CA2371106A1

CA2371106A1 - Method for inducing a thermal gradient in an optical fiber

Info

Publication number: CA2371106A1
Application number: CA 2371106
Authority: CA
Inventors: Tristan Ruthers; Philippe Pepin; Richard Lachance
Original assignee: Teraxion Inc
Current assignee: Teraxion Inc
Priority date: 2002-02-07
Filing date: 2002-02-07
Publication date: 2003-08-07

Description

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.

2 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.

3 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

4 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 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 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 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.

Naturally, the present invention is not limited to the preferred embodiment and materials presented herein for illustration purposes.