Electrically Variable Optical Attenuator using All-Fiber Sagnac Loop ...

XXIX ENFMC - Annals of Optics 2006
Electrically Variable Optical Attenuator using All-Fiber Sagnac
Loop Interferometer
Michael Fokine, Helio Carvalho, Paula M. P. Gouvêa, Danays M. González, I. C. S. Carvalho
Physics Dept., PUC-RIO
Maria Cristina R. Carvalho
CETUC, PUC-RIO
Oleksandr Tarasenko, Walter Margulis
ACREO-Sweden
Rua Marquês de São Vicente, 225 Gávea.
22452-970 Rio de Janeiro - RJ
michael@fokine.net
Abstract
In this work a variable optical attenuator, based on the all-fiber Sagnac loop interferometer is
examined. The intensity of the transmitted signal at the output of the interferometer is controlled
by changing the polarization state of the light in the loop of the interferometer. Three different
means of controlling the state of polarization of the light in the loop are tested, including
mechanical polarization control as well as electrical polarization control by using the fibers with
internal electrodes.
Introduction
As high-speed optical telecommunication systems continue to advance to higher capacities, the need for
more functional optical components and devices increase. One important component is the variable optical
attenuator (VOA). There has been a strong demand for Variable Optical Attenuators (VOAs) in e.g. DWDM
networks due to their variable attenuation range [1] and possibility to tailor their wavelength characteristics.
Typical applications are e.g. to balance the signal intensities in WDM networks, optimizing gain in erbium
doped amplifiers, or to protect devices and systems from damage due to high intensities.
Today, there are several types of commercial VOAs, such as microelectromechanical systems (MEMS) [2],
liquid crystals based devices [3]. These systems however, increase the use of fiber-to-bulk coupling that require
additional points where the light needs to be coupled out of or into the fiber. The key advantages for using allfiber
VOA’s are high stability, potentially high speed devices, simplicity in design and low loss. An important
technique for controlling the transmitted signal in a fiber loop reflector or Sagnac loop interferometer has been
previously shown by Mortimore [4], presenting an interesting result on how to control the transmitted signal by
inducing birefringence.
In this work we explore a VOA based on the Sagnac loop interferometer in which the intensity of the
transmitted signal at the output of the interferometer can be controlled by controlling the polarization state, or
birefringence, in the loop of the interferometer [5]. Three different methods of controlling the state of
polarization of the light in the loop are examined; (a) a mechanical fiber polarization controller, to characterize
the fundamental behavior of the VOA Sagnac loop Interferometer, (b) a fiber based Kerr phase modulator, and
(c) thermally poled fiber. The latter two methods are based on fibers containing electrodes positioned along the
core of the fiber, enabling electrical control of the VOA with fast response as well as providing all-fiber
solutions.
Experimental Setup
The experimental setup consists of a Sagnac loop Interferometer configuration as shown in figure 1. The fiber
Sagnac interferometer is made by combining the two output ports of a 3 dB coupler, at a wavelength of 1550 nm,
to form the loop. The type of polarization controller under study is then placed inside the loop. A laser at 1550
nm is launched into port 1, while the intensity of the signal is monitored at port 2 using an InGaAs photodetector
and an oscilloscope. The 1550 nm laser is modulated using a function generator, which was also used to trigger
the oscilloscope. The intensity of the laser was approximately -10 dBm, avoiding any non-linear effects from the
light in the interferometer.

XXIX ENFMC - Annals of Optics 2006
The mechanical polarization controller was a Newport F-Pol-PC device. This device works on the principle of a
mechanical fiber squeezer mechanism, which is rotated about the fiber. This allows conversion of any input
polarization to any desired output polarization. The Kerr phase modulator and the poled fiber device are based
on metal-electrode filled two-hole fibers. The metal electrodes are inserted into the holes of the fiber by a
temperature-pressure process described in detail elsewhere [6]. For the Kerr phase modulator a low-temperature
alloy of Bi-Sn was used and typical fiber-electrode lengths of 1-2 meters [7]. For the poled fiber device, either
lead (Pb) or Au-Sn alloy was used. In this case the length of the fiber-electrode is much shorter, typically 10-15
cm. Thermal poling of electrode containing fiber is described in detail by Myrén et. al. [8]
Figure 1: Schematic diagram of Sagnac loop interferometer and experimental setup (MPC-
Mechanical polarization controller, EPC-electric polarization controller)
Results and Discussions
Figure 1 shows the output response of the Sagnac loop interferometer as a function of the angular variation of the
mechanical polarization controller at a set value of applied pressure to the fiber squeezer. The function of the
rotation of the mechanical polarizer corresponds approximately to a corresponding rotation of a half wave plate,
showing a maxima or minima every 45 degrees. As can be seen, the rotation of the mechanical polarizer results
in very repeatable results with minima very close to zero, representing a signal extinction ratio of more than
99%, indicating the potential as a wide dynamic range device. As for long term operation, this device shows very
good stability, an intrinsic property of the Sagnac loop interferometer. Further, a comparison between the
different means of polarization control will be presented including details on the overall electrical and optical
performance of the different devices.
Intensity (a.u.)
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Measurement data
Fitted (sin) data
0 40 80 120 160 200 240 280
Rotating angle (deg)
Figure 2: Typical response of the output from the Sagnac loop interferometer when
altering the polarization of the light in the loop using the mechanical polarization controller.

XXIX ENFMC - Annals of Optics 2006
Conclusions
Variable optical attenuators, based on the all-fiber Sagnac loop interferometer are examined. The intensity of the
transmitted signal at the output of the interferometer is controlled by changing the polarization state of the light
in the loop of the interferometer by using various methods of controlling the state of polarization of the light in
the loop. The different methods include mechanical polarization control as well as electrical polarization control
by using the fibers with internal electrodes.
The rotation of the polarization based on the mechanical polarizer resulted in a reproducible, stable and wide
dynamic device with an extinction ratio of more than 99%. The development of a VOA Sagnac loop
Interferometer is further investigated when different means of polarization control are considered based on Kerr
phase modulator and thermally poled fibers.
Acknowledgements
Michael Fokine acknowledges the financial support from CNPq
References
1. S. Cohen and P. Melman, “New breakthrough design for VOAs based on electro-optic materials,”
Lightwave, January (2000).
2. C. Chen, C. Lee, Y.-J. Lai, “Novel VOA Using In-Plane Reflective Micromirror and Off-Axis Light
Attenuation,” IEEE Optical Communications 516-520 (2003).
3. Y.-H. Wu, X. Liang, Y.-Q. Lu, F. Du, Y.-H. Lin, S.-T. Wu, “Variable optical attenuator with a polymerstabilized
dual-frequency liquid crystal”, Appl. Opt., 44, 4394 (2005).
4. D. B. Mortimore, Fiber Loop Reflectors, Journal of Lightwave Technology, 6, 1217 (1988).
5. Valente, D. C. B., Costa, E. T. M., Gouvêa, P. M. P., Carvalho, I. C. S., Carvalgo, M. C. R., and Margulis,
W., "A Sagnac Interferometer as a Variable Optical Attenuator", XXVI National Meeting of Condensed
Matter Physics, p. 249, Caxambu, Brazil, May (2003).
6. Fokine, M., Nilsson, L-E., Claesson, A., Berlemont, D., Kjellberg, L., Krummenacher, L., and Margulis, W.,
"Integrated fiber Mach Zehnder interferometer for electro-optic switching", Optics Letters, 27 (18), p. 1643
(2002).
7. Fokine, M., Kjellberg, L., Helander, P., Myrén, N, Norin, L., Olsson, H., Sjödin, N., and Margulis, W., "A
Fibre-Based Kerr Switch and Modulator"., 30th European Conference on Optical Communications
(ECOC´2004)., Tu4.3.3, Stockholm, Sweden, Sept (2004).
8. Myrén, N, Olsson, H., Norin, L., Sjödin, N., Helander, P., Svennebrink, J., and Margulis, W., "Wide wedgeshaped
depletion region in thermally poled fiber with alloy electrodes", Optics Express, 12(25), 6093.
(2004)