Highly efficient all-fiber tunable polarization
filter using torsional acoustic wave
Kwang Jo Lee*, Hyun Chul Park, Hee Su Park, and Byoung Yoon Kim
Department of Physics, Korea Advanced Institute of Science and Technology
373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea
* [email protected]
http://fiber.kaist.ac.kr
Abstract: We demonstrate an all-fiber tunable polarization filter with high
coupling efficiency based on acousto-optic coupling between two optical
polarization modes of the LP01 mode propagating in a highly birefringent
single mode optical fiber. An over-coupling between the two polarization
modes is realized over the wavelength range from 1530 nm to 1610 nm
using traveling torsional acoustic wave. The measured 3-dB optical
bandwidth of the filter was 4.8 nm at the wavelength around 1550 nm. The
details of the filter transmission and the coupling characteristics are
discussed.
©2007 Optical Society of America
OCIS codes: (060.2310) Fiber optics; (230.1040) Acousto-optical devices.
References and links
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tunable bandpass filter,” Opt. Express 15, 2987-2992 (2007).
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1. Introduction
Tunable optical filters are key components in optical communication networks and optical
sensor systems. In particular, all-fiber acousto-optic tunable filters (AOTFs) have attracted
considerable interest because of their advantages such as wide and fast wavelength tuning and
variable attenuation with simple electronic control [1]. Practical AOTFs with a simple
structure for various applications such as optical switches and wavelength tuning devices have
been realized as notch type [2] or bandpass type [3]. Most of reported all-fiber acousto-optic
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Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
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(AO) devices are based on wavelength selective AO coupling between several spatial modes
of light propagating in an optical fibers using traveling flexural acoustic wave. On the other
hand, it has also been known that AO coupling between two optical polarization modes in a
highly birefringent (HB) optical fiber can be realized using traveling torsional acoustic wave,
which is one of the three acoustic modes propagating in a cylindrical optical fiber without
cutoff when the acoustic wavelength is much longer than the dimension of the fiber core [4, 5].
Several researchers have investigated the potential of the polarization mode coupling by
torsional acoustic wave theoretically [6] and experimentally [7, 8]. However, the optical
coupling efficiency between two polarization modes demonstrated so far were only 6% and
12% [7, 8], which is too small for practical applications and therefore it has not been seriously
pursued.
In this paper, we demonstrate, for the first time to our knowledge, that a full coupling can
be achieved in an all-fiber acousto-optic tunable polarization filter (AOTPF) using the lowest
torsional acoustic mode traveling in a HB single mode optical fiber. The complete optical
power transfer between two polarization modes is realized over the wavelength range from
1530 nm to 1610 nm by employing a newly designed transducer that efficiently generates
torsional acoustic wave in the fiber. The fabricated AOTPF can operate as the notch type or
the bandpass type according to the polarization direction of the output polarizer. The coupling
characteristics between two optical polarization modes by the lowest torsional acoustic mode
and the transmission characteristics of the fabricated filter are discussed in detail.
2. Principles of operation
A cylindrical optical fiber, whose diameter is smaller than acoustic wavelengths, can support
longitudinal, flexural and torsional acoustic fundamental modes traveling along the fiber
without cut-off [4, 5]. Among them, the torsional acoustic mode having only the
circumferential angular displacement component in the cross-section of the fiber can
efficiently produce AO coupling between two optical polarization eigenmodes in a HB optical
fiber [6]. The periodic twists of the optical polarization eigenaxes in the HB fiber induced by
the torsional acoustic wave perturb the incident polarization eigenstate of the LP01 mode and
cause the energy to be transferred efficiently between two polarization eigenmodes. The
torsional acoustic modes can exist in the optical fiber with the various radial orders which are
defined as the number of nodes of the acoustic energy distribution in the radial direction [4].
The lowest torsional acoustic mode propagates for all frequencies and wave numbers with a
constant velocity equal to the shear-wave velocity, while the torsional acoustic modes of
higher radial orders are dispersive only above their cutoff frequencies of a few hundreds MHz
[6]. For the acoustic frequencies of a few MHz of our interest that provide phase matching
condition for efficient polarization coupling, only the lowest torsional acoustic mode can
propagate along the fiber. Coupling between the two polarization eigenmodes caused by the
periodic twists of the HB optical fiber can be described by the coupled mode equations [6],
and the coupling coefficient κ can be expressed as following,
κ = 2π
Φ
1
(1 + n 2 p44 ) .
2
LB
(1)
Here, Φ denotes the angular displacement in the cross-section of the fiber and LB is the
optical beatlength between two polarization eigenmodes. The refractive index of n and the
photo-elastic tensor element of p44 are 1.46 and -0.075 for fused silica, respectively. The first
term of the Eq. (1) corresponds to the perturbation between two polarization eigenstates of the
incident LP01 mode caused by the birefringence effect due to the periodic angular deviation of
the polarization eigenaxes in the HB optical fiber. The second term, (1/2)n2p44, accounts for
the refractive index changes caused by the photo-elastic effect due to the induced strain in the
periodically twisted fiber, which has the numerical value of -0.08 and hence tends to reduce
the mode coupling [9]. Therefore, the main contribution of AO coupling by the lowest
#84902 - $15.00 USD
(C) 2007 OSA
Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 12363
torsional acoustic mode is from the geometrical twist effect which is reduced by 8% due to the
elasto-optic effect. As is well known, the most efficient coupling is achieved when phase
matching condition is satisfied where the acoustic wavelength is the same as LB.
3. Experiment and analysis
Figure 1 shows the schematic of a proposed AOTPF. The device is composed of a torsional
acoustic transducer, two polarizers, and a HB single mode optical fiber (LB = 2.8 mm @ 1550
nm, the stress induced birefringence produced by an elliptical stress member in the cladding,
3MTM). The input polarization state of the LP01 mode was aligned to one of the polarization
eigenstate using the in-line fiber polarizer. The torsional acoustic wave was generated by the
combination of two shear mode lead zirconate titanate (PZT) plates attached to the end of an
acoustic horn with epoxy adhesive, as shown in Fig. 2. The two PZT plates were arranged so
that they oscillate 180 degree out of phase. The torsional acoustic wave generated by the
acoustic transducer was coupled to a bare section of the fiber bonded to the central hole in the
acoustic horn, and was absorbed by an acoustic damper at the end of the interaction region.
Torsional acoustic wave
HB optical fiber
Polarized light
Output Polarizer
Acoustic damper
Acoustic transducer
I
I
0
I
0
λ
0
λ
λ
Fig. 1. Schematic of an all-fiber acousto-optic tunable polarization filter.
Vibration direction
Shear mode PZT
Front view
Glass horn
Side view
Fig. 2. Transducer design for generation of torsional acoustic wave. Two oppositely poled PZT
elements are attached to glass horn.
The incident eigen polarization of the LP01 core mode is converted to the other eigen
polarization of the same core mode at resonant wavelength satisfying the phase matching
condition, as the following:
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Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 12364
β 01, x − β 01, y = 2π LB ,
(2)
where, LB is the beatlength and each β denotes the wave number for two eigen polarizations of
LP01 mode, respectively. When the phase matching condition is satisfied, the acoustic
wavelength is the same as beatlength between two polarization eigenmodes. As can be seen in
Eq. (2), the beatlength is a function of wavelength and, for a given acoustic frequency, a
specific wavelength component will be filtered in the device. The resonant wavelength and
the transmitted power of the filter can be tuned by adjusting the frequency and the magnitude
of the applied electric signal, respectively. Since the converted polarization mode can be
selected or removed by adjusting the polarization direction of the output polarizer, the
fabricated filter can be operated as the notch type or the bandpass type as illustrated in Fig.1.
For instance, when the polarization directions of the input and the output polarizers are
oriented parallel to each other, the AOTPF operates as a notch filter. On the other hand, when
the polarization direction of the output polarizer is perpendicular to that of the input polarizer,
the AOTPF operates as a bandpass filter.
Figure 3 shows the measured transmission spectra of the polarization filters operating as
the notch type (Fig. 3(a)) and as the bandpass type (Fig. 3(b)) at the applied acoustic
frequency of 1.337 MHz. The measured 3-dB optical bandwidth for a 60-cm-long AO
interaction region was 4.80 nm at the wavelength around 1550 nm, which agrees well with the
theoretical value of 4.85 nm calculated with the equation in [10]. The variation of 3-dB
bandwidth was only about 0.5 nm for optical wavelength range from 1530 to 1610 nm.
0
(a)
-3
-6
-9
-12
-15
Transmission [dB]
Transmission [dB]
0
(b)
-5
-10
-15
-20
-25
-18
1526
1533
1540
1547
1554
1561
1568
1526
1533
1540
1547
1554
1561
1568
Wavelength [nm]
Wavelength [nm]
Fig. 3. Measured transmission spectra of the all-fiber AOTPF operating as (a) the notch type
and as (b) the bandpass type at the acoustic frequency of 1.337 MHz.
Figure 4 shows the dependence of the optical coupling efficiency between two
polarization eigenmodes to the magnitude of the applied voltage. The coupling strength
increased with the increase of applied voltage and then decreased after complete optical power
transfer between two polarization modes because of an over-coupling between the two
eigenmodes. The achieved maximum coupling efficiency was 100% at the wavelength around
1550 nm. The angular displacement of the torsional wave, Φ can be calculated from the
coupling length Lc = π/(2κ) for a 100% coupling as;
−1
Φ=
1 LB ⎛ 1 2 ⎞
⎜ 1 + n p44 ⎟ .
4 Lc ⎝ 2
⎠
(3)
For a 60 cm-long coupling length used in this experiment, Φ is estimated to be 0.073° for a
complete coupling. This corresponds to 11.3 mW of average acoustic power propagating
along the fiber and that is 5.65% of the electrical power applied to the transducer in our
experiment. The values are comparable to the reported efficiency of acoustic transducer for
all-fiber AOTF using flexural acoustic wave [11]. The device can be reduced in length by
#84902 - $15.00 USD
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Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 12365
using the HB fiber of smaller diameter because the coupling coefficient κ is inversely
proportional to the square of the fiber radius [6] and the AO interaction length Lc can be
expressed as Lc = π/2κ with the coupling coefficient κ. Figure 5 shows the center wavelength
change of the AOTPF as a function of the acoustic wavelength showing an almost linear
relationship.
Coupling efficiency [%]
100
80
60
40
20
0
0
10
20
30
40
50
60
70
Applied voltage (peak to peak) [V]
Fig. 4. Optical coupling efficiency between two polarization eigenmodes versus the magnitude
of the applied voltage.
Center wavelength [nm]
1600
1590
Experiment
Theory
1580
1570
1560
1550
1540
1530
2.78 2.80 2.82 2.84 2.86 2.88 2.90 2.92
Acoustic wavelength [mm]
Fig. 5. Center wavelength of the all-fiber AOTPF as a function of the acoustic wavelength.
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Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 12366
In order to investigate the influence of fiber tension to the filter performance, we induced
tension to the AO interaction length of the device and measured the transmission spectra of
the filter. Preliminary experiment results show 0.15 nm shift of the center wavelength for
0.1% strain and the detailed study is underway.
4. Conclusion
In conclusion, we have demonstrated a practical all-fiber tunable polarization filter based on
AO coupling between two polarization eigenmodes of the LP01 mode in a HB single mode
optical fiber using traveling torsional acoustic wave. The fabricated filter was operated as the
notch type or the bandpass type according to the polarization direction of the output polarizer.
Complete coupling between two polarization eigenmodes was achieved and the measured 3dB bandwidth was 4.8 nm at the wavelength around 1550 nm. The center wavelength could be
tuned continuously and almost linearly by tuning the acoustic frequency over the whole tuning
range of 1530 - 1610 nm limited by the light source. The transmission characteristics of the
filter and the coupling characteristics between two optical polarization modes by the lowest
torsional acoustic mode were investigated in detail. Many new active polarization controlling
devices would be possible for applications in optical communications and sensing.
#84902 - $15.00 USD
(C) 2007 OSA
Received 5 Jul 2007; revised 8 Sep 2007; accepted 11 Sep 2007; published 13 Sep 2007
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 12367