Abstract:
An optical attenuator and method, the optical attenuator having a core which is covered by a cladding material with a section of polymer cladding of an identifiable length. The polymer cladding has an index of refraction that varies with temperature. A thermal device such as a thermoelectric heater or cooler is placed adjacent to the polymer cladding to control the temperature of the cladding. The index of refraction of the polymer cladding is manipulated by changing its temperature by supplying power to the thermal device. The index of refraction of the polymer cladding will range from values below and above the index of refraction of the core material. A light wave transmitted through the core will experience a degree of attenuation due to leakage into the polymer cladding material when the index of refraction of the polymer cladding is equal to or greater than the index of refraction of the core. The light wave may be either attenuated or blocked entirely.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optical waveguides and, more particularly, to the materials used to construct optical waveguides. 
     BACKGROUND OF THE INVENTION 
     Current communications networks throughout the world have embraced the use of optical fiber waveguide technology to provide a conduit of transmission components for voice, video, and data signals. Optical networks offer far greater bandwidth and reliability than do conventional electronic networks. As a consequence, current research efforts have been directed to expanding the capabilities of optical waveguide technology at reduced cost to aid in the acceleration of the conversion of the present electrical communications networks to optical communications networks. 
     These optical communications networks are comprised of many different components. These include optical fiber cable, switches, attenuators, couplers, and many more such devices. Typically, these devices are comprised of a core surrounded by a cladding material. Both the materials used for the core and the cladding include silica or doped silica as well as many other similar materials. These materials are employed because they have a desirable index of refraction and as well as other properties which facilitate their use. 
     Of ten times it is desirable to create specific effects on the propagation of the optical signal transmitted through these devices. For example, one such effect is to either attenuate or interrupt an optical signal. Consequently, there is a need for new optical structures which will facilitate the attenuation and interruption of an optical signal in an optical waveguide. 
     SUMMARY OF THE INVENTION 
     The present invention entails an optical attenuator having a core which is covered by a cladding material with a section of polymer cladding of an identifiable length. The polymer cladding has an index of refraction that varies with temperature. A thermal device such as a thermoelectric heater or cooler is placed adjacent to the polymer cladding to control the temperature of the cladding. The index of refraction of the polymer cladding is manipulated by changing its temperature by supplying power to the thermal device. The index of refraction of the polymer cladding will range from values below and above the index of refraction of the core material. A light wave transmitted through the core will experience a degree of attenuation due to leakage into the polymer cladding material when the index of refraction of the polymer cladding is equal to or greater than the index of refraction of the core. The light wave may be partially or entirely attenuated. 
     The present invention may also be viewed as a method for attenuating an optical signal in a waveguide comprising the steps of transmitting an optical signal having an initial power strength through an optical attenuator having a section of polymer cladding of identifiable length, and determining a desired optical signal strength at the output of the optical attenuator. Finally, the step of controlling the temperature of the polymer cladding to attenuate the optical signal to the desired optical signal strength is performed. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a drawing showing a conventional optical fiber waveguide; 
     FIG. 2A is a drawing showing an optical waveguide; 
     FIG. 2B is a drawing showing a sectional view of the optical waveguide of FIG. 2A; 
     FIG. 3 is a chart of the indexes of refraction of example polymers as a function of temperature; 
     FIG. 4A is a drawing showing a second optical waveguide; 
     FIG. 4B is a drawing showing a sectional view of the second optical waveguide of FIG. 4A; 
     FIG. 5 is a drawing showing an optical waveguide system that employs a thermo-electric heater with the optical waveguide of FIG. 2A; 
     FIG. 6 is a drawing showing an optical waveguide system that employs a laser source with the optical waveguide of FIG. 2A; 
     FIG. 7 is a drawing showing an optical attenuator according to an embodiment of the present invention; 
     FIG. 8 is a drawing showing the dissipation of heat from the optical attenuator of FIG. 7; 
     FIG. 9 is a drawing showing an optical attenuator system employing the optical attenuator of FIG. 7; and 
     FIG. 10 is a graph of showing an example of the attenuation of a light wave in an optical attenuator of FIG. 7 as a function of the index of refraction of the polymer cladding. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The Use of Polymer Material in Optical Waveguide Structures 
     Turning to FIG. 1, shown is a waveguide which comprises a conventional optical fiber  50 . The optical fiber  50  comprises a core  55  surrounded by a cladding  60 . The core is comprised of a material with an index of refraction of n a . The cladding  60  is comprised of a material with an index of refraction of n b . The optical fiber  50  operates as a waveguide for light radiation  65  when n a  is greater than n b  as is known by those skilled in the art. When n a  is less than or equal to n b , the light radiation  65  leaves the core and will not propagate along the core  55 . 
     With these concepts is mind, reference is made to FIGS. 2A and 2B which show a waveguide structure  100  according to an embodiment of the present invention. In FIG. 2A, the waveguide structure  100  includes a core  105  which is formed on a substrate  110 . The core  105  is surrounded by a silica cladding material  115  which encloses the remaining sides of the core  105  not bounded by the substrate  110 . In a section of identifiable length L, a polymer cladding material  120  encloses the remaining sides of the core  105  instead of the silica cladding material  115 . The core  105  has an index of refraction of n R , the substrate  110  has an index of refraction of n S , the silica cladding material  115  has an index of refraction of n C , and the polymer cladding material  120  has an index of refraction of n P . A cutaway line  122  is shown around the waveguide structure  100 . In FIG. 2B, shown is a sectional view of the waveguide structure  100  taken along the cutaway line  122  (FIG. 2A) in the middle of the polymer cladding material  120 . Note that the waveguide structure  100  is generally a planar waveguide structure. However, the invention may also be accomplished using optical fiber structures based on the principles described herein. 
     The polymer cladding material  120  is taken from the general category of materials classified as polymers which generally are chemical compounds with high molecular weight comprising a number of structural units linked together by covalent bonds. Polymers which qualify for use as the polymer cladding  130  should generally possess the optical characteristics including an index of refraction that varies with temperature as will be discussed. 
     The core  105  may comprise doped silica and the substrate  110  may comprise silica, however, it is understood that other materials may be employed as known by those skilled in the art. Consequently, an exhaustive list of possible materials used to create these components is not offered herein. 
     It is understood that the waveguide structure  100  is for illustrative purposes and is not the only structural configuration possible. It may be possible for example, that the polymer cladding material  120  only contact the core  105  in identifiable regions such as on the upper surface of the core  105 , for example. According to embodiments of the invention, the design of the actual waveguide structure  100  is such that the polymer cladding material  120  comes into contact with the core  105  so that the propagation of light radiation through the core  105  can be manipulated by controlling the index of refraction of the polymer cladding material  120  relative to the index of refraction of the core  105  to achieve certain advantages. 
     The polymer cladding material  120  features a relatively high thermo-optic coefficient          ∂     n   p         ∂   T                            
     due to the fact that the index of refraction of polymers can vary significantly with changing temperature. For example, the thermo-optic coefficient          ∂     n   p         ∂   T                            
     generally may be as high as −0.0001C −1  and even up to −0.0003C −1 , where n P  is the refractive index of the polymer and T is temperature. In contrast, the thermo-optic coefficient of silica is much lower, being on the order of 0.00001C −1 . Consequently, the index of refraction of fused silica and other similar materials will not change significantly when subjected to heat, while the index of refraction of the polymer will change significantly. 
     Referring to FIG. 3, shown is a graph depicting the index of refraction as a function of temperature in degrees Celsius of three example polymers which may be used as the polymer cladding material  120 . Line  130  depicts the index of refraction of F/CA polymer which has a thermal coefficient of −0.00002C −1 , line  135  depicts the index of refraction of D-PMMA/D-FA polymer which has a thermal coefficient of 0.0001C −1 , and line  140  depicts the index of refraction of FA polymer which has a thermal coefficient of −0.0003C −1 . Note that the starting point at n=1.46 and Temperature=−20° C. were chosen arbitrarily. Ultimately, the indexes of refraction of various polymers depend upon their composition and can vary over a relatively wide range as a function of temperature. 
     The change of the index of refraction of a polymer cladding as contemplated herein provides distinct advantages. For example, a change in the propagation constant βof the guided wave can be made by changing the temperature of the polymer cladding. Also, the propagation of light radiation through the core may be diminished or stopped by raising the index of refraction of the polymer cladding above that of the core. 
     Turning next to FIG. 4A, shown is a waveguide structure  150  according to another embodiment of the present invention. The waveguide structure  150  features a polymer core  155  formed on a substrate material  160  and surrounded on the remaining sides by a cladding material  165 . The cladding material  165  may be another polymer or other material that has an index of refraction that allows the propagation of light through the polymer core  155 . The relative indexes of refraction of the polymer core  155  and the cladding material  165  are manipulated to achieve the desired propagation through the waveguide structure  150 . A cutaway line  167  is shown around the middle of the waveguide structure  150 . FIG. 4B shows is a sectional view of the waveguide structure  150  taken along the cutaway line  167  (FIG.  4 A). 
     Referring to FIG. 5, shown is a waveguide system  200  according to another embodiment of the present invention. The waveguide system  200  features the waveguide structure  100  (FIG. 2B) which includes the polymer cladding material  120  with the core  105  formed on a substrate  110 . The waveguide system  200  further includes a thermo-electric heater  205  and a thermoelectric cooler  210 . The thermo-electric heater  205  is electrically coupled to a voltage source V 1  and may be of the chrome strip type. Other types of thermoelectric heaters  205  may include electrically conducting glass materials. The thermoelectric cooler  210  is electrically coupled to a voltage source V 2 . The waveguide system  200  may be constructed with the thermo-electric heater  205  alone or with the thermoelectric cooler  210  alone depending on the ambient temperature and the desired range for the index of refraction of the polymer material. The waveguide system  200  is accomplished preferably on an integrated optical circuit which are well known by those skilled in the art and not discussed here in detail. 
     Referring next, to FIG. 6, shown is a second waveguide system  300  according to an additional embodiment of the present invention. The waveguide system  300  also features the waveguide structure  100  (FIG. 2B) which includes the polymer cladding material  120  with the core  105  formed on the substrate  110 . In addition, the waveguide system  300  includes a laser source  305  which produces laser radiation  310 . The laser source  305  is positioned such that the laser radiation  310  falls onto the polymer cladding material  120 . The laser radiation  310  heats up the polymer cladding material  120  resulting in a corresponding change in the index of refraction of the polymer cladding material  120 . Note that a thermoelectric cooler  210  (FIG. 5) may be included in the waveguide system  300  similar to the waveguide system  200 . 
     Use of Polymer Material in an Optical Attenuator 
     The foregoing discussion relates to the use of the polymer material in a waveguide. The following discourse relates to the use of the polymer material in the specific application of an optical attenuator. Turning then to FIG. 7, shown is an optical attenuator  400 . The optical attenuator  400  is comprised of a core  405  formed on a substrate  410 . The three sides of the core  405  not in contact with the substrate  410  are covered with a cladding material  415  and a section of polymer cladding  420 . The section of polymer cladding  420  has an identified length L along the axis of the core  405 . Disposed adjacent to the polymer cladding  415  is a thermal device  425 . Note that the thermal device  425  may actually be a thermoelectric heater  205  (FIG. 5) or a thermoelectric cooler  210  (FIG. 5) as previously discussed. Also, a laser source  305  (FIG. 6) may be employed in place of the thermal device  210 . A cutaway line  427  is shown around the optical attenuator  400 . 
     The operation of the optical attenuator  400  is as follows. An optical signal or light wave is transmitted through the core  405 . When in an unattenuated state, the light wave is guided through core  405  with no significant loss or attenuation other than the inevitable loss imparted by the presence of the polymer cladding  420 . In this case, the temperature of the polymer cladding  420  is increased so that its index of refraction is lowered to be approximately equal to the index of refraction n C  of the cladding material  415 . The actual temperature of the polymer cladding  420  that results in an index of refraction n P  which is equal to the index of refraction n C  of the cladding material  415  depends upon the type of polymer material  415  chosen as seen in FIG.  3 . When attenuation of the light wave transmitted through the optical attenuator  400  is desired, the optical attenuator  400  is put into an attenuation state in which the polymer cladding  420  is cooled, for example, thereby raising the index of refraction n P  to be approximately equal to or greater than the index of refraction n R  of the core  405 . When this occurs, the light is no longer contained in the core  405  and begins to leak out into the polymer cladding  420 . The attenuation of the light wave varies depending on the difference between the index of refraction n P  of the polymer cladding  420  and the index of refraction of the core  405 . The attenuation of the light wave also varies with the identified length of the polymer cladding  420 . The longer the polymer cladding length L, the greater the attenuation and vice versa. The actual length L of the polymer chosen depends upon the particular application as determined by one skilled in the art. The polymer cladding  420  may be cooled or allowed to cool to a point where the index of refraction n P  of the polymer cladding is high enough that the light wave leaves the core  405  through the cladding  420  entirely, stopping the transmission of the light wave out of the optical attenuator  400 . In this manner, the optical attenuator  400  may perform the function of an optical switch. 
     Turning next to FIG. 8, shown is a sectional view of the optical attenuator  400  taken from the cutaway line  427  (FIG. 7) in which the thermal device  425  is generating heat  430  that leaves the thermal device  425  and enters the polymer cladding  420 . The heat  430  then is dissipated from the polymer cladding  420  into the surrounding environment, including through the core  405  and the substrate  410 . If the thermal device  425  were a thermoelectric cooler  210  (FIG. 5) the heat  430  would generally flow toward the thermal device  425  rather than away from it. Thus, a given optical attenuator  400  has a particular rate of heat loss given the nature of the optical attenuator structure and the temperature of the surrounding environment. Consequently, the optical attenuator  400  may be designed with these concepts in mind. That is to say, where a higher index of refraction n P  of the polymer cladding  420  is desired, it may be necessary to employ a thermoelectric cooler  210  or similar device where the natural heat dissipation of the optical attenuator structure is inadequate or where the temperature of the surrounding environment of the optical attenuator  400  is too high to allow proper heat dissipation. If a lower index of refraction n P  of the polymer cladding  420  is desired requiring higher polymer cladding temperatures, it may be necessary to employ a thermo-electric heater  205  (FIG. 5) where the natural heat absorption of the optical attenuator structure is inadequate or where the temperature of the surrounding environment of the optical attenuator  400  is too low resulting in greater heat dissipation. In some cases, both a thermoelectric heater  205  and a thermoelectric cooler  210  can be used concurrently. 
     Referring next, to FIG. 9, shown is an optical attenuator system  500  which employs the optical attenuator  400  in a temperature control arrangement which employs a feedback loop. The optical attenuator system  500  comprises the optical attenuator  400  with a thermo-electric heater  205  and a thermoelectric cooler  210  positioned adjacent to the polymer cladding  420 . 
     A thermal control device  505  is electrically coupled to both the thermoelectric heater  205  and the thermoelectric cooler  210  and transmits a power signal to these devices, thereby adding or subtracting heat to or from the polymer cladding  420  as needed. The power source receives a reference signal V REF  which is proportional to the desired temperature of the polymer cladding  420 , which, as discussed previously, is a function of the desired attenuation. A photo detection device  510  or similar device is optically coupled to the output of the optical attenuator  400  and is electrically coupled to the thermal control device  505 . The photo detection device  510  generates a feedback signal which is transmitted to the thermal control device  505 . This feedback signal allows the thermal control device  505  to maintain a steady temperature of the polymer cladding  420 . In this manner, the incoming optical signal  515  is attenuated by leaking into the polymer cladding  420 , resulting in an outgoing optical signal  520  with diminished power. 
     Turning to FIG. 10, shown by way of an example is a graph which depicts the attenuation of a light wave in an optical attenuator  400  (FIG. 7) as a function of the index of refraction of the polymer cladding  420  (FIG.  7 ). The optical attenuator  400  data was analyzed with the following parameters: 
     wavelength, λ=1.55 μm 
     core index or refraction n R =1.45 
     cladding material index of refraction n C =1.444 
     core width d=5 μm        V   =         2      π                 d     λ                n   R   2     -     n   C   2         .                              
     A Gaussian shaped beam was assumed transmitted into the core  405  of the attenuator  400  whose full width at the 1/e points of the power distribution is w=6.75 μm. At the output of the optical attenuator  400 , the light wave power was collected over an area whose width is 14.3 μm. The widths of the incident light beam and of the collection area were chosen to ensure that the light beam traveled through the core  405  without width oscillations and that 99% of the power is captured when the index of refraction n P  (FIG. 7) of the polymer cladding  420  equals the index of refraction nc (FIG. 7) of the cladding material  415 . FIG. 10 shows a first curve C 1  and a second curve C 2  which represent the ratio of collected light (P OUT ) to input light power (P IN ) as functions of the difference between the index of refraction n C  of the cladding material  415  and the index of refraction n P  of the polymer cladding  420 . For the first curve C 1 , the length of the polymer section L is equal to 0.5 mm and for the second curve C 2 , the length of the polymer section L is equal to 1 mm. 
     Many variations and modifications may be made to the embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.