BIREFRINGENCE OF GLASS-AIR PHOTONIC CRYSTAL FIBRE INDUCED BY ELLIPTICAL MICROHOLES OF CLADDING
a)
b)
c)
Fig. 1. Subpreform with rectangular air holes: a) general view, b) core, c) single hole.
Rys. 1. Szklana subpreforma o prostokątnych otworach: a) widok ogólny, b) obszar rdzenia, c) pojedynczy otwór.
Fig. 2. The photos of obtained PCF.
Rys. 2. ZdjĊcia wytworzonego wáókna.
cal hole features very dif¿cult (Fig. 2). To specify the obtained
structure, we rely on the following de¿nitions: the lattice
constant along the X-axis ȁx; the lattice constant along the
Y - axis ȁy; the lattice constant ratio ȡ = ȁy /ȁx; the ellipticity of the air holes Ș = dx/dy (where dx and dy represent the
minor and major axis of the air-¿lled ellipse) and the linear
¿lling factors fx and fy given by fa = da /ȁa , when a is X or Y.
The lattice constants of our ¿bre are ȁx = 1.35 ȝm and
ȁy = 2.25 Pm. The size of the elliptical holes varies slightly
depending on the location in the structure (Fig. 2). The holes
have a minor and major axes dx = 735 nm and dy = 960 nm,
respectively, yielding linear ¿lling factors of fx = 0.54 and
fy = 0.43 and Ș = 0.76.
The base material for our PCF is a borosilicate glass
labelled NC-21A. This multicomponent glass is synthesized
in-house at the ITME and has an oxide composition detailed
in Table 1.
This glass is selected to verify our stack and draw technology for elliptical holes fabrication. The main physical
properties of NC21A are:
– refractive index nD = 1.533,
– density ȡ = 2.50 g/cm3,
– coef¿cient of thermal expansion Į20–300 = 82·10í7 Kí1,
– glass transition temperature Tg = 500°C,
– softening point DTM = 530°C.
Fig. 3. The curve of viscosity for NC-21A.
Rys. 3. Krzywa lepkoĞciowa dla szkáa NC-21A.
Table 1. Oxide composition of borosilicate NC-21A glass.
Tabela 1. Skáad tlenkowy borokrzemianowego szkáa NC-21A.
Oxide composition of borosilicate NC-21A glass [wt%]
SiO2
Al2O3
B2O3
Li2O
Na2O
K2O
As2O3
55.0
1.0
26.0
3.0
9.5
5.5
0.8
Fig. 4. Spectral transmission of NC-21A glass.
Rys. 4. Transmisja spektralna szkáa NC-21A.
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 64, 1, (2012)
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I. KUJAWA, R. BUCZYēSKI, D. PYSZ, T. MARTYNKIEN, H. THIENPONT
The curve of viscosity for NC-21A is present in Fig. 3.
The transmission of NC-21A glass is limited to the
range 380–2700 nm (Fig. 4) with a moderate attenuation
of 4–6 dB/m.
For the preform fabrication, we use ellipse-like glass
capillaries with axis aspect ratio of 0.33 and linear ¿lling
factors of fx = 0.90 and fy = 0.75 ordered in a rectangular lattice. The core of the ¿bre is formed with 3 rectangular rods.
During subpreform and ¿bre drawing, we use a low-speed
drawing process to ensure homogenous heat distribution in
the subpreform and a relatively low pulling temperature of
730 °C to preserve the ellipticity of the air holes. The drawing process is performed on a 6-m-long ¿bre drawing tower.
We use a feeding speed of 1.5 mm/min, while the pulling
speed is 0.6 m/min. Adjusting those drawing parameters is
essential to obtaining elliptical holes. For our ¿nal ¿bre (Fig.
2), we intentionally chose a rectangular cross section, which
allows easily identifying the main axis and avoiding twisting
of the ¿bre in the measurement set-ups.
3. Measurement results
The parameters characterizing a photonic crystal ¿bre
with the global anisotropy are the phase birefringence B
de¿ned as the difference between the propagation constants
Ex and Ey of the two orthogonally polarized components HE11x
and HE11y of the fundamental mode:
B
O
Ex Ey
2S
neff X neffY
O
Lb
(1)
and the group birefringence G de¿ned as:
G
B-O
dB
dO
(2)
B and G are different for PCFs, while they are almost the
same for conventional ¿bres [8]. We used the spectral interferometric method with crossed polarizers to determine the
group birefringence. The interferometer is shown in Fig. 5.
Polarized supercontinuum light obtained with a highly nonlinear PCF pumped with a femtosecond Ti:Sapphire laser
is launched into our ¿bre. A ¿rst polarizer is aligned so that
both polarization modes were equally excited. At the output
of our ¿bre, an analyzer is oriented at 90° with respect to the
input polarizer. The output signal is registered using a spectrum analyzer and monitored with a CCD camera. The CCD
camera allows verifying proper light coupling into the core
of the PCF. The spectrum analyzer records the modulation
of the intensity as a function of wavelength, which results
Fig. 5. Group birefringence measurement set-up.
Rys. 5. Ukáad pomiarowy.
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MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 64, 1, (2012)
Fig. 6. The measured interferogram, which shows interference
between polarization components in the PCF (1.2 —m is cut-off
wavelength for single mode regime).
Rys. 6. Interferogram uwidaczniający interferencje skáadowych
polaryzacji we wáóknie PCF (1,2 —m jest dáugoĞcią odciĊcia).
from the interference between the polarized components of
the propagating mode.
Maximum intensity occurs when:
d'M
dO
r2S
(3)
where 'ij is the phase shift corresponding to successive
fringes in the output spectrum represented by their maxima,
and 'Ȝ is the distance between successive fringes. The group
birefringence given by (2) can then be calculated as:
G
O2 d'M
˜
2SL dO
O2
'OL
(4)
where Ȝ is an average wavelength between two successive
fringes, and L is the length of the measured ¿bre. A measurement interferogram is depicted in Fig. 6.
Fig. 7. Values of group birefringence obtained from interferogram
(Fig. 6) for two regimes: left: two-modal, and right: single mode.
Rys. 7. WartoĞci dwójáomnoĞci grupowej uzyskane z interferogramu
(Rys. 6) dla zakresu dwumodowego (krzywa po lewej) i jednodomowego (krzywa po prawej).
BIREFRINGENCE OF GLASS-AIR PHOTONIC CRYSTAL FIBRE INDUCED BY ELLIPTICAL MICROHOLES OF CLADDING
Using an image from the actual structure obtained with
a Scanning Electron Microscope (SEM) we performed numerical analysis of the structure by means the ¿nite element
method (FEM). Simulations show that 2 modes are guided
in the ¿bre (1.2 —m is cut-off wavelength for obtained structure). Moreover for the fundamental mode at the wavelength
1.5 —m the phase birefringence reaches the value of 2.2·10-4
(calculated) and the group birefringence 3.0·10-4 (measured).
Acknowledgements
Fig. 8. Dispersion of NC-21A glass.
Rys. 8. Dyspersja szkáa NC-21A.
Measured values of a group birefringence obtained from
the interferogram and an equation (4) are presented in Fig.
7. The group birefringence for the fundamental mode at the
wavelength 1.5 Pm reaches the values of 3.0·10-4.
Using in-house software based on the ¿nite element
method (FEM), we calculated the phase birefringence using
the actual structural parameters as obtained from the SEM
photography (Fig. 2). In the simulation we took into account
the material dispersion of NC-21A glass (Fig. 8).
Calculations con¿rmed that 2 modes are guided in the
structure. The phase (B) birefringence for the fundamental
mode at the wavelength 1.5—m was calculated, and reaches
the values of 2.2·10-4.
4. Conclusions
In this paper, we present a rectangular lattice photonic
crystal ¿bre with elliptical like holes made of the high quality
silicate glass NC-21A. The manufactured ¿bre has a lattice
pitch ȁx = 1.35 —m and ȁy = 2.25 —m for main axes X and Y,
respectively, and elliptical holes with ellipticity Ș = 0.76. The
average ¿lling factors of 0.5 is obtained. The core of the ¿bre
is rectangular with dimension 3.3 —m x 5.4 —m.
This work was supported in part by Polish Ministry
of Science and Information Society Technologies grant
NN515244737, by the COST 299 action, and by an internal
scienti¿c grant of ITME. Some of the numerical results were
obtained using computer resources of the Interdisciplinary
Centre for Mathematical and Computational Modelling (ICM),
University of Warsaw.
The authors of the paper would like to thank Dr R. StĊpieĔ
(from ITME) for synthesis of NC-21A glass.
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Received 21 September 2011; accepted 15 December 2011
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 64, 1, (2012)
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