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Publication numberWO2001035133 A1
Publication typeApplication
Application numberPCT/DK2000/000627
Publication date17 May 2001
Filing date10 Nov 2000
Priority date11 Nov 1999
Publication numberPCT/2000/627, PCT/DK/0/000627, PCT/DK/0/00627, PCT/DK/2000/000627, PCT/DK/2000/00627, PCT/DK0/000627, PCT/DK0/00627, PCT/DK0000627, PCT/DK000627, PCT/DK2000/000627, PCT/DK2000/00627, PCT/DK2000000627, PCT/DK200000627, WO 0135133 A1, WO 0135133A1, WO 2001/035133 A1, WO 2001035133 A1, WO 2001035133A1, WO-A1-0135133, WO-A1-2001035133, WO0135133 A1, WO0135133A1, WO2001/035133A1, WO2001035133 A1, WO2001035133A1
InventorsOle Sigmund, Martijn Beukema, Jens Engholm Pedersen
ApplicantKoheras A/S
Export CitationBiBTeX, EndNote, RefMan
External Links: Patentscope, Espacenet
Compact athermal optical waveguide using thermal expansion amplification
WO 2001035133 A1
Abstract
A method of temperature stabilising optical waveguides having positive thermal optical path length expansion, in particular fiber Bragg gratings or optical fiber DFB lasers or optical fiber DBR lasers, comprising affixing the optical waveguide to at least two points of a negative expanding fixture; said negative expanding fixture comprising one or two V-shaped displacement amplifiers made from a material with a higher positive CTE mounted in a frame made of a material with lower positive CTE; the V-shaped displacement amplifier is mounted on a support in such a manner that its motion in directions substantially perpendicular to the axis of the mounted fiber device is restrained by the support and transferred to (amplified) displacement along the fiber axis.
Claims  (OCR text may contain errors)
Claims:
1. A method for controlling the temperature sensitivity of an optical waveguide having a positive thermal optical path length expansion where a first part of the optical waveguide is affixed to a point on a displacement amplifier and a second part is affixed to a another point at a distance from the first part, said optical waveguide being pre-strained between the two affixing points such that it forms a well defined axis c h a r a c t e r i s e d in that the displacement amplifier is mounted on a support in such a way that its motion in directions substantially perpendicular to the axis of the optical waveguide is constrained by the motion of the support.
2. A method according to claim 1, c h a r a c t e r i s e d in that the second part constitutes the support.
3. A method according to claim ^ c h a r a c t e r i s e d in that the second part constitutes another displacement amplifier.
4. A method according to claims 1-3, c h a r a c t e r i s e d in that the displacement amplifier has two beams forming a V-shape and two adjoining beams forming a base for attachment to the support.
5. A method according to claims 1-4, c h a r a c t e r i s e d in that the links between the beams forming the V-shape and two adjoining beams form hinges.
6. A method according to claims 1-5, c h a r a c t e r i s e d in that the support consists of a frame or a plate.
7. A method according to claims 1-6, c h a r a c t e r i s e d in that the first part of the optical waveguide is affixed on a point on the upper side of one of the beams.
8. A method according to claim 7, c h a r a c t e r i s e d in that the point on the upper side is in the center of symmetry of the V-shape.
9. A method according to claims 1-8, c h a r a c t e r i s e d in that the coefficient of thermal expansion of the displacement amplifier is higher than the coefficient of thermal expansion of the support.
10. A method according to claims 1-9, c h a r a c t e r i s e d in that the V-shape of the displacement amplifier, where the first part of the optical waveguide is fixed, points towards the other fixation point of the optical waveguide.
11. A method according to claims 1-8, c h a r a c t e r i s e d in that the coefficient of thermal expansion of the displacement amplifier is lower than the coefficient of thermal expansion of the support.
12. A method according to claims 1-8,11, c h a r a c t e r i s e d in that the V-shape of the displacement amplifier, where the first part of the optical waveguide is fixed, points away from the other fixation point of the optical waveguide.
13. A method according to claims 1-12, c h a r a c t e r i s e d in that the displacement amplifier is made of a material exhibiting a low degree of mechanical hysteresis.
14. A method according to claims 1-13, c h a r a c t e r i s e d in that the thermal expansion between the fixation points of the optical waveguide lies in the range from -2 O"6/C to -12 O"6/C, preferably in the range from -610"6/C to -910"6/C.
15. A method according to claims 1-14, c h a r a c t e r i s e d in that a controlled biastension is applied to the optical waveguide prior to fixation.
16. A method according to claims 1-15, c h a r a c t e r i s e d in that a further controlled biastension is applied to the optical waveguide after affixing the optical waveguide, by adjusting the position of the displacement amplifier in the axial direction of the optical waveguide.
17. A method according to claims 1-16, c h a r a c t e r i s e d in that the optical waveguide is an optical fiber, e.g. a single mode fiber.
18. A method according to claims 1-16, c h a r a c t e r i s e d in that the optical waveguide is an optical fiber device.
19. A method according to claim 18, c h a r a c t e r i s e d in that the optical fiber device is a fiber Bragg grating.
20. A method according to claim 18, c h a r a c t e r i s e d in that the optical fiber device is an optical fiber laser.
21. A method according to claim 20, c h a r a c t e r i s e d in that the optical fiber laser is an optical fiber distributed feed back laser or an optical fiber distributed Bragg reflector laser.
22. A method according to claims 20-21, c h a r a c t e r i s e d in that the optical fiber laser is doped with one or more rare elements, including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and having UV-induced Bragg gratings.
23. A method according to claims 18-19, c h a r a c t e r i s e d in that the optical fiber device is polarisation insensitive.
24. Device according to claims 20-22, c h a r a c t e r i s e d in that the fiber laser is polarisation stable.
25. A method according to claim 24, wherein the optical fiber laser is spliced to a polarisation maintaining fiber, and the axes of both have been aligned by twisting the polarisation maintaining fiber and the optical fiber device or the optical fiber laser relative to each other prior to both the optical fiber device or the optical fiber laser and the polarisation maintaining fiber being affixed to the device.
26. Device according to claims 18-25, c h a r a c t e r i s e d in that the optical fiber device is temperature stable with a temperature response numerically lower than 3 pm/K, preferably lower than 1 pm/K.
27. A method according to claims 1-9,13, c h a r a c t e r i s e d in that the V-shape of the displacement amplifier, where a first part of the optical waveguide is fixed, points away from the other fixation point of the optical waveguide.
28. A method according to claims 1-9,13,27, c h a r a c t e r i s e d in that the thermal expansion between the fixation points of the optical waveguide is higher than the corresponding thermal expansion of the support.
Description  (OCR text may contain errors)

COMPACT ATHERMAL OPTICAL WAVEGUIDE USING THERMAL EXPANSION AMPLIFICATION.

BACKGROUND OF THE INVENTION.

The present invention relates to a method for controlling the temperature sensitivity of an optical waveguide having a positive thermal optical path length expansion where a first part of the optical waveguide is affixed to a point on a displacement amplifier and a second part is affixed to a another point at a distance from the first part, said optical waveguide being pre- strained between the two affixing points such that it forms a well defined axis.

The Technical Field.

It is well known in the field of optics that the performance of optical components depends on temperature. This dependence is due to a change of refractive index (thermo-optic effect) and strain with temperature. Typically the thermo-optic effect yields the dominant contribution, and for most optical materials the thermo-optic coefficient is positive, i.e. the refractive index increases with increasing temperature. In silica this increase is of the order of +11 -10"6/C at near- to mid-infrared wavelengths. For components based on UV-written Bragg gratings in fibers or planar waveguides this results in a temperature dependence of the center wavelength of approximately 0.01 nm/C. Although this number is approximately 10 times lower than for semiconductor based optical components it is still too high for a range of important applications. A notable example is found in optical communication systems based on dense wavelength division multiplexing where the channel spacing may be e.g. 50 GHz / 0.4 nm and system administration requires that the wavelength stays within at least 25% of the channel spacing. I e the wavelength should drift no more than 0.1 nm corresponding to temperature change of approximately 10 C. In real systems the temperature may vary by much more that 10 C and it is thus necessary to reduce the temperature dependent wavelength drift by somewhere around a factor of 10, corresponding to a wavelength dependence no higher than 0.001 nm/C.

Various methods of stabilising the wavelength have been suggested. In one method the temperature of the device is stabilised actively, e.g. by measuring the device temperature and controlling it through a suitable feedback. A disadvantage of this method is that energy is produced which will dissipate to the rest of the system.

In other methods the thermo-optic coefficient is manipulated to balance the thermal expansion, or vice versa.

Generally, the temperature (T) dependence of the center wavelength λ of a Bragg grating in an optical fiber is given by the following equation (1 ):

X dλ X δn X dn δε 1 δA δε λ dT n δT n δε δT A δε δT

where n, α and ε are the values for the refractive index, the thermal expansion and the strain. Λ is the Bragg grating period. The 1st term which includes the thermo-optic coefficient 3n/9T represents the change in refractive index with temperature, the 2nd term is the coefficient of thermal expansion (CTE) of the optical fiber, the 3rd term which includes the elasto- optic coefficient dnldε represents the change in refractive index with strain, and the last term represents the change in the Bragg grating period with strain.

This equation suggests the following methods for temperature stabilisation:

The material is modified to tune the thermo-optic coefficient so that it cancels out the contributions from thermal expansion and strain. In most fiber optical materials these two effects act together to increase the center wavelength with temperature. However, by tailoring the optical material to provide a negative thermo-optic coefficient, the positive contribution from the remaining terms is balanced to provide a stable center wavelength. A disadvantage of this method is that it is difficult to produce an optical material with a negative thermo-optic coefficient while maintaining other properties of the material.

Alternatively the optical fiber is mounted on a substrate under tension in such a way that its effective thermal expansion becomes negative to compensate the normally positive contributions from the thermo-optic and photo-elastic effects. When the optical fiber is mounted under tension equation (1 ) reduces to:

X dλ X dn f X δ X dn

(2) 1 + -af λ dT dT s V n dε j n dε

where αs and αf are the CTE's of the substrate and the optical fiber respectively. The CTE of the substrate can be made negative by the following methods.

In a first method (hereafter referred to as the first method), the substrate is composed of two materials of different length and having different positive CTE's. The shortest piece of material is made from the material with the highest positive CTE and the longest piece is made from the material with the lowest positive CTE. By fixing one end of the short piece to one end of the long piece, the other ends of the two pieces will approach each other as the temperature is increased. This presumes that the lengths and material parameters are balanced correctly. When an optical fiber is mounted under tension between these ends its effective thermal expansion becomes negative. A disadvantage of this method is that the substrate must be quite long to obtain the desired behaviour. Specifically the substrate will be longer than the fiber device by the length of the material with the highest positive CTE. In a second method (hereafter referred to as the second method), the substrate consists of a single material with an intrinsic negative CTE. An optical fiber is mounted under tension on the substrate. By designing a substrate material with a suitable value of the negative CTE, the positive contributions from the thermo-optic and photo-elastic coefficients of the optical fiber are compensated. This method has the advantage that once the correct material composition has been provided no further adjustments are required in order to achieve a stable center wavelength. Thus, this method has the advantage of simplicity in the mounting process; the exact length of the fiber is not important. A disadvantage of this method lies in the limited selection of materials that combine a proper negative CTE with other required material properties, eg. insensitivity to humidity, proper stiffness and mechanical robustness. Another disadvantage of this method is its relative inflexibility: one substrate material will match only one value of the thermo-optic coefficient.

Prior Art Disclosures.

Chu et al., "Multilayer dielectric materials of SiOxTa2O5/SiO2 for temperature-stable diode lasers", Materials Chemistry and Physics, 42

(1995), pp. 214-216, discloses a SiOx/Ta2O5/SiO2 sandwiched waveguide design with an effective negative thermo-optic coefficient applied to temperature stabilising diode lasers. Nothing is disclosed about temperature stabilising optical waveguides.

US patent 5 042 898 discloses a method wherein two pieces of different materials with different CTE's and different length are arranged to balance the thermo-optic coefficient of an optical fiber (the first method described above). This method has the disadvantage of requiring full control over the process used to fix the optical fiber to the substrate. Also, the substrate must be quite long to obtain the desired behaviour. Specifically the substrate will be longer than the fiber device by the length of the material with the highest positive CTE. This technique makes use solely of the longitudinal forces that arise due to the differential CTE's of the two materials forming the substrate.

International application WO 97/26572 discloses a method using a single substrate material with an intrinsic negative CTE (corresponding to the second method described above) combined with a particular class of substrate material with an intrinsic negative CTE: lithium-alumina-silica type ceramic glasses heat treated to develop the beta eucryptite crystal phase. Beta eucryptite being a ceramic glass is potentially fragile. It exhibits thermal expansion anisotropy which results in microcracks and hysteresis in the heating/cooling curve for compensated fiber components.

US patent 5 694 503 discloses a method using a single substrate material with an intrinsic negative CTE (corresponding to the second method described above) combined with a particular class of substrate material with an intrinsic negative CTE: Zr-tungstate and/or Hf-tungstate. The thermal expansion can be tailored by admixture of positive CTE material (e.g., AI2O3, SiO2) to the negative CTE material (e.g., ZrW2O8), or by a variety of other techniques. This mixture being a ceramic is potentially fragile.

US patent 5 841 920 discloses a method wherein a mechanical structure, consisting of a compensating and a tension adjusting member, exhibits a negative thermal expansion on which an optical fiber component, such as a grating, can be mounted. The tension adjusting member and the compensating member are formed of materials selected so that as the temperature of the device decreases, the tension adjusting member contracts more than the compensating member so as to control the deformation of the compensating member and thereby impose an axial strain on the grating. This method is based on deflection of the fiber device and hence requires the fiber to rest firmly on a support whereas in some applications the fiber should be free to move along the whole length of the device without any contact. International application WO 99/27400 discloses a method using a single substrate material with an intrinsic negative CTE (corresponding to the second method described above) combined with a particular class of substrate material with an intrinsic negative CTE, the substrate material being fiber based composite materials using fibers with negative CTE in a resin matrix. Although the design of the composite material offers flexibility in the choice of the negative CTE and the material is mechanically rugged it is also sensitive to environmental influences such as humidity.

Ole Sigmund, "Design of thermomechanical actuators using topology optimization", Proc. of the Second World Congress on Structural and Multidisciplinary Optimization, Zakopane, Poland, May 1999, pp. 393-398, discloses a design of thermal actuators with enhanced thermal elongation. The applications discussed are within the field of MicroElectroMechanicalSystems (MEMS); nothing is disclosed about temperature stabilising optical waveguides.

DISCLOSURE OF THE INVENTION.

It is an object of the present invention to provide a method of controlling the temperature response of an optical waveguide. More specifically it is an object to provide a method of temperature stabilising an optical waveguide having a positive thermal optical path length expansion, in particular fiber Bragg gratings and fiber Bragg grating based optical fiber lasers, and thus to provide a robust temperature stabilised optical waveguide. It is another object to provide a method of amplifying the temperature sensitivity of an optical waveguide (with possible applications in sensors or for amplified temperature induced stretch tuning of fiber Bragg grating based components).

This object is achieved by providing a method for controlling the temperature sensitivity of an optical waveguide having a positive thermal optical path length expansion where a first part of the optical waveguide is affixed to a point on a displacement amplifier and a second part is affixed to a another point at a distance from the first part, said optical waveguide being pre-strained between the two affixing points such that it forms a well defined axis.

This method is characterised in that the displacement amplifier is mounted on a support in such a way that its motion in directions substantially perpendicular to the axis of the optical waveguide is constrained by the motion of the support.

This method has, amongst other things, the advantage that the substrate can be made much shorter than the substrate known from the prior art.

It is expedient that the second part constitutes the support or another displacement amplifier as stated in claims 2,3 respectively.

Preferably, as stated in claim 4, the displacement amplifier has two beams forming a V-shape and two adjoining beams forming a base for attachment to the support.

By adjustment of the distance between fixation points as well as the off-set of the fixation points from the bottom of the V-shaped displacement amplifiers, it is possible to adjust the negative CTE of the fixture accurately to a desired value to match the thermal response of different types of components.

The V-shaped displacement amplifier(s) is (are) mounted in such a way that, as stated in claim 10, the V-shape of the displacement amplifier, where the first part of the optical waveguide is fixed, points towards the other fixation point of the optical waveguide.

It is furthermore preferred, as stated in claim 13, that the displacement amplifier is made of a material exhibiting a low degree of mechanical hysteresis. This is in order to minimise hysteresis in the wavelength response of the packaged device. Examples of such materials are Macor for the V-shaped displacement amplifiers (high CTE, easy to machine).

In a preferred embodiment, a controlled tension is applied to the optical waveguide prior to affixing it to the negative thermal expansion fixture. The tension is adjusted to such a level that the thermal expansion of the waveguide is determined solely by the thermal expansion of the fixture and not by the thermal expansion of the waveguide itself over the temperature interval specified for the device.

The center wavelength of fiber optic devices such as fiber Bragg gratings and fiber Bragg grating based fiber lasers is adjusted, as stated in claim 15, by applying a controlled bias tension to the optical waveguide prior to fixation.

Post tuning of this center wavelength is possible, as stated in claim 16, by applying a further controlled bias tension to the optical waveguide after fixation, whereby the position of the displacement amplifier is adjusted in the axial direction of the optical waveguide.

In a preferred embodiment, cf. claim 17 the optical waveguide is an optical fiber, e.g. single mode fiber. The properties of axial symmetry and the flexibility of such an optical fiber makes it particularly simple to temperature stabilise by affixing it under tension to a negative thermal expansion fixture.

In another preferred embodiment, cf. claim 18 the optical waveguide is an optical fiber device, such as, cf. claim 19, a fiber Bragg grating.

Particularly preferred optical waveguides include optical fiber lasers, cf. claim 22, that are doped with one or more rare earth elements, including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and that have UV-induced Bragg gratings. Further preferred are stable polarisation mode optical fiber DFB lasers or optical fiber DBR lasers. Stable single polarisation mode operation of these devices is necessary for a number of important applications including optical communication where external modulation requires the use of polarisation sensitive devices such as lithium niobate modulators.

In a preferred embodiment the optical fiber laser is, as stated in claim 25, spliced to a polarisation maintaining fiber, and the polarisation axes of both have been aligned by twisting the polarisation maintaining fiber and the optical fiber laser relative to each other prior to both the optical fiber laser and the polarisation maintaining fiber being affixed to the device. In this way the polarisation extinction is optimised at the other end of the polarisation maintaining fiber so that the optical output is in a controlled single linear polarisation state.

Further expedient embodiments for the invention are expressed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS.

In the following, the invention is described in more detail with reference being made to the drawings in which

Figure 1 shows the schematics of an ideal negative thermal expansion fixture comprising a support (in this figure the support consists of a frame) made of a material with a low positive CTE and one V-shaped displacement amplifier made of a material with a high positive CTE. An UV-grating based optical fiber component is mounted on the negative thermal expansion fixture;

Figure 2 shows a practical implementation of the negative thermal expansion fixture; Figure 3 shows a negative thermal expansion fixture including two V- shaped displacement amplifiers for use with longer components;

Figure 4 shows another practical embodiment of implementation of a negative thermal expansion fixture where the support consists of a base plate;

Figure 5 shows graphs of the contributions to the CTE between fiber component mounting points on an ideal negative thermal expansion fixture made from a single V-shaped displacement amplifier.

Figure 6 shows a graph of the thermal response of 1 ) a non-compensated fiber Bragg grating, and 2) a fiber Bragg grating mounted on a negative thermal expansion fixture with two V-shaped displacement amplifiers;

Figure 7 shows the variation of the CTE as a function of the fiber mounting point position on the V-shaped displacement amplifier relative to the bottom of the V-shaped displacement amplifier.

DETAILED DESCRIPTION.

As illustrated in figure 1 , the fixture 1 consists of a support 2 made from a material with low positive CTE and one (or two) V-shaped displacement amplifiers 3. The V-shaped displacement amplifier 3 typically consists of the actual V-shape (convex shape) 4 and an outer region 5 for fixation to the support 2. With denoting the distance between fixation points 7 for the fiber component 6, l2 denoting the displacement amplifier 3 position on the support 2 relative to the opposite fiber fixation point 7, w denoting the distance between V-shape-to-support fixation points 8, w2 denoting the V- shape (convex part) 3 width, and αi and α2 denoting the CTE (coefficient of thermal expansion) of the support 2 and the V-shaped displacement amplifier 3, respectively, the thermal expansion between the fixture points 7 can be approximated as (for a single V-shaped displacement amplifier):

g. = d2 + a2 (l, - l2) | w.w^a, - a2) 4(/2 -/,)/,

Without the contribution from the structure of the fixture 3 w-i = w2 = 0), the behaviour is governed by the left part of equation (3) corresponding to a simpler system (described in US patent 5 042 898 and corresponding to the first method described above) containing materials with different CTE's but no displacement amplification via geometry. This system makes use solely of the longitudinal forces that arise due to the differential CTE's of the two materials forming the substrate. With the displacement amplifiers, because of the particular structure of the device, the negative thermal expansion is amplified when the displacement amplifier 3 position l2 approaches the fiber component 6 length . This amplification arises because the motion of the displacement amplifier 3 in directions substantially perpendicular to the axis of the optical fiber component 6 mounted on the negative thermal expansion fixture 1 is constrained by the motion of the support 2. Hence, by way of example, if the negative thermal expansion fixture 1 is heated, the displacement amplifier 3 will want to expand more than the support 2.

However, because it is fixed to the more rigid support 2, tension builds up resulting in a push on the V-shaped displacement amplifier 3 in the direction of the fiber 6 axis and the bottom of the V 7. The total displacement along the fiber 6 axis will therefore be larger than the displacement due to thermal elongation alone. The negative thermal expansion fixture 1 of the new invention can therefore be made much shorter than the negative thermal expansion mount of the invention described in US patent 5 042 898.

The difference between the invention described in US patent 5 042 898 and the new invention is demonstrated by the graph in Figure 5. For a specific choice of materials and lengths (Macor for the single V-shaped displacement amplifier (3 in figure 1 ), αMacor = 9.8 O"6 / C, and invar for the support (2 in figure 1 ), αjnVar = 1.5 O"6 / C, w1 = 15 mm, w2 = 11 mm and /? = 33 mm), the contributions to the CTE of the fixation points (7 in figure 1 ) are plotted in figure 5 as a function of the displacement amplifier position, l2. Plots are given for the longitudinal contribution (1 in figure 5) to the CTE corresponding to the invention described in US patent 5 042 898 and to the first term of equation (3), the structure contribution (2 in figure 5) corresponding to the second term of equation (3), and the total (3 in figure 5) value for the CTE. In order to obtain a negative thermal expansion of α = -8.5 10"6 / C (a typical value at which the thermo-optic effect is balanced by the expansion of the substrate at wavelengths around 1550 nm; marked with a line (4 in figure 5)), the length of the negative thermal expansion mount of the invention described in US patent 5 042 898 must be approximately l2 = 73 mm whereas the length of the negative thermal expansion fixture 1 of the new invention is approximately l2 = 34 mm. In other words the length along the fiber axis of the V-shaped displacement amplifier is approximately 1 mm and the length of the package is only slightly longer than the device. As l2 increases, the CTE of the negative thermal expansion fixture according to the invention approaches that of the longitudinal contribution corresponding to the first term in (3) and to the first method described above.

For the ideal fixture, the effective thermal expansion goes to minus infinity when the displacement amplifier position l2 approaches the fiber length . The ideal fixture assumes that the V-shaped displacement amplifiers are build with loss-free hinges and rigid bars. In practical realizations, the V- shaped displacement amplifier is flexible and can be built with compliant hinges (see figures 2,3,4) which degrades the performance. This results in the CTE curve bending back towards positive values as the displacement amplifier position l2 approaches the fiber length . Furthermore, the stiffness of the V-shaped displacement amplifier approaches zero when the displacement amplifier position l2 approaches the fiber length /?. In order to predict the thermal expansion and stiffness of a practical fixture, the device must be modeled using e.g. Finite Element analysis. Different practical realizations of the fixture are shown in figures 2, 3 and 4. In figure 2 the V-shaped displacement amplifier 3 is attached within a frame by screws 9 or other means. The frame can be machined out of one piece of material. The V-shaped displacement amplifier 3 can be machined, casted or extruded followed by drilling and threading. The fiber component

6 can be attached 7 to the V-shaped displacement amplifiers 3 by glueing, soldering, welding or other means. Figure 3 shows a practical realization of the fixture including two V-shaped displacement amplifiers 3. This way a numerically higher negative CTE is obtained for the same distance between V-shaped displacement amplifiers 3. This makes it possible to compensate longer components with un-changed thermo-optic coefficient. In figure 4 a different practical realization using a base plate for support is illustrated. A base plate is easier to fabricate than a frame. This is especially important when the base plate material is difficult to machine such as is the case e.g. for quartz.

A temperature stabilised optical waveguide, such as a UV-written Bragg grating based component, is realized by affixing the optical waveguide under controlled tension onto two points of a V-shaped displacement amplifier fixture with a negative CTE in such a manner that the positive thermal wavelength response of the waveguide is balanced and the thermal response of the packaged component ideally reduces to zero nm/K.

In particular optical fiber Bragg gratings and optical fiber DFB or DBR lasers can be thermally stabilised using the V-shaped displacement amplifier fixture of the invention.

The V-shaped displacement amplifier fixture consists of a support made from a material with a low thermal expansion onto which is mounted a V- shaped displacement amplifier made from a material with a high thermal expansion. Provided that geometrical and material parameters are selected according to special criteria, the bottom point of the V-shaped displacement amplifier and a point on the frame will approach each other when the temperature is increased. When an optical waveguide is mounted under tension between these two points, its effective thermal expansion becomes negative. By choosing all parameters correctly the effective negative thermal expansion can be designed to balance the positive thermal wavelength response of the waveguide thus providing an athermal waveguide.

Two V-shaped displacement amplifiers can be mounted in opposite directions on the support / in the frame to obtain numerically higher values of the negative thermal expansion. For a small angle of the V-shaped displacement amplifier, a low displacement amplification factor is achieved resulting in a numerically low value of the effective negative thermal expansion. For a large angle of the V-shaped displacement amplifier, a large amplification of the negative thermal expansion behaviour is obtained. This method has the advantage that the substrate can be made short, and indeed with a length not greatly exceeding the length of the waveguide component. Furthermore, the method has the advantage that different values of the CTE can be obtained within a certain range by mounting the waveguide off center in the V-shaped displacement amplifier - thus providing a means for fine-tuning the device.

Besides providing a short package with a numerically high negative CTE, other qualities of the V-shaped displacement amplifier fixture include high mechanical stability, the possibility of using different mounting methods including soldering, and the option of very precise tuning of the center wavelength of the fiber device after mounting.

The negative CTE, αs, required for the fixture to balance the positive thermal wavelength response of the waveguide is obtained from eq.2:

X dn where: P n dε Typical values are (assuming the waveguide is a silica fiber and that wavelengths close to 1550 nm are considered):

n = 1.45 (refractive index) dn/dT = 1 1 0"6/C (thermo-optic coefficient) pe = 0.22 (photo-elastic constant)

<xf = 0.55 O"6/C (CTE)

The required value for the CTE of the V-shaped displacement amplifier fixture is therefore approximately (depending on exact optical waveguide parameters):

αs = -8.5-10"6 /C

Realistic implementations of the V-shaped displacement amplifier fixture will produce negative CTE's in a range around this value as illustrated in figure 5.

A temperature stabilised optical waveguide is obtained by affixing the optical waveguide under controlled tension on the V-shaped displacement amplifier mount. Specifically, a controlled tension is applied to the optical fiber in an amount so that both the desired center wavelength is obtained and the fiber remains under positive tension over the entire temperature interval specified for its function. This interval may typically be between -40 C and +70 C. I.e., if the optical fiber is affixed to the V-shaped displacement amplifier fixture at room temperature, say 20 C, then it should still be under tension when heated to 70 C. With a temperature sensitivity of the center wavelength of e.g. 0.01 nm/C in a free optical fiber, tension to the optical fiber must therefore be applied in such an amount that the center wavelength moves at least +0.50 nm. After tension is applied, the optical fiber is affixed to the V-shaped displacement amplifier mount. EXAMPLES.

Example 1.

Temperature stabilising an optical fiber Bragg grating based notch filter:

A fiber Bragg grating was photo induced in a UV-sensitive fiber using an excimer laser operating at 248 nm and a phase mask. A temperature stabilised optical fiber Bragg grating was obtained by affixing the optical fiber under controlled tension on a negative expanding fixture made according to the invention. It was fabricated with a 19 mm wide frame made of stainless steel, two opposing 15 mm wide V-shaped displacement amplifiers made of aluminium. The distance between the bottom points of the V-shapes was 25 mm and the distance between the mounting points of V-shapes was 27 mm. Tension was applied so that both the correct center wavelength was obtained and the fiber remained under positive tension over the temperature interval specified for the component. After tension was applied, the fiber was glued to the negative expanding fixture. As shown in figure 6 the temperature stablized grating experienced a wavelength variation of less than 15 pm over the interval between room temperature and 70 C. This should be compared with a temperature sensitivity of approximately 0.01 nm/C of the free grating.

Example 2.

Tuning of the CTE:

The CTE of the package (1 in figures 1-4) is sensitive to the position (l2 in figures 1-4) of the V-shaped displacement amplifier (3 in figures 1-4) on the support (2 in figures 1-4). This provides a method for fine-tuning the CTE of the device (1 in figures 1-4). In addition the CTE can be tuned by placing the optical fiber with the grating (6 in figures 1-4) on different positions on the convex part of the V-shaped displacement amplifier (4 in figures 1-4). The CTE increases towards positive values as the fixation point (7 in figures 1-4) is moved away from from the bottom of the V-shaped displacement amplifier towards the outer region of the V-shaped displacement amplifier (5 in figures 1-4). This is illustrated by the graph in figure 7. In this way the package can compensate the thermo optic effect for different types of optical fibers.

Example 3.

Affixing the optical fiber to the V-shaped displacement amplifiers:

The optical fiber can be affixed to the V-shaped displacement amplifiers using adhesives such as polymeric adhesives, soldering or welding, depending on the performance requirements. Using a polymeric adhesive, such as a thermally curing epoxy adhesive, e.g. EpoTek 353ND, a thin bond line should be used to reduce creep to a minimum.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
WO1997026572A1 *7 Aug 199624 Jul 1997Corning IncorporatedAthermal optical device
US5042898 *26 Dec 198927 Aug 1991United Technologies CorporationIncorporated Bragg filter temperature compensated optical waveguide device
US5841920 *18 Mar 199724 Nov 1998Lucent Technologies Inc.Fiber grating package
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
WO2017050767A1 *20 Sep 201630 Mar 2017fos4X GmbHLight guide clamping device, fibre optic sensor and production method
EP1679497A1 *30 Dec 200512 Jul 2006Fibersensing - Sistemas Avanados de Monitorizao S.A.Passive athermal fibre Bragg grating strain gage
Classifications
International ClassificationG02B6/36
Cooperative ClassificationG02B6/0218
European ClassificationG02B6/02G8R2B
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