WO2002073268A2

WO2002073268A2 - Thermal compensator for optical communications circuits and the like

Info

Publication number: WO2002073268A2
Application number: PCT/GB2002/000945
Authority: WO
Inventors: Stuart Cameron Holdsworth; Neil Duncan Holdworth
Original assignee: Stuart Cameron Holdsworth; Neil Duncan Holdworth
Priority date: 2001-03-09
Filing date: 2002-03-04
Publication date: 2002-09-19
Also published as: GB0105774D0; GB0204945D0; GB2373870A; WO2002073268A3; AU2002236053A1

Abstract

There is described a thermal compensator for a grading embedded in an optical fibre (6). The thermal compensator comprises first and second members fixedly connected to the optical fibre (6) at first and second fixing points either side of the grating. The first and second members have respective first and second bodies (9, 10) disposed parallel to each other and to the optical fibre (6), the first and second members being formed with juxtaposed surfaces disposed transversely to the longitudinal axis of the fibre (6). An expansion body (11) is captivated between the juxtaposed surfaces such that expansion of the expansion body (11) creates a force that urges the first and second fixing points towards each other, and contraction of the expansion body (11) creates a force that urges the first and second fixing points away from each other.

Description

Thermal Compensator for Optical Communications Circuits and the like

This invention relates to a thermal compensator for an embedded grating optical wavelength light filtering device, for use in optical communication networks, components and the like. Such a device is commonly known as a "Bragg Grating" and performs a significant function in optical communications and sensing. However Bragg gratings are sensitive to temperature induced changes in wavelength response which adversely affect their performance.

The invention comprises a device for the mounting of a fibre Bragg grating so as to compensate for the inherent change in wavelength with respect to temperature which is characteristic of free gratings.

Fibre Bragg gratings have a multiplicity of uses, including applications in telecommunications, as optical filters, as frequency stabilisers for optical fibre lasers and optical amplifiers. In addition they are used as sensors for strain measurement and as temperature sensors. Further uses include chromatic dispersion compensation and optical gain control.

A modern fibre optic communications network relies on its ability simultaneously to transmit a large number of light signals of differing wavelengths through a single optic fibre. The discrete wavelength supporting any communication channel must maintain frequency within a narrow band and the Bragg grating is particularly suited to the manipulation of such signals.

All optical communication systems need to selectively filter wavelengths in order to successfully abstract or add information into the network, without interference between the various wavelength divisions in-order to maximize the available communication channels within the system. This requires stable wavelengths, within a narrow margin, to be maintained throughout a range of temperatures. Such systems are known as Wavelength Division Multiplexing (WDM) or, where the number of channels is increased, as Dense Wavelength Division Multiplexing (DWDM). Increasing the number of channels reduces the wavelength separation and increases the risk of co-channel interference due to thermal drift.

The device utilized to set the discrete wavelengths within the systems and components is often known as a Fibre Bragg Grating or FBG. Such devices are commonly formed by exposing photosensitive fibres through a regular mask, creating a grating by a permanent change in the refractive index of the fibre core.

Variations in temperature have three discrete affects on such an FBG component. The wavelength of the fibre is shifted by an amount that is a function of the coefficient of linear expansion of the fibre material, and by two other characteristics, the thermo-optic effect and the photo-elastic constant. Additionally any changes in fibre strain will affect the period of the refractive index grating and hence the wavelength response.

Neglecting second order effects these characteristics can be expressed using the following relationship:

Δλ/λ=(α_fibre+ζ).ΔT+(1-p_e).Δε (I) where Δλ/λ = ratio of the change in wavelength and the original wavelength; coefficient of linear expansion of the fibre; ζ= the thermo-optic co-efficient;

ΔT= change in temperature in degrees Celsius; p_e= photoelastic constant; Δε=change in strain.

Substituting the appropriate values yields a sensitivity for an unrestrained grating typically in the range 0.01-0.12nm per °C at room temperature. This is an unacceptable response for many applications.

It is therefore important to devise a method for reducing the temperature induced drift to a level that is compatible with the intended application. In particular in the field of optical telecommunications the wavelength of the grating must remain stable over an operating range which can extend from -40° to +80°C.

This need has hitherto been addressed by the use of active devices incorporating Thermo Electric Coolers (TECs) that continually monitor and adjust the package temperature to compensate for thermal drift, and by a variety of passive devices.

Passive devices typically comprise a material or combination of materials that by mechanical interaction adjust a pre-tension applied to the fibre to compensate for the thermal effects.

Two clearly distinct types of passive device are known. A first type utilises a substrate material with negative thermal expansion properties to which the pre- tensioned fibre is affixed. A rise in temperature causes the substrate to contract thereby reducing the strain in the fibre and maintaining the desired wavelength.

However, materials with the required negative thermal expansion coefficient are few and are generally difficult to machine without affecting the thermal properties. Consistency both in material and response is difficult to achieve for production volumes.

A second approach utilises a material (or combination of materials) of varying thermal expansion coefficient, which interact either by a differential linear response or by a change in bending of an affected bimaterial member to vary the applied pre-tension in the fibre in such a manner as to maintain the desired wavelength response of the grating.

The general principle requires two materials with substantially differing coefficients of thermal expansion. Typically a material of low thermal expansion (eg various superalloys, silica etc) would be combined with a material of higher linear expansion (such as stainless steel, aluminium etc) to produce an overall differential expansion which stabilises the frequency response of the grating. One such device is disclosed in US 6, 01 ,301.

The known approaches described above are not entirely satisfactory. Apart from the limitations and disadvantages already referred to, these approaches may not give the desired degree of compensation for thermal effects, may be expensive to implement and/or may be bulky and lacking in compactness.

There has now been devised a thermal compensator for use in applications such as those described above, which overcomes or substantially mitigates the above- mentioned or other disadvantages of the prior art.

According to the invention, there is provided a thermal compensator for a grating embedded in an optical fibre, said thermal compensator comprising first and second members fixedly connected to the optical fibre respectively at first and second fixing points either side of the grating, the first and second members having respective first and second bodies disposed parallel to each other and to the optical fibre, the first and second members being formed with juxtaposed surfaces disposed transversely to the longitudinal axis of the optical fibre between which an expansion body is captivated such that expansion of the expansion body creates a force that urges said first and second fixing points towards each other, and contraction of the expansion body creates a force that urges said first and second fixing points away from each other.

This device according to the invention is advantageous primarily in that it compensates for the wavelength shift caused by changes in temperature within the fibre grating. The device may be of an overall length only slightly greater than the fibre grating requiring stabilising. The device consists of three primary components (viz the first and second bodies and the expansion body), the geometry and materials of which are so chosen that the actions of one material when subjected to a change in temperature will counteract the effect of the other material's change in temperature and compensate for this change. The amount of compensation is so proportioned as to be directly comparable to the amount of adjustment in movement required to ensure an almost zero change in wavelength in the fibre grating, typically no more than ±0.5nm around a centrally proscribed wavelength, when subjected to the same amount of change in temperature.

The device is compact and easy to incorporate into an external package, which may further be hermetically sealed. The stresses to which the device is subjected are low which results in a minimal maintenance requirement and long life. Sealed in a hermetic package and utilised with a telecommunications network the device may be so constructed that it will perform the desired temperature correction function for the lifetime of the attached optical grating or other attached device. In this regard the device may be so fashioned as to meet the stringent testing requirements of the telecommunications industry.

In a first embodiment of the thermal compensator according to the invention, the first and second members are manufactured of the same material, or of materials with similar thermal expansion properties, while the expansion body is of a material with a higher thermal expansion coefficient.

In an alternative approach, all of the principal components may be manufactured in the same or similar materials, but the thermal expansivity of the first and second members along the axis of relative movement of those components may be reduced by folding or otherwise forming them into an appropriate geometry.

The expansion body preferably acts upon the ends of the first and second members opposite to those at which the first and second members are attached to the fibre.

The effect of the thermal compensator according to the invention is preferably such as to maintain the optical properties of the grating substantially constant. This may involve counteracting the thermal expansion of the fibre grating exactly, so that the overall length of the grating remains constant, or may involve overcompensation for thermal expansion of the grating, in cases in which terms in formula (I) other than the coefficient of linear expansion of the fibre are significant.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which

Figure 1 is a perspective view of a package, showing a typical optical fibre installation incorporating a thermal compensator according to the invention;

Figure 2 is a perspective view of the thermal compensator, with the housing of the package removed for clarity;

Figure 3 is an exploded view of the thermal compensator; and

Figure 4 is a schematic illustration of geometry that may be utilised in order to achieve the desired thermal expansion characteristics using a single material.

Referring to the Figures, a Fibre Bragg Grating (12) is mounted within a package enclosed in a housing (1) which may be used in a telecommunications application. The Fibre Bragg Grating (12) is written into an optical fibre (6) and is susceptible to changes in temperature, which if left uncontrolled would typically cause a change in wavelength of the order of 1.4nm for 110°C temperature rise if the grating wavelength was 1550nm. This change in frequency may be evaluated by reference solely to the first part of the formula (I) when the term (1-p_e).Δε is zero.

This change in wavelength is undesirable and would not achieve the required accuracy for telecommunications requirements, typically ± 0.05nm around a centrally proscribed wavelength.

The fibre (6) enters the housing (1) via spouts (7) formed at each end thereof. At these points a hermetic seal may be achieved, boots (3) of plastics material being placed over the spouts (7) in order to give strain relief to the fibre (6) and prevent unwanted damage. Between the housing (1) and the mount for the Bragg grating (12) a further means of fibre thermal compensation is present in order to ensure that the fibre (6) is not strained and does not interfere with operation of the compensating device. This further means of thermal compensation is in the form of slack portions (5) of the fibre (6). The whole assembly is enclosed by a lid (4), which may sealed if required.

The housing (1) is provided with fixing flanges (8) - only one of which is visible in Figure 1 - by which the housing (1) can be fixed to an external support.

The thermal compensator, generally designated (2) and mounted within the housing (1), is shown in greater detail in Figures 2 and 3. The thermal compensator consists of a first member (9) holding one end of the Bragg Grating (12) by means of a clamp or similar device (13a) applied to a pillar (9a) extending perpendicularly at one end of the first member (9) and fixed by bolts (14a). A second member (10), made from the same material as the first member (9), is similarly attached to the other end of the Bragg Grating (12) by a clamp or the like (13b) applied to a pillar (10a) and secured by bolts (14b).

A cylindrical central body (11) is enclosed between the first member (9) and the second member (10). The central body (11) is made from a material having a higher thermal expansion coefficient than the material of the first member (9) and the second member (10).

The first member (9) and the second member (10) are attached to opposite ends of the central body (11) by means of cap-screws (17) or similar. One cap-screw (17) passes through an opening (21) in a fixing lug formed at the end of the first member (9) remote from the pillar (9a) and engages in a threaded bore (22) in the end of the central body (11). With the second member (10) then positioned about the central body (11), a second cap-screw (17) is engaged with a threaded bore in the other end of the central body (11), the second cap-screw (17) being passed through an opening (15) adjacent the pillar (9a) and an opening (23) in the end of the second member (10) remote from the pillar (10a). The first member (9) is fixed at one of its ends to the floor of the housing (1) by a bolt or screw passing through the hole marked (19). However, the other end of the first member (9) is free and so the first member (9) is free to expand lengthways as the temperature varies. The central body (11) will similarly expand with the first member (9), as it is attached at one end to the first member (9) and to the other end to the second member (10). This will in turn move the second member (10) relative to the first member (9). Additionally, changes in length of the central body (11), which is of higher thermal expansivity, will add to this movement.

As the temperature induced movement of the central body (11) is greater than that of the first member (9) and second member (10), by virtue of the choice of material, the distance between the points (13a, 13b) at which the Fibre Grating (12) is attached to the first member (9) and the second member (10) can be made to vary to suit the properties required of the Fibre Grating (12).

Adjustment of the frequency over a small range may be made by means of a taper screw set (16) mounted on the end of the first member (9) remote from the end at which the first member (9) is fixed to the housing (1). This acts against the first member (9) and forces a small change in angle between the base of the first member (9) and the pillar (9a) to which the fibre (6) is attached by means of the clamp (13a). Alternatively an offset screw may be used to put a pillar with the attached fibre into bending and thus make small adjustments possible.

The lengths of the second member (10), the first member (9) and the central body (11) may be calculated from the properties of the fibre.

In the embodiment described above, the differential thermal expansion characteristics of the first member and second member are achieved by the use of materials having different thermal expansion coefficients. An alternative approach is illustrated in Figure 4. This shows simply that the linear expansion of components corresponding to the second member and the first member may be formed of the same material as the central body, but the linear expansion may be reduced by forming the material into a non-linear form. In the illustrated arrangement, the material is formed into a "W" shape, the linear expansion then being a function of the sine of the included angle α.

Claims

1. According to the invention, there is provided a thermal compensator for a grating embedded in an optical fibre, said thermal compensator comprising first and second members fixedly connected to the optical fibre respectively at first and second fixing points either side of the grating, the first and second members having respective first and second bodies disposed parallel to each other and to the optical fibre, the first and second members being formed with juxtaposed surfaces disposed transversely to the longitudinal axis of the optical fibre between which an expansion body is captivated such that expansion of the expansion body creates a force that urges said first and second fixing points towards each other, and contraction of the expansion body creates a force that urges said first and second fixing points away from each other.

2. A thermal compensator according to any preceding claim, wherein the expansion body is of elongated shape.

3. A thermal compensator according to Claim 2, wherein the expansion body is of cylindrical shape.

4. A thermal compensator according to any preceding claim, wherein the juxtaposed surfaces of the first and second members are at the opposite ends of the respective members to those ends at which the respective members are fixed to the fibre.

5. A thermal compensator according to any preceding claim, wherein the first and second members respectively comprise first and second pillars at which the first and second members fixedly connect to the optical fibre.

6. A thermal compensator according to Claim 5, wherein the first and second pillars extend perpendicularly to the respective first and second bodies.

7. A thermal compensator according to any one of Claims 5 and 6, wherein the first and second pillars are fixedly connected to the optical fibre by means of a clamp.

8. A thermal compensator according to any preceding claim, wherein the first and second bodies are formed with lateral extensions, which extensions have the juxtaposed surfaces between which the expansion body is captivated.

9. A thermal compensator according to Claim 8, wherein the lateral extensions are located at the ends of the first and second bodies that are remote from the first and second fixing points.

10. A thermal compensator according to any preceding claim, wherein the expansion body is fixed to the juxtaposed surfaces.

11. A thermal compensator according to Claim 10, wherein the expansion body is fixed to the juxtaposed surfaces by means of screws.

12. A thermal compensator according to any preceding claim, wherein the thermal compensator is enclosed within a housing.

13. A thermal compensator according to Claim 12, wherein one of the first and second members is fixed at one end to the housing.

14. A thermal compensator according to any preceding claim, wherein the expansion body is of a material with a higher thermal expansion coefficient than the first and second members.

15. A thermal compensator according to Claim 14, wherein the first and second members are of materials of similar thermal expansion properties.

16. A thermal compensator according to Claim 14, wherein the first and second members are of the same material.

17. A thermal compensator according to any preceding claim, wherein the expansion body and the first and second members are manufactured in materials with similar thermal expansion properties, but the thermal expansivity of the first and second members along the longitudinal axis of the optical fibre is reduced by folding or otherwise forming them into an appropriate geometry.