Abstract:
A Mach-Zender interferometer employing a section of polymer cladding in one branch. The polymer cladding has an index of refraction that varies with temperature. The temperature of the section of polymer cladding is adjusted to cause a corresponding change in the phase of the laser light flowing through the waveguide core bounded by the polymer cladding to effect a desired switching or modulation of the laser light.

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 as well as other properties which facilitate their use. 
     Often 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 modulate an optical signal or switch an optical signal from one waveguide to another. Consequently, there is a need for new optical structures which will facilitate the switching and modulation of an optical signal in an optical waveguide. 
     SUMMARY OF THE INVENTION 
     The present invention entails a Mach-Zender interferometer with a first waveguide core and a second waveguide core. The first and second waveguide cores are brought close together at a first directional coupler and a second directional coupler. The first waveguide core is surrounded by a first cladding and the second waveguide core is surrounded by a second cladding. The first cladding includes a section of polymer cladding in contact with the first waveguide core between the first and second directional couplers. Outside the polymer cladding section, the first waveguide core is surrounded by a standard cladding material which may include a substrate upon which the first waveguide core is formed and a silica cladding material covering the remaining surface area of the first waveguide core. The second waveguide core is similar to the first waveguide core except a standard cladding material is employed throughout its length. The polymer cladding has an index of refraction that varies with temperature. 
     A laser light which is transmitted into an input port may be transferred from the first waveguide core to the second waveguide core and vice versa in an identified proportion by altering the phase in one of the waveguides between the directional couplers. This is accomplished by altering the temperature of the polymer cladding resulting in a change in the index of refraction. Depending on the length of the section of polymer cladding, the phase of the laser light in the waveguide core bounded by the polymer cladding is altered by the desired amount. 
     The present invention may also be viewed as a method for switching laser radiation in an optical circuit comprising the steps of optically coupling a first waveguide core and a second waveguide core with a first directional coupler and a second directional coupler forming first and second joints, and then covering the first waveguide core with a first cladding that includes an identifiable area of a polymer cladding material, the identifiable area being between the first and second joints. Next the second waveguide core is covered with a second cladding and a laser beam is transmitted into the first waveguide core. Thereafter, further steps include splitting the laser beam in the first directional coupler at the first joint into a first split beam directed to the first waveguide core and a second split beam directed to the second waveguide core, and then controlling the temperature of the polymer cladding, thereby changing the index of refraction of the polymer cladding resulting in a change in the phase of the first split beam. Finally, the first and second split beams are recombined in the second directional coupler at the second joint, and the laser beam is transmitted out of the first and second waveguide cores according to a power ratio based upon the degree of the phase change of the first split beam. 
     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 side view of a conventional optical fiber; 
     FIG. 2 is a drawing showing a Mach-Zender interferometer according to an embodiment of the present invention; 
     FIG. 3 is a drawing showing a sectional view of the Mach-Zender interferometer of FIG. 2 taken along the cutaway line in FIG. 2; 
     FIG. 4 is a graph showing the index of refraction of selected polymers used in the Mach-Zender interferometer of FIG. 2 as a function of temperature; 
     FIG. 5 is a graph showing the length of the polymer section in the Mach-Zender interferometer of FIG. 2 as a function of the index of refraction of the polymer material for a phase change of 180°; 
     FIG. 6 is a drawing showing a partial sectional view of the Mach-Zinder interferometer of FIG. 2, further including a thermoelectric heater; 
     FIG. 7 is a drawing showing a partial sectional view of the Mach-Zinder interferometer of FIG. 2, further including a laser employed as a heating device; and 
     FIG. 8 is a temperature control system according to an embodiment of the invention, including a feedback loop to control the temperature of the polymer cladding of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 1 . The cladding  60  is comprised of a material with an index of refraction of n 2 . The optical fiber  50  operates as a waveguide for light radiation  65  when n 1  is greater than n 2  as is known by those skilled in the art. When n 1  is less than or equal to n 2 , the light radiation  65  leaves the core  55  and will not propagate along the core  55 . 
     With these concepts in mind, reference is made to FIG. 2 which shows a Mach-Zender interferometer  100  according an embodiment of the present invention. The Mach-Zender interferometer  100  comprises a first waveguide core  105  and a second waveguide core  110  essentially in parallel. The first and second waveguide cores  105  and  110  approach each other at two points forming a first 3 dB directional coupler  115  and a second 3 dB directional coupler  120 . The first and second waveguide cores  105  and  110  are the same length between the first and second 3 dB directional couplers  115  and  120 . The first waveguide core  105  has an input port A and an output port C. The second waveguide core  110  has an input port B and an output port D. 
     Both the first and second waveguide cores  105  and  110  are formed on a substrate. The remaining surface area of the first waveguide core  105  not bounded by the substrate is encased in a cladding which comprises both a standard cladding  125  and a section of polymer cladding  130 . The section of polymer cladding  130  covers the first waveguide core  105  for an identifiable length L and is positioned between the first and second 3 dB directional couplers  115  and  120 . The remaining surface area of the second waveguide core  110  not in contact with the substrate is encased by the standard cladding  125 . Note that the substrate and the standard cladding  125  have similar properties and together can be considered a single cladding material for purposes of the operation of the Mach-Zender interferometer  100 . Also, the Mach-Zender interferometer  100  is accomplished preferably on a planar waveguide optical circuit which are well known by those skilled in the art and not discussed here in detail. A cutaway line  133  is drawn across the middle of the Mach-Zender interferometer  100 . 
     The polymer cladding  130  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. 
     Turning to FIG. 3, shown is a sectional view taken along the cutaway line  133  (FIG. 2) of the Mach-Zender interferometer  100 . The first and second waveguide cores  105  and  110  are square in shape having sides of dimension d. The bottom side of the first and second waveguide cores  105  and  110  contact a substrate  135  which has an index of refraction n S . The remaining sides of the second waveguide core  110  are in contact with the standard cladding  125 . The remaining sides of the first waveguide core  105  are in contact with the polymer cladding  130  as shown, although the remaining sides of the first waveguide core  105  beyond the length L of the polymer cladding  130  are in contact with the standard cladding  125 . The first and second waveguide cores  105  and  110  have an index of refraction n WC . The standard cladding  125  has an index of refraction n CL , and the polymer cladding  130  has an index of refraction n P . 
     Note that the polymer cladding  130  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 −0.0001 C. −1  and even up to −0.0003 C. −1 , where n P  is the refractive index of the polymer and T is temperature. In contrast, the thermo-optic coefficient of silica, for example, is much lower, being on the order of 0.00001 C. −1 . Consequently, the index of refraction of fused silica and other similar materials do not change significantly when subjected to heat, while the index of refraction of the polymers do change significantly. In addition, according to the preferred embodiment, the core  105  comprises doped silica and the substrate  135  comprises silica. However, it is understood by those skilled in the art that other materials may be employed. Consequently, an exhaustive list of possible materials used to create these components is not offered herein. 
     Referring to FIG. 4, 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 for the polymer cladding  130  (FIG.  2 ). Line  140  depicts the index of refraction of F/CA polymer which has a thermal coefficient of −0.00002 C. −1 , line  145  depicts the index of refraction of D-PMMA/D-FA polymer which has a thermal coefficient of −0.0001 C. −1 , and line  150  depicts the index of refraction of FA polymer which has a thermal coefficient of −0.0003 C. −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. 
     Turning back to FIG. 2, the operation of the Mach-Zender interferometer  100  is explained. A light beam  155  enters the first waveguide core  105  at input port A and is directed to the first 3 dB directional coupler  115 . Note that the light beam  155  might enter the second waveguide core  110  at input port B rather than only the first waveguide core  105  at input port A. The light beam  155  is depicted as entering the first waveguide core  105  at input port A for the purposes of illustration herein. 
     Upon encountering the first 3 dB directional coupler  115 , half of the light beam  155  is transferred to the second waveguide core  110 , resulting in a first split light beam  160  transmitted through the first waveguide core  105  and a second split light beam  165  transmitted through the second waveguide core  110 . At this point, the second split light beam  165  trails the first split light beam  160 , the first and second split light beams  160  and  165  being 90° out of phase. Given that the first and second waveguides  105  and  110  are the same length between the first and second 3 dB directional couplers  115  and  120 , the second split light beam  165  is transferred back into the first waveguide core  105  by the second 3 dB directional coupler  120  and the light beam  155  emerges from the output port C as shown in FIG.  2 . However, if the phase of the first split light beam  160  transmitted through the first waveguide core  105  is shifted by 180°, then the light beam  155  emerges from port D. Thus, the Mach-Zender interferometer  100  can be made to act as a switch, where the phase of one of the split light beams  160  or  165  can be shifted appropriately. The first and second 3 dB directional couplers  115  and  120  can also be replaced by directional couplers which will result in a particular proportion of light transmitted out of both output ports C and D, depending on the degree of the phase shift. 
     The phase of a light beam can be shifted by altering the propagation constant β of the waveguide through which the light beam travels. According to an embodiments of the present invention, in the case of the Mach-Zender interferometer  100 , the propagation constant β of the first waveguide core  105  is altered by changing the index of refraction of the polymer cladding  130 . Thus, in order to switch the light beam  155  from exiting the first waveguide core  105  at output port C to exiting the second waveguide core  110  at output port D, a change in the propagation constant Δβ is made to occur. Given the length L of the polymer cladding  130 , then the relationship between the change in the propagation constant can be written as ΔβL=π. 
     Accordingly, the length L required for switching is L=π/Δβ. 
     Referring then, to FIG. 5, shown is a graph which depicts the length L of the polymer cladding  130  (FIG. 2) as a function of the index of refraction n P  of the polymer cladding  130  that results in a phase shift of 180°. The index of refraction n P  of the polymer cladding  130  is determined by altering its temperature accordingly. The graph is generated where the frequency of the light wave λ=1.55 μm, the index of refraction n WC =1.45276, and the index of refraction n S =1.4441. According to the graph, the phase changes by 180° when the index of refraction n p  of the polymer cladding  130  rises from n p =1.4441, the value of the index of refraction n s  of the substrate  135  (FIG.  3 A), to the value indicated on the x-axis of the graph of FIG. 5, if the length of the polymer cladding  130  is equal to the length indicated on the vertical y-axis. The graph of FIG. 5 assumes the structure of the Mach-Zender interferometer  100  (FIG. 2) which includes the square waveguide core  105  with sides of dimension d=6 μm being bounded by the substrate  135  (FIG. 3) on one side and the polymer cladding  130  (FIG. 3) on the remaining three sides as shown in FIG.  3 . Note it would be possible to employ other physical structures as well and compute a similar graph to that shown in FIG.  5 . In the case where the Mach-Zender interferometer  100  (FIG. 2) is used as an optical switch, the temperature of the polymer cladding  130  should be stable within 10% of the value to keep any leakage at below 1% out of whichever output port C or D is switched off. 
     Turning to FIG. 6, shown is a partial sectional view  170  of the waveguide core  105  surrounded by the polymer cladding  130  and the substrate  135 . Disposed adjacent to the polymer cladding  130  is a thermo-electric heater  175  which is electrically coupled to a voltage source V 1 . The temperature of the polymer cladding  130  is raised by raising the temperature of the thermo-electric heater  175  by application of voltage source V 1  appropriately. To lower the temperature of the polymer cladding, heat is allowed to dissipate from the Mach-Zender interferometer  100  (FIG.  2 ), or a thermo-electric cooler may be employed. 
     Referring then, to FIG. 7, shown is a partial sectional view  180  of the waveguide core  105  surrounded by the polymer cladding  130  and the substrate  135 . Also shown is a laser source  185  which is positioned to project laser radiation  190  onto the polymer cladding  130 . When applied, the laser radiation  190  causes the temperature of the polymer cladding  130  to rise. To lower the temperature of the polymer cladding, heat is allowed to dissipate from the Mach-Zender interferometer  100  (FIG.  2 ), or a thermo-electric cooler may be employed as was the case with the thermo-electric heater  175  (FIG.  6 ). 
     Finally, reference is made to FIG. 8 which shows a temperature control system  200  to control the temperature of the polymer cladding  130 . The temperature control system  200  includes a thermal control  205  which is employed in a feedback loop. A reference voltage V REF  is input into the thermal control  205  which indicates the desired phase shift such as 180° for a switching action in the Mach-Zender interferometer  100  (FIG. 2) as discussed previously. The thermal control  205  is electrically coupled to both a thermo-electric heater  175  and a thermo-electric cooler  210 . The thermo-electric heater  175  and a thermo-electric cooler  210  are positioned, for example, adjacent to the polymer cladding  130  to facilitate heat transfer to and from the polymer cladding. The polymer cladding  130  covers the waveguide core  105  as described previously. The temperature of the polymer cladding  130  is raised by causing the thermo-electric heater  175  to add heat  215 . The temperature of polymer cladding  130  is cooled by activating the thermo-electric cooler  210  thereby drawing heat  220  out of the polymer cladding  103 , and by taking into account the natural heat loss  225  of the Mach-Zender interferometer  100  to the surrounding atmosphere. Laser light  230  is directed through the waveguide core  105 , which is surrounded by the polymer cladding  130 . Before encountering the polymer cladding  130 , the laser light  230  is at an initial phase  235  and leaves the polymer cladding  130  with a shifted phase  240 . The laser light  230  is ultimately routed out of the Mach-Zender interferometer  100  out of output ports C and/or D. The laser light output can be measured by a photo-detection device  245  and, accordingly, it can be determined whether the appropriate phase change has occurred due to the change in temperature of the polymer cladding  130  as discussed previously. The thermal control  205  receives a feedback signal which relays the phase information and generates an appropriate signal to the thermo-electric heater  175  or the thermo-electric cooler  210 . 
     Many variations and modifications may be made to the various 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.