Note: Descriptions are shown in the official language in which they were submitted.
CA 02371106 2002-02-07
METHOD FOR INDUCING A THERMAL GRADIENT IN AN OPTICAL FIBER
FIELD OF THE INVENTION
The present invention relates to Bragg gratings in optical fiber components.
s It concerns more particularly the dynamical tuning of the optical properties
of the
grating by means of a controlled temperature gradient. An exemplary
application
of this invention is the active tuning of the chromatic dispersion of the
grating.
BACKGROUND OF THE INVENTION
io It is known in the art to impose a temperature gradient to a Fiber Bragg
Grating (FBG) for various purposes.
More particularly, U.S. patent no. 5,671,307 (LAUZON et al.) discloses the
use of a temperature gradient to impose a chirp on a FBG. The temperature
gradient is realized by providing heat conductive means such as a thin brass
plate
is to hold the portion of the fiber provided with the Bragg grating, and pairs
of Pettier
effect plates sandwiching each end of the fiber to selectively apply and
dissipate
heat to end from the ends of the fiber. Lauzon suggests that the device might
be
used as an accurately tunable dispersion compensator for optical fiber
communication links, but does not disclose any energy efficient manner of
2o realizing such an embodiment.
European patent no. 0 867 736 (FARRIES et al.) also discloses a
temperature-based device and method for wavelength and bandwidth tuning of an
optical grating. This patent combines the application of a temperature
gradient and
an optical strain to modify the optical properties of the grating. This device
2s however implies gluing the fiber to a metallic block along its entire
length, which in
practice is a technologically challenging operation.
There is therefore a need for a practical and power efficient method for
applying a temperature gradient to a FBG that may be used for practical
applications.
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SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method for inducing a
temperature gradient in an optical fiber in order to change the characteristic
spectral response of a fiber Bragg grating. Use of a thermally conductive
recirculation bar allows a heat transfer between the opposite ends of a
natural
gradient rod, into which a temperature gradient can be set and dynamically
tuned
with a minimal heat loss. This principle allows the rapid and efficient tuning
of the
optical properties of the optical fiber Bragg grating.
The present invention allows for the manufacture of pratical power efficient
to devices for a plurality of applications. In accordance with a first present
embodiment, the invention may be applied to make a tunable dispersion
compensator, a tunable gain flattening filter or tunable optical filters in
general.
IS
Any other devices where a temperature gradient could be advantageously applied
to a Bragg grating may also benefit from the teachings of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side view of a preferred embodiment of the present
invention.
2o DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Referring to FIG .1, there is shown an optical device 10 having a fiber Bragg
grating 12 written in a length of optical fiber 11. The optical fiber 11 is in
close
contact with an elongated element, hereinafter designated as "the natural
gradient
rod 13". This rod, preferably made out of a good metallic conductor, allows a
2s uniform heat transfer along its length and thus creates a temperature
gradient
along adjacent fiber 11. The fiber can be coupled to this rod by numerous
means,
using for example a lateral groove with a thermal compound to improve thermal
contact. The optical fiber 11 is positioned in rod 13 such that the portion of
the
fiber containing the Bragg grating 12 is located at the center of the length
of the
3o rod.
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Preferably, the natural gradient rod 13 is embodied by a thin cylindrical
tube, preferably made of a heat conductive metal, with a small hole along its
longitudinal axis into which the fiber 11 rests freely. This preferred
embodiment
isolates the fiber 11 from surrounding perturbations. A thermal compound is
not
s required to ensure a good replication of the temperature profile along the
natural
gradient rod 13 in the fiber 11. Moreover, the optical properties of the Bragg
grating 12 remain unaffected by the contact between the optical fiber 11 and
the
natural gradient rod 13. Finally, long term reliability is increased since no
mechanical stress is applied to the optical fiber 11 at any time. Within this
to preferred embodiment, the fiber 11 remains unaffected by the thermal
expansion
(or contraction) of the metallic rod 13, since they are not mechanically
coupled to
one another. Only the thermal change in the index of refraction of the fiber
11 will
affect the optical properties of the Bragg grating 12.
The natural gradient rod 13 shall be thermally isolated from the
is surroundings in order to ensure the linearity of the induced thermal
gradient. A
dewar type thermos system, with an inner shield to improve radiation
isolation, can
be used for this purpose. A low emissivity construction, using for example a
rod
with a mirror finish surface, will further improve the performance of the
device.
Two heat pumping elements 14 are fixed in close physical contact at both
2o ends of the natural gradient rod 13, using an appropriate method like
pressure
mounting with a thermal compound, thermal gluing, or soldering. The heat
pumping elements 14 are preferably Pettier effect Thermo Electric Coolers,
referred hereafter as TECs. These elements pump heat from one side of their
body to the other to fix the temperature of the extremities of the attached
2s conductive rod 13 (T~ and T2), into which will settle a natural temperature
gradient
0T = T~ - T2.
On top of each TEC 14 is fixed a temperature sensor element 15, such as a
thermistor or a resistance temperature detector (RTD), in close proximity to
the
natural gradient rod 13. These sensors 15 are fixed in close contact with an
3o appropriate method, using for example a thermally conductive epoxy. Signals
from
these sensors are used as input to a servo control system not shown in FIG .1
to
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precisely control (fix and maintain) the temperature at each end of the
grating.
Such means for temperature control are well known in the art, comprising
appropriate control electronics and drive such as TEC controllers with PID
servo-
control for optimum dynamic operation.
s Both TECs 14a and 14b are mounted on a thermally conductive metallic
recirculation bar 16. This bar acts as a "heat tank", i.e. as a local heat
sink into
which a TEC can dump heat or as a heat source from which a TEC can extract
heat.
In order to change the optical properties of fiber grating 12, an appropriate
to thermal gradient 0T is induced in the natural gradient rod 13 by setting
temperatures T~ and T2 at its extremities with heat pumping elements 14. The
following scenario is intended as an example illustrating the principle of
operation
of the invention. Let's assume for the purpose of demonstration that the left
end of
bar 13 at temperature T~ (point A in FIG .1 ) is hotter than the right end at
is temperature TZ (point B), i.e. T> > TZ . The difference in temperature
creates a
temperature gradient inside the rod and a heat flux ensues, flowing from hot
point
A to cold point B. Ensuring that the heat loss along the rod is small compared
to
the heat flux in the rod keeps the temperature gradient along the rod nearly
linear.
In order to maintain the temperature gradient, heat must be supplied to the
rod at
2o point A and extracted from the rod at point B. In this case, left TEC 14a
extracts
heat from the recirculation bar 16 at point D and pumps it into the natural
gradient
rod 13 at point A. At the other end, right TEC 14b extracts heat from rod 13
at
point B and drops it into the recirculation bar 16 at point C. The heat taken
out of
rod 13 is thus sinked into recirculation bar 16 rather than dissipated in air
with a
2s regular heat sink. A second temperature gradient, opposite that existing in
the
natural gradient rod 13, is therefore created in recirculation bar 16. As
indicated by
arrows in FIG .1, heat flows from point A to B in the conductive rod 13, and
from
point C to the D in the recirculation bar 16. This continuous heat flow is
sustained
by TEC 14a maintaining a temperature difference between points A and D and by
3o TEC 14b maintaining a temperature difference between points B and C.
Recycling
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the heat extracted from rod 13 rather than dissipating it into the
surroundings
makes the system more power efficient.
A main advantage of the invention follows from this idea of a recirculation
loop, identified in FIG .1 as the heat recirculation region 19, which allows
the
s continuous exchange of heat between the natural gradient rod 13 and the
recirculation bar 16. When the system is properly isolated, the power required
to
maintain the temperature gradient is minimal and serves only to counteract
natural
heat losses. This avoids the unnecessary loss of power in a large heat sink
that
wastes energy and affects efficiency. This principle of operation applies of
course
io for any other combination of temperatures T~ and T2, and is not limited to
the case
T~ > T2 given in the example.
Finally, supplementary base TECs 17 can be fixed to the recirculation bar
16 to dissipate excess heat from the bar into a heat sink 20, if needed, in
order to
maintain the average temperature of the system. This situation is most likely
to
Is occur during rapid transitions, when the temperature gradient is quickly
inverted by
changing the heat flow direction within TECs 14. The recirculation bar 16 can
also
overheat or get too cold in the advent of external or environmental
temperature
changes. The base TECs 17 then pump heat out of (or into) the system to bring
TECs 14 within their optimal temperature range of operation. The heat sink 20
can
2o consist in a standard dissipative heat sink with fins or more simply in a
large heat
dissipation plate. It can even be the metallic casing of a packaged device.
The
temperature of the recirculation bar 16 is monitored with a temperature sensor
18
connected to an appropriate control system not shown in the drawing of FIG .1.
In a properly implemented embodiment of the invention operated under
Zs normal conditions, the role of the base TECs 17 is minimal, as the
temperature
gradient is self maintained by the heat exchange via the recirculation region
19
between the rod 13 and the bar 16. Proper implementation requires minimizing
heat losses, achieved by using low emissivity materials, by thermally
isolating the
device and by ensuring a good thermal contact between the heat pumping
3o elements 14 and the rod 13 and bar 16.
CA 02371106 2002-02-07
Naturally, the present invention is not limited to the preferred embodiment
and materials presented herein for illustration purposes.
