Temperature Compensating Device for Fiber Gratings and a Package Therefor

This invention relates to optical fiber gratings used in optical communication systems and, more particularly, to an improved temperature compensating device for such optical fiber gratings.

Optical fiber gratings are formed by exposing a photosensitive fiber such as, for example, boron-doped germanosilicate fiber to ultraviolet light so as to create permanent refractive-index perturbations at selective sections along the core of the fiber. An optical fiber grating is a wavelength-selective reflector having a reflectance response curve with at least one well-defined peak. In other words, an optical fiber grating reflects light of a particular wavelength or a narrow band of wavelengths back along the original propagation direction, while permitting other wavelengths of light to propagate undisturbed. The reflected wavelength of light is often referred to as the grating wavelength.

Optical fiber gratings may be used in Wavelength Division Multiplexing optical systems for high-precision selective wavelength filtering so that signals propagating through an optical fiber can be separated, combined, and/or rerouted. They can also be used as feedback elements for a fiber optic laser or as external laser mirrors. For all such uses, it is essential that the grating wavelength remains constant over an expected temperature range of, for example, from -20.degree. C. to 80.degree. C. (i.e., .DELTA.T=100.degree. C.).

The grating wavelength .lambda..sub.g (or Bragg wavelength) is related to the effective guided mode index, n, of the optical fiber and the spacing of the grating sections, .LAMBDA. (also known as the grating period), in the following way:

This equation shows that the effective guided mode index, n, and the grating period, .LAMBDA., are inversely proportional to each other. Therefore, in order to maintain .lambda..sub.g constant, an increase, for example, in the effective guided mode index, n, requires a proportionate decrease in the grating period, .LAMBDA..

For optical communication systems, it is essential that the grating wavelength remains constant over the expected temperature range. But this requirement is not so easily satisfied since the effective guided mode index of a fiber varies rather significantly over an expected temperature range of, for example, from -20.degree. C. to 80.degree. C. (i.e., .DELTA.T=100.degree. C.), primarily due to the temperature dependence of the fiber's refractive index. It has been reported that over this temperature range, the grating wavelength shift of an uncompensated 1550 nm grating can exceed 1 nm, which can be detrimental to an optical communication system.

Fortunately, it can be readily shown that in order to hold .lambda..sub.g constant over a temperature range, an increase in temperature must be accompanied by a corresponding decrease in strain in the fiber grating and vice versa. Stated in a different way, the change in strain (.DELTA..epsilon.) and change in temperature (.DELTA.T) in a fiber grating are inversely and linearly related to each other, that is:

Accordingly, to compensate for, or counteract, an unwanted shift in grating wavelength, one could vary the grating period, .LAMBDA., through selective adjustment of strain in the fiber. Thus, for example, when the ambient temperature of the fiber grating rises, one may decrease the strain in the grating to maintain the same grating wavelength as was set at the initial temperature condition. Conversely, when the ambient temperature of the fiber grating decreases, the strain in the grating may be increased to maintain the grating wavelength constant.

U.S. patent application Ser. No. 09/023,425, filed Feb. 13, 1998, and assigned to the assignee of the present invention, discloses a compact temperature compensating device for optical fiber gratings which includes a pair of cylindrical expansion members which are coaxial with each other and with the fiber grating. A base member secures a distal end of the first and second expansion members to a distal end of the fiber grating, and a lever flexibly connects a proximal end of the first and second expansion members to a proximal end of the fiber grating. The coefficients of thermal expansion of the expansion members are different, so that a change in temperature causes the first and second expansion members to expand and contract differentially, thereby pivoting the lever to vary the axial strain in the fiber grating, maintaining the grating wavelength of the fiber grating substantially constant throughout a desired temperature range. While effective, it has been found that the disclosed device is not particularly easy to manufacture or install in modular packages. Further, once the prior device is manufactured, it is not easily adjusted. Accordingly, a need exists for a temperature compensating device for optical fiber gratings which is easy to manufacture, allows for multiple, modular devices to be stacked together in a common package, and is adjustable after manufacture.

According to the present invention, there is provided a temperature compensating device for a prestrained fiber grating of an optical fiber. The temperature compensating device extends along a longitudinal axis of the fiber grating and comprises first and second expansion members and first and second levers, wherein the expansion members, the levers and the fiber all lie substantially in a single plane. The first expansion member is formed of a material having a first coefficient of thermal expansion and extends parallel to the longitudinal axis of the fiber grating, with a length between first and second ends at least as long as the fiber grating. The second expansion member is formed of a material having a second coefficient of thermal expansion and extends parallel to the longitudinal axis of the fiber grating, with a length between first and second ends at least as long as the fiber grating. The first and second coefficients of thermal expansion are different from each other. The first lever has first and second ends and an intermediate portion and is flexibly connected to a first end of the first expansion member proximate the first end of the first lever and to a first end of the second expansion member proximate the intermediate portion of the first lever and to a first end of the fiber grating proximate the second end of the first lever. The second lever has first and second ends and an intermediate portion, and is flexibly connected to a second end of the first expansion member proximate the first end of the second lever and to a second end of the second expansion member proximate the intermediate portion of the second lever and to a second end of the fiber grating proximate the second end of the second lever. Accordingly, a change in temperature of the device causes the first and second expansion members to expand and contract differentially, thereby pivoting the first and second levers to vary the axial strain in the fiber grating so as to maintain a grating wavelength of the fiber grating substantially constant throughout a desired temperature range.

In accordance with an aspect of this invention, the first and second levers are formed unitarily with the second expansion member.

In accordance with another aspect of this invention, the first and second expansion members are spaced from each other and the temperature compensating device further comprises an adjustment member extending between the midpoints of the first and second expansion members. The adjustment member is adapted to selectively vary the spacing between the first and second expansion members at their midpoints.

In accordance with a further aspect of this invention, the temperature compensating device further comprises a spring element interposed between the first and second expansion members. The spring element is adapted to provide a yieldable force to bias the first expansion member away from the second expansion member.

In accordance with yet a another aspect of this invention, the second expansion member comprises first and second spaced elements extending parallel to the longitudinal axis of the fiber grating and a plurality of struts joining the first and second elements.

In accordance with yet a further aspect of this invention, a package for holding a plurality of the inventive temperature compensating devices is provided. The package includes a support formed of a material having the second coefficient of thermal expansion. The second expansion members of the plurality of temperature compensating devices are clamped to the support and are separated by spacers having the second coefficient of thermal expansion so that they do not interact.

The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein:

FIG. 1 is a side view of a temperature compensating device according to the present invention;

FIG. 2 is a plan view showing the mounting of four temperature compensating devices of the type shown in FIG. 1 into a modular package; and

FIG. 3 is an elevational view taken along the line 3--3 in FIG. 2.

FIG. 1 illustrates a temperature compensating device, designated generally by the reference numeral 10, which holds an optical fiber grating 12 which is an integral portion of an optical fiber 14. The temperature compensating device 10 is substantially coplanar with the fiber 14 and includes a first expansion member 16, a second expansion member 18, a first lever 20, and a second lever 22. As shown, the first expansion member 16 is a separate piece from the unitary structure including the second expansion member 18, the first lever 20 and the second lever 22, and is secured to the lower ends of the levers 20, 22 by welding at the joints 24, 26, respectively. The first expansion member 16 is formed of a material having a high coefficient of thermal expansion, illustratively stainless steel. The second expansion member 18 and the levers 20, 22 are formed of a material having a low coefficient of thermal expansion, illustratively Kovar.

The first expansion member 16 is a bar which extends parallel to the longitudinal axis of the grating 12 and, illustratively, is at least as long as the fiber grating 12. The length of the bar 16 is chosen for convenience of welding and can be shorter than the fiber grating 12 if the location of the flexures 38, 40 (to be discussed hereinafter) is lowered. The second expansion member 18 is also at least as long as the grating 12, but is preferably formed as a first element 28 spaced from the first expansion member 16 and extending parallel to the longitudinal axis of the fiber grating 12, a second element 30 extending parallel to the longitudinal axis of the grating 12 and spaced from the first element 28, and a plurality of struts 32 joining the elements 28, 30. This construction of the second expansion member 18 provides a rigid structure which insures that stresses induced by changes in temperature translate in the direction parallel to the fiber grating 12, and not orthogonal thereto. The second expansion member 18 is further formed at each of its two ends with a respective through-hole 34, 36 extending orthogonally to the plane of the temperature compensating device 10. These holes 34, 36 are used for mounting the temperature compensating device 10 to a package, as will be described hereinafter.

The levers 20, 22 are formed unitarily with the second expansion member 18 and are joined at intermediate portions thereof to opposite ends of the expansion member 18 by flexures 38, 40, respectively. The lower ends of the levers 20, 22 are joined by flexures 42, 44, respectively, to welding portions 46, 48. The welding portions 46, 48 are welded to opposite ends of the first expansion member 16 at the joints 24, 26, respectively.

At their upper ends, each of the levers 20, 22 is bifurcated so as to have a pair of prongs 50, 52; 54, 56, respectively. The prongs 50, 52; 54, 56 are used for attaching the fiber grating 12 to the temperature compensating device 10 through the quartz blocks 58, 60. It has been shown that superior stability can be achieved by attaching the fiber grating 12 to the temperature compensating device 10 using a glass frit material. This glass frit attaches most readily to quartz. Accordingly, quartz blocks 58, 60 are provided and the ends of the fiber grating 12 are attached to the blocks 58, 60 by glass frit 62, 64, respectively. To hold the blocks 58, 60, each of the prongs 50, 52, 54, 56 is formed with a respective channel 66, 68, 70, 72 at its distal end, with the channels 66, 68 facing each other and the channels 70, 72 facing each other. The prongs with their channels are used to clamp the quartz blocks 58, 60 to the temperature compensating device 10, since it is known that attaching the blocks to the temperature compensating device by epoxy undesirably results in unwanted wavelength shifts. Preferably, the inner clamping surfaces of the channels 66, 68, 70, 72 are gold plated to provide compliance at rough spots to avoid damaging the quartz. Each of the quartz blocks 58, 60 is substantially trapezoidal in shape and the channels 66, 68, 70, 72 are formed so that there are four line contacts, two on angled surfaces, for each quartz block to provide full constraint against motion of the quartz block. Before installing the quartz blocks 58, 60, the prongs 50, 54 are slightly bent toward the prongs 52, 56, respectively. Then, the prongs 50, 54 are pulled away from the prongs 52, 56, respectively, to provide room for the blocks 58, 60 to be inserted into the channels pairs 66, 68; 70, 72. The prongs 50, 54 are then released and they spring back to firmly hold the quartz blocks 58, 60.

The aforedescribed construction possesses a number of advantages. Initially, it is noted that the temperature compensating device 10 is generally planar, which allows for easier manufacturing and also allows for multiple, modular temperature compensating devices to be stacked together in a common package, as will be described. The disclosed construction is also double-ended. This is advantageous since, by allowing both ends to rotate, twice as much compensating motion can be transmitted to the fiber grating 12 as from an identical single-ended design. Since the temperature compensating coefficient of the temperature compensating device 10 is very sensitive to the distance between the middle and lower flexures 38, 42; 40, 44, making all the flexures unitarily with the second expansion member 18 and the levers 20, 22 provides for a precise control of this distance. In contrast, if the lower flexures 42, 44 were part of the first expansion member 16, the relative locations of the flexures 42, 44 are limited by the accuracy of the welding process, which is approximately a twenty five fold degradation over the accuracy of the process for machining the flexures from the unitary construction of the second expansion member 18 and the levers 20, 22.

According to the present invention, the first expansion member 16 has a flexure 74 at its midpoint, and an adjusting screw 76 attaches this midpoint of the first expansion member 16 to the midpoint of the second expansion member 18. Because of the left/right symmetry of the temperature compensating device 10, there is no relative horizontal motion between the first and second expansion members 16, 18 at their midpoints. The use of the adjustment screw 76 to join the expansion members 16, 18 has two purposes. Firstly, attachment of the relatively thin first expansion member 16 to the more robust second expansion member 18 limits deformation of the first expansion member 16 during shock, and increases its natural frequency during vibration. Secondly, the screw 76 can intentionally and controllably deflect the first expansion member 16 toward the second expansion member 18, increasing the tension on the fiber grating 12, thereby producing a tuning capability. Because of the high mechanical advantage of the tuning mechanism (typically at least 50:1 relative to screw motion), precise plus permanent stable tuning is possible, assuming that the screw 76 is locked into position after tuning is complete. Alternatively, the fiber grating 12 can be tuned during operation by permanently attaching a suitable actuator (e.g., DC servomotor) to the screw 76. Motion of the first expansion member 16 toward the second expansion member 18 is opposed by the spring members 78, 80, which are formed unitarily with the second expansion member 18. The spring members 78, 80 preload the first expansion member 16 against the head of the screw 76, preventing motion toward the second expansion member 18 under shock conditions.

FIGS. 2 and 3 show a modular package holding four temperature compensating devices 10 of the type shown in FIG. 1. The temperature compensating devices 10 are arranged in two rows, with each row having two devices arranged with their fiber gratings 12 adjacent to each other. The package includes a support 82 formed of a material having the same coefficient of thermal expansion as the second expansion member 18. Illustratively, this support 82 is made of Kovar. To hold the devices 10 to the support 82, four clamping members 84 are provided, each having the same coefficient of thermal expansion as the support 82 and the second expansion member 18. The clamping members 84 extend through the holes 34, 36 of the devices 10 and through appropriately positioned openings 86 in a planar portion 88 of the support 82. Between each stacked pair of devices 10, there is provided a spacer element 90 and between each of the lower devices 10 and the planar portion 88 is a spacer element 92. The spacer elements 90, 92 are likewise formed of a material having the same coefficient of thermal expansion as the support 82 and the second expansion member 18. The clamping members 84 are held in place by washers 94 and C-rings 96. The support 82 is formed of sheet stock material and is cut and bent so as to have walls surrounding the temperature compensating devices 10. Although not shown in the drawings, a lid can be welded to the walls after assembly of the modular package. The pigtail optical fibers 14 pass through oversized openings in bend limiting devices 98 disposed in opposed end walls of the support 82. This allows the fibers 14 to slide in and out of the package as the temperature compensating devices 10 vary the lengths of the gratings 12 during temperature changes. The bend limiting devices 98 hold the fibers 14 so that when viewed in a plane orthogonal to the longitudinal axis of the gratings 12, the gratings 12 are each at a respective corner of a rectangle. The aforedescribed package is attached to a substrate 100 by standoff legs 102 formed as part of the support 82. These legs 102 provide compliance so that thermally induced motion in the substrate 100 is isolated from the temperature compensating devices 10, while being stiff enough to avoid resonance during vibration testing, and plastic deformation or substantial force/displacement magnification during shock testing.

The disclosed device can also be used in applications where it is desired to change the fiber grating wavelength as a function of temperature, either monotonically increasing or monotonically decreasing. For such an application, the lever action would be changed by a careful selection of the distance between the middle and lower flexures 38, 42; 40, 44 to tailor the strain-temperature relationship.

Accordingly, there has been disclosed an improved temperature compensating device for optical fiber gratings, along with a package therefor. While an illustrative embodiment of the present invention has been disclosed herein, it is understood that various adaptations and modifications to the disclosed embodiment are possible and it is intended that this invention be limited only by the scope of the appended claims. Thus, for example, the disclosed and claimed temperature compensating device is not limited to use with fiber Bragg gratings, but is usable with other types of fiber devices as well.