Note: Descriptions are shown in the official language in which they were submitted.
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POWER EFFICIENT ASSEMBLIES FOR Pa,PPL'~iNG A
TEMPERATURE GRADIENT TO A REFRACTIVE 1NDE~
GRATING
FIELD OF THE INVENTION
The present invention generally relates to optical fiber Bragg gratings, and
more particularly concerns the dynamical tuning of the optical properties of a
grating
1o 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
A temperature gradient can be induced in an optical fiber containing a fiber
Bragg grating (FBG) in order to change the characteristic spectral response of
the
grating. Such thermally adjustable devices show great potential for optical
communication systems. It is known in the art how to impose a temperature
change
or gradient to a FBG for various purposes. Uniform heating along the length of
the
grating allows to shift the spectral response of the device, while a variable
heating
along said length allows to adjust the bandwidth and/or dispersion of the
grating.
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 realised with a heat conductive substrate, such as a thin brass plate
holding the
portion of fiber containing the Bragg grating, and Pettier effect plates
heating one end
of the fiber and cooling the other. Lauzon suggests that the device might be
used as
a tuneable dispersion compensator for optical fiber communication links, but
does
not disclose any energy efFicient embodiment of such a device.
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European patenfi No. 0 867 736 (FARRIES et al.) also discloses a
temperature-based device and method for wavelength and bandwidfih tuning of an
optical grating. This patent combines the application of a temperature
gradient and
a mechanical strain to modify the optical properties of the grating. This
device
requires gluing the fiber to a metal block along its entire length, which in
practice is
a technologically challenging operation.
U.S. Patent No. 6,351,385 (AMUNDSON et al.) presents a method for
enhancing the performance of thermally adjustable fiber grating devices by
disposing
them within a vessel that eliminates detrimental air currents around the
fiber. This
invention requires the application of a special resistive coating to the fiber
itself for
heating purposes. The coating thickness must be varied in a well controlled
manner
along the fiber in order to produce a desired temperature gradient.
As requirements of optical communication systems get more and more
demanding, near ideal grating performance becomes critical in many
applications.
A practical method for efiFiciently applying an accurately controlled
temperature
gradient to a FBG that may be used in many applications is therefore needed,
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a practical
and
power efficient assembly for inducing a temperature gradient in a FBG.
Lt is a preferable object of the present invention to provide such a power
efficient assembly which minimizes heat losses in the application of the
temperature
gradient to a Bragg grating.
3o It is another preferable object of the invention to allow the rapid and
energy-efficient
tuning of the spectral response of an optical fiber Bragg grating.
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It is another preferable object of the present invention to minimise energy
losses through radiation in an assembly inducing a temperature gradient in a
FBG.
According to a first aspect of the present invention, there is provided a
power
efficient assembly for applying a temperature gradient to a refractive index
grating
provided in a section of optical fiber. The assembly includes a heat
conductive
elongated element having opposite first and second ends and a longitudinal
axis
extending therebetween. The elongated element is provided with a fiber-
receiving
area along its longitudinal axis shaped for receiving the section of optical
fiber
_ Y., therealong_ in continuous thermal contact with the elongated element,
The_ assembly. ._ _ .. ._
also includes a first heat pumping device for maintaining the first end of the
elongated element at a first temperature and a second heat pumping device for
maintaining the second end of the elongated element at a second temperature
different from fihe first temperature, thereby applying the temperature
gradient to the
refractive index grating. Each of the first and second heat pumping devices
has a top
side in thermal contact with a corresponding end of the elongated element and
a
bottom side opposed thereto. In operation, the first heat pumping device pumps
heat
from the top to the bottom side thereof and the second heat pumping device
pumps
heat from the bottom to the top side thereof. Moreover, the assembly is also
provided
2 0 with a heat recirculation member having opposite first and second ends
respectively
in thermal contact with the bottom sides of the first and second heat pumping
devices. In operation, the heat recirculation member recuperates heat from the
bottom side of the first heat pumping device and recirculates the heat to the
bottom
side of the second heat pumping device.
Preferably, the heat conductive elongated element is a tube made out of a
metallic conductor and provided with a cavity extending therethrough along the
longitudinal axis for freely receiving the section of optical fiber, thereby
thermally
insulating the latter. The heat conductive elongated element thus assumes two
3o functions, i.e. heating the optical fiber and isolating it from air
currents or thermal
perturbations,
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According to another aspect of the present invention, there is also provided
another power efficient assembly for applying a temperature gradient to a
refractive
index grating provided in a section of optical fiber. The assembly includes a
heat
conductive elongated element having opposite first and second ends and a
longitudinal axis extending therebetween. The elongated element is provided
with a
cavity extending therethrough along its longitudinal axis for freely receiving
the
section of optical fiber therein in continuous thermal contact with the
elongated
element. The assembly also includes a heat exchanging sysfiem for maintaining
the
first end of the elongated element at a first temperature and the second end
of the
to elongated element at a second temperature different from the first
temperature, . _ -
- ~ ~the~eby applying said temperature gradient to the refractive index
grating. The heat
exchanging system comprises a first and a second heat pumping device
respectively
operationally connected to the first and second ends of the elongated element.
In
operation, the first heat pumping device pumps heat out of the first end 'of
the '
Z5 elongated element and the second heat pumping device pumps heat in the
second
end of the elongated element. iliioreover, the assembly is also provided with
a
thermal insulating enclosure provided around at feast a portion of the
elongated
element between the first and second ends thereof. The insulating enclosure
includes a vacuum chamber surrounding the portion of the elongated element.
Thus,
2 0 the thermal gradient inside the elongated element is then controlled
solely by the
temperature set values of the extremities thereof, without being affected by
the
ambient temperature. This improves the linearity of the thermal gradient along
the
elongated element.
2 5 Advantageously, the present invention allows for the manufacture of
practical
devices for a plurality of applications. In accordance with the preferred
embodiments,
the invention may be applied to mafce a tunable dispersion compensator, or
tunable
optical fitters in general. Any device requiring a highly linear temperature
gradient to
be applied along a fiber Bragg grating or along any other type of filiform
optical
3o component will also benefit from the teachings of the present invention.
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Other aspects and advantages of the present invention will be better
understood upon reading preferred embodiments thereof with reference to the
appended drawings.
5 BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become
apparent upon reading the detailed description and upon referring to the
drawings
in which:
FIGURE '! is a schematic side view of a power efficient assembly according
to a preferred embodiment of the present invention, ,
FIGURE 2 is a graph illustrating the discrepancy from an ideal linear
temperature gradient caused by heat loss to the surroundings in a non-isolated
system.
FIGURE 3 is a graph showing the normalised temperature gradient for
different insulation schemes.
FIGURE 4 is a schematic partial side view of another power efFcient assembly
according to another preferred embodiment of the present invention.
FIGURE 5 is a schematic side view of another power efficient assembly in
which thermal insulation is provided by a vacuum region contained in a thermos-
like
device according to another preferred embodiment of the present invention.
FIGURE 6 is a schematic side view of another power efficient assembly in
which thermal insulation is provided by a vacuum region contained in a thermos-
tike
device according to another preferred embodiment of the present invention.
FIGURE 7 is a schematic side view of a radially symmetric implementation of
another power efficient assembly according to another preferred embodiment of
the
present invention.
FIGURE 8 is a schematic partial side view of another power efficient assembly
3o according to another preferred embodiment of the present invention.
While the invention will be described in conjunction with an example
embodiment, it will be understood that it is not intended to limit the scope
of the
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invention to such embodiment. On the contrary, it is intended to cover all
alternatives
modifications and equivalents as may be included as defined by the appended
claims.
DESCRIPTION OF A PREFERRED EMBODIMENT
In the following description, similar features in the drawings have been given
similar reference numerals and in order to weigh down the figures, some
elements
are not referred to in some figures if they were already identified in a
preceding
figure.
The present invention concerns a practical and power efficient assembly for
applying.a temperature gradient to a refractive index grating. Such a device
allows
for the dynamical tuning of the optical properties of a grating such as, for
example,
the chromatic dispersion of the grating.
Referring to FIGURE 1, there is shown a power efficient assembly ~ for
applying a fiemperature gradient to a refractive index grating 5 provided in a
section
2 0 of optical fiber 3. The assembly 1 includes a heat conductive elongated
element T
having opposite first and second ends 29, 3'I and a longitudinal axis 33
extending
therebetween. The elongated element 7 has a fiber-receiving area 35 along the
longitudinal axis 33 shaped for receiving the section of optical fiber 3
therealong in
continuous thermal contact with the elongated element 7. Preferably, the
elongated
element 7 is made out of a metallic conductor for allowing an uniform transfer
of heat
therealong and, thus, creating a temperature gradient along the adjacent fiber
3. In
the illustrated embodiment, the fiber-receiving area 35 includes a groove 45
provided
along the heat conductive elongated element 7 and a thermal compound extending
therein for providing the continuous thermal contact between the section of
optical
3 0 fiber 3 and the heat conductive elongated element 7. Preferably, the
portion of the
fiber 3 containing the Bragg Grating 5 is located at the center of the length
of the
elongated element 7. In another preferred embodiment which is illustrated in
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FIGURE 4, the heat conductive elongated element 7 is a tube provided with a
cavity
along the longitudinal axis 33 defining the fiber receiving area 35 and freely
receiving
the section of optical fiber 3. This preferred embodiment isolates the fiber 3
from
surrounding perturbations. Moreover, a thermal compound is not required to
ensure
a good replication of the temperature profile along the elongated element 7 in
the
fiber 3. Furthermore, the optical properties of the Bragg grating remain
unaffected
by the contact between the optical fiber 3 and the elongated element 7.
Finally, long
term reliability is increased since no mechanical stress is applied to the
optical fiber
3 at any time. Within this preferred embodiment, the fiber 3 remains
unaffected by
the thermal expansion (or contraction) of the elongated element 7, since they
are not
mechanically coupled to one another. Only the thermal change in the refractive
index
of the fiber 3 will affect the optical properties of the Bragg grating 5. For
further
improve the performance of the device, a low emissivity construction of the
tube may
advantageously be used, such as, for example, a tube having an exterior
surface
l5 provided with a mirror finish. Advantageously, the optical fiber 3 shall be
recoated
with an acrylic jacket in order to prevent any contact between the sensing
material
of the fiber 3, which generally consists of glass, and the metallic material
of the
elongated element 7. Such a recoating thus prevents deterioration of the fiber
by
microcracks that could lead to a breakage of the fiber 3, and consequently
improves
2 o the reliability of the system. One can use standard recoating methods
which are well
known in the art and which won't be further exposed therein.
Referring back to FIGURE 1, the assembly 1 is also provided with a first heat
pumping device 9 for maintaining the first end 29 of the elongated element 7
at a first
temperature, and a second heat pumping device 11 for maintaining the second
end
25 31 of the elongated element 7 at a second temperature different from the
first
temperature, thereby applying the temperature gradient to the refractive index
grating
5. Each of the first and second heat pumping devices 9, 11 has a top side 37
in
thermal contact with a corresponding end 29, 31 of the elongated element 7 and
a
bottom side 39 opposed thereto. The heat pumping devices 9, 11 are mounted in
3 o thermal contact with the elongated element 7 with a pressure mounting
means. Such
a mounting means may be a thermal gluing, a soldering or even a pressure
method
mounting with a thermal compound. Preferably, the heat pumping devices 9, 11
are
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Pettier Effect Thermo-Electric coolers, referred hereafter as TECs. The heat
pumping
elements 9, 11 pump heat from one side of their body to the other to fix and
maintain
the temperature of the ends 29, 31 of the elongated element T, into which will
settle
a natural temperature gradient OT~= T~ - T2. Thus, In operation, the first
heat
pumping device 9 pumps heat from the top to the bottom side thereof and the
second heat pumping device 11 pumps heat from the bottom to the top side
thereof.
It is of course immaterial to the invention from which side heat is pumped out
of or
into.
The assembly 1 also includes a heat recirculation member 17 having opposite
ZO first and second ends 41, 43 respectively in thermal contact with the
bottom sides 39
of the heat pumping devices 9, 11. In operation, the heat recirculation member
17
recuperates heat from the bottom side 39 of the first heat pumping device 9
and
recirculates the heat to the bottom side 39 of the second heat pumping device
11.
In other words, the heat recirculation member 17 acts as a "heat exchanger"
into
which a TEC 9, 11 can dump or extract heat.
For allowing an appropriate control of the temperature gradient applied to the
FBG 5, each of the heat pumping devices 9, 11 is advantageously operationally
connected to a temperature sensor 13, 15 mounted in close proximity to the
corresponding end 29, 31 of the heat conductive elongated element 7. The
2o temperature sensors 13, 15 may be thermistors or resistance temperature
detectors
(RTD), for example. These sensors 13, 15 are fixed in close contact with an
appropriate method, using for example a thermally conductive epoxy.
The assembly 1 may also advantageously include a servo-control system 47
connected to each of the heat pumping devices 9, 11 for precisely controlling
the first
and second temperatures. Moreover, signals from the sensors 13, 15 are
advantageously used as input to the servo-control system 47 to precisely
control (fix
and maintain) the temperature at each end of the grating 5. Such servo-control
systems 47 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.
Moreover, The power efficient assembly 1 may be provided with at least one
additional heat pumping device having a top side 37 arranged in thermal
contact with
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the heat recirculation member 17. Such an assembly 1 may further include a
heat
exchanging means 27; preferably a heat sink, arranged in thermal contact with
the
bottom side of each of the at least one additions( heat pumping device for
exchanging heat between the heat recirculation member 17 and surroundings
thereof. In the case where a single additional heat pumping device is used, it
could
advantageously be mounted in the middle of the heat recirculation member 17.
In the
case illustrated in FIGURE 1, the power efficient assembly 1 is provided with
a third
and a fourth additions! heat pumping devices 19, 21. Each of them has a tap
side 37
respectively arranged in thermal contact with the first and second ends 41, 43
of the
heat recirculation member 17. The illustrated assembly 1 further includes a
heat sink
27 arranged in thermal contact with the bottom side 39 of each of tile
additional heat
pumping devices 19, 21.
In order to change the optical properties of fiber grating 5, an appropriate
thermal gradient dT is induced in the elongated element 7 by setting
temperatures
T~ and T2 at its first and second ends 29, 31 with heat pumping elements 9,
11. The
following scenario is intended as a non-restrictive example illustrating the
principle
of operation of the invention. 1_et's assume for the purpose of demonstration
that the
first end 29 of the elongated element 7 at temperature T~ ( point A in Figure
1) is
hotter than the second end 31 at temperature T2 (point B), i.e. T~ > T2. The
difference
in temperature creates a temperature gradient inside the elongated element 7
and
a heat flux ensues, flowing from hot point A to cold point B. Ensuring that
the heat
loss along the elongated element 7 is small compared to the heat flux in the
elongated element 7 keeps the temperature gradient along the elongated element
7 nearly linear. In order to maintain the temperature gradient, heat must be
supplied
to the elongated element 7 at point A and extracted from the elongated element
7
at point B. In this case, the TEC 9 extracts heat from the heat recirculation
member
17 at poinfi D and pumps it into the elongated element 7 at point A. At the
other end,
the TEC 11 extracts heat from the elongated element 7 at point 8 and drops it
into
the heat recirculation member 17 at point C. The heat taken out of the
elongated
element T is thus sunk into the recirculation member 17 rather than dissipated
in air
with a regular heat sink. A second temperature gradient, opposed to the one
existing
in the elongated element 7, is therefore created in the heat recirculation
member 17.
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As indicated by arrows in FIGURE 1, heat flows from point A to B in the
elongated
element 7, and from point C to the D in the recirculation element 97. This
continuous
heat flow is sustained by TEC 9 maintaining a temperature difFerence between
points
A and D and by TEC 11 maintaining a temperature difference between points B
and
5 C. Recycling the heat extracted from the elongated element 7 rather than
dissipating
it into the surroundings makes the system more power efficient.
A main advantage of the present invention follows from this idea of a
recirculation loop, identified in FIGURE 1 as the heat recirculation region
25, which
allows the continuous exchange of heat between the elongated element 7 and the
10 recirculation element 17. 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 for
any other combination of temperatures T~ and T2, and is not limited to the
case T~
> T~ given in the example.
The at least one additional heat pumping device, which is fixed to the
recirculation element 17 can be used to dissipate excess heat from the
recirculation
member 17 into the heat exchanging means 27, if needed, in order to maintain
the
average temperature of the system. This situation is most likely to occur
during rapid
transitions, when the temperature gradient is quickly inverted by changing the
heat
flow direction within TECs 9, 11. The heat recirculation element 17 can also
overheat
or get too cold in the advent of external or environmental temperature
changes. The
additional TEC then pumps heat out of the system, or into the system, to bring
TECs
9, 11 within their optimal temperature range of operation. As a first example,
one can
apply first and second temperatures to the corresponding ends of the elongated
element 7 which are lower than the surrounding. In that case, the additional
TEC wilt
evacuate the heat excess of the heat recirculation member 17. In a second
example
where the first and second temperatures are higher than the temperature of the
surrounding, the additional TEC will help keeping the heat recirculation
member 17
3o to its average temperature, which depends on the first and second
temperatures.
Such an embodiment will thus provide a more rapid tuning of the spectral
response
of the grating 5. The heat exchanging means 27 can consist in a standard
dissipative
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heat sink with fins or more simply in a large heat dissipation plate. It can
even be the
metallic casing of a packaged device. Advantageously, the temperature of the
heat
recirculation member 17 may be monitored with a temperature sensor 23
operatively
connected to the servo-control system 47 described above.
In a properly implemented embodiment of the present invention operated
under normal conditions, the role of the additional TEC is minimal, as the
temperature gradient is self maintained by the heat exchange via the
recirculation
region 25 between the elongated element 7 and the recirculation bar 17. 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 elements 9, 11 and the elongated element 7 and the
recirculation element 17.
In many applications, the thermal gradient applied to the grating should
ideally ,
be linear. In principle, a linear temperature gradient can be created between
the ends
of an elongated element if the ends are maintained at different temperatures
and if
heat transport takes place only between these ends. In practice, heat toss
fram the
elongated element to the surroundings distorts the thermal gradient which no
longer
remains linear.
Heat loss from the elongated element to the surroundings can result from
three different mechanisms, i.e. conduction, convection, and radiafion.
Conductive
heat transport consists in the microscopic transfer of kinetic energy, through
direct
contact, between neighbouring atoms or molecules. Air, being a tenuous medium,
is a good fihermal insulator that gives rise to little conduction. Connective
heat
transport results from the macroscopic motion of a fluid between a warmer
location
and a cooler one. For example, an air current can pick up some heat from the
conductive elongated element and take it away. A warm body can also lose heat
through radiation, i.e. by emitting electromagnetic waves. Radiative heat
transport
does not require a material support, since electromagnetic waves can travel in
vacuum.
In order to improve the linearity of the thermal gradient along the conductive
elongated element , these heat loss mechanisms between the elongated element
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and the surroundings should be minimised. In the case at hand, the low
emissivity
of the metallic elongated element reduces radiative losses. As a result, the
heat loss
from the conductive elongated element mainly stems from convection. Neglecting
radiation heat loss, the temperature distribution along the elongated element
is then
given by Equation 1
T(x) = Z'~ -~- ~(~'~ - Z,~ ) ~6z ! 81 ) sinh (m x~ + sinh~m ~L -- x)~
sink (m L)
where el =T -T~ , e2 =Tz -T~ , m = hp/k A , 0 <_ x <_ L is the position along
the
elongated element 7, L being the length of the elongated element, A and P are
1 o respectively the area and perimeter of the elongated element cross-
section, T~ and
T~ are the temperature of the ends of the elongated element at x = 0 and x =
L,
respecfiively, T~ is the ambient temperature away from the elongated element
7, k
is the thermal conductivity of the material constituting the elongated element
7 and
h is the convection heat transfer coefficient. FIGURE 2 illustrates the effect
of
convective heat loss on the temperature gradient along the elongated element
when
both ends of the elongated element are warmer than the surroundings (T2 > T~ >
T~).
The heat loss is seen to distort the thermal gradient, the temperature
distortion being
indicated as bT in the figure.
According to Equation 1, the linearity of the gradient depends on the ratio
2 0 between the convecfiive heat loss (~hP) and the heat flux in the elongated
element
(~kA) through factor m. Equation 1 actually reduces to:
( ) _ (L-x)Ti +xTz (2)
T x Im-~0 - L
when m is small, which is the expression for the ideal linear gradient. The
linearity can therefore be improved by reducing the heat loss to the
surroundings
2 5 and/or increasing the heat flux in the conductive element 7. In order to
achieve low
power consumption, reducing the heat loss is the preferred course of action.
FIGURE
3 illustrates the effect of thermally insulating the conductive element 7 on
the
normalised temperature distribution U(x) along fihe elongated element 7,
defined as:
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U~x) = Tax) (3)
CTazT Jx+Ti
where T(x) is given by Equation 2. (The normalised temperature distribution
for the ideal linear gradient is therefore equal to U(x) = 1.) These
distributions were
computed using a finite elements analysis software and confirmed by numerical
analysis. They clearly show that strengthening the thermal insulation around
the
conductive element 7 improves the linearity of the thermal gradient along the
element
7.
The insulation schemes considered in FIGURE 3 will be discussed in more
details below, after a presentation of another preferred embodiment of the
present
1o invention using a vacuum insulation.
According with another aspect of the present invention, there is also provided
another preferred embodiment of another power efficient assembly providing
isolation from the surrounding environment. In order to improve the control of
the
optical response of the grating, this assembly allows to decouple the desired
temperature gradient from ambient temperature fluctuations. Referring now to
FIGURES 5 to 7, there is shown different embodiments of a power efficient
assembly
9 for applying a temperature gradient to a refractive index grating 5 provided
in a
section of optical fiber 3. The assembly '9 includes a heat conductive
elongated
element 7 having opposite first and second ends 29, 3'i and a longitudinal
axis 33
2o extending therebetween. Preferably, the elongated element 7 is made out of
a
metallic conductor for allowing an uniform transfer of heafi therealong and
thus
creating a temperature gradient along the adjacent fiber 3. The elongated
element
7 is provided with a cavity 6~ extending therethrough along the longitudinal
axis 33
for freely receiving the section of optical fiber 3 therein in continuous
thermal contact
with the elongated element 7. As described above, the cavity 6~B isolates the
optical
fiber 3 from surrounding perturbations. The heat conductive elongated element
7
thus assumes two functions, i.e. healing the optical fiber and isolating it
from air
currents or thermal perturbations. This differs from the invention disclosed
in patent
No. 6,351,385, where these functions are carried out by separate components,
i.e.
the resistive coating and the isolating vessel. As also already explained
above, for
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further improve the performance of the device, a low emissivity construction
of the
elongated element 7 may advantageously be used, such as, for example, a tube
having an exterior surface provided with a mirror finish.
Still referring to FIGURES 5 to 7, the assembly 1 is also provided with a heat
exchanging system which includes a first and a second heat pumping device 9,
11
respectively operationally connected to the first and second ends 29, 31 of
the
elongated element 7. The heat exchanging system allows to maintain the first
end
29 of the elongated element 7 at a first temperature and the second end 31 of
the
elongated element 7 at a second temperature different from the first
temperature,
to thereby applying the temperature gradient to the refractive index grating
5. In
operation, the first heat pumping device 9 pumps heat out of the first end 29
of the
elongated element 7 and the second heat pumping device 11 pumps heat in the
second end 31 of the elongated element 7.
The power efficient assembly 1 also includes a thermal insulating enclosure
67 provided around at least a portion of the elongated element 7 between the
first
and second ends 29, 31 thereof. The insulating enclosure 67 includes a vacuum
chamber 69 surrounding said portion of the elongated element 7. Preferably,
the
thermal insulating enclosure 67 is made of glass.
As stated above, heat loss from the conductive elongated element 7 to the
surroundings must be minimised in order to preserve the linearity of the
thermal
gradient created therein. The conductive elongated element 7 can be thermally
insulated by enclosing it in a cylinder made of a low density material. For
example,
insulating foams with a very low thermal conductivity (k ~ 0.03 W/m~K) can be
used
efficiently to improve the linearity of the thermal gradient. The necessary
thickness
2 5 of insulating material can be determined from existing art. For example,
it is found
that a cylinder of foam that is too thin actually worsens the heat loss
because of the
increase in exposed surface with respect to the gain in insulation. Over a
certain
thickness, however, insulating foam does reduce the heat loss from the
conductive
elongated element 7. The achievable gain in performance can then be weighted
3 o against the increase in volume of the device to determine an optimum foam
thickness.
At ambient temperature, air is an even better insulator than foam. In view of
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volume limitations, it may be preferable in some cases to replace the foam
cylinder
by a thin layer of air confined in a tube. Convection within the air layer
must be
avoided at all cost, because it will severely degrade the thermal insulation.
To this
end, the air gap must be kept thin enough that buoyancy forces cannot overcome
the
5 resistance imposed by the viscous forces of air. The maximum allowable air
thickness can be determined from existing art. This type of thermal
insulation,
discussed in U.S. Patent No. 6,351,585, represents a good compromise between
cost and complexity.
Even better insulation can be achieved by surrounding the conductive
10 elongated element 7 with vacuum, using a thermal insulating enclosure 67,
for
example a vacuum dewar. Neither conduction nor convection can occur in a
complete vacuum. As a result, heat loss can only result from radiation. In
practice,
small losses can be caused by conduction in end walls 71, 73 of the insulating
enclosure 67. The amaunt of radiation emitted by the conductive elongated
element
15 7 can be reduced by polishing its outer surface to a mirror finish, Another
advantage
of this preferred embodiment is that a vacuum region can be significantly
thinner
than an air gap or a foam cylinder while still maintaining its insulation
properkies.
FIGURE 5 illustrates an embodiment of this approach where the conductive
elongated element 7 is surrounded by a vacuum chamber 69. In this embodiment,
2o the thermal insulating enclosure 67 includes two end walls 71, 73, each
being
provided with a hole 75 therein for receiving the heat conductive elongated
element
7 therethrough. The thermal insulating enclosure 67 also includes a Tubular
portion
77 thermally sealed to the end walls 71, 73 and extending therebetween. The
end
walls 71, 73 are thermally sealed to the heat conductive elongated element 7.
Thereby, the end walls 71, 73, the tubular portion 77 and the heat conductive
elongated element 7 form together a closed area defining the vacuum chamber
69.
An appropriate seal between the end walls 71, 73 and the conductive elongated
element 7 is required in order to provide an airtight fit. The end walls 71,13
and the
tubular portion 77 can be made of different materials or from a common
material. To
3o further minimise radiative heat losses, the tubular portion 17 andlor the
ends walls
71, 73 may advantageously be provided with a heat reflective coating 79
extending
outwards or inwards the vacuum chamber 69. Preferably, the coating 79 is a
metallic
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coating with a high emissivity such as, for example, gold or aluminium. The
thermal
insulating enclosure 67 is further provided with an airtight valve for
creating and
maintaining vacuum in the vacuum chamber 69.
Another implementation of the vacuum insulation is shown in FIGURE 6,
wherein the thermal insulating enclosure 67 includes an inner and an outer
tubular
walls 83, 85 sealed together at extremities thereof for defining the vacuum
chamber
69 therebetween. The inner wall 83 forms a longitudinal channel 87 inwardly
thereof
extending centrally through the enclosure 67 for receiving the heat conductive
elongated element 7. When the insulating enclosure 67 is made out of glass,
the
inner tubular wall 83 that gets heated by the conductive element 7 will
radiate
strongly, given the large emissivity of glass. A metallic heat reflective
coating 79
extending on the outer wall 85 can be used to limit radiative heat loss. Such
a
reflective coating 79 may be applied on the interior or on the exterior of the
outer wall
85. As in the previous described embodiment, vacuum is made in the vacuum
chamber 69 by means of an appropriate airtight valve 81, which can be an
airtight
fusioned valve for example.
FIGURE 7 presents another preferred embodiment of a power efficient
assembly 1 that has a radial symmetry. In this embodiment, the thermal
insulating
enclosure 67 includes two opposed end walls 71, 73, each of them having a hole
75
therein for receiving the heat conductive elongated element 7 therethrough.
The
insulating enclosure 67 also includes a tubular portion 77 hermetically fixed
to the
end walls 71, 73 by, for example, but not limited to, an airtight welding 91.
The end
walls 71, 73 are thermally sealed to the heat conductive elongated element 7
by any
appropriate means such a soldering, or they can be non-conductively attached
to the
conductive elongated element 7 by an appropriate airtight joint. Another
option to
minimise losses is to 'provide end walls 71, 73 that are made of an insulating
material. Thus, this assembly 1 constitutes an airtight construction enclosing
the
conductive elongated element 7. Any other appropriate means providing an air
tight
construction around the portion of the elongated element 7 containing the
optical
grating 5 could also be envisaged and the present invention intends to cover
any
equivalent of such a means. Air is pumped out of this enclosure and vacuum is
maintained by an airtight valve 81 that can be a crimped valve for example.
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Advantageously, an inner shield 89 is used to increase radiation isolation and
further
improve the pertormance of the device. An outer casing 93 can also be used to
provide additional protection to the assembly 1 from surrounding
perturbations. In
this illustrated case, heat is transferred to and taken out from the
conductive
elongated element 7 via heat distributors 95 in contact with circular TECs 9,
11
provided with a hole in their center mounted perpendicularly on the axis of
the
assembly 1. The assembly 1 further comprises a heat reservoir mounted in a
thermal
contact with each of the TECs 9, 11. For example, heat sinks 63, 65 may be
used
to dissipate heat in the ambient air. In another preferred embodiment of the
invention
which is not illustrated, the assembly 1 includes a heat recirculation member
17 in
thermal contact with the TECs 9, 11 for recuperating and recirculating heat,
thereby
further improving the efficiency and performances of the present assembly 1.
Of
course, the embodiment presented in FIGURE 7 may also benefit from
advantageous features described for the embodiment of FIGURE 1. For example,
15- the assembly 1 may advantageously be provided with a servo-control system
47
connected to the TECs 9, 11 for controlling the temperatures at the ends 29,
31 of
the elongated element 7.
FIGURE 8 presents another preferred embodiment of a power assembly 1
that is provided with a heating block 97 having a longitudinal cavity 99
therethrough
for receiving the elongated element 7 therein. Preferably, the heating block
97 is
made out of a metallic conductor, such as, for example, copper, for allowing
an
uniform firansfer of heat therealong. More preferably, the heating block 97 is
cylindrically shaped in order to be the most power efficient as the surface of
this
heating block is minimized. The cavity 99 is preferably slightly larger then
the
diameter of the elongated element 7 and is precisely aligned in order to avoid
any
physical contact of the elongated element 7 with the heating block 97. The
assembly
1 also includes a heating means '101 for heating the heating block 97 and
maintaining a temperature thereof at a fixed value. The heating means '101 may
include a resistive heating wire embedded into the heating block 97. Such a
heating
3o wire can be glued or roiled onto the heating block 97. The heating means
10'1 may
advantageously be operatively connected to the servo-control system 47
described
above in order to precisely control the temperature of the heating block 97.
Of
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course, an independent confirol sysfiem could also be envisaged for
controlling the
temperature of the heating block 97.
Thus, in this assembly, the heafiing block 97 fixes an exterior temperature at
a fixed value T3, chosen for example as the mean of the two heating TECs.9, 11
(T3
= (T~ + T2)l2). This presenfis the major advantage of rendering the assembly 1
independent of the exterior temperature variations. With this third
temperature value
in the assembly, fibs thermal gradienfi in fibs grating 5 still will not be
perfectly linear,
as explained by the previous equations, buff it will be more constant, which
will
provide a better repeatability of operation of the assembly 1 whatever the
1o temperature fluctuations of surroundings. Advantageously, a correction in
the grating
curvature ifiself could be made to compensate for this non-linearity of the
gradient.
Of course, such a heating black may be used in fibs other presented preferred
embodimenfis, and more particularly in fibs embodiment presented in FIGURE 1.
FIGURE 3 illustrates the performance of the various insulation schemes
presented above on the linearifiy of the thermal gradient. The vacuum
insulation
approach clearly gives the best results. Moreover, the assembly 1 allows to
provide
a much more power efficient device which is much more compact than existing
device. In the case of insulation by an air gap, the gap thickness was taken
as the
maximum allowable to maintain a convectionless heat transfer. In terms of
thermal
2o insulation, this corresponded fio a 10-mm layer of foam for the specific
configuration
studied. This radius can change in function of the length and the exfierior
diameter
of the conductive elongated element 7 and the temperatures involved.
Although preferred embodimenfis of fibs present invention have been
described in detail herein and illustrated in the accompanying drawings, it is
to be
understood that the invenfiion is not limited to these precise embodiments and
fihat
various changes and modificafiions may be effected therein without departing
from
the scope or spirit of the present invention. For example, a preferred
applicatian of
fibs present invention is the acfiive tuning of the chromatic dispersion of an
optical
3o fiber grafting; but it must be understood that the present invention is
intended to cover
a power efficient assembly for applying a highly linear temperature gradient
to any
other suitable filiform element not limited to an opfiical fiber grating.
