WO2002037156A1

WO2002037156A1 - Use of highly oriented lcp's for low cost negative thermal expansion polymeric materials for photonics applications

Info

Publication number: WO2002037156A1
Application number: PCT/US2001/042645
Authority: WO
Inventors: Everett W. Coonan; Russell A. Crook
Original assignee: Corning Incorporated
Priority date: 2000-11-01
Filing date: 2001-10-09
Publication date: 2002-05-10
Also published as: AU2002211902A1

Abstract

The present invention provides a packaged temperature compensating fiber Bragg grating (1), which includes a fiber (1) having a fiber Bragg grating with a grating period (Μ) and a temperature compensating package (9). The temperature compensating package (9) includes at least one liquid crystal polymer material having a negative coefficient of thermal expansion. The temperature compensating package keeps the grating period (Μ) of the fiber Bragg grating (1) substantially constant when the fiber Bragg grating (1) is exposed to varying temperatures. The present invention also provides a method of manufacturing a packaged temperature compensating fiber Bragg grating (1). The method includes the steps of providing a temperature compensating package (9), the package including at least one liquid crystal polymer having a negative coefficient of thermal expansion and attaching a fiber (1) having a fiber Bragg grating to the temperature compensating package (9).

Description

USE OF HIGHLY ORIENTED LCP'S FOR LOW COST NEGATIVE THERMAL EXPANSION POLYMERIC MATERIALS FOR PHOTONICS

APPLICATIONS

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention is directed generally to devices for photonic communications, and more particularly to fiber Bragg gratings.

2. Description of the Related Art

Fiber Bragg gratings are fabricated by producing periodic, permanent perturbations of the index of refraction within a fiber. The grating reflects light of a particular wavelength or particular band of wavelengths. The non-reflected wavelengths are transmitted through the grating and continue through the optical fiber. This property makes fiber Bragg gratings useful as filters in optical communications.

Whether a wavelength is reflected or transmitted is determined as a function of the spacing between the perturbations, i.e. the grating period Λ. Even very small changes in the grating period can result in a significant shift in the reflected and transmitted wavelengths. In practice, it is very difficult to control the grating period. Strains in the fiber cause variations in the grating period and hence, shifts in the reflected and transmitted light.

Strains in the fiber may be introduced mechanically during fabrication of the fiber Bragg grating or be the result of thermal fluctuations during operation. Typically, after the perturbations in the index of refraction are written into the optical fiber, the fiber is covered with a cladding layer to improve the transmission characteristics of the fiber. Additional protective layers may be added as well as final packaging. Strains may be introduced during any of these processing steps.

In addition to the strain introduced during fabrication, strains due to thermal fluctuations during operation are of great concern. The grating period is very sensitive to thermal strains and even small fluctuations in temperature can affect the grating period. In fact, this sensitivity to temperature has been exploited with the production of extremely sensitive temperature sensors and gauges.

There have been many efforts to keep the grating period as constant as possible. Prior art methods include (1) using cladding layers with a lower coefficient of thermal expansion (CTE) than the fiber, (2) mounting the fiber on substrate with a lower coefficient of thermal expansion than the fiber, (3) using multiple cladding layers with different coefficients of thermal expansion, (4) mounting the fiber on a ceramic substrate with a negative coefficient of thermal expansion, and (5) attaching thermoelectric coolers/heaters to the fiber package.

All of these methods provide some degree of control of the grating period. All of them, however, have drawbacks. Cladding or mounting materials having a lower, but positive, coefficient of thermal expansion reduce the rate of variation of the grating period but do not keep the grating period constant. Multiple cladding layers further reduce the rate of variation relative to single cladding layers. However, the grating period is not constant under typical working conditions. Mounting on a ceramic substrate with a negative coefficient of thermal expansion shows great promise, but the ceramics are exotic and costly. Furthermore, the package design suffers because of its inability to protect the fiber attachment joint between the negative expansion substrate and the fiber during damp heat exposure as well as thermal and mechanical shock treatment. Ceramic materials such as beta- eucryptite have problems with dimensional stability and varying coefficients of thermal expansion in moist environments because of their tendency to absorb water. Attaching a thermoelectric cooler/heater provides significant control of the fiber grating. However, thermoelectric devices tend to be bulky and require attention. They are not suitable for large communications arrays where it would be advantageous to have devices which could be left unattended for long periods of time.

The prior art methods for controlling the grating period of a fiber

Bragg grating suffer various problems as discussed above. Therefore, it would be desirable to have a simple, effective, inexpensive method for controlling the grating period of a fiber Bragg grating.

SUMMARY OF THE INVENTION

The present invention provides a packaged temperature compensating fiber Bragg grating comprising a fiber having a fiber Bragg grating with a grating period Λ, and a temperature compensating package including at least one liquid crystal polymer material having a negative coefficient of thermal expansion, wherein the temperature compensating package keeps the grating period of the fiber Bragg grating substantially constant when the fiber Bragg grating is exposed to varying temperatures. The present invention also provides a method of manufacturing a packaged temperature compensating fiber Bragg grating comprising the steps of providing a temperature compensating package, the package including at least one liquid crystal polymer having a negative coefficient of thermal expansion and attaching a fiber having a fiber Bragg grating to the temperature compensating package.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features, aspects and advantages of the present invention will become apparent from the following description, appended claims and the exemplary embodiments shown in the drawings, which are briefly described below.

Figure 1 is a cross section of a fiber Bragg grating according to a first embodiment of the invention. Figure 2 is a cross section of a first embodiment of the invention illustrating the definition of the grating period Λ.

Figure 3 is a plot of the thermal expansion characteristics of Vectra A950™ as a function of temperature. Figure 4 is a plot of the thermal expansion characteristics of Vectra

A950™ as a function of draw ratio.

Figure 5 is a cross section of a fiber Bragg grating according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a cross section of a fiber Bragg grating 1 according to a first embodiment of the invention. The fiber Bragg grating 1 has a core 3 comprising a material suitable for optical communications. Example materials include, but are not limited to silica, germanium doped silica, silicon oxynitride and germanium doped silicon oxynitride. Any other material suitable for optical communications may be used. Preferably, the core 3 is surrounded by a cladding layer 5 which has an index of refraction lower than the index of refraction of the core.

Within the core 3, is a region 7 which contains a regular array of periodic, permanent perturbations 13 of the index of refraction. The array of perturbations 13 form an optical grating having a grating period Λ. Figure 2 illustrates the grating period Λ. Methods of making the array of perturbations 13 are well known in the art.

Surrounding the optical core 3 and the cladding layer 5 is a temperature compensating package 9. The temperature compensating package is preferably made of a single material having a negative coefficient of thermal expansion. It is preferable that the package material comprise a liquid crystal polymer (LCP). Example liquid crystal materials include Vectra™, Kevlar™ and Xydar™. Other liquid crystal polymers, such as thermosetting LCP's, may be used. The above examples are for illustration purposes only and should not be considered limiting. Because the materials comprising the optical core 3 and the cladding layer 5 typically have positive coefficients of thermal expansion, they expand when heated and contract when cooled. In contrast, materials with a negative coefficient of thermal expansion contract when heated and expand when cooled. By wrapping the core 3 and the cladding layer 5 in a package 9 with a negative coefficient of thermal expansion, a packaged fiber Bragg grating is produced in which the induced thermal stresses in the package 9 balance the induced thermal stresses in the fiber such that the grating period does not significantly change. With a judicious selection of core, cladding and packaging material, a temperature compensating fiber Bragg grating can be fabricated in which the grating period remains essentially constant over a wide range of operational temperatures.

The temperature compensating fiber Bragg grating can be readily manufactured. Preferably, the liquid crystal starting material is extruded and/or drawn to form a tube. Then the fiber Bragg grating 7 is centered in the tube. The ends of the tube are then thermally formed to seal the fiber in the polymer. To increase bonding between the fiber and the tube, it is preferable to put an adhesive on the fiber prior to inserting the fiber into the tube. If an adhesive is used, it is preferable to mill a narrow groove into the tube to offset swelling from the adhesive. The completed temperature compensating fiber Bragg grating can then be further packaged in a hermetic package for increased protection from the environment. Additionally, it is advantageous to thermally anneal the fiber to remove residual stresses in the fiber.

Figure 3 illustrates the thermal expansion characteristics of A950 Vectra™ over a temperature range of -40 to 120 °C. Vectra™ has been found to be a particularly advantageous material for use in the temperature compensating package 9. Vectra™ is a material that possesses low water vapor permeability, good environmental durability and a coefficient of thermal expansion which can be easily controlled. This combination of properties results in a package which not only has superior optical performance over the prior art, but also has superior durability and resistance to moisture. Liquid crystal polymers are highly anisotropic and may have coefficients of thermal expansion which vary from highly positive to highly negative. Additionally, the coefficient of thermal expansion of components manufactured from a liquid crystal polymer is strongly dependent on the method of manufacture. In fact, depending on the processing, a particular component may have a highly positive coefficient of thermal expansion in one direction and a highly negative coefficient of thermal expansion in another direction.

In accordance with the present invention, it is desirable to process the liquid crystal polymer of the temperature compensating package so as to have a 0 negative coefficient of thermal expansion. Methods of producing a desired negative coefficient of thermal expansion by extrusion and drawing are well known in the art.

Table 1 summarizes the results of a first series of experiments to optimize the coefficient of thermal expansion of A950 Vectra™ by extruding and drawing. The samples were extruded through a circular die and then drawn to produce rods of different diameter.

Table 1

As can be seen in Table 1, the coefficient of thermal expansion varies with the amount of drawing. Additionally, the coefficient of thermal expansion tends to be more variable as the extruded material is drawn.

Figure 4 illustrates the results of a second series of experiments to determine the effect of drawing on the thermal expansion of A950 Vectra™. The data points are an average of three samples. As can be seen in the figure, the coefficient of thermal expansion becomes more negative as the draw ratio increases. Additionally, the rate of change in the coefficient of thermal expansion decreases as the draw ratio increases.

Figure 5 illustrates a second embodiment of the invention. This embodiment is similar to the first embodiment in many respects. However, the second embodiment includes glass frits 11 on the cladding layer 5. The frits 11 are added to improve adhesion by forming a secure mechanical bond of the package 9 to the fiber. In general, liquid crystal polymers do not adhere well to typical fiber materials. If no cladding layer 5 is used, the frits 11 may be formed directly on the core 3 Of course, adding frits is not the only way to increase adhesion.

Solder balls or any other material which can be molded into bumps may be added to the fiber. In addition, chemical adhesion promoters may be coated on the fiber to improve the chemical bonds between the fiber and the package. One particularly advantageous adhesion promoter is Chipshield 2400™ , a UV curable adhesive. The use of a liquid crystal polymer with a negative coefficient of thermal expansion as packaging to form a temperature compensating fiber Bragg grating has many advantages over the prior art. For example, the device is sealed from the environment and protected from damage during handling and service life. The device is easy to manufacture and inexpensive. Additionally, because there are no brittle components (e.g. ceramics), there is no need for the shock absorbing materials frequently used in conventional fiber Bragg gratings. Moreover, fiber /LCP adhesion and durability can be easily enhanced by chemical adhesion promoters and/or mechanical interlocks.

The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The drawings and description were chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims

WE CLAIM:

1. A packaged temperature compensating fiber Bragg grating comprising: a fiber having a fiber Bragg grating with a grating period Λ; and a temperature compensating package including at least one liquid crystal polymer material having a negative coefficient of thermal expansion; wherein said temperature compensating package keeps the grating period of said fiber Bragg grating substantially constant when said fiber Bragg grating is exposed to varying temperatures.

2. A packaged temperature compensating fiber Bragg grating according to claim 1, wherein the grating period is maintained substantially constant over a temperature range from -40 °C to 100 °C.

3. A packaged temperature compensating fiber Bragg grating according to claim 1, wherein the grating period is maintained substantially constant over a temperature range from 0 °C to 100 °C.

4. A packaged temperature compensating fiber Bragg grating according to claim 1, wherein the grating period is maintained substantially constant over a temperature range from 10 °C to 80 °C.

5. A packaged temperature compensating fiber Bragg grating according to claim 1, wherein said temperature compensating package provides a self contained protective jacket for the fiber Bragg grating.

6. A packaged temperature compensating fiber Bragg grating according to claim 1 , wherein said fiber Bragg grating further comprises at least one frit on the fiber for mechanically bonding said fiber to said temperature compensating package.

7. A packaged temperature compensating fiber Bragg grating according to claim 1 , wherein said fiber Bragg grating further comprises at least one solder ball on the fiber for mechanically bonding said fiber to said temperature compensating package.

8. A packaged temperature compensating fiber Bragg grating according to claim 1, wherein said fiber Bragg grating further comprises at least one chemical adhesion promoter on the fiber for bonding said fiber to said temperature compensating package.

9. A packaged temperature compensating fiber Bragg grating according to claim 9, wherein said temperature compensating package further comprises a groove.

10. A packaged temperature compensating fiber Bragg grating according to claim 1, further comprising a hermetic package.

11. A method of manufacturing a packaged temperature compensating fiber Bragg grating comprising the steps of: providing a temperature compensating package, the package including at least one liquid crystal polymer having a negative coefficient of thermal expansion; and attaching a fiber having a fiber Bragg grating to the temperature compensating package.

12. The method of claim 11, wherein the step of providing a temperature compensating package includes forming a temperature compensating polymer by a process selected from the group consisting of extruding, transfer molding, compression molding, blow molding and injection molding.

13. The method of claim 11, further comprising the step of forming at least one frit on the fiber Bragg grating for mechanically bonding said fiber to said temperature compensating package.

14. The method of claim 11, further comprising the step of forming at least one solder ball on the fiber Bragg grating for mechanically bonding said fiber to said temperature compensating package.

15. The method of claim 11, further comprising the step of adding at least one chemical adhesion promoter to the fiber for bonding said fiber to said temperature compensating package.

16. The method of claim 15, further comprising the step of forming a groove in said temperature compensating package.

17. The method of claim 11, further comprising the step of sealing the fiber Bragg grating in the temperature compensating package.

18. The method of claim 17, wherein the step of sealing comprises thermal forming the extruded liquid crystal polymer material around the fiber Bragg grating.

19. The method of claim 11, further comprising the step of annealing the fiber to remove residual stresses.

20. The method of claim 11, further comprising the step of sealing said temperature compensating package in a hermetic package.