Note: Descriptions are shown in the official language in which they were submitted.
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FIBER BRAGG GRATING DISPERSION COMPENSATOR
TECHNICAL FIELD
[01] This invention relates to fiber Bragg grating dispersion compensation,
and
particularly to a thermally tunable fiber Bragg grating dispersion
compensator.
BACKGROUND OF THE INVENTION
[02] One of the key issues in modern high-speed optical networks is the
necessity
to compensate for the optical pulse broadening caused by optical fiber
chromatic
dispersion. With the advance of new generations of fast networks (40 Gb/sec
and
higher), the ability to precisely compensate for the dispersion becomes
critical for the
network operation thus necessitating dispersion compensation components with
variable
dispersion capabilities.
[03] Efforts to compensate for chromatic dispersion have involved thus far the
use of etalon-based systems, dispersion compensating fibres, dispersion
compensating
gratings, e.g. fiber Bragg gratings (FBG), or a combination of both. A device
described
in a paper "Implementation and characterization of fiber Bragg gratings
linearly chirped
by a temperature gradient", J. Lauzon et al, Optics Letters, Vol. 19, No. 23,
pp. 2027-
2029, Dec. 1994, has a heat distributor and thermoelectric coolers to control
the end
temperatures of the distributor.
[04] Various dispersion compensating systems are also described in patent
literature, e.g. US Patent 5,671,307 issued to Lauzon et al., US Patent
6,148,127 issued
Nov. 14, 2000 to Adams et al, USP 5,694,501 issued Dec. 2, 1997 to Alavie et
al. (now
assigned to the present assignee), and USP 6,307,988 issued Oct. 23, 2001 to
Eggleton
et al.
[05] It is desirable to provide a tunable dispersion compensator (DC),
preferably
over a broad dispersion range. The key element of a popular type of a DC is a
linearly
chirped fiber Bragg grating (FBG), a diffractive grating with a linearly
varying pitch
(refractive index perturbation) written inside an optical fibre. Optical pulse
broadening
comes from the fact that the pulse's frequency components travel with
different
velocities, so that the longer wavelength components lag the shorter ones. In
a chirped
FBG, the location of resonant Bragg condition (reflection point) will be
wavelength
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dependent. This causes a time-of-flight difference between longer and shorter
wavelength equivalent to a chromatic dispersion added to the pulse.
[06] Since silica used for fibre manufacturing has a temperature dependent
refractive index and the fibre itself has certain thermal expansion
coefficient, the grating
local resonant wavelength becomes temperature dependent and varies as 0~. =
STOT ,
where ST -1 Opm/K is the grating thermo-optical sensitivity.
[07] In a linearly chirped grating, i.e. grating with the reflection position
varying
linearly with wavelength, group delay will be a linear function of wavelength
and, after
differentiation, yields uniform dispersion across the grating wavelength range
(bandwidth). Any deviations of group delay from the linear profile called
group delay
ripple (GDR) distort the shape of an optical pulse and thus they are highly
undesirable.
[08] If one creates a uniform temperature gradient along a linearly chirped
FBG,
the grating chirp changes but remains linear thus giving rise to a different
dispersion
value. Based on this fact one can design a dispersion compensator with a
thermally
tuneable dispersion. Unfortunately, as is commonly known, an elongated object
heated
at two ends, due to thermal losses, will exhibit a non-linear temperature
profile, the
temperature deviation from linearity being greatest in the middle. In the case
of a
distributor housing a chirped Bragg grating, such thermal losses (dubbed here
"temperature sagging") amount to an undesirable group delay ripple (GDR).
[09] It is desirable to provide a DC capable of maintaining a uniform.
temperature gradient on the FBG, typically a chirped FBG.
[10] It is further desirable to provide a DC with means for varying the
temperature gradient at a relatively high rate, preferably maintaining the
temperature
gradient linearity.
SUMMARY OF THE INVENTION
[11] In accordance with one aspect of the invention, there is provided a
dispersion compensator comprising: a length of a waveguide including a grating
region
having two opposite ends, a heat distributing body extending along the grating
region
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and adjacent to the grating region, and a heating strip mounted to the body
and
extending along the grating region for controlled heating of the entire
grating region.
[12] The compensator may have a temperature sensor disposed intermediate the
ends of the grating region, preferably at the center of the grating region,
for generating
a signal indicative of the temperature of the respective region of the
distributor.
Preferably, the grating region is a chirped Bragg grating, e.g. a linearly
chirped Bragg
grating to afford an efficient compensation of chromatic dispersion.
[13] The compensator may further include a longitudinally variable heating
means adjacent to and extending along the length of the distributing body and
the
grating region for effecting a longitudinally varying heating of the grating
region, the
heating means having a monotonic heating-intensity variance along the length
of the
grating region.
[14] In one embodiment, the compensator has two terminal heating/cooling
means adjacent the ends of the grating region for heating at least the end
parts of the
grating region.
(15] As indicated above, it is desired to create a linear temperature profile
along
the Bragg grating. To this end, according to the invention, the grating is
disposed in a
close proximity of a heat distributor, preferably inside a heat distributor.
Tests have
shown that the provision of the distributor, made of a material of high
thermal
conductivity and preferably but not necessarily with a thermal expansion
coefficient
(CTE) identical or close to CTE of the material of the grating (glass), is
beneficial in
maintaining a linear variance of temperature along the grating.
[16] Dispersion dependence on temperature difference at the ends of the
grating
can be expressed as
D _ I + cST _0T
Do 2n L
where Do is the dispersion of the grating without temperature gradient
("nominal"
dispersion), L is the grating length, n is refractive index of the optical
fibre in which the
grating is imprinted, ST is as explained above and c is the speed of light.
Based on the
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desired dispersion tuning range, one can calculate the required temperature
range. In
order to maintain the device's bandwidth centred at a particular wavelength,
the grating
center temperature should remain constant. A sensor, e.g. a thermistor placed
at the
centre of the distributor provides the necessary feedback for the uniform
heater control
loop. For a linear temperature profile, the temperature at the middle of the
distributor
(and thus at the middle of the grating region) should be maintained at (T1
+T2)12 where
Tl and T2 are the temperatures at the respective ends of the distributor. A
central
control unit may be provided to respond to the signal generated by the sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[17] The invention will be explained in more detail by way of the following
description in conjunction with the drawings in which
Fig. 1 is a schematic side view of the dispersion compensator o the invention,
Fig. 2 is a schematic cross-sectional view of the compensator of Fig. 1,
Fig. 3 represents a partial top view of the distributor,
Fig. 4 is a top view of another embodiment of the compensator,
Fig. 5 is a cross-sectional view of the embodiment of Fig. 4, and
Fig. 6 is a schematic top view of the distributor showing the channel and the
varying wall thickness.
DETAILED DESCRIPTION OF THE INVENTION
(18] As shown in Figs. 1 and 2, the dispersion compensator 10 has an elongated
heat distributor 12 made e.g. from a CaW alloy or another material with high
thermal
conductivity, which features two end parts 14 and 16 and a middle portion 18.
The
distributor forms a channel 17 as seen in Fig. 2, Fig. 5 and Fig. 6. It is
preferable that
the cross-section of the channel 17 is constant through the length of the
channel. The
thickness of the side walls of the distributor 12, and/or the bottom wall
thereof, may be
varied, e.g. profiled as shown in Fig. 6 so that the thickness is smallest in
the mid-region
of the distributor. This provision is intended to make the heat flux
throughout the
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distributor relatively constant so that the temperature gradient can be
relatively constant
as well. The narrowing may be in the range of a few per cent, depending on the
dimensions of the components, and may be determined by experimenting or
modeling.
[79] The compensator is preferably placed inside a chamber, or casing (not
illustrated for clarity), made of a heat-conductive material, so that a
relatively uniform
ambient temperature is maintained in the vicinity of the dispersion
compensator.
[20] Two thermoelectric coolers (TEC) 20, 22 are disposed at, and attached to,
the end parts 14 and 16 of the heat distributor 12. The TECs are electrically
connected
to a source of electric energy and to a central control unit (not shown).
[21 ] A length of a waveguide 24 for reflecting an optical signal in need of
dispersion compensation is placed in the channel 17 of the distributor 12 and
secured to
the distributor at two ends thereof. Aside from the physical (mechanical)
contact
between the waveguide and the distributor at the two ends of the latter, there
is only
thermal, non-mechanical contact between the grating region and the
distributor.
[22] The waveguide has a linearly chirped Bragg grating region 26 co-extensive
with the distributor 12, the length of the distributor approximately matching
the length
of the grating region.
[23] A temperature sensor, e.g. a thermistor 28 for sensing the temperature of
the
middle of the grating 26 is disposed approximately in the middle of the
distributor 12,
and is connected to the central control unit, not shown. Two thermistors 29
are mounted
at the ends of the distributor. The TECs are mounted to a heat sink 31.
[24] A strip-shaped resistive heating element 30 (Fig. 2), of uniform heat
generation capacity along its length, is attached to the surface of the
distributor
coextensively with the grating region 26.
[25] Because of the heat exchange between the distributor and the
surroundings,
temperature profile deviates from linear resulting in GDR and non-uniform
dispersion.
The strip-shaped heating element 30 is provided in order to compensate for the
thermal
losses from the distributor by delivering uniform heat. It was found that
maintaining the
centre of the distributor at exactly half the temperature span between the end
points
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Tl + TZ
T~,;~ _
2
minimises the temperature sagging and GDR.
[26] It is desirable to periodically dither, i.e. introduce small variations
in the
temperature profile (gradient) of the grating, and the resulting change in
network
performance (i.e. eye closure, or bit error rate) can serve as a feedback to
make a
decision on tuning the dispersion up or down. Typically one would require a
dithering
rate of ~ 1 Hz over few ps/nm of dispersion change. Ideally, during the
dithering cycle
no additional GDR should be introduced, i.e. the shape of the grating
temperature
profile should remain linear. In order to introduce the capability of fast
change of the
grating's temperature gradient, the invention provides linearly-variable
heating means.
Two versions are proposed and illustrated. In the first version, illustrated
in Fig. 2, two
resistive strip heaters 32, 34 are attached to the distributor over the open
side of the
channel 17. The heaters 32 and 34 are enveloped between two layers of a
supporting
material 35, e.g. Kapton and connected each to a controlled voltage source.
When
operated, the heat from the heaters 32, 34 is transferred to the grating
region 26.
[27] As can be seen in Fig. 2 and 3, the shape of each heater 32, 34 is
tapered,
with the taper directions being opposite. The shape of each taper is trimmed
monotonically in such a way that the amount of resistive heat produced varies
linearly
along the distributor. By periodically switching the heaters on and off one
after another,
small linear change can be introduced in the grating temperature.
[28] Alternatively, as illustrated in Fig. 4 and Fig. 5, the linearly variable
heating
means are provided by way of two uniform heating wires 36, 38 which are
arranged at a
variable spacing from the grating region, preferably but not necessarily
inside the
channel 17, the spacing defining two tapers of opposite direction as seen in
Fig. 4.
[29] While not described in detail herein, it is conceivable within the
present
state of the art to provide other linearly variable heating means for the
above purpose.
[30] It has been found that the provision of the uniform heating strip (30)
and the
temperature control (28) of the middle of the distributor, and hence of the
grating
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region, is beneficial in maintaining a desirable linear temperature profile of
the fiber
grating.
[31] Various alternatives and modifications of the above may occur to those
skilled in the art, without departing from the scope and spirit of the
invention as defined
by the appended claims.
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