Abstract:
Waveguide amplifiers having high gain dynamical range, methods for amplifying optical signals, and methods for fabricating wave guide amplifiers are provided. The waveguide amplifiers include a substrate, lower cladding, upper cladding, and a core having a varying cross-section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/467,143 filed May 2, 2003, and is incorporated by reference in its entirety herein. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention generally relates to optical amplifiers, and more particularly, to optical waveguide amplifier configurations having high gain dynamical range.  
       BACKGROUND  
       [0003]     Optical waveguide amplifiers have been attracting great interest because of their promising integration capability with other passive and active optical components. Further, waveguide amplifiers can be realized at a low cost, be multi-functional, and be combined with advanced integrated light circuits. A conventional waveguide amplifier  100 , shown in  FIG. 1 , includes a substrate  10 , a lower cladding layer  20 , a core  30 , and an upper cladding layer  40 . In conventional waveguide amplifier  100 , the cross-section of core  30  and the numerical aperture (NA) are constant.  
         [0004]     In standard operation, a weak optical signal is generally coupled into waveguide amplifier  100  from a single mode optical fiber. Amplification is accomplished by stimulated emission of active ions (e.g., Erbium, Yitterbium, Nd, Cr, etc.) in core  30 . The active ions are excited by an optical pump at a suitable wavelength. The amplified signal is then coupled out of the waveguide amplifier into another single mode fiber. For a given set of material parameters, an optimum small signal gain can be achieved with small core sizes, i.e., less than 2 μm. High power signal gain can be achieved with larger core sizes, i.e., larger than 2 μm. However, because the material and waveguide properties of conventional waveguide amplifiers are constant throughout the medium, conventional waveguide amplifiers are suitable for only a limited range of input signal power values. This results in low gain dynamic range.  
         [0005]     Thus, there is a need to overcome these and other problems of the prior art and to provide waveguide amplifiers that enhance amplifier performance of both small signal and high power values.  
       SUMMARY OF THE INVENTION  
       [0006]     According to various embodiments, there is provided a waveguide amplifier comprising a substrate and a lower cladding disposed on the substrate. The waveguide amplifier further comprises a core disposed on the lower cladding, wherein a core cross-section varies along a length of the core and an upper cladding disposed on the lower cladding and the core.  
         [0007]     According to various embodiments, there is further provided a wave guide amplifier comprising a substrate, a lower cladding disposed on the substrate, a core disposed on the lower cladding, and an upper cladding disposed on the lower cladding and the core. The core comprises a first section, wherein a cross-section of the first section decreases continuously from a first end of the first section to a second end of the first section. The core further comprises a second section, wherein a cross-section of the second section increases continuously from a first end of the second section to a second end of the second section. The core further comprises a third curved section, wherein the third curved has a constant cross-section, a first end of the third curved section adjacent the second end of the first section and a second end of the third section adjacent the first end of the second section.  
         [0008]     Also according to various embodiments, there are provided methods for making a waveguide amplifier comprising providing a substrate and depositing a lower cladding layer on the substrate. A core layer is deposited on the lower cladding layer and a shadow photomask is deposited on the core layer. The shadow photomask is exposed to ultraviolet light. The core layer is etched to form a core comprising a varying cross-section and to expose a portion of the lower cladding. An upper cladding layer is then deposited on the core and the exposed portion of the lower cladding.  
         [0009]     According to various embodiments, a method for amplifying an optical signal is provided. The method comprises coupling the optical signal from a first optical fiber into a core of a waveguide amplifier, wherein the core of the waveguide amplifier comprises a varying cross-section to form a range of mode-field regions. The optical signal is amplified by stimulated emission as the optical signal propagates through the mode-field regions. The optical signal is then coupled from the core of the waveguide amplifier into a second optical fiber.  
         [0010]     According to various embodiments, a method for making a waveguide amplifier is provided. The method comprises lithographically fabricating a master including a core shape having a varying dimension. The master is used to form a stamper, the stamper including a negative of the core shape. A lower cladding layer and a core layer are provided, and the stamper is used to form a core having a varying dimension from the core layer. A portion of the lower cladding is exposed and an upper cladding layer is deposited on the core and the exposed portion of the lower cladding layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention.  
         [0012]     In the drawings,  
         [0013]      FIG. 1  schematically depicts a conventional waveguide amplifier.  
         [0014]      FIG. 2   a  schematically depicts a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0015]      FIG. 2   b  depicts a side view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0016]      FIG. 3   a  schematically depicts a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0017]      FIG. 3   b  depicts a top view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0018]      FIG. 3   c  depicts a side view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0019]      FIG. 4   a  schematically depicts a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0020]      FIG. 4   b  depicts a top view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0021]      FIG. 4   c  depicts a side view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0022]      FIG. 5   a  schematically depicts a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0023]      FIG. 5   b  depicts a top view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0024]      FIG. 5   c  depicts a side view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0025]      FIG. 6   a  schematically depicts a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0026]      FIG. 6   b  depicts a top view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0027]      FIG. 6   c  depicts a side view of a waveguide amplifier in accordance with an exemplary embodiment of the invention.  
         [0028]      FIG. 7  is a graph showing simulation results of convention waveguide amplifiers and waveguide amplifiers in accordance with an exemplary embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0029]     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration a specific exemplary embodiment in which the invention may be practiced. This embodiment is described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense.  
         [0030]      FIGS. 2-8  disclose waveguide amplifier structures, methods of use, and methods of manufacture, in accordance with an exemplary embodiment of the present invention. The exemplary waveguide amplifier structures can propagate an input optical signal. Further, the exemplary waveguide amplifiers can amplify the input optical signal in a range of mode-field regions. The amplification can enhance performance in both small signal and high input power regimes. As used herein, the term “mode field region” or “mode field diameter” refers to a measure of radial intensity distribution of propagating light in a waveguide and means the distance from the waveguide center at which intensity drops to a value of 1/e 2 =0.135 of a peak value, where e is Euler&#39;s number (also known as Napier&#39;s constant).  
         [0031]     According to various embodiments, as shown in  FIGS. 2   a - 2   b , waveguide amplifier  200  can include a substrate  210 , a lower cladding  220 , a core  230 , and an upper cladding  240 . Lower cladding  220  is disposed on substrate  210  and core  230  is disposed on a portion of lower cladding  220 . Upper cladding  240  is disposed on core  230  and on portions of lower cladding  220  not covered by core  230 .  
         [0032]     Substrate  210  provides a relatively flat platform for forming subsequent layers. Further, the polarization and thermal properties of the resulting waveguide amplifier can be adjusted based on a difference between a coefficient of thermal expansion (CTE) of the material of substrate  210  and a CTE of the materials of waveguide amplifier  200 . In certain embodiments, the material of substrate  210  can be a conventional material, such as, for example, silicon, glass, or polymer.  
         [0033]     As shown in  FIGS. 2   a  and  2   b , core  230  can have a cross-section, for example comprising a width and/or a height, that continuously increases from an input end  231  to an output end  239 . In various embodiments, core  230  can have a width and a height that continuously increases along a length of waveguide amplifier  200  from input end  231  to output end  239 .  
         [0034]     The difference in the refractive index (An) between the material of core  230  and the material of the cladding layers (lower cladding  220  and upper cladding  240 ) confines an optical signal (i.e., light) inside of core  230 . Core  230 , lower cladding  220 , and upper cladding  240  can be an optical material, such as, for example, glass or polymer.  
         [0035]     Generally, lower cladding  220  can be deposited on substrate  210  by spin coating, dipping, spraying, molding, stamping, nanoreplication, physical vapor deposition, chemical vapor deposition, sputtering, or other methods known in the art. In various embodiments, substrate  210  can have a coefficient of thermal expansion similar to the coefficient of thermal expansion of lower cladding layer  220 . A core layer can then be deposited on lower cladding  220  by spin coating, dipping, spraying, molding, stamping, nanoreplication, physical vapor deposition, chemical vapor deposition, sputtering, or other methods known in the art. The width and/or height of core  230  can then be varied. In various embodiments, the width and/or height of core  230  can be varied using a shadow photomask. For example, the shadow photomask can be formed by depositing a photoresist layer on the core layer. A desired pattern of core  230 , including variations in width, can be obtained by applying ultraviolet (UV) light through the shadow photomask to expose the photoresist. The shadow photomask can also affect the photoresist exposed to UV light so as to obtain height variations of the core. In various embodiments, the UV transmission profile of the shadow photomask can be linearly varied across a predetermined length to obtain the continuously increasing height across that length of core  230 . The shape of core  230  can be formed on a portion of lower cladding  220  by wet or dry etching techniques, such as, reactive ion etching (RIE). In certain embodiments, the height of core  230  follows the photoresist layer thickness pattern. Upper cladding  240  can then be deposited on core  230  and on exposed portions of lower cladding  220  by spin coating, dipping, spraying, physical vapor deposition, chemical vapor deposition, sputtering, or other methods known in the art.  
         [0036]     According to various embodiments, waveguide amplifier  200  can be fabricated via nanoreplication process, which utilizes, a master, a stamper, and replicas. The master can be fabricated by lithographic methods described herein (e.g., via shadow mask) on various substrates including but not limited to silicon, glass, and quartz. Once the master with a waveguide structure is generated, it can be used to form a stamper. The stamper is cured via, including but not limited to, ultra violet (UV) light curing and hot embossing processes. Furthermore, an appropriate substrate can also be utilized as a lower cladding  220  to reduce the fabrication steps. The stamper is then used to form core  230  from lower cladding  220  having a core layer on lower cladding  220 . Upper cladding  240  can be formed on core  230  and lower cladding  220  by, for example, spin coating, dipping, spraying, physical vapor deposition, chemical vapor deposition, sputtering processes, or other methods known in the art. The formed waveguide amplifiers are then cured and the pattern features permanently fixed.  
         [0037]     According to various embodiments, as shown in  FIGS. 3   a - 3   c , waveguide amplifier  300  can include a substrate  310 , a lower cladding  320 , a core  330 , and an upper cladding  340 . As depicted in  FIG. 3   a , core  330  can include a first section  332 , a second section  336 , and a third section  338 . First section  332  can have a cross-section that continuously decreases from an input end  331  to an end adjacent second section  336 .  FIG. 3   b , which depicts a top view of waveguide amplifier  300 , shows a width of first section  332  that continuously decrease from first end  331  to the end adjacent second section  336 . Further, as shown in  FIG. 3   c , which is a side view of waveguide amplifier  300 , the height of first section  331  can also continuously decrease from first end  331  to the end adjacent second section  336 .  
         [0038]     In various embodiments, second section  336  can have a constant cross-section. As shown in  FIG. 3   b , a width of second section  336  can be constant. Further,  FIG. 3   c , shows the height of second section  336  can be constant.  
         [0039]     In various embodiments, third section  338  can have a width that continuously increases from an end adjacent second section  336  to an output end  339 , as shown in  FIG. 3   b . Third section  338  can also have a continuously increasing height from the end adjacent second section  336  to the output end  339 , as shown in  FIG. 3   c.    
         [0040]     The structure, such as the shape, of core  330  can be formed by the shadow photomask technique described herein. The exemplary three section configuration can be formed by a shadow photomask having a UV light transmission profile consistent with the shape of the core height profile shown in  FIG. 3   c . In various embodiments, a length of each core section can be different. Moreover, in various embodiments, the variation in height of first section  332  can differ from the variation in height of second section  336  and/or third section  338 . In addition, the structure of waveguide amplifier can be fabricated by the nanoreplication process described herein.  
         [0041]     Propagating the input signal in various core structures having different lengths, widths, and/or heights, can change the mode field diameter and the confinement of the signal in the waveguide structure. A waveguide design using embodiments disclosed herein can be beneficial for high An waveguide amplifiers. For example, these structures can take advantage of both small and large core amplifier characteristics, as well as match the mode-field diameters to the single mode fiber at the interfaces. Further, efficient coupling to a single mode fiber can be accomplished by matching the mode field diameters of waveguide  300  to an input single mode fiber and an output single mode fiber (not shown). Thus, the cross-section of first section  332  at input end  331 , the cross-section of second section  336 , the cross-section of third section  338  at output end  339 , and the length of each section can be changed to adjust the amplification and the coupling to the single mode fibers. Input and output fibers need not be restricted to single mode fibers but can also include multimode fibers.  
         [0042]     According to various embodiments, as shown in  FIGS. 4   a - 4   c , waveguide amplifier  400  can include a substrate  410 , a lower cladding  420 , a core  430 , and an upper cladding  440 . As depicted in  FIG. 4   a , core  430  can include a first section  432 , a second section  436 , and a third section  438 .  FIG. 4   b , which is a top view of waveguide  400 , depicts first section  432  having a cross-section that continuously increases from an input end  431  to an end adjacent second section  436 . Further, as shown in  FIG. 4   c , which is a side view of waveguide amplifier  400 , the height of first section  432  can also continuously increase from first end  431  to the end adjacent second section  436 .  
         [0043]     In various embodiments, second section  436  can have a constant cross-section. As shown in  FIG. 4   b , the width of second section  436  can be constant. Further, as shown in  FIG. 4   c , the height of second section  436  can be constant.  
         [0044]     In various embodiments, as shown, for example in  FIG. 4   b , third section  438  can have a width that continuously decreases from an end adjacent second section  436  to an output end  439 . Further, as shown in  FIG. 4   c , the height of third section  438  can further have a continuously decreasing height from the end adjacent second section  436  to the output end  439 .  
         [0045]     Moreover, as shown in  FIG. 4   b , third section  438  can have a width that continuously decreases from an end adjacent second section  436  to an output end  439 . Third section  438  can further have a continuously decreasing height from the end adjacent second section  436  to the output end  439 , as shown in side view  FIG. 4   c.    
         [0046]     The structure of core  430  can be formed by a shadow photomask technique described herein. The exemplary three section configuration can be formed by a shadow photomask having a UV light transmission profile consistent with the shape of the core height profile shown in  FIG. 4   c . In various embodiments, the length, width, and/or height of each core section can be different. In addition, the structure of core  430  can be fabricated via nanoreplication process, as described herein.  
         [0047]     Propagating the input signal in different core structures with different lengths, widths, and/or heights can change the signal in terms of its mode field diameter and its confinement in the waveguide structure. A waveguide design using this type of structure can be beneficial for low An waveguide amplifiers. For example, these structures can take advantage of both small and large core amplifier characteristics, as well as match the mode-field diameters to the single mode fiber at the interfaces. The cross-section of first section  432  at input end  431 , the cross-section of second section  436 , the cross-section of third section  438  at output end  439 , and the length of each section can be changed to adjust the amplification and the coupling to the single mode fibers.  
         [0048]     According to various embodiments, as shown in  FIGS. 5   a - 5   c , waveguide amplifier  500  can include a substrate  510 , a lower cladding  520 , a core  530 , and an upper cladding  540 . As depicted in  FIG. 5   a , core  530  can include a first section  532 , a second section  536 , and a third section  538 . First section  532  can have a constant cross-section. Third section  538  can also have a constant cross-section.  
         [0049]     Referring to  FIGS. 5   b  and  5   c , second section  536  can include a middle portion  534  having a constant cross-section that can be smaller than the cross-sections of first section  532  and third section  538 . Second section  536  can further include a first end portion  533  adjacent to first section  532 . As shown in  FIG. 5   b , which is a top view of waveguide amplifier  500 , a cross-section of first end portion  533  decreases from a cross-section similar to first section  532  to a cross-section similar to middle portion  534 . To reduce mode field mismatch losses and scattering loss that can result from a sudden decrease in core size, a gradual decrease in core diameter can be used. In various embodiments, first end portion  533  can have a concave shape to gradually change the mode size of the light. The concave shape can be formed using two photomasks by methods known to one of skill in the art. Second section  536  can further include a second end portion  535  adjacent to third section  538 . As shown in  FIG. 5   b , a cross-section of second end portion  535  increases from a cross-section similar to middle portion  534  to a cross-section similar to third section  538 . To reduce mode field mismatch losses and scattering loss that can result from a sudden increase in core size, a gradual increase in core diameter can be used. In various embodiments, second end portion  535  can have a convex shape to gradually change the mode size of the light. The convex shape can be formed using two photomasks by methods known to one of skill in the art.  
         [0050]     Fabrication of waveguide amplifiers can be accomplished using methods similar to those disclosed herein. However, in certain embodiments, a shadow photomask may not be required, and an addition process step can be used. In particular, a separate photomask and a reactive ion etch process can be used for each core height. Similarly, in nanoreplication fabrication process, masters can be fabricated via similar method without a shadow mask on substrates of, for example, silicon, glass, and quartz. Once a master with a waveguide structure is generated, it can be used to form hundreds of stampers via, including but not limited to, ultra violet (UV) light curing and hot embossing processes.  
         [0051]     For example, a first photomask can provide the greater core height of first section  532  and third section  538 , in an embodiment where first section  532  and third section  538  have a similar core height. A second photomask can then be provided to allow etching of core  530  so that the height of second section  536  is lower than the height of first section  532  and third section  538 . In various embodiments, a more complex structure can be made with additional layers by using additional photomasks and etching processes. Similarly, in nanoreplication fabrication processes, masters can be fabricated via similar methods without a shadow mask on substrates, such as, for example, silicon, glass, and quartz. Once a master with a waveguide structure is generated, it can be used to form hundreds of stampers via, including but not limited to, ultra violet (UV) light curing and hot embossing processes.  
         [0052]     Propagating the input signal in waveguide amplifier  500  having various core structures with different lengths, widths, and/or heights, can change the mode field diameter and the confinement of the signal in the waveguide structure. A waveguide design using embodiments disclosed herein can be beneficial for low Δn waveguide amplifiers by taking advantage of both small and large core amplifier characteristics, as well as matching the mode-field diameters to the single mode fiber at the interfaces. The cross-section of first section  532 , the cross-section of second section  536 , the cross-section of third section  538 , and the length of each section can be changed to adjust the amplification and the coupling to the single mode fibers.  
         [0053]     According to various embodiments, as shown in  FIGS. 6   a - 6   c , waveguide amplifier  600  can include a substrate  610 , a lower cladding  620 , a core  630 , and an upper cladding  640 . As depicted in  FIG. 6   a , core  630  can include a first section  632 , a second section  636 , and a third section  638 . First section  632  can have cross-section that continuously decreases from an input end  531  to an end adjacent to second section  636 . Third section  638  can have cross-section that continuously increases from an end adjacent to second section  636  to an output end  639 . Second section  636  can have a constant cross-section.  
         [0054]     Referring to the top view of waveguide amplifier  600  as shown in  FIG. 6   b , first section  632  can have a width that increases from input end  631  to an end adjacent to second section  636 . As shown in side view of waveguide amplifier  600  in  FIG. 6   c , first section  632  can have a height that decreases from input end  631  to an end adjacent to section  636 .  
         [0055]     Referring to  FIGS. 6   b  and  6   c , second section can be curved and can have a constant height. Referring to the top view of waveguide amplifier  600  in  FIG. 6   b , third section  638  can have a width that increases from an end adjacent to second section  636  to input end  631 . As shown in side view of waveguide amplifier  600  in  FIG. 6   c , third section  638  can have a height that increases from an end adjacent to second section  636  to output end  639 .  
         [0056]     Fabrication of waveguide amplifier  600  can be accomplished using methods similar to those disclosed herein. In particular, the waveguide amplifier structure can be made using a shadow photomask having a UV light transmission profile similar to a height of core  630  as shown in  FIG. 6   c . In various embodiments, a more complex structure can be made with additional layers by using additional photomasks and etching processes. Because the input signal propagates within waveguide amplifier  600  in various core structures with different lengths, widths, and/or heights, the signal will be changed in terms of its mode field diameter and its confinement in the waveguide structure. A waveguide design using embodiments disclosed herein can be beneficial for high An waveguide amplifiers by taking advantage of both small and large core amplifier characteristics, as well as matching the mode-field diameters to the single mode fiber at the interfaces. The cross-section of first section  632  at input end  631 , the cross-section of second section  636 , the cross-section of third section  638  at output end  639 , and the length of each section can be changed to adjust the amplification and the coupling to the single mode and/or multimode fibers. In addition, waveguide amplifier can be fabricated via nanoreplication processes, as disclosed herein.  
         [0057]      FIG. 7  shows an exemplary simulated amplifier performance for four waveguide amplifiers. The performance of first conventional waveguide amplifier having a core height of 0.5 μm, a core width of 0.5 μm, and a core length of 7 cm is shown in  FIG. 7  by the circles. The performance of second conventional waveguide amplifier having a core height of 1.0 μm, a core width of 1.0 μm, and a core length of 7 cm is shown in  FIG. 7  by the squares. As shown by  FIG. 7 , the conventional waveguide amplifier having the larger core size of 1.0 μm×1.0 μm×7 cm provides lower efficiency in a small signal regime (Pin&lt;0.1 mW) and a higher efficiency in a saturation regime (Pin&gt;1 mW) compared to the conventional waveguide having the smaller core size of 0.5 μm×0.5 μm×7 cm.  
         [0058]      FIG. 7  further shows simulated amplifier performance for two exemplary waveguide amplifiers having a core with two sections that increases in size. A first exemplary wave guide has a first core section of 0.5 μm height, 0.5 μm width, and 3.5 cm height and a second core section of 1.0 μm height, 1.0 μm width, and 3.5 cm length, shown in  FIG. 7  by the stars. A second exemplary wave guide has a first core section of 0.5 μm height, 0.5 μm width, and 5.0 cm height and a second core section of 1.0 μm height, 1.0 μm width, and 2.0 cm length, shown in  FIG. 7  by the stars. As shown in  FIG. 7 , the two exemplary waveguide amplifiers with varying core sizes provide a higher dynamic range in input power values by performing better in both the small signal and the saturation regime.  
         [0059]     It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed process without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered exemplary only, with a true scope of the invention being indicated by the following claims.