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Electrically tunable dispersion compensator using fibre
Bragg grating
Ast/P John Ngo Quoc Nam, Li Songyang, Zheng Ruitao, A/P Tjin Swee Chuan, A/P Shum Ping
One of the most critical challenges in nextgeneration
high-speed long-haul optical transmission
systems is to overcome the fibre’s chromatic dispersion.
The tunable dispersion compensator (TDC) can
be used to optimise network performance. We have proposed
and developed a novel TDC that can electrically
adjust the chirp of the FBG while maintaining a fixed
centre wavelength. This new method can minimise signal
distortion and crosstalk in a WDM system.
This electrically tunable TDC consists of a uniform
FBG in a fibre that is etched to a prescribed diameter profile,
an on-fibre thin-film heater whose local resistance
varies with position in a prescribed manner along the
grating length, and a negative thermal expansion coefficient
(NTEC) ceramic in which the FBG is mounted [see
Figure 1]. Electrical current flowing through the thin film
will generate a temperature gradient because of the pre-
+
λB1
Fiber with FBG in the center Copper thin-film coating
Conductive paint layer
DC
λB0 λB2
Epoxy
Negative thermal-expansion coefficient (NTEC) material
Figure 1: Schematic diagram of the proposed tunable dispersion compensating
FBG with fixed central wavelength. When current flows through the thin-film
coating and the conductive paint layer, the bandwidth and chirp of the FBG can
be tuned while the center wavelength is kept fixed.
_
scribed thickness profile of the metal film. Shrinkage of
the NTEC ceramic imposes a strain gradient on the FBG
because of the prescribed diameter profile of the fibre
with FBG. Both the temperature gradient and the strain
gradient affect the chirp and hence dispersion of the FBG.
At the same time, the center wavelength is kept fixed because
the effects of temperature rise and of compression
of the FBG are such that they offset each other.
To our knowledge, this is the first proposal to combine
the temperature gradient and the strain gradient as a dispersion
tuning technique. As an example to demonstrate
the effectiveness of the proposed method, we have developed
a TDC of 25 mm length, which has a dispersion tuning
range of −178 ps/nm to −302 ps/nm with a central
wavelength shift of as small as 0.17 nm and an applied
electrical power smaller than 0.68 W.
Fiber
1cm
Epoxy
Electrodes
Conductive paint layer
on surface of NTEC
ceramic
Epoxy
NTEC ceramic
with FBG inside
Figure 2: Photograph of a packaged tunable
FBG dispersion compensator.
Group Delay (ps)
250
200
150
100
50
0
B: Volt=0.4 V; Dispersion= -190 ps/nm
-50
1549.0 1549.2 1549.4 1549.6 1549.8 1550.0 1550.2 1550.4 1550.6
Wavelength (nm)
Figure 3: Measured group delay characteristics
of the tunable dispersion compensator.
FIBRE AND LASER OPTICS RESEARCH September 2003 11