CA2383807A1 - Method for improving a thermal gradient in an optical fiber - Google Patents

Description

METHW FDFt~fIAPRDV(N~ A THERI~Li4L ~RAQ1ENT IN AN QPTICAL FIBER
FIELD OF-THE INVENTION
The present invention generally relates to optical fiber Bragg gratings, and s more particularly concerns the thermal corrrtrol of the optical properties of such gratings as described in commonly own Canadian Patent application no.

2,371,106 filed on February 7t" 2002.
BACKGROUND OF~'fHE INVENTION
to A temperature gradient can be induced in an optical fiber containing a fiber Bragg grating (FBG) in order to changewthe characteristic spectral response of said 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 Is 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.
In particular, 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 2o realised with a heat conductive substrate, such as a thin brass plate holding the portion of fiber cvntainirng 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 cvrrrpensator for optical fiber communication links, but does not disclose any energy efficient embodiment of such a device.
2s European patent No. 0 867 736 (FARRIES et al.) also discloses a temperature-based device and method for wavelength arnd bandwidth tuning of an optical grating. This patent combirres-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 black along its entire length, which in practice is 3o a technologically challenging operation.

U.S. Patent No. 6,351,385 (AMUNDSON et al.) presents a method for enhancing the performance of themrally adjustable fiber grating devices by disposing them within a vessel that eliminates detrimental air currents around the fiber. This invention requires the appli~ion of a special resistive coating to the s fiber itself for heating purposes. The coating thickness must be varied in a well controlled manner along the fiber in order to pn~rJuce a desired temperature gradient.
As n~quirements of optical communication systems get mare and more demanding, near ideal grating performance becomes critical in many applications.
to A practical method for applying an accurately controlled temperature gradient to a FBG is therefore needed that may be used in applications.
SUMMARY OFTHE INVENTION
The above-mentioned previously filed Carnadian patent application no Is 2,371,106 discloses a practical and power efficient system for inducing a temperature gradient in a FBG. The temperarture gradient is produced in a conductive rod by elements controlling the temperature of the ends of said rod. A
bar, also made of a conductive material, allows the n=circulation of heat between the ends of said rod. As a result, a temperature gradient can be set between the 2o ends of the rod and dynamically tuner! with a minimal heat loss. This principle allows the rapid and energy-efficient tuning of the spectral response of an optical fiber Bragg grating.
The present invention further improves on this system by providing isolation from the surrounding environment. Accordingly, the present invention provides a 2s method to decouple the desired temperature gradient from ambient temperature fluctuations, in order to improve the control of the optical n=spvnse of a fiber Bragg grating found therein.
A first aspect of the invention consists in placing the fiber grating inside the conductive rod through a small hole drilled along the longitudinal axis of said rod.
3o The conductive rod thus assumes two functions, i.e. heating the optical fiber and isolating it from air currents orthem7al perturbations. This differs from the invention

3 disclosed in patent No. 6,351,385, where these functions are carried out by separate components, i.e. the resistive cerating arid the isolating vessel.
A second aspect of the present invention consists in isolating the central section of the conductive heating rod from its surroundings with a them~os-like s insulation system. The thermal gradiertt inside the rod is then controlled solely by the temperature set values at the extremities of said rod, without being affected by the ambient-tentperature. This improves the linearity of the themTal gradient along the conductive rod.
Advantageously, the present invention allows for the manufacture of to practical devices for a plurality of applications. In accordance with the disclosed embodiment, the invention may be applied to make a tuneable dispersion compensator, or tuneable optical filters in general. Any device s=quiring a highly linear temperature gradient to be applied along a fiber Bragg grating or along another type of filiform optical component will also benefit from the teachings of is the present invention.
BRIEF DESCRIPTIDN QF~fHE 6RAWINGS
The present invention will be described in mere detail below with references to the accompanying drawings, in which:
2o FIG. 1 shows the discrepancy from an ideal linear temperature gradient caused by heat loss to the surrourndings in a non-isolated system.
FIG. 2 shows the nomralised temperature gradient for different insulation schemes.
FIG. 3 is a schematic side view of various implementations of a packaging 2s arrangement for a thgmTally adjustable optical fiber grating device; FIG.
3a depicts a device with no thermal insulation; FIG. 3b shows a device thermally insulated by foam; FIG. 3c and d presErrt the preferred embodiment in which thermal insulation is provided by a vacuum region contained in a thermos-like device.
FIG. 4 is a schematic side view of a radially symmetric implementation of 3o the invention using a shielded thermos.

4 DESCRIPTIZ7N QF--PREFERF~ED EMBODIMENTS C7F THE INVENTION
Theoretical considerations The present invention addresses-the generation of a themral gradient along s an optical fiber in order~to control the spectral response of a Bragg grating written in this fiber. In many applications, the thermal gradient shauld ideally be linear. In principle, a linear temperature gradient can be m=ated between the ends of a conductive rod if said ends are maintained at different temperatures and if heat transport takes place only between these ends. In practice, heat loss from the Io conductive rod to the surrourrdirrgs distorts the thermal gradient which no longer remains linear.
Heat loss fn~m the corrductive nod to the surraurndings can result from three different mechanisms, i.e. conduction, convection, and radiation. Conductive heat transport consists in the micnJSCOpic transfer of kinetic energy, through direct is contact, between neighbouring atoms or molecules. Air, being a tenuous medium, is a good thermal insulator that gives rise to little conduction. Convective heat transport results-fn~m 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 rod and take it away. A warm body can also lose heat through 2o 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 themral gradient along the conductive rod, these heat loss mechanisms between the rod and the surroundings should be minimised. In the case at hand, the low emissivity of the metallic rod reduces 2s radiative losses. As a result, the heat loss from the conductive rod mainly stems from convection. Neglecting radiation heat lass, the temperature distribution along the conductive rod is then given by Equation 1 T(x)=T~+ ~T,-T~~~BZ~e~~sinh~mx~+sinh~m~L-x~~
sinh ~m L
where e, = T - T~ , ez = TZ - T~ , m = h P/k A , 0 <_ x s L is the position along the 3o rod, L being the length of said rod, A and P are respectively the area and perimeter of the rod crass-section, T~ and T2 are the temperature of the ends of the rod at x = 0 and x = L, respectively, T~ is the ambient temperature away from the rod, k is the thermal carrductivity of the material constituting the rod and h is the convection heat transfer coefficient. FIG. 1 illustrates the effect of convective s heat loss on the temperature gr~dient~alvng-the rod when both ends of the rod are warmer than the surrourndings (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 between the convective heat loss (~hP) and the heat flux in the rod (~kA) through to factor m. Equation 1 actually e=duces to:
T (x)I _ (L - x)T~ + xTz rn-a0 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 and/or increasing the heat flux in the rod. In order to achieve low power Is consumption, reducing the heat loss is the preferred course of action. FIG.

illustrates the effect of thermally insulating the conductive rod on the normalised temperature distribution U(x) along said rod, defined as:
U(x) _ _T (x) (3) CTz T ~x+T
L
where T(x) is given by Equation 2. (The normalised temperature distribution 2o for the ideal linear gradient is therefore equal to U(x) = 1.) (II serait pent-etre bon de donner les parametres utilises pour 1e calcul. -~ Dons Section Experimental).
These distributions were computed using a finite elf analysis software and confirmed by numerical analysis. They clearly show that strengthening the thermal insulation around the conductive rod improves the linearity of the thermal gradient 2s along said rod. The insulation schemes considered in FIG. 2 will be discussed in more details below, afters presentation of the elements user! to create the thermal gradient in the first place.
Heating system s The basic system 10 for establishing a thermal gradient along an optical fiber 11 is illustrated in FIG. 3a. It includes an elongated element 13, called herein the thermal gradient rod, that is in close contact with the optical fiber 11.
The thermal gradient rod 13, preferably made out of a heat conductive metal, allows a uniform heat transfer along its length in order to create a temperature gradient in to adjacent fiber 11. A fiber Bragg grating 12 is written in the optical fiber 11. These elements are positioned such that the Bragg grating 12 and the them~ral gradient rod 13 are co-centred longitudinally.
In a preferred embodiment, the optical fiber 11 rests freely in a small hole 14 located at the centerof~the transversE cross-section of the themTal gradient rod Is 13 and extending along the lorrgitudirral axis of said rod. This geometry has many advantages. A thEm~tal compound is not rewired to ensure a good replication in the fiber 11 of the temperature profile existing along the rod 13. As a result, the optical properties of the Bragg grating 12 are not affected by the contact between the optical fiber 11 and said rod 13. This configuration also minimises the 2o detrimental effect of air currents on the temperature distribution created along the fiber 11. Within this embodiment, the fiber 11 remains unaffected by the thermal expansion (or contraction) of the therrrral gradient rod 13, since they are not mechanically coupled to one another. Only the thermal change in the index of refraction of the fiber 11 and the thermal expansion of said fibEr will affect the 2s optical properties of the Brag grating 12. The evolution of the optical properties of the Bragg grating 12 with temperature can then be predicted more accurately.
Finally, long term reliability is improved since this arrangement leaves the fiber 11 free of any mechanical stress.
Two heat pumping elements 15 are fixed in close physical contact with both 3o ends of the thermal gradient rod 13, using an appropriate method like pressure mounting with a thermal compound, thermal gluing, or soldering. The heat pumping elements 15 are preferably Pettier effect-thErmo-electric coolers, referred to hereafter as TECs. These elements pump heat from one side of their body to the other to fix the temperature T1 and T2 of the ends of the rod 13. The temperature difference 0T = T~ - T2 is responsible for the creation of a thermal s gradient along the rod 13.
A temperature sertsvr element 16, such as a thermistor or a resistance temperature detector (RTD), is located on top of each TEC in close proximity to the thermal gradient rod 13. The sensors 16 are fixed in close contact with an appropriate method, usirng for example a thermally conductive epoxy. Signals from to these sensors are used as input to a servo control system not shown in FIG.
3 to precisely control (fix arnd maintain) the temperature at each end of the rod 13.
Such means for temperature control are well known in the art, and include appropriate control electn~nics and drive such as TEC controllers with PID
servo-control for optimum dynamic operation.
Is The heat pumping elements 15 can be mounted on heat sinks acting as heat reservoirs (illustrated by 17) or on a thermally conductive n=circulation bar, as described in Canadian patent application no 2,371,106.
Thermal Insulation 2o As stated above, heat loss fn~m the thermal gradient and 13 to the surroundings must be minimised in order to preserve the linearity of the thermal gradient created therein. The rod 13 can be thermally insulated by enclosing it in a cylinder 20 made of a low density material, as depicted in FIG. 3b. For example, insulating foams with a very Ivw themral conductivity (k ~ 0.03 W/m2K) can be 2s used efficiently to improve the linearity of the thermal gradient. The necessary thickness of insulating material can be determined from existing art. For example, it is found that a cylirnder 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 dues reduce the heat loss from 3o the thermal gradient rod 13. The achievable gain in p~rfvrmance can then be weighted against the increase in volume of the device to determine an optimum foam thickness.
At ambient temperature, air is an even better insulatvrthan foam. In view of volume limitations, it may be preferable in some cases to replace the foam cylinder s by a thin layer of air confirred in a tube. Convection within the air layer must be avoided at all cyst, 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 resistance imposEd by the viscous forces of air. The maximum allowable air thickness can be determined from existing art. This type of thermal insulation, Io discussed in U.S. Patent 6,351,585, represents a good compromise between cost and complexity.
Even better insulation can be achieved by sumaunding the thermal gradient rod with vacuum, using for example a vacuum dewar. Vacuum flasks made out of glass and forming a Themrros~-type insulator can be used efficiently to this end.
Is Neither conduction nor convection can occur in a complete vacuum. As a result, heat loss can only resultwfn~m radiation. In practice, small lasses can be cause by conduction in lateral walls of the dewar. The amount of radiation emitted by the thermal gradient rod 13 can be reduced by polishing its outer surface to a mirror finish. The radiative heat loss can be further minimised by applying a heat 2o reflective coating on the inner or outer-wall of the vacuum cavity enclosing the rod.
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 properties.
FIG. 3c illustrates an embodiment of this approach where the thermal 2s gradient rod is surrounded by a vacuum tube 30 with insulating end walls 31 that come into contact with said rod. The end walls 31 and the tube 30 can be made of different materials, as illustrated by 31a or~from a common material as indicated by 31 b. Vacuum is made in the cavity by means of an appropriate airtight valve 33.
An appropriate seal between the end walls 31 and the thermal gradient rod is 3o required in order to provide an airtight fit. A reflective coating 32 can be used on the outer tube to minimise losses.

Another implementation of the vacuum insulation is shown in FIG. 3d, where the thermal gradient rod is located in the central hole 42 of a separate thermos tube 40. WhEn the themms 40 is made out of glass, the inner wall of the tube that gets heated by the thermal gradient rod will radiate strongly, given the s large emissivity of glass. A metallic heat reflective coating 41 applied on the thermos cylinder outer wall can be used to limit radiative heat loss. Vacuum is made in the cavity by means of an appn~priate airtight fusiorred valve 43.
FIG. 2 compares the efFect of these various insulation schemes on the linearity of the thermal gradient. The vacuum insulation approach clearly gives the to best results. In the case of insulation by an air gap, the gap thickness was taken as the maximum allowable to maintain a cQnvectionless heat transfer. In terms of thermal insulation, this corresponded to a 10-mm layer of foam for the specific configuration studied. This radius can change in function of the length and the exterior diameter of-the cvrrductive rod acrd the temperatures involved.
Is FIG. 4 presents another exemplary embodiment of the invention that has a radial symmetry. Heat is trarrsferred to arnd taken out from the thermal gradient rod 13 via heat distributors 56 in contact with circular TECs 15 (with a hole in their center) mounted p~rperrdicularly on the axis of the device. Heat sinks 17 in close contact with the TECs dissipate heat in the ambient air. An outer cylinder 50 is 2o fixed hermetically to end walls 51 by an airtight welding 53. The end walls 51 are either made of an irtsulatirrg material orr~ron-conductively attached to the rod 13 by an appropriate airtight joint (or soldering) 54. This ensemble constitutes an airtight construction enclosing the thermal gradient rod 13. Air is pumped out of this enclosure and vacuum is maintained by an airtight crimped valve 55. An optional 2s inner shield 52 can be used to increase radiation isolation and further improve the performance of the device. An outer device casing 57 can be used to provide additional pnJtection to the device~from-surrounding perturbations.
Naturally, the present invention is not limited to the pn3fem~d embodiments and materials presented herein far illustration purposes.

Claims