Download In-line polarization controller that uses a hollow

November 15, 2004 / Vol. 29, No. 22 / OPTICS LETTERS
2605
In-line polarization controller that uses a hollow optical fiber
filled with a liquid crystal
Young-Hoon Oh, Min-Suk Kwon, and Sang-Yung Shin
Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology,
373-1 Kusong-dong, Yusung-gu, Taejon 305-701, South Korea
S. Choi
LG Electronics PRC, 19-1 Cheongho-ri, Jinwuy-myun, Pyungtak-si, Kyunggi-do, 401-713, South Korea
K. Oh
Department of Information and Communications, Gwangju Institute of Science and Technology,
1 Oryong-dong, Buk-gu, Kwangju 500-712, South Korea
Received June 17, 2004
We demonstrate an in-line polarization controller based on a hollow optical fiber filled with a nematic liquid
crystal by fabricating thin-film electrodes on the cladding of a fiber. The polarization controller consists of
three control sections with fixed optic axes, which operate as phase retarders. The phase retardation in each
section is controlled by the magnitude of the applied electric field. The full wave retardation voltage of the
polarization controller is ⬃85 V. © 2004 Optical Society of America
OCIS codes: 060.2340, 160.3710, 230.3720, 260.5430.
Communication systems with polarization-induced
problems need dynamic polarization controllers that
may be implemented by liquid crystals,1 – 3 fiber squeezers,4 Faraday rotators,5 and LiNbO3 waveguides.6
Liquid-crystal- (LC-) based devices may offer some
potential advantages: high electro-optic response,
low power consumption, and low-cost fabrication.7
LC-based polarization-controlling devices have traditionally been demonstrated by use of fiber ferrules2 or
LC cells that consist of glasses, spacers, and LCs.1,3 In
these cases the polarization controllers require complex hybrid packaging of ferrule– LC– ferrule or
fiber –LC cell– f iber, which results in alignment loss.
Recently the index modulation of a LC inside a hollow
optical fiber (HOF) was reported.8 Jeong et al.8 have
shown the tunability of a long-period f iber grating
by using an external long-period-combed electrode.
Also, they reported an in-line-type polarizer that uses
polymer-dispersed LC and external electrodes.9 In
the research of Jeong et al. the f ibers are aligned
with a glass tube to produce the in-line-type device,
and the function of the polarizer is obtained from
the scattering of a polymer-dispersed LC. However,
the total insertion loss was not given, although they
mentioned that the in-line polarizer has the advantage
of easy alignment and low transmission loss.
In this Letter we report an in-line polarization
controller with thin-f ilm electrodes on a HOF f illed
with a nematic LC. The thin-f ilm electrodes were
fabricated on the cladding of the HOF. A thin-f ilm
electrode may be preferable to an external electrode
because it facilitates compact packaging, simple exact
angle control of optic axes, and easy fabrication of
electrode pairs. The proposed device has the following features: simple alignment-free fabrication, low
cost, and compact design.
0146-9592/04/222605-03$15.00/0
Nematic LCs with aligned directors exhibit uniaxial birefringent properties, and their optic axes
can be realigned by the applied electric field. This
controllability is useful for polarization-controlling
devices. The polarization controller should transform
an arbitrary input state of polarization (SOP) to a
desired output SOP. Our polarization controller
has three control sections of the phase retardation
type, following the bulk-type polarization controller
of Zhuang et al.1 Figure 1 is a schematic diagram
of the proposed device with the principal axes of the
wave plates f ixed. The principal axis of the second
control section is rotated by 45± with respect to that
of the first section and the third section, whereas the
principal axis of the first section is parallel to that
of the third section. The principal axes of the three
control sections are formed by an average arrangement
of LC directors. Therefore the principal axes of the
control sections are determined by the positions of
Fig. 1. Schematic diagram of the proposed device.
© 2004 Optical Society of America
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OPTICS LETTERS / Vol. 29, No. 22 / November 15, 2004
the electrodes fabricated on the cladding, as shown in
Fig. 1.
The initial alignment of directors depends highly on
the capillary interface interaction of the liquid crystal.
We use the HOF made of the silica capillary to make
directors align along the axis of the fiber.10 Figure 2
shows the alignment states of directors in the hollow
core of the HOF in two cases: Fig. 2(a) without an
external electric field and Fig. 2(b) with it. As shown
in Fig. 2(b), it is assumed that the directors lie in the
x z plane and that they are initially in the direction
of the HOF’s axis (z axis). First, the two-dimensional
electrostatic f ield formed by the concave electrodes
that lie on the x axis was simulated by using CST EM
STUDIO. From the simulation result, the electric
field in the hollow core region could be approximated
to only an x component field because the hollow core
is much smaller than the cladding. Therefore we may
assume uniform electric field distribution within the
hollow core simply to analyze the principle of operation, although the HOF has a cylindrical geometry.
If an electric field is applied in the direction of the
x axis, the directors deviate from the z axis, making
an angle u. Thus the x-polarized light becomes the
extraordinary wave in a uniaxial medium whose optic
axis is along the LC directors, while the y-polarized
light is the ordinary wave whose refractive index is
no . Therefore the refractive index of an x-polarized
wave is given by11
1
cos2 u ,
sin2 u
1
苷
nx 2 共u兲
ne 2
no 2
(1)
where ne and no are the extraordinary and the ordinary indices, respectively, of a nematic LC. Phase
retardation G is a function of tilt angle u, which is controlled by the magnitude of the applied electric field.
It is given by
G苷
2pL
关nx 共u兲 2 no 兴 ,
l
10 mm. Finally, we completed the device by filling
the hollow core with a nematic LC (ZLI 1800-100, from
Merck Korea) by capillary action.
The device was pigtailed with standard single-mode
fiber by use of UV-curable epoxy. The fiber-to-f iber
insertion loss of the 35-mm-long device was ⬃8 dB,
which includes mode mismatch loss between the HOF
and the single-mode-f iber and scattering loss that is
due to the directors. The measured propagation loss
of the HOF f illed with the LC was ⬃1 dB兾cm. From
the result we estimate that the loss that is due to mode
mismatch and misalignment was ⬃2 dB on average
at the input and output ends. To reduce the total
insertion loss required optimization of HOF design
parameters along with reduction of the loss that is due
to the LC. We measured the amplitude and the frequency of the applied voltage of the fabricated device.
First we applied a low-frequency sinusoidal signal
with a peak-to-peak voltage from 285 to 185 V to
each of three control sections to test whether the
control sections operate as phase retarders. Figure 4
shows variations of the output SOP of the f irst control
section and the second on the Poincaré sphere, which
Fig. 2. Alignment of nematic directors of the nematic LC
(a) without and (b) with an external electric f ield. Angle
u increases with applied voltage.
(2)
where L is the length of the electrodes and l is the
free-space wavelength of light. The explicit mathematical expression for G1 , G2 , and G3 is given in Ref. 1;
Stokes parameters are used for given input and target
SOPs.
The HOF diameters of cladding, core, and hollow
core are approximately 125, 9, and 4 mm, respectively. Figure 3 shows the procedure for fabrication
of thin-f ilm electrodes on the cladding. First, for deep
reactive-ion etching (DRIE) a SiO2 window mask was
formed upon a thermally oxided (100) silicon wafer by
standard photolithography. Next, the substrate was
etched to a depth of ⬃150 mm by deep reactive-ion
etching. Then anisotropic wet etching was done in
an aqueous solution of 50-wt. % potassium hydroxide
(KOH) at 80 ±C. To form the electrodes of proposed
device, the HOF was inserted into the diamond-shaped
hole of the structure. Cr with a thickness of ⬃100 nm
was deposited upon the cladding of the HOF by
electron-beam evaporation. The width of the fabricated electrodes was 60 mm, and their length was
Fig. 3. Procedure for fabrication of thin-f ilm electrodes on
the cladding.
November 15, 2004 / Vol. 29, No. 22 / OPTICS LETTERS
Fig. 4. Variation of output SOP in the first and the second
control sections.
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for acquiring 10± phase retardation is ⬃10 V, and
the full wave retardation voltage is ⬃85 V. Using
these data shown in Fig. 5, we obtained the adjusting
voltages from the appropriate phase retardations G1 ,
G2 , and G3 with which to transform the input SOP into
the desired SOP. Next we measured the frequency
response of the device by applying the same sinusoidal
signal to one of the control sections. If the bandwidth
is def ined with reference to the point where the
response has fallen by 3 dB, that is, at half of the total
excursion, the bandwidth of the polarization controller
is ⬃40 Hz, which is typical in LC-based devices.
We have proposed and demonstrated an in-line
polarization controller that uses a hollow optical fiber
filled with a nematic liquid crystal. The polarization
controller has a full wave retardation voltage of ⬃85 V
and a 3-dB bandwidth of 40 Hz. One may achieve
a reduction of operating voltage and improvement
in modulation speed by decreasing the cladding
thickness.
This research was partially supported by Novera
Optics, Inc. Y.-H. Oh’s e-mail address is yhoh@
eeinfo.kaist.ac.kr.
References
Fig. 5. Phase retardation in one of the control sections as
a function of applied voltage.
were measured at a wavelength of 1.55 mm by a
conventional polarization analyzer (HP 8509B). As
shown in Fig. 4, the rotation axis of the first control
section, V1 , is almost perpendicular to that of the
second section, V2 , graphically. From that fact we
knew that the thin-f ilm electrodes of the second
section were formed well at a 45± angle with respect
to that of the first. The phase retardation versus
the applied voltage in one of the control sections
is plotted in Fig. 5. The threshold voltage needed
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