PROCESS FOR INDUCING OR ENHANCING SHEAR STRESS-OPTICAL PROPERTIES AND REDUCTION OR ELIMINATION OF STRESS BIREFRINGENCE IN OPTICALLY TRANSMISSIVE MATERIALS Field of the Invention This invention relates to a process for inducing or enhancing shear stress-optical properties and reduction or elimination of stress birefringence in optically transmissive material and to devices comprising or incorporating material that has been subjected to such process. More particularly but not exclusively, the present invention relates to a process for producing an optically transmissive material having shear stress-optical properties for use in an optical waveguide such as an optical fiber.

Background of the Invention Most laser sources, a diode laser for example, used in optical communications are inherently polarized. A polarized laser light traveling in the core (10) of a standard optical fiber has no specific preference in the alignment of its polarization axis because the core (10) is perfectly axi-symmetric and hence zero birefringence. Any external perturbation to the fiber or the fiber cable will therefore randomly rotate this axis of polarization in the laser light along the length of the fiber. It results in an unpredictable and random alignment of the polarization axis of the laser light along the length of and at the exit end of the optical fiber.

Random rotation of the laser light along the optical fiber can cause many undesired effects in an optical communication system. For example, in an erbium doped optical fiber amplifier, if the optical pump laser light is not polarized in the same direction as the signal laser light along an erbium doped fiber, then the erbium doped fiber amplifier suffers from polarization dependent gain. Random polarization at the fiber exit will cause variation in coupled power if the device to be coupled to, is polarization dependent, such as a typical planar optical wave-guide. In these situations, a polarization scrambler is desired.

Polarization scramblers can be roughly divided into two categories: (1) active and (2) passive. Active polarization scrambler requires electrical power to operate whereas passive polarization scrambler does not.

The principle of active polarization scrambler is based on active rotation of the polarization axis of the laser light. This is typically accomplished by the utilization of electro-optic or magneto-optic (Faraday) effect. One can find hundreds, if not thousands, of published papers in the area of active polarization scrambling. Typically electro-optic or magneto-optic materials in the form of planar wave guide or micro-optic elements or optical fibers are placed in the light path. They are then driven by either an electrical or

magnetic signal to ensure that the total range of polarization rotation in the light passing through these waveguides or micro-optic elements or fiber meets or exceeds 360 degree.

As a result, a polarized laser beam becomes randomly polarized or scrambled (i. e. its polarization axis is time averaged in all directions). Active polarization scrambler works for both broadband and narrow band laser sources. Unless it is an all-fiber based device, it is generally, bulky because it then usually deploys micro-optic device technology. This also means higher insertion loss due to fiber pig-tailing. More importantly, the need of electrical power to function makes purely passive polarization scrambler attractive.

Passive polarization scramblers are generally made of optical fibers although some may appear in micro-optic form. Their polarization scrambling efficiency depends heavily on the spectral width and/or coherent length of the laser light. Micro-optic passive polarization scrambler based on Faraday rotator mirror is inherently bulky and a higher insertion loss.

The first all-fiber passive depolarizer known to the optical fiber industry is the Lyot fiber depolarizer which is made from splicing a number of sections of birefringent fibers with 45 degree rotational off-set between the two fibers'birefringent axes. To operate, the depolarizer requires a broadband input light such that the various spectral components of the polarized input light are subjected to different retardations.

Consequently, at the depolarizer output, each spectral component exhibits a different polarization state. When averaged over the selected bandwidth, the output appears depolarized. The 45-degree splicing between different fiber sections ensures that the operation of the depolarizer is independent of the input polarization state. If depolarization is not achieved in the first fiber section, then it will be achieved in the subsequent fiber sections. To ensure this is the case, each fiber section has a different fiber length. It is clear that 45 degree splicing of a number of birefringent fiber sections does add a considerable amount of insertion loss to the depolarizer.

Another all-fiber passive depolarizer is based on incoherent cascaded fiber rings.

It is made by cascading a number fiber coupler rings of which is formed by connecting an input and output port of a 2 by 2 fiber coupler. To operate, the length of the fiber ring delay line is much longer than the coherence length of the light source to be depolarized.

Under this condition, the state of polarization of each recirculating field at the emerging port could be different. The output of the depolarizer is a superposition of different polarization states with different intensities. Depolarization is achieved by averaging over the many different polarization states of the recirculating beams. Once again, due to the added splice loss and the coupler insertion loss, this device has a considerable amount of insertion loss. Furthermore, the existence of 2 x 2 couplers means larger device package.

In a standard optical fiber, both the core and the cladding are circular, i. e. no fiber birefringence. Fig. l shows the cross-section of a standard optical fiber. It consists of a core (10) and cladding (11), and the laser light is guided in the core (10) because its index of refraction is higher than that of the cladding (11).

The low index cladding (11) region is usually made of silica and the high index core (10) region is usually made of germanosilicate, for example. Hence, the core has a

lower softening temperature and a higher linear coefficient of thermal expansion (CTE) than those of the silica cladding (11).

During fiber drawing, the preform is heated in a furnace until it is soft enough to be drawn into fiber. When the fiber temperature is between the softening temperatures of the cladding (11) and the core (10), the cladding (11) solidifies with the expanded fluidic core (10). As the fiber is drawn away from the furnace, the fiber cools down further. The fluidic core (10) solidifies with a solid cladding (11) boundary. The shrinkage of the expanded fluidic core (10) leads to frozen-in uniform radial tensile stress and hence the core (10) possesses no shear stress. Zero shear stress means that lights polarized in one direction will not be easily coupled to the orthogonal direction if no external forces, such as bends, or twists, etc. are applied. Coupling of light from one polarization to the orthogonal polarization in a uniform unperturbed medium requires the non-zero off- diagonal terms in the dielectric tensor which relate to the shear stress components in the stress tensor.

Since the core (10) of a standard optical fiber possesses no shear stress components and hence stress anisotropy or stress birefringence, polarized light traveling in the core (10) therefore has no specific preference in the alignment of its polarization axis. Any external perturbation to the fiber or the fiber cable, such as bending, will rotate this direction of polarization, or state of polarization, along the length of the fiber. It results in an unpredictable state of polarization of the light along the length and at the exit end of the optical fiber. Hence, in order to maintain polarization state, as required in many high bit rate polarization sensitive devices and systems, highly birefringent polarization maintaining optical fibers and devices are required.

Figure 2 depicts a prior art birefringent optical fiber. Fig. 2 shows the cross- section of an elliptical inner cladding circular core optical fiber. The core (10) is perfectly circular and hence there is no waveguide or form birefringence. The thermoelastic property of the elliptical cladding (12) is different from that of the core (10) and/or cladding (11). Due to this difference in thermoelastic properties, such as softening temperature and linear coefficient of thermal expansion, as well as the two-fold elliptical geometry of the elliptical inner cladding (12) in the fiber cross-section, the circular core (10) possesses stress birefringence or birefringence for short. The directions of the two principal stress axes, (13) and (14) are therefore the two principal birefringent axes seen by any light traveling in the core (10). In short, a standard non-polarized fiber as shown in Fig. 1 is now transformed into a birefringent fiber shown in Fig. 2 as if the core is crystal.

Elliptical inner cladding circular core optical fiber provides the least birefringence comparing to Panda (Fig. 3) and Bowtie (Fig. 4) birefringent optical fibers. As shown in Fig. 3 and Fig. 4, the diametrically located isolated stress modifying regions (15) maximize the internal thermal stress such that the core (10) appears even more stress anisotropic and hence higher birefringence.

The key to these birefringent fibers is therefore in the introduction of principal birefringent stresses for the creation of fiber birefringence. Polarized light aligned to one

of the two birefringent axes (13 and 14) will have their state of polarization maintained, along the entire length of the birefringent optical fiber.

For two optical modes to couple power to each other, they need to be in the same plane of polarization as well as having very similar propagation constants. This is very similar to the well know experiment of coupling pendulums in which two pendulums are tied to the same string. One of the pendulums is then made to swing while the other is not disturbed. If the two pendulums have the same mass and length, then the energy from the swinging pendulum will be transferred or coupled to the other stationary pendulum. This energy continues to be transferred back and forth between the pendulums until absorbed by the loss in the system. In other words, the initially swinging pendulum will become stationary while the initially stationary pendulum will swing, and so forth. If the two pendulums have different mass and lengths, then little or no energy from the swinging pendulum will be transferred to the other stationary pendulum.

In birefringent fibers, the propagation constants for the two polarization modes are different and their planes of polarizations are orthogonal. Hence, the two polarization modes do not easily couple to each other if there is no other form of perturbation such as bends, twists, etc. Since birefringent optical fiber has much higher internal stress fields than that of a standard optical fiber, it has higher resistivity to external mechanical perturbations. Thus it is understandable why birefringent optical fibers are made to maintain polarizations.

Consider a piece of birefringent optical fiber being clamped at 45 degree to the birefringent axes. The birefringence will be reduced due to this clamping because it increases the magnitude of the shear stress in the fiber and hence the off diagonal terms in the corresponding dielectric tensor. It is a shear loading with respect to the principal birefringent axis because the direction of applied forces is not normal to the planes of the birefringent axis. Reduction in birefringence also means a reduction in the difference between the propagation constants of the two polarization modes. As a result, polarization mode coupling is enhanced. Hence, the polarization maintaining property of a birefringent fiber will degrade if shear stress is introduced or enhanced in the fiber core (10).

Note that existence of the two birefringent axes always accompanied by a two- fold symmetry in the fiber geometry and stress distribution in the fiber cross-section.

Hence, existence of shear stress and the absence of birefringent axes or two fold symmetry in optical fibers will ensure that the fiber becomes more susceptible to polarization mode coupling through external perturbation such as bends, twists, etc.

It is not surprising that there are hundreds, if not thousands, of published research papers, patents, etc. in the area of maximizing principal birefringent stresses in birefringent optical fibers. However, historically, no attempt has ever been made to maximize shear stress-optical effect in optical fibers.

The present invention overcomes the problem of undesired polarization of light used in optical communications inherent in prior art designs by setting forth a process for inducing or enhancing shear stress-optical properties and reducing or eliminating stress birefringence in optically transmissive materials and or optical fibers, micro-optic or bulk

optical devices comprising or incorporating materials that have been subjected to such process.

SUMMARY OF THE INVENTION The present invention provides a process to induce or enhance shear stress-optical properties and to reduce or eliminate stress birefringence in optically transmissive material and to optical fiber, fiber devices, micro-optical devices, bulk optical devices comprising or incorporating material that has been subjected to such process, which results in guaranteed zero birefringent axes. The shear stress inducing or enhancing or modifying process can be intrinsic or extrinsic, or a combination of both. The optically transmissive materials can be of any shape, form or dimensions.

It is thus an object of the present invention to provide a method for inducing or enhancing shear stress-optical properties in optically transmissive materials comprising providing the condition such that no principal birefringent axes exist in the optically transmissive material.

It is another object of the present invention to provide a method wherein providing the condition that no principal birefringent axes exist in the fiber core comprises providing an odd number of internally, externally, or a combination of applied forces in rotational symmetry in the fiber cross-section if the stress profile in the fiber cross-section possesses axisymmetry.

It is yet another object of the present invention to provide a method wherein providing the condition that no principal birefringent axes exist in the fiber core comprises providing any number of internally, externally, or a combination of, strategically applied forces with or without rotational symmetry in the fiber cross-section if the stress profile in the fiber cross-section possesses non-axisymmetry.

It is yet another object of the present invention to provide a method wherein providing the condition that no principal birefringent axes exist in the region of light propagation in the optically transmissive material comprises providing any number of internally, externally, or a combination of, applied forces in rotational symmetry in a plane orthogonal to the direction of light propagation in the cross-section, where the cross-section of the optically transmissive material possesses axisymmetry in the plane orthogonal to the direction of light propagation.

It is yet another object of the present invention to provide a method wherein providing the condition that no principal birefringent axes exist in the region of light propagation in the optically transmissive material comprises providing any number of internally, externally, or a combination of, applied forces in a plane orthogonal to the direction of light propagation in the cross-section where the cross-section of the optically transmissive material possesses non-axisymmetry in the plane orthogonal to the direction of light optical propagation.

Therefore there is provided a system and a process for inducing or enhancing shear stress-optical properties and reduction or elimination of stress birefringence in optically transmissive materials and to devices comprising or incorporating material that has been subjected to such process. More particularly the present invention relates to a fiber optic cable, comprising an optical fiber, the fiber comprising a core having a light propagation region disposed about its length, and a cladding encasing the core wherein said fiber optic cable possess shear stresses in said fiber core in a plane orthogonal to said light propagation region.

Further features and advantages of the invention as well as the structure and operation of the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like numbers indicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which : Figure 1 shows a diagram of the cross-section of a standard prior art optical fiber.

Figure 1 a shows a diagram of residual forces in the cross-sections of optical fibers with regular and irregular cross-section as well as concentric and non-concentric cores.

Figure 2 shows the cross-section of a prior art elliptical inner cladding circular core optical fiber.

Figure 3 shows prior art Panda configuration birefringent optical fibers.

Figure 4 shows prior art Bowtie configuration birefringent optical fibers.

Figure 5 shows a diagram of a fiber cross-section having an inner cladding.

Figure 6 shows a diagram of a fiber cross-section having isolated stress lobes.

Figure 6a shows a diagram of a fiber cross-section having isolated stress lobes.

Figure 7 shows a diagram of a fiber cross-section having isolated stress lobes.

Figure 8 shows a diagram of a fiber cross-section having stress-relieving pits.

Figure 9 shows a diagram of a fiber cross-section having stress-relieving pits.

Figure 10 shows a diagram of a fiber cross-section having stress inducing regions with a high number of rotational symmetries.

Figure 11 shows a diagram of a fiber cross-section having stress inducing regions with a high number of rotational symmetries.

Figure 12 shows a diagram of a bulk or micro optic polarization scrambler in the form of a circular disc.

Figure 13 shows a diagram of fiber cross-section having an extrinsic stress modification design, with a five fold rotational symmetry, in the case of a circular disc shaped optically transmissive material.

Figure 14 shows a diagram of a practical micro-optics depolarizing device.

Figure 15 shows a diagram of a practical example of depolarization device having a functional bulk element, such as an optical filter included.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION While the present invention may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, an embodiment with the understanding that the present description is to be considered and exemplification of the principles of the invention and is not intended to limit the invention to those embodiments as illustrated and described herein.

The foregoing description of the various application specific inventions is by way of illustration of the practical application of the present invention of the process to induce or enhance shear stress-optical properties and reduction or elimination of stress birefringence in optically transmissive material. Modifications, obvious to those skilled in<BR> the art can be made thereto without departing from the scope of the present inventions.

Passive Polarization Scramblers The present invention can be used to produce passive polarization scramblers in the form of an optical fiber or micro-optic element or bulk optical device. These passive depolarizers can be very small in size (in a single fiber form without the need of 45 degree splicing or any splicing), no other added optical component such as fiber couplers or Faraday mirror and hence reduce insertion loss. It works for both narrow band and broadband polarized light since it does not rely on time or spatial averaging nor dependence on coherence length of the laser light. Since the optical beam never leaves the core in the all-fiber design, and the beam experience no intermediate fusion splices, it is therefore extremely low loss. Comparing with the prior designs described above, the present invention is novel and superior.

The principle of operation of the present invention of all-fiber polarization scrambler is based on the use of the process of inducing shear stress into the fiber core.

Hence, the core possesses zero birefringence and depolarization occurs due to the presence of the shear stress field in the core.

In accordance with the present invention, in order to maintain depolarization, the medium in which the laser light is guided must possess shear stress field and hence with no birefringent axes, whether this medium is in the form of an optical fiber or a bulk optical element or a micro-optical element. Furthermore, these optical fibers or bulk optical elements or micro-optical elements can have other photonic functions such as spectral filtering, gain, switching, etc.

Turning now to Fig. 5 there is shown a fiber cross-section in which an inner cladding (16) provides the condition such that no principal birefringent axes exist in the core (10). This is due to the three-fold rotational symmetry (120 degree angular spacing between 17 and 18,18 and 19,19 and 17) of the inner cladding (16) in the fiber cross- section. In fact, the condition of zero principal birefringent axes holds for any odd number of rotational symmetry, if the cross-section in the optically transmissive material has an axi-symmetry. If the cross-sectional boundary is irregular, for instance, providing the condition of zero birefringent axes requires that shear stresses be applied to the core wherein the stress inducing regions are strategically shaped. The stress inducing regions may or may not be placed about the core in the said three-fold rotational symmetry, depending on the cross-sectional boundary shape.

The distribution of, forces to create shear stresses in the cross-section of the light guiding region in an optically transmissive material, such as the core in an optical fiber, is not always in rotational symmetry. The three-fold rotational symmetry of intrinsic shear stress inducing region (16) as shown in Fig. 5 is due to the fact that the original stress distribution in the axi-symmetric fiber core as shown in Fig. 1 possesses zero shear stress.

However, should the original stress distribution in an optical fiber core deviates from axi- symmetry and/or possesses some residual non-uniform shear stress, such as off-axis core and/or irregular cross-sectional boundary such as those examples shown in Fig. la, the distribution of forces to create shear stresses in the cross-section of the light guiding region of the optically transmissive material, are not applied in rotational symmetry. This is due to the fact that, in these situations, the forces must be strategically placed in order to counter act the preexisting non-uniform shear stresses, in order to produce a uniform resultant shear stress distribution in the light guiding region-of the optically transmissive material.

The fiber design depicted in Fig. 5 includes (1) it breaks the two-fold symmetry required to support two principal birefringent axes, (2) it introduces shear stresses into the core and hence the corresponding off diagonal terms in the dielectric tensor to facilitate efficient mode coupling and (3) the odd number (three in this example) of rotational symmetry of the stress modifying regions (16) in the fiber cross-section ensures that birefringence is reduced or eliminated. This can be easily understood by visualizing the decomposition of radial tensile forces acting on the core into two orthogonal components.

For three equal radial forces (120 degree apart), the difference in the resultant forces along the two orthogonal directions is 13%. In the case of clamping forces (180 degree apart), the difference in the resultant forces along the two orthogonal directions is almost 100%-as in highly birefringent fibers.

To induce large shear stress field in the core, isolated stress lobes as shown in Fig. 6 and Fig. 7 can be used. Alternately, if the optically transmissive material has

appreciable inherent thermoelastic property, instead of isolated stress lobes, hollows or pits can also be used to create the required shear stress field. Examples of these alternate designs are shown in Fig. 8 and Fig. 9 where stress-relieving pits are illustrated.

For an optically transmissive material possessing an axi-symmetric cross-section in a plane orthogonal to the direction of light propagation, the higher the odd number in the rotational symmetry of the stress inducing regions, the more evenly is the shear stress distributed in the cross-section. Examples of these stress inducing region designs with higher number of rotational symmetries are shown in Fig. 10 and Fig. 11.

When the stress inducing regions (16,20,21,22,28) are located in the fiber cladding, it is known as intrinsic stress inducing technique. The intrinsic stress inducing technique is not limited to optical fibers. In the case of bulk or micro optically transmissive material, the intrinsic stress inclusion technique can still be employed but the physical dimensions of the optically transmissive material becomes larger than that of an optical fiber. For example, a preform slab bearing the cross-section of that shown in Fig. 6 may be used. A free-space polarized optical beam of diameter less than that of the preform core (10) will experience shear stress-optical effect when passing through. the preform core (10).

Drawing of the present invention fiber will result in fiber having the same shear stress profile in the fiber cross-section along the entire length of the drawn fiber. To maximize mode coupling by shear stress optical effect along the length of the drawn fiber, the optical preform, or fiber, or both, can be rocked, spun, or a combination of, in a random or non-random manner, during fiber drawing. As a result, it increases the shear stress optical effect along the length of the fiber. In other words, as seen by the light traveling the length of the fiber, the fiber cross section now possesses a multi fold rotational symmetry (greater than three-fold rotational symmetry) This feature can be further described with respect to Fig. 6 and 6a. Turning to Fig. 6 there is shown an example of the present invention in a form having isolated stress lobes which posses a three fold rotational symmetry having 120 degree angular spacing between radii 17 and 18, 18 and 19,19 and 17. In Fig. 6, radius 17 is shown positioned as extending from the axis at a position of 0 degrees. Turning now to Fig. 6a, radius 17 is shown positioned as extending from the axis at a position of 90 degrees. As stated above, the stress lobes located along radii 17,18 and 19, while having been rotated in a clockwise direction 90 degrees, they have maintained the same relationship with respect to each other. The possible rotations of the isolate stress lobes along the axis of the material are infinite, and while not shown, it would be obvious to one skilled in the art to randomly rotate the stress lobes about the axis a random amount along the length.

The technique of moving the preform in an irregular spinning or rocking motion is applicable to intrinsic loading in the case of the manufacture of fiber optic cable. In the case of micro-optic devices, such as the glass rod optical medium depicted in fig 12, shear stresses can be randomly distributed along the device axis using extrinsic techniques, i. e. application of external forces, along the length of the medium in a random manner as described above with respect to the intrinsic method.

Instead of the intrinsic stress inducing technique, the extrinsic stress inducing technique can be used particularly but not exclusively in the case of bulk or micro optical devices. Stress birefringence can be introduced into an optically homogeneous and isotropic transmissive rod by radial clamping. The clamped rod thus has optical behavior similar to the prior art cores (10) shown in Fig. 2, Fig. 3 and Fig. 4, a two-fold cross- sectional stress distribution with two birefringent axes. In other words, in the extrinsic stress modifying technique, the internal shear stress producing regions described earlier are replaced by strategically applied external forces transverse to the plane of light propagation. Mechanical clamps, can produce these external forces during fiber, micro or bulk optical device packaging, for example.

As an example, Fig. 12 shows a circular disc (29) made of optically transmissive material, with a diameter slightly larger than that of the bulk optical beam propagating normal to this disc (i. e. into or out of the paper). On the rim of this disc, three mechanical forces (30,31 and 32) are applied corresponding to the case of stress inducing regions with a three fold rotational symmetry. In a manner similar to those situations described earlier, the disc possesses almost no birefringent axes. Hence, modal coupling is enhanced due to similar values in their propagation constants. Due to the three-fold rotational symmetry of the applied forces, shear stress (with respect to the two fold symmetry) is introduced in the cross-section of the disc.

Once again, any odd number of rotational symmetry in the externally applied forces can create the condition of zero birefringent axes and shear stress field in a cross- section possessing axi-symmetry in the plane orthogonal to the direction of light propagation. An example of an extrinsic stress modification design, with a five fold rotational symmetry, in the case of a circular disc shaped optically transmissive material, is shown in Fig. 13. It must be noted that rotational symmetry in the externally applied forces becomes irrelevant in creating the condition of zero birefringent axes and shear stress field in a cross-section, which possesses non-axisymmetry in the plane orthogonal to the direction of light propagation. In optically transmissive materials possessing non- axisymmetric cross-section, externally applied forces must be strategically positioned to accomplish the condition of zero birefringent axes and shear stress field in the region where light is propagating in the cross-section as explained above.

The optically transmissive material can be of any shape, as long as the externally applied forces are such that the condition of zero birefringent axes is met by the creation of shear stress field. The externally applied forces also need not to be along the entire thickness of the optical material since it is not the condition of zero birefringent axes, rather the amount of influence of shear stress onto the optical beam passing through the material. Furthermore, the condition of zero birefringent axes does not guarantee the condition of shear stress distribution, as in the case of a standard non-birefringent circular optical fiber, but the converse is true. Hence, the present invention claims the process of inducing or enhancing shear stress-optical properties and reduction or elimination of stress birefringence in optically transmissive material.

Fig. 5 shows an example of the invention used as a passive polarization scrambler in a form possessing an axi-symmetry in the plane orthogonal to the direction of light propagation, with a continued stress-modifying region possessing a three fold rotational

symmetry. Whereas, Fig. 6 and Fig. 7 show examples of the invention in a form with isolated stress lobes which possesses a three fold rotational symmetry. Examples of these alternate designs are shown in Fig. 8 and Fig. 9 with hollows as stress modifying regions.

Examples of these designs with higher number of rotational symmetry are shown in Fig. 10 and Fig. 11.

Although all passive optical fiber depolarizer is clearly preferred in the field of compact optical component, there are applications where a bulk or micro-optical design is preferred. The present invention can also be applied to bulk or micro optic polarization scrambling. The method in inducing shear stress distribution in these devices can be either intrinsic or extrinsic. The intrinsic technique involves inclusion of stress modifying regions. In the extrinsic technique, strategically located externally applied forces in the plane orthogonal to the propagating beam are used to ensure the plane possesses shear stress field. Mechanical clamping, during device packaging, for example, can produce these external forces.

Fig. 12 shows a bulk or micro optic polarization scrambler in the form of a circular disc (29). On the rim of this disc, three mechanical forces (30,31 and 32) are applied. In a manner similar to those situations described earlier, the disc possesses no birefringent axes and a shear stress is created in the disc. Through the stress optic relations, polarized bulk beam becomes polarization scrambled after passing through this disc.

Fig. 14 shows an example of a practical micro-optics depolarizing device. A fiber pigtailed (33) half pitch GRIN lens (29) configuration may be used for beam expansion and collimation. For a half pitch GRIN lens (29), its beam diameter is a maximum in the middle of the GRIN lens. The beam path inside the GRIN lens is indicated by (34).

Strategically positioned forces (30, 31, etc.) are applied to the GRIN lens to create shear stress field inside the GRIN lens (29) and not to create principal birefringent axes.

Fig. 15 shows another practical example of depolarization application where a functional bulk element, such as an optical filter (29) for example, is included. In this example, the optical beam from an optical fiber (33) is expanded and collimated by a quarter pitch GRIN lens (35). The functional optical element (29) is usually inserted into the collimated beam path. Depolarization characteristic can be introduced in this type of packaging arrangement by strategically positioned and applied forces (30) to the optical element.

1. Zero Polarization Effect (PMD. PDG & PDL) Optical Fiber It is known that the principal polarization effects that lead to transmission impairments are polarization dependent loss (PDL), polarization dependent gain (PDG) and polarization mode dispersion (PMD).

PDL is caused by the strong polarization dependence in optical components such as WDM couplers, isolators and optical switches. PDG is caused by polarization hole

burning in erbium doped fiber amplifiers. Finally, polarization mode dispersion (PMD) is principally caused by the randomly varying birefringence in an optical fiber, which breaks the degeneracy of the fundamental spatial mode into two orthogonal polarization modes, introducing a time delay between them that gives rise to signal distortion.

Because of the constantly varying distributed environmental conditions of optical cables, polarization effects are stochastic due to random mode coupling which occurs in standard optical fibers. Hence, unlike chromatic dispersion, PMD effects are constantly varying and cannot be easily compensated by usual equalization techniques such as chirped gratings and dispersion-shifted fibers. Moreover, because birefringence effects are wavelength dependent, and this feature, which is referred to as second-order PMD, is similar to the chromatic dispersion, and due to its stochastic character, becomes the major limiting factor to the bandwidth and linearity of optical systems.

A standard single mode optical fiber supports two polarization modes. These two modes result from either intrinsic non-cylindrical symmetric core shape or from the environmental perturbations such as bending and twisting. Hence, for a cabled field deployed standard single mode optical fiber, both PMD and PDL are present and both fluctuate with wavelength and time.

Accordingly, the reader will see that the present invention can reduce or even eliminate PMD and PDL, due to the fact that the optical fiber described herein possesses zero birefringence. In addition, due to the existence of high internal shear stress introduced by the intrinsic stress modifying regions, the fiber is less susceptible to <BR> <BR> external perturbations such as bends and twists. If this zero polarization effect optical<BR> fiber has an erbium-doped core, then the corresponding erbium doped fiber amplifier will have a zero PDG.

While the invention has been particularly shown and described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.